experimental kaon physics

57

Upload: ngodan

Post on 04-Jan-2017

234 views

Category:

Documents


0 download

TRANSCRIPT

COURSE

EXPERIMENTAL KAON PHYSICS

Louis Fayard

Laboratoire de l�Acc�el�erateur Lin�eaire

IN�P��CNRS et Universit�e de Paris�Sud� BP ��� F���� Orsay Cedex

c� ��� Elsevier Science BV All rights reserved

Photograph of Lecturer

Contents

�� Preamble ��� Introduction �

���� De�nitions and historical introduction ����� Phenomenology of the neutral kaon system ������ K � �� amplitudes ��

�� CP violation ��

���� ���� discussion and experimental status ������ The NA�� and E�� experiments ������ Future of ���� KTeV� NA�� and KLOE ��

�� New measurements of the neutral kaon properties� CP violation and CPT tests ����� The CPLEAR experiment ����� The E� experiment at Fermilab ������ Combining Results from m on ��� from di�erent experiments ������ Search for CP violation in K�

� �� ���� Decays forbidden in the Standard Model ��

���� KL � �e ������ Other decays forbidden in the Standard Model �

�� Radiative kaon decays �� Suppressed decay modes ��

��� The K� � ����� mode ����� K� � ���� ��

�� CP�violating decays ������ T�violating polarization in K� � ����� ������ KL � ��e�e� ������ KL � ������ ������ KL � ����� ��

�� Conclusions ��References ��

�� Preamble

This set of lectures is supposed to complement the theoretical reviews thatwere given in this school by a rapid overview of experimental K physics�The aim of this review is not to be exhaustive� In addition to the otherlectures given in this school� standard texbooks ��� and references quotedin the text� the curious reader is invited to consult reviews on CP violation������� rare K decays ����� and related subjects ��������Valuable information can also be found in the proceedings of workshops

on K physics ��������

Among the main objectives of K physics�

i� low energy tests of QCD and applications of non perturbative methods lattice� chiral perturbation theory�

ii� measurements of parameters of the CKM matrix like jVtdj or �

iii� physics beyond the Standard Model going from supersymmetry to trulyexotic physics related to CPT or Quantum Mechanics violations

iv� CP violation and rare decays�the main emphasis will be on topic iv� in particular present and futuremeasurements of ����� Very little will be said on CPT violation ������ andunfortunately the importance deserved by Physics beyond the StandardModel ��� will no appear in the following pages�

Several other important topics closely related to K physics will also notappear�

� asymmetric � factories ����

� Novosibirsk � factory with round beam ����

� L Fayard

� some byproducts of K experiments like �� branching ratios

� nuclear physics like the search for hypernuclei K�stop �

AXZ �A� XZ � �

that will occur at FINUDA FIsica NUcleare a DA�NE� using very lowenergy Kaons from � decay ����

� quantum mechanics tests at DA�NE ������

Section � contains an historical introduction and few reminders on phe�nomenology of the Neutral Kaon System� CP violation is the topic ofsections � and � while rare decays will be covered in sections � to ��

�� Introduction

���� De�nitions and historical introduction

The development of high energy concepts in the last � years owes a lotto K physics� Understanding of this physics generated a succession ofdiscoveries�experimental and theoretical� Since very good historical reviewsexist �������� we will just use the various discoveries in the �eld as a guideto some de�nitions used later�Studying elastic scattering of charged tracks on electrons in emulsions

at Largentiere French Alps�� Leprince�Ringuet and Lh�eritier ���� deducedthe existence of a new particle of mass about �� times the electron�s mass�In retrospect� this appears to be the �rst measurement of the K� or Kaon�Rochester and Butler recorded in ��� an event which we would char�

acterize now as a neutral kaon decaying into two pions� This event waspublished one year later ���� together with a �kink� event � a charged Kdecay in �ight�This is the real birth of K physics�

Decays of charged strange mesons were identi�ed in the early ����s�The masses of particles decaying to various modes clustered more and moreclosely about a single value now known as m K�� � ����� MeV� Howevertwo distinct decays� called �� � ���� and �� � ������ could notcorrespond to the same particle if weak interactions were invariant underP�Indeed� while the parity of �� was ���J � it was shown ���� that the ��

had JP � O�� ��� ���� This �� � � puzzle� was solved in ��� by Lee andYang ���� who realized that no conclusive test had yet been performed ofparity conservation in weak interactions� They suggested such tests and itwas soon proven that� indeed� P is violated by weak interactions�

EXPERIMENTAL KAON PHYSICS �

Meanwhile neutral kaon mixing was proposed by Gell�Mann and Pa�is���� It had been proposed ���� that kaons had isospin ���� implying theexistence of two kinds of neutral kaons� K� � d�s� and �K� � �ds� withstrangeness ��� One combination of neutral kaons KS � K� � �K� coulddecay to ���� �with CP � �� while the other KL � K� � �K� could not�The existence of the KL with its characteristic long lifetime and dominantthree�body decay mode� was con�rmed experimentally ����� At the sametime the proposal of regeneration of KS by KL passing through matterwas made ����� The origin of this phenomena is the di�erence betweeninteractions of K� and �K� with matter � for instance �K�p � ��� has nocorrespondance for K�� Let us consider incident KL on nucleus with f ��and �f ��� the di�usion amplitudes of K� and �K�� The initial state is

jini � jKLi � �p� � � j��j�� � � � ��jK�i � �� ���j �K�i� ����

and the �nal state

jouti � �p����j�j�� � � � ��f ��jK�i � �� ��f ��j �K�i�

� f����f���� jKSi� f����f���

� jKLi ����

The appearance of KS is clear since f �� �� �f ��� Di�erent modes ofregeneration exist ���� � inelastic� di�ractive and coherent�In ��� Christenson� Cronin� Fitch and Turlay found ��� the CP vi�

olating decay KL � ���� with �� events corresponding to a branchingratio BR KL � ����� � �� � ��� ��� in agreement with the mostrecent measurements ��� BR KL � ����� � ���� ���� ���� Thehistory of this experiment have been described in several places ������The apparatus was a two�armed spectrometer shown in �gure ��Cerenkov and scintillation counters� operated in coincidence� triggered

spark chambers� which were recorded photographically� The decay to twopions was distinguished from copious three�body decay in two ways�

� cos� � � for two pions decay where � � angle ��P a�a�����KL�� Indeed in

this case the sum of the momenta of the two particles a� and a� will lineup with the direction of the incident KL� In general this will not happenfor three�body decay�

� m a�a�� � m KL� if it is a two pions decay a � ���

� L Fayard

The angular distribution of the events found in three di�erent massregions is shown in �gure �� About � events are found in the forwardpeak in the proper mass interval � where the background is ���

Fig� �� Plan view of the original CP Violation detector ����

Fig� �� Angular distribution of the events measured by the original CP Violation exper�iment in three relevant mass regions ����

EXPERIMENTAL KAON PHYSICS �

Shortly after its discovery it was pointed by Sakharov �� that CP viola�tion is a key ingredient � together with interactions violating baryon numberconservation and with departure from thermal equilibrium � in understand�ing why the observed universe contains more baryons than antibaryons� Itwas in ���� that Kobayashi and Maskawa proposed ��� a Cabibbo gener�alization of �x� unitary mixing matrix which� in the approximation of theWolfenstein parametrization ��� can be written as�

V �

�� Vud Vus Vub

Vcd Vcs VcbVtd Vts Vtb

�A

�� �� ��

� A� � i��

� �� ��

� A�

A� �� � i�� �A� �

�A� ��

In ���� Gaillard and Lee ��� computed the box diagram of �gure �which corresponds to the KL �KS mass di�erence �mK

�mK � � Re �hKjHS� j �Ki� � �m�c cos

��c�� m�

� K� � ���� ����

and were able to predict a charm mass of few GeV six months before theJ� discovery� This original computation considers only charm quark ex�change in �gure � and neglected long distance contributions� We now knowthat the top quark contribution is small since jV �

tdVtsj� m�t � jV �

cdVcsj� m�c

and that the long distance contributions are small ��� � �! of �mK��One may note that the short distance term charm and top� has beenrecently computed ���� at the NLO Next to Leading Order��

s W d

c c

d W s

s c d

d c s

W W

Fig� �� Feynman graphs contributing to �mK

More recent results on Kaons will be partly described in the followingchapters� The experiments that will be described later are shown on �gure�� They correspond to about half a dozen places where Kaons are produced�

�� L Fayard

FermilabE621E731, E773, E799IE799II, E832 (KTeV)

CERNNA31 NA48CPLear

Dubna, Serpukhov

BrookhavenE777, E851, E865E787E791, E871E845E926

FrascatiDAØNE

KEK137162246

Fig� �� K�physics experiments

���� Phenomenology of the neutral kaon system

Only selected results concerning the phenomenology of the K� �K� systemwill be given here� A full discussion including phases conventions can befound elsewhere� An arbitrary neutral K�meson state

ajK�i� bj �K�i ����

is governed by the time�dependent Schr�odinger equation

id

dt

�ab

�� H

�ab

���M � i�

�� ab

� ����

where M and � are �x� hermitian matrices� The hermitian part M iscalled the mass matrix while the anti�hermitian part �i��� describes theexponential decay of the K��meson system�

