Heavy Ion Physics - Indico · from p+p I but: have to scale p+p result by number of N-N collisions...

Hard processes Small collision systems Other fun stu� with heavy ions Summary

Heavy Ion Physicspart II

Korinna Zapp

Lund University

ESHEP 2019

Korinna Zapp (Lund University) Heavy Ion Physics ESHEP 2019 1 / 48


First lecture: summary

I heavy ion collisions: unique opportunity to study matter atextreme conditions (temperature and pressure)

I formation of new phase of QCD matter: quark-gluon plasma→ decon�nement, chiral symmetry restoration

I QGP: only strongly coupled system of Standard Modelmicroscopic degrees of freedom

I what we have learned so farI QGP created in heavy ion collisions at RHIC and LHCI �rapid hydrodynamisation�I QGP is a strongly coupled liquid



Summary: stages of a heavy ion collision

I initial state:I geometry (→ Glauber model)I gluon saturation (→ colour glass condensate)

I pre-equilibirium dynamics:I successful descriptions at weak (kinetic theory) and strong

coupling (AdS/CFT)I leads to hydrodynamic regime on timescales O(1 fm/c)

I hydrodynamic expansion:I the QGP �ows → collective behaviourI least dissipative system known (η/s & 1/4π)

I hadronisation: phenomenological modeling

I hadronic re-scattering: hydrodynamics or transport theorymore important at lower beam energies



Outline of today's lecture

Hard processesFactorisation and nuclear PDFsQuarkoniaElecroweak bosonsJets

Small collision systems

Other fun stu� with heavy ions

Summary



Outline




Summary



Factorisation and nuclear PDFs

Hard processes: timescale

~-0.5 fm/c ~0.1 fm/c ~1 fm/c ~10 fm/c time

I hard processes: large momentum transfer Q

I corresponds to short time scales ∆t ∼ 1/Qdue to uncertainty principle

I �rst processes to happen in heavy ion collision

I time/length scales of production processes too short to feelnuclear environment

I produced hard particles traverse QGP




Factorisation of the cross section

I factorisation of the cross section:

σ(P1,P2) =∑i ,j

1∫0

dx1 dx2 fi (x1,Q2)fj(x2,Q

2)σ̂ij(x1P1, x2P2, αs,Q2)

I σ̂ij : partonic cross sectionI has perturbative expansion in αs

known: NNLO for di-jets, NLO for up to ∼ 5 jetsI short distance physics: insensitive to nature of incoming hadrons

i.e. no nuclear modi�cations

I fi (x ,Q2): parton distribution function

nuclear pdf �ts available




Nuclear PDFs: EPPS16

Eskola, Paakkinen, Paukkunen, Salgado, Eur. Phys. J. C 77 (2017) no.3, 163

I bound proton PDF de�ned as

fp/Ai (x ,Q2) = RA

i (x ,Q2)f pi (x ,Q2)

I bound neutron PDF from isospin symmetry

I Q2 dependence: DGLAP with2-loop splitting functions

I parametrise RAi and �t for

chosen free proton PDF (inthis case CT14NLO)

I other nPDF sets:nCTEQ15, DSSZ

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

10-4

10-3

10-2

10-1

1

antishadowing maximum

EMC minimum

small-x shadowing

xa xe

ye

ya

y0

EPPS16

x

RA i(x,Q

2 0)




Nuclear PDFs


I consistent �t to all available data

I constraining data sparse → sizeable uncertainties

I modi�cations less important at higher momentum scales




Nuclear PDFs

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10-4

10-3

10-2

10-1

1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10-4

10-3

10-2

10-1

1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10-4

10-3

10-2

10-1

1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10-4

10-3

10-2

10-1

1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10-4

10-3

10-2

10-1

1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

10-4

10-3

10-2

10-1

1

EPPS16nCTEQ15

EPPS16nCTEQ15

EPPS16nCTEQ15

EPPS16nCTEQ15

EPPS16nCTEQ15

EPPS16nCTEQ15

x x x

x x x

RPb

uV(x,Q

2=10

GeV

2)

