search for m - au → e - au conversion with sindrum ii

““Flavour in the era of the LHC” Flavour in the era of the LHC” 3rd Workshop CERN, May 20063rd Workshop CERN, May 2006

W. Bertl, PSIW. Bertl, PSI

Search for Search for AuAu →→ eeAuAu conversion conversion with with SINDRUM IISINDRUM II

Collaboration (2000):Collaboration (2000):

RWTH Aachen (1)RWTH Aachen (1)

PSI (Paul Scherrer Institut) (2)PSI (Paul Scherrer Institut) (2)

University Zurich (3)University Zurich (3)

Authors:Authors:

W. Bertl (2)W. Bertl (2)

R. Engfer (3)R. Engfer (3)

E.A. Hermes (3)E.A. Hermes (3)

T. Kozlowski (2+3)T. Kozlowski (2+3)

G. Kurz (3)G. Kurz (3)

J. Kuth (1)J. Kuth (1)

A. BadertscherA. Badertscher, J.Bagaturia, M.Begalli, C.Dohmen, W.Dzhordzhadze, , J.Bagaturia, M.Begalli, C.Dohmen, W.Dzhordzhadze, J.Egger, S.Eggli, Ch.Findeisen, D.Gaewiler, M.Grossmann-J.Egger, S.Eggli, Ch.Findeisen, D.Gaewiler, M.Grossmann-Handschin, K.D.Groth, H.Haan, P.Hawelka, B.Heer, W.Herold, Handschin, K.D.Groth, H.Haan, P.Hawelka, B.Heer, W.Herold, J.Hofmann, W.Honecker, D.Junker, D.Kampmann, J.Kaulard, J.Hofmann, W.Honecker, D.Junker, D.Kampmann, J.Kaulard, B.Krause,N.Lordong, G.Melitauri, A.Mtchedlishvili, F.Muheim, B.Krause,N.Lordong, G.Melitauri, A.Mtchedlishvili, F.Muheim, U.Mueller, C.B.Niebuhr, S.Playfer, H.S.Pruys, D.Renker, U.Mueller, C.B.Niebuhr, S.Playfer, H.S.Pruys, D.Renker, L.Ricken,M.Rutsche, M.Salzmann, R.Seeliger,L.Simons, L.Ricken,M.Rutsche, M.Salzmann, R.Seeliger,L.Simons, B.Steinruecken, M.Starlinger, O.Szavits, D.Vermeulen, B.Steinruecken, M.Starlinger, O.Szavits, D.Vermeulen, H.K.WalterH.K.Walter,,

ETH ZurichETH ZurichTbilisi State UniversityTbilisi State University

……and earlier members of the SINDRUM II collaboration:and earlier members of the SINDRUM II collaboration:

G. Otter (1)G. Otter (1)

F. Rosenbaum (2+3)F. Rosenbaum (2+3)

N.M. Ryskulov (2)N.M. Ryskulov (2)

A. van der SchaafA. van der Schaaf (3) (3)

P. Wintz (3)P. Wintz (3)

I. Zychor (3)I. Zychor (3)



LLepton epton FFamily-number amily-number CConservationonservation

• LFC experimentally well confirmed. BUT: Is there an underlying symmetry?LFC experimentally well confirmed. BUT: Is there an underlying symmetry?

• LFC in LFC in SStandard tandard MModel is a consequence of the assumption model is a consequence of the assumption m=0=0

• mm≠ ≠ 0 leads to LFC0 leads to LFC violation (LFV), but decays are suppressed by violation (LFV), but decays are suppressed by

• most extensions of the SM predict additional sources of LFVmost extensions of the SM predict additional sources of LFV

• many searches done for LFV processes in decays of many searches done for LFV processes in decays of ,K,B,D,W,Z,K,B,D,W,Z

• LFV would be an unambiguous sign of new physicsLFV would be an unambiguous sign of new physics

• highest sensitivities reached in highest sensitivities reached in ,K and recently ,K and recently experimentsexperiments

• most popular muon decays: most popular muon decays: →e→e, , →eee, →eee, -e -e conversionconversion

4Wmm



– – e conversion in muonic atomse conversion in muonic atoms

1-ZA,ZA,

ZA,ZA, ee

• muonic atoms muonic atoms --(A,Z) formed in their ground state after stopping (A,Z) formed in their ground state after stopping -- in matter ( in matter (OO((1ns))1ns))