EXPERIMENTAL KAON PHYSICS ��

The element of the mass matrix are given by

Mij � m� ij � hijHS�jji�Xf

PhijHS�jfihf jHS�jji

m� �Ef ���

where P stands for principal part� the index i � � �� denotes K� �K�� andthe sum is taken over all possible intermediate states�

The elements of the decay matrix are given by

�ij � ��Xf

hijHS�jfihf jHS�jji mo �Ef � ����

where the sum is taken over only real �nal states� If the hamiltonian isinvariant under T� CPT or CP we have the following conditions�

T � jM�� � i���� j � jM��� � i

����� j

CP � jM�� � i���� j � jM��� � i

����� j " M�� �M�� " ��� � ���

CPT � M�� �M�� " ��� � ���

����

From now on we assume CPT invariance� The physical states KS

and KL are usually parametrized as� using jK�i � �p� jK�i � j �K�i� and

jK�i � �p� K�i � j �K�i�

KS � �p����j�j�� � � � ��jK�i� �� ��jK�i� � pjK�i� q �K�i

� �p��j�j� jK�i� �jK�i�

KL � �p����j�j�� � � � ��jK�i� �� ��j �K�i� � pjK�i � qj �K�i

� �p��j�j� jK�i� �jK�i�

����

We may note that pq � � and Hij are phase�dependent values and not

physical quantities� If � is the proper time� the KS and KL will evolvewith an exponential time dependance low

jKLi � e�i�L jKLi jKSi � e�i�S jKSi ����

where �L � mL � i�L� and �S � mS � i�S�

�� L Fayard

The hamiltonian H can then be written

H ��

� �L � �S�

pq �L � �S�

qp �L � �S� �L � �S�

� �����

���� K � �� amplitudes

It is convenient to express j����i and j����i in terms of well�de�nedisospin jI � i and jI � �i states I � � is forbidden by Bose statistics�

j����i �q

�� j�� I � �i�

q�� j�� I � ��i

j����i � �q

�� j�� I � �i�

q�� j�� I � ��i

�����

Usually one de�nes

h�� I � �jHW jKoi � a�ei��

h�� I � �jHW j �K�i � a��ei��

h�� I � ��jHW jK�i � a�ei��

h�� I � ��jHW j �K�i � a��ei��

�����

where � and � are the strong phases� Using

a��S�L� � h�� I � �jHW jKS�L�ia��S�L� � h�� I � ��jHW jKS�L�i

and de�ning � � a��L�a��S " �� � a��L�a��S" � � a��S�a��S one obtains

��� � h����jHW jKSih����jHW jKLi � �� ���

��p��

��� � h����jHW jKSih����jHW jKLi � �� ��

����p�

with �� � �p������ � �

� �����

One may also write � and �� in another way� usingt� � Im a�� � Re a�� and t� � Im a�� � Re a��

� � ��it���i�t�

�� � ip�

Re�a��Re�a��

�� ��� t��t����i�t���

ei������� �����

Several comments can be made ��������

EXPERIMENTAL KAON PHYSICS ��

� Up to now there are four observations of CP violations � KL ������ ����� the semileptonic charge asymmetry as � �� KL � ���X��� KL � ���X����� KL � ���X� � � KL � ���X�� � � Re ��� � � j�j��and the interference between KL and KS in ������ They all can be de�scribed by a single CP violation parameter �� One can show that Re �� isrelated to CP violation in mixing�

� It is clear from formula ����� that �� vanishes as a� and a� have thesame phases� i�e� t� � t� � � One can show that Re �

�� is related to CPviolation in decay direct CP Violation�� One may note that ���� Phase ��� Phase i�ei�������� ��� ���

� All usual phase conventions assume t�� t� � �� to � is the historicalWu�Yang convention ���� rarely used now in theoretical papers�

� We have experimentally ja��a�j �� ����� ���� It is the �I � ��� rule�Using it one has ��� �� ��� and ��� � �� ���

� One has �m��� � mL �mS�� �S � �L� � ������ ����

� If ��� and ��� are the phases of ��� and ��� and �SW �atan ��m���� � ����� ���� being the superweak phase one has

��� ��� Phase �� �SW

these equalities being correct ��������� with a precision � ���� Since thevalues of the phases of � and �� are similar one has ���� Re �����

� After the discovery ��� of the importance of electroweak penguins� therehave been several computations ���������� of ���� at the NLO in the Stan�dard Model� Let us just say here that there is a general #� agreementthat

��few ���� � ���� ���

�� L Fayard

�� CP violation

���� ���� � discussion and experimental status

The measurement of ���� is one of the most challenging in High EnergyPhysics� There are two correlated di$culties in measuring the double ratio�

R �� KL � ����� � KS � ������ KS � ����� � KL � �����

�� Re ����� ����

� There is a need for a low statistical error and therefore a high rate�in particular for the rarest CP violating channel KL � ����� One has�neglecting the statistical errors on the other channels

d ����� � dR� �

�qN��KL

whereN��KLis the number of detectedKL � ���� WithN��

KL� � �� � � �

one gets d ����� � � ��� �����

� There is a need for a low systematic error� Since R is a double ratio alarge number of the systematic errors cancel�

The current results are coming from the E��� experiment at Fermilab andfrom NA�� at CERN that will be described below�

E��� has published�

� In ����� the results ���� from a ���� test run with converted photons

���� � ��� ��� ��� ���

� In ���� the analysis ���� of data taken between August ���� and February���� including reanalysis ��� of �! of the data�taking�

���� � ���� ���� ���� ���

NA�� has published�

� In ���� the analysis ���� of ��� data

���� � ��� �� �� ���

EXPERIMENTAL KAON PHYSICS �

� In ���� the analysis ���� of ����� ���� data

���� � �� �� �� ������� � ��� ��� ��� including ��� data�

The error for E��� is dominated by statistical uncertainties while thesystematic error dominates in the NA�� result� The NA�� result tends to�nd evidence for direct CP violation� while it is not the case for the E���result� Although the results di�er by less than two standard deviations�the conclusions are su$ciently di�erent that� as we will see in

H���� three

new experiments have been built to re�measure �����

���� The NA�� and E��� experiments

The NA�� experiment is described in detail elsewhere ����� We list belowsome of the important characteristics�

� There are two di�erent running modes � the KL beam and the KS beam�They alternate every � hours�

� The decay distributions are a priori di�erent � for an energy of � GeVone has ��c�S � ���m and ��c�L � ��� km� In order to have similar ac�ceptances inside the large �dutial length � � m�� the KS target and theassociated collimator move on a train of �� stations in � � m� This isshown in �gure ��

� There is no magnetic �eld so the charged mode resolution on the kaonmass is only � �� MeV� This resolution is important in order to cure asmall residual asymmetry between KL and KS � the charged backgroundis negligible in the KS case and not in the KL case KL � ������� KL ��e�� KL � ����� In NA�� the KL background is � ��!�

� The neutral modes are measured with a Liquid�Argon calorimeter withan energy resolution E in GeV��

�EE�

�����

E

��

����pE

��

� ����

����

and a position resolution of �� mm�

�� L Fayard

Fig� �� Schematic layout of KL�KS beams of NA�� ����

The positions and energies of the four photons are measured� If D isthe distance between the decay point and the detector� the invariantmass of an event with n photons with energy Ei and relative distancedij �

p xi � xj�� � yi � yj�� is

m� ��

D�

Xi�j

EiEj d�ij ����

Under the assumption that an event with four photons detected is a ���

decay from a kaon� the vertex position Zvertex � Zcalo�D can be obtained see �gure �

Zvertex � Zcalo �D � Zcalo � �

mK

sXi�j

EiEj d�ij ����

This vertex can then be used in order to compute the masses of the two�� candidates and reject the KL � ��� decays in which two of the sixphotons escaped detection in the detector� The �� mass resolution is

EXPERIMENTAL KAON PHYSICS ��

� ��� MeV and the background under KL � ��� is � ���!� We cannote that� since there are two �� candidates� a mass resolution � timessmaller would give a background �� � � times smaller� One sees from �g�ure � � showing the KS beam train of NA�� � that the beginning of theKS decay region is de�ned by an anticounter � a small scintillator counterwith � mm of lead placed directly in front of it� This counter situated �cm downstream from the KS collimator vetoes upstream decays and estab�lishes a well�de�ned leading edge to the decay distribution� which allowsto calibrate the energy scale of the liquid�argon calorimeter� using the fact see formula ���� that the distance D is proportional to the energy E�

Z vertex Z calo

D

Z

E1, x1, y1

E3, x 3, y3

E2, x 2, y2

E4, x 4, y4

Fig� �� Reconstrution of the vertex in the neutral mode

� All kaon CP experiments have some sensitivity to accidental activity �activity occuring on top of a triggered event � caused by other K decay orby interactions in the beam pipe� The e�ects are di�erent for neutral andcharged modes since the detectors are di�erent� In NA��� in addition� theyare di�erent for KS and KL since the KL beam is more intense� It is avery di$cult problem that is solved computing the e�ect ���� by overlaying�random events� to triggered events�

