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CMS Sensitivity to Quark Contact Interactions with Dijets

Selda Esen (Brown) Robert M. Harris (Fermilab)

DPF MeetingNov 1, 2006

Selda Esen, Brown 2

Quark Contact Interactions New physics at a scale

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 t - channel

QCD

Quark Contact Interaction

M ~

Quark Compositeness New Interactions

M ~

Dijet Mass <<

q

q

q

q

q

q q

q

Selda Esen, Brown 3

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.

Selda Esen, Brown 4

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.

cos *

QCD Background

Signal

0 1

dN/

dco

s *

*

Center of MomentumFrame

Parton Parton

Jet

Jet

Selda Esen, Brown 5

The CMS Detector and Jets

Hadronic

Electromagnetic

CMSCalorimeters

Measure jets in the CMS Calorimeter

Energy inside a cone of radius R centered on jet axis

5.022 R 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 is2

212

21 )()( ppEE

Selda Esen, Brown 6

Sensitive Variable for Contact Interactions Dijet Ratio = N(||<0.5) / N(0.5<||<1)

Number of events in which each leading jet has ||<0.5, divided by the number in which each leading jet has 0.5<||<1.0

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 signaland background.

Selda Esen, Brown 7

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 =5, 10 & 15 TeV

Calculate 2 for significance estimates.

Selda Esen, Brown 8

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 . (included)

Relative Energy Scale Energy scale in ||<0.5 vs. 0.5 < || < 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 We’ve used CTEQ6.1 uncertainties. Systematics on ratio less than 0.02.

Selda Esen, Brown 9

Significance of Contact Interaction Signal

Significance found from 2 5 Discovery 95% CL Exclusion

Effect of dijet ratio systematics on the significance is small.

Selda Esen, Brown 10

Left-HandedQuark Contact

Interaction

+ for100 pb-1

(TeV)

+ for 1 fb-1

(TeV)

+ for 10 fb-1

(TeV)

95% CLExclusion

Stat Only 6.4 10.6 15.1

All Sys 6.2 10.4 14.8

5σ Discovery

Stat Only 4.7 8.0 12.2

All Sys 4.7 7.8 12.0

Sensitivity to Contact Interactions

Numbers in red are our sensitivity estimates. Published Limit from D0:+ > 2.7 TeV at 95% CL (hep-ex/980714).

Selda Esen, Brown 11

Conclusions CMS plans to use the dijet ratio to search for quark contact interactions

We’ve 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 5discovery sensitivity to a quark contact interaction. Including both statistical and systematic uncertainties. CMS can discover a quark contact interaction = 12 TeV with 10 fb-1. Corresponds to a quark radius of order 10-18 cm if quarks are composite.