Quenching at RHIC and LHC

Oliver Busch

University of Tsukuba Heidelberg University

1 • Nuclear modification factor

• Event plane dependence

• Semi-inclusive jet- yields

• Jet shapes, subjets, jet mass

• Identified jet fragmentation

2 • Nuclear modification factor

• Event plane dependence

• Semi-inclusive jet-hadron yields

• Jet shapes, subjets, jet mass

• Identified jet fragmentation

selection strongly biased towards: • light flavour jet results • LHC • ALICE

3 Hadron nuclear modification factor

• high-pT hadron suppression both at RHIC and LHC

• hadron observable biased towards leading fragment → study the effect for fully reconstructed jets • theory: energy loss ⊗ leading hadron fragmentation → parton shower ⊗ energy loss ⊗ hadronisation

PHENIX, Nucl Part. Phys 38 (2011) 124016 CMS, JHEP 04 (2017) 039

4 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

jet area ~ 0.5 (R = 0.4) central peripheral 5 Experimental aspects / differences

• 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 6 complementarity of LHC experiments

• ALICE: ‘charged’ jets from charged particle tracking + ‘full’ jets from tracking + em. Calorimeter • ATLAS/CMS: em. + hadronic calorimetry (+tracking)

• complementary jet pT reach, ALICE typically lower constituent pT cutoff CMS, PRD 87 (2012) 112002 ALICE, Phys. Lett. B 722 (2013) 262

7 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, 072302

• JEWEL and YaJEM jet quenching models reproduce suppression

Jet nuclear modification factor: ALICE

• ALICE full jet RAA at √sNN = 2.76 TeV, R = 0.2 Phys.Lett. B746 (2015) 1 const,ch Cluster lead, ch • pT > 150 MeV, E > 300 MeV, pT > 5 GeV/c JEWEL: PLB 735 (2014) • maybe hint for weak pT dependence YaJEM:PRC 88 (2013) 014905

• JEWEL and YaJEM jet quenching models reproduce suppression

9 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/1609.05383 10 Angular 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) 013 11 Rapidity dependence

• expect stronger energy loss for than for

• 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, 072302

12 √s depencence ALICE, HP 2016 • comparison of different √s constrains energy density dependence • no significant difference • increased energy loss compensated by flatter parton spectrum ?

ATLAS, QM 2017

13 Jet Azimuthal Anisotropy

14 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

) 200 background fluctuations : c Pb-Pb sNN = 2.76 TeV ALICE separately for spectra Single event in- and out-of-plane 150 ) (GeV/ ϕ ( ch ρ 100

50 0.15 < p < 5 GeV/c, |η | < 0.9 T, track track ρ (ϕ) ρ (1+2v cos(2[ϕ-Ψ ])) ch 0 2 EP, 2 ρ ρ (1+2v cos(3[ϕ-Ψ ])) 0 0 0 3 EP, 3 0123456 ϕ (rad) Phys. Lett. B753 (2016) 511

\ 15 Charged jet v2: results

nd ch jet • quantify azimuthal asymmetry via 2 Fourier harmonic v2

ch jet • central collisions: 1.5 - 2 sigma from v2 = 0 → consistent with 0, but maybe hint for effect of initial density fluctuations ?

ch jet • non-zero v2 in semi-central collisions 0-5% 30-50%

ALICE ch jet ALICE v ch jet 0-5%, Stat unc. v 30-50%, Stat unc. 0.2 2 0.2 2 |>0.9 } |>0.9 } Pb-Pb s = 2.76 TeV Pb-Pb s = 2.76 TeV Syst unc. (shape) NN Syst unc. (shape) NN η η Syst (correlated) R = 0.2 anti-k , |η |<0.7 R = 0.2 anti-k , |η |<0.7 ∆ ∆ Syst (correlated) T T jet Syst unc. (correlated) jet Syst unc. (correlated) {EP, | {EP, | 0.1 0.1 ch jet 2 ch jet 2 v v

0 0 (a) p > 0.15 GeV/c, p > 3 GeV/c (b) 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 pch jet (GeV/c) pch jet (GeV/c) T T Phys. Lett. B753 (2016) 511 16 Comparison to previous results

• ALICE + CMS single particles, ATLAS full jets : different energy scales !