Let us now turn to the E��� experiment which is described in detail else�where ������������� In fact the same apparatus shown is �gure � is usedfor three experiments�

�� L Fayard

Fig� � The NA�� KS train ����

� E��� with one regenerator� the aim being the measurement of �����

� E��� with two regenerators� looking at ���� ��� and �m�

� E���I with no regenerator� dedicated to rare decays search�

Some speci�cations of E��� are listed below�

� There is one KL beam producted by � GeV protons� Part of the KL

beam is going through two interaction lengths of boron carbide B�C� pro�viding coherently regeneratedKS � Downstream� the �� decay rate is givenby

d�dz �

jj� e�z�� c�S � j�j� e�z�� c�L

� � jj j�j coshm z� c � �� � ��

ie�z����S����L���� c

����

where z is the distance from the downstream end of the regenerator and is the coherent regeneration amplitude� Since jj�j�j � ��� the KS decay

EXPERIMENTAL KAON PHYSICS ��

Fig� �� E�� apparatus

term dominates�

Each spill each minute� the regenerator is moved from one beam toanother� so the same part of the apparatus sees KL and KS beams�

� There are two sets of data taking� neutral and charged� In each set KL

and KS are recorded� so the e�ect of �accidental activity� is small� �! ofthe data taking was done ��� with the four modes at the same time�

� There are large acceptance corrections� For KL the distance from thetarget is ������ m for ���� and ������ m for ����� For KS the distanceconsidered is ������� m�

� The charged mode is detected with a very good charged spectrometerwith P kick

T � � MeV� The momentum resolution is�P�P � ���! �� � P����� GeV�� The charged background in theKL beamis � ��!� The neutral modes are detected with a Lead Glass calorimeter�where the main problem is the monitoring of the linearity�

�� L Fayard

� In the neutral mode� there is some background � ���! in KS� ���! inKL� due to the scattering in the regenerator�

The comparison of the systematic errors in NA�� on E��� is shown intable � adapted from ref ����

���� Future of ���� � KTeV� NA� and KLOE

Three signi�cant e�orts to measure ���� with a precision of � to � ��� overthe new few years are underway at Fermilab KTeV�� CERN NA��� andFrascati KLOE�� They will be described below�

������ The KTeV experiment

The KTeV Kaons at the TeVatron� consists ��� in two physics pro�grams� E���� addressing the ���� measurement and E���II studying rarekaon decays� The E��� con�guration is shown on �gure �� The main dif�ferences with E��� are�

� E���� like E���� uses a fully active regenerator�

� There is a better charged spectrometer with a new magnet P kickT � �

MeV� and new chambers giving � mK�� � MeV�

� There is a new calorimeter with undoped� CsI cristals of ���x���x� cm�

or �x�x� cm�� The e�� di�erence is much smaller with this calorimeterthan with the Lead Glass that was used in E���� The energy resolution���� is better than �! for energies larger than � GeV�

KTeV began to take data in October ���� the run ending in September����� Part of the running time was devoted to the rare decay experimentE���II� In this case the detector con�guration di�ered from that used forE��� in several respects�

� The regenerator was moved out of the beams� resulting in two parallelKL beams�

� Transition radiation detectors were added� Providing a ��e rejection of� �� � � for an electron e$ciency of ��� they will help to reject back�grounds to ��e�e��

EXPERIMENTAL KAON PHYSICS ��

Table �

Comparison of systematic errors from NA�� and E���

FNAL CERN NA��E�� ����� data �� data

STATISTICS

KL � ���� ��� k ��� k ��� kKS � ���� ��� k ���� k ��� kKL � ���� �� k �� k ��� kKS � ���� ���� k ���� k ���� k

SYSTEMATIC ERRORS ����

������

acceptance ��� �� ��calibration ��� ��� ���

accidentals ��� ��� ���

backgrounds

��� �� ��� ����e� ��� �� �� ���

regenerator ���ine�ciencies���� ��� ��� ���

RESULTS �����������

�� � ��� � ��� �� � � � � �� � � ��� � ��� ���� � ���

� The intensity was increased to improve the rare decay sensitivity�

During the �������� run E��� collected about � millions KL � �����This data sample should result in a statistical uncertainty of around ��� ��� on ����� The experiment hopes to reduce the systematic error to abouthalf of the statistical error� though achieving this goal may require a few

�� L Fayard

Fig� �� Plan view of KTeV detector �E��� con�guration�

years of e�ort�In order to reach the design sensitivity on ���� ���� E��� proposed to

run � weeks in ���� with several improvements� In particular a new slowspill length of � sec� currently sec�� with a � sec� �at top increasedfrom � sec�� will give a �! increase in kaon yield per hour withoutincreasing the instantaneous rate in the detector�

������ The NA� experiment

The NA�� detector ���� is shown on �gure �� It is very di�erent fromNA�� contrary to the Fermilab e�ort�� A neutral KL beam is derived at��� mrad from a Be target hit by �� GeV protons� Only a small fraction � ���� of the primary proton beam is necessary for KS production if onewants a similar rate of K � �� decays from both beams in a �ducial vol�ume of about three KS decay lengths� It is possible to deviate this amountfrom the primary proton beam by channeling in a silicon bent crystal �����Then there are two simultaneous and nearly collinear KS and KL beams�Some important parts of the detector are described below �

� There is a magnetic spectrometer with � P ��P � ��! � ��PGeV !giving a Kaon mass resolution � m� � �� MeV that should put the Ke�background at a level � �� ! compared to �! in NA����

EXPERIMENTAL KAON PHYSICS ��

Fig� ��� Layout of NA��

� The neutral modes are detected with a Liquid Krypton homogeneousionization calorimeter with � �� cells of �x�x�� cm�� This device isvery stable� The position resolutions are �x�y

� � mm� the time resolutionof a ��� event is � t�

� � ps and the energy resolution is E in GeV�

� E��E � ���!�pE � ��!� ����E�� The KL � ��� background will

be at a level� ��! instead of ���! in NA����

� KL and KS are distinguished by tagging the protons producing the KS

component� For both neutral and charged modes� this kaon tagging isrealised by making a time coincidence between the detectors measuringthe kaon into � pions decays and a highly segmented tagger where the rateis �� MHz�

In order to minimize acceptance corrections� it is planned to weight KL

decays according to the measured z position by a factor e�z���P � where P � � Pc�S

m � �� �S��L�� The z distributions of KS and weighted� KL

decays are thereby made similar�

�� L Fayard

This weighting procedure decreases a lot the uncertainty due to theacceptance correction� However� as a drawback� the e�ective number ofusable KL is decreased and NA�� needs to work at a higher kaon rate thanE����

About � KL � ��� have been registered in the short run periodof September and October ����� The statistics will probably be multipliedby � � in ����������We can stress that the accidental rate is very important in the CERN

experiment� about � times more important than at Fermilab" in particu�lar�

� The e�ective spill occupancy is smaller by a factor ��� at CERN � � ���! ��� sec�each ���� sec�� than at Fermilab � � � �! �� sec� each sec���

� There are � � �� K��mn at CERN � � times larger than the ����� K��spill at Fermilab�

������ The KLOE experiment at DA�NE

DA�NE Double Annular � factory for Nice Experiments� is a high lu�minosity electron�positron collider ��� that will operate at the energy ofthe � resonance �� MeV in the center of mass�� The luminosity requiredfor physics see below� is � � ��� cm�� s�� corresponding to �� bunchesand �A stored in each beam� With �� � ��� ���� cm� one obtains ������ � mesons in one �year� � �� sec���

We have ����

j�i � jK� �K� C � oddi

� �p�

�jK� %z� �K� �%z�i � j �K� %z�K� �%z�i� ����

where %z �%z� is the direction of the momenta of the kaons in the cm system�Without assuming CPT invariance one has

jKSi � p� jK�i� q� j �K�i

jKLi � p jK�i� q j �K�i ���

EXPERIMENTAL KAON PHYSICS �

and formula ��� becomes

j�i � �p� qp� � q�p�

jKL %z� KS �%z�i � jKS %z� KL �%z�i

����

Note that this formula does not assume CPT invariance� It is just there�ection of the fact that � is a C�odd particle conserving C while decaying�One can note�

� That DA�NE is a source of pure backgroundless beams of KL and KS�the C�even backgrounds e�e� � K� �K�� and e�e� � �� � K� �K� are atthe ��� and ��� levels�

� That the beams are monochromatic with mean decay paths dS � ���cm for KS and dL � �� cm for KL�

The KLOE KLOng Experiment� detector ���� is shown in cross sectionin �gure ��� The main motivation of this detector is the measurement ofRe ������The dimensions of the detector are dictated mainly by one parameter �

dL� The detector consists of a � m radius drift chamber� specially designedin order to minimize regeneration surrounded by a Lead�scintillator �berselectromagnetic calorimeter� The energy resolution is � E��E � ���! at� GeV and the timing resolution is � t� � �� ps� also at � GeV� Sincethe produced K mesons are monochromatic� one can measure the �ightpath of neutral kaons by time of �ight� There are two methods to measureRe ������

The double ratio method

Like in other experiments one has

R �� KL � ����� � KS � ������ KL � ����� � KS � �����

� �� Re ��

��

with the values of the mean decay paths quoted above the number of KS

is larger the number of KL� With one year at full luminosity� one will have� � KL � ��� corresponding to an uncertainty of ��� ��� on Re ������

The time evolution method

Let us consider the decay chain �� KS KL � ��������� The di�er�

�� L Fayard

Fig� ��� Cross section of the KLOE detector at DA�NE �distances are in mm�

ential rate of events with a ���� pair produced at distance dc from theinteraction vertex and a ���� pair produced at distance dn is ��������

N dc� dn� � j���j� exp � dcdL� dn

dS

� j���j� exp � dndL� dc

dS

� �exp � �

�S dc�dn

dS

Reh�������exp

�im�S dc�dn

dS

�i ����

where � � �S � �L��� and �m � mL �mS �

EXPERIMENTAL KAON PHYSICS ��

The measurement of Re ��

�can be obtained using �d � dc � dn � d

asymetries� One has

A �N d � ��N d � �

N d � � �N d � � �Re�

� ����

For one year at full luminosity the expected statistical error on Re �����is � ��� ���� The commissionning of the detector should happen in thebeginning of �����

�� New measurements of the neutral kaon properties� CP viola�

tion and CPT tests

A large part of the new measurements of K properties are due to theCPLEAR experiment that will be described below�

���� The CPLEAR experiment

This experiment studied at LEAR at CERN neutral K mesons producedin equal numbers in proton�antiproton annihilations at rest

p�p � K���K�

p�p � K��� �K� ����

The charge of K� ��� tags the strangeness S of the neutral K at t��Antiprotons of � MeV�c are stopped inside a high pressure gaseous hy�drogen target about � per second�� The cylindrical detector �gure ���is placed inside a solenoid of � m radius� �� m length which provides amagnetic �eld of ��� T �����

������ Measurement of �m

The experiment has producted a new measurement of the KL�KS massdi�erence �m using semileptonic decays of neutral kaons� If � is the decayeigentime of the kaon one de�nes an asymmetry ����

Am��� � �R����� �R������� �R�����R������R����� �R������� �R�����R�����

�� cos ��m�e�

������L����S�

�� � �Re x��e����S � ��� �Re x��e����L ����

where x � A �K� � �������A K� � ������ describes the amountof violation of the �S � �Q rule and R�� �R�� R�� �R� are decay rates

�� L Fayard

Fig� ��� The CPLEAR experiment

depending of the strangeness of the neutral kaon at the production timeand on the charge of the decay lepton e� or e��

R� �� � R�K� � � �� e����� ���R� �� � R� �K� � � �� e������ ��R� �� � R�K� � � �� e������ ���R� �� � R� �K� � � �� e����� ��

����

The asymmetry versus the decay time is shown on �gure ��� The valuefound ��� is �m � ������� ���� ������s��������� Direct measurement of the T and CPT Violation Parameters

Amplitudes which violate �S � �Q are expected to be very smallwithin the Standard Model� Semileptonic decays might therefore be usedto tag the strangeness of the neutral kaon at the time of its decay� Thedecay�rate asymmetry between a K� decaying as �K� and T�conjugatedprocess �K� decaying as K�� can be measured if we also know the initialstrangeness� A similar asymmetry is constructed between CPT�conjugated

EXPERIMENTAL KAON PHYSICS ��

Fig� ��� Asymmetry �m� as a function of the decay eigentime measured by CPLEAR����

process� Assuming x � one has

AT �� ��R�����R�����R�����R����

� R� �K��K������R�K�� �K�����R� �K��K������R�K�� �K�����

� �Re �T �

ACPT �� ��R�����R�����R�����R����

� R� �K�� �K������R�K��K�����R� �K�� �K������R�K��K�����

����

The measurements ���� are hACPT i � ��� ���� ������� and hAT i � �������������� which is the �rst direct evidence of T violation at ��!C�L�

������ Measurement of ���

If the rates for initially pure K� and �K� decaying into a ���� �nalstate are R �� and �R ��� a precise measurement of ��� � j���jei��� isobtained by �tting the time�dependent decay�rate asymmetry

A�� �� ��R�����R����R�����R���

� �� j���j e�

�� ��S� ��L

�cos ��m� ����

� � j���j� e����S����L� ����

�� L Fayard

The �t takes into account the residual background contribution and thenormalization factor � �� � �Re ���� where � is the tagging e$ciency of�K� relative to K�� A preliminary result on the full statistics is �������

j���j � ����� ���� ��� ���

��� � ����� ��� ��� ���� ���

where the last error is due to the uncertainty on �m�

���� The E��� experiment at Fermilab

E��� is seeH���� a modi�ed E��� set�up� with a downstream regenerator

added� A KL beam striking a regenerator results in a coherent superpo�sition of the form KL � KS� The decay rate of this mixture is givenby

Rate K � ����� � jj�e����S � j���j�e����L��jjj���jcos �m� � �� � ����e

� �

���S

� ��L

� ����

Experimentally one can easily extract �� � ���� In order to get ���one needs a means of correcting for the regeneration phase ��� Doing thisthe E��� experiment obtains �����

��� � ������ ���� ����� ����

where the value has been averaged with the E��� result ���� The pro�cedure of correcting by �� has produced a living discussion ����� Onecan note that this experiment produced also the best measurement of�� � ��� � ��� � ���� �����

Another important measurement ���� by E��� is the interference be�tween K�

S and K�L in K� � ����� which had been previously ���� ob�

served by E���� An elegant point is that with the interference one trulymeasures�

��� �A KL � ������CP violating�A KS � ������CP conserving�

����

Figure �� shows the ����� decay probability�The results obtained are j����j � ����� � ��� ��� and ���� � ���� � ������ Comparison with ��� and ��� gives excellent agreementwhich implies that all one sees is the CP impurity in the K states�

EXPERIMENTAL KAON PHYSICS ��

Fig� ��� The decay probability for K � ����� measured ����� by E� as a functionof proper time

���� Combining Results from �m on ��� from dierent experiments

The CPLEAR collaboration has performed ������� an analysis for obtain�ing the best value for �m and ��� taking properly into account the factthat di�erent experiments have di�erent correlations between the two vari�ables� In addition to the CPLEAR� E��� and E��� data quoted aboveprevious results are also used ���� and shown on �gure ���The results of the global �t are �m � ������ � �������� s�� and

��� � �������� in agreement with �SW � ��������� These resultsmay be combined ���� to limit the K� � �K� mass di�erence� yielding

jmK��m �K�jmK� � �mj���j

mK� sin �sw

j��� � �sw ��� j

� � ���� �! C�L�

����

���� Search for CP violation in K� � ��

There are two �� decay modes for each of the neutral K mesons

K� � ��� CP � �� I � �� �K� � ������ CP � �� I � �� �

CP � � I � � � �����

�� L Fayard

Fig� ��� A compilation of � m and ��� results

The CP violating parameters are

���� �A KS � ����

A KL � �������� �

A KS � �������CP violating�A KL � �������

�����

������ � �� mode

The observation of KS � ��� will be very di$cult and would be a newobservation of indirect CP violation� DA�NE will be get � � events peryear� The best published measurement of ���� comes from ITEP ���

Re ����� � ���� ��� Im ����� � ���� ����

Very recently improved values were obtained by CPLEAR �����

Re ����� � ���� ���� � Im ����� � ���� ��� �������� ������ mode

The best result is coming from the CPLEAR experiment ���� by �t�ting the time dependent K� �K� decay rate asymmetry� which is a direct

EXPERIMENTAL KAON PHYSICS ��

measurement of the KS �KL interference term� This asymmetry is givenas

A��� �� ��R��� �� �R��� ���R��� �� �R��� ��

� �Re ��� ��Re �����cos �m��

�Im �����sin �m���e� �

�� ��S� ��L

�����

where �R��� �� and R��� �� are the �K� and K� decay rates� The result

Re ����� � ��� ������ ���Im ����� � ��� ������ ���

�����

is shown on �gure � together with the Fermilab E�� result ���� �obtainedby �xing Re ����� � Re ����

Fig� ��� ��� from CPLEAR ��� and E��� ���

CPLEAR ���� and E�� ���� have also measured the amplitude of theCP�conserving decay

� ��� �A KS � �������CP conserving�

A KL � ������� �����

�� L Fayard

which gives the branching ratio B��� � B K�S � ������� CP � ��

B��� � ���������� � ������� E��

B��� � ���������������� ��

�� CPLEAR ����

in agreement with the predictions ���� of B��� � ���� �������

�� Decays forbidden in the Standard Model

The �rst part of this chapter will be devoted to the search ���� of KL � �e�where the limit is the lowest upper limit ever reached in K�physics �eld�Then other decays K� � ����e��KL � ���e�KL � e�e����� will beshortly discussed�

���� KL � �e

Some remarks can be made on this decay�

� It is absolutely forbidden in the Standard Model with no neutrino masses�

� The branching ratio is expected ����� to be very small B � ����� inSU ��L � U �� with non�zero neutrino masses� so the observation of thisdecay would therefore be a clear indication of new physics� Examples ofnew physics are shown on �gure �� � exchange of a heavy X boson or of aleptoquark LQ� The KL � �e decay through X echange of �gure �� a�can be compared the K� � ��� decay ����� Assuming the same V�A formof the interaction one has

� KL � �e�

� K� � �����

�ff ��M�

�g�sin�c�M�

W

�� ����

where g is the electroweak coupling� �c the Cabibbo angle� MW the Wmass� Assuming f � f � � g one obtains

M�� �� TeV

�����

B KL � �e�

� ��

����

EXPERIMENTAL KAON PHYSICS �

s µ

d e

LQ

+

-

(b)

s µ

d e

X+

-

(a)

f f'

Fig� �� Diagrams relevant to KL � �e �a� exchange of a heavy X boson� �b� exchangeof a leptoquark LQ

In this scenario� the current limit of ��� ���� implies a lower bound of� TeV on MX � One may also notice ����� that� assuming that the B or Kcouplings to this heavy X are equal� one �nds�

BR B� � �e�

BR KL � �e� �B mB

�L mK� � ��� ����

This indicates that rare kaon decays are more likely to be sensitive tothis kind of new physics than rare B decays�

� There is a huge background from KL � � � ���e�� In this case themaximum reconstructed mass is

Memax� � m�

K �m�� �m�

���� � mK � ���� MeV ����

In order to use this bound� the experiments need to have a very goodmass resolution� KEK��� ����� and BNL��� ����� have mass resolutions measured for KL � ��� of ��� and ��� MeV� The lack of signal in theregion corresponding to M �e� � MK allows �! con�dence level upperlimits to be set� These are B KL � �e� � ��� ���� for KEK��� andB KL � �e� � ��� ���� for BNL���� The data from this experiment isshown in �gure �� as a scatter plot of the reconstructed mass M�e versusp�T � where pT is the component of the �nal state momentum transverse tothe initial KL direction�The BNL��� experiment� which is shown in �gure �� has a double arm spec�trometer� with the neutral beam passing between the two arms down thecenterline� There is momentum measurement using two analyzing magnetsand redundant particle identi�cation both for electrons and muons� Thereare large background rates in downstream detectors due to neutral beamdecay�

�� L Fayard

The successor of this experiment � BNL���� shown in �gure � has im�plemented several changes ����� in order to make better use of the increasedbeam intensity�

Fig� ��� Distribution of M�e versus p�T

for BNL �� �����

Fig� ��� The BNL �� detector

EXPERIMENTAL KAON PHYSICS ��

Fig� ��� The BNL �� detector

The neutral beam is stopped inside the spectrometer by a novel hadronicbeam plug� Straw chambers � mm diameter straws with a fast CF��ethanedrift gas� provide tracking in the upstream section of the spectrometerwhere the rates are the highest� Downstream of the beam stop� where therates are lower� conventional drift chambers are used� The ��� experimentran �� weeks in ���� and � weeks in ���� implying a sensitivity of ���� ����� close to the design goal�

���� Other decays forbidden in the Standard Model

� The current limit on K� � ����e� is B � �� ���� �! C�L fromthe BNL��� experiment ����� This experiment is continued as experimentBNL�� ����� which� from the �������� data set� expect a single eventsensitivity of � �� ���� that should decrease by a factor of � in the ��������� run�

� Finally we quote the �! con�dence level limits B KL � ���e� ���� ��� and B KL � e�e������ � ��� ��� obtained by the Fermi�lab experiment E���I ������

�� Radiative kaon decays

This chapter summarizes the experimental results on radiative decays�where decays into a virtual �& are included� Table � shows the results

�� L Fayard

Table �

Results on radiative decays� The limits quoted are at ��� con�dence level�

Decay mode Experiment ResultKL � �� NA�� �������� B�KL � ��� � ���� � ��� �������

KS � �� NA�� ��������� B�KS � ��� � ���� ������

KL � e�e�� NA�� ����� B�KL � e�e��� � ���� ������

BNL��� �����KL � ����� E��I ����� B�KL � ����� � ���� � ��� �������

NA�� ����� B�KL � ����� � ��� � �� ������

KL � e�e��� E��I ����� B�KL � e�e���� � ���� ��� ������

BNL��� ����� B�KL � e�e���� � ���� �������

KL � e�e�e�e� E��I ���� B�KL � e�e�e�e�� � ���� ������

BNL��� �����NA�� �����KEK�� �����

KL � e�e����� E��I ����� B�KL � e�e������ ��������

��������

�� event observed

KL � ���� NA�� ����� B�KL � ����� � ��� � ��� ����

E��I ����� B�KL � ����� � ���� � � � �� ����

KL � �� NA�� ����� B�KL � ��� � ��� ����

KL � ����� NA�� ����� B�KL � ������ � ��� ����

K� � ���� BNL� ����� B�K� � ����� � ���� � ��� � �� ����

��� MeV�c � p�� � ��� MeV�cKL � �e�� NA�� ����� agreement with bremsstrahlungK� � ��e��� ISTRA at agreement with bremsstrahlung

Serpukhov �����K� � ������ BNL� ����� B�K� � �������

� ��� � �� � ������

K� � ��e�e� BNL��� ����� B�K� � ��e�e��� ��� � ��� ��� ����

on these decays� Most of these decays provide stringent tests of chiralperturbation theories� Details of the comparison between data and theorycan be found elsewhere ���� It should be stressed that NA�� and evenmore E���II will in several case provide large improvements of these re�sults� In particular E���II� in the run ending in September ���� must have� � KL � e�e��� events which corresponds to an increase of � �compared to what was used in table ��It is important to note that the decay KL � ����e�e� which is ex�

pected to proceed dominantly through a ������ intermediate state hasbeen recently observed by KEK�� ���� and E���II ������ which has re�ported a branching ratio of ��������� with � �� events corresponding

EXPERIMENTAL KAON PHYSICS ��

to � � day of data� A preliminary analysis has been performed by E���IIon a three�week data set and is shown on �gure ���

Fig� ��� KL � ����e�e� mass peak from E��II three week data set ���

This decay is very important because the distribution of the angle �between the ���� and e�e� decay planes is predicted to contain a CP�violating term proportional to sin ��� Unlike other CP�violating e�ectspreviously observed� this e�ect is predicted to be important ������ �!�The E���II experiment hopes ���� to measure this asymmetry with anuncertainty of ���! using the run that ended in September ����� Theultimate error� with data that will be registered in ���� should be � ���!�

�� L Fayard

� Suppressed decay modes

���� The K� � ����� mode

This decay mode is very interesting ����� because it �rst arises at the oneelectroweak loop level in the Standard Model as shown in �gure ���

Fig� ��� Feynman graphs of K� � ����� in the Standard Model

As a consequence it is suppressed by some nine orders of magnitude withrespect to the kinematically identical Ke� decay� This mode gives theo�retically very clean information since it is short�distance dominated� Theambiguity due to hadronic matrix elements can be removed by taking theratio with Ke� using isospin symmetry� Corrections to this approximationhave been extensively studied ������ The next to leading QCD correctionshave been computed ������ In the current experimental and theoreticalsituation� the most interesting potential is that of determining jVtdj froma measurement of B K� � ������� The intrinsic theoretical uncertaintyin jVtdj is � �!� However� since jVtdj is not yet tied down the presentprediction of the branching ratio is more uncertain �����

B K� � ������ ���� ��������The BNL��� experiment has recently announced ���� the observation

of a strong candidate for K� � ����� with the analysis of their ���� data�The experiment uses K� that are stopped in a scintillating �ber target inthe center of the detector shown on �gure ���The detector itself covers close to �� solid angle and is in a �kG mag�

netic �eld� The signature for K� � ����� is a K� decay to a �� ofmomentum p � ��� MeV�c and no other observable product� The major

EXPERIMENTAL KAON PHYSICS ��

Fig� ��� The Brookhaven E� experiment

background sources are the copious two�body decays K� � ���� K���with a �! branching ratio and p � �� MeV�c and K� � ���� K���with a ��! branching ratio and p � ��MeV�c� The experiment requiredan identi�ed K� to stop in the target followed� after a delay of at least�ns by a �� whose range and energy must be between the K�� and K��peaks� The range and energy of event candidates passing all other cuts isshown in �gure ���One event consistent with the decay K� � ����� was observed� The

expected background from all sources is ����� events� This event is ina �golden region� where the expected background is ��� �� and with��! of the acceptance of the signal region� A reconstruction of the eventis shown in �gure ���The decay timesK � �� � � � and �� e are ������� ns� ������ ns

and ������ �� ns respectively� The clean �� � �� decay can be seen in

�� L Fayard

Fig� ��� Final event candidate ����� for K� � ����� a� data b� Monte�Carlo

Fig� ��� K� � ����� event �����

the upper insert of �gure ��� There is no signi�cant activity anywhere elsein the detector at the time of the K� decay� The branching ratio impliedby this observation is B K� � ������ � ����������� �

��� about four timesmore than the Standard Model predictions� The expected sensitivity fromthe data already on tape ���������� is � ��� times that of the ���� dataalone ���������� If the Standard Model prediction is correct BNL��� isunlikely to get more than a handful of events by the end of ����� Sincethe relative error on jVtdj is about �

� that on B K� � ������ a signal

EXPERIMENTAL KAON PHYSICS ��

of � events would determine jVtdj to about ��!� There are future plans����� at Brookhaven for upgrades that should allow in year �� to measurejVtdj with a precision better than ��!� At this time scale there is also apossibility ����� to measure K� � ����� in �ight at the Main Injector ofFermilab using a �velocity spectrometer� in which RICH�s would be usedselect incident K� and to measure the daughter pion velocity�

���� K� � ����

Table � shows the status of the K� � ���� results� One may note thatB KL � ����� is close to the �unitary bound� ����� given by the longdistance absorptive imaginary� part of amplitude Babs KL � ����� � ��� ������ and it will be almost impossible to extract the short distancepart� which is expected ���� to be very small � ����� The BNL���experiment is analyzing its �������� data set� A preliminary study ofKL � ���� shows ����� � � events� Probably BN��� should observeseveral K�

L � e�e� events� which is expected to have a branching ratio offew ���� in the standard Model�

Table �

Results on K� � ����� The limits quoted are at ��� con�dencelevel�

Decay mode Experiment ResultKL � ���� BNL�� ����� B�KL � �����

KEK������� � ���� � ���� ����

KL � e�e� BNL�� ����� B�KL � e�e�� � �� �����

KS � e�e� Dubna ����� B�KS � e�e�� � �� ����

KS � ���� CERN PS ����� B�KS � ����� � �� ����

� CP�violating decays

��� T violating polarization in K� � �����

The transverse polarization PT normal to the decay plane is known since along time ���� to be proportional to T�violating term that is expected tobe zero in the Standard Model� It is therefore a good probe of CP�violatingphases arising from the Higgs sector ����� and� anyway� the existence of non�zero value of this polarization will be a de�nite signature of new physics�

�� L Fayard

The current experimental result is �����

PT � ����� ��� ��� ����

There is a new experiment KEK�� and a proposal at BrookhavenBNL��� which aim ������� at a sensitivities ��� and ��� respec�tively�

��� KL � ��e�e�

K� � ���� � ��e�e� is forbidden by CP�invariance so KL � ��e�e�

can proceed�

� through direct CP violation� The contribution of this part to theKL � ��e�e� branching ratio is proportional to the CKM parameter� and has been estimated �������� to be ���� �������

� through indirect CP violation� The contribution of this part to thebranching ratio is proportional to j�K j� and to B KS � ��e�e�� which isnot known ����� It is estimated ���� to be in the range � � � ����

� through a CP conserving part via two virtual � process KL � ���� ���e�e�� The contribution is estimated �������� to be in the range �� �� �����These three contributions are similar and three order of magnitude

smaller than the current �! C�L� experimental limits

B KL � ��e�e�� � ��� ��� Fermilab E���I����� ��� ��� BNL �������

����

The single event sensitivity SES� of KTeV ����� is ��� ���� with the�������� data� The expected SES for the data that KTeV will take up to���� is ��� ����� The long term future of this physics will be done by theKAMI collaboration ��� which will continue the experimental program ofrare kaon decay physics at Fermilab using the Main Injector� The �� GeVMain Injector beam will provide neutral kaon beams which are two ordersof magnitude more intense than those currently available at Fermilab�This increase of �ux will give for KL � ��e�e� a SES of � ����� It willbe necessary to upgrade the existing KTeV detector with�� �ber tracking for decay modes like ��e�e� with charged particles

EXPERIMENTAL KAON PHYSICS �

� hermetic photon vetos used in searches for KL � ����� see later��

One may remark that� once detected� untangling the various contributionsto the decay� particularly in the presence of the KL � e�e��� backgroundwill be a signi�ant experimental challenge�

��� KL � ������

This mode is of interest for similar reasons� The KL � ���� backgroundis expected to be less severe� However the CP conserving amplitude may besigni�cantly larger than for ��e�e� because there is no helicity suppression�The branching ratio for this decay is also predicted to be on the order of���� and the current best limit is from Fermilab E���I �����

B KL � ������� � ��� ��� �! C�L� ����

The SES expected from KTeV and KAMI are ��� ���� and � �����

��� KL � �����

The great virtue of KL � ����� is that it proceeds almost exclusivelythrough direct CP violation ���� and as such is the cleanest decay tomeasure this important phenomenon� It also o�ers a clean determinationof the Wolfenstein parameter � while K� � ����� was sensitive to both and �� The theoretical uncertainties present in K� � ����� due to thecharm mass are absent� One has �����

B KL � ������ � ��� ����h �

���

i� � mt mt�

�� GeV

���� � jV cbj��

�� ����

Scanning equation ����with the CKM matrix elements one obtains

B KL � ������ � ���� �������� ����

The main problems with this decay are entirely experimental� The bestpublished limit comes from E���I at Fermilab ���� using Dalitz decays of�� � �� � e�e��

B KL � ������ � ��� ��� �! C�L�

However it is clear that in order to eventually reach Standard Modelsensitivity levels� it will be necessary to use the eighty�times more commonneutral decay �� � ���

�� L Fayard

A special half�day E���II run at the end of ��� was devoted ���� toa test of this method with a single narrow beam � � cm� on the CsIcalorimeter�� The two missing neutrinos and the lack of any charged in�formation make the analysis di$cult because there are only two handlesavailable to separate signal from background�

� The z vertex determined from the two photon clusters� assuming the ��

mass� which must reconstruct within the �ducial volume of the detector "and

� The transverse momentum� p� of the reconstructed ��� with respect tothe beam direction� The maximum possible p� from KL � ����� decays is��� MeV�c� A narrow kaon beam is necessary in order to obtain good p�resolution since the assumption is made that the decay originated at thetransverse center of the beam�

The p� distribution of events inside the �ducial volume of the detectorafter �nal cuts is shown in �gure ��

Fig� ��� The p� distribution of KL � ����� candidates from E��II ���

The one remaining event in the signal search region was found to beconsistent with beam neutron interactions� Based upon one observedevent E���II set a preliminary upper limit on the ����� branching ratioB KL � ������ � �� �� at the �! C�L� The expected SES from KTeVat the end of ���� will be ��� ��� with �� � e�e�� and � ��� with

EXPERIMENTAL KAON PHYSICS ��

�� � ��� still three orders of magnitude above the Standard Model predic�tions� In order to see the signal and go to a SES smaller than ��� thereare three projects�

� KAMI at Fermilab�

� A series of experiments at KEK using� like KAMI� pencil KL beam �����A pilot experiment already approved at the present �� GeV Proton Syn�chrotron under the name E���A could be extended to a high sensitivityone at the � GeV Proton Synchroton of the future JHP Japan HadronFacility��

� The experiment BNL�� at Brookhaven ��������� which is shown in �g�ure ���

Fig� �� Schematic of the proposed BNL ��� detector

They are planning to tag the KL momentum by using the time di�erencebetween the protons tightly bunched and the arrival of the photons in thedetector� Determining theKL momentum makes it possible to transform tothe KL center of mass system so that the �

� from the primary background�KL � ���� has a unique energy and can be suppressed kinematicallywithout excessive signal loss�

�� L Fayard

�� Conclusions

There is still a large physics program in the kaon �eld with particular em�phasis�

� on measurement of ���� where it is likely to be the last round of attemptsto see a non�zero value�

� on rare decays where the emphasis will probably shift toward StandardModel studies and could probably compete with the B sector�

It is hoped that the general budget issues in the High Energy Physicscommunity will allow this program to continue�

Acknowledgements

It is a pleasure to acknowledge Rajan Gupta and Andr�e Morel for theorganization of this school and for having convinced me to write thesenotes� I also thank Guillaume Unal for a careful reading of this paperand the LAL Scienti�c Secretariat for having transformed my handwrittennotes into this paper�

EXPERIMENTAL KAON PHYSICS ��

References

��� R�E Marshak et al�� Theory of Weak Interactions in Particle Physics �Wiley� �����T�D� Lee� Particle Physics and Introduction to Field Theory �Harwood� �����R�G� Sachs� The Physics of Time Reversal �The University of Chicago Press�����J�F� Donoghue� E� Golowich and B�R� Holstein� Dynamics of the Standard Model�Cambridge University Press� �����L�B� Okun� Leptons and Quarks �North Holland� �����T�P� Cheng and L�F� Li� Gauge Theory of elementary particle physics �ClarendonPress� �����

��� T�D Lee� R Oehme and C� N� Yang� Phys� Rev� ��� ����� ���

��� T�T Wu and C�N� Yang� Phys� Rev� Lett� �� ������ ���

��� J�S Bell and J� Steinberger� Weak Interactions of Kaons� Proc� Oxford Int� Conf�on Elementary Particles ������ ����

��� T�D Lee and L� Wolfenstein� Phys� Rev� ��� ������ B����

��� T�D� Lee and C�S� Wu� Ann� Rev� Nucl� Sci� �� ������ ���� E� ����� ���

�� G� Charpak and M� Gourdin� CERN ���� �����

��� L� Caneschi and L Van Hove� CERN ��� �sept� ����

��� L� Wolfenstein� Proc� of the ���� Int� School of Physics �Ettore Majorana� ed� A�Zichichi �Academic Press� �����

���� K� Kleinknecht� Ann� Rev� Nucl� Sci� �� ������

���� L�L� Chau� Phys� Rep� �� ������ �

���� W� Grimus� Fortschr� Phys� �� ������ ���

���� CP Violation� Advanced Series on Detectors in High Energy Physics� Vol �� ed�C� Jarlskog �World Scienti�c� �����

���� T� Nakada� PSI�PR������ �January �����

���� Y� Nir� Proc� of the ��th Annual SLAC Summer Institute� SLAC Report ����Stanford CA ��������

���� T� Nakada in �Lepton and Photon Interactions� �AIP Conference proceedingsn� ���� ed� by P� Drell and D� Rubin �AIP Press� New York� �����

��� M� Hayakawa and A� I� Sanda� Phys� Rev� D�� ������ ����

� L Fayard

���� B� Winstein and L� Wolfenstein Rev� Mod� Phys� �� ������ �����

���� P� Debu in �Ecole d��et�e Physique des Particules Gif ��� �IN�P�� Paris�

���� CP Violation and the limits of the Standard Model� Proc� of the ���� TheoreticalAdvanced Study Institute in Elementary Particle Physics� TASI �� ed� J�F�Donoghue �World Scienti�c� �����

���� G� D�Ambrosio and G� Isidori� LNF������������� submitted to Int� J� Mod� Phys�A

���� T� Ruf� CERN�PPE������� ������

���� T� Nakada� �������� Academic Training Lecture� CERN

���� A�J� Buras� Proc� of the ��th Int� Conference on High Energy Physics� Warsaw���� ed� Z� Ajduk and A�K� Wroblewski �World Scienti�c� ����

���� J� Lee�Franzini and P� Franzini LNF������ �sept� ����

���� D� Bryman� Int� J� Mod� Phys� A� ������ �

��� R� Battiston� D� Cocolicchio� G�L� Fogli and N� Paver� Phys� Rep� ��� ������ ���

���� L� Littenberg and G� Valencia� Ann� Rev� Nucl� Part� Sci� �� ������ ��

���� J� Ritchie and S� Wojcicki� Rev� Mod� Phys� �� ������ ����

���� P� Buchholz and B� Renk� Prog� Part� Nucl� Phys� �� ����� ���

���� J� Gasser and H� Leutwyler� Ann� Phys� ��� ������ ���� Nucl� Phys� B��� ���������

���� G� Buchalla� A�J� Buras and M�E� Lautenbacher� Rev� Mod� Phys� �� ������ ����

���� Proc� of the Workshop on CP Violation in Particle Physics and Astrophysics�Blois� ���� ed� by J� Tran Than Van �Editions Fronti�eres�

���� The DA�NE Physics Handbook� ed� by L� Maiani� G� Pancheri and N� Paver

�INFN� Frascati� �����

���� Proc� of the Workshop on K physics� Orsay� France� ���� ed� by L� Iconomidou�Fayard �Editions Fronti�eres�

���� C�D� Buchanan et al�� Phys� Rev� D�� ������ ����

��� L� Maiani in ��

���� R� Adler et al�� Phys� Lett� B��� ������ ���

EXPERIMENTAL KAON PHYSICS �

���� J� Ellis� P� Huet� A� Kostelecky and L� Okun in ��

���� Y� Grossman� Y� Nir and R� Rattazi CERN�TH������� ������

���� D�B� Cline in ��

���� Yu� M� Shatunov in ��

���� T� Bressani in ��

���� G�C� Ghirardi� R� Grassi and R� Ragazzon in ��

���� A� di Domenico� Nucl� Phys� B��� ������ ���

���� A� di Domenico in ��

��� A� Pais� �Inward Bound� �Oxford University Press� New York� �����

���� �The Experimental Foundations of Particle Physics�� ed� R�N� Cahn and G�Goldhaber �Cambridge University Press� �����

���� L� Wolfenstein �ed�� �Current Physics Sources and Comments vol � CP violation��North Holland� �����

���� L�M� Brown� A� Pais and B� Pippard �ed�� �Twentieth Century Physics� �Ameri�can Institute of Physics� �����

���� V�V Ezhela et al�� �Particle Physics ��� years of discoveries� �AmericanInstitute of Physics� �����

���� L� Leprince�Ringuet et M� Lh�eritier� Compt� Ren� ��� ������ ���See� however a criticism H�A� Bethe� Phys� Rev� � ������ ���

���� G�D� Rochester and C�C� Butler� Nature ��� ����� ���

���� R�H� Dalitz et al�� Phys� Rev� �� ������ ����

���� T�D� Lee and C�N� Yang� Phys� Rev� ��� ������ ���

���� M� Gell�Mann and A� Pais� Phys� Rev� � ������ ���

��� M� Gell�Mann� Phys� Rev� �� ������ ���T� Nakano and K� Nishijima� Prog� Theo� Phys� �� ������ ���

���� K� Lande et al�� Phys� Rev� ��� ������ ����W� F� Fry et al�� Phys� Rev� ��� ������ ����

���� A� Pais and O� Piccioni� Phys� Rev� ��� ������ ���

� L Fayard

���� K� Kleinknecht� Fortsch� der� Physik �� ����� �

���� L� K� Gibbons� PhD thesis� the University of Chicago ������

���� J� H� Christenson� J� W� Cronin� V� L� Fitch and R� Turlay� Phys� Rev� Lett� �������� ���

���� Review of Particle Properties� Phys� Rev� D�� ������ �

���� V� L� Fitch� Rev� Mod� Phys� �� ������ ��

���� R� Turlay in ��R� Turlay� History of Original Ideas and Basic Discoveries in Particle Physics� ed�by H� B� Newman and T� Ypsilantis� NATO ASI ��� �Plenum �����

���� A� Sakharov� JETP Lett� � ����� ��

��� M� Kobayashi and T� Maskawa� Prog� Theor� Phys� �� ����� ���

���� L� Wolfenstein� Phys� Rev� Lett� �� ������ ����

���� M� K� Gaillard and B� W� Lee� Phys� Rev� D�� ����� ��

��� J� Bijnens� J� M� Gerard and G� Klein� Phys� Lett� B�� ������ ���

��� S� Herrlich and U� Nierste� Nucl� Phys� B��� ������ ���S� Herrlich and U� Nierste� Phys� Rev� D�� ������ ����A� J� Buras� M� Jamin and P� H� Weisz� Nucl� Phys� B�� ������ ���S� Herrlich and U� Nierste� Nucl� Phys� B�� ������ �

��� Y� Nir in ��

��� E� Chell and M� G� Olsson� Phys� Rev� D�� ������ ���

��� R� A� Briere� PhD thesis� the University of Chicago ������

��� V� V� Barmin et al�� Nucl� Phys� B�� ������ ���� EB��� ������ �

��� J� M� Flynn and L� Randall� Phys� Lett� B��� ������ ���� EB��� ������ ���

�� P� Soldan and S� Bertolini in ��J� Heinrich� E� A� Paschos� J� M� Schwarz and Y� L� Wu� Phys� Lett� B�� ���������S� Bertolini� J� O� Eeg and M� Fabbrichesi� Nucl� Phys� B�� ������ ���M� Ciuchini� E� Franco� G� Martinelli and L� Reina� Phys� Lett� B��� ������ ���M� Ciuchini et al�� Z� Phys� C�� ������ ���M� Ciuchini� CERN�TH���� �January ����A� J� Buras� M� Jamin and M� E� Lautenbacher� Nucl� Phys� B��� ������ ���

EXPERIMENTAL KAON PHYSICS �

��� M� Woods et al�� Phys� Rev� Lett� �� ������ ����

��� L� K� Gibbons et al�� Phys� Rev� Lett� � ������ ����L� K� Gibbons et al�� Phys� Rev� D�� ����� ����

���� J� R� Patterson et al�� Phys� Rev� Lett� �� ������ ����

���� H� Burkhardt et al�� Phys� Lett� B��� ������ ���

���� G� D� Barr et al�� Phys� Lett� B�� ������ ���

���� H� Burkhardt et al�� Nucl� Inst� Meth A��� � ����� ���

���� E� Aug�e� m�emoire d�habilitation LAL ����� � �����L� Iconomidou�Fayard� m�emoire d�habilitation LAL ����� ������O� Perdereau� m�emoire d�habilitation LAL ����� ������

���� J� R� Patterson� PhD thesis� The University of Chicago ������B� Schwingenheuer� PhD thesis� The University of Chicago ������

���� R� S� Kessler in Proc� of the ���� Workshop on Heavy Quarks a Fixed Targed ed�by L� Kopke �Frascati Physics Series�V� D�Dell in ��R� Ben�David� Fermilab�CONF� �����E �Oct� ����

��� E� Cheu et al�� Proposal to Continue the Study of Direct CP Violation and RareDecay Processes in KTeV in ���� �Dec� ����

���� M� Holder in ��C� Ebersberger� Dissertation J� Gutenberg Universitat Mainz �����S� Schanne� th�ese Universit�e Paris �octobre ����B� Hay� thesis RAL�TH������ �June ����R� Moore� thesis RAL�TH������ �October ����

���� A� Baurichter et al�� Nucl� Intr� Meth� B��� ������ ��

���� M� Preger in ��

���� I� Dunietz� J� Hauser and J� L� Rosner� Phys� Rev� D�� ����� ����

���� V� Patera in ��

���� V� Patera and A� Pugliese in ��V� Patera in Proc� of the Workshop on Physics and Detectors for DA�NE�Frascati ���� ed� by G� Pancheri �INFN Frascati�

���� R� Adler et al�� Nucl� Inst� Meth A�� ������ �

� L Fayard

���� R� Adler et al�� Phys� Lett� B��� ������ ��

���� R� Le Gac in ��

��� R� Adler et al�� Phys� Lett� B��� ������ ���

���� A� Schopper� presented at the B Physics Conference� Hawa� � March ���

���� B� Schwingenheuer et al�� Phys� Lett� � ������ ���

����� L� K� Gibbons et al�� Phys� Rev� Lett� � ������ ����

����� K� Kleinknecht and S� Luitz� Phys� Lett� B��� ������ ���R�A� Briere and B� Winstein� Phys� Rev� Lett� � ������ ���K� Kleinknecht� Phys� Rev� Lett� � ������ ���R�A� Briere and B� Winstein� Phys� Rev� Lett� � ������ ���R�A� Briere� K� Kleinknecht� A� Martin� B� Winstein in ��

����� J�N� Matthews et al�� Phys� Rev� Lett� � ������ ����

����� E�J� Ramberg et al�� Phys� Rev� Lett� � ������ ����

����� R� Adler et al�� Phys� Lett� B��� ������ ��

����� S� Gjesdal et al�� Phys� Lett� B�� ����� ���C� Geweniger et al�� Phys� Lett� B�� ����� ��C� Geweniger et al�� Phys� Lett� B�� ����� ���M� Cullen et al�� Phys� Lett� B�� ����� ���W�C� Carithers et al� Phys� Rev� Lett� �� ����� ����R� Carosi et al�� Phys� Lett� B�� ����� ���

����� V�V� Barmin et al�� Phys� Lett� B��� ������ ���

���� R� Adler et al�� Phys� Lett� B�� ����� ��� � Phys� Lett� B�� ������ �� � Phys�Lett� B�� ������ ���

����� Y� Zou et al�� Phys� Lett� B��� ������ ���

����� Y� Zou et al�� Phys� Lett� B��� ������ ���

����� J� Kambor� J� Missimer and D� Wyler� Phys� Lett� B��� ���������

����� P� Langacker� S� Uma Sankar and K� Schilcher� Phys� Rev� D�� ������ ����

����� W� J� Marciano in Rare Decay Symposium� Vancouver� Canada� ���� ed� D�Bryman et al� �World Scienti�c�

����� T� Akagi et al�� Phys� Rev� Lett� � ������ ����

EXPERIMENTAL KAON PHYSICS

����� A� Arisaka et al� Phys� Rev� Lett� � ������ ����

����� J� Ritchie in ��

����� A� M� Lee et al�� Phys� Rev� Lett� �� ������ ���

���� M� Zeller in ��

����� S� Schnetzer in ��

����� H� Burkhardt et al�� Phys� Lett� B��� ����� ���

����� G� D� Barr et al�� Phys� Lett� B��� ������ ��

����� G� D� Barr et al�� Phys� Lett� B��� ������ ���

����� K� E� Ohl et al�� Phys� Rev� Lett� �� ������ ���

����� M� B� Spencer et al�� Phys� Rev� Lett� � ������ ����

����� V� Fanti et al�� Z� Phys� C� ����� ���

����� T� Nakaya et al�� Phys� Rev� Lett� � ������ ����

����� W� M� Morse et al�� Phys� Rev� D�� ������ ��

���� P� Gu et al�� Phys� Rev� Lett� � ����������

����� M� R� Vagins et al�� Phys� Rev� Lett� � ������ ��

����� G� D� Barr et al�� Z� Phys� C�� ������ ���

����� T� Akagi et al�� Phys� Rev� D�� ������ ����

����� P� Gu et al�� Phys� Rev� Lett� � ������ ����

����� G� D� Barr et al�� Phys� Lett� B��� ������ ��� � Phys� Lett� B��� ������ ���

����� V� Papadimitriou et al�� Phys� Rev� D�� ������ R��

����� G� D� Barr et al�� Phys� Lett� B��� ������ ���

����� G� D� Barr et al�� Phys� Lett� B��� � ����� ���

����� N� Sasao� presented at the ��th SLAC Summer Institute on Particle Physics�August ��� �Stanford�

���� L� M� Sehgal and M� Wanninger� Phys� Rev� D�� ������ ����� ED�� ������ ����P� Heiliger and L� M� Sehgal� Phys� Rev� D�� ������ ����

� L Fayard

J� K� Elwood� M� Wise and M� J� Savage� Phys� Rev� D�� ������ ����J� K� Elwood� M� Wise� M� J� Savage and J� W� Walden� Phys� Rev� D�� ���������

����� P� Kitching et al�� Phys� Rev� Lett� � ����� ���

����� F� Leber et al�� Phys� Lett� B��� ������ ��

����� V� N� Bolotov et al�� Sov� J� Nucl� Phys� �� ������ ��

����� S� Adler et al�� Phys� Rev� Lett� � ����� ���

����� L� Littenberg in ��

����� W� J� Marciano and Z� Parsa� Phys� Rev� D�� ������ R�

����� G� Buchalla and A� J� Buras� Nucl� Phys� B��� ������ ���

����� A� J� Buras in ��

����� S� Adler et al�� Phys� Rev� Lett� � ����� ����

���� L� Littenberg� BNL Preprint ����� �February �����

����� S� Kettell� BNL Preprint ����� �January �����

����� A� P� Heinson et al�� Phys� Rev� D�� ������ ���

����� T� Akagi et al�� Phys� Rev� D�� ������ ����

����� K� Arisaka et al�� Phys� Rev� Lett� � ������ ����

����� A� M� Blick et al�� Phys� Lett� B��� ������ ���

����� S� Gjesdal et al�� Phys� Lett� B�� ����� ��

����� L� M� Sehgal� Phys� Rev� ��� ������ ����

����� C� Alliegro et al�� Phys� Rev� Lett� �� ������ ��

����� J� J� Sakurai� Phys� Rev� ��� ������ ���

���� G� B�elanger and C� Q� Geng� Phys� Rev� D�� ������ ���R� Garisto and G� Kane� Phys� Rev� D�� ������ ����P� Castoldi� J� M� Fr�ere and G� L� Kane� Phys� Rev� D�� � ����� ����

����� S� R� Blatt et al�� Phys� Rev� D� ������ ����

����� Y� G� Kudenko in ��

EXPERIMENTAL KAON PHYSICS �

����� M� V� Diwan� BNL preprint ����� �January �����

����� A� Pich in ��

����� There are limits on B�KS � ��e�e�� coming from NA�� at CERNB�KS � ��e�e�� � �� ���� ��� C�LG� Barr et al�� Phys� Lett� B��� ������ ���and from E��� at Fermilab B�KS � ��e�e�� � �� ���� ��� C�LG� Thomson in ��

����� P� Heiliger and L� M� Sehgal� Phys� Rev� D� ������ ����G� Ecker� A� Pich and E� de Rafael� Nucl� Phys� B��� ������ ���J� F� Donoghue and F� Gabbiani� Phys� Rev� D�� ������ ���C� Bruno and J� Prades� Z� Phys� C� ������ ���

����� K� E� Ohl et al�� Phys� Rev� Lett� �� ������ ���

����� D� A� Harris et al�� Phys� Rev� Lett� � ������ ����

����� E� Cheu et al�� Fermilab�PUB������ �September ����

���� D� A� Harris et al�� Phys� Rev� Lett� � ������ ����

����� L� S� Littenberg� Phys� Rev� D�� ������ ����

����� M� Weaver et al�� Phys� Rev� Lett� � ������ ���

���� T� Inagaki� KEK preprint ������ �March ����

���� Proc� of the Workshop on Fixed Target Physics at the Main Injector� ed�G� J� Bock and J� G� Mor�n� Fermilab�CONF����� �September ����

���� A� Angelopoulos et al�� CERN�EP������ �January �����