1 New Physics with Dijets at CMS Selda Esen and Robert M. Harris Fermilab Kazim Gumus and Nural Akchurin Texas Tech Physics Colloquium at Texas Tech April 20, 2006

Robert Harris, Fermilab2 Outline l Introduction to the Physics l CMS Jet Trigger and Dijet Mass Distribution l CMS Sensitivity to Dijet Resonances l CMS Sensitivity to Quark Contact Interactions l Conclusions

Robert Harris, Fermilab3 Standard Model of Particle Physics l In the standard model nature contains è 6 quarks à u and d quark make nucleons in atom è 6 leptons à electrons complete the atoms è 4 force carrying particles   electromagnetism à W & Z : weak interaction à g : color (nuclear) interaction. è Higgs particle to give W & Z mass à Higgs not discovered yet. l Tremendously successful. è Withstood experimental tests for the last 30 years. à “Tyranny of the Standard Model”. l Why should there be anything else ? è Other than the Higgs.

Robert Harris, Fermilab4 Beyond the Standard Model l The Standard Model raises questions. l Why three nearly identical generations of quarks and leptons? è Like the periodic table of the elements, does this suggest an underlying physics? l What causes the flavor differences within a generation? è Or mass difference between generations? l How do we unify the forces ?  , Z and W are unified already. è Can we include gluons ? è Can we include gravity ? è Why is gravity so weak ? l These questions suggest there will be new physics beyond the standard model. è We will search for new physics with dijets. ? ? ?

Robert Harris, Fermilab5 Dijets in the Standard Model l What’s a dijet? l Parton Level  Dijets result from simple 2  2 scattering of “partons” è quarks, anti-quarks, and gluons. l Particle Level è Partons come from colliding protons (more on this later) è The final state partons become jets of observable particles via the following chain of events à The partons radiate gluons. à Gluons split into quarks and antiquarks à All colored objects “hadronize” into color neutral particles.  Jet made of , k, p, n, etc. l Dijets are events which primarily consist of two jets in the final state. ** Parton Level Picture q, q, g Particle Level Picture Jet Proton

Robert Harris, Fermilab6 Models of New Physics with Dijets l Two types of observations will be considered. è Dijet resonances are new particles beyond the standard model. è Quark contact Interactions are new interactions beyond the standard model. l Dijet resonances are found in models that try to address some of the big questions of particle physics beyond the SM, the Higgs, or Supersymmetry  Why Flavor ?  Technicolor or Topcolor  Octet Technirho or Coloron  Why Generations ?  Compositeness  Excited Quarks  Why So Many Forces ?  Grand Unified Theory  W’ & Z’  Can we include Gravity ?  Superstrings  E6 Diquarks  Why is Gravity Weak ?  Extra Dimensions  RS Gravitions l Quark contact interactions result from most new physics involving quarks. è Quark compositeness is the most commonly sought example.

Robert Harris, Fermilab7 Dijet Resonances l New particles that decay to dijets è Produced in “s-channel” è Parton - Parton Resonances à Observed as dijet resonances.  Many models have small width  à Similar dijet resonances (more later) X q, q, g Time Space Mass Rate Breit- Wigner  M 11 11 22 11 11 1+1+ ½+½ J P qq0.01SingletZ ‘Heavy Z q1q2q1q2 0.01SingletW ‘Heavy W qq,gg0.01SingletGR S Graviton qq,gg0.01Octet  T8 Octet Technirho qq0.05OctetCColoron qq0.05OctetAAxigluon qg0.02Tripletq*Excited Quark ud0.004TripletDE 6 Diquark Chan  / (2M) ColorXModel Name

Robert Harris, Fermilab8 Quark Contact Interactions New physics at large scale  è Composite Quarks è New Interactions l Modelled by contact interaction  Intermediate state collapses to a point for dijet mass << . l Observable Consequences è Has effects at high dijet mass. è Higher rate than standard model. è Angular distributions can be different from standard model. è We will use a simple measure of the angular distribution at high mass (more later). Quark Contact Interaction  M ~  Composite QuarksNew Interactions M ~  Dijet Mass <<  q q q q q q q q

Robert Harris, Fermilab9 The Large Hadron Collider l The LHC will collide protons with a total energy of  s =14 TeV. è The collisions take place inside two general purpose detectors: CMS & ATLAS. l Protons are made of partons. è Quarks, anti-quarks and gluons. è Three “valence” quarks held together by gluons. è The anti-quarks come from gluons which can split into quark- antiquark pairs while colliding. l The collisions of interest are between two partons è One parton from each proton. l Also extra pp collisions (pile-up). CMS ATLAS Proton q, q, g

Robert Harris, Fermilab10 Dijet Cross Sections at the LHC l Product of 3 probabilities  f a (x a ): parton of type a with fractional momentum x a.  f b (x b ): parton of type b with fractional momentum x b   (a b  12): subprocess cross section to make dijets. l Falls rapidly with total collision energy, equal to final state mass, m. Jet 1 Jet 2 ^ TeV

Robert Harris, Fermilab11 Standard Model Background: QCD l Dijet Mass from Final State l QCD Prediction for Dijet Mass Distribution è Expressed as a cross section è Rate = Cross Section times Integrated Luminosity ( ∫Ldt ) In  m= 0.1 TeV for ∫Ldt = 1 fb -1 è ~1 dijet with m = 6 TeV è ~10 5 dijets with m = 1 TeV è ~10 8 dijets with m = 0.2 TeV l Will need a trigger to prevent a flood of low mass dijets! | jet  | < 1

Robert Harris, Fermilab12 The CMS Detector Hadronic Electro- magnetic Calorimeters Protons

Robert Harris, Fermilab13 CMS Barrel & Endcap Calorimeters (r-z view, top half)          HCAL BARREL ECAL BARREL SOLENOID HCAL END CAP ECAL END CAP HCAL END CAP ECAL END CAP Z                     HCAL OUTER 3 m HCAL > 10  I ECAL > 26 0

Robert Harris, Fermilab14 Calorimeter & Jets l Jets are reconstructed using a cone algorithm è Energy inside a circle of radius R centered on jet axis is summed:   This analysis requires jet |  | < 1. è Well contained in barrel. l Jet energy is corrected for è Calorimeter non-linear response è Pile-up of extra soft proton-proton collisions on top of our event à Event is a hard parton-parton collision creating energetic jets. è Correction varies from 33% at 75 GeV to 7% at 2.8 TeV. à Mainly calorimeter response.   Jet 2 Jet 1 

Robert Harris, Fermilab15 CMS Jet Trigger & Dijet Mass Distribution

Robert Harris, Fermilab16 Trigger l Collision rate at LHC is expected to be 40 MHz è 40 million events every second ! è CMS cannot read out and save that many. l Trigger chooses which events to save è Only the most interesting events can be saved l Two levels of trigger are used è Level 1 (L1) is fast custom built hardware à Reduces rate to 100 KHz: chooses only 1 event out of 400 è High Level Trigger (HLT) is a PC farm à Reduces rate to 150 Hz: chooses only 1 event out of 700. l Trigger selects events with high energy objects  Jet trigger at L1 uses energy in a square  x  = 1 x 1 è Jet trigger at HLT uses same jet algorithm as analysis. 4 x 10 7 Hz 1 x 10 5 Hz 1.5 x 10 2 Hz Event Selection CMS Detector L1 Trigger HLT Trigger Saved for Analysis

Robert Harris, Fermilab17 Design of Jet Trigger Table for CMS l The jet trigger table is a list of jet triggers CMS could use. è We consider triggers that look at all jets in the Barrel and Endcap  Requires a jet to have E T = E sin  > threshold to reduce the rate. è Jet triggers can also be “prescaled” to further reduce the rate by a factor of N. à The prescale just counts events and selects 1 event out of N, rejecting all others. l Guided by Tevatron experience, we’ve designed a jet trigger table for CMS. è Chose reasonable thresholds, prescales, and rates at L1 & HLT. è Evolution of the trigger table with time (luminosity) l Driven by need to reconstruct dijet mass distribution è To low mass to constrain QCD and overlap with Tevatron. è For realistic search for dijet resonances and contact interactions. l Running periods and sample sizes considered è Luminosity = cm -2 s -1. Month integrated luminosity ~ 100 pb ? è Luminosity = cm -2 s -1. Month integrated luminosity ~ 1 fb ? è Luminosity = cm -2 s -1. Month integrated luminosity ~ 10 fb ?

Robert Harris, Fermilab18 Path L1HLTANA E T (GeV) Pre- scale Rate (KHz) E T (GeV) Rate (Hz) Dijet Mass (GeV) Low NONE Med High Super > 1800 L = pb -1 L = fb -1 L = fb -1 Ultra Add New Threshold (Ultra). Increase Prescales by 10. Analysis done for 3 values of integrated lum: 100 pb -1 1 fb fb -1 Each has new unprescaled threshold Mass ranges listed are fully efficient for each trigger Add New Threshold (Super). Increase Prescales by 10. Jet Trigger Table and Dijet Mass Analysis l HLT budget is what constrains the jet trigger rate to roughly 10 Hz. l Table shows L1 & HLT jet E T threshold and analysis dijet mass range.

