Jet Physics at the LHC

Transcription

1 Jet Physics at the LHC Oliver Busch University of Tsukuba Heidelberg University 1

2 pp jet cross sections and fragmentation pp identified jet fragmentation Nuclear modification factor Event plane dependence Strangeness in jets in Pb-Pb 2

3 pp jet cross sections and fragmentation pp identified jet fragmentation Nuclear modification factor Event plane dependence Strangeness in jets in Pb-Pb selection biased towards: light flavour jet results ALICE 3

4 Introduction 4

5 Jets: seeing quarks and gluons jet: collimated bunch of hadrons quasi-free parton scattering at high Q 2 : the best available experimental equivalent to quarks and gluons 5

6 Jet Fragmentation initial hard (high Q 2 ) scattering: high-pt, quasi-free partons Q 2 evolution: parton shower, gluon cascade hadronization at ~ΛQCD jets allow to study QCD at various scales 6

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

8 Jets at ALICE (LHC run 1) charged particle tracking: - Inner Tracking System (ITS) - Time Projection Chamber - full azimuth, η < 0.9 pt > 150 MeV/c ZDC ZDC EMCal : - neutral particles - Δφ = 107, η <0.7 cluster ET > 300 MeV jet trigger with EMCal and TRD charged (tracking) jets and full jets tracking jets: suitable especially under harsh pile-up conditions 8

9 Results from pp Collisions 9

10 pp charged jet cross sections measured in minimum bias collisions at = 7 TeV good agreement with ATLAS charged jet measurements (despite slightly different acceptance and track pt range) R = 0.4 R = 0.6 PRD 91 (2015)

11 Complementarity of LHC experiments ALICE: full jets from tracking + em. Calorimeter ATLAS/CMS: em. + hadronic calorimetry (+tracking) complementary jet pt reach, ALICE typically lower constituent pt cutoff ALICE, Phys. Lett. B 722 (2013) 262 CMS, PRD 87 (2012)

12 Charged jet cross sections: run 2 measured in minimum bias pp collisions at = 5 TeV important as reference for Pb-Pb collisions at same NN POWHEG+PYTHIA8 allows to compare charged jet cross section to NLO calculations 12

13 ATLAS jet cross section at 8 TeV ATLAS jet cross sections at = 8 TeV comparison to NLO + nonpert. corr. as well as POWEHG calculations compatible within each other, and consistent with data over wide range of measured jet pt ATLAS, hep-ex/ (2017) 13

14 pp jet fragmentation at 7 TeV distributions of charged particles in charged jets at = 7 TeV bulk production at low z: ~ 5-10 charged particles per jet for z > 0.2 distributions consistent for all jet pt: scaling ALICE, PRD 91,

15 z scaling in QCD MLLA: analytic parton shower calculation, NLL resummation hadronization not explicitly included low-ξ (approximate) scaling a feature of parton shower (with some caveats about validity as ξ 0) CDF, PRD 68 (2003)

16 PID in jets: TPC Coherent Fit particle identification via specific ionization in TPC ('de/dx'): TPC coherent fit: use energy loss model parameterization as input, adjust model parameters and particle fractions on the fly during fit regularization requiring continuity of particle fractions 16

17 Multi-Template Fit TPC multi-template fit - best possible description of de/dx from external reference - parametrize dependences on η, TPC nclusters - templates in transverse momentum (z, xi) slices de/dx in one z slice (0.6 < z < 0.65), GeV/c fitted with 4 templates complementary and consistent with TCF 34 17

18 Particle identified fragmentation identified charged hadrons in charged jets at = 7 TeV π, K, p, 5 < pt ch jet < 20 GeV/c z scaling for all species: no strong hadronization effects 18

19 Particle ratios in jets strangeness content strongly enhanced for z ch 1 leading baryons suppressed 19

20 Event generator comparison comparison to PYTHIA6 (pt ordered parton shower, Lund string fragmentation) data reasonably well described well reproduced by Perugia0 NoCR tune without color reconnections 20

21 Jets in Heavy-Ion Collisions 21

22 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 90, energy density from the lattice: rapid increase around TC, indicating increase of degrees of freedom (pion gas quarks and gluons) TC = 154 +/- 9 MeV EC = 340 +/- 45 MeV/fm 3 22

23 QCD matter at the LHC direct photons: prompt photons from hard scattering + thermal radiation from QCD matter low-pt inverse slope parameter: Teff = 297 ± 12 stat. ± 42 syst. MeV/c indicates initial temperature way above TC Phys. Lett. B 754 (2016)

