1 Probing New Physics with Jets at the LHC Robert M. Harris Fermilab Los Alamos Seminar November 28, 2007

Robert Harris, Fermilab2 Outline l Motivation l Introduction è Jets è Description of Jet Algorithms on Los Alamos Request è New Physics with Jets l CMS Jet Trigger and Jet Backgrounds from QCD l CMS Sensitivity to Signals of New Physics è New Particles: Dijet Resonances è New Forces: Contact Interactions 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 CMS Physics TDR Volume II à J. Phys. G: Nucl. Part. Phys. 34: è CMS Notes (2006 / 069, 070 and 071) l Ph.D thesis of two students è Selda Esen now a postdoc at Brown è Kazim Gumus from Texas Tech è Both mentored at the LPC l Demonstration of physics at LPC è Working within the CMS collaboration l Thanks to many CMS colleagues!

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 collaborators, 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 towers in a cone of radius CMS Barrel & Endcap Calorimeters proton  =-1 proton Jet 1 Jet 2  =1  Transverse  =0 

Robert Harris, Fermilab11 Iterative Cone Algorithm l Simplest cone algorithm. Used for this analysis, and for CMS trigger. è Consider towers in calorimeter with E T > seed threshold. Start with highest E T à Include all towers with E T > tower threshold within a cone centered on the seed è Calculate the jet Lorentz vector from the vector sum of tower energies. à Include all towers over threshold in new cone centered on jet momentum vector. à Iterate until towers in jet don’t change. Found a “stable cone” = jet in this algorithm. è Remove all the towers used from the list. Repeat until you run out of seeds. l Cookie cutter cone algorithm è First seed takes all towers in cone E T > Seed Threshold (1.0 GeV at CMS) E T > Tower Threshold (0.5 GeV at CMS) Example Calorimeter Towers Jet 1 Jet 2

Robert Harris, Fermilab12 JETCLU Cone Algorithm l First algorithm with splitting and merging, in use at CDF. è Similar to iterative cone, except towers are not removed from list. è Towers are part of all overlapping stable cones found from seeds until the jets are split or merged. à Merged if overlap p T is greater than 75% of lower p T jet. à Otherwise split, with towers going to nearest jet. Jet 1 Jet 2 Here overlap towers are “split” between jet 1 and jet 2. This example gave a larger Jet 2 than the Iterative cone. Overlap

Robert Harris, Fermilab13 Midpoint Cone Algorithm l Infrared safer algorithm from the Tevatron and briefly used by CMS. è Also starts with the stable cones found from seeds. è Includes mid-points between stable cones as seeds for more stable cones. è Concludes with usual splitting and merging based on overlap p T. l In this example the midpoint seed has given a single large jet. è Note that jet would have been found regardless of energy between the two seeds. è More infrared safe. à Stable for infrared radiation between two partons. à Up to NLO for dijets. “Infrared” Radiation Jet 1 Midpoint Seeded Cone

Robert Harris, Fermilab14 Seedless Infrared Safe Cone Algorithm l Infrared safe algorithm which will be the CMS standard cone algorithm for analysis. l Finds jets from ALL possible stable cones and does NOT use seeds. è Also an iterative cone algorithm with splitting and merging è Similar to midpoint algorithm but infrared safe. à Uses all possible stable cones and includes jets without a high E T seed. l Here it finds same large jet as midpoint plus an extra low energy jet. Jet 1 Jet 2 For high p T dijets and R=0.5 all cone algorithms discussed give very similar results in CMS simulations. There are also K T algorithms, which I am not familiar with, and will not discuss.

Robert Harris, Fermilab15 l Many signals are also large  Either large PDFs or  or both. Dijet Rates and Cross Sections l Rate = Cross Section x Luminosity è Luminosity (L) is rate of protons / area supplied by the LHC. è Design L=10 34 cm -2 s -1 ~ 10 fb -1 /month l Cross section from two factors è Parton distributions functions (PDFs) à Probability of finding partons in proton with fractional momentum x à Valence quarks u and d have large PDFs at high x (high dijet mass).  Parton scattering cross section  Jet 1 Jet 2 ^ | jet  | < 1 l QCD dijet cross section is large.   from color force is large ^ ^ Proton PDF(x a ) PDF(x b ) ~10 events/ month at design L ~10 7 events/ month at design L

Robert Harris, Fermilab16 Introduction to New Physics with Jets

Robert Harris, Fermilab17 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 Searching for more dijets in center of the CMS Barrel than at the edge. è More about this when we discuss sensitivity to 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, Fermilab18 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 CDF Run 1 ( Data – Fit ) / Fit time space

