1 Probing New Physics with Dijets at CMS Robert M. Harris Fermilab HEP Seminar Johns Hopkins University March 26, 2008

Robert Harris, Fermilab2 Outline l Motivation l Introduction è Jets at CMS è New Physics with Jets è Best limits on new physics from Tevatron l CMS Search Plans for Contact Interactions è Jet Rate: Inclusive Jet P T è Jet Angle: Dijet Ratio l CMS Search Plans for Dijet Resonances è Jet Rate: Dijet Mass è Jet Angle: Dijet Ratio l Conclusions

Robert Harris, Fermilab3 Credits l Study done at LHC Physics Center (LPC) è A center of CMS physics at Fermilab l CMS approved, publicly available è Initial study completed in April 2006 and published in CMS Physics TDR Vol II à J. Phys. G: Nucl. Part. Phys. 34: à CMS Notes (2006 / 069, 070 and 071) è Update study completed in December 2007 à physics/public/SBM pas-v3.pdf l Demonstration of physics at LPC è Working within the CMS collaboration l Thanks to my many CMS colleagues! è Involved postdocs and grad students: M. Cardaci, S. Esen, K. Gumus, M. Jha, K. Kousouris, D. Mason. P T D R U p d a t e

Robert Harris, Fermilab4 CMS ATLAS In 2008 science will start to explore a new energy scale 14 TeV proton-proton collisions will allow us to see deeper into nature than ever before Two large detectors will observe the collisions Large Hadron Collider at CERN Geneva Switzerland

Robert Harris, Fermilab5 l ATLAS è 22 x 44 meters, 6000 tons è 2000 physicists, 34 nations l CMS è 15 x 22 meters, tons è 3000 collaborators, 37 nations

Robert Harris, Fermilab6 What will the LHC detectors see? l But we hope to see more than the standard model ! l We expect to see the “standard model”: è The particles and forces already catalogued... è... & perhaps a Higgs particle that remains to be discovered.

Robert Harris, Fermilab7 Questions in the Standard Model l Can 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 Why three nearly identical generations of quarks and leptons? è Like the periodic table of the elements, does this suggest an underlying physics? è Is it possible that quarks and leptons are made of other particles? l These & other questions suggest new physics beyond the standard model. è We can discover this new physics with simple measurements of jets at the LHC. l The simple picture of the standard model raises many fundamental questions.

Robert Harris, Fermilab8 Introduction to Jets

Robert Harris, Fermilab9 Jets at LHC in Standard Model Proton q, q, g Proton q, q, g The LHC collides protons containing colored partons: quarks, antiquarks & gluons. q, q, g The dominant hard collision process is simple 2  2 scattering of partons off partons via the strong color force (QCD). Jet Each final state parton becomes a jet of observable particles. The process is called dijet production.

Robert Harris, Fermilab10 Experimental Observation of Jets Calorimeter Simulation   ETET 0 1 Jet 1 Jet 2 Dijet Mass = 900 GeV l Dijets are easy to find è Two jets with highest P T in the calorimeter. è A jet is the sum of calorimeter energy in a cone of radius CMS Barrel & Endcap Calorimeters proton  =-1 proton Jet 1 Jet 2  =1  Transverse  =0 

Robert Harris, Fermilab11 Jet Response & Corrections Jets in Barrel have uniform response vs  & are sensitive to new physics  Jet response changes smoothly and slowly up to | jet  | = 1.3 Measure relative response vs. jet  in data with dijet balance è Data will tell us what is the region of response we can trust. l Measured jet p T in the calorimeter is less than true jet p T (particles in cone) l Measured jets are corrected so p T is the same as true jet p T è Scales Jet (E,p x,p y,p z ) by à ~1.5 at p T = 70 GeV à ~1.1 at p T = 3 TeV à for jets in barrel region |  |<1.3 |  |<1 Jet Response vs  relative to Barrel CMS Preliminary Jet Response vs p T in Barrel Jets we use (GeV)

Robert Harris, Fermilab12 Introduction to New Physics with Jets

Robert Harris, Fermilab13 Quark Compositeness and Scattering l Three nearly identical generations suggests quark compositeness. è Compositeness is also historically motivated.  Molecules  Atoms  Nucleus  Protons & Neutrons  Quarks  Preons ? l Scattering probes compositeness. l In 1909 Rutherford discovered the nucleus inside the atom via scattering.  Scattered  particles off gold foil.  Too many scattered at wide angles to the incoming  beam è Hit the nucleus inside the atom! l A century later, we can discover quark compositeness in a similar way ! è Rate: more jets at high p T than QCD. è Angle: more dijets in center of the CMS barrel than at the edge. Measured with dijet ratio (more later) è Today we model quark compositeness with contact interactions.   Gold Detector Discovery of Nucleus More of this Than of this Quark Compositeness Signal q q q q q q q q

