Measurement of inclusive charged jet production in pp and Pb-Pb

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1 Measurement of inclusive charged jet production in pp and Pb-Pb collisions at S NN 5. 02TeV with ALICE Run2 Data Yan Li for the ALICE collaboration Central China Normal University CLHCP /12/2016

2 Outline Motivation ALICE experiment Analysis flow Results Jet cross section in pp at 5.02 TeV Underlying event subtraction in PbPb collisons at 5.02 TeV Jet yield in Pb-Pb Nuclear modification Factor RAA Summary

3 Jets in Heavy Ion Collisions ( Hard Probes of the QGP ) What's a Jet ü Collimated spray of hadrons produced by the hard scattering of partons at the initial stage of the collision 2 ü high Q process Why Jets s ü The QGP lifetime is so short ( ) that characterisation by external probes is ruled out self-produced probes ü Occur at early stage : ~ 1/ Q probe the entire medium evolution ü Production rate calculable within pqcd well calibrated probes ü Large cross-section at the LHC copious production ü Reconstructed jet enables to access 4-momentum of original parton jet structure (energy re-distribution) Jet Quenching ü Attenuation or disappearance of observed Jets in Pb-Pb due to partons' energy loss in the QGP jet shape broadening ü Evaluation of the degree of the attenuation allows to assess QGP properties

4 ALICE Jet Quenching Measurements in Pb-Pb R AA ü Nuclear modification factor : if R AA = 1, No modification R AA >1, enhancement R AA <1, suppression ü High-pT hadrons strong suppression : R AA ~ 0.2 proxy for jet ( parton ) : p T 10GeV / c fragmentation of quenched partons ü Jets Strong suppression : R AA ~ 0.4 Jet shape broadens? where is the lost energy?

5 ALICE Jet Quenching Measurements in Pb-Pb ü Nuclear modification factor : if = 1, No modification R AA R AA ü High-pT hadrons strong suppression : R AA ~ 0.2 proxy for jet ( parton ) : p T 10GeV / c fragmentation of quenched partons ü Jets Strong suppression : R ~ 0.4 p AA Jet T, shape collision broadens? where is the lost energy? ü We are interested in quantifying the jet suppression (parton energy loss) as a function of Jet energy and centrality. ü For higher s NN, denser, hotter and longer-lived QGP is created. => stronger jet suppression

6 Jet Measurement in LHC-ALICE

7 Analysis Flow ü Dataset s = 5.02 TeV, pp and Pb-Pb collisions ü MB triggered events ü Charged track selection η < 0.9, p track T 0.15GeV / c ü Jet reconstruction Anti-kt jet reconstruction algorithm R = 0.2, 0.4 η < 0.7, ü Unfolding p lead T 5GeV / c To correct for detector effects ü Inclusive jet spectrum Fully corrected to charged particle level Assess nuclear modification for Pb-Pb collision Event Selection Track Selection Jet Reconstruction Unfolding pp charged Jet Spectrum R AA UE density: UE Subtraction Unfolding Pb-Pb charged Jet Spectrum UE fluctuation: p T

8 Unfolding correction for jets ü Detector response matrix for detector effects ü The result of MC closure test for unfolding is reilable for correcting the jet spectrum

9 pp Inclusive Jet Cross Section ü Jet cross section Well described by POWHEG NLO calculations within systematic uncertainties. ü Ratio of cross sections (R 0.2) / ( R 0.4) Stronger collimation at high pt

10 Underlying Event Density Challenge in Heavy-Ion Collisions ü large background contribution to jet energy ü ( 0-10% centrality ) dn ch Jet Background Subtraction Background density : median / d ~ 1300 k T Background subtraction excluding the highest two clusters pt medina{ A background is estimated event-by-event and subtracted from each jet p rec T, jet raw T, jet Minimum leading constituent combinatorial jets in low momentum p, i i } A p T 5GeV / c rec jet requirement suppresses

11 Underlying Event Fluctuation p T UE fluctuation : p T is used as a measurement for background fluctuations Random Cone Method 1) random selection 2) RC apart from leading jet ( ) p T p RC track T pt, i i to reduce jet component. r ( - RC 3) use randomised tracks A r jet ) ( RC jet to exclude flow effect width (magnitude of UE fluctuation) fluctuations larger in central than in peripheral collisions ~5 GeV/c for R=0.2, 0-10% centrality ) 2

12 Inclusive Jet Cross Section ü pp Jet cross section ( reference for R AA ) pp reference run ( S NN TeV ) POWHEG simulation ü Pb-Pb Jet cross section 4 centrality bins (0-10%, 10-30%, 40-50%, 50-90%)

13 Nuclear Modification Factor : R AA ü R AA in each centrality bin Increased suppression from peripheral ~0.8 to central ~0.4 ü Difference of pp reference pp data / POWHEG simulation Consistent within uncertainties

14 Summary ü First measurement of jet Charged jet, R=0.2, p lead RAA at T 5Gev / c pp cross section, σ(r=0.2) / σ(r=0.4) well described by POWHEG NLO simulation S NN 5. 02TeV Evaluation of Underlying Event density / fluctuation large fluctuating underlying event in most central collisions Nuclear Modification Factor : R AA strong suppression in most central collisions Effect of flattening of the spectrum compensated by stronger jet suppression