Robert Harris, Fermilab19 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, Fermilab20 Example Resonance Model: Excited Quarks l If quarks are composite particles then excited states, q*, are expected è Excited quarks are produced when a ground state quark absorbs a gluon.  q* decay to the ground state q by re-emitting a gluon (qg  q*  qg). l Cross section is large because the interaction is from the color force (QCD). è Similar number of events produced as the QCD background ! g q q* q g Initial State Resonance Final State l Excited states are common in nature. Hydrogen Atom ground state Excited State Light from Excited States of Hydrogen

Robert Harris, Fermilab21 CMS Jet Trigger & Dijets from QCD

Robert Harris, Fermilab22 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 amounts by end of 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, Fermilab23 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, Fermilab24 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

Robert Harris, Fermilab25 Uncertainties on Dijet Cross Section l Statistical Uncertainties è Simplest measure of our sensitivity to new physics as a fraction of QCD background è < 3% in prescaled region. è As luminosity increases our statistical error shrinks at high mass. l Systematic Uncertainties are large è Dominated by uncertainty in jet energy measurement. è Correlated with dijet mass. à Smooth changes, not bumps.

Robert Harris, Fermilab26 CMS Sensitivity to Dijet Resonances

Robert Harris, Fermilab27 Resonances and Background (CMS Simulation) 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, Fermilab28 Resonances and QCD Statistical Errors 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)  u d  D  u d l Others may be at the edge of our sensitivity.

Robert Harris, Fermilab29 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, Fermilab30 Sensitivity to Resonance Cross Section l Cross Section for Discovery or Exclusion è Including systematics. è Shown here for 1 fb -1 è Also done 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.

Robert Harris, Fermilab31 Sensitivity to Dijet Resonance Models l Discover models up to 5 TeV in 10 fb -1 è 1 month at design luminosity = 10 fb -1 è Discovery up to 2.5 TeV with 100 pb -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 l Wide exclusion sensitivity è Extending Tevatron exclusions (<1 TeV) 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 Mass (TeV) 95% CL Sensitivity to Dijet Resonances CMS 10 fb -1

Robert Harris, Fermilab32 CMS Sensitivity to Quark Contact Interactions

Robert Harris, Fermilab33 Contact Interactions & Compositeness New physics at a large scale  è For example composite quarks.  Intermediate state looks like a point for dijet mass << . è Giving a contact interaction. M ~  Composite Quarks New Interactions M ~  q q q q Quark Contact Interaction  q q q q l Increases rate at high dijet mass. è But the signal in rate alone is hard to find due to uncertainties in jet energy & parton distributions. Dijet Mass <<  l We will use a ratio of two rates. è Look at angles like Rutherford did !

Robert Harris, Fermilab34 Dijet Ratio and New Physics 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 Simplest measurement of angle dist. è Most sensitive part for new physics è It was first introduced by D0 Jet 1 Jet 2 Numerator Sensitive to New Physics Signal Denominator Dominated by QCD Background Jet 1 Jet 2 (rare) or  = z z

Robert Harris, Fermilab35 Dijet Ratio and Uncertainties (Smoothed CMS Simulation) l QCD Background Simulation is flat l Signal rises with mass è Clear statistical sensitivity to contact signal We find 5  discovery and 95% CL exclusion sensitivities for . è Including both the statistical and systematic uncertainties. l Small systematic uncertainties. è Cancel in the ratio

Robert Harris, Fermilab36 Left-Handed Quark Contact Interaction (Eichten, Lane & Peskin)  + for 100 pb -1 (TeV)  + for 1 fb -1 (TeV)  + for 10 fb -1 (TeV) 95% CL Exclusion σ Discovery CMS Sensitivity to Contact Interactions Published Limit (D0):  + > 2.7 TeV at 95% CL è CMS could quadruple this limit in  can be translated roughly into the radius of a composite quark.  h =  x  p ~ (2r) (  / c)  r = cm-TeV /   For  ~ 10 TeV, r ~ cm Composite Quark Preon r

Robert Harris, Fermilab37 Conclusions l The LHC will probe exciting new physics with simple measurements of jets. l We have presented a jet trigger table and dijet analysis developed at LPC. l CMS can discover a strongly produced dijet resonance up to several TeV. è An excited quark, an E 6 diquark, or even an unanticipated new particle! CMS can discover a quark contact interaction up to  + =12 TeV with 10 fb -1. è Corresponds to a quark radius of order cm if quarks are composite. l The LHC Physics Center at Fermilab is active in jet physics analysis. è Mentoring the postdocs and graduate students who will analyze first CMS data. è Well integrated in the CMS physics organization. l We are looking forward to exciting discoveries at the TeV energy scale!

Robert Harris, Fermilab38 Dijet Resonance Cross Sections l Resonances produced via color force, or from valence quarks in each proton, have the highest cross sections.