Single boson production at LEP Guillaume Leibenguth On behalf of the LEP experiments Institut de...

Single boson production at LEPSingle boson production at LEP

Guillaume Leibenguth

On behalf of the LEP experiments

Institut de physique nucléaire

Université catholique de Louvain

DIS’04, Štrbské Pleso

G. Leibenguth Single Boson Production at LEP 2

Two ChannelsTwo Channels

• W exchange:– Test of the Standard Model: IR – Study of the trilinear gauge coupling– Measurement of W mass at the next linear collider

• Z/ exchange:– Electroweak Compton scattering process– Cross section refers to 48 Feynman Diagrams Diagrams involving Higgs boson are neglected


Single W Production: ContextSingle W Production: Context

W pair production

Single W production:fermion-antifermioncomes from the W Boson,t-channel diagram

96 Charged or NeutralCurrents


And Difficulties…And Difficulties…

Single W production is in Yellow!

Data taken between 161 and 209 GeV

• qq+isr: photon emitted along the beam pipe

• qq: little missing energy

• W pair: 4q: no missing energy, lqq: high energetic lepton visible

fully leptonic: 2 leptons alone!

• Z pair: irreducible bkg ee• single W boson (signal We!)

• Zee (second part of the talk)


Signal Definition L3 (single W)Signal Definition L3 (single W)Generated phase space = SIGNAL:

• |cos e+ | > 0.997

• Min (Ef, Ef’) > 15 GeV

• for ee only: |cos e- | < 0.75

for Z ()ee rejection

Analysis relies on:

• Two fermions and Pmiss

e+ in the beam pipe

MC: e+qq’

Angular acceptance of LEP detectors


L3 Single W AnalysisL3 Single W Analysis

Hadronic channel

• Two acoplanar jets

• Ee.m > 15 GeV

• 0.3 < Evis/s < 0.65

• ET > 0.2 Evis

• |cos miss | < 0.92

• Acoplanarity > 11º• No high Elepton

• Anti ZZ cut

• Neural Network (NN)

Leptonic channel

• High energetic lepton

• No other activity in the detector


Signal Definition AlephSignal Definition Aleph

Generated phase space requirement:• |cos e | > 0.9994• Mqq’ > 60 GeV/c² for eqq’OR• |cos l | < 0.95 and El > 20 GeV for el

Monte Carlo signal : Grc4f

A candidate:


ALEPH Single W AnalysisALEPH Single W Analysis

Hadronic channel

• |cos miss | < 0.90

• Acollinearity between the two hemisphere momentum < 165º

• Energy of a wedge of 30º centered on missing momentum direction

• Energy in a cone of 12º (E12) around the beam pipe < 0.025 s

• Invariant hadronic mass < 60 GeV/c²

• Neural Network

• High energetic isolated lepton• No E12 energy, missing

momentum unbalance

Leptonic channel


Signal Definition Opal-DelphiSignal Definition Opal-Delphi

Graph content (and not generated phase space):

• t-channel diagram only

• Mqq > 45 GeV/c2

• |cos e- | < 0.95, El+ > 20 GeV and |cos e+ | > 0.95 for ee

Monte Carlo Signal: Wphact (Delphi), KoralW, Grc4f, Excalibur (Opal)

MC Background: 2 fermions: KK2f (Delphi) KoralZ (Opal), e+e- : TEEGG & BHWIDE, PYTHIA 6.143 & BDK (Delphi), Pythia, Herwig, Phojet, twogen, Phot02 (Opal)

The LEP experiments have agreed to use this definition as LEP wide.