CMS, PRL 109 (2012) 022 • non-zero v2 up to high pT ATLAS, PRL 111 (2013) 152 ALICE, Phys. Lett. B753 (2016) 511 ALICE, Phys. Lett. B719 (2013) 18 jet 2

ALICE ch jet jet 2 ALICE ch jet v v2 0-5%, Stat unc. v v 0.3 Pb-Pb s = 2.76 TeV 2 30-50%, Stat unc. , NN 0.3 Pb-Pb s = 2.76 TeV Syst unc. (shape) , NN Syst unc. (shape) R = 0.2 anti-k T, |η |<0.7 R = 0.2 anti-k , |η |<0.7 jet Syst unc. (correlated) T jet part 2 Syst unc. (correlated) part 2 calo jet calo jet v

ATLAS v 5-10% v 2 ATLAS v 2 30-50% part 0.2 CMS v {|∆η|>3} 0-10% 0.2 part 2 CMS v 2 {|∆η|>3} 30-50% part part ALICE v 2 {|∆η|>2} 0-5% ALICE v 2 {|∆η|>2} 30-50%

0.1 0.1

0 0 p > 0.15 GeV/c, p > 3 GeV/c p > 0.15 GeV/c, p > 3 GeV/c (a) T, track T, lead (b) T, track T, lead 0 50 100 150 0 50 100 150 part jet p part, p jet (GeV/c) p , p (GeV/c) T T T T

17 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 ch jet v 30-50%, JEWEL ALICE v 0-5%, JEWEL ALICE 2

ch jet 2 2 ch jet 2 0.2 ch jet v ch jet v 30-50%, Stat unc. 0.2 v 2 Pb-Pb sNN = 2.76 TeV v 2 0-5%, Stat unc. Pb-Pb sNN = 2.76 TeV R = 0.2 anti-k , |η |<0.7 Syst unc. (shape) R = 0.2 anti-k T, |η |<0.7 Syst unc. (shape) T jet jet Syst unc. (correlated) Syst unc. (correlated)

0.1 0.1

0 0 lead (a) p > 0.15 GeV/c, p > 3 GeV/c lead T, track T (b) p > 0.15 GeV/c, p > 3 GeV/c T, track T 20 30 40 50 60 70 80 90 100 ch jet 20 30 40 50 60 70 80 90 100 p (GeV/c) ch jet T p (GeV/c) T

Phys. Lett. B753 (2016) 511

18 Semi-Inclusive Hadron-Jet Distributions

19 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) 170

20 ΔIAA

• Δrecoil divided by PYTHIA reference: significant suppression observed

• subtraction technique allows for large R up to 0.5 with constituent pT > 0.150 GeV/c, no leading constituent bias

ALICE, JHEP 09 (2015) 170

21 ΔIAA: R dependence

• R dependence as expected for vacuum fragmentation (PYTHIA)

• no medium-induced broadening observed for recoil jets

ALICE, JHEP 09 (2015) 170 22 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) 170

23 Recoil jets at RHIC

• STAR charged jets, mixed event background subtraction

• ICP(central-peripheral): jet suppression observed

• estimate E-loss through spectral ‘shift’ ΔE : energy transported out-of-cone smaller at RHIC than LHC

STAR, nucl-ex/1702.01108 24 Recoil jets at RHIC: R dependence

• no evidence of broadening

STAR, nucl-ex/1702.01108 25 Intrajet Observables and Subjets

26 Jet shapes

• radial moment ‘girth’ g, longitudinal dispersion pTD, difference leading - subleading pT LeSub

• shapes in pp collisions at 7 TeV: - constrain QCD calculations of small-R jets (‘microjets’: M. Dasgupta, F. Dreyer, G. Salam, G. Soyez hep-ph/1602.01110)