24 Partons in heavy-ion collisions hard partons are produced early and traverse the hot and dense QGP expect enhanced parton energy loss : jet quenching (mostly) due to medium-induced gluon radiation vacuum expectation calculable by pqcd : calibrated probe of QGP jets sensitive to properties of the medium (energy density,, mean free path, coupling... )... but also jet-medium interaction not trivial (strong / weak coupling, parton mass / type, fireball dynamics...) JET collaboration, PRC 90,

25 Hadron nuclear modification factor high-pt hadron suppression hadron observable biased towards leading fragment study the effect for fully reconstructed jets theory: parton energy loss leading hadron fragmentation parton shower energy loss hadronisation PHENIX, Nucl Part. Phys 38 (2011) CMS, JHEP 04 (2017)

26 Underlying event jet reconstruction in heavy-ion collisions : difficult due to the high underlying event background not related to hard scattering correct spectra for background fluctuations and detector effects not possible down to lowest jet pt fake jets central jet area ~ 0.5 (R = 0.4) peripheral 26

27 Jet reconstruction in HI collisions jet reconstruction: EM + hadron calorimeter based (ATLAS) charged track + EMCal based (STAR + ALICE) particle flow (CMS) fake jet rejection: constituent pt cuts leading constituent bias / track jet matching di-jet (hadron-jet) coincidence subtraction. consistently applied to the reference, but may introduce physics bias background subtraction: median density from clusters (ALICE, STAR) iterative geometrical (ATLAS, CMS) corrections: for detector effects (efficiency, resolution) and background fluctuations (resolution-like) typical: full corrections to particle level sometimes: detector and/or background effects applied to a reference 27

28 Jet nuclear modification factor: ATLAS ATLAS jet RAA at snn = 2.76 TeV, R = 0.4 strong suppression observed, similar to hadron RAA parton energy not recovered inside jet cone stronger suppression for more central events weak pt dependence ATLAS, PRL 114, JEWEL and YaJEM jet quenching models reproduce suppression 28

29 Jet nuclear modification factor: ALICE ALICE full jet RAA at snn = 2.76 TeV, R = 0.2 Phys.Lett. B746 (2015) 1 pt const,ch > 150 MeV, E Cluster > 300 MeV, pt lead, ch > 5 GeV/c maybe hint for weak pt dependence JEWEL: PLB 735 (2014) YaJEM:PRC 88 (2013) JEWEL and YaJEM jet quenching models reproduce suppression 29

30 Jet nuclear modification factor: CMS CMS jet RAA at snn = 2.76 TeV, R = 0.2, 0.3, 0.4 constituent pt > 150 MeV/c, fake jet spectrum from background events subtracted no significant R dependence for pt > 70 GeV/c CMS, nucl-ex/

31 Cone size dependence R dependence sensitive to broadening of transverse jet profile ATLAS RCP double ratio: R dependence seen, in particular for low pt and high R not observed in ALICE (but small R difference) ATLAS, PLB 719 (2013) 220 ALICE, JHEP 03 (2014)

32 Rapidity dependence expect stronger energy loss for gluons than for quarks ATLAS: no significant rapidity dependence of RAA, despite change in q/g balanced by parton spectral slope, y-dependence of energy density? role of fluctuations / biases? ATLAS, PRL 114,

33 s dependence ALICE, HP 2016 compare snn = 2.76 TeV and 5 TeV constrains energy density dependence no significant difference increased energy loss compensated by flatter parton spectrum? ATLAS, QM

34 Jet Azimuthal Anisotropy 34

35 Reaction plane dependence different medium thickness in- and out-of plane sensitive to path length dependence of jet quenching: pqcd radiative E-loss : ~L 2 collisional E-loss : ~L strong coupling (ADS/CFT) : ~L 3 35

36 Local background subtraction ALICE jet v2: event plane from forward/backward V0 scintillators account for flow-modulation of background via event-by-event fit and subtraction of local background density unfolding to account for background fluctuations : separately for spectra in- and out-of-plane (ϕ) (GeV/c) Pb-Pb s NN Single event = 2.76 TeV ALICE ρ ch 100 Phys. Lett. B753 (2016) < p < 5 GeV/c, η < 0.9 T, track track ρ (ϕ) ch ρ 0 ρ (1+2v cos(2[ϕ-ψ ])) 0 2 EP, 2 ρ (1+2v cos(3[ϕ-ψ ])) 0 3 EP, ϕ (rad) \ 36

