W physics at lep
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W physics at LEP. E.Barberio Southern Methodist University PIC2003 Zeuthen 28 th June 2003. 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

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W physics at LEP

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W physics at lep

W physics at LEP

E.Barberio

Southern Methodist University

PIC2003 Zeuthen 28th June 2003


The lep program

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:

E.Barberio


Ww events

WW events

WWlnln

leptonic channel

10.6%

large missing energy

semileptonic channel

43.8%

missing energy

low background

WWqqln

hadronic channel

45.6%

large background

ambiguity in assigning jets to W

WWqqqq

E.Barberio


W pair cross section

=0.9980.006(stat)0.007(syst)

W pair cross section

+

+

preliminary LEP

clear evidence of WWg and

WWZ vertices: probe of the

non-Abelian structure of the

Standard Model

1%measurement

E.Barberio


W branching fractions

= 1.000 0.021

= 1.052 0.029

= 1.052 0.028

SM: 10.83%

hadronic branching fraction:

Br(Wqq’) = 67.92  0.27%

SM: 67.51%

W branching fractions

test of lepton universality at 3%

(less precise than LEP1)

E.Barberio


Triple gauge couplings ww g wwz

W

W

g

Z

W

W

SM values

triple gauge couplings WWg WWZ

general WWg and WWZ

interaction: 14 parameters

applying C and P invariance

& use low-energy constraints

we are left with 3 parameters

relation with the static W properties:

magnetic dipole moment

electric quadrupole moment

E.Barberio


Measuring the coupling at lep2

W-

qW

e-

e+

f

W+

q

W

f

measuring the coupling at LEP2

WW production:

most constraining

sensitive observables

W+W- production angle cosW

W decay angles (helicity)

W rest frame

q and  of W

decay products

E.Barberio


Ww production decay angular distributions

WW production/decay angular distributions

E.Barberio


Single w

+

OPAL preliminary

  • - single W

  • WW angles

  • sWW

  • combined

8% precision

kg

Single W

single W production

smaller cross section

than WW:

but it is very constraining for kg:

E.Barberio


Tgc 1 parameter fit results

TGC 1-parameter fit results

- ALEPH

- DELPHI

- L3

- OPAL

- LEP

(partial statistics)

g1Z, kg 2-5% measurement

dominant systematics O(em)

g1Z,lg: 0.015 kg: 0.039

E.Barberio


Tgc 3 d parameter fit results

TGC 3-D parameter fit results

2D contour: 3rd parameter

at the minimum

joint minimization of

statistical error

E.Barberio


W polarization

sL/s =0.2430.0270.012

SM: 0.240 at s=197 GeV

cosqh*

OPAL

L

sL=r00ds/dcosqWdcosqW

sT=(r+++r--)ds/dcosqWdcosqW

cosqW

W polarization

in the SM  W boson longitudinally polarized

unfold decay angle distribution

spin density matrix

sL/s =0.2100.0330.016

evidence for WL at 5s level !

E.Barberio


Quartic gauge coupling

Quartic Gauge Coupling

in SM these couplings exist but too

small to be seen at LEP

look for anomalous contributions

parameterised by additional terms

in the Lagrangian

new OPAL analysis of WWg

  • 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

E.Barberio


Mass of the w boson

excellent mass resolution comes from

kinematic fit:

constrain total (E,p) to (s,0)

need for precise knowledge of the

beam energy from LEP

raw mass

mass of the W boson

measure mW and mtop prediction of mH

direct reconstruction:

mW from the invariant mass calculated using the W decay products

WW  qqqq and WW  qqln

(ALEPH and OPAL also WW  lnln)

E.Barberio


Reconstructed mass distributions

L3 tnqq

ALEPH 4q

OPAL

mnqq

DELPHI

enqq

reconstructed mass distributions

E.Barberio


W physics at lep

mW spectrum

W

observation (DETECTOR)

W production and decay

Pert.QCD

hadronisation

decay

reconstructed mass distorted!

