w physics at lep

W physics at LEP

E.Barberio

Southern Methodist University

PIC2003 Zeuthen 28th June 2003

June 2003 E.Barberio

the LEP program

LEP1: 18 Million Z boson decays (89-95)LEP2: 36 Thousand W pairs (96-00)

• W pair production• triple and quartic gauge couplings• W mass and width measurements • final state interactions

this talk:


WW events

semileptonic channel

43.8%missing energylow background

hadronic channel

45.6%large backgroundambiguity in assigning jets to W

leptonic channel 10.6%large missing energy

WWll

WWqql

WWqqqq


W pair cross section

1% measurement

clear evidence of WW and WWZ vertices: probe of the non-Abelian structure of the Standard Model

+ +

theo

WW

σ

σ=0.9980.006(stat)0.007(syst)

preliminary LEP


W branching fractions

SM: 10.83%

= 1.000 0.021

= 1.052 0.029

= 1.052 0.028

test of lepton universality at 3% (less precise than LEP1)

SM: 67.51%

hadronic branching fraction:Br(Wqq’) = 67.92 0.27%


triple gauge couplings WW WWZ

W

W

W

W

Zgeneral WW and WWZinteraction: 14 parameters

electric quadrupole moment

magnetic dipole moment

1W

m2e

W

2W

m

eW

q

applying C and P invariance& use low-energy constraints we are left with 3 parameters

relation with the static W properties:

SM values

0

1z1

g

1


measuring the coupling at LEP2

sensitive observables

WW production:most constraining

We-

e+

W+

W-

W

f

f

W decay angles (helicity)

W+W- production angle cosW

W rest frame and of W decay products


WW production/decay angular distributions


Single W

single W production +

8% precision

but it is very constraining for k

smaller cross section than WW:

OPAL preliminary

- single W- WW angles- WW

- combined

k


TGC 1-parameter fit results

dominant systematics O(em)

g1Z,: 0.015 : 0.039

(partial statistics)

- ALEPH- DELPHI- L3- OPAL - LEP

g1Z, k 2-5% measurement


TGC 3-D parameter fit results

joint minimization of statistical error

2D contour: 3rd parameter at the minimum


W polarizationin the SM W boson longitudinally polarized

spin density matrix

evidence for WL at 5 level !

OPAL

cosW

LL=00d/dcosWdcosW

T=(+++--)d/dcosWdcosW

L/ =0.2430.0270.012

SM: 0.240 at s=197 GeV

cosh*

L/ =0.2100.0330.016

unfold decay angle distribution


Quartic Gauge Couplingin SM these couplings exist but too small to be seen at LEP

look for anomalous contributionsparameterised by additional termsin the Lagrangian

couplings a0, ac, an; physics scale

-0.020 < a0/2 < 0.020 GeV-

2

-0.053 < ac/2 < 0.037 GeV-

2

-0.16 < an/2 < 0.15 GeV-2

new OPAL analysis of WW


excellent mass resolution comes from

kinematic fit:

constrain total (E,p) to (s,0)

need for precise knowledge of the beam energy from LEP

mass of the W boson

direct reconstruction :

mW from the invariant mass calculated using the W decay

productsWW qqqq and WW qql (ALEPH and OPAL also WW ll)

raw mass

measure mW and mtop prediction of mH


reconstructed mass distributions

DELPHIeqq

ALEPH 4q

L3 qq

OPAL qq


mW spectrum

had

ron

isat

ionW

p

rod

ucti

on

an

d d

ecay

Pert

.QC

Dd

eca

y

W

ob

serv

ati

on

(D

ETEC

TO

R)

mW extraction calibrated

with Monte Carlo simulation

reconstructed mass distorted! - initial state radiation E0<Ebeam

- mW(jet/recon. lepton) mW(quark/lepton)


LEP: latest results

direct measurements

mH<210 GeV @ 95% C.L. SM fitmH > 114 GeV direct limit

mW(GeV)

mWworld=80.4260.034 GeVW constrained to SM

relationship with mW:


Systematic errors

WWqqqq weight channel in the combination: 9%

experimentschannelsyears

qqlv qqqq comb. corr.e c y

CR - 90 9 e - y

BE - 35 3 e - y

other 4 5 4 - - -

rad. corrections 8 8 8

fragmentation 19 18 18 e c y

detector 14 10 14 - c y

LEP energy 17 17 17 e c y

systematics 31 101 31statistical 32 35 29

total 44 107 43

cross-LEP effort in progress to address these errorsderive them from data whenever is possible


radiative corrections

a new OPAL analysis tries to estimate on data the contribution of real production using WW events

mW calibrated on Monte Carlo with O() photon radiation but not all diagrams are completely included:

estimated mass shift dueto real photon production from data ~ 6-8 MeV


final state interactions (only 4q)

possible interaction between the two W decays products not in the simulation apparent shift in mw

only phenomenological modelsfm

Colour Reconnection (CR):

• W decay~0.1fm<< hadronization scale~1fm colour flow between Ws

• seen at ep,pp colliders (rapidity gaps) and in heavy meson decays

Bose Einstein Correlation (BEC):

• favours production of pairs/multiplets of identical particles close together

• well established in single Z and W


CR: particle flow in 4-jet events at LEP2

L330%

RN=(A+C)/(B+D) is

used to compare with models:

various models and parameters! one experiment can exclude only extreme cases LEP combination

CR: modifies particle flow between Ws:

W W


particle flow: LEP combination

r=RNdata/RN

no-CR

r=0 no CR, r0 CR

preferred value in data Precmin

~49%r

between various models SK1 gives the largest mW

bias: vary reconnection fraction

mass bias calculated from Prec

min+1 used in the mW

combination: mass shift increases (90 MeV) but data driven


mW and CR

strategies to reduce CR bias: - hybrid cone jet cone

algorithm- remove low energy particle pcut

SK1 parameter

most probably LEP will use these strategies for the final mW

trade statistics for systematics:

all CR model used behave as SK1!

it also reduces BEC systematics!systematics are under study

~ factor 2-3 in CR shift, 2 in BEC shift ~ 20% loss in statistics


CR with mW

combination with colour flow (almost uncorrelated)

mW(no-CR)–mW

CR to study CR

- higher sensitivity than colour flow- mass difference still use the qqqq channel to measure mW!

use this combination to get the CR systematics for the W mass:the exact procedure is under discussion

all experiments are working on similar analyses

it will be difficult to achieve a 5discovery for CR in WW events


Bose Einstein Correlations

hadronic parts of qqln

rotate/boost

mix ‘WW’ event

measure BEC between W comparing (Q) (2-particle density) in 4q and ‘mixed’ WW events:

R2(Q)=ρ(4q) /ρ(mix WW)noBE

Δρ = ρ(4q)- ρ(mix WW)

ALEPH, L3: no sign of BEC between WsDELPHI: small BEC between Ws

propagate results on BEC between Ws into mW systematics: work in progresshowever mass shift due to BEC is expected to be smaller than CR


measuring the W widthfit simultaneously for mW and W direct measurement of W

SM 2.095 GeV

wworld=2.1390.069 GeV


conclusions and outlook• LEP met the expectations and exceeded them

• many properties of the W boson are measured • triple gauge coupling are well determined

•5 evidence of the longitudinal polarisation of the W

• for the measurements of the W mass and width

there are good prospects to improve the results and for mW to meet the 35 MeV error goal

• so far good agreement with the Standard Model predictions• final analyses still going on …

0.0240.020λ

0.0550.943

0.998g

γ

λ

0.0230.025

Z1

κ

091.0150.2Γ

0.046GeV80.426m

W

W


CKM unitarity and Vcs

ji,

2

ijs

ji V)πα

(1)qq'BR(W

|Vcs| = 0.966 ± 0.013dominated by the error on the Br

measurement of Vcs the least know CKM element before LEP2 (11%):

flavour changing transitionsW on-shell

b)s,(d,j

c)(u,i 0.0252.039V2

ij

CKM unitarity for elements not involving the top quark

2V2

ij

dominated by the error on the Br

∝|Vqq|2

qW

q’

w physics at lep

Documents

w physics

w pairs

sm w boson

w decays products

w pair production triple

static w properties

quartic gauge couplings

w decay productsww qqqq