CMS Sensitivity to Quark Contact Interactions with Dijets

1
CMS Sensitivity to Quark Contact Interactions
with Dijets

• Selda Esen (Brown)
• Robert M. Harris (Fermilab)
• DPF Meeting
• Nov 1, 2006

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Quark Contact Interactions
Quark Compositeness
New Interactions

• New physics at a scale L above the observed dijet
  mass is effectively modeled as a contact
  interaction.
• Quark compositeness.
• New interactions from massive particles exchanged
  among partons.
• Contact interactions sometimes look different
  than QCD.
• QCD is predominantly t-channel

q
q
M L
M L
q
q
Dijet Mass ltlt L
Quark Contact Interaction
q
q
L
q
q
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Contact Interactions in Mass Distribution

• Contact interaction produces rise in rate
  relative to QCD at high mass.
• Observation in mass distribution alone requires
  precise understanding of QCD cross section.
• Hard to do
• Jet energy uncertainties give large cross section
  uncertainties
• Parton distribution uncertainties are significant
  at high mass high x and Q2.

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Contact Interactions in Angular Distribution

• Contact interaction is often more isotropic than
  QCD.
• For example, the standard contact interaction
  among left-handed quarks introduced by Eichten,
  Lane and Peskin (Phys. Rev. Lett. 50, 811-814
  (1983)).
• Angular distribution has much smaller systematic
  uncertainties than cross section vs. dijet mass.
  
• We use a simple single measure (one number) for
  the angular distribution as a function of dijet
  mass.
• See the effect emerge at high mass.

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The CMS Detector and Jets

• Measure jets in the CMS Calorimeter
• Energy inside a cone of radius R centered on jet
  axis

Hadronic
CMS Calorimeters
Electromagnetic

• Jet energy calibrated to equal the energy of
  particles in jet cone
• Dijet is the two leading jets in the event
• Two jets with highest transverse momentum, PT.
• Dijet mass m is

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Sensitive Variable for Contact Interactions

• Dijet Ratio
• N(hlt0.5) / N(0.5lthlt1)
• Number of events in which each leading jet has
  hlt0.5, divided by the number in which each
  leading jet has 0.5lthlt1.0
• Dijet Ratio is the variable we use
• Simple measure of the most sensitive part of the
  angular distribution.
• We measure it as a function of mass.
• It was first introduced by D0 (hep-ex/980714).
• We will show systematics on the dijet ratio are
  small.

Our lowest order calculation of both signal and
background.
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Dijet Ratio and Statistical Uncertainty(Smoothed
CMS Simulation)

• CMS simulation of QCD is flat at 0.6
• Error bars are the expected statistical
  uncertainty for 100 pb-1, 1 fb-1 and 10 fb-1
• This uses expected jet triggers and prescales
  (CMS PTDR vol. II, 2006)
• We compare to the contact interaction signal for
  L5, 10 15 TeV
• Calculate c2 for significance estimates.

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Dijet Ratio and Systematic Uncertainty

• Systematics are small
• Because they cancel out in the ratio.
• Upper plot shows systematics statistics.
• Lower plot shows zoomed vertical scale.
• Absolute Jet Energy Scale
• No effect on QCD dijet ratio flat vs. dijet
  mass.
• Causes 5 uncertainty in L. (included)
• Relative Energy Scale
• Energy scale in hlt0.5 vs. 0.5 lt h lt 1.
• Estimate /- 0.5 is achievable in Barrel.
• Changes ratio between /-.01 and /-.03.
• Resolution
• No change to ratio when changing resolution
• Systematics bounded by MC statistics 0.02.
• Parton Distributions

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Significance of Contact Interaction Signal

• Significance found from c2
• 5s Discovery
• 95 CL Exclusion
• Effect of dijet ratio systematics on the
  significance is small.

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Sensitivity to Contact Interactions
Left-Handed Quark Contact Interaction Left-Handed Quark Contact Interaction L for 100 pb-1 (TeV) L for 1 fb-1 (TeV) L for 10 fb-1 (TeV)
95 CL Exclusion Stat Only 6.4 10.6 15.1
95 CL Exclusion All Sys 6.2 10.4 14.8
5s Discovery Stat Only 4.7 8.0 12.2
5s Discovery All Sys 4.7 7.8 12.0

• Numbers in red are our L sensitivity estimates.
• Published Limit from D0 L gt 2.7 TeV at 95 CL
  (hep-ex/980714).

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Conclusions

• CMS plans to use the dijet ratio to search for
  quark contact interactions
• Weve estimated the signal and QCD background
  from 0.3 to 6.5 TeV
• From jet triggers for 100 pb-1, 1 fb-1 and 10
  fb-1
• Using the full CMS detector simulation.
• We have presented CMS 95 CL exclusion and 5s
  discovery sensitivity to a quark contact
  interaction.
• Including both statistical and systematic
  uncertainties.
• CMS can discover a quark contact interaction L
  12 TeV with 10 fb-1.
• Corresponds to a quark radius of order 10-18 cm
  if quarks are composite.