13 experimental papers: alice (7), atlas (2), cms (3...

p–Pb Collisions at the LHC: a brief overview

Anton Andronic – GSI Darmstadt

13 experimental papers: ALICE (7), ATLAS (2), CMS (3), LHCb (1)

(...and “innumerable” theoretical ones :)

• Initial state (shadowing/saturation) ...reference measurementsIS2013, http://indico.cern.ch/conferenceTimeTable.py?confId=239958

• Final state? (to flow or not to flow?)

Rencontres QGP-France 2013 - Etretat, Sep. 9-11

Reference measurements

2 A.Andronic@GSI.de

ALICE, PRL 110 (2013) 032301 PRL 110 (2013) 08230230 citations 32 citations

(GeV)NNs210 310

rtN⟨

)/η/d

5) NSDppp (p

Central AA

BRAHMS

PHENIX

PHOBOS

0.10NN

0.11NN

0.15NN

pPb ALICE

dAu PHOBOSpAu NA35

) INELppp (p

PHOBOS

(GeV/c)T

p0 2 4 6 8 10 12 14 16 18 20

1.8 ALICE, charged particles

| < 0.3cms

η = 5.02 TeV, NSD, | NN

| < 0.8η = 2.76 TeV, 0-5% central, | NNsPb-Pb

| < 0.8η = 2.76 TeV, 70-80% central, | NNsPb-Pb

p-Pb data exhibit generic features of (gluon) saturation (NB: Ncoll (p-Pb) = 8)

Reference measurements + theory

3 A.Andronic@GSI.de

ALICE, PRL 110 (2013) 032301 PRL 110 (2013) 082302

-2 0 2

DPMJET [32]

Sat. Models:IP-Sat [5]KLN [3]rcBK [7]

HIJING:2.1 no shad. [6]

=0.28 [6]g2.1 s2.0 no shad. [4]BB2.0 with shad. [4]BB

ALICE NSD

= 5.02 TeVNNsp-Pb,

0 2 4 6 8 10 12 14 16 18 200.4

1.8 = 5.02 TeVNNsp-Pb

| < 0.3cms

ηALICE, NSD, charged particles, |

Saturation (CGC), rcBK-MCSaturation (CGC), rcBKSaturation (CGC), IP-Sat

0 2 4 6 8 10 12 14 16 18 20

1.8 )0πShadowing, EPS09s (

LO pQCD + cold nuclear matter

(GeV/c)T

p0 2 4 6 8 10 12 14 16 18 20

1.8 HIJING 2.1=0.28gs

=0.28gDHC, s

DHC, no shad.

DHC, no shad., indep. frag.

Binary collision scaling: “cold” matter / EW observables

4 A.Andronic@GSI.de

)2 (GeV), or mass (GeV/cT

(GeV/c), ET

p0 10 20 30 40 50 60 70 80 90 100

2 = 5.02 TeV, NSD (ALICE)

NNs, p-Pb chN

= 2.76 TeV, 0-10% (CMS)NN

s, Pb-Pb γ

= 2.76 TeV, 0-10% (CMS)NNs, Pb-Pb ±W

= 2.76 TeV, 0-10% (CMS)NN

s, Pb-Pb 0Z

“cold” nuclei (p–Pb):shadowing / saturation(gluon distr. func.) ...at low-x

binary coll. scaling, pT > 2 GeV/c

exhibited by various observables

ALICE, PRL 110 (2013) 082302

CMSγ, PLB 710 (2012) 256W±, PLB 715 (2012) 66Z0, PRL 106 (2011) 212301

ATLAS: PRL 110 (2013) 002301

...and relevance to jet quenching at the LHC

4 A.Andronic@GSI.de

)2 (GeV), or mass (GeV/cT

(GeV/c), ET

p0 10 20 30 40 50 60 70 80 90 100

2 = 5.02 TeV, NSD (ALICE)