Robert Harris, Fermilab14 Quark Contact Interactions New physics at large scale  è Composite Quarks è New Interactions l Modeled by contact interaction  Intermediate state collapses to a point for dijet mass << . è For example, the standard contact interaction among left- handed quarks introduced by Eichten, Lane and Peskin  Excluded for   < 2.7 TeV (D0) l Observable Consequences è Effects at high p T & dijet mass. à Rate: Higher rate than QCD à Angle: Angular distributions can be very different from QCD. Quark Contact Interaction  M ~  Composite QuarksNew Interactions M ~  Dijet Mass <<  q q q q q q q q L = ± [2  /   ] (q   q) (q   q)

Robert Harris, Fermilab15 Dijet Resonances X q, q, g Mass Rate M New particles, X, produced in parton-parton annihilation will decay to 2 partons (dijets). They are observed as dijet resonances: mass bumps. Tevatron has searched but not found any dijet resonances so far: D0 Run 1 time space CDF Run 1 ( Data – Fit ) / Fit

Robert Harris, Fermilab16 Why Search for Dijet Resonances? l Experimental Motivation è LHC is a parton-parton resonance factory in a previously unexplored mass region à With the higher energy we have a good chance of finding new physics. è Nature may surprise us with previously unanticipated new particles. à We will search for generic dijet resonances, not just specific models. è One search can encompass ALL narrow dijet resonances. à Resonances narrower than jet resolution produce similar mass bumps in our data. è We can discover dijet resonances if they have a large enough cross section. l Theoretical Motivation è Dijet Resonances found in many models that address fundamental questions.  Why Generations ?  Compositeness  Excited Quarks  Why So Many Forces ?  Grand Unified Theory  W ’ & Z ’  Can we include Gravity ?  Superstrings & GUT  E6 Diquarks  Why is Gravity Weak ?  Extra Dimensions  RS Gravitons  Why Symmetry Broken ?  Technicolor  Color Octet Technirho  More Symmetries ?  Extra Color  Colorons & Axigluons

Robert Harris, Fermilab17 Dijet Resonance Details 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 mainly t - channel QCD Background X q, q, g Dijet Resonance s - channel Signal and background will have very different angular distributions

Robert Harris, Fermilab18 CDF Dijet Resonance Search in Run II l Preliminary Results Just Released è Best limits on dijet resonances è Thanks to Ken Hatakeyama ! ModelExcluded (GeV)ModelExcluded (GeV) A or C D  T W ’ q* Z ’

Robert Harris, Fermilab19 CMS Search Plans for Contact Interactions

Robert Harris, Fermilab20 Rate: Inclusive Jet p T l Inclusive jet p T is a QCD measurement that is sensitive to new physics.  Counts all jets inside a p T bin and  interval, and divides by bin width and luminosity. l Corrected calorimeter jets (CaloJets) agree with particle level jets (GenJets). è CaloJets before corrections shifted to lower p T than GenJets è Ratio between corrected CaloJets and GenJets is “resolution smearing”: small at high p T. l Simple correction for resolution smearing in real data is to divide rate by this ratio. Resolution Smearing Inclusive Jet Cross Section

Robert Harris, Fermilab21 Rate: Jet P T and Contact Interactions l Contact interactions create large rate at high P T and immediate discovery possible è Error dominated by jet energy scale (~10%) in early running (10 pb -1 )   E~ 10% not as big an effect as   = 3 TeV for P T >1 TeV. è PDF “errors” and statistical errors (10 pb -1 ) smaller than E scale error With 10 pb -1 we can see new physics beyond Tevatron exclusion of   < 2.7 TeV. Rate of QCD and Contact Interactions Sensitivity with 10 pb -1 Sys Err. PDF Err.

