3/25/2007 High-pT Physics at the LHC 1 Jet Correlations in ATLAS Nathan Grau Columbia University, Nevis Labs RHIC Correlations and High- p T Measurements with ATLAS

3/25/2007 High-pT Physics at the LHC 2 Outline A very brief review of some key RHIC data on high-p T (> 5 GeV trigger, mostly) correlations  Questions rather than answers Move on to possibly finding answers:  Need event-by-event data, not integral/average quantities  i.e. LHC jet studies at ATLAS

3/25/2007 High-pT Physics at the LHC 3 Jet Quenching at RHIC: Singles Dramatic quenching relative to binary-scaled p+p is observed How sensitive is R AA to the details of energy loss? T. Renk Hard Probes 2006

3/25/2007 High-pT Physics at the LHC 4 Jet Quenching at RHIC: Pairs Why are high-p T two-particle away-side correlation yields suppressed but their shape is not broadened? 8 < p T (trig) < 15 GeV/c

3/25/2007 High-pT Physics at the LHC 5 I AA vs R AA Why is I AA ~ R AA ? D. Magestro, QM 2005 J. Jia, QM 2006 Cu+Cu

3/25/2007 High-pT Physics at the LHC 6 I AA vs R AA : An Interesting Implication Is this “factorized” e-loss? R AA dominated by geometry, so is I AA ? Pair suppression

3/25/2007 High-pT Physics at the LHC 7 I AA vs R AA : Is It True? 00-20% : R AA trig=0.60, R AA assoc= % : R AA trig=0.75, R AA assoc= % : R AA trig=0.90, R AA assoc=0.80 D AA h-h Correlations from Au+Au 2-3 GeV/c trigger Chun Zhang, QM 2006

3/25/2007 High-pT Physics at the LHC 8 Jet Medium Interactions: The Ridge in STAR J. Putschke, QM 2006 STAR has measured a very pronounced ridge structure in Au+Au And it’s always there… p t,assoc. > 2 GeV

3/25/2007 High-pT Physics at the LHC 9 Jet Medium Interactions: The Ridge in PHENIX Seems at higher associated p T ridge even goes away? Where is the ridge the 4-5 GeV/c bin? h-h Correlations from 0-20% Au+Au Chun Zhang, QM 2006 Near Side

3/25/2007 High-pT Physics at the LHC 10 Jet Medium Interactions: Near-side Energy Loss Increased yield in radiative tail of j T distribution measured in Cu+Cu  Not the ridge since this is hard (perturbatively calculable) radiation Near side jet loses energy: not surface emission. H Pei, QM2006 ~1/Ntrigg dN/dj T jTjT

3/25/2007 High-pT Physics at the LHC 11 Jet Medium Interactions: The Mach Cone Know v 2 depending on trigger’s RP orientation. J. Bielcikova et al, Phys. Rev. C69:021901, 2004 J. Jia, QM % Au+Au x h-h I think PHENIX has their v 2 systematics under control. Is this just momentum conservation?

3/25/2007 High-pT Physics at the LHC 12 Jet Medium Interactions: Cone or Bent Jet Two-particle can possibly rule out a bent-jet scenario: seems like dip is filled in by the jet. But this is not event-by-event information M. Horner, QM 2006

3/25/2007 High-pT Physics at the LHC 13 p T -Dependence of Jet Modification NG QM2006,  0 -h in Au+Au How do di-hadrons vary with p T : Conical  extinction  punch-through?

3/25/2007 High-pT Physics at the LHC 14 Summary of the Introduction Discovery of jet quenching occurred at RHIC!  But that was in What more can we say since then about  Medium properties?  Energy-dependence of energy loss?  Path-length dependence of energy loss? To borrow a phrase from Mike: “We are still on a long learning curve.” What is missing: A clear event view of a ridge/mach cone/broadened jet etc. a la Paris 1982 Where can we do this: at the LHC with ATLAS (and CMS and ALICE)

3/25/2007 High-pT Physics at the LHC 15 The ATLAS Detector

3/25/2007 High-pT Physics at the LHC 16 The ATLAS Detector Full azimuthal acceptance in all detectors Unprecedented pseudorapidity coverage for A+A

3/25/2007 High-pT Physics at the LHC 17 Tracking with the Inner Detector Reconstructed tracks with |  |<1 Inner detector has full azimuthal coverage within |  |<2.5 and consists of  Pixel detector  Silicon tracking detector  Transition radiation tracker Results from p+p tracking algorithm optimized for HI environment. Tracking down to 500 MeV. Important for D(z) and v 2.

3/25/2007 High-pT Physics at the LHC 18 ATLAS Calorimetry Hadronic Barrel Hadronic EndCap EM EndCap EM Barrel Forward

3/25/2007 High-pT Physics at the LHC 19 Longitudinal Segmentation: 3-d Jets Sampling of a 100 GeV jet (no background) Note the  region is 0.8x0.8: a typical jet size Sampling aides in energy resolution. Important for EM vs. hadronic energy separation.

3/25/2007 High-pT Physics at the LHC 20 Approach to Cone Jet Reconstruction Original cells Cloned cells Original towers Subtracted cells New towers Reconstructed jets Backgroundsubtraction (exclude seeds) Cells: single readout channel Tower: 0.1x0.1  sums of constituent cells Different background Techniques are possible: E-by-E cell level E-by-E tower level E-avg cell level E-avg tower level Iteration also possible. Need to understand the fluctuations in the different methods Inherent limitations on jet measurements. Use standard ATLAS code, except for the background subtraction.

