Heavy Ion Physics Hot and dense QCD after the first LHC running - - PowerPoint PPT Presentation

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Heavy Ion Physics Hot and dense QCD after the first LHC running - - PowerPoint PPT Presentation

Aug 07, 2023 219 likes •785 views

Heavy Ion Physics Hot and dense QCD after the first LHC running period Carlos A. Salgado Universidade de Santiago de Compostela The European School of High-Energy Physics Garderen - the Netherlands - June 2014 @CASSalgado @HotLHC Some of

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Heavy Ion Physics

The European School of High-Energy Physics Garderen - the Netherlands - June 2014

Carlos A. Salgado Universidade de Santiago de Compostela

@CASSalgado @HotLHC

Hot and dense QCD after the first LHC running period

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Some of the questions accessible with heavy-ion collisions

nucleus A

What is the structure of hadrons/nuclei at high energy? color coherence effects in the small-x partonic wave function fix the initial conditions in well-controlled theoretical framework Is the created medium thermalized? How? presence of a hydrodynamical behavior what is the mechanism of thermalization in a non-abelian gauge theory? What are the properties of the produced medium? identify signals to characterize the medium with well-controlled observables what are the building blocks and how they organize? is it strongly-coupled? quasiparticle description? phases? Initial State Final State

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Menu

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Splitting probability: the building block

The splitting probability of an off-shell parton computed in pQCD Soft and collinear divergent Large probability to emit soft and collinear gluons Divergencies need to be resumed (renormalization techniques) The picture is a shower of partons produced by subsequent splittings

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Heuristic: Collision “counts” partons

Lifetime of the fluctuation of the order of the size of the probe The probe cannot resolve smaller fluctuations (stay virtual) Harder probes resolve smaller components (basic idea of pQCD factorization) (Incoherent) cross section proportional to the number of partons in hadron Quantum fluctuations put on-shell by the probe ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Heuristic: Collision “counts” partons II

Saturation of partonic densities (gluon fusion) - aka Color Glass Condensate Color correlations among different partons in the proton/nucleus Coherent cross section: the probe can interact with more than one parton TAMES the cross section ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Quantum fluctuations: Linear/non-linear dynamics

Different kinematical regions: dominated by different dynamics Large-Q : Linear Small-x : Non-linear (eventually) Where is the boundary? (Information from experiment needed) ESHEP - Garderen - June 2014 Heavy Ion Collisions

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ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Collinear factorization A hard cross section is the convolution of universal PDFs and partonic cross sections

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Collinear factorization

Factorization of long-distance and short distance terms in the cross section Short-distance (perturbative) in the partonic cross section Long-distance (non-perturbative) in the PDFs and Fragmentation Functions (FF) ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Global fits for nucleus

Plots from Hannu Paukkunen

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RA

i (x, Q2) = f p/A i

(x, Q2) f p

i (x, Q2)

Ratios of the PDF of a proton inside a nucleus over that in a free proton Isospin effects may be important (e.g. W production in pPb@LHC) ESHEP - Garderen - June 2014 Heavy Ion Collisions

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DGLAP approach - Some recent results I

Agreement of EPS09 with neutrino DIS data

0.9 1 1.1 1.2 NuTeV CTEQ6.6 EPS09 0.9 1 1.1 1.2 CHORUS CTEQ6.6 EPS09 0.8 0.9 1 1.1 1.2 10-2 10-1 1 CDHSW CTEQ6.6 EPS09

x Neutrino beam

RAverage RAverage RAverage

0.9 1 1.1 1.2 NuTeV CTEQ6.6 EPS09 0.9 1 1.1 1.2 CHORUS CTEQ6.6 EPS09 0.8 0.9 1 1.1 1.2 10-2 10-1 1 CDHSW CTEQ6.6 EPS09

x Antineutrino beam

RAverage RAverage RAverage

Collinear factorization works - universal set of nPDFs Neutrino data important for proton global fits

[Paukkunen, Salgado, 2013]

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ESHEP - Garderen - June 2014 Heavy Ion Collisions

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DGLAP approach - Some recent results II

Dijet data in proton-nucleus collisions at LHC - CMS

Preliminary CMS data “by eye”

Doga Gulhan, IS2013, Spain

[Eskola, Paukkunen, Salgado, 2013] [Plots from Paukkunen - LHeC workshop - Jan 2014]

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ESHEP - Garderen - June 2014 Heavy Ion Collisions Provide new constraints to gluon distributions

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DGLAP approach - Some recent results II

Dijet data in proton-nucleus collisions at LHC - CMS

Preliminary CMS data “by eye”

Doga Gulhan, IS2013, Spain

Preliminary CMS data “by eye”

[Eskola, Paukkunen, Salgado, 2013] [Plots from Paukkunen - LHeC workshop - Jan 2014]

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ESHEP - Garderen - June 2014 Heavy Ion Collisions Provide new constraints to gluon distributions

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So that the S-matrix is

Saturation in the dipole picture

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Students day - QM14 IS: observables and concepts A convenient way of discussing the problem is the dipole picture A dipole measures the color correlations in transverse plane

W(x) = P exp

  • i

⇤ dx−A+(x⊥, x−) ⇥

Propagator of the quark - Wilson line

|α; β⇥ Sαβαβ|α; β⇥ = Wαα(x⇥)W †

ββ(¯

x⇥)|α; β⇥

P q¯

q tot =

⇤ 2 − 2 NC Tr

  • W(x⊥)W †(¯

x⊥) ⇥⌅

and the total interaction probability (cross section w/ needed factors)

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Medium averages

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Students day - QM14 IS: observables and concepts All the medium properties are encoded in the averages of Wilson lines Several prescriptions used. Here, just focus on a simple one

1 N Tr

  • W(x⊥)W †(¯

x⊥) ⇥ ≈ exp ⇤ −1 8Qsat(x⊥ − ¯ x⊥)2 ⌅

2

[up to logs: McLerran, Venugopalan 1994]

N(r) = 1 − exp

  • −1

8Q2

satr2

⇥ = ⇒ φ(k) = ⇤ d2r 2πr2 eir.kN(r)

φ = φ(k2/Q2

sat) The dipole “counts” the number of gluons, the unintegrated gluon distribution Two important consequences Qsat cuts-off the low momentum Geometric scaling

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QCD evolution

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Students day - QM14 IS: observables and concepts A way of including QCD evolution in the dipole picture (in x) Boost the dipole: the splitting probability can be computed Use the large-N limit

[Balitsky-Kovchegov eqs]

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Fits using non-linear evolution eqs.

2 4 6 8 10 12 14 16 18 20 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 = 5.02 TeV

NN

s p-Pb | < 0.3

cