Properties of the QGP with hard probes Oliver Busch for the ALICE - PowerPoint PPT Presentation

Properties of the QGP with hard probes Oliver Busch for the ALICE collaboration 1 Oliver Busch – TGSW 2016 /09/17

Outline • introduction • jet azimuthal anisotropy • jet shapes 2 Oliver Busch – TGSW 2016 /09/17

Introduction 3 Oliver Busch – TGSW 2016 /09/17

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 4 Oliver Busch – TGSW 2016 /09/17

Jet fragmentation • initial hard scattering: high-p T partons • cascade of (anti-)quarks and gluons: parton shower • at soft scale (O( Λ QCD )): hadronization 5 Oliver Busch – TGSW 2016 /09/17

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, HotQCD, PRD 90, 094503 chirally symmetric matter • energy density from the lattice: rapid increase around T C , indicating increase of degrees of freedom (pion gas -> quarks and gluons) • T C = 154 +/- 9 MeV ε C = 340 +/- 45 MeV/fm 3

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, arXiv: 1312.5003 7

LHC aerial view 8

Jets at ALICE (LHC run 1) • charged particle tracking: - Inner Tracking System (ITS) - Time Projection Chamber - full azimuth, | η |< 0.9 p T > 150 MeV/c • EMCal : - neutral particles - Δφ = 107°, | η |<0.7 cluster E T > 300 MeV • jet trigger with EMCal and TRD • ‘charged’ (tracking) jets and ‘full’ jets • full jets from charged particle tracking and EM energy: conceptually different and complementary to traditional approach 9

Underlying event in heavy-ion collisions • 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 via unfolding • not possible down to lowest jet p T jet area ~ 0.5 (R = 0.4) central peripheral

Jet nuclear modification factor • strong suppression observed, similar to hadron RAA → parton energy not recovered inside jet cone Phys.Lett. B746 (2015) 1 • increase of suppression JEWEL: PLB 735 (2014) with centrality YaJEM:PRC 88 (2013) 014905 • weak p T dependence • JEWEL and YaJEM jet quenching models reproduce suppression

Jet azimuthal anisotropy Oliver Busch – TGSW 2016 /09/17 12

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 Oliver Busch – TGSW 2016 /09/17 13

Jet v 2 : results • quantify azimuthal asymmetry via 2 nd Fourier harmonic v 2ch jet • central collisions: 1.5 - 2 sigma from v 2ch jet = 0 → consistent with 0, but maybe hint for effect of initial density fluctuations ? • non-zero v 2ch jet in semi-central collisions |>0.9 } |>0.9 } ALICE ALICE ch jet ch jet v 30-50%, Stat unc. v 0-5%, Stat unc. 2 2 0.2 0.2 Pb-Pb s = 2.76 TeV Pb-Pb s = 2.76 TeV Syst unc. (shape) Syst unc. (shape) NN NN η η Syst (correlated) = 0.2 anti- , | |<0.7 R = 0.2 anti- k , | |<0.7 R k η ∆ Syst (correlated) η ∆ T T jet jet {EP, | {EP, | Syst unc. (correlated) Syst unc. (correlated) ch jet 0.1 ch jet 0.1 2 2 v v 0 0 (a) (b) p > 0.15 GeV/ c , p > 3 GeV/ c p > 0.15 GeV/ c , p > 3 GeV/ c T, track T, lead T, track T, lead 20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100 ch jet ch jet p (GeV/ c ) p (GeV/ c ) T T Phys. Lett. B753 (2016) 511 Oliver Busch – TGSW 2016 /09/17 14

Comparison to previous results • ALICE + CMS single particles, ATLAS full jets : different energy scales ! • non-zero v2 up to high p T CMS, PRL 109 (2012) 022 ATLAS, PRL 111 (2013) 152 ALICE, Phys. Lett. B753 (2016) 511 ALICE, Phys. Lett. B719 (2013) 18 jet jet 2 ALICE 2 ALICE ch jet v v 0-5%, Stat unc. ch jet 0.3 v 30-50%, Stat unc. v 2 0.3 Pb-Pb s = 2.76 TeV , 2 Pb-Pb s = 2.76 TeV , NN Syst unc. (shape) NN part Syst unc. (shape) R = 0.2 anti- k , | η |<0.7 part R = 0.2 anti- k , | |<0.7 η T jet T Syst unc. (correlated) jet Syst unc. (correlated) 2 2 v calo jet ATLAS 5-10% v calo jet v ATLAS v 30-50% 2 2 0.2 0.2 part part CMS v {| ∆ η |>3} 0-10% CMS v {| |>3} 30-50% ∆ η 2 2 part part ALICE v {| ∆ η |>2} 0-5% ALICE v {| ∆ η |>2} 30-50% 2 2 0.1 0.1 0 0 p > 0.15 GeV/ c , p > 3 GeV/ c p > 0.15 GeV/ c , p > 3 GeV/ c (b) (a) T, track T, lead T, track T, lead 0 50 100 150 0 50 100 150 part jet , (GeV/ ) part jet p p c p , p (GeV/ c ) T T T T Oliver Busch – TGSW 2016 /09/17 15