RPb

dV(x,Q

2=10

GeV

2)

RPb

u(x,Q

2=10

GeV

2)

RPb

d(x,Q

2=10

GeV

2)

RPb

s(x,Q

2=10

GeV

2)

RPb

g(x,Q

2=10

GeV

2)





Nuclear PDFsdσ

(yZ−

0.4)

/dσ

(−y Z−

0.4)

yZ − 0.4

EPPS16

No nuclear effects

60 GeV < M`+`− < 120 GeV

|η`±lab| < 2.4

pT(`±) > 20 GeV

CMS data

Z production, pPb,√s = 5.02 TeV

dσ(η−

0.465)/dσ(−η−

0.465)

η − 0.465

EPPS16

nCTEQ15

DSSZ

|ηjetlab| < 3.0

psubleadingT > 30 GeV

pleadingT > 120 GeV

CMS data

dijetspPb√s = 5.02 TeV




Quarkonia

Quarkonium suppression: idea

I in decon�ned medium: colour screeningI attractive force between quark and anti-quark screened

-1000

-500

0

500

1000

0 0.2 0.4 0.6 0.8 1 1.2 1.4

F(L,T)[MeV]

L[fm]

T/Tc T/Tc

1.01 1.02 1.03 1.10 1.15 1.22 1.32

1.43 1.66 1.86 2.16 2.45 2.79

Ewerz, Kaczmarek, Samberg, JHEP 1803 (2018) 088 [arXiv:1605.07181]

I should lead to dissociation of bound statesI suitable states to observe this: charmonium & bottomonium

Matsui & Satz, 1986



Quarkonia

J/Ψ suppression

⟩ part

N ⟨0 50 100 150 200 250 300 350 400

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4 12%±<4 global sys.= y = 2.76 TeV), 2.5<

NNsALICE (PbPb

9.2%±|<2.2 global sys.= y = 200 GeV), 1.2<|NN

sPHENIX (AuAu

12%±|<0.35 global sys.= y = 200 GeV), |NN

sPHENIX (AuAu

ALICE, Phys. Rev. Lett. 109 (2012) 072301 [arXiv:1202.1383]

3 clear suppression of J/Ψ

7 weaker suppression at higher cms energy



Quarkonia

J/Ψ regeneration

I charm quarks produced earlyin a hard processes

too heavy for thermal production

I quark move quasi-freely inQGP phase

picture: J. Stachel

I at phase boundary J/Ψ production by combinatorics

I J/Ψ production rate depends sensitively on charm cross section

I e�ect increases rapidly with cms energy



Quarkonia

Sequential suppression

state J/Ψ Ψ′ Υ Υ′ Υ′′

mass [GeV] 3.10 3.68 9.46 10.02 10.36

∆E [GeV] 0.64 0.05 1.10 0.54 0.20

r [fm] 0.25 0.45 0.14 0.28 0.39

I expect dissociation at di�erent temperatures

I �QGP thermometer�



Quarkonia

Sequential suppression

CMS, Phys. Rev. Lett. 109 (2012) 222301 [Erratum: Phys. Rev. Lett. 120 (2018) no.19, 199903]

Guillermo Breto Rangel for CMS, Quark Matter 2012

I there is a clear trend

I the story is actually much more complicatedstatic potential over-simpli�ed, quarkonia not produced at rest, . . .



Elecroweak bosons

Nuclear modi�cation factors

I standard procedure for quantifying QGP e�ects: divide by resultfrom p+p

I but: have to scale p+p result by number of N-N collisionshard processes scale like Ncoll

RAA({·}) =

dσXd{·}

∣∣∣AA

〈Ncoll〉 dσXd{·}

∣∣∣pp

I deviations from unity signal e�ects beyond incoherentsuperposition of nucleon-nucleon collisions

I in case of electroweak processes: have to take di�erent isospincomposition into account