• decay of decay of --(A,Z) mostly by (A,Z) mostly by

““muon decay in orbit (MIO)” muon decay in orbit (MIO)”

and “nuclear muon capture (MC)”and “nuclear muon capture (MC)”

• MIO rate ↓ for Z ↑ ; MC rate ∞ZMIO rate ↓ for Z ↑ ; MC rate ∞Z44

• if if -e conversion - -e conversion - leaving the nucleus in its ground state -leaving the nucleus in its ground state - exists, it’s boosted by the coherent action of all quarks over the exists, it’s boosted by the coherent action of all quarks over the MC process (excited states experimentally not accessible)MC process (excited states experimentally not accessible)

• -e conversion results in a two body final state -e conversion results in a two body final state → → electrons are electrons are monoenergeticmonoenergetic

A

Al Ti AuPb

Nuclear dependence of Nuclear dependence of →→ e conversion e conversion

binding energybinding energy atomic recoil energyatomic recoil energy

MeVAuE e 56.95

ARZBcmE e 2

T.S. Kosmas, I.E. Lagaris: T.S. Kosmas, I.E. Lagaris:

J. Phys. G: Nucl. Part. Phys. J. Phys. G: Nucl. Part. Phys. 2828 (2002) 2907 (2002) 2907



Background : a) muon inducedBackground : a) muon induced

- decay in orbit (MIO) :- decay in orbit (MIO) : Radiative Muon Capture (RMC) :Radiative Muon Capture (RMC) :

Au e Au

Au Pt

efollowed by+ e

((R. Watanabe et al.: Atom.Data and Nucl. Data R. Watanabe et al.: Atom.Data and Nucl. Data Tab. Tab. 5454 (1993), 165) (1993), 165)

(Lead spectrum, corrected for shift in gold endpoint energy)(Lead spectrum, corrected for shift in gold endpoint energy)Kinematic endpoint ofKinematic endpoint of photon spectrum photon spectrum only 0.7 MeV below Eonly 0.7 MeV below Ee e , , but strongly but strongly

suppressed at high energies.suppressed at high energies.

Check contribution by investigating Check contribution by investigating positron spectrum ! ( see below )positron spectrum ! ( see below )

Theoretical spectrum :Theoretical spectrum :

Muon decay-in-flight :Muon decay-in-flight :

With pWith p< 65 MeV/c always below E< 65 MeV/c always below Eee

prop. (Eprop. (Eee – E – EMIOMIO))55 resolution of 1-2 MeV resolution of 1-2 MeV (FWHM) sufficient(FWHM) sufficient



Background : b) pion inducedBackground : b) pion inducedRadiative Pion Capture (RPC) : Au Pt efollowed by

+ e

Kinematic endpoint ofKinematic endpoint of photon spectrum around 130 MeV ! Branching ratio of order 2%.photon spectrum around 130 MeV ! Branching ratio of order 2%.

Suppression by Suppression by

• fast beam veto (RPC is a strong interaction process)fast beam veto (RPC is a strong interaction process)

• pulsed beampulsed beam

• keep keep stop rate in target below 10 stop rate in target below 10 -9-9 (chosen in this experiment) (chosen in this experiment)

Achieved by a combination of degrader (10Achieved by a combination of degrader (10 -6-6) and decay path (8m ) and decay path (8m →10→10-3-3))

E5

PMC

SINDRUMProbability of Probability of with p=52MeV/c to cross a CH with p=52MeV/c to cross a CH22 degrader degrader

BUT: distributions broadened by finite beam momentum resolutionBUT: distributions broadened by finite beam momentum resolution

tune beamline to tune beamline to suppress high suppress high momentum tailmomentum tail



Background: c) electron inducedBackground: c) electron induced

Electrons in the beam may scatter from the targetElectrons in the beam may scatter from the target

Suppression bySuppression by

• beam momentum much below 90 MeV/cbeam momentum much below 90 MeV/c

However, However,

electrons from pion decayselectrons from pion decays in flight before the degrader in flight before the degrader

and and

from RPC in the degrader from RPC in the degrader

may have higher momenta and represent a serious background source. may have higher momenta and represent a serious background source.

Suppression achieved by using their time relation with the proton beam’s Suppression achieved by using their time relation with the proton beam’s rf-structure together with a reduction in phase space (see below)rf-structure together with a reduction in phase space (see below)



Background: d) cosmic inducedBackground: d) cosmic induced

Cosmic radiation produce eCosmic radiation produce e-- which may fake a which may fake a e event.e event.