- validate MC simulations

• shapes in Pb-Pb probe of quenching of low-pT jets: characterise fragment distributions and are sensitive to medium induced changes of intra-jet momentum flow

• ‘event-by-event’ measure, sensitive to fluctuations

• infrared (& collinear) safe

27 Jet shapes in Pb-Pb

jet • R=0.2, 40 < pT < 60 GeV/c, no leading constituent cut, fully corrected to charged particle level

• g shifted to smaller values compared to PYTHIA reference → indicates more collimated jet core

30 g ALICE Preliminary

Pb-Pb sNN = 2.76 TeV 25 Anti-k T charged jets, R = 0.2

dN/d ALICE Data 40 < pjet,ch < 60 GeV/c T Shape uncertainty jets 20 Correlated uncertainty PYTHIA Perugia 11 1/N 15

10

5

0 0 0.02 0.04 0.06 0.08 0.1 0.12 g ALI−PREL−101580

28 • larger pTD in Pb-Pb compared to PYTHIA → indicates fewer constituents in quenched jets (or ‘less democratic’ splitting)

• LeSub in Pb-Pb in good agreement with Pb-Pb: → hardest splittings likely unaffected

6 0.2

D ALICE Preliminary ALICE Preliminary T Pb-Pb s = 2.76 TeV 0.18 Pb-Pb s = 2.76 TeV p NN NN 5 Anti-k charged jets, R = 0.2 Anti-k T charged jets, R = 0.2 T /GeV) 0.16 ALICE Data 40 < pjet,ch <60 GeV/c c 40 < pjet,ch < 60 GeV/c T Shape uncertainty ( 0.14 T dN/d 4 ALICE Data Correlated uncertainty 0.12 jets PYTHIA Perugia 11 Shape uncertainty 3 0.1 Correlated uncertainty 1/N LeSub 0.08 PYTHIA Perugia 11 2 0.06 dN/d 0.04

1 jets 0.02

0 1/N 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) ALI−PREL−101584 T ALI−PREL−101588

29 Model comparison

• trends reproduced by JEWEL → indicates collimation through emission to large angles

• ‘recoils off’: no medium recoils partons in final state (note R=0.2)

30 Subjets at LHC • declustering and soft drop grooming to identify hard jet substructure

• subjet momentum balance

• in vacuum, dσ/dzg ~ splitting function

• CMS: strongest suppression for jet lower pTj at high zg

CMS PAS HIN 16-006 31 Subjets in STAR • select dijet pairs matching to ‘hard core’ jets reconstructed with high const constituent cut pT > 2 GeV/c

• no suppression observed

• role of different kinematics, STAR selection bias, subjet ΔR cut ?

STAR, HP 2016 32 ALICE subjets • charged jets, kt declustering

• subjettyness ͳN: how consistent is a jet with having N subjets

• ͳ2/ ͳ1 : no significant modification

ALICE, QM 2017 33 ALICE subjet ΔR • subjet distance ΔR

• data uncorrected for det. effects and background fluctuations compared to PYTHIA embedded reference

• no significant modification observed relative to reference, full correction to particle level in progress

ALICE, QM 2017

34 Jet Mass

35 36 Mass and virtuality

• invariant mass of jet constituents, related to virtuality of initial parton

• parton from hard scattering produced off-shell

• in vacuum: virtuality decreases at each emission

• in medium, virtuality can rise due to scatterings

→ quenching observable (A. Majumder, J. Putschke, nucl-th 1408.3404)

• soft constituents far from jet axis within cone → larger mass

• few hard constituents → smaller mass 37 Results: Pb-Pb

• jet Mass in Pb-Pb collisions at √sNN = 2.76 TeV fully corrected for detector effects and background fluctuations via 2D unfolding

• small √s dependence is expected ( / composition)

• compare the ratio Pb-Pb / pPb to the ratio in PYTHIA at the 2 energies

ALICE, nucl-ex / 1702.00804 38 Ratio Pb-Pb / p-Pb

• slope indicates that Pb-Pb distribution is shifted towards smaller masses with respect to pPb

• overall small modification - cancellations ?

