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 1

2. QCD t - channel 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 << L Quark Contact Interaction q q L 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. Center of Momentum Frame Jet q* Parton Parton Jet QCD Background dN/ dcos q* Signal 0 1 cos q* 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. Selda Esen, Brown

5. The CMS Detector and Jets • Measure jets in the CMS Calorimeter • Energy inside a cone of radius R centered on jet axis CMS Calorimeters Hadronic 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 Selda Esen, Brown

6. Sensitive Variable for Contact Interactions • Dijet Ratio = N(|h|<0.5) / N(0.5<|h|<1) • Number of events in which each leading jet has |h|<0.5, divided by the number in which each leading jet has 0.5<|h|<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 signal and 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 L+=5, 10 & 15 TeV • Calculate c2 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 L. (included) • Relative Energy Scale • Energy scale in |h|<0.5 vs. 0.5 < |h| < 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 c2 • 5s Discovery • 95% CL Exclusion • Effect of dijet ratio systematics on the significance is small. Selda Esen, Brown

10. Sensitivity to Contact Interactions • Numbers in red are our L sensitivity estimates. • Published Limit from D0: L+ > 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 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. Selda Esen, Brown