NNs, p-Pb chN

= 2.76 TeV, 0-10% (CMS)NN

s, Pb-Pb γ

= 2.76 TeV, 0-10% (CMS)NNs, Pb-Pb ±W

= 2.76 TeV, 0-10% (CMS)NN

s, Pb-Pb 0Z

, Pb-Pb (ALICE)chN, Pb-Pb (CMS)chN

= 2.76 TeV, 0-5% NNs

reaches a factor of about 7 aroundpT=7 GeV/c

(stronger than previously measuredat RHIC,

√sNN=200 GeV)

remains substantial even at 50-100GeV/c

ALICE, PLB 720 (2013) 52

CMS, EPJC (2012) 72

seen also with jets

ATLAS, PLB 719 (2013) 220

CMS, PLB 712 (2012) 176PLB 718 (2013) 773 (γ-jet)

ALICE preliminary

Long-range correlations

5 A.Andronic@GSI.de

pp p-Pb

η∆4

φ∆0

∆,η

( 2101

<3.0GeV/cT

110, 1.0GeV/c<p≥(d) CMS N

η∆-4

φ∆0

4φ∆

dη∆d

N1 1.6

110≥ trk

offline = 5.02 TeV, NNNsCMS pPb

< 3 GeV/cT

long range in η

CMS, JHEP 1009 (2010) 091, PLB 718 (2013) 795, ALICE, PLB 719 (2013) 29

Interpretations (long range in η): flow (EPOS MC), initial state (CGC/Glasma)Werner et al., PRL 106 (2011) 122004; Dusling, Venugopalan, PRL 108 (2012) 262001

aligned flux tubes connecting valence quarks (pp), Bjorken, Brodsky, Goldhaber, arXiv:1308.1435

Long-range correlations

5 A.Andronic@GSI.de

pp p-Pb p–Pb

η∆4

φ∆0

∆,η

( 2101

<3.0GeV/cT

110, 1.0GeV/c<p≥(d) CMS N

η∆-4

φ∆0

4φ∆

dη∆d

N1 1.6

110≥ trk

offline = 5.02 TeV, NNNsCMS pPb

< 3 GeV/cT

long range in η ...and in φ (jet-subtracted distr.)

CMS, JHEP 1009 (2010) 091, PLB 718 (2013) 795, ALICE, PLB 719 (2013) 29

Interpretations (long range in η): flow (EPOS MC), initial state (CGC/Glasma)Werner et al., PRL 106 (2011) 122004; Dusling, Venugopalan, PRL 108 (2012) 262001

aligned flux tubes connecting valence quarks (pp), Bjorken, Brodsky, Goldhaber, arXiv:1308.1435

Initial state model (CGC) vs. data

6 A.Andronic@GSI.de

near side (CMS) near and away side (ALICE)

0 1 2 3 4 5 6

pTtrig=pT

asc [GeV]

Associated Yield

p+Pb: CMS data

p+p: CMS data

0-20% 20-40% 40-60%

Away-Side, Symmetric Near-Side, Symmetric

Away-Side, Off-DiagonalNear-Side, Off-Diagonal

Dusling, Venugopalan, PRD 87 (2013) 094034

the saturation model (CGC) is successful

Collective flow in p–Pb collisions?

7 A.Andronic@GSI.de

ALICE, PLB 719 (2013) 29

[GeV]⟩T

PbEΣ⟨20 40 60 80 100 120

xxxxxxxxxxxxxxxxxxxxxxxxxxxx

xxxxxxxxxxxx

{2}2v{4}2v{2PC}2v

hydro{2}2v

ATLAS -1bµ= 1

int = 5.02 TeV, LNNsp+Pb,

< 5 GeVT

0.3 < p| < 2.5η|

{2}2v{4}2v{2PC}2v

hydro{2}2v

ATLAS, PLB 725 (2013) 60

ATLAS, PRL 110 (2013) 182302; CMS, arXiv:1305.0609

also hydrodynamics can explain data...a heated saturation/hydrodynamics debate (and pursuit with data and models)

Flow in p–Pb (d–Au) collisions?

8 A.Andronic@GSI.de

P.Bozek, PRC 85 (2012) 014911 (hydrodynamics)ε2, initial eccentricity, from MC Glauber

PHENIX, arXiv:1303.1794

Pushing the frontiers of deconfined matter?

