PowerPointPrsentation

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Search for the Unexpected at HERA
Friederike Januschek, DESY International
School of Subnuclear Physics, Erice, September
2007
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Why it is interesting...
Hierarchy problem
qeqp
The Standard Model describes precisely the known
phenomena, but
Why 3 families?
Our understanding is still far from being
complete...
How to include gravity?
Thus Look for New Physics? The Unexpected
Hints?!?
H1 Collab., A. Aktas et al., Eur Phys J C31
(2003) 17
H1 Collab., C. Adloff et al., Z. Phys. C74 (1997)
191
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Outline

• HERA and ZEUS
• Motivation
• Multi-Leptons
• Processes
• Selection
• Results
• Conclusion

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HERA-Accelerator

• Running Periods
• ? HERA-I 1994-00
• ? HERA-II 2003-07

ep-collisions at a CMS energy of vs 300 and
vs 320 GeV
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ZEUS Detector
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Multileptons in HERA-I
H1 Collab., A. Aktas et al., Eur Phys J C31
(2003) 17
High mass region -Excess of Di-Electron events
reported by H1 (3 events seen, SM-expectation
of 0.30.04) -No excess found in ZEUS data (2
events seen, 1.20.1 expected from SM)
Here Model-Independent Search
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SM Dilepton-Production at HERA

• Precise QED predictions
• Dominated by Bethe-Heitler-Process ??--gtll-
• Even few events (at high invariant mass) could be
  the signature of new physics

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Analysis of ZEUS HERA-II e-p-Data
Di-Electron-Selection
It was looked for 2 electrons, at least 2 in
the central region of the detektor (0.3ltTlt2.86)
Required was a transverse momentum ptgt10
GeV for one e and ptgt5 GeV for the other e .

• Both electron candidates were isolated and
  matched to a track.

p
e
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Multi-Electron Results
No excess in high mass regions!
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Di-Electron High Mass Events
Event with the highest invariant mass of 167 GeV

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Cross section for Di-electrons
NiEvents in bin AiAcceptance in bin LLuminosity
Differential Cross Sections in pb/GeV
Total Cross Section1.740.3pb MC 2.17 pb
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Multi-Electron HERA Results (1 fb-1)
-Combined ep and e-p data from ZEUS and
H1 -Good agreement with Standard Model -But
Slight excess of high Minv events in the ep-data
(see next slide)
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Di-Electron ep High Mass Events
SM Probability for 8 events 1 (2.6s)
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Summary

• A search for multi-electron events was performed
  on ZEUS HERA-II e-p data
• Investigated data agrees reasonably well with
  Standard Model
• Combining all available data an intriguing excess
  of multi-electron events over the Standard Model
  remains in high Minv-region for ep-data
• ZEUS and H1 have each collected 0.5fb-1
  luminosity of which the analysis is ongoing and
  more interesting results will follow

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References

• Multileptons
• H1 Collab., A. Aktas et al., Eur Phys J C31
  (2003) 17 , 07/03
• H1 Collab., A. Aktas et al., Phys. Lett. B583
  (2004) 28 , 11/03
• ZEUS-prel-04-019
• ZEUS-prel-06-017
• H1prelim-07-166, ZEUS-prel-07-024
• Isolated Leptons
• ZEUS Collab., J.Breitweg et al., Phys. Lett. B
  471, (2000) 4, 411-428
• H1 Collab., V. Andreev et al., Phys. Lett. B561
  (2003) 241
• ZEUS-prel-06-021, H1prelim-07-162

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Back-Up-Slides
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ZEUS and H1 Detector
Multi-purpose detectors
Uranium-Scintillator Calorimeter
Liquid Argon Calorimeter
sE/E18/vE (ele)
sE/E12/vE?1 (ele)
sE/E35/vE (had)
sE/E55/vE?1 (had)
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Deep Inelastic Scattering (DIS)
NCDIS ?/Z0-exchange scattered lepton and jet
measured in the final state kinematic
reconstruction based on electron energy and
angles and 4-momentum conservation
CCDIS W-exchange only jet measured in the final
state kinematic reconstruction based on hadronic
final state and 4-momentum conservation only
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DIS variables

• Q2 virtuality of the exchanged boson

• x fraction of proton momenta carried by
  struck quark

• y inelasticity, fraction of lepton energy
  transferred in the proton rest frame

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Leptoquarks

• Leptoquarks carry both baryon and lepton numbers,
  thus connecting quark and leptonic sector
• At HERA leptoquarks could be resonantly produced
  in the s-channel
• Signature one lepton, one jet

Resonanz-Produktion für e-p
SM process
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Leptoquark-Results

• No evidence of leptoquarks
• Limits on all LQs set
• Main improvement at low mass resonant production
  region
• For HERA-II (more e-p-data) improvement for
  leptoquarks with F2

FL3B
H1-prel-06-061
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Isolated Leptons

• Search for events with isolated leptons with high
  transverse momentum pt and missing pt in the
  event
• In the SM these events are expected mainly from
  W-boson production, where the W is decaying
  leptonically

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Isolated Leptons HERA Results (1 fb-1)

• No excess in full sample
• For ep-data 1.8s-deviation from the SM for
  events with ptxgt25 GeV

Full Sample
ep Sample
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Monte Carlo
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Di-lepton Production
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Di-Muon-Events
No excess found!
Data SM MC Events 44
45.60.9
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Di-Muon-Events for HERA-I
No excess observed for di-muon events in either
experiment.
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Di-Tau-Events
tt-gteµ???? Channel
Data SM MC Events 3 2.20.8
ZEUS-prel-06-017