Elecroweak bosons

W & Z production

partN0 50 100 150 200 250 300 350 400

AA

R

0

0.5

1

1.5

2

2.5 = 2.76 TeV

NNsCMS

> 0 GeV/cT

, |y| < 2.0, pµµ →Z , 0-100% centralityµµ →Z

> 0 GeV/cT

ee, |y| < 1.44, p→Z ee, 0-100% centrality→Z

pp luminosity uncertainty

CMS, JHEP 1503 (2015) 022 [arXiv:1410.4825] ATLAS arXiv:1907.10414

I no signi�cant deviation from unity

I W and Z don't interact strongly → leave dense interactionregion without re-scattering

I important con�rmation of Ncoll scaling and nPDFs



Jets

Second main discovery of heavy ion physics

Jet quenching: Hard jets are suppressed and their structure modi�ed.

I proton-proton collisions: 2 jets with balancing transversemomentum

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



Jets

Suppression of single-inclusive jets

[GeV]T

p

AA

R

0

0.5

1

1.5

100 200 300 400 500 600 900000

0 - 10 %30 - 40 %60 - 70 %

PreliminaryATLAS = 5.02 TeVNNs = 0.4 jets, R tkanti-

| < 2.8y|

-12015 Pb+Pb data, 0.49 nb-1 data, 25 pbpp2015

ATLAS-CONF-2017-009

I suppression of jets by factor 2 relative to expectation from p+pneed to scale p+p reference by number of hard N+N collisions



Jets

Di-jet momentum asymmetry

0 0.2 0.4 0.6 0.8 1

Eve

nt F

ract

ion

0

0.05

0.1

0.15

0.2

0.25

0.3

70-100%

CMS

0 0.2 0.4 0.6 0.8 1

Eve

nt F

ract

ion

0

0.05

0.1

0.15

0.2

0.25

0.3

20-30%

0 0.2 0.4 0.6 0.8 1E

vent

Fra

ctio

n0

0.05

0.1

0.15

0.2

0.25

0.3 = 2.76 TeVNN

sPbPb

-1bµLdt = 150 ∫

= 2.76 TeVspp -1

Ldt = 231 nb∫PYTHIA+HYDJET

50-70%

)T,2

+pT,1

)/(pT,2

-pT,1

= (pJA0 0.2 0.4 0.6 0.8 1

Eve

nt F

ract

ion

0

0.05

0.1

0.15

0.2

0.25

0.3

10-20%

(e) 0 0.2 0.4 0.6 0.8 1

Eve

nt F

ract

ion

0

0.05

0.1

0.15

0.2

0.25

0.3

30-50%

(PFlow), R = 0.3TAnti-k

> 120 GeV/cT,1

p

> 30 GeV/cT,2

p

π32 >

12φ∆

0 0.2 0.4 0.6 0.8 1E

vent

Fra

ctio

n0

0.05

0.1

0.15

0.2

0.25

0.3

0-10%

CMS, Phys. Lett. B 712 (2012) 176

I enhancement of asymmetric con�gurations



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



Jets

Intra-jet energy distribution: fragmentation function

z =p⊥,hp⊥,J

cos(∆RhJ)

z

-110 1D

(z)

R0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

0-10%/60-80%

>100 GeVjet

Tp

R=0.4Tanti-k

ATLAS=2.76 TeVNNsPb+Pb

-10.14 nb

z

-110 1

D(z

)R

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

30-40%/60-80%


-10.14 nb

z

-110 1

D(z

)R

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

DataSystematic Uncertainty

10-20%/60-80%


-10.14 nb

z

-110 1D

(z)

R0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

40-50%/60-80%


-10.14 nb

z

-110 1

D(z

)R

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

20-30%/60-80%


-10.14 nb

z

-110 1

D(z

)R

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

50-60%/60-80%


-10.14 nb

ATLAS, Phys. Lett. B 739 (2014) 320

I distribution of hadrons inside jets

I suppression at intermediate & enhancement of soft momenta



Jets

A �rst lesson

Summary of experimental results

I jet production suppressed by factor ∼ 2 up to p⊥'s of 1 TeV

I hard structures inside jets survive largely unmodi�ed

I enhancement of soft activity far away from jet axis

A �rst lesson to be learned

I energy loss from jets driven by peeling o� ofsoft gluons

I rare, semi-hard emissions not an importante�ect

I both were equivalent in early calculations

Casalderrey-Solana, Milhano, Wiedemann, J. Phys. G 38 (2011) 035006 [arXiv:1012.0745]