9 A.Andronic@GSI.de

Au–Au, b=4 fm d–Au

8 -8-6

MC-Glauber

x [fm]

y [fm]

8 -8-6

IP-Glasmao=0.2 fm/c

x [fm]

y [fm]

MC-Glauber

-4 -2 0 2 4

x [fm]

]-4 -2 0 2 4

x [fm]

IP-Glasma

Gale, Jeon, Schenke, arXiv: 1301.5893; Bzdak et al., arXiv:1304.3403

10 A.Andronic@GSI.de

hydrodynamics makes large strides...

0 1 2 3p

T (GeV/c)

ATLAS pPb 0-2% v2

CMS pPb 0-2% v3

0-1% v2

0-2% v2

0-5% v2

0-1% v3

0-2% v3

0-5% v3

0 1 2 3p

T (GeV/c)

ALICE pPb 0-20% v2

ALICE pPb 0-20% v3

0-20% v2

0-20% v3

Qin, Muller, arXiv:1306.3439

0 1 2 3 40

0.2p-Pb 5.02TeV CMS Data 0-2%nv

[GeV/c]T

Bozek, Broniowski, Torrieri, arXiv:1307.5060

...after it trully predicted collective flow (v2, v3)Bozek, Broniowski, arXiv:1211.0845, PLB 718 (2013) 1557

(rad)ϕ∆-1

0 1 23 4

∆dη∆d

c < 4 GeV/T,trig

c < 2 GeV/T,assoc

p 1 < = 5.02 TeVNNsp-Pb

-2-1.5

-1-0.5

2.0-4.0 GeV/ctrigg

Tp 1.0-2.0 GeV/cassoc

η∆φ∆

EPOS3.074

ϕ∆-1

∆dη∆d

c < 4 GeV/T,trig

c < 2 GeV/T,assoc

p 1 < = 5.02 TeVNNsp-Pb

(0-20%) - (60-100%)

-2-1.5

-1-0.5

2.0-4.0 GeV/ctrigg

Tp 1.0-2.0 GeV/cassoc

η∆φ∆

EPOS3.074

experiment makes strides ...and theory great claims

)c (GeV/T

p0.5 1 1.5 2 2.5 3 3.5 4

0.25ALICE

= 5.02 TeVNNsp-Pb

(0-20%) - (60-100%)

ALICE, 1307.3237

0.5 1 1.5 2 2.5 pt

EPOS3.074πKp

ALICEπKp

Werner et al., arXiv:1307.4379

EPOS 3.074: full hydrodynamical treatment (+Pomerons:)

more points to hydrodynamics ...

0 0.5 1 1.5 20

0.18 p-Pb 5.02TeV ALICE Data 0-20%

[GeV/c]T

(ALICE, 1307.3237)20 40

a) 3+1D Hydro

p-Pb 5.02 TeV

ALICE Data (preliminary)

ηdN/d

1.5 b) HIJING

-π+π

ηdN/d

Bozek, Broniowski, Torrieri, arXiv:1307.5060

p–Pb in perspective

chN0 20 40 60 80 100

0.9 ALICE, charged particlesc<10.0 GeV/

Tp|<0.3, 0.15< η|

= 7 TeVspp = 5.02 TeVNNsp-Pb

= 2.76 TeVNNsPb-Pb

ALICE, 1307.1094

p–Pb exhibits features of both pp andPb–Pb

system√sNN (TeV) 〈Nch〉〈pT〉 (GeV/c)

pp 7 4.42±0.22 0.622±0.021

p–Pb 5.02 11.9±0.5 0.696±0.024

Pb–Pb 2.76 259.9±5.9 0.678±0.007

Nch > 14 corresp. to 10%, 50%, 82% upper cross

section for pp, p–Pb, Pb–Pb

Nch > 40 corresp. to upper 1% (70%) of the cross

section p–Pb (Pb–Pb)

0 20 40 60 80 100

PYTHIA 8, tune 4C

without CR

with CR

ALICE, charged particles

c<10.0 GeV/T

p|<0.3, 0.15<η|

= 7 TeVspp

0 20 40 60 80 100

EPOSDPMJETHIJINGAMPTGlauber MC

= 5.02 TeVNNsp-Pb

chN0 20 40 60 80 100

= 2.76 TeVNNsPb-Pb

ALICE, 1307.1094

CR: color reconnection - a “collective”effect of hadronizing strings(a probability parameter)

EPOS LHC: parametrized flow(pp, p–Pb, Pb–Pb)Pierog et al., arXiv:1306.0121

CGC strikes back

0 20 40 60 80 100 120 140N

ALICE, Pb+Pb 2.76 TeVALICE, p+p 7 TeVALICE, p+Pb 5.02 TeVb-CGC (Saturation)

| η| < 0.3, 0.15< pT [GeV]<10

Rezaeian, 1308.4736 (3 parameters)

CGC strikes back

...or does it?