3/25/2007 High-pT Physics at the LHC 21 Hard Radiation in p+p Collisions Vacuum radiation is important  Changes “typical” R-cut in cone and k T algorithms, i.e. R<0.7  Splitting/Merging of proto-jets is important in p+p Simulated pythia event with hard scattering from GeV

3/25/2007 High-pT Physics at the LHC 22 Radiation From E-Loss: More Subjets The distribution of radiated gluons related to Hard radiation in the tail  subjets Requires a precise measure of the jet direction

3/25/2007 High-pT Physics at the LHC GeV Cone Jet Position Resolutions Very good position resolution of jets Position resolution not dominant error on j T  5% uncertainty in the angular measurement, p T is about 2%   GeV GeV

3/25/2007 High-pT Physics at the LHC 24 Energy Resolution Energy Resolution of jets necessary for  Jet spectrum  show jet cross section binary scales  Fragmentation functions since error in z is dominated by error on jet E T This is using the most brute force cell background subtraction  Improvements on the method are being made  Calibrations due to subtraction have not been applied. 

3/25/2007 High-pT Physics at the LHC 25 Utilizing the k T Algorithm See W. Holzmann’s talk for all of the details Cacciari and Salam: Fast k T  Not a new k T algorithm, just improved the algorithms speed  Fast enough to run on HI events before subtracting the background event! Use background jets to discriminate real jets: i.e. their properties are different

3/25/2007 High-pT Physics at the LHC 26 “Fast” k T Finder: Discriminating Jets and Background Each color is a different “jet” defined by the algorithm “Real” jets and “Background” jets look different. Real Jets appear as narrow towers “Fake” Jets appear flat and broad

3/25/2007 High-pT Physics at the LHC 27 Advantages of k T Algorithm Not a fixed cone size  Easier to handle the hard radiation from jets No seeds are required  All the energy in a high multiplicity (central) heavy ion event results in a jet Changed the problem of subtracting a background to discrimination of clustered energy  Could suffer less from background fluctuations

3/25/2007 High-pT Physics at the LHC 28 The More Perfect Way:  -jet Barrel EMCal front layer finely segmented in  for vectoring H   events and  0 rejection. Example of jet embedded in central b=2 fm HIJING event. Jet Background Single  slice 0.1 rad Jet Back ground All too wide for single photons Single  slice 0.1 rad EM Layer 1 E T (GeV)  Single  slice 0.1 rad  -jet event embedded

3/25/2007 High-pT Physics at the LHC 29 Separation from  0 Decay Cuts to discriminate  /  0 before isolation Inverting these cuts results in  0 measurement Energy of a 2nd maximumFractional energy outside core

3/25/2007 High-pT Physics at the LHC 30  /  0 Separation Rejection of  0 with appropriate cuts on previous variables Efficiency in p+p is ~90%, flat with E T and  Some rejection prior to isolation from jet!

3/25/2007 High-pT Physics at the LHC 31 Summary & Conclusion RHIC high-p T discoveries and measurements are interesting and the I don’t understand all of the measurements at the moment. ATLAS will provide useful and necessary measurements complimentary to the RHIC program and extend these results and, ideally, help to answer the questions I have.  Single jet, di-jet, and  -jet Unfortunately I could not cover all aspects of ATLAS high-p T program: heavy flavor bound states, heavy flavor energy loss, v 2, etc.  Let’s have more than 1 ATLAS and 1 CMS talk next time?

3/25/2007 High-pT Physics at the LHC 32 ATLAS Heavy Ion Working Group A. Ajitanand 10, A. Angerami 3, G. Atoian 11, M. Baker 1, P. Chung 10, B. Cole 3, R. Debbe 1, A. Denisov 5, J. Dolejsi 2, N. Grau 3, J. Hill 7, W. Holzmann 3, V. Issakov 11, J. Jia 10, H. Kasper 11, R. Lacey 10, A. Lebedev 7, M. Leltchouk 3, A. Moraes 1, R. Nouicer 1, A. Olszewski 6, A. Poblaguev 11, V. Pozdnyakov 8, M. Rosati 7, L. Rosselet 4, M. Spousta 2, P. Steinberg 1, H. Takai 1, S. Timoshenko 9, B. Toczek 6, A. Trzupek 6, F. Videbaek 1, S. White 1, B. Wosiek 6, K. Wozniak 6, M. Zeller 11 1 Brookhaven National Laboratory, USA 2 Charles University, Prague 3 Columbia Unversity, Nevis Laboratories, USA 4 University of Geneva, Switzerland 5 IHEP, Russia 6 IFJ PAN, Krakow, Poland 7 Iowa State University, USA 8 JINR, Dubna, Russia 9 MePHI, Moscow, Russia 10 Chemistry Department, Stony Brook University, USA 11 Yale University, USA

3/25/2007 High-pT Physics at the LHC 33 Backup Slides

3/25/2007 High-pT Physics at the LHC 34 I AA from x E distributions in Cu+Cu

3/25/2007 High-pT Physics at the LHC 35 I AA from x E distributions in Cu+Cu

3/25/2007 High-pT Physics at the LHC 36 Tracking to Lower p T Work extending p T reach important for p+p and A+A.  dN/d  in both cases  v 2 in A+A Ongoing with high energy and heavy ion participation Efficiency: red/black Fake rate: red/green Preliminary Minimum bias p+p