Heavy ion physics · 2018. 11. 22. · Pb+Pb 2.76 A TeV LHC ALICE VISHNU p K p Song, Bass & Heinz,...

Introduction Hydrodynamic behaviour Jet quenching Small systems Conclusions

Heavy ion physics

Korinna Zapp

LIP (Lisbon) & CERN

1st Workshop on High Energy Theory and GenderCERN 26.�28. 09. 2018

Korinna Zapp (LIP & CERN) Heavy ion physics 26. 09. 2018 1 / 22


A heavy ion collision

I ∼ 1600 primary charged hadrons per unit rapidity

I p+p at comparable√s: dNch/dη ' 4− 5



Heavy ion collisions: bird's eye view

I heavy ion collisions create strongly interacting system of high density

I Bjorken's energy density estimate:

ε0 '1

πR2τ0

dE⊥dη

∣∣∣∣η=0

' 25GeV/fm3

Bjorken, Phys. Rev. D 27 (1983) 140

ALICE, Phys. Rev. C 94 (2016) 034903

I there has to be re-scattering in the �nal state

I scattering drives a system towards thermal equilibrium

I How and to what extent does the �nal state in heavy ion collisionsequilibrate?

I What are �material properties� of hot and dense QCD matter?



Three main discoveries in heavy ion physics

1. hydrodynamic behaviour

2. jet quenching

3. similarity between p+p and A+A



Discovery # 1: Hydrodynamic behaviour

Soft particles in A+A collisions behave like an almost perfect �uid.soft = low transverse momentum

event displays from G. Roland, CMS

I anisotropy due to di�erent pressure gradients

→ collective �ow



Collective �ow

O'Hara et al.,

Science 298 (2002) 2179



Collective �ow

O'Hara et al.,

Science 298 (2002) 2179

0

0.1

0.2

v2

π

Kp

0 1 2

pT(GeV)

0

0.1

0.2

v2

0 1 2

pT(GeV)

p

kp

10-20% 20-30%

40-50% 50-60%

Pb+Pb 2.76 A TeV

LHC

ALICE VISHNU

π

Kp

Song, Bass & Heinz, Phys. Rev. C 89 (2014) no.3, 034919

v2 = 〈cos(2φ)〉I at large coupling hydrodynamics correct description

I mass ordering due to common velocity



Shear viscosity and black holes

I e�ciency of v2 generationsensitive to shear viscosity η

I heavy ion collisions leastdissipative system known

Romatschke & Romatschke, Phys. Rev. Lett. 99 (2007) 172301

I conjectured lower bound form obtained with AdS/CFT techniques

η

s≥ 1

4π

saturated by �eld theories with gravity duals

Kovtun, Son, Starinets, Phys. Rev. Lett. 94 (2005) 111601



Higher harmonics

no odd harmonics �uctuations generate odd harmonicsAlver, Roland, Phys. Rev. C 81 (2010) 054905



Higher harmonics

no odd harmonics �uctuations generate odd harmonicsAlver, Roland, Phys. Rev. C 81 (2010) 054905

I very central collisions

I v2 small

ALICE, Phys. Lett. B 708 (2012) 249



Discovery # 2: Jet quenching

Jet quenching: Hard jets are suppressed and their structure modi�ed.hard = high transverse momentum

I proton-proton collisions: 2 jets with balancing transverse momentum

I in heavy ion collisions: signi�cant softening of jets

→ thermalisation of a far-from-equilibrium system

→ jet quenching informs us about equilibration in QCD



What happens to jets in medium?

Scenario I: hard partons don't resolve quasi-particles

I interactions between jet & medium at large coupling

I AdS/CFT techniques

Scenario II: hard partons do resolve quasi-particles

I jet � medium interactions at weak(ish) coupling

I perturbative techniques

I thermalisation through elastic re-scattering (slow)

I parton energy loss through QCD bremsstrahlung

I destructive interference in multiple scattering LPM e�ect

relevant scale: momentum transfer q between hard parton and medium



Jets and thermalisation

In both scenarios jet quenching related to thermalisation:

I early stages of HIC and jets are far-from-equilibrium systems

I jet quenching can be seen as �jet thermalisation�

I e�ective kinetic theory describing thermalisationArnold, Moore, Ya�e, JHEP 0301 (2003) 030

Kurkela, Zhu, Phys. Rev. Lett. 115 (2015) no.18, 182301

−dfpdτ

= C1↔2[fp] + C2↔2[fp] + Cexp[fp]