37 Charged jet v2: results quantify azimuthal asymmetry via 2 nd Fourier harmonic ch jet v2 central collisions: sigma from v2 ch jet = 0 consistent with 0, but maybe hint for effect of initial density fluctuations? non-zero v2 ch jet in semi-central collisions 0-5% 30-50% {EP, η >0.9 } ch jet v ch jet v 2 0-5%, Stat unc. Syst unc. (shape) Syst (correlated) Syst unc. (correlated) ALICE Pb-Pb s NN = 2.76 TeV R = 0.2 anti-k T, η <0.7 jet {EP, η >0.9 } ch jet v ch jet v %, Stat unc. Syst unc. (shape) Syst (correlated) Syst unc. (correlated) ALICE Pb-Pb s NN = 2.76 TeV R = 0.2 anti-k T, η <0.7 jet 0 (a) p > 0.15 GeV/c, p > 3 GeV/c T, track T, lead ch jet p (GeV/c) T Phys. Lett. B753 (2016) (b) p > 0.15 GeV/c, p > 3 GeV/c T, track T, lead ch jet p (GeV/c) T 37

38 Comparison to previous results ALICE + CMS single particles, ATLAS full jets: different energy scales! non-zero v2 up to high pt CMS, PRL 109 (2012) 022 ATLAS, PRL 111 (2013) 152 ALICE, Phys. Lett. B753 (2016) 511 ALICE, Phys. Lett. B719 (2013) 18 jet, v 2 part v ALICE Pb-Pb = 2.76 TeV s NN R = 0.2 anti-k T, η <0.7 jet ch jet v 2 0-5%, Stat unc. Syst unc. (shape) Syst unc. (correlated) calo jet 5-10% part ATLAS v 2 CMS v 2 { η >3} 0-10% part ALICE v 2 { η >2} 0-5% jet, v 2 part v ALICE Pb-Pb = 2.76 TeV s NN R = 0.2 anti-k T, η <0.7 jet ch jet v %, Stat unc. Syst unc. (shape) Syst unc. (correlated) ATLAS v 2 calo jet 30-50% part CMS v 2 { η >3} 30-50% part ALICE v 2 { η >2} 30-50% (a) p > 0.15 GeV/c, p > 3 GeV/c T, track T, lead part jet p, p (GeV/c) T T 0 (b) p > 0.15 GeV/c, p > 3 GeV/c T, track T, lead part jet p, p (GeV/c) T T 38

39 Comparison to JEWEL good agreement with JEWEL in semi-central collisions clear indication of path-length dependence of energy loss caveat: no transverse expansion in JEWEL ch jet v ch jet v 2 ch jet v 2 0-5%, JEWEL 0-5%, Stat unc. Syst unc. (shape) Syst unc. (correlated) ALICE Pb-Pb s NN = 2.76 TeV R = 0.2 anti-k T, η <0.7 jet ch jet v ch jet v 2 ch jet v %, JEWEL 30-50%, Stat unc. Syst unc. (shape) Syst unc. (correlated) ALICE Pb-Pb s NN = 2.76 TeV R = 0.2 anti-k T, η <0.7 jet (a) ch jet p (GeV/c) Phys. Lett. B753 (2016) 511 lead p > 0.15 GeV/c, p T, track T T > 3 GeV/c 0 (b) lead p > 0.15 GeV/c, p T, track T ch jet p (GeV/c) T > 3 GeV/c 39

40 Strangeness Production in Jets 40

41 Charged particle fragmentation ATLAS, CMS: enhancement at low z observed for unidentified charged particles in high-pt jets pt ch > 2 GeV/c (ATLAS), 1 GeV/c (CMS) ATLAS, PLB 739 (2014)

42 Strangeness production in nuclear collisions Inclusive strangeness production in Pb-Pb: Baryon / Meson ratio enhanced - collective effects? - parton recombination? - jet fragmentation? Phys. Rev. Lett. 111 (2013) measurement of identified particles in jets helps to constrain hadronisation and energy loss scenarios 42

43 Strangeness in jets neutral strange particles reconstructed via decay topology ( V 0 ): V 0 - jet matching signal extraction via invariant mass corrections for efficiency, feed-down, UE background + fluctuations 43

44 (Λ+Λ)/2K 0 s ratio in jets ratio in jets significantly lower than for inclusive hadrons compare Pb-Pb results to reference from p-pb collisions at 5.02 TeV: 2 agreement within uncertainties p-pb 0 Λ)/2K (Λ + S ALICE Preliminary Pb-Pb Pb Pb, s NN in jets, p in jets, p = 2.76 TeV, 0 10 % jet,ch T jet,ch T > 10 GeV/c > 20 GeV/c feed-down uncertainty 0 inclusive Λ/K, ALICE, S (0 5 %, y < 0.5) 0 V η 0 < 0.7 V anti-k t, R = 0.2 η < 0.5 jet,ch p T track T p leading track > 5 GeV/c > 150 MeV/c 0.2 ALI-PREL ALI PREL p (GeV/c) T 44