Jets

What is unique about jet quenching

I �calibrated� probe: well understood in p+p�xed order matrix elements + resummation (parton showers)

I jet quenching allows to observe process of equilibrationsoft observables see result of equilibration

I jets give access to scale dependence of medium properties



Jets

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

Jet quenching at strong coupling

Advantages:

1. exact solution of gauge theory at strong coupling

2. no uncertainty in jet-medium interaction

Fundamental problems:

1. jets are a weak-coupling phenomenon

2. QCD is not N = 4 SYM

�holographic jet�:qq̄ pair in QGP dual to classicalstring in black hole geometry

jet

hydrodynamicmodes

boundary

horizon

falling string

Chesler, Jensen, Karch, Ya�e, Phys. Rev. D 79 (2009) 125015

Chesler & Rajagopal, JHEP 1605 (2016) 098



Jets

How long does it take to radiate a gluon?

kµ = (ω, ~k⊥, k‖)

p′µ = (E ′, ~p′⊥, p′‖)

pµ = (E , 0, 0, p‖)

= (kµ + p′µ)

θ

assume: massless quark

I virtual state: p2 = E 2 − p2 6= 0 → m2virt = p2

I uncertainty principle: 1 = ∆t ∆E = ∆t mvirt

I gluon formation time: tform = ∆t × (boost factor)

tform =1

mvirt

E

mvirt=

E

2pµkµ' E

ωEθ2' ω

k2⊥

I time for entire jet evolution: O((1− 10) fm)I (transverse) size of medium: O((1− 10) fm)



Jets

Bremsstrahlung in medium: heuristic discussionBaier, Dokshitzer, Mueller, Peigne, Schi�, Nucl. Phys. B 483 (1997) 291

Baier, Schi�, Zakharov, Ann. Rev. Nucl. Part. Sci. 50 (2000) 37ω, k⊥

E

E � ω � k⊥, q⊥

Brownian motion of the gluon:⟨k2⊥⟩

= q̂Lformation time of the radiated gluon:

tf 'ω

k2⊥' ω

q̂tf⇒ tf =

√ω

q̂and Ncoh =

tfλ

gluon energy spectrum:

d2I coh

dωdy' 1

Ncoh

d2I incoh

dωdy∝ αs

ωλλ

√q̂

ω= αs

√q̂ ω−3/2 radiative energy

loss: ∆E =

L∫0

dy

ωc∫0

dω ωd2I

dωdy∝ αsq̂L

2



Jets

Beyond parametric estimates

Eikonal (straight line) propagation

I interactions result in colour phase rotation

I in-medium propagator: Wilson line

W (xf +, xi+; x⊥) = P exp

ig

xf +∫xi+

dξ A−(ξ, x⊥)

path ordering medium colour �eld

Further developments

I non-eikonal propagation

I 2-gluon emission

I resummation of incoherent emissions

I . . .Korinna Zapp (Lund University) Heavy Ion Physics ESHEP 2019 30 / 48


Jets

From partons to jets: antennae

I consider colour singlet dipole in medium

I in vacuum colour coherence leads to angular ordering of radiation

I re-scattering in medium introduces new scale: q⊥I medium modi�cations depend on whether dipole is resolved

d⊥

q⊥

d⊥

q⊥

I in particular: unresolved structures remain unperturbed

Casalderrey-Solana, Mehtar-Tani, Salgado, Tywoniuk, Phys. Lett. B 725 (2013) 357



Jets

From partons to jets: antennae

I decoherence time scale: τd ∼(

1

q̂θ2qq̄

)1/3

I decoherence opens up phase space

I in soft limit: geometric separationMehtar-Tani, Salgado, Tywoniuk, JHEP 1204 (2012) 064 doi:10.1007/JHEP04(2012)064 [arXiv:1112.5031]