Robert Harris, Fermilab19 Rates for Measuring Cross Section (QCD + CMS Simulation) l Analyze each trigger where it is efficient. è Stop analyzing data from trigger where next trigger is efficient l Prescaled triggers give low mass spectrum at a conveniently lower rate. l Expect the highest mass dijet to be è ~ 5 TeV for 100 pb -1 è ~ 6 TeV for 1 fb -1 è ~ 7 TeV for 10 fb -1

Robert Harris, Fermilab20 Dijet Mass Cross Section (QCD + CMS Simulation) l Put triggers together for dijet mass spectrum. l Prescaled triggers give us the ability to measure mass down to 300 GeV. l Plot dijet mass in bins equal to our mass resolution.

Robert Harris, Fermilab21 Statistical Uncertainties l Simplest measure of our sensitivity to new physics as a fraction of QCD. l Prescaled Triggers è 1-3% statistical error to nail QCD l Unprescaled Triggers è ~1% statistical error at threshold è 1 st one begins at mass=670 GeV à Overlaps with Tevatron measurements.

Robert Harris, Fermilab22 Systematic Uncertainties l Jet Energy è CMS estimates +/- 5 % is achievable. è Changes dijet mass cross section between 30% and 70% l Parton Distributions è CTEQ 6.1 uncertainty l Resolution è Bounded by difference between particle level jets and calorimeter level jets. l Systematic uncertainties on the cross section vs. dijet mass are large. è But they are correlated vs. mass. The distribution changes smoothly.

Robert Harris, Fermilab23 CMS Sensitivity to Dijet Resonances

Robert Harris, Fermilab24 Motivation l Theoretical Motivation è The many models of dijet resonances are ample theoretical motivation. è But experimentalists should not be biased by theoretical motivations... l Experimental Motivation è The LHC collides partons (quarks, antiquarks and gluons). à LHC is a parton-parton resonance factory in a previously unexplored region à The motivation to search for dijet resonances is intuitively obvious. ð We must do it. è We should search for generic dijet resonances, not specific models. à Nature may surprise us with unexpected new particles. It wouldn’t be the first time … è One search can encompass ALL narrow dijet resonances. à Resonances more narrow than the jet resolution all produce similar line shapes.

Robert Harris, Fermilab25 Resonance Cross Sections & Constraints l Resonances produced via color force, or from valence quarks in each proton, have the highest cross sections. l Published Limits in Dijet Channel in TeV : è q* > (D0) è A or C > 0.98 (CDF) è E 6 Diq > 0.42 (CDF)   T8 > 0.48 (CDF) è W ’ > 0.8 (D0) è Z ’ > 0.6 (D0) CDF: hep-ex/ D0: hep-ex/

Robert Harris, Fermilab26 Signal and Background l QCD cross section falls smoothly as a function of dijet mass. l Resonances produce mass bumps we can see if xsec is big enough.

Robert Harris, Fermilab27 Signal / QCD l Many resonances give obvious signals above the QCD error bars è Resonances produced via color force à q* (shown) à Axigluon à Coloron  Color Octet  T è Resonances produced from valence quarks of each proton à E6 Diquark (shown) l Others may be at the edge of our sensitivity.

Robert Harris, Fermilab28 Statistical Sensitivity to Dijet Resonances l Sensitivity estimates è Statistical likelihoods done for both discovery and exclusion 5  Discovery  We see a resonance with 5  significance à 1 chance in 2 million of effect being due to QCD. l 95% CL Exclusion è We don’t see anything but QCD è Exclude resonances at 95% confidence level. Plots show resonances at 5  and 95% CL è Compared to statistical error bars from QCD. 5 TeV 2 TeV 0.7 TeV 2 TeV0.7 TeV

Robert Harris, Fermilab29 Systematic Uncertainties l We include all these systematic uncertainties in our likelihood distributions l Uncertainty on QCD Background è Dominated by jet energy uncertainty (±5%). à Background will be measured. l Trigger prescale edge effect è Jet energy uncertainty has large effect at mass values just above where trigger prescale changes. l Resolution Effect on Resonance Shape è Bounded by difference between particle level jets and calorimeter level jets. l Radiation effect on Resonance Shape è Long tail to low mass which comes mainly from final state radiation. l Luminosity

Robert Harris, Fermilab30 Sensitivity to Resonance Cross Section l Cross Section for Discovery or Exclusion è Shown here for 1 fb -1 è Also for 100 pb -1, 10 fb -1 l Compared to cross section for 8 models l CMS expects to have sufficient sensitivity to  Discover with 5  significance any model above solid black curve è Exclude with 95% CL any model above the dashed black curve. l Can discover resonances produced via color force, or from valence quarks.

