Jet Properties in Pb-Pb collisions at ALICE

Transcription

1 Jet Properties in Pb-Pb collisions at ALICE Oliver Busch University of sukuba Heidelberg University for the ALICE collaboration Oliver Busch LHC Seminar 5/216 1

2 Outline Introduction Jets in heavy-ion collisions Strangeness production in jets Jet shapes Summary Oliver Busch LHC Seminar 5/216 2

3 Introduction Oliver Busch LHC Seminar 5/216 3

4 QCD phase transition in heavy-ion collisions at ultra-relativistic energies, a quasi macroscopic fireball of hot, strongly interacting matter in local thermal equilibrium is created lattice QCD predicts phase transition to deconfined, chirally symmetric matter HotQCD, PRD 9, 9453 energy density from the lattice: rapid increase around C, indicating increase of degrees of freedom (pion gas quarks and gluons) C = 154 ± 9 MeV EC = 34 ± 45 MeV/fm 3

5 QCD matter at LHC direct photons: prompt photons from hard scattering + thermal radiation from QCD matter low-p inverse slope parameter: eff = 297 ± 12 stat. ± 42 syst. MeV/c indicates initial temperature way above C Phys. Lett. B 754 (216) 235 Oliver Busch LHC Seminar 5/216 5

6 Partons in heavy-ion collisions hard partons are produced early and traverse the hot and dense QGP expect enhanced parton energy loss, (mostly) due to medium-induced gluon radiation: jet quenching jet: collimated bunch of hadrons the best available experimental equivalent to quarks and gluons vacuum expectation calculable by pqcd: calibrated probe of QGP Oliver Busch LHC Seminar 5/216 6

7 Parton fragmentation initial hard scattering: high-p partons with high virtuality virtuality evolution through parton shower hadronisation at a scale (O(ΛQCD)) jets probe the medium at a variety of scales and are sensitive to its properties (energy density,, mean free path, coupling... ) Oliver Busch LHC Seminar 5/216 7

8 Hadrons in heavy-ion collisions high- p hadrons proxy for jet jet quenching for charged hadrons, Pb-Pb collisions at snn = 2.76 ev PLB 72 (213) 25 ALICE Pb-Pb s NN = 2.76 ev charged particles, η <.8 1 R AA 1 1 ALI DER p (GeV/c) -5% 7-8% Oliver Busch LHC Seminar 5/216 8

9 9 Identified hadrons in heavy-ion collisions baryons / meson RAA a probe of gluon / quark energy loss? R AA would expect stronger radiative energy loss for gluons than for quarks - subtle cancellations? - hadron observable biased towards hard fragmentation? arxiv: ALICE -5% Pb-Pb - π + +π + - K +K p + p Charged s NN =2.76 ev (GeV/c) study jets to improve our ALI DER understanding of parton energy loss: - PID in reconstructed jets mitigates fragmentation biases - enhanced sensitivity to medium effects measuring soft particles in jets note: medium effects likely strongest at scales of ~ medium emperature (J.G. Milhano, K. C. Zapp, hep-ph/ ,. Renk, Phys. Rev. C 81, 1496, B. Mueller, hep-ph/ ) p

10 Jets at ALICE (LHC run 1) charged particle tracking: - Inner racking System (IS) - ime Projection Chamber - full azimuth, η <.9 p > 15 MeV/c EMCal : - neutral particles - Δφ = 17, η <.7 cluster E > 3 MeV full jets: assess full parton energy charged (tracking) jets: - full azimuthal coverage - measures parton energy deposited into charged fragments - good definition of jet axis: well suited for fragmentation, jet structure, PID 1

11 Underlying event in heavy-ion collisions jet reconstruction in heavy-ion collisions : high underlying event background not related to hard scattering background is dominant at low jet and constituent p background fluctuations are important central jet area ~.5 (R =.4) peripheral

12 Strangeness Production in Jets Oliver Busch LHC Seminar 5/216 3

13 Strange hadron reconstruction neutral strange particles reconstructed via decay topology ( V ): signal extraction from invariant mass distributions Oliver Busch LHC Seminar 5/216 13

14 Strangeness production in nuclear collisions Baryon / Meson ratio enhanced in Pb-Pb and p-pb collisions - collective effects? - parton recombination? - jet fragmentation? measurement of identified particles in jets helps to constrain hadronisation and energy loss scenarios Phys. Rev. Lett. 111 (213) 2231 Phys. Lett. B 728 (214) 25

15 Strangeness in jets candidate - jet matching (V in jet cone) jet R =.2, acceptance η V <.7 candidate - bulk matching: underlying event V signal extraction from invariant mass distributions correct for efficiency and feed-down subtract underlying event from spectra Oliver Busch LHC Seminar 5/216 15