- initial state radiation E0<Ebeam

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

mW extraction calibrated

with Monte Carlo simulation

E.Barberio


Lep latest results

direct measurements

LEP: latest results

mWworld=80.4260.034 GeV

GW constrained to SM relationship with mW:

mH<210 GeV @ 95% C.L. SM fit

mH > 114 GeV direct limit

mW(GeV)

E.Barberio


Systematic errors

qqlv

qqqq

comb.

corr.

e c y

rad. corrections

8

8

8

e c y

fragmentation

19

18

18

-

c y

detector

1

4

10

1

4

e c y

LEP energy

17

17

17

e

-

y

CR

-

90

9

BE

-

35

3

e

-

y

-

-

-

other

4

5

4

systematics

31

101

31

statistical

3

2

3

5

29

total

44

107

4

3

Systematic errors

experiments

channels

years

WWqqqq weight channel in

the combination: 9%

cross-LEP effort in progress to address these errors

derive them from data whenever is possible

E.Barberio


Radiative corrections

radiative corrections

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

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

estimated mass shift due

to real photon production

from data ~ 6-8 MeV

E.Barberio


Final state interactions only 4q

final state interactions (only 4q)

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

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

only phenomenological models

fm

E.Barberio


Cr particle flow in 4 jet events at lep2

W

W

L3

30%

CR: particle flow in 4-jet events at LEP2

CR: modifies particle flow between

Ws:

RN=(A+C)/(B+D) is used to compare with models:

various models and parameters!

one experiment can exclude only extreme cases  LEP combination

E.Barberio


Particle flow lep combination

r

particle flow: LEP combination

between various models SK1 gives the largest mW bias:

vary reconnection fraction

preferred value in data Precmin~49%

mass bias calculated from Precmin+1s used in the mW combination: mass shift increases (90 MeV) but data driven

r=RNdata/RNno-CR

r=0 no CR, r0 CR

E.Barberio


M w and cr

mW and CR

strategies to reduce CR bias:

- hybrid cone jet cone algorithm

- remove low energy particle pcut

all CR model used behave as SK1!

it also reduces BEC systematics!

systematics are under study

SK1 parameter

most probably LEP will use these strategies for the final mW  trade statistics for systematics:

~ factor 2-3 in CR shift, 2 in BEC shift

~ 20% loss in statistics

E.Barberio


Cr with m w

CR with mW

- higher sensitivity than colour flow

- mass difference  still use the

qqqq channel to measure mW!

mW(no-CR)–mWCR  to study CR

combination with colour flow

(almost uncorrelated)

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

5s discovery for CR in WW events

E.Barberio


Bose einstein correlations

Δρ = ρ(4q)- ρ(mix WW)

hadronic parts of qqln

rotate/boost

Bose Einstein Correlations

measure BEC between W comparing r(Q) (2-particle density) in 4q

and ‘mixed’ WW events:

R2(Q)=ρ(4q) /ρ(mix WW)noBE

mix ‘WW’

event

ALEPH, L3: no sign of BEC between Ws

DELPHI: small BEC between Ws

propagate results on BEC between Ws

into mW systematics: work in progress

however mass shift due to BEC is

expected to be smaller than CR

E.Barberio


Measuring the w width

measuring the W width

fit simultaneously for mW and GW

 direct measurement of GW

Gwworld=2.1390.069 GeV

SM 2.095 GeV

E.Barberio


Conclusions and outlook

conclusions and outlook

  • LEP met the expectations and exceeded them

  • many properties of the W boson are measured

  • triple gauge coupling are well determined

  • 5s 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 …

E.Barberio


Ckm unitarity and v cs

q

∝|Vqq|2

W

q’

CKM unitarity and Vcs

CKM unitarity for elements not involving the top quark

flavour changing transitions

W on-shell

dominated by the error on the Br

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

|Vcs| = 0.966 ± 0.013

dominated by the error on the Br

E.Barberio


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