0 20 40 60 80 100 120 140N

ALICE, Pb+Pb 2.76 TeVALICE, p+p 7 TeVALICE, p+Pb 5.02 TeVb-CGC (Saturation)

| η| < 0.3, 0.15< pT [GeV]<10

Rezaeian, 1308.4736 (3 parameters)

)-1 (fm1/2)TS /chN(0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

0.9 ALICE, charged particlesc<10.0 GeV/

Tp|<0.3, 0.15<η|

= 5.02 TeVNN

= 7 TeVspp

= 2.76 TeVspp

ALICE, 1307.1094

geometric scaling (ST = πR2), McLerran et al., arXiv:1306.2350;

R ∼ (dNg/dy)1/3 ≃ (1.5 · dNch/dη)

1/3, sat. at 1.54 (2.39) fm for pp (p–Pb)

claim of success based on CMS data, arXiv:1307.3442 ...in a smaller Nch range

)c (GeV/T

00.20.40.60.8

11.21.41.61.8

22.22.4

0 2 4 6 8

60-80%

= 5.02 TeVNNsALICE, p-Pb,

0 2 4 6 8

80-90%

= 2.76 TeVNNsALICE, Pb-Pb,

0-5% central p–Pb collisions look like 60-70% central Pb–Pb(dNch/dη ∼1.7 lower)

ALICE, arXiv:1307.6796

η/dchNd10 210 310

)- π +

= 200 GeVNNsSTAR, Au-Au,

= 200 GeVNNsPHENIX, Au-Au,

= 200 GeVNNsBRAHMS, Au-Au,

= 200 GeVNNsSTAR, d-Au,

η/dchNd10 210 310

)- π +

= 200 GeVNNsPHENIX, Au-Au,

= 200 GeVNNsBRAHMS, Au-Au, = 5.02 TeVNNsALICE, p-Pb,

More flow arguments

η/dchNd10 210

Λ + Λpp +

- + K+K

-π + +π

β⟨0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

= 7 TeV (with Color Reconnection)sPYTHIA8,

= 7 TeV (without Color Reconnection)sPYTHIA8,

0 2 4 6 8 10

]-2 )c

(100x)-π + +π

(10x)- + K+K

(1x)pp +

(0.1x)0SK

(0.01x)Λ + Λevent class 5-10%

Blast-Wave

EPOS LHC

Krakow

DPMJET

)c (GeV/T

p0 2 4 6 8 10

1.5 -π + +π

0 2 4 6 8 10

1.5 - + K+K

0 2 4 6 8 10

1.5 pp +

0 2 4 6 8 10

1.5 0SK

0 2 4 6 8 10

1.5 Λ + Λ

ALICE, 1307.6796

Flow or Cronin effect or saturation?

(GeV/c)T

p0 2 4 6 8 10 12 14 16 18 20

1.8charged particles

| < 0.3cms

η = 5.02 TeV, NSD, | NN

sALICE, p-Pb

| < 0.5η = 0.2 TeV, | NNsSTAR, d-Au

| < 0.18η = 0.2 TeV, | NNsPHENIX, d-Au

LHC vs. RHIC data

ALICE, PRL 110 (2013) 082302

• flow: blue-shift of spectralarger at LHC

• Cronin effect: “re-distribution” oflow-pT hadrons at higher pT dueto multiple (parton) scattering

larger at RHIC

• saturation: depletion of spectra atlow pTlarger at LHC

still need some strategy to distinguish...a challenge: reduce (dominant) systematic uncertainties (?)

Charmonium in p–Pb

ALICE, arXiv:1308.6726; LHCb, arXiv:1308.6729

-4 -3 -2 -1 0 1 2 3 4

1.4 c<15 GeV/T

p, 0<-µ+µ→ψ= 5.02 TeV, inclusive J/NNs p-Pb

-1<3.53)= 5.0 nbcms

y (2.03<int

, L-1<-2.96)= 5.8 nbcms

y (-4.46<intL

EPS09 NLO (Vogt)

CGC (Fujii et al.)