I C1↔2: splitting/merging rate in presence of multiple scatteringincluding LPM e�ect

I C2↔2: elastic scattering rateI both processes also responsible for parton energy loss



Jet quenching: setting the stage

Jets in p+p

I jets in proton-proton collisions fairly well understood in pQCDI jet productionI jet sub-structureI jet reconstruction using dedicated algorithms

most commonly used at LHC: anti-k⊥



Jet quenching: setting the stage

Jets in p+p

I jets in proton-proton collisions fairly well understood in pQCDI jet productionI jet sub-structureI jet reconstruction using dedicated algorithms

most commonly used at LHC: anti-k⊥

Jets in A+A

I jet production in heavy ion collisions understoodonly nuclear modi�cation in pdf's

I cross-check: vector boson production

I observed: strong modi�cations of jets

I due to re-scattering in �nal state



What happens to jets in medium?

I time it takes to radiate a gluon: tform ≈ ω/k2⊥I time needed for jet evolution ∼ medium size

I jet evolution & re-scattering happen simultaneously

I multi-scale problem

I cannot be solved exactly analytic results for special cases



What is known analytically

I single gluon emission o� a hard parton in eikonal limitmultiple soft or single hard scattering

h

Gyulassy, Wang, Nucl. Phys. B 420 (1994) 583

Baier, Dokshitzer, Mueller, Peigne, Schi�, Nucl. Phys. B 484 (1997) 265

Zakharov, JETP Lett. 65 (1997) 615

Wiedemann, Nucl. Phys. B 588 (2000) 303

Gyulassy, Levai, Vitev, Nucl. Phys. B 594 (2001) 371

Wang, Guo, Nucl. Phys. A 696 (2001) 788

I non-Abelian LPM e�ect plays important role

I 2-gluon emission Arnold, Chang, Iqbal, JHEP 1610 (2016) 124

Casalderrey-Solana, Pablos, Tywoniuk, JHEP 1611 (2016) 174

I radiation o� colour dipoleMehtar-Tani, Salgado, Tywoniuk, JHEP 1204 (2012) 064 & JHEP 1210 (2012) 197

I non-eikonal kinematics Apolinário, Armesto, Milhano, Salgado, JHEP 1502 (2015) 119



A Monte Carlo model for jet quenching: JEWEL

Zapp, Krauss, Wiedemann, JHEP 1303 (2013) 080

I jet production in initial N+N collisions: ME+PS






I re-scattering: ME+PSI generates elastic & inelastic processesI with leading log correct relative ratesI general kinematics






I re-scattering: ME+PSI generates elastic & inelastic processesI with leading log correct relative ratesI general kinematics

I emission with shortest formation time is realisedI all emissions (vacuum & medium induced) treated equallyI hard structures remain unperturbed





I jet production in initial N+N collisions: ME+PSI re-scattering: ME+PS

I generates elastic & inelastic processesI with leading log correct relative ratesI general kinematics

I emission with shortest formation time is realisedI all emissions (vacuum & medium induced) treated equallyI hard structures remain unperturbed

I LPM interference Zapp, Stachel, Wiedemann, JHEP 1107 (2011) 118

I also governed by formation timesI without kinematic restrictions



Jet shape and jet sub-structure observables

I observables built from jet constituentsparticles, partons, calorimeter cells, . . .

I characterise distribution of momentum & �nd structures inside jet

image from David Krohn

I various grooming techniques studied in p+p to separate hard structurefrom soft contaminations �ltering, trimming, pruning, . . .

I shapes/sub-structure of quenched jets sensitive to medium's reactionto energy & momentum deposited by jets



Intra-jet energy distribution: Jet pro�le

CMS, Phys. Lett. B 730 (2014) 243

I suppression of activity at intermediate r

I increase near the edge of the jet



Theory results

0.0 0.1 0.2 0.3∆r

0.4

0.6

0.8

1.0

1.2

1.4

1.6

ρ(∆r)PbPb/ρ(∆r)pp

Pb-Pb @ 2.76TeV (0-10%)anti-kT R=0.3p jetT >100GeV/c, p trkT >1GeV/c0.3<|ηjet |<2.0

without recoil

with recoil (pcut=4T)