Suppression:Suppression:

• Most of them identified by hits not belonging to main track (see below).Most of them identified by hits not belonging to main track (see below).

• →→ee++ee- - pair production in the targetpair production in the target : :

use passive shielding use passive shielding

veto counter systemsveto counter systems

small target masssmall target mass



Experimental Set-upExperimental Set-up

Proton beam: 590 MeV, time structure: bursts 0.3 ns wide every 19.75 nProton beam: 590 MeV, time structure: bursts 0.3 ns wide every 19.75 n

E5-beam line: 52 or 53 MeV/c with ± 2% FWHM (limiting high momentum tail)E5-beam line: 52 or 53 MeV/c with ± 2% FWHM (limiting high momentum tail)

Transport solenoid PMC: 1.1 T, lead collimator with 60mm hole followed by a 8mm CHTransport solenoid PMC: 1.1 T, lead collimator with 60mm hole followed by a 8mm CH22 degrader degrader



SINDRUM II SpectrometerSINDRUM II Spectrometer

SINDRUM II

1m

A

B

CD ED

F

G

H

H

I

J

ABCDE

FGHIJ

exit beam solenoidgold targetvacuum wallscintillator hodoscopeCerenkov hodoscope

inner drift chamberouter drift chambersuperconducting coilhelium bathmagnet yoke

tracks left by a 100 MeV etracks left by a 100 MeV e-- PMCPMC

Schematic event display:Schematic event display:rr rz rz

Gold Target (B) (produced by galvanic process to avoid low Z contaminations): Gold Target (B) (produced by galvanic process to avoid low Z contaminations):

• diameter ~ 40 mmdiameter ~ 40 mm

• wall thickness 75 mg/cmwall thickness 75 mg/cm22

• length 65 cm (no beam focus exists)length 65 cm (no beam focus exists)



Determine Determine stops: a) Principle stops: a) Principle

Muonic X-ray detection from Muonic X-ray detection from --Au captureAu capture Method:Method:

• using a using a Germanium diodeGermanium diode to monitor yield of to monitor yield of 4f4f5/25/2→→3d3d3/23/2 transition (899 keV) transition (899 keV)

or (respectively)or (respectively)• using a large using a large NaI crystalNaI crystal detector to monitor yield of detector to monitor yield of

2p2p→→1s transitions (5765 and 5595 keV)1s transitions (5765 and 5595 keV)• Well known X-ray -intensities (F.J.Hartmann et al. TU Well known X-ray -intensities (F.J.Hartmann et al. TU

München) and -energies (B.Robert-Tissot et al. Univ. München) and -energies (B.Robert-Tissot et al. Univ. Fribourg)Fribourg)

Calibration:Calibration:• Acceptance for Ge-diode measured with calibrated Acceptance for Ge-diode measured with calibrated

sources : sources : 137137Cs (662 keV), Cs (662 keV), 6060Co (1117,1333 Co (1117,1333

keV)keV)

at 3 different z positionsat 3 different z positions

Reproduced by simulation within ±3%Reproduced by simulation within ±3%• Simulation used to determine acceptance for gold Simulation used to determine acceptance for gold

transition (899keV) & measured stop distributiontransition (899keV) & measured stop distribution

AAGeGe = (1.02 ± 0.05 = (1.02 ± 0.05statstat ± 0.07 ± 0.07 syssys) ) 10 10-6-6

• Acceptance for NaI (5765,5595 keV) from Acceptance for NaI (5765,5595 keV) from (simultaneous ) Ge measurement(simultaneous ) Ge measurement

AANaINaI = (1.01 ± 0.06 = (1.01 ± 0.06statstat ± 0.08 ± 0.08 syssys) ) 10 10--

55

SINDRUM II spectrometer + X-ray detector (Ge)SINDRUM II spectrometer + X-ray detector (Ge)

(NaI detector not shown)(NaI detector not shown)



Determine Determine stops: b) Results stops: b) ResultsExample of a raw mExample of a raw m - -Au X-ray spectrum taken during a 3 hour runAu X-ray spectrum taken during a 3 hour run

Total Total -stops in Au-target:-stops in Au-target:

NN = ( 4.37 = ( 4.37 ± 0.27± 0.27stat stat ± 0.17± 0.17sys sys ) x 10) x 101313



Event simulationEvent simulation

-e conversion, MIO and -e conversion, MIO and -decay-in-flight events simulated with GEANT-decay-in-flight events simulated with GEANT