I can be generalised to jets

Casalderrey-Solana, Mehtar-Tani, Salgado, Tywoniuk,

Phys. Lett. B 725 (2013) 357 [arXiv:1210.7765]



Jets

Medium response

I medium response: medium's reaction to energy & momentumdeposited by jet

I gives rise to additional soft activity

I momentum conservation → additional particles follow jetdirection

I correlated background

background

jetI correlated background cannot

and should not be subtracted

I rather, it is part of jet

I �uctuations matter



Jets

Medium response





background

jetbackground

correlated I correlated background cannotand should not be subtracted





Jets

Medium response





background

jet correlatedbackground

I correlated background cannotand should not be subtracted





Jets

Particle distribution outside jets

0 0.2 0.4 0.6 0.8 1

r∆ddNje

tN1

Y=

0

10

20

30

40 pp reference

< 1 GeVtrk

T0.7 < p

< 2 GeVtrk

T1 < p

< 3 GeVtrk

T2 < p

< 4 GeVtrk

T3 < p

< 8 GeVtrk

T4 < p

< 12 GeVtrk

T8 < p

< 16 GeVtrk

T12 < p

< 20 GeVtrk

T16 < p

< 20 GeVtrk

T0.7 < p

0 0.2 0.4 0.6 0.8 1

r∆ddNje

tN1

Y=

0

10

20

30

40 PbPb50-100%

0 0.2 0.4 0.6 0.8 1

r∆ddNje

tN1

Y=

0

10

20

30

40 PbPb30-50%

0 0.2 0.4 0.6 0.8 1

r∆ddNje

tN1

Y=

0

10

20

30

40 PbPb10-30%

0 0.2 0.4 0.6 0.8 1

r∆ddNje

tN1

Y=

0

10

20

30

40 PbPb0-10%

r∆0 0.2 0.4 0.6 0.8 1

pp -

YP

bPb

Y

0

5

10

15PbPb - pp50-100%

r∆0 0.2 0.4 0.6 0.8 1

0

5

10

15

PbPb - pp30-50%

r∆0 0.2 0.4 0.6 0.8 1

0

5

10

15

PbPb - pp10-30%

r∆0 0.2 0.4 0.6 0.8 1

0

5

10

15

PbPb - pp0-10%

CMS r∆Particle yield vs. (5.02 TeV)-1bµ (5.02 TeV) PbPb 404 -1pp 27.4 pb

|<1.6jet

η> 120 GeV, |T

R=0.4 jets, pTanti-k

0 0 0 1

CMS,CMS-HIN-16-020,arXiv:1803.00042

I enhancement of soft particles far away from jet



Jets

Some 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

)PbPb/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



Outline




Summary



Third main discovery of heavy ion physics

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



Strangeness enhancement

I strong increase of strangeparticle production withmultiplicity

I traditional view: strangenessproduction in p+p suppressiondue to s quark mass

I strangeness practicallyequilibrated in Pb+Pbcollisions at LHC

I strangeness enhancementcounted as signal forformation of hot system

ALICE, Nature Phys. 13 (2017) 535



Unmodi�ed observables

[GeV/c]T

p30 40 50 100 200 300

pPb

R*

0.5

1

1.5 = 5.02 TeV NNs pPb

| < 0.5, R = 0.3CM

ηCMS (0-100%), |

| < 0.3, R = 0.4CM

ATLAS (0-90%), | y

[GeV]T

p

10 15 20 25 30 35 40pP

bR

0.6

0.8

1

1.2

1.4

1.6

1.8-1 = 5.02 TeV, L = 28 nbNNs+Pb, p

-1 = 5.02 TeV, L = 25 pbs, pp

ATLASψPrompt J/

-2.0 < y* < 1.5

CMS, Eur. Phys. J. C 76 (2016) no.7, 372 [arXiv:1601.02001]

ATLAS, Eur. Phys. J. C 78 (2018) no.3, 171 [arXiv:1709.03089]

I no suppression of jets and J/Ψ in p+Pb

I large suppression in Pb+Pb collisions



Possible explanations for vn's in p+p

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

Alternative explanations

models inspired by p+p physics

I string shoving Bierlich, Gustafson, Lönnblad, Phys. Lett. B 779 (2018) 58 [arXiv:1710.09725]

