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Progress on jet quenching simulations

Progress on jet quenching simulations. Marco van Leeuwen, LBNL. Plots/simulations by J ö rn Putschke. Quenching simulation. Simulations: AliPythiaQuench, PQM with fragmentation of radiated gluons (A. Morsch). Includes full geometry & energy loss fluctuations. Fragmentation function.

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Progress on jet quenching simulations

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  1. Progress on jet quenching simulations Marco van Leeuwen, LBNL Plots/simulations by Jörn Putschke

  2. Quenching simulation Simulations: AliPythiaQuench, PQM with fragmentation of radiated gluons (A. Morsch) Includes full geometry & energy loss fluctuations Fragmentation function Radial profile Idealised simulation: All particles, no background R=1 Energy loss depletes high-zand populates low-z Low-z fragments (from gluon radiation)show up at large R This model: in-medium energy loss leads to redistribution of particles in the cone Model has the main phenomenology included; use as benchmark Caveat: Model/theory needs to become more quantitative to extract physics

  3. Heavy-ion backgrounds Background energy fluctuations Jet cone energy pT > 2 GeV ALICE PPR Vol II 100 GeV 50 GeV Dominant source: Impact parameter fluctuations already taken out for these plots 50 GeV jets: need to restrict cone size >100 GeV jets: can use larger size (R=0.5-1) Note: also here large uncertainty: need first data to know background level

  4. ALICE+EMCal in one LHC year Simulation: Full spectrum of jets (Pythia) No detector simu ratio Background added in‘by hand’ Parameters R = 0.4 pT > 1 GeV effect on E-res not not simulated Ratios of quenched/unquenched fragmentation function measures quenching

  5. Trigger gain Jet trigger Jet trigger statistics gain largest for small systems (p+p, p+Pb, peripheral Pb+Pb) p+p runs 10x longer, so main need for stats in p+Pb and peripheral Pb+Pb Need all reference systems for a complete systematic study Includes acceptance, efficiency, dead time, energy resolution

  6. Jet yields: one LHC year Jet yield in 20 GeV bin Large gains due to jet trigger Large variation in statistical reach for different reference systems

  7. p+Pb reference 125 GeV 225 GeV • With EMCal: provides • Jet trigger gives much greater ET reach • Larger fraction of jet-E measured (better reso) Benefit of better E-res not obvious, further study needed

  8. Peripheral Pb+Pb reference Peripheral Pb+Pb reference has smallest stat; reach to well over 125 GeV with EMCAL

  9. Compare R=0.4 and R=0.8 • Larger cone: • Smaller deviation from “ideal” at moderate  but effect is small • Larger background: reduced significance at large 

  10. Open questions • Why are jet resolution effects (TPC vs.EMCAL) not clearly visible in x-distLarger effect in z-distribution? • Average jet energy correction not sufficient. Need to refine • Evaluate different jet-finding algorithms(Cone, kt, hybrid)  Improve jet energy resolution? • Different bkg. models (HIJING vs. HYDJET) • Full GEANT simulations • Phenomenological studies of APQ to understand in more detail sensitivity of various probes (single jets, di-jets, g-jet) to geometry, medium density

  11. Jet splitting Jet splitting in EMCal acceptance Reconstructed energy R=0.3, pt>2GeV all particles, R=0.3, pT > 2GeV all particles charged+pi0 charged # Jets • input • - Njets,rec=1 • - Njets,rec>=1 highest jet • Njets,rec>=1 mid-cone • - Njets,rec>=1 sum Fraction of events Njets,rec.>1 Jet Energy (GeV) Jet Energy (GeV) Jet shape fluctuate; need large cone, or merging algorithm to recover all energy Ideal task for kT algorithm (non-circular ‘cones’)?

  12. kT algorithm M Cacciari and G Salam Example p+p event Result Resulting spectrum after bkg subtraction fairly similar to input Use background jets to subtract uncorrelated particles Large tail due tobkg fluctuations No pT-cut needed?

  13. Extra slides

  14. What does the calorimeter bring Jet energy response ALICE PPR, part II 100 GeV Jets R=0.4 Note: tail due to jet-splitting Charged particle tracking only sees ~50 % of jet energy TPC+EMCal recovers large fraction of jet energy So far: have not been able to quantify benefit of larger reconstructed fraction But of course: EMCal provides important trigger capability

  15. Finding the working point With background, split jets, and in-medium modifications, need to trade-off effects to optimise sensitivity to physics p+p events Pb+Pb and p+p events R=0.3, pT > 2 GeV Best resolution for EMC+TPC: 15% With these cuts, influence of background is small Background pT-cut, smaller R, worsens resolution by 5-10% Caveat: Large cut-space; Final tune of cuts can only be made with data

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