Comparison to JEWEL • in semi-central collisions, good agreement with JEWEL (collisional + ‘pQCD’ radiative energy loss) • clear indication of path-length dependence of energy loss ch jet ch jet ALICE ch jet ALICE v 30-50%, JEWEL ch jet v 0-5%, JEWEL 2 2 2 2 0.2 0.2 ch jet v ch jet v v 30-50%, Stat unc. Pb-Pb s = 2.76 TeV v 0-5%, Stat unc. Pb-Pb s = 2.76 TeV 2 NN 2 NN R = 0.2 anti- k , | |<0.7 R = 0.2 anti- k , | |<0.7 η Syst unc. (shape) η T Syst unc. (shape) T jet jet Syst unc. (correlated) Syst unc. (correlated) 0.1 0.1 0 0 (a) (b) lead p > 0.15 GeV/ c , p > 3 GeV/ c lead p > 0.15 GeV/ c , p > 3 GeV/ c T, track T T, track T 20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100 ch jet p (GeV/ c ) ch jet p (GeV/ c ) T T Phys. Lett. B753 (2016) 511 Oliver Busch – TGSW 2016 /09/17 16

Jet Shapes Oliver Busch – TGSW 2016 /09/17 17

Jet shapes • radial moment ‘girth’ g, longitudinal dispersion p T D, difference leading - subleading p T LeSub • shapes in Pb-Pb as probe of quenching of low-p T 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 – TGSW 2016 /09/17 18

Jet shapes in Pb-Pb • fully corrected to charged particle level • compare to PYTHIA reference, validated with results from pp collisions at 7 TeV → • g shifted to smaller values indicates more collimated jet core 30 g ALICE Preliminary dN/d Pb-Pb s = 2.76 TeV NN 25 Anti- k charged jets, R = 0.2 T ALICE Data jet,ch jets 40 < p < 60 GeV/ c Shape uncertainty T 20 1/N Correlated uncertainty PYTHIA Perugia 11 15 10 5 0 0 0.02 0.04 0.06 0.08 0.1 0.12 g ALI − PREL − 101580 Oliver Busch – TGSW 2016 /09/17 19

• larger p T D in Pb-Pb compared to PYTHIA → indicates fewer constituents in quenched jets • LeSub in Pb-Pb in good agreement with Pb-Pb: → hardest splittings likely unaffected 0.2 6 /GeV) D ALICE Preliminary ALICE Preliminary T 0.18 Pb-Pb s = 2.76 TeV p Pb-Pb s = 2.76 TeV NN NN dN/d 5 Anti- charged jets, = 0.2 Anti- k charged jets, R = 0.2 k R 0.16 T T ALICE Data c jet,ch jet,ch 40 < p < 60 GeV/ c 40 < p <60 GeV/ c ( 0.14 Shape uncertainty T T 4 LeSub ALICE Data jets Correlated uncertainty 0.12 Shape uncertainty PYTHIA Perugia 11 1/N 3 0.1 Correlated uncertainty PYTHIA Perugia 11 dN/d 0.08 2 0.06 jets 0.04 1 1/N 0.02 0 0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 p D LeSub (GeV/ c ) T ALI − PREL − 101584 ALI − PREL − 101588 Oliver Busch – TGSW 2016 /09/17 20

Jet shapes: 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 1303 (2013) 080 Oliver Busch – TGSW 2016 /09/17 21

Summary • hard probes allow to probe properties of the QGP • first insights on dynamics parton of energy loss from jet nuclear suppression factor and jet shape measurements • non-zero jet v 2 indicates path-length dependence of jet quenching • run2: extended calorimetry allows to assess new observables 22 Oliver Busch – TGSW 2016 /09/17

- Backup - 23 Oliver Busch – TGSW 2016 /09/17

Jet reconstruction • Establish correspondence between detector measurements / final state particles / partons • two types of jet finder: - iterative cone - sequential recombination (e.g. anti-k T ) • resolution parameter R hep-ph/0802.1189 24

QCD matter at LHC • direct photons: prompt photons from hard scattering + thermal radiation from QCD matter • low-p T inverse slope parameter: T eff = 297 +/- 12 stat. +/- 42 syst. MeV/c • indicates initial temperature way above T C arXiv 1509.07324 [nucl-ex] 25 Oliver Busch – TGSW 2016 /09/17