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



Important di�erences between small and large systems

I multiplicity generated in di�erent ways:I Pb+Pb: requiring high multiplicity biases towards central eventsI p+p/Pb: requiring high multiplicity biases towards jetty events

I not clear what multi-particle correlations measure in jetty events

dijetη

-2 -1 0 1 2 3

dije

tηd

dije

tdN

<0]

dije

tη[

dije

tN

1

0

0.2

0.4

0.6

0.8

1

1.2

-1CMS pPb 35 nb = 5.02 TeVNNs

> 120 GeV/cT,1

p > 30 GeV/c

T,2p

/3π > 21,2

φ∆

(b)< 2020 - 2525 - 3030 - 40> 40

classes (GeV): |<5.2η4<|

TE

CMS, Eur. Phys. J. C 74 (2014) no.7, 2951

I strong auto-correlations insmall systems, e.g. betweenjets and forward multiplicity

I example: rapidity shift ofdi-jets due to energyconservation

Armesto, Gülhan, Milhano, Phys. Lett. B 747 (2015)

441 [arXiv:1502.02986]



Outline




Summary



Ultra-peripheral collisions

I relativistic ions are sources of strong electric �elds

I evidence for γγ → γγscattering

~v −c

=

Z

Zem−fields

em−fields

~

v c~~Pb

Pb

. .

Pb

Pb

. .

Pb

Pb

. .

Pb82+

Pb82+

Pb82+

Pb82+

X

γ

γ

acoplanarityγγ0 0.01 0.02 0.03 0.04 0.05 0.06

Eve

nts

/ 0.0

05

0

2

4

6

8

10

12

14 ATLAS

= 5.02 TeVNNsPb+Pb

Signal selectionno Aco requirement

-1bµData, 480 MC γγ→γγ

MC -e+e→γγ MC γγCEP

ATLAS, Nature Phys. 13 (2017) no.9, 852 [arXiv:1702.01625]



Search for the chiral magnetic e�ect

I spectator protons generate strong B �eld (1018 − 1020 G)I quark interactions with topological gluon con�gurations can

induce chirality imbalance and local parity violationI leads to electric charge separation along direction of magnetic

�eld → chiral magnetic e�ectI leads to charge dependent elliptic �ow

Belmont, Nucl. Phys. A 931 (2014) 981 [arXiv:1408.1043]

ALICE, Phys. Rev. C 93 (2016) no.4, 044903 [arXiv:1512.05739]



Outline




Summary



Wrap-up

I heavy ion collisions allow to study fundamental questions instrong interaction physics

I three main discoveries of heavy ion physics:1. hydrodynamic behaviour

I QGP behaves as an almost perfect liquidI lowest η/s observed, close to theoretical lower bound of 1/4π

2. jet quenchingI partial thermalisation of a far-from-equilibrium systemI informs us about how the QGP came into beingI gives access to scale dependence of QGP properties

3. similarity between p+p/p+A and A+A collisionsI exciting, but no �rm conclusions yet

I many more exciting phenomenasome of which I discussed, others I had to leave out



Wrap-up

I heavy ion physics is new physics

I rich and diverse phenomenology

I have to develop new theoretical tools usingI classical �eld theoryI AdS/CFTI thermal �eld theoryI kinetic theory/transport theoryI relativistic hydrodynamicsI e�ective �eld theoriesI zero-temperature �eld theory (in particular perturbation theory)I phenomenological models

I also new experimental techniques needede.g. variety of correlations, background subtraction for jets, . . .



Wrap-up

I heavy ion physics is��new unexplored physics






Wrap-up

I heavy ion physics is��new unexplored physics




I heavy ion physics is a broad, multifaceted and exciting �eld


Heavy Ion Physics - Indico · from p+p I but: have to scale p+p result by number of N-N collisions...

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Transcript of Heavy Ion Physics - Indico · from p+p I but: have to scale p+p result by number of N-N collisions...