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W Physics at LEP. Paolo Azzurri Pisa (ALEPH-CMS ) Hanoi August 7, 2004 5th Rencontres du Vietnam New Views in Particle Physics. Outline. W-pair production at LEP2 W Gauge Couplings W Decay Couplings Other four fermion processes W mass. Introduction.

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W Physics at LEP

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W physics at lep

W Physics at LEP

Paolo Azzurri

Pisa (ALEPH-CMS)

Hanoi August 7, 2004

5th Rencontres du Vietnam

New Views in Particle Physics


Outline

Outline

  • W-pair production at LEP2

  • W Gauge Couplings

  • W Decay Couplings

  • Other four fermion processes

  • W mass

P.Azzurri - W Physics at LEP


Introduction

Introduction

The study of W-pair production at LEP2

was a nice and exciting work that provided:

  • a clear evidence of the presence of Standard Model

    triple gauge couplings and of their SU(2)U(1) structure

  • the first direct measurements of the W decay branching ratios, and from these:

  • a direct test of the W lepton family and lepton-quark universality to 1% and 0.5%

  • a test of CKM unitarity and best current determination of the Vcs amplitude

  • an independent W mass measurement at production threshold (200 MeV accuracy)

  • a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy

P.Azzurri - W Physics at LEP


W pair production at lep2

W-pair production at LEP2

P.Azzurri - W Physics at LEP


W pair production

LEP2 produced about 10000 WW events/exp. with ~700pb-1/exp.

Expect:

per experiment.

W-pair production

CC03 diagrams (not gauge invariant)

P.Azzurri - W Physics at LEP


W pair event selection

… sophisticated selections

Channel

efficiency

purity

bkg

ll

50-80%

80-90%

,,ll

eqq

75-90%

~90%

qq,Zee

qq

75-90%

~95%

qq

qq

50-80%

80-85%

qq,We

qqqq

80-90%

75-80%

qq

W-pair event selection

Clear event decay ...

P.Azzurri - W Physics at LEP


W pair cross sections

W-pair cross sections

Clear proof of SU(2)xU(1) gauge couplings !

WW (pb)

s (GeV)

Precision

better than 1%

Test of the SM radiative corrections to the CC03 diagrams

P.Azzurri - W Physics at LEP


W gauge couplings

W Gauge Couplings

P.Azzurri - W Physics at LEP


Triple gauge couplings

triple and quartic SU(2) gauge boson self couplings

The signature of the non-abelian SU(2) electroweak structure !

Triple Gauge Couplings

Standard electroweak theory

U(1)

SU(2)

P.Azzurri - W Physics at LEP


Triple gauge couplings1

Triple Gauge Couplings

The most general Lorentz Invariant WWV (V=,Z) vertex has 7 complex couplings

P.Azzurri - W Physics at LEP


W pair angular distributions

W-pair angular distributions

The WWZ and WW gauge couplings can be measured in W-pair events, fitting the W-pair event rates and the W production and decay angular distributions.

P.Azzurri - W Physics at LEP


Triple gauge couplings2

Relaxing all constraints and fitting for any of the 28 WWZ and WW couplings

one-dimensional fit results

for the SM non-zero TGC values

(ALEPH data only)

… all other 24 couplings are

consistent with zero (within 5-20%) !

Triple Gauge Couplings

Standard measurements of three TGC couplings that conserve C and P, U(1)em and global SU(2)L⊗U(1)Y

one dimensional fit results (LEP):

P.Azzurri - W Physics at LEP


W decay couplings

W Decay Couplings

P.Azzurri - W Physics at LEP


W leptonic couplings

Direct test of W lepton universality

at the 1% precision level

W leptonic couplings

P.Azzurri - W Physics at LEP


W hadronic couplings

Direct test of W quark-lepton universality

at the 0.6% precision level

W hadronic couplings

P.Azzurri - W Physics at LEP


Other four fermion processes

Other four fermion processes

P.Azzurri - W Physics at LEP


Z pairs

Z-pairs

No gauge self couplings involved

in Standard Model Z-pair production

In agreement with SM within 5% precision

P.Azzurri - W Physics at LEP


Single w and z

Single W and Z

In agreement with SM within 8% precision

In agreement with SM within 7% precision

P.Azzurri - W Physics at LEP


W mass

W mass

P.Azzurri - W Physics at LEP


W mass1

W mass

Direct kinematic reconstruction of the W mass in qqqq, qql  (and l l ) final states

Improve the W invariant mass resolution:

kinematic fit

energy-momentum conservation

(equal mass constraints)

  • mW value extracted with different methods

  • Breit-Wigner fit (with bias correction)

  • Monte Carlo reweighting

  • (with different observables M1,δM)

  • Probability Density function P(M1,M2,..)

Statistical power of the LEP2 data: ΔmW(stat)=21 MeV

P.Azzurri - W Physics at LEP


W mass systematics

mW(qqqq)= 80.420±107 MeV

mW(qql )= 80.411± 44 MeV

(ρ=0.18)

ΔmW(qqqq-qql )= +22±43 MeV

without CR and BE errors

W mass systematics

The fully hadronic channel is de-weighted to 0.10

for possible final state interconnection effects !

P.Azzurri - W Physics at LEP


Hadronic final state interactions

Colour Reconnection effects:

measure particle flow in regions

between W’s / inside W’s

upper limits: ki<2.13  Prob(CR)<0.65

 ΔmW=90 MeV

Hadronic final state interactions

Bose-Einstein correlations:

measure two particle correlations

between W’s / inside W’s

full effect ΔmW=35 MeV

measured fraction ΔmW=15 MeV

P.Azzurri - W Physics at LEP


Cr insensitive jet reconstructions

CR insensitive jet reconstructions ?

Prospects

Can reduce the CR mass shifts by a factor 2-3

deteriorating the statistical precision by 20%

CR systematics : ΔmW=90  40 MeV

statistical error: ΔmW=35  40 MeV

total mW(qqqq) error:ΔmW=110  60 MeV

mW(qqqq) weight in combination : 0.10  0.30

combined mW error :42  38 MeV

  • Ways to reduce the CR bias:

  • Remove low energy particles (pcut)

  • Hybrid cone algorithm (R)

P.Azzurri - W Physics at LEP


W mass and width

W mass and width

W measured with the same methods used for the mW extraction

mW= 80.412±0.042 GeV/c2

W= 2.150±0.91 GeV/c2

P.Azzurri - W Physics at LEP


Standard model fit

Standard Model Fit

All this and more in Roberto Tenchini’s talk tomorrow !

P.Azzurri - W Physics at LEP


Conclusions introduction

Conclusions = Introduction

The study of W-pair production at LEP2

was a nice and exciting work that provided:

  • a clear evidence of the presence of Standard Model

    triple gauge couplings and of their SU(2)U(1) structure

  • the first direct measurements of the W decay branching ratios, and from these:

  • a direct test of the W lepton family and lepton-quark universality to 1% and 0.5%

  • a test of CKM unitarity and best current determination of the Vcs amplitude

  • an independent W mass measurement at production threshold (200 MeV accuracy)

  • a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy

P.Azzurri - W Physics at LEP


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