Robert Harris, Fermilab22 Dijet Ratio: Simple Angular Measure l Dijet angular distributions are sensitive to new physics. è Contact Interactions & Resonances l Dijet Ratio = N(|  |<0.7) / N(0.7<|  |<1.3)  Number of events in which each leading jet has |  |<0.7, divided by the number in which each leading jet has 0.7<|  |<1.3  Numerator is sensitive to new physics at low cos  *.  Denominator is dominated by QCD at high cos  *. l Simplest measurement of angular distribution  Uses detector variable  è Uses same mass bins as resonance search in rate vs. mass. Jet 1 Jet 2 Numerator Sensitive to New Physics |cos    ~ 0 Denominator Dominated By QCD |cos  *| ~ 0.7, usually Jet 1 Jet 2 (rare) or  = z z

Robert Harris, Fermilab23 Angle: Dijet Ratio from QCD l We have optimized the dijet ratio for a contact interaction search in barrel  Old dijet ratio used by D0 and PTDR was N(|  |<0.5) / N(0.5<|  |<1.0)  New dijet ratio is N(|  |<0.7) / N(0.7<|  |<1.3) l Dijet ratio from QCD agrees for GenJets and Corrected CaloJets è Flat at 0.6 for old ratio, and flat at 0.5 for new ratio up to around 6 TeV. Old Dijet RatioNew Dijet Ratio

Robert Harris, Fermilab24 l Optimization dramatically increases sensitivity to contact interactions. è Raising the signal and decreasing the QCD error bars. Old Dijet Ratio (D0 and PTDR  Cuts)  + (TeV) QCD New Dijet Ratio (Optimized in Barrel)  + (TeV) QCD Angle: Dijet Ratio from Contact Interactions

Robert Harris, Fermilab25 Dijet Ratio and Systematic Uncertainties l Systematics (red) are small è They cancel in the ratio. l Relative Energy Scale  Energy scale in center vs edge of barrel in . è Estimate +/- 0.5 % is achievable in barrel. è Determined with dijet balance l Parton Distributions è We’ve used CTEQ6.1 uncertainties P T D R

Robert Harris, Fermilab26 CMS Search Plans for Dijet Resonances

Robert Harris, Fermilab27 Dijet Mass Resolution l First high statistics study of CMS dijet resonance mass resolution. Gaussian core of resolution for |  |<1 and |  |<1.3 is similar. l Resolution for corrected CaloJets è 9% at 0.7 TeV è 4.5% at 5 TeV è Better than in PTDR 2 study. 2 TeV Z’ |η| < 1.3 Corrected CaloJets GenJets Natural Width Resolution

Robert Harris, Fermilab28 Rate vs. Dijet Mass and Resonances l Measure rate vs. corrected dijet mass and look for resonances. è Use a smooth parameterized fit or QCD prediction to model background l Strongly produced resonances can be seen è Convincing signal for a 2 TeV excited quark in 100 pb -1 à Tevatron excluded up to 0.87 TeV. QCD Backgound Resonances with 100 pb -1

Robert Harris, Fermilab29 Rate: Systematic Uncertainties l Jet Energy è CMS estimates +/- 5 % is achievable by 1 fb -1 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. P T D R

Robert Harris, Fermilab30 Rate: 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. P T D R

Robert Harris, Fermilab31 Rate: 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 P T D R

Robert Harris, Fermilab32 Rate: Exclusion Sensitivity to Models E6 Diquark Excited Quark Axigluon or Coloron Color Octet Technirho W ’ R S Graviton Z ’ Tevatron 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 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. P T D R

Robert Harris, Fermilab33 Angle: Dijet Resonances with Dijet Ratio l All resonances have a more isotropic decay angular distribution than QCD  Spin ½ (q*), spin 1 (Z’), and spin 2 (RS Graviton) all flatter than QCD in dN / dcos  *. l Dijet ratio is larger for resonances than for QCD.  Because numerator mainly low cos  *, denominator mainly high cos  * QCD Dijet Ratio vs Mass Dijet Angular Distributions

Robert Harris, Fermilab34 Angle: Dijet Resonances with Dijet Ratio l Dijet ratio from signal + QCD compared to statistical errors for QCD alone  Resonances normalized with q* cross section for |  |<1.3 to see effect of spin. l Convincing signal for 2 TeV strong resonance in 100 pb -1 regardless of spin. l Promising technique for discovery, confirmation, and eventually spin measurement. Dijet Ratio for q* Dijet Ratio for Spin ½, 1, 2

Robert Harris, Fermilab35 Conclusions l We’ve described CMS search plans for new physics with dijets. l Inclusive jet p T could give a convincing contact interaction signal at startup!  Can discover  + = 3 TeV in 10 pb -1 even if jet energy errors are 10%. l Dijet ratio will probe contact interactions in dijet angular distributions  Can discover  + = 4, 7, 10 TeV in 10, 100, 1000 pb -1 with small systematics. l Dijet mass can be used to discover a 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. l Dijet ratio can discover or confirm a dijet resonance with small systematics. è Gives a convincing signal for a 2 TeV q* with 100 pb -1. è Eventually it will be used to measure the resonance spin. l CMS is preparing to discover new physics at the TeV scale using dijets.