CMS 0-10%

Park,Jeon,Gale,arXiv:1807.06550

��

��

��

��

��

��

��

��

��

��

��2.76 TeV��

pTjet > 100�GeV/c��R= 0.3

q̂q,0= 1.7�GeV2/fm��ωcut = 1.0�GeV/c

pTtrk, hyd > 1.0�GeV/c

ρ PbPb(r)/ρpp(r)

r

��0-10 %�

Tachibana,Chang,Qin,

Phys.Rev.C95(2017)no.4,044909

b

b

b

b

b

b

0 0.05 0.1 0.15 0.2 0.25 0.30.4

0.6

0.8

1

1.2

1.4

1.6

w/o Recoils

w/ Recoils 4MomSub

CMSb

anti-k⊥ R=0.3, |ηjet| < 2

pjet⊥ > 100 GeV

JEWEL+PYTHIA (0− 10%), Pb+Pb√s = 2.76 TeV

r from jet axis

ρ(r)P

bPb/ρ

(r)p

p

Kunnawalkam

Elayavalli,Zapp,

JHEP1707(2017)141

CMS

sNN = 2.76 TeVR = 0.3, 0.3<È Η È<2

pT > 100 GeV

centrality 0-10%

0.00 0.05 0.10 0.15 0.20 0.25 0.300.4

0.6

0.8

1.0

1.2

1.4

1.6

r

Ρ HrLPbPb

Ρ HrLpp

All effects

CNM+RAA

CNM only

Chien,Vitev,

JHEP1605(2016)023



Discovery # 3: similarity between p+p and A+A

I (partial) equilibration of �nal state in A+AI expectation: p+p qualitatively di�erent from A+A

p+p collisions too dilute to develop collectivity

I observed:

Soft particle production in high-multiplicity p+p and p+A closelyresembles A+A, but so far no jet quenching is observed.

offlinetrkN

0 50 100 150

2v

0.05

0.10 = 13 TeVspp

< 3.0 GeV/cT

0.3 < p

| < 2.4η|

CMS

|>2}η∆{2, |sub2v{4}2v{6}2v{8}2v{LYZ}2v

offlinetrkN

0 100 200 300

2v0.05

0.10 = 2.76 TeVNNsPbPb

< 3.0 GeV/cT

0.3 < p

| < 2.4η|

offlinetrkN

0 100 200 300

2v

0.05

0.10 = 5 TeVNNspPb

< 3.0 GeV/cT

0.3 < p

| < 2.4η|

CMS,Phys.Lett.B

765(2017)193



Possible explanations

Final state interactions

azimuthal anisotropies due to response to initial geometry

I hydrodynamics Weller, Romatschke, Phys. Lett. B 774 (2017) 351

I kinetic theory Kurkela, Wiedemann, Wu, Phys. Lett. B 783 (2018) 274

Initial state correlations

initial state correlations imprinted on �nal state

I color-glass condensate Schenke, Schlichting, Venugopalan, Phys. Lett. B 747 (2015) 76

e�ective �eld theory of saturated gluons generated by color sources in nuclei

Alternative explanations

models inspired by p+p physics

I string shoving Bierlich, Gustafson, Lönnblad, arXiv:1612.05132

at hadronisation densely packed strings repel each other



The escape mechanism

I kinetic theory set-up:Kurkela, Wiedemann, Wu, Phys. Lett. B 783 (2018) 274

I initially isotropic particle distributionI with spatial anisotropyI number of scatterings O(1)I scattering isotropises particlesI anisotropic scattering probabilityI generates vn's closely resembling hydrodynamic vn's

I also observed in transport code AMPTHe, Edmonds, Lin, Liu, Molnar, Wang, Phys. Lett. B 753 (2016) 506

I have to reassess hydrodynamic interpretation of A+A data



Conclusions

1. Soft particles in A+A collisions behave like an almost perfect �uid.I very well described by hydrodynamics with η/s & 1/4πI indications of at least partial equilibration

2. Jet quenching: Hard jets are suppressed and their structure modi�ed.I jet quenching closely related to thermalisationI chance to resolve quasi-particles in background mediumI jet shape/sub-structure may be sensitive to medium response

chance to observe thermalisation of deposited energyI no consensus about relevant processes

3. Soft particle production in high-multiplicity p+p (and p+A) closelyresembles A+A, but so far no jet quenching is observed.

I largely unresolvedI possible explanations: �nal state interactions, initial state correlations,

hadronisation e�ectsI questions our understanding of soft particle production in p+p and/or

collectivity in A+A


Heavy ion physics · 2018. 11. 22. · Pb+Pb 2.76 A TeV LHC ALICE VISHNU p K p Song, Bass & Heinz,...

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Transcript of Heavy ion physics · 2018. 11. 22. · Pb+Pb 2.76 A TeV LHC ALICE VISHNU p K p Song, Bass & Heinz,...