Charged jets in p Pb collisions measured with the ALICE detector (CERN) for the ALICE collaboration (25.03.2015) Rencontres de Moriond, QCD and High Energy Interactions, La Thuile
Motivation for p Pb Study of cold nuclear matter effects Initial state effects (Nuclear PDFs,...) Final state effects (Nuclear absorption,...) PRL 110 (2013) 082302 Necessary for interpretation of heavy-ion collisions Disentangle cold nuclear matter & hot nuclear matter effects QGP formation not expected in p-pb collisions Detailed analyses carried out for minimum bias and centrality dependent data Final state effects expected to be strongest in central collisions 2
Outline Experimental setup and charged jet reconstruction with the ALICE detector Centrality determination in p Pb collisions Correction techniques Results 3
Experimental setup and charged jet reconstruction with the ALICE detector
The ALICE detector ITS: Inner Tracking System (silicon detectors) ZDC: 116 m distance to interaction point TPC: Time Projection Chamber (gas detector) ZDC ITS ZDC: Zero-Degree Calorimeter for centrality classification (hadronic calorimeters) ZDC ITS TPC Charged jet constituents: Tracks mainly reconstructed with ITS+TPC ITS+TPC coverage: η < 0.9 full azimuth pt > 150 MeV/c 5
Jet physics Conceptually, a jet is the final state of collimated hadrons that fragmented from a hard scattered parton Jets can be used to investigate the very early stage of a hadron collision and to probe system properties (jet quenching in PbPb) There is no unambiguous jet definition The reconstructed jet observable is defined by the jet finding algorithm used to clusterize tracks and calorimeter clusters into jets 6
Jet reconstruction FastJet1 is used for jet finding anti-kt algorithm for signal jets kt algorithm for background correction Resolution parameters R = 0.2 & R = 0.4 Analyzed only jets fully contained in detector Background density and fluctuation correction applied Jet spectra corrected for detector effects Charged jets = charged part of full jet 1 Cacciari, Salam, Soyez. EPJC72 (2012) 1896 [arxiv:1111.6097] 7
Centrality determination in p Pb collision with ALICE Pb 208 ZNA p
Centrality determination in p Pb Centrality = measure for collision geometry Much work done by ALICE on hybrid approach using ZDC (see PRC 91, 064905(2015)) Procedure: 1. Event classification Use deposited energy in ZDC neutron calorimeter (ZN) for divide data sample into centrality classes 2. Estimate number of binary collisions per event Several different assumptions can been utilized to calculate Ncoll. For the jets, we use: Charged particle multiplicity (at midrapidity) on the Pb-going side is proportional to Ncoll 9
Correction techniques
Background density correction Default background approach1 Corrected jet-by-jet clusterize kt jets (susceptible to background) take median of the pt densities arxiv:1603.03402 neglecting the first two leading kt jets (how dense or sparse the event is) correct for event occupancy 1 As in arxiv:1207.2392 [hep-ex] 11
Background fluctuations & detector effects Fluctuations and detector effects corrected in unfolding procedure Corrected via unfolding Background fluctuates within one event Estimated by randomly placing cone with radius R into acceptance. arxiv:1603.03402 Summing up pt in cone and subtracting background Finite detector resolution (pt resolution, tracking efficiency ) Detector effects estimated by calculating response matrix PLB749 (2015) 68 81 12
Results
Charged jet cross sections Minimum bias R = 0.4 Fully corrected charged jet production cross sections measured from 20 GeV/c up to 120 GeV/c for R = 0.2 and R = 0.4 Good agreement with NLO calculations from POWHEG+PYTHIA8 R = 0.2 Plots from PLB749 (2015) 68 81 14
Charged jet cross sections Centrality-dependent cross sections Fully corrected charged jet production cross sections measured from 20 GeV/c up to 120 GeV/c for R = 0.2 and R = 0.4 R = 0.4 Note: Spectra not scaled with number of binary collisions Ncoll R = 0.2 Plots from arxiv:1603.03402 15
Nuclear modification factor RpPb = ppb yield/pp yield normalized by Nıcoll R = 0.2 R = 0.4 Proton-proton reference Reference created from 7 TeV charged jets, scaled to 5 TeV using NLO calculations. Also used for the centrality-dependent results Minimum bias Charged jet nuclear modification factor measured from 20 to 120 GeV/c Within the uncertainties, no significant nuclear modification is visible for both resolution parameters Plots from PLB749 (2015) 68 81 16
Nuclear modification factor Centrality-dependent factor R = 0.4 Charged jet nuclear modification factor measured from 20 to 120 GeV/c. Within the uncertainties, neither significant centrality dependence nor nuclear modification is observed for both resolution parameters R = 0.4 R = 0.2 Plots from arxiv:1603.03402 17
Nuclear modification factor Comparison to ATLAS full jets arxiv:1603.03402 R = 0.4 ATLAS found centralitydependent deviations of jet production for large rapidities in the proton-going (y* > 0) direction and pt, jet > 100 GeV/c Our ηlab < 0.5 corresponds to -0.96 < y* < -0.04 In the comparable rapidity region, ALICE charged and ATLAS full jets consistent in central and peripheral collisions 18
Jet cross section ratio PLB749 (2015) 68 81 arxiv:1603.03402 Charged jet cross section ratio R = 0.2/R = 0.4 in p-pb collisions Simplest measure for changes in radial jet narrowing/ broadening (note: only indirectly sensitive to large changes) No significant nuclear effects measured in minimum bias (left) No significant centrality dependence (right) No strong changes in the radial jet structure (as already expected from Pb Pb) 19
Summary Charged jets measured in p-pb collisions at snn = 5.02 TeV ranging from 20 GeV/c to 120 GeV/c Good agreement of minimum bias spectra to NLO pqcd calculations with POWHEG+PYTHIA8 Nuclear modification factors RpPb, QpPb show no significant effect no strong nuclear modification no strong centrality dependence Charged jet cross section ratios In minimum bias, the ratios agree with ALICE 7 TeV pp jet cross section ratio Several Monte Carlo calculations In bins of centrality, the ratios agree with Each other & minimum bias Thank you for your attention 20
Backup
Analysis references ALICE measured charged jet production in p-pb collisions at snn = 5.02 TeV up to 120 GeV/c Minimum bias (arxiv: 1503.00681, PLB 749 (2015) 68 81) Centrality-dependent (arxiv: 1603.03402, submitted to EPJC) Centrality approach based on arxiv:1412.6828 22
pp reference for QpPb General approach: 5 TeV pp measurements not available Measured 7 TeV pp jets are scaled down to 5.02 TeV Scaling done bin-by-bin with: Yields taken from NLO pqcd calculations, including ppb boost Additionally tested: Bin-by-bin power-law extrapolations using 2.76 and 7 TeV jets 23
Compatibility with charged particle spectra arxiv:1412.6828 Charged particle QpPb shows qualitatively the same behavior 24
Biased centrality estimators ALICE-PUBLIC-2016-001 Note: The final jet results do not make use of the strongly biased estimators In the ALICE p-pb centrality paper (1412.6828), several centrality estimators were analyzed. The classical estimators and the Slow Nucleon Model (SNM) estimator are strongly biased. See paper for a detailed explanation. The behavior of the jets is qualitative the same as that of the particles 25