1 / 42

Jets and High- p T Physics with ALICE at the LHC

Jets and High- p T Physics with ALICE at the LHC. Andreas Morsch CERN. Workshop on High p T Physics at the LHC, Jyv ä skylä, March 25, 2007. Outline. Jet reconstruction in heavy ion collisions Modified fragmentation functions with reconstructed jets Di-Hadron Correlations at LHC.

manny
Download Presentation

Jets and High- p T Physics with ALICE at the LHC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Jets and High-pT Physics with ALICE at the LHC Andreas Morsch CERN Workshop on High pT Physics at the LHC, Jyväskylä, March 25, 2007

  2. Outline • Jet reconstruction in heavy ion collisions • Modified fragmentation functions with reconstructed jets • Di-Hadron Correlations at LHC

  3. Jet Physics at RHIC p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (300 GeV in R< 1.0) and the relatively low accessible jet energies (< 20 GeV). Use leading particles to tag the jet.

  4. STAR Phys. Rev. Lett. 91, 072304 (2003). Pedestal&flow subtracted Evidence for Jet Quenching • In central Au+Au • Strong suppression of inclusive hadron yield in Au-Au collisions • Disappearance of away-side jet • No suppression in d+Au • Hence suppression is final state effect.

  5. Eskola et al., hep-ph/0406319 RAA~0.2-0.3 for broad range of q Sensitivity to medium parameters • RAA measurements are consistent with pQCD-based energy loss calculations. However, they provide only a lower bound to the initial color charge density. Use 2-hadron correlation, 3-hadron correlation … multi-hadron correlation = Reconstructed Jets !

  6. From toy model Reconstructed Jet Jet Physics at LHC: Motivation • Study of reconstructed jets increases sensitivity to medium parameters by reducing • Trigger bias • Surface bias • Using reconstructed jets to study • Modification of the leading hadron • Additional hadrons from gluon radiation • Transverse heating. x = ln(Ejet/phadron)

  7. central Pb–Pb pp Jet reconstruction:New Challenges for ALICE • Existing TPC+ITS+PID • |h| < 0.9 • Excellent momentum resolution up to 100 GeV • Tracking down to 100 MeV • Excellent Particle ID • New: EMCAL • Pb-scintillator • Energy resolution ~15%/√E • Energy from neutral particles • Trigger capabilities

  8. Signal fluctuationsResponse function for mono-chromatic jets ET = 100 GeV, R = 0.4 DE/E ~ 50% DE/E ~ 24%

  9. Expected resolution including EMCAL Assumes conservative multiplicity: dN/dy = 6000

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

  11. Background energy • In cone of R = 1 • RHIC: 300 GeV • LHC: 1500 GeV • However jet energies up to ~250 GeV accessible ! • Provides lever arm to measure the energy dependence of the medium induced energy loss. • 104 jets needed to study fragmentation function in the z > 0.8 region.

  12. Background energy • How to reconstructs jets above a large fluctuation background (DEBg) ? • Restrict identification and reconstruction to domain in which Emeas >> DEBg • Cone size R < 1 • pT-cut • Limiting case R=0: leading particle • Advantage: background free by construction

  13. Jets reconstructed from charged particles: Energy contained in sub-cone R Need reduced cone sizes and transverse momentum cut ! Optimal Cone Size Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1). R and pT cut have to be optimized according to the background conditions. E ~ R2

  14. Background Fluctuations • Background fluctuations limit the energy resolution. • Fluctuations caused by event-by-event variations of the impact parameter for a given centrality class. • Strong correlation between different regions in h-f plane • ~R2 • Can be eliminated using impact parameter dependent background subtraction. • Poissonian fluctuations of uncorrelated particles • DE = N[<pT>2 +DpT2] • ~R • Correlated particles from common source (low-ET jets) • ~R