Exact description of geometry:Exact description of geometry:

• target positiontarget position

• active elementsactive elements

• flanges of vacuum tubeflanges of vacuum tube

Light propagation & attenuation in scintillator and Light propagation & attenuation in scintillator and ČČerenkov counterserenkov counters

Discriminator threshold & time resolution of each individual counter (from observed values)Discriminator threshold & time resolution of each individual counter (from observed values)

Variation of DC1 anodes & cathodes efficiencies during measurement time taken into accountVariation of DC1 anodes & cathodes efficiencies during measurement time taken into account

B-field map used for event reconstruction and simulationB-field map used for event reconstruction and simulation

-stop distribution over target adjusted to measurement-stop distribution over target adjusted to measurement

Black: dataBlack: data

Red: Monte CarloRed: Monte Carlo



Trigger conditionTrigger conditionCircular track triggerCircular track trigger Straight track triggerStraight track trigger

Endcap detectorEndcap detector

Scintillator Scintillator

hodoscopehodoscope

Driftchamber DC1Driftchamber DC1Used for main data taking and Used for main data taking and energy calibration (see below)energy calibration (see below)

Cosmic muon trigger used for drift Cosmic muon trigger used for drift chamber calibration once a daychamber calibration once a day



Event reconstruction and selectionEvent reconstruction and selectionTrajectory reconstruction:Trajectory reconstruction:

• search for DC1 track elements in drift time patterns of search for DC1 track elements in drift time patterns of sense wire and cathode strips, independentlysense wire and cathode strips, independently

• sense wire & cath. track elements combined pair wise sense wire & cath. track elements combined pair wise to maximize number of coincident signalsto maximize number of coincident signals

•Drift timeDrift time conversion to r x conversion to r x coordinatescoordinates

→ → check that track elementscheck that track elements point to hodoscope point to hodoscope hithit

• pairs of track elements are combined into turns and pairs of track elements are combined into turns and trajectories are fitted (taking account multiple scattering)trajectories are fitted (taking account multiple scattering)

Drift time Drift time ↔ position relation described by 6 parameters ↔ position relation described by 6 parameters received from “straight track calibration” runsreceived from “straight track calibration” runs



Energy calibration & resolutionEnergy calibration & resolution

e

e

Method:Method:

use euse e++ spectrum from spectrum from

stopped in the targetstopped in the target

Requires:Requires:

• mirror B-field (emirror B-field (e+ + track instead etrack instead e-- track) track)

• lower B-field (by 52/95)lower B-field (by 52/95)

• check B-field linearity (Hall-probe)check B-field linearity (Hall-probe)

• change beam line polaritychange beam line polarity

• lower beam momentum (lower beam momentum (→→44 MeV/c)44 MeV/c)

Check resolutionCheck resolution (at (at e settings): e settings): Compare radius of helical trajectories Compare radius of helical trajectories between 1. and 2. turnbetween 1. and 2. turn

Edge rise over 3mm gives Edge rise over 3mm gives ppt t /p/ptt ≈ 1% FWHM≈ 1% FWHM

Edge position very sensitive Edge position very sensitive to material budgetto material budget

BBee = 1.069 = 1.069 ± 0.001 T± 0.001 T

(central value)(central value)



Cosmic ray induced backgroundCosmic ray induced background

•

3 classes of cosmic background events:3 classes of cosmic background events:

1)1) a track element from a high energetic cosmic muon is present a track element from a high energetic cosmic muon is present together with a knock-on electron circular trajectorytogether with a knock-on electron circular trajectory

2)2) an electron enters the tracking region from outside → track does an electron enters the tracking region from outside → track does not origin in the target or has an early signal in one of the fast not origin in the target or has an early signal in one of the fast counterscounters

3)3) a “cosmic” photon creates an asymmetric ea “cosmic” photon creates an asymmetric e++ee-- pair in the target pair in the target

Minimized by:Minimized by:

• shielding from magnet return yoke and lateral lead wallsshielding from magnet return yoke and lateral lead walls

• low target masslow target mass

• a hole in the yoke on top (for supply lines) excluded in acceptance (loss ~4%)a hole in the yoke on top (for supply lines) excluded in acceptance (loss ~4%)