DELPHI single W analysisDELPHI single W analysis

Hadronic selection• Ee.m/s < 50 %• Evis/s > 20 %• ET > 0.2 Evis • |cos miss | < 0.98• Acoplanarity > 10 deg• No E.M Cluster > 45 GeV• Multiplicity < 50• 30 <Mvis < 110 GeV/c2 • E(e,) < 7.5 (10) s % • Neural Network

Leptonic channel• One Lepton, El (Et) /s< 45 (12)%• | IPR | < 0.2 cm for muon


OPAL Single W AnalysisOPAL Single W Analysis

Hadronic channel• Two non collinear jets

recoiling against the neutrino in the transverse plane

• 120 > Mqq > 40 GeV/c²• 15 % < Ee.m/s < 50 %• WW eqq removed• No lepton isolated• |cos miss | < 0.96• Acoplanarity > 7.5 º• Likelihood selection

Leptonic selection very sophisticated

ee

e


Combining …Combining …

• Take the Delphi-Opal definition (rescale the phase space cut L3, Aleph to it)

• Correlation between uncertainties are taken into account

• Only a preliminary analysis of 189 GeV data from OPAL

Systematic errors are treated in 4 different ways!

Several sources, but a few really contribute:

• Limited amount of MC stat

• Background estimate• Luminosity• For the muon channel,

triggering issue (ALEPH and L3)

• 5 % accuracy of signal modelling


Accuracy of SM predictionAccuracy of SM prediction

Good agreement in all experiment between the expected and the observed number of events.

Luminosity used by Aleph : 683.4 pb-1

Delphi : 619.3 pb-1

L3 : 675.5 pb-1

Opal : 174.7 pb-1


Comparison Grc4f-WphactComparison Grc4f-Wphact


Sensitivity to Sensitivity to

Assuming SU(2)xU(1) symmetry

-> 3 couplings

0.90 < < 1.32 (L3)

The sensitivity to z is weaker.

For a more complete discussion on TGC, see U. Parzelfall’s talk


Z/Z/ exchange exchange

Main diagrams contributing to the “ single Z ” production


DELPHI Zee AnalysisDELPHI Zee Analysis

Requirement on the phase space

(COMMON LEP Definition!) un < 12 º, 12 º < sc<120º

and Esc > 3 GeV

• Mqq or M > 60 GeV/c2

Analysis based on:• A loose preselection• Identification of an isolated electron• Cut on Mqq or M


ALEPH Zee AnalysisALEPH Zee Analysis

Hadronic channel

• A loose preselection

• Identification of an isolated electron with

E> 3GeV

• cos e < 0.8

• Qe cos z > -0.85

• Qz cos miss > 0.9

• Mhad > 50 GeV/c²

• Emiss / s > 25 %Leptonic channel

• Three Leptons

• Charge = +/- 1


L3 Zee AnalysisL3 Zee Analysis

Hadronic channel

• An electron identified in the BGO with an energy of 3 GeV

• 50<Mhad < 130 GeV

• Qe cos z > -0.5

• Qz cos miss > 0.94

• Two jets (Durham)

• Opening angle of

the two jets > 150• 55 < M < 145 GeV/c²• -0.5 < q cos < 0.93, q cos miss > 0.5• Q cos z < 0.40

:


OPAL Zee AnalysisOPAL Zee Analysis

Hadronic channel• Two jets• EM. Cluster larger than 1 GeV• dE/dx consistent with electron• Qe cos miss 0• Mqq’ > 25 GeV/c² +Kin. Fit.• Eem/ s < 0.93• |Pmiss| > 35 GeV/c• Isolation of the electron: nothing in 0.6 rad cone• Qe cos miss 0.95, Emax in forward detector < 30 GeV• - Qe cos e 0.65 or Ee 0.35 E

Eqq

Ee

Mqq


Comparison Grc4f -WphactComparison Grc4f -Wphact


ConclusionsConclusions

• Standard model is in a very good shape• Even when these difficult corner of phase space are

studied and tested to a 5% accuracySo• L3 has already published its final results• ALEPH and DELPHI are really close to finalizing their

publications• OPAL has only analyzed 189 GeV data set• Final LEP word on this issue hopefully in summer 2004And• The single W cross section is rising! These channels are of

importance in view of the next linear collider

Single boson production at LEP Guillaume Leibenguth On behalf of the LEP experiments Institut de...

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