ALICE, nucl-ex / 1702.00804

Oliver Busch – HI café, Tokyo 2017/01 39 Model comparison • data lies in between PYTHIA and JEWEL with ‘recoils off’ • Q-PYTHIA and JEWEL with ‘recoils on’ produce too large mass

ALICE, nucl-ex / 1702.00804

• Q-PYTHIA: radiative energy loss modelled by enhanced splitting functions (N. Armesto, L. Cunqueiro, C. A. Salgado, hep-ph/0907.1014)

Oliver Busch – HI café, Tokyo 2017/01 40 Strangeness Production in Jets

41 Charged particle fragmentation

• ATLAS, CMS: enhancement at low z observed for unidentified charged particles in high-pT jets

ch • pT > 2 GeV/c (ATLAS), 1 GeV/c (CMS)

ATLAS, PLB 739 (2014) 320

42 Strangeness production in nuclear collisions

• Inclusive strangeness production in Pb-Pb: Baryon / Meson ratio enhanced - collective effects ? Phys. Rev. Lett. 111 (2013) 223001 - parton recombination ? - jet fragmentation ?

• measurement of identified particles in jets helps to constrain hadronisation and energy loss scenarios 43 Strangeness in jets

• neutral strange particles reconstructed via decay topology (‘V0’):

• V0 - jet matching

• signal extraction via invariant mass

• corrections for efficiency, feed-down, UE background + fluctuations 44 0 (Λ+Λ)/2K s ratio in jets

• ratio in jets significantly lower than for inclusive

• compare Pb-Pb results to reference from p-Pb collisions at 5.02 TeV: Pb-Pb agreement within uncertainties 0 S 2 ALICE Pb−Pb, s = 2.76 TeV, 0−10 % )/2K NN

Λ 1.8

Preliminary jet,ch

+ in jets, p > 10 GeV/c

T

Λ jet,ch ( 1.6 in jets, p > 20 GeV/c T feed-down uncertainty 1.4 p-Pb 0 inclusive Λ/KS, ALICE,

1.2 (0−5 %, |y 0| < 0.5) V 1 |η 0| < 0.7 V anti-k t, R = 0.2 0.8 |η | < 0.5 jet,ch pleading track > 5 GeV/c 0.6 T ptrack > 150 MeV/c 0.4 T

0.2

0 0 2 4 6 8 10 12 p (GeV/c) T ALI−PREL−93799 ALI-PREL-80712

45 Strange particle spectra in jets

0 • spectra of K S and Λ particles in jets: more differential observable to increase sensitivity to potentially modified fragmentation

0 • K S spectra in jets follow similar slope as predicted by PYTHIA simulations

• Λ shape different ? More reliable reference needed !

0 0 Pb−Pb, s = 2.76 TeV, 0−10 % Ks, stat. unc., (x 1.5) Pb−Pb, s = 2.76 TeV, 0−10 % Ks, stat. unc., (x 1.5)

/GeV) NN /GeV) NN

c (Λ+Λ)/2, stat. unc. c (Λ+Λ)/2, stat. unc. ( (

T syst. unc. T syst. unc. 1 ALICE Preliminary 0 1 ALICE Preliminary 0 p p full markers Ks, (x 1.5) full markers Ks, (x 1.5) /d open markers (Λ+Λ)/2 /d open markers (Λ+Λ)/2 N PYTHIA 8 - tune Monash N PYTHIA 8 - tune Monash ) d ) d

2 PYTHIA 6 - tune Perugia 2011 2 PYTHIA 6 - tune Perugia 2011

R PYTHIA 6 - tune Perugia NoCR R PYTHIA 6 - tune Perugia NoCR

π jet π jet pjet smeared with true σ (δp ) pjet smeared with true σ (δp ) T T T T jets jets N pjet,ch > 10 GeV/c N pjet,ch > 20 GeV/c