45 Strange particle spectra in jets spectra of K 0 S and Λ particles in jets: more differential observable to increase sensitivity to potentially modified fragmentation K 0 S spectra in jets follow similar slope as predicted by PYTHIA simulations Λ shape different? Excess? More reliable reference needed! (c/gev) ) dn/dp 2 1/(N jets πr T Pb Pb, = 2.76 TeV, 0 10 % s NN ALICE Preliminary p jet,ch T 0 Ks, stat. unc., (x 1.5) (Λ+Λ)/2, stat. unc. syst. unc. 0 s full markers K, (x 1.5) open markers (Λ+Λ)/2 PYTHIA 8 - tune Monash PYTHIA 6 - tune Perugia 2011 PYTHIA 6 - tune Perugia NoCR p jet T smeared with true σ (δp > 10 GeV/c jet T ) (c/gev) ) dn/dp 2 1/(N jets πr T Pb Pb, = 2.76 TeV, 0 10 % s NN ALICE Preliminary p jet,ch T 0 Ks, stat. unc., (x 1.5) (Λ+Λ)/2, stat. unc. syst. unc. 0 s full markers K, (x 1.5) open markers (Λ+Λ)/2 PYTHIA 8 - tune Monash PYTHIA 6 - tune Perugia 2011 PYTHIA 6 - tune Perugia NoCR p jet T smeared with true σ (δp > 20 GeV/c jet T ) ALI PREL η V 0 < 0.7 anti-k t, R = 0.2 η < 0.5 p T track T p jet,ch leading track > 5 GeV/c > 150 MeV/c p (GeV/c T ) ALI PREL η V 0 < 0.7 anti-k t, R = 0.2 η < 0.5 p T track T p jet,ch leading track > 5 GeV/c > 150 MeV/c p (GeV/c T ) 45

46 Summary jet cross sections in pp particle identified fragmentation LHC inclusive jet RAA reaction plane dependence of jet production strange particles in jets 46

47 - Backup - 47

48 Semi-Inclusive Hadron-Jet Distributions 48

49 Hadron triggered recoil jets charged jets recoiling from charged hadron hadron biased towards surface Δrecoil: difference between hadron trigger pt classes further fake jet removal ALICE, JHEP 09 (2015)

50 ΔIAA Δrecoil divided by PYTHIA reference: significant suppression observed subtraction technique allows for large R up to 0.5 with constituent pt > GeV/c, no leading constituent bias ALICE, JHEP 09 (2015)

51 ΔIAA: R dependence R dependence as expected for vacuum fragmentation (PYTHIA) no medium-induced broadening observed for recoil jets ALICE, JHEP 09 (2015)

52 Medium-induced acoplanarity? Δφ hadron-jet: potentially sensitive to large-angle scattering data compared to embedded PYTHIA reference no significant effect within present uncertainties ALICE, JHEP 09 (2015)

53 STAR charged jets, mixed event background subtraction ICP(central-peripheral): jet suppression observed Recoil jets at RHIC estimate E-loss through spectral shift ΔE : energy transported out-of-cone smaller at RHIC than LHC STAR, nucl-ex/

54 Recoil jets at RHIC: R dependence no evidence of broadening STAR, nucl-ex/

55 full jets, pp at 2.76 TeV JES uncertainty ~ 3.6% at pt jet = 100 GeV/c ALICE jet response Phys. Lett. B 722 (2013) 262 JHEP 03 (2014) 013 charged jets: Pb-Pb JE resolution at low pt dominated by background, at high pt by detector effects 55

56 q/g fraction Spousta, Cole, hep-ph/

57 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? Phys. Rev. C 93, ALICE 0-5% Pb-Pb - π + +π + - K +K p + p Charged s NN =2.76 TeV 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 p (GeV/c) T note: medium effects likely strongest at scales of ~ medium Temperature (J.G. Milhano, K. C. Zapp, hep-ph/ , T. Renk, Phys. Rev. C 81, , B. Mueller, hep-ph/ ) 57

58 Underlying event subtraction subtract underlying event contribution to K 0 S, Λ spectra in jets various methods with different sensitivity to acceptance, event plane correlations, presence of additional jets, apply a correction to account for background density fluctuations 58

59 Event generator comparison low-pt jets rather well described by PYTHIA (pt ordered parton shower, Lund string fragmentation) Perugia2010NoCR OK, no need to evoke MPI coherence 5 GeV/c hadron clusters look jetty and can be described by single hard scattering + fragmentation, however non-perturbative effects are important (even at high z?!) 59

60 z scaling: low-pt jets onset from about 10 GeV/c charged jet pt deviations for 5-10 GeV/c jets low-pt jets and their properties important in the context of pp UE studies (high-multiplicity pp) 60