16 Underlying event subtraction subtract underlying event contribution to K S, Λ spectra in jets various methods with different sensitivity to acceptance, event plane correlations, presence of additional jets, differences used to estimate systematic uncertainty Oliver Busch LHC Seminar 5/216 16

17 (Λ+Λ)/2K s ratio in jets Pb-Pb collisions at = 2.76 ev jet R =.2 p jet > 1 GeV/c (2 GeV/c) leading constituent bias p leading > 5 GeV/c to reject fake jets S (Λ + Λ)/2K ALICE Preliminary Pb Pb, s NN in jets, p in jets, p = 2.76 ev, 1 % jet,ch jet,ch > 1 GeV/c > 2 GeV/c feed-down uncertainty inclusive Λ/K, ALICE, S ( 5 %, y <.5) V no significant jet p jet dependence ratio in jets significantly lower than for inclusive case η <.7 V anti-k t, R =.2 η <.5 jet,ch p track p leading track > 5 GeV/c > 15 MeV/c.2 ALI PREL (GeV/c) p Oliver Busch LHC Seminar 5/216 17

18 Comparison to p-pb compare Pb-Pb results to reference from p-pb collisions at 5.2 ev: agreement within uncertainties ongoing efforts to improve systematics for lowest K S, Λ p p-pb Λ)/2K (Λ + S ALICE Preliminary Pb-Pb Pb Pb, s NN in jets, p in jets, p = 2.76 ev, 1 % jet,ch jet,ch > 1 GeV/c > 2 GeV/c feed-down uncertainty inclusive Λ/K, ALICE, S ( 5 %, y <.5) V η <.7 V anti-k t, R =.2 η <.5 jet,ch p track p leading track > 5 GeV/c > 15 MeV/c.2 ALI-PREL-8712 ALI PREL (GeV/c) p Oliver Busch LHC Seminar 5/216 18

19 Jet Shapes Oliver Busch LHC Seminar 5/216 3

20 Jet nuclear modification factor strong suppression, similar to hadron RAA parton energy not recovered inside jet cone increase of suppression with centrality, weak p dependence JEWEL: - microscopic pqcd parton shower + gluon induced emissions YaJEM: - detailed fireball model - parameterisation of radiative and collisional energy loss Phys.Lett. B746 (215) 1 JEWEL: PLB 735 (214) YaJEM:PRC 88 (213) 1495 different models reproduce observed jet suppression study jet quenching through more differential measurements 2

21 Jet shapes radial moment girth g, longitudinal dispersion pd, difference leading - subleading p LeSub shapes in pp collisions at 7 ev: - constrain QCD calculations of small-r jets ( microjets : M. Dasgupta, F. Dreyer, G. Salam, G. Soyez) - validate MC simulations shapes in Pb-Pb as IRC safe probe of quenching of low-p jets: characterise fragment distributions and are sensitive to medium induced changes of intra-jet momentum flow event-by-event measure, sensitive to fluctuations Oliver Busch LHC Seminar 5/216 21

22 Jet shapes as quenching signatures compare quarks and gluons: gluon jets broader and softer g is p weighted width of the jet: - broadening (collimation) enhanced (reduced) g pd measures p dispersion: - less constituents / more democratic splitting reduced pd LeSub characterises hardest splitting, insensitive against background dn/dg jets 1/N ALICE Simulation Anti-k charged jets, R=.2 jet, ch 4 < p < 6 GeV/c PYHIA Perugia pp s = 2.76 ev Gluon jets Quark jets D dn/dp jets 1/N ALICE Simulation Anti-k charged jets, R=.2 jet, ch 4 < p < 6 GeV/c PYHIA Perugia pp s = 2.76 ev Gluon jets Quark jets (c/gev) dn/dlesub jets 1/N ALICE Simulation Anti-k charged jets, R=.2 jet, ch 4 < p < 6 GeV/c PYHIA Perugia pp s = 2.76 ev Gluon jets Quark jets ALI SIMUL g ALI SIMUL Oliver Busch LHC Seminar 5/216 p D ALI SIMUL LeSub (GeV/c)

23 Analysis details charged jets from charged particle tracks, p const > 15 MeV/c in pp MinB at 7 ev and Pb-Pb 1% central at 2.76 ev R=.2, 4 < p jet < 6 GeV/c, no leading constituent cut novel background subtraction methods (Pb-Pb) - area subtraction (G. Soyez et al, Phys. Rev. Lett 11 (213) 16) - constituent subtraction (P. Berta et al, JHEP 146 (214) 92) 2D unfolding to correct for background fluctuations and detector effects 2 dn/dg jet /N ALICE simulation R=.2, Anti-k charged jets jet,ch 4 < p <6 GeV/c Pythia Det. Level Pythia Embedded Area. Sub (2nd order) Pythia Embedded Const. Sub Pythia Embedded Unsubtracted ALI-SIMUL uncorrected g 23

