1. Jet studies in preparation for ATLAS: from pp to heavy ion collisions Arthur M. Moraes Brookhaven National Laboratory (on behalf of the ATLAS Collaboration) APS – April Meeting 2006 Dallas, April 22nd – 25th 2006.

2. Outline: • LHC and ATLAS • Jet studies at the LHC: • Measuring jets in pp collisions • Heavy ion collisions & jet physics • From RHIC to the LHC • Jets in LHC heavy ion collisions • Finding jets in central Pb-Pb events • Reconstruction performance • Summary

4. •Pb-Pb collisions at √s = 5.5TeV (per colliding nucleon pair) •bunch crossing every 100 ns(10 MHz) • LHC heavy ion programme: one month of running per year of operation. • Pb-Pb luminosity: L ≈ 1027cm-2s-1 (limited by the quenching of the magnets) •p-Pb and lighter ion collisions are also foreseen. • first runs with heavy-ion collisions are expected in 2008. More info: CERN/LHCC/2004-009 LHC (Large Hadron Collider): •p-p collisions at √s = 14TeV •bunch crossing every 25 ns(40 MHz) • low-luminosity: L ≈ 2 x 1033cm-2s-1(L ≈ 20 fb-1/year) • high-luminosity: L ≈ 1034cm-2s-1 (L ≈ 100 fb-1/year)

5. ATLAS: AToroidal LHC AparatuS Muon Detectors Tile Calorimeter Liquid Argon Calorimeter TRT Tracker Toroid Magnets Pixel Detector SCT Tracker Solenoid Magnet ATLAS is well adapted to perform high-pT measurements in LHC pp collision as well as physics studies in heavy ion collisions. •Multi-purpose detectors coverage up to |η| = 5; design to operate at L= 1034cm-2s-1 • Calorimetry: well suited for jet quenching studies. Best granularity and hadronic energy resolution compared to other LHC detectors. • Most of the detector subsystems will be operational for the study of heavy ion collisions. • Muon System: quieter than in pp collisions at high-luminosity. Can be used for efficient μ identification and b-jet tagging. • Inner Detector: Pixel & SCT will be able to provide track measurements. TRT is unusable for nucleus-nucleus (Pb-Pb) studies due to high occupancy.

6. Jets in pp collisions at √s=14TeV • Large event rate reduces statistical errors quickly. • SM candles (W, Z, γ-jet and top production) will be used to calibrate the detector. • Systematic errors are expected to dominate. σ (nb) Events/year (L=10 fb-1) pp collisions at √s=14TeV Sources of systematic errors: • jet algorithm, • calorimeter response (jet energy scale), • jet trigger efficiency, • luminosity (dominant uncertainty 5% -10% ), • the underlying event. Inclusive Jet Production pt > 200 GeV 100 ~109 pt > 1 TeV 0.1 ~106 pt > 2 TeV 10-4 ~103 pt > 3 TeV 1.3×10-6 ~10 Jet physics at the LHC • Essentially all physics at LHC are connected to the interactions of quarks and gluons (small & large transferred momentum). p • Expect multi-jet final states. • This requires a solid understanding of QCD. p

7. Jets in pp collisions at √s=14TeV Reconstructed jet event (pp collisions):Atlantis event display Some applications of jet measurements in pp collisions: • Test of pQCD in an energy regime never probed! • The measurement of di-jets and their properties (ET and η1,2) can be used to constrain p.d.f.’s. • Inclusive jet cross section: αS(MZ) measurement with 10% accuracy. ETjet #2 = 1.16TeV ( can be reduced by using the 3-jet to 2-jet production ) • Multi-jet production is important for several physics studies: • tt production with hadronic final states • Higgs production in association with tt and bb • Search for R-parity violating SUSY (8 – 12 jets). ETjet #1 = 1.63TeV - - -

8. Jets in heavy ion collisions • Results from RHIC experiments suggest that a hot and dense QCD matter, which may be the quark-gluon plasma (QGP), is formed in Au-Au collisions at √s=200GeV per colliding nucleon pair. • RHIC experiments also show that hard scattered quarks inside the QGP radiate gluons and therefore modify the jet properties such as energy angular distribution and suppression of high-pT hadrons (jet quenching). From RHIC to the LHC • The LHC will collide heavy nuclei such as Pb at √s=5.5TeV/nucleon pair. At these energies it will be possible to produce an even higher temperature QCD matter. STAR, PRL 91 (2003) 072304 • The parton energy loss is directly related to the initial gluon density of the system, which is expected to be over a factor of ten higher at the LHC than at RHIC. • Hard scattering cross-sections will increase significantly and the resulting jets could be used as probes of the QCD matter.

9. Jets in heavy ion collisions at the LHC Pb-Pb at √s = 5.5TeV (per colliding nucleon pair) • One month (~106s) run with Pb-Pb at L=41026 cm-2 s-1 • Expected numbers of jet events at ATLAS for |η| < 4.9: And also: ~106 + jet events ~500 Z0() + jets with ET > 40 GeV

10. Finding jets in central Pb-Pb event • Jet reconstruction in heavy ion environment is more challenging than in pp collisions because of the large background of low-pT particles. pp collisions: jet ET threshold ~10(20)GeV for a jet of size ΔR=0.4 central Pb-Pb: jet ET threshold ~50GeV for a jet of size ΔR=0.4 • Jet energy calibration may be different for pp and heavy ion events. • Reconstruction: • background is estimated in rings DhxDf=0.1x2π and subtracted from calorimeter towers. • sliding window algorithm is used to find jet candidates, which are reconstructed inside cones of size ΔR=0.4. • jet candidates are accepted if ETjet>40GeV. • other methods are currently being investigated (e.g. cone fit algorithm).

11. ET=75GeV Jet reconstruction performance Jet reconstruction efficiency Pb-Pb (b=0 – 1 fm) • The efficiency of the reconstruction is evaluated by counting reconstructed jets (PYTHIA+HIJING) matching the generated PYTHIA jets within a cone of size ΔR=0.2. • Fake jets: comparison between jets in HIJING (background) matching the direction of jets on PYTHIA+HIJING. • Two jet finder algorithms testet up to now - Sliding Window and Cone Fit • for ET > 75GeV: efficiency > 95%, fake < 5% • good energy and angular resolution • The worsening in energy resolution introduced by the background has nearly the same effect when pile-up and electronic noise is added to high luminosity proton-proton runs (see ATLAS-TDR).

12. Summary: • Jet measurements are crucial to most LHC studies. This applies not only to physics from proton-proton collisions but also to the heavy ion programme. • Measuring jets in pp collisions: precision limited by systematic and not by statistical uncertainties! • Measuring jets in heavy ion collisions: challenging environment (low-pT background), but ATLAS will be able to deliver precise measurements. • Jet physics studies in LHC heavy ion collisions at ATLAS will contribute immensely to the understanding of jet quenching. • jet reconstruction is possible despite the additional background • jet energy resolution comparable to pp for jet ET>100 GeV • Combining calorimeter measurements with tracking and muon system there is also the possibility to study separately light and heavy-quark jets • ATLAS can make a valuable and significant contribution to the LHC’s heavyion physics programme