Robert Harris, Fermilab36 Backup Slides

Robert Harris, Fermilab37 The CMS Detector Hadronic Electro- magnetic Calorimeters Protons

Robert Harris, Fermilab38 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, Fermilab39 Trigger and Luminosity 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 l Two levels of trigger are used to reduce rate in steps è Level 1 (L1) reduces rate by a factor of 400. è High Level Trigger (HLT) reduces rate by a factor of 700. l Trigger tables are intended for specific luminosities è We’ve specificied a jet trigger table for three luminosities è L = cm -2 s -1. Integrated luminosity ~ 100 pb -1. à LHC schedule projects this after ~1 months running. è L = cm -2 s -1. Integrated luminosity ~ 1 fb -1. à LHC schedule projects these after ~ 1 year of running. è L = cm -2 s -1. Integrated luminosity ~ 10 fb -1. à One months running at design luminosity. 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, Fermilab40 Path L1HLTANA E T (GeV) Pre- scale Rate (Hz) E T (GeV) Rate (Hz) Dijet Mass (GeV) Low Med High Super L = pb -1 Ultra L = fb -1 Add New Threshold (Ultra). Increase Prescales by 10. Mass values are efficient for each trigger, measured with prior trigger L = fb -1 Add New Threshold (Super). Increase Prescales by 10. Jet Trigger Table and Dijet Mass Analysis l CMS jet trigger saves all high E T jets & pre-scales the lower E T jets. è Prescale means to save 1 event out of every N events. As luminosity increases new trigger paths are added Each with new unprescaled threshold.

Robert Harris, Fermilab41 Trigger Rates & Dijet Cross Section (QCD + CMS Simulation) l Include data from each trigger where it is efficient in dijet mass. è Stop analyzing data from trigger where next trigger is efficient l Prescaled triggers give low mass spectrum at a convenient rate. è Measure mass down to 300 GeV è Overlap with Tevatron measurements. l Trigger without any prescaling saves all the high mass dijets l Expect the highest mass dijet event to be è ~ 7.5 TeV for 10 fb -1 è ~ 5 TeV for 100 pb -1 è LHC will open a new mass reach early! l Put the triggers together to form a cross section. |jet  |<1 Prescaled Trigger Samples P T D R

Robert Harris, Fermilab42 |  |<1.3 |  |<1 Jet response vs  relative to |  |<1.3 CMS Preliminary Jet  Region l Barrel jets have uniform response & sensitive to new physics  Jet response changes smoothly and slowly up to | jet  | = 1.3  CaloTowers with |  |<1.3 are in barrel with uniform construction.  CaloTowers with 1.3<|  |<1.5 are in barrel / endcap transition region  Some of our analyses use | jet  |<1.3, others still use | jet  |<1  All are migrating to | jet  |<1.3 which is optimal for dijet resonances Measure relative response vs. jet  in data with dijet balance è Data will tell us what is the region of response we can trust. Barrel Jet (|  |<1.3) Probe Jet (any  ) Dijet Balance  = 1.3 HB HE Hcal towers and  cuts Transition Region  = 1

Robert Harris, Fermilab43 Dijet Event Cleanup l Dijet events do not usually contain large missing E T  A cut at MET /  E T 99% efficient for P T > 100 GeV è Won’t change the QCD background to new physics. l Most unphysical background contain large missing E T è Catastrophic detector noise, cosmic ray air showers, beam-halo backgrounds  A simple cut at MET /  E T < 0.3 should remove most of these at high jet P T. è This cut is our first defense, simpler and safer than cutting on jet characteristics. 99% Efficiency Cut & Chosen Cut MET /  E T for QCD Dijets and Cut

Robert Harris, Fermilab44 Dijet Resonances: Optimization of  cut QCD cross section rises dramatically with |  | cut due to t-channel pole.  Z’ signal only gradually increases with |  | cut  optimal value at low |  |. Optimal cut is at |  | < 1.3 for a 2 TeV dijet resonance. è Optimization uses Pythia Z’ angular distribution for the resonance.  cut and cross section  cut and sensitivity

Robert Harris, Fermilab45 CDF Run II Resonance Search l Dijet resonance shapes are similar l Separate limits for W’ and Z’ models l Systematic uncertainties reduced from run 1. è Much lower jet energy scale error