Robert Harris, Fermilab31 Discovery Sensitivity for Models l Resonances produced by the valence quarks of each proton è Large cross section from higher probability of quarks in the initial state at high x.  E6 diquarks (ud  D  ud) can be discovered up to 3.7 TeV for 1 fb -1 l Resonances produced by color force è Large cross sections from strong force è With just 1 fb -1 CMS can discover à Excited Quarks up to 3.4 TeV à Axigluons or Colorons up to 3.3 TeV à Color Octet Technirhos up to 2.2 TeV. l Discoveries possible with only 100 pb -1 è Large discovery potential with 10 fb -1 Mass (TeV) E6 Diquark Excited Quark Axigluon or Coloron Color Octet Technirho CMS 100 pb -1 CMS 1 fb -1 CMS 10 fb -1 5  Sensitivity to Dijet Resonances

Robert Harris, Fermilab32 Sensitivity to Dijet Resonance Models E6 Diquark Excited Quark Axigluon or Coloron Color Octet Technirho W ’ R S Graviton Z ’ Published Exclusion (Dijets) CMS 100 pb -1 CMS 1 fb -1 CMS 10 fb -1 Mass (TeV) 95% CL Sensitivity to Dijet Resonances l Resonances produced via color interaction or valence quarks. è Wide exclusion possibility connecting up with many exclusions at Tevatron à CMS can extend to lower mass to fill gaps. l Resonances produced weakly are harder. è But CMS has some sensitivity to each model with sufficient luminosity. è Z’ is particularly hard. à weak coupling and requires an anti-quark in the proton at high x.

Robert Harris, Fermilab33 CMS Sensitivity to Quark Contact Interactions

Robert Harris, Fermilab34 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 are multiplied by factor of ~6-16 to get cross section uncertainties è Parton distribution uncertainties are significant at high mass = high x and Q 2.

Robert Harris, Fermilab35 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. l Angular distribution has much smaller systematic uncertainties than cross section vs. dijet mass. l But we want 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

Robert Harris, Fermilab36 Sensitive Variable for Contact Interactions 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). 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 è We will show systematics on the dijet ratio are small. Jet 1 Jet 2 Numerator |cos    ~ 0 Denominator |cos  *| ~ 0.6, usually Jet 1 Jet 2 (rare) or  = z z

Robert Harris, Fermilab37 Dijet Ratio l Lowest order (LO) calculation. è Both signal and background. l Same code as used by CDF in 1996 paper è hep-ex/ è but with modern parton distributions (CTEQ 6L). l Signal emerges clearly at high mass è QCD is pretty flat

Robert Harris, Fermilab38 Dijet Ratio and Statistical Uncertainty (Smoothed CMS Simulation) l Background Simulation is flat at 0.6 è Shown here with expected statistical errors for 100 pb -1, 1 fb -1, and 10 fb -1. l Signals near edge of error bars   ~5 TeV for 100 pb -1   ~10 TeV for 1 fb -1   ~15 TeV for 10 fb -1 Calculate  2 for significance estimates.

Robert Harris, Fermilab39 Dijet Ratio and Systematic Uncertainty l Systematics are small è The largely cancel 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 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 +/-.013 and +/ l Resolution è No change to ratio when changing resolution è Systematic bounded by MC statistics: l Parton Distributions è We’ve used CTEQ6.1 uncertainties. è Systematic on ratio less than 0.02.

Robert Harris, Fermilab40 Significance of Contact Interaction Signal Significance found from  2  5  Discoveries è 95% CL Exclusions l Effect of dijet ratio systematics on the significance is small.

Robert Harris, Fermilab41 Left-Handed Quark Contact Interaction  + for 100 pb -1 (TeV)  + for 1 fb -1 (TeV)  + for 10 fb -1 (TeV) 95% CL Exclusion σ Discovery Sensitivity to Contact Interactions Published Limit from D0:  + > 2.7 TeV at 95% CL (hep-ex/980714).  can be translated roughly into the radius of a composite quark.  h =  x  p ~ (2r) (  / c)  r = cm-TeV /   For  ~ 10 TeV, r ~ cm è Proton radius divided by 100,000 ! Composite Quark Preon r

Robert Harris, Fermilab42 Conclusions l We’ve described a jet trigger for CMS designed from Tevatron experience. è It will be used to search for new physics with dijets. l CMS is sensitive to dijet resonances and quark contact interactions è We’ve presented sensivity estimates for 100 pb -1, 1 fb -1 and 10 fb -1  Capability for discovery (5  ) or exclusion (95% CL) including systematics. l CMS can discover a strongly produced dijet resonance up to many TeV. è Axigluon, Coloron, Excited Quark, Color Octet Technirho or E 6 Diquark è Produced via the color force, or from the valence quarks of each proton. 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. l We are prepared to discover new physics at the TeV scale using dijets.