Single%20boson%20production%20at%20LEP

1
Single boson production at LEP

• Guillaume Leibenguth
• On behalf of the LEP experiments
• Institut de physique nucléaire
• Université catholique de Louvain
• DIS04, Štrbské Pleso

2
Two 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

3
Single W Production Context

• W pair production

96 Charged or Neutral Currents
Single W production fermion-antifermion comes
from the W Boson, t-channel diagram
4
And Difficulties

• Single W production is in Yellow!
• Data taken between 161 and 209 GeV
• qqisr photon emitted along the beam pipe
• qq little missing energy
• W pair 4q no missing energy, l?qq 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)

5
Signal Definition L3 (single W)

• Generated phase space SIGNAL
• cos ?e gt 0.997
• Min (Ef, Ef) gt 15 GeV
• for e?e? only cos ?e- lt 0.75
• for Z (???)ee rejection
• Analysis relies on
• Two fermions and Pmiss

MC e?qq
Angular acceptance of LEP detectors
e in the beam pipe
6
L3 Single W Analysis

• Hadronic channel
• Two acoplanar jets
• Ee.m gt 15 GeV
• 0.3 lt Evis/?s lt 0.65
• ET gt 0.2 Evis
• cos ?miss lt 0.92
• Acoplanarity gt 11º
• No high Elepton
• Anti ZZ cut
• Neural Network (NN)

• Leptonic channel
• High energetic lepton
• No other activity in the detector

7
Signal Definition Aleph

• Generated phase space requirement
• cos ?e gt 0.9994
• Mqq gt 60 GeV/c²
• for e?qq
• OR
• cos ?l lt 0.95 and
• El gt 20 GeV for e?l?
• Monte Carlo signal
• Grc4f

A candidate
8
ALEPH Single W Analysis

• Hadronic channel
• cos ?miss lt 0.90
• Acollinearity between the two hemisphere momentum
  lt 165º
• Energy of a ? wedge of 30º centered on missing
  momentum direction
• Energy in a cone of 12º (E12) around the beam
  pipe lt 0.025 ?s
• Invariant hadronic mass lt 60 GeV/c²
• Neural Network

Leptonic channel

• High energetic isolated lepton
• No E12 energy, missing momentum unbalance

9
Signal Definition Opal-Delphi

• Graph content (and not generated phase space)
• t-channel diagram only
• Mqq gt 45 GeV/c2
• cos ?e- lt 0.95, El gt 20 GeV and cos ?e gt
  0.95 for e?e?
• Monte Carlo Signal Wphact (Delphi), KoralW,
  Grc4f, Excalibur (Opal)
• MC Background 2 fermions KK2f (Delphi)
  KoralZ (Opal), ee- ? 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.

10
DELPHI single W analysis

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

• Leptonic channel
• One Lepton, El (Et) /?slt 45 (12)
• IPR? lt 0.2 cm for muon

11
OPAL Single W Analysis

• Hadronic channel
• Two non collinear jets recoiling against the
  neutrino in the transverse plane
• 120 gt Mqq gt 40 GeV/c²
• 15 lt Ee.m/?s lt 50
• WW ? e?qq removed
• No lepton isolated
• cos ?miss lt 0.96
• Acoplanarity gt 7.5 º
• Likelihood selection

e?e?
e???

• Leptonic selection very sophisticated

12
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

13
Accuracy 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
14
Comparison Grc4f-Wphact
15
Sensitivity to ??

• Assuming SU(2)xU(1) symmetry
• -gt 3 couplings
• 0.90 lt ?? lt 1.32 (L3)
• The sensitivity to ?z is weaker.
• For a more complete discussion on TGC, see
  U. Parzelfalls talk

16
Z/? exchange
Main diagrams contributing to the single Z
production
17
DELPHI Zee Analysis

• Requirement on the phase space
• (COMMON LEP Definition!)
• ?un lt 12 º, 12 º lt ?sclt120º
• and Esc gt 3 GeV
• Mqq or M?? gt 60 GeV/c2
• Analysis based on
• A loose preselection
• Identification of an isolated electron
• Cut on Mqq or M??

18
ALEPH Zee Analysis

• Hadronic channel
• A loose preselection
• Identification of an isolated electron with
• Egt 3GeV
• cos ?e lt 0.8
• Qe cos ?z gt -0.85
• Qz cos ?miss gt 0.9
• Mhad gt 50 GeV/c²
• Emiss / ?s gt 25

• Leptonic channel
• Three Leptons
• Charge /- 1

19
L3 Zee Analysis

• Hadronic channel
• An electron identified in the BGO with an energy
  of 3 GeV
• 50ltMhad lt 130 GeV
• Qe cos ?z gt -0.5
• Qz cos ?miss gt 0.94
• Two jets (Durham)
• Opening angle of
• the two jets gt 150?

??

• 55 lt M?? lt 145 GeV/c²
• -0.5 lt q cos ? lt 0.93, q cos ?miss gt 0.5
• Q cos ?z lt 0.40

20
OPAL Zee Analysis
Eqq

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

Mqq
Ee
21
Comparison Grc4f -Wphact
22
Conclusions

• Standard model is in a very good shape
• Even when these difficult corner of phase space
  are studied and tested to a 5 accuracy
• So
• 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
  2004
• And
• The single W cross section is rising! These
  channels are of importance in view of the next
  linear collider