1 CMS Sensitivity to Quark Contact Interactions with Dijets Selda Esen (Brown) Robert M. Harris (Fermilab) DPF Meeting Nov 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. è Quark compositeness. è New interactions from massive particles exchanged among partons. l Contact interactions sometimes look different than QCD. è QCD is predominantly t- channel t - channel QCD Quark Contact Interaction  M ~  Quark CompositenessNew Interactions M ~  Dijet Mass <<  q q q q q q q q

Selda Esen, Brown 3 Contact Interactions in Mass Distribution l Contact interaction produces rise in rate relative to QCD at high mass. l Observation in mass distribution alone requires precise understanding of QCD cross section. l Hard to do è Jet energy uncertainties give large cross section uncertainties è Parton distribution uncertainties are significant at high mass = high x and Q 2.

Selda Esen, Brown 4 Contact Interactions in Angular Distribution l Contact interaction is often more isotropic than QCD. è For example, the standard contact interaction among left-handed quarks introduced by Eichten, Lane and Peskin (Phys. Rev. Lett. 50, (1983)). l Angular distribution has much smaller systematic uncertainties than cross section vs. dijet mass. l We use a simple single measure (one number) for the angular distribution as a function of dijet mass. è See the effect emerge at high mass. cos  * QCD Background Signal 01 dN  / dcos  * ** Center of Momentum Frame Parton Jet

Selda Esen, Brown 5 The CMS Detector and Jets Hadronic Electromagnetic CMS Calorimeters l Measure jets in the CMS Calorimeter l Energy inside a cone of radius R centered on jet axis l Jet energy calibrated to equal the energy of particles in jet cone l Dijet is the two leading jets in the event è Two jets with highest transverse momentum, P T. l Dijet mass m is

Selda Esen, Brown 6 Sensitive Variable for Contact Interactions l 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 l Dijet Ratio is the variable we use è Simple measure of the most sensitive part of the angular distribution. è We measure it as a function of mass. è It was first introduced by D0 (hep-ex/980714). è 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) l CMS simulation of QCD is flat at 0.6 è Error bars are the expected statistical uncertainty for 100 pb -1, 1 fb -1 and 10 fb -1 è 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 l Systematics are small è Because they cancel out in the ratio. è Upper plot shows systematics & statistics. è Lower plot shows zoomed vertical scale. l Absolute Jet Energy Scale è No effect on QCD dijet ratio: flat vs. dijet mass.  Causes 5% uncertainty in . (included) l Relative Energy Scale  Energy scale in |  |<0.5 vs. 0.5 < |  | < 1. è Estimate +/- 0.5 % is achievable in Barrel. è Changes ratio between +/-.01 and +/-.03. l Resolution è No change to ratio when changing resolution è Systematics bounded by MC statistics: l Parton Distributions è We’ve used CTEQ6.1 uncertainties. è Systematics on ratio less than 0.02.

Selda Esen, Brown 9 Significance of Contact Interaction Signal Significance found from  2  5  Discovery è 95% CL Exclusion l Effect of dijet ratio systematics on the significance is small.

Selda Esen, Brown 10 Left-Handed Quark Contact Interaction  + for 100 pb -1 (TeV)  + for 1 fb -1 (TeV)  + for 10 fb -1 (TeV) 95% CL Exclusion Stat Only All Sys σ Discovery Stat Only All Sys 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 l CMS plans to use the dijet ratio to search for quark contact interactions è We’ve estimated the signal and QCD background from 0.3 to 6.5 TeV è From jet triggers for 100 pb -1, 1 fb -1 and 10 fb -1 è Using the full CMS detector simulation. We have presented CMS 95% CL exclusion and 5  discovery sensitivity to a quark contact interaction. è Including both statistical and systematic uncertainties.  CMS can discover a quark contact interaction   = 12 TeV with 10 fb -1. è Corresponds to a quark radius of order cm if quarks are composite.