1/( T 1/( T 10−1 10−1

−2 −2 10 |η | < 0.7 10 |η | < 0.7 V0 V0 anti-k t, R = 0.2 anti-k t, R = 0.2 |η | < 0.5 |η | < 0.5 jet,ch jet,ch pleading track > 5 GeV/c pleading track > 5 GeV/c T T ptrack > 150 MeV/c ptrack > 150 MeV/c T T 10−3 10−3 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 10 p (GeV/c ) p (GeV/c ) T T ALI−PREL−112798 ALI−PREL−112802 46 Summary

• LHC inclusive jet RAA and recoil jet measurements at RHIC and LHC

• subjet momentum balance at LHC and RHIC

• jet shapes

• jet mass

• strange particles in jets

47 - Backup -

48 ALICE jet response Phys. Lett. B 722 (2013) 262 • full jets, pp at 2.76 TeV • JES uncertainty ~ 3.6%

jet at pT = 100 GeV/c

JHEP 03 (2014) 013

• charged jets: Pb-Pb

• JE resolution at low pT dominated

by background, at high pT by detector effects 49 Jet Structure : Model Comparison

• trends reproduced by JEWEL jet quenching model

JEWEL: K.C. Zapp, F. Kraus, U.A. Wiedemann, JHEP 1303 (2013) 080

2750 q/g fraction

Spousta, Cole, hep-ph/1504.06169

51 jet mass: pPb

• jet mass in pPb collisions at √sNN = 5.02 TeV, charged jets with R=0.4

• overall well described by PYTHIA with some tension in the tails

60 < p (GeV/c) < 80 80 < p (GeV/c) < 100 100 < p (GeV/c) < 120 T, ch jet T, ch jet T, ch jet /GeV) /GeV) /GeV) 2 2 2

c c c p-Pb s = 5.02 TeV 0.2 ALICE Preliminary 0.2 0.2 NN ( ( ( ch jet ch jet ch jet N N N Systematic p-Pb d d d M M M d d d

Anti-k T, R = 0.4 PYTHIA Perugia 2011 jets jets jets 1 1 1 N N N 0.1 0.1 0.1

0 0 0 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 24 M (GeV/c2) M (GeV/c2) M (GeV/c2) ALI−PREL−113418 ch jet ch jet ch jet

4 4 4 60 < p (GeV/c) < 80 80 < p (GeV/c) < 100 100 < p (GeV/c) < 120 T, ch jet T, ch jet T, ch jet ALICE Preliminary 3 3 3 p-Pb / PYTHIA(5.02TeV) PYTHIA Perugia 2011 p-Pb s = 5.02 TeV Data/PYTHIA Data/PYTHIA NN Data/PYTHIA (Sys p-Pb) / PYTHIA(5.02TeV)

Anti-k T, R = 0.4 2 2 2

1 1 1

0 0 0 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 20 22 0 2 4 6 8 10 12 14 16 18 20 22 24 M (GeV/c2) M (GeV/c2) M (GeV/c2) ALI−DER−113422 ch jet ch jet ch jet 52 Identified hadrons in heavy-ion collisions

• baryons / meson RAA a probe of AA R Phys. Rev. C 93, 034913 gluon / quark energy loss? 1

ALICE 0-5% Pb-Pb sNN=2.76 TeV π++π- 0.8 - • would expect stronger radiative K++K p + p energy loss for gluons than for quarks Charged - subtle cancellations? 0.6

- hadron observable biased 0.4 towards hard fragmentation? 0.2

study jets to improve our 0 2 4 6 8 10 12 14 16 18 20 • ALI−DER−86149 p (GeV/c) understanding of parton energy loss: T - 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 Temperature (J.G. Milhano, K. C. Zapp, hep-ph/1512.0819, T. Renk, Phys. Rev. C 81, 014906, B. Mueller, hep-ph/1010.4258) 53 generalized angularities

54 Underlying event subtraction

0 • 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, …

• apply a correction to account for background density fluctuations

55