  15. Background Fluctuations Evt-by-evt background energy estimation

  16. Jet reconstruction in reduced domain:Why does it work ? • Measure only fraction of jet energy but measure it well • In this case • Ejet Emeas • Since ds/dEjet ~ 1/ET5.7, Ejet >> Emeas unlikely • Ejet Erec with relative small fluctuations • Jets are biased into the domain in which they are reconstructed = “Trigger Bias” • Works even for leading particle “jet reconstruction” • pTrig = 0.6 Ejet • For ideal calorimetry and R=0.4: pTrig = 0.9 Ejet • Further restriction of domain in ALICE • Charged particles only, in region without EMCAL coverage • Trigger bias: enhanced charged particle component • TPC + EMCAL: charged + g, only small fraction of energy from neutrons and K0L

  17. Reduction of the trigger biasby collecting more energy from jet fragmentation… Unbiased parton energy fractionproduction spectrum induced bias

  18. Another good reason for jet reconstruction:Statistics ! • Strong bias on fragmentation function • … which we want to measure • Low selectivity of the parton energy • Very low efficiency, example: • ~6% for ET > 100 GeV • 1.1 106 Jets produced in central Pb-Pb collisions (|h| < 0.5) • No trigger: ~2.6 104 Jets on tape • ~1500 Jets selected using leading particles

  19. Unquenched Quenched (AliPythia) Quenched (Pyquen) pT < 2 GeV Largest effect seen in low-pT particles. Jet reconstruction in restricted domain:What can go wrong ? • Correction factors to go from measured to reconstructed jet energy unknown in AA ! • Radiation • Mainly soft particles • Part of the energy goes outside of the jet cone • Needs • Good low-pT capabilities • Measurement of the transverse jet structure. • Theoretical understanding of the transverse jet-structure.

  20. ALICE performance studiesand preparation for first analysis • Full detector simulation and reconstruction of HIJING events with embedded Pythia Jets • Implementation of a core jet analysis frame work • Reconstruction and analysis of charged jets. • Quenching studies with fragmentation function • TPC only and TPC + EMCAL

  21. Energy spectrum from charged jets Cone-Algorithm: R = 0.4, pT > 2 GeV Selection efficiency ~30% as compared to 6% with leading particle ! No de-convolution, but GaussE-n ~ E-n

  22. Afterburner A Pythia hard scattering Initial and Final State Radiation Afterburner B Pythia Hadronization Afterburner C . . . Modification of the fragmentation function: Toy Model Nuclear Geometry (Glauber) Jet (E) → Jet (E-DE) + n gluons (“Mini Jets”) • Quenching of the final jet system and radiation of 1-5 gluons. (AliPythia::Quench using Salgado/Wiedemann - Quenching weights)

  23. ratio RAA(x)

  24. Example: p+Pb reference With EMCal: jet trigger+ improved jet reconstruction provides much greater ET reach

  25. Trigger Bias Production spectrum weighted response matrix: Out-of-domain fluctuations are damped with 1/En symmetrizing the distribution.

  26. Jet energy resolution and dN/dz Model1 Model2 Direct comparison with the MC truth for the same selected track. The dotted line shows the point spread function for z = 0.4.

  27. unquenched quenched Systematic shift in RAA(x) Erec = 100 GeV • More energy is radiated outside the cone. • On average the input energy has to be higher in order to give a reconstructed energy of 100 GeV. • As a consequence x is shifted to lower values. • Systematics has to be controlled using measurements of the transverse jet structure and RAAJet(ET)

  28. Systematic Error fromBackground Subtraction Soft Background 2 GeV

  29. Background Fluctuation log(dN/dE) Background fluctuates up Background fluctuates down Bias towards higher Bg Jet input spectrum log(E/GeV)

  30. Influence on Jet axis dR Under Ideal detector response – Not quenching R. Diaz Valdes

  31. Influence for a jet input spectrum <Etinput> <Etrec/Etinput> p-p 120.0 ± 17.23 0.856 ± 0.0815 Pb-Pb 116.2 ± 19.21 0.894 ± 0.1169 R. Diaz Valdes