24 Jet shapes in pp fully corrected to charged particle level fair agreement with PYHIA simulations: validates PYHIA as reference for Pb-Pb dn/dg jets 1/N ALICE Preliminary pp s = 7 ev Anti-k charged jets, R =.2 jet,ch 4 < p < 6 GeV/c ALICE Data Shape uncertainty Correlated uncertainty PYHIA Perugia 11 PYHIA Perugia D dn/dp jets 1/N ALICE Preliminary pp s = 7 ev Anti-k charged jets, R =.2 jet,ch 4 < p < 6 GeV/c ALICE Data Shape uncertainty Correlated uncertainty PYHIA Perugia 11 PYHIA Perugia dn/dlesub (c/gev) jets 1/N.14 ALICE Preliminary pp s = 7 ev.12 Anti-k charged jets, R =.2 jet,ch 4 < p < 6 GeV/c ALICE Data Shape uncertainty Correlated uncertainty PYHIA Perugia 11 PYHIA Perugia Data/MC LI PREL g Data/MC ALI PREL 1151 p D Data/MC ALI PREL 1152 LeSub (GeV/c) Oliver Busch LHC Seminar 5/216 24

25 Jet shapes in Pb-Pb fully corrected to charged particle level g shifted to smaller values compared to PYHIA reference indicates more collimated jet core dn/dg jets 1/N ALICE Preliminary Pb-Pb s NN = 2.76 ev Anti-k charged jets, R =.2 4 < p jet,ch < 6 GeV/c ALICE Data Shape uncertainty Correlated uncertainty PYHIA Perugia ALI PREL g Oliver Busch LHC Seminar 5/216 25

26 larger pd in Pb-Pb compared to PYHIA indicates fewer constituents in quenched jets LeSub in Pb-Pb in good agreement with Pb-Pb: hardest splittings likely unaffected D dn/dp jets 1/N ALI PREL ALICE Preliminary Pb-Pb Anti-k 4 < p s NN = 2.76 ev charged jets, R =.2 jet,ch <6 GeV/c ALICE Data Shape uncertainty Correlated uncertainty PYHIA Perugia p D (c/gev) dn/dlesub jets 1/N ALI PREL ALICE Preliminary Pb-Pb Anti-k 4 < p s NN = 2.76 ev charged jets, R =.2 jet,ch < 6 GeV/c ALICE Data Shape uncertainty Correlated uncertainty PYHIA Perugia LeSub (GeV/c) Oliver Busch LHC Seminar 5/216 26

27 Jet structure: model comparison trends reproduced by JEWEL jet quenching model: collimation through emission of soft particles at large angles JEWEL: K.C. Zapp, F. Kraus, U.A. Wiedemann, JHEP 133 (213) 8 Oliver Busch LHC Seminar 5/216 27

28 Qualitiative discussion characterise degree of dispersion and broadening in terms of quark-like and gluon-like observed effects favour quark-like scenario quenching mechanism or change of quark/gluon composition caveat: jet p not equal parton p ALI-PREL-1168 ALI-PREL

29 Qualitiative discussion II jet quenching = jet p shift + vacuum fragmentation? if yes, would expect shapes to agree with vacuum shapes from higher p jets g agrees qualitatively with this picture, however pd does not ALI-DER-1213 ALI-DER

30 Summary strangeness production in jets in Pb-Pb collisions - significant difference between the Λ/K S ratio of inclusive particles and the ratio in charged jets measurement of jet shapes - characterise modifications of intra-jet momentum flow by QGP - results indicate that jet cores in Pb-Pb are narrower and harder and have fewer constituents than PYHIA pp reference - results in qualitative agreement with quark-like fragmentation and described by quenching models like JEWEL Oliver Busch LHC Seminar 5/216 3

31 - Backup - Oliver Busch LHC sukuba Seminar 2145/216 /3/13 28

32 LHC run 2 LHC run 2: , heavy-ion run November 215 increased CMS energy for Pb-Pb collisions from ev quenching strength ~ s ~ ε 3/4 expect (modest) increase in ε, measure energy density dependence of jet quenching ALICE, PRL 15, note: also a dependence on parton input spectrum (increased RAA??? ) Oliver Busch GSW 215 /9/3

33 ALICE in run 2: DCal run 2: DCal upgrade - significantly extended jet acceptance - back-to-back in azimuth (di-jet topology) Oliver Busch LHC GSW Seminar 2155/216 /9/3 3

34 Jet structure jet structure ratio R=.2 / R=.3 for charged jets sensitive to potential broadening of jet shape consistent with PYHIA pp: no modification observed within small radii (jet core) JHEP 3 (214) 13

35 Establish correspondence between detector measurements / final state particles / partons two types of jet finder: - iterative cone - sequential recombination (e.g. anti-k) resolution parameter R Jet reconstruction hep-ph/

36 Oliver Busch LHC Seminar 5/216