Cuts applied in Cuts applied in reconstruction reconstruction are shownare shown



Pion induced backgroundPion induced background

a)a) pion stops in target pion stops in target → → negligible (beam tuning!)negligible (beam tuning!)

b)b) rad. rad. capture in degrader + pair prod. capture in degrader + pair prod.

c)c) decay in flight just before degraderdecay in flight just before degraderee- - scattering off the targetscattering off the target

Beam correlated region containing pion induced eventsBeam correlated region containing pion induced events

(width given by beam momentum spread)(width given by beam momentum spread)

b) is charge symmetric, c) notb) is charge symmetric, c) not

Expected: Expected: flat background between 80 and flat background between 80 and 100 MeV/c (O(10) events)100 MeV/c (O(10) events)

ee++,e,e-- tracks with p>87MeV/c tracks with p>87MeV/c

(signal region excluded)(signal region excluded)



Final momentum spectraFinal momentum spectra

2 event classes2 event classes defined based on defined based on polar anglepolar angle and and rf-phaserf-phase ttrfrf ::

1)1) coscos < 0.4 or |t < 0.4 or |trf rf -10 ns| > 4.5 ns-10 ns| > 4.5 ns

2)2) coscos > 0.4 and > 0.4 and ||ttrfrf -10 ns| -10 ns| < 4.5 ns< 4.5 ns

Class 1:Class 1: No sign for No sign for -e conversion events-e conversion events

Class 2:Class 2: Approximately flat high energy Approximately flat high energy component as expected from pion induced component as expected from pion induced backgroundbackground



Single event sensitivity SSingle event sensitivity See

icapt

ie RN

S

1

131 10)2.08.2( eS

i= 1,2i= 1,2

NN = ( 4.37 = ( 4.37 ± 0.32 ) x 10± 0.32 ) x 101313 total number of muons stoppedtotal number of muons stopped

RRcapt capt = 0.9717 = 0.9717 ± 0.0002 ± 0.0002 probability of muon capture in goldprobability of muon capture in gold

= 0.44= 0.44 spectrometer acceptancespectrometer acceptance

= 0.19= 0.19 efficiency for events of efficiency for events of

class 1class 1

11 = 0.014 = 0.014 efficiency for events of class 2efficiency for events of class 2

ii : derived from simulation with most parameters : derived from simulation with most parameters

adjusted from measurements. Spectrum shape well adjusted from measurements. Spectrum shape well reproduced. Finally lowered by ~10% to equate numbers reproduced. Finally lowered by ~10% to equate numbers of measured and expected MIO eventsof measured and expected MIO events

122 10)2.07.3( eS



Likelihood analysisLikelihood analysis4 contributions to the momentum spectra of class 1 and 2 events considered:4 contributions to the momentum spectra of class 1 and 2 events considered:

• muon decay in orbit (MIO)muon decay in orbit (MIO)

• – – e conversione conversion

• contribution from processes with intermediate photons (like RMC) (taken from observed econtribution from processes with intermediate photons (like RMC) (taken from observed e+ + spectrum)spectrum)

• a flat component (a flat component (→e→e decay-in-flight or cosmic backgrd.)decay-in-flight or cosmic backgrd.)

Likelihood distribution Likelihood distribution LLtottot = = LL11 x x LL2 2 ((LL11 : class 1, : class 1, LL22 : : class 2)class 2)::



Final Result Final Result AuAueAuB capturesge

..

..%90

0

9.0

LCeB

totdB

L

Likelihood analysis Likelihood analysis ► ► zero zero -e conversion events are most likely -e conversion events are most likely

The upper limit at 90% confidence level is given by: The upper limit at 90% confidence level is given by:

13107 AueB

Improvement :Improvement :by 2 orders of magnitude to the previous best limitby 2 orders of magnitude to the previous best limit W. Honecker et al.,(SINDRUM) Phys. Rev. Lett. 76,W. Honecker et al.,(SINDRUM) Phys. Rev. Lett. 76, (1996) 200 (1996) 200and 3 orders of magnitudes to the best pre-SINDRUM limitand 3 orders of magnitudes to the best pre-SINDRUM limitS. Ahmad et al. (TRIUMF) Phys. Rev. D 38 (1988) 2102.S. Ahmad et al. (TRIUMF) Phys. Rev. D 38 (1988) 2102.

(Accepted for publication in European (Accepted for publication in European Physical Journal C)Physical Journal C)

search for m - au → e - au conversion with sindrum ii

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