/fm (Arleo et al.)2=0.075 GeV0

ELoss, q

EPS09 NLO + ELoss, q

(GeV/c)T

p0 2 4 6 8 10 12 14

EPS09 NLO (Vogt)

ELoss, q

EPS09 NLO + Eloss, q

< 3.53cms

y, 2.96 < -µ+µ→ψ= 5.02 TeV, inclusive J/NNsp-Pb

theory (nobody’s perfect:) shadowing and Eloss ∼ OK ...CGC seems ruled out

tantalizing implication for Pb–Pb: RAA > 1 (at low pT) if-no-shadowing...cannot turn off shadowing, but means we may see this at the top LHC energy

Summary

• interesting / puzzling features in p–Pb collisions

• both initial state and final state (collective flow?) play a role

... would we be able to disentangle them?

...(if so) is CGC the correct description of the initial state?

• what are the implications for Pb–Pb?...normalize Pb–Pb to p–Pb(min.B)?

• would p–Pb (and pp) help elucidating thermalization and hadronization?

Extra slides

Elliptic flow (non-central collisions)

x1 A.Andronic@GSI.de

density of binary collisionsU.Heinz, arXiv:0901.4355

−10 −5 0 5 10

x (fm)

...arising from different initial gradi-ents along x and y

“self-quenching” (develops early)

determined by the spatial eccentricity

ε(b) = <y2−x2><y2+x2>

with energy dens. as weight

...transformed into momentumanisotropy in φ (wrt reaction plane)

Fourier coef. v2 = 〈cos(2(φ−ΨRP ))〉quantifies collective (elliptic) flow

Higher order harmonics (central collisions)

Sorensen, JPG 37 (2010) 094011; Alver, Roland, PRC 81 (2010) 054905 ...collision geometry fluctuations

)φ∆C

Pb-Pb 2.76 TeV, 0-2% central

< 2.5 GeV/ctT

2 < p < 2 GeV/ca

T1.5 < p

| < 1.8η∆0.8 < |

/ndf = 33.3 / 352χ

[rad]φ∆0 2 4

n1 2 3 4 5 6 7 8

0.35 Centrality

< 2.5 GeV/ct

T2 < p

< 2 GeV/ca

T1.5 < p

ALICE, PLB 708 (2012) 032301; PRL 107 (2011) 032301ATLAS, PRC 86 (2012) 014907 ; PHENIX, PRL 107 (2011) 252301event-by-event distributions: ATLAS, arXiv:1305.2942constrain initial state and η/s (small) in hydrodynamic models

Identified hadrons at the LHC

)c (GeV/T

)- π +

0 1 2 3

60-80%

0 1 2 3

80-90%

)c (GeV/T

)- π +

0 1 2 3

60-80%

0 1 2 3

80-90%

ALICE, 1307.6796 ...

0-5% central p–Pb collisions look like 60-70% central Pb–Pb

Identified hadrons at the LHC

)c (GeV/T

00.20.40.60.8

11.21.41.61.8

22.22.4

0 2 4 6 8

60-80%

0 2 4 6 8

80-90%

η/dchNd10 210 310

)- π +

= 200 GeVNNsSTAR, Au-Au,

)-π + +π) / (Λ + ΛSTAR, ( = 5.02 TeVNNsALICE, p-Pb,

ALICE, 1307.6796 ...

0-5% central p–Pb collisions look like 60-70% central Pb–Pb(dNch/dη ∼1.7 lower)

Identified hadrons at RHIC (d–Au collisions)

-π/p 0-10%10-20%20-40%40-60%60-92%p+p

+πp/ = 200 GeVNNsAu+Au

(GeV/c)T

p0 1 2 3 4 5 6

-π/p 0-20%20-40%0-100%40-60%60-88%p+p

+πp/ = 200 GeVNNsd+Au

(GeV/c)T

p0 1 2 3 4 5 6

PHENIX, arXiv:1304.3410

... 0-20% central d–Au collisions look like 60-92% central Au–Au

13 experimental papers: alice (7), atlas (2), cms (3...

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