SEARCH FOR A UNIVERSAL EXTRAD SIGNAL AT LHC

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SEARCH FOR A UNIVERSAL EXTRA-D SIGNAL AT LHC
Pierre-Hugues Beauchemin McGill
University and Université de Montréal
PRINCIPAL REFERENCES 1. C. Macesanu et al.
Phys.Rev. D66 (2002) 015009 2. T. Rizzo
Phys.Rev. D64 (2001) 095010 FUTHER REFERENCES 1.
Applequist et al. Phys.Rev. D64 (2001) 035002 2.
Lykken et al. Phys.Rev. D59 (1999) 105006 3.
Dienes et al. Nucl.Phys. B537, (1999) 47
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MOTIVATION OF THIS STUDY

• In General
• String-inspired model that predicts non-standard
  phenomenology at a low energy scale (TeV)
  all particles in the bulk
• Dont need mechanism to constrain Standard
  fields to a 3-brane ? smaller dependence of
  the low energy phenomenology on the new
  physics
• Loops of KK-bosons can provide
• gauge coupling unification at TeV scale (ref. 1)
• some mechanisms for SUSY breaking
• Weaker collider bounds than the scenario where
  only gauge bosons propagate in the TeV-1
  Extra-D.
• In particular
• We can design UED model which are consistent
  with SLED scenario (see previous talk)
• This can provide gauge unification mechanism to
  SLED

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GOAL OF THE ANALYSIS

• Setup effective criteria for a discovery of a
  UED signal at ATLAS
• Evaluate the ATLAS sensitivity to the lowest
  KK-mode mass of partons

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PHENOMENOLOGY

• KK-parton production
• Conservation of momentum in the UED
• ? involve only vertices that conserve the
  KK-mode number
• ? No indirect evidence for KK-mode, only
  direct production can manifest KK effects at
  tree-level
• KK number conservation is broken at the one-loop
  level, except for the lowest-lying KK excitations
  which can remain stable

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• Two distinct KK quark towers corresponding to a
  left-handed and a right-handed zero-mode, can be
  produced (same signal)
• In the presence of the Higgs mechanism, the mass
  of the 1st KK-mode is
• The signal of a two KK-parton final state will
  be two slow high-ionizing tracks
• ?
• Suggestions
• TOF (scintillator has good timing response)
• dE/dx

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• Final state KK-parton decay
• SLED scenario predicts a violation in KK number
  conservation, allowing the lowest-lying KK
  excitation to decay into the associate SM parton
  plus bulk particle.
• In the minimal scenario (only the graviton
  channel is available) the vertex will then be
  multiplied by a form factor
• The signature would be 2-jets and a lot of
  missing
• In more sophisticated models, KK-partons may not
  be degenerate and can undergo a cascade of
  SM-like decay, before emitting graviton
• Macesanu et al., hep-ph/0207269
• H. Przysiezniak and D. Goujdami

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PHYSICAL PREDICTION

• LAGRANGIAN
• The Lagrangian is given by a 5D generalization of
  4D QCD
• We focus of the hadronic sector of KK-pair
  production
• The 5D Lagrangian density for the kinetic and
  quark-gluon interactions is
• To project to a 4D theory we must
• Fourier expand the 5D fields in terms of the
  extra-D coordinate y
• integrate over y (restrict the fermions to a
  S1/Z2 orbifold)

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In the unitary gauge, we can apply the gauge
choice
In the limit of massless SM quarks, the two quark
towers are defined as
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• The 4D Lagrangian density for the 1st
  KK-excitation is

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CROSS-SECTIONS

• We RECALCULATED the differential cross section
  for every process of gluon (g) and KK quark (q
  and q) pair production in proton-proton
  collisions at LHC.
• (Macesanu et al. in Phys.Rev. D66 (2002) 015009
  had many mistakes!)
• The total cross section for the process j, (j
  1,..,14) (with n1) is given by

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• The processes of interest with their computed
  cross sections are

We use -CTEQ 5L as PDF -PYTHIA to perform
numerical integration and generate events
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ANALYSIS

• (Using ATLFAST)
• The signal will be 2-Jets (gravitons)
• The standard backgrounds are (with pT gt 100 GeV)

(will do W/Z 2 jets later)

• For MKK 1.3 TeV, the signal cross section is
  2.2 pb
• Two obvious cuts (already studied in
  hep-ph/0401125)
• CUT 1 number of leptons 0
• CUT 2 accept

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• Other possible cuts

CUT 3 GeV
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RESULTS

• With 100fb-1
• B 27300 events (PT gt 100GeV )
• ?
• S 146000 events for and MKK 1.3 TeV )
  Significance of 883 !
• ?
• Can be discovered with ATLAS
• We have to do the analysis for different value of
  MKK and this will give the ATLAS sensitivity to
  such UED signal.
• For a 5s discovery, we need

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CONCLUSION

• If we have a brane with a small thickness
  UED will be allowed
• SLED is consistent with UED ? gauge unification
• We can study direct production of KK-partons
• Because of SLED they will further decay into
  gravitons SM parton
• We can expect a signal at ATLAS (at least for MKK
  1.3 TeV)
• We still have to evaluate the maximum mass for
  which ATLAS is sensitive