  32. Bias on RAA(x) Corrections should be applied on p-p distribution to compare it with quenched Pb-Pb jet fragmentation R. Diaz Valdes

  33. PYTHIA 6.2 Di-hadron Correlations:from RHIC to LHC • Di-hadron correlations will be studied at LHC in an energy region where full jet reconstruction is not possible (E < 30 GeV). • What will be different at LHC ? • Number of hadrons/event (P) large • Leads to increased signal and background at LHC • Background dominates, significance independent of multiplicity • Increased width of the away-side peak (NLO) • Wider h-correlation (loss of acceptance for fixed h-widow) • Power law behavior ds/dpT ~ 1/pTn with n = 8 at RHIC and n = 4 at LHC • Changes the trigger bias on parton energy See also, K. Filimonov, J.Phys.G31:S513-S520 (2005)

  34. RHIC/STAR-like central Au-Au (1.8 107 events) LHC/ALICE central Pb-Pb (107 events), no-quenching Scaling From RHIC to LHC From STAR pTtrig = 8 GeV/c pTtrig > 8 GeV • S/B and significance for away-side correlations • Scale rates between RHIC and LHC • Ratio of inclusive hadron cross-section • N(pT) ~ pT4

  35. Di-hadron Correlations STAR LHC, ALICE acceptance HIJING Simulation 4 105 events M. Ploskon, ALICE INT-2005-49 O(1)/2p “Peak Inversion”

  36. Summary • Copious production of jets in Pb-Pb collisions at the LHC • Jets can be reconstructed over the background from the underlying event • Sufficient dynamic range (50 – 250 GeV) to make systematic studies of energy dependence. • Background conditions require jet identification and reconstruction in reduced domain R = 0.4. • We will measure jet structure observables (jT, fragmentation function, jet-shape) for reconstructed jets. • In AA, high-pT (calorimetry) and low-pT capabilities needed for unbiased measurement of parton energy. • Strength of ALICE • Excellent low-pT capabilities to measure particles from medium induced radiation. • PID to measure the particle composition of quenched jets • Dedicated pp experiments have larger ET reach

  37. Jet Finder based on cone algorithms • Input: List of cells in an h-f grid sorted in decreasing cell energy Ei • Estimate the average background energy Ebg per cell from all cells. • For at least 2 iterations and until the change in Ebg between 2 successive iterations is smaller than a set threshold: • Clear the jet list • Flag cells outside a jet. • Execute the jet-finding loop for each cell, starting with the highest cell energy. If Ei – Ebg > Eseed and if the cell is not already flagged as being inside a jet: • Set the jet-cone centroid to be the center of the jet seed cell (hc, fc) = (hi, fi) • Using all cells with (hi-h)2+(fi-f)2 < Rc of the initial centroid, calculate the new energy weighted centroid to be the new initial centroid. • Repeat until difference between iterations shifts less than one cell. • Store centroid as jet candidate. • Recalculate background energy using information from cells outside jets.

  38. Jet Finder in HI Environment:Principle Rc Loop1: Background estimation from cells outside jet cones Loop2: UA1 cone algorithm to find centroid using cells after background subtraction

  39. Putting things together:Intrinsic resolution limit Ejet = 100 GeV Background included pT > 0 GeV 1 GeV 2 GeV Resolution limited by out-of-cone fluctuations common to all experiments !

  40. ALICE Set-up Size: 16 x 26 meters Weight: 10,000 tons TOF TRD HMPID TPC PMD ITS Muon Arm PHOS

  41. Trigger performance Trigger on energy in patch Dh x Df Background rejection set to factor of 10 =>HLT Centrality dependent thresholds

  42. Summary of statistical reach Large : ~10% error requires several hundred signal events (Pb central) and normalization events (pp,pA). Large z>0.5 requires several thousand events • The EMCAL • extends kinematic range by 40–125 GeV • improves resolution (important at high z) • Some measurements impossible w/o EMCAL

More Related