Davide Boscherini

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High Pt Leptons and W Productionat HERA

• Davide Boscherini

I.N.F.N.-Bologna and DESY-Hamburg
On Behalf of the H1 and ZEUS Collaborations
Introduction ZEUS search for W ? ?? H1 search for
W ? ?? H1 search for leptons and Pt ZEUS search
for leptons and Pt Summary
International Europhysics Conference High Energy
Physics 99 Tampere (Finland), 15-21 July 1999
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The HERA Collider
Processes with ? lt 1pb are accessible
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W Production
The dominant process for W production is e p ?
e W? X Main contribution at leading order is from
?(e p ? e W? X) ? 1.0pb
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Process Signature
The W ? ?? decay channel is the cleanest
signal In this case the process signature is
a high-pt lepton Pt
The same signature marks also some exotic process
like SUSY
selectron squark production
stop sbottom production
W production is the main background source for
these processes
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Background from SM processes
Neutral Current DIS
Charged Current DIS
? ? 105 pb
? ? 102 pb
Bethe-Heitler
Photoproduction
? ? 107 pb
? ? 102 pb
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ZEUS Event Selectionfor W?e? (W???)
Cleaning cuts to remove non-ep events
Missing transverse momentum
Ptmiss gt 20 (15) GeV
High-pt track pt gt 10 (5) GeV
Acoplanarity (track - hadronic system) ?? gt 0.3
radians
Polar angle of the candidate electron (muon) ? lt
1.5 (2.0) radians
Lepton identification energy deposit in
Calorimeter
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ZEUS W Results
1994-97 data ep
Compare selected events to W MC (EPVEC)
MC luminosity is 1000x data
Events selected show agreement with W kinematics
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W Candidate from ZEUS
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ZEUS W Results
Selection efficiency determined using EPVEC MC
38 for e-channel, 13 for
?-channel
(lower than for e due to trigger request on Pt )
1994-97 data ep
From the observed events
(SM expectation is 0.95 pb)
Upper limits can be set
?(e p ? e W? X) lt 3.3 pb at 95 CL (from
electron channel)
?(e p ? e W? X) lt 3.7 pb at 95 CL (from
muon channel)
Events observed are consistent with SM prediction
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Anomalous WW? Couplings
Contribution at leading order comes from
Triple Gauge boson Coupling
Effects of anomalous WW? couplings (??, ?) can
be investigated at HERA In Standard Model ??0,
?0
Larger mean values of the hadronic Pt would be
expected in presence of anomalous WW? couplings
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ZEUS Limits on Anomalous Couplings
Combining e and ? channels and using larger
hadronic Pt cut (Pt gt20) a 95 CL upper limit of
0.58 pb have been set for W production
cross-section
Bounds on anomalous couplings can be set by
comparison to theoretical calculations ?4.7 lt ??
lt 1.5 (?0) ?3.2 lt ? lt 3.2 (??0)
Combined LEP and TeVatron results ?0.14
lt ?? lt 0.48 (?0) ?0.22 lt ? lt 0.13
(??0)
With 1000 pb-1 ?0.38 lt ?? lt 0.38 (?0) ?1.65
lt ? lt 1.66 (??0) (From Proceedings of the
Workshop 1995/96 Future Physics at HERA, vol. 1,
p.190)
Interesting item for high luminosity running
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H1 Search for W Production
Main selection cuts
Calorimetric missing transverse momentum PtCAL gt
12 GeV
High-pt track pt gt 10 GeV
Results
e-channel
?-channel
? and hadronic decay channels also
searched still low sensitivity to W production
Agreement with SM in e-channel 3 outstanding
events in the ?-channel
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H1 Analysis forHigh-pt Leptons and PtEvent
Selection
Cleaning cuts to remove non-ep events
Calorimetric missing transverse momentum PtCAL gt
25 GeV
High-pt track pt gt 10 GeV
distance is defined in pseudorapidity-azimuth
plane
Distance from closest track
Distance from closest jet
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H1 Selection Results
In the region Dtrk gt 0.5, Djet gt 1
After lepton identification energy deposit in
Calorimeter match to Instrumented Iron signal
1994-97 data 36.5pb?1 of e?p
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pt? 28 GeV PTX 67 GeV PTmiss 43 GeV
Muon Event from H1
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H1 AnalysisComparison to SM Expectation
1994-97 data
observed 1 e 5
? expected 2.40.5 e (1.70.5
from W) 0.80.2 ? (0.50.1 from
W)
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H1 AnalysisInvestigation of Event Properties
Compare Pt vs MT?? for data with the dominant
processes (W and ??)
MC luminosity is 500x data
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H1 Selection Results for e?p and Comparison to SM
Expectation
1998-99 data 13.6pb?1, e?p at ?s320 GeV
observed 0 e 0
? expected 1.00.2 e (0.630.19
from W) 0.40.1 ? (0.230.05
from W)
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ZEUS Event Selection(á la H1)
Cleaning cuts to remove non-ep events
Calorimetric missing transverse momentum PtCAL gt
20 GeV
High-pt track pt gt 10 GeV
Hadronic jet with Et gt 5 GeV
Acoplanarity (track - hadronic system) ?? gt 0.2
radians
Track isolation Dtrk gt 0.5, Djet gt 1
Lepton identification energy deposit in
Calorimeter matching the high-pt track
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ZEUS Selection Results
1994-97 data ep
observed 3 e 0 ? expected e 3.50.7 (0.9 from
W) ? 2.00.4 (0.4 from W)
1998-99 data e?p
observed 2 e? 0 ? expected e 0.80.4 (0.3
from W) ? 0.80.1 (0.3 from W)
consistency with SM expectations
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An Interpretation of the H1 Events?
Following T.Kon et al. (hep-ph/9601338) in the
framework of MSSM with RP violation
Kinematics not incompatible for Msbottom? 100 GeV
and Mstop ? 200 GeV
Possible with e?, suppressed with e?
Search for W hadronic decay jetjetPtmiss Selec
tion PT(both jets) gt 25 GeV
set limit Nobs(W ??) lt 0.36 at 90 CL
4 observed ? events unlikely due to this process
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Summary

• W production cross-section has been measured by
  ZEUS for the first time in lepton-hadron
  collisions using 47.7 pb?1 of e?p data and is
  consistent with SM prediction (DESY 99-054)
• Searching for events with isolated leptons and
  missing Pt in 36.5 pb?1 of e?p data, H1 observes
  an excess of events w.r.t. SM expectation
  3 ? events
  properties unlikely explainable with W
  production
  (H1 Collab., Eur. Phys. J. C 5 (1998)
  575-584)
• H1 searched for events with W production in 36.5
  pb?1 of e?p data. The same 3 events as above are
  observed in excess to SM prediction in the
  ?-channel, whilst agreement is found in the
  e-channel
• ZEUS search with similar selection as H1 gives
  consistent results with SM both in e- and
  ?-channel (DESY 99-054)
• In 15 pb?1 of e?p 1998-99 data neither H1 nor
  ZEUS observes any excess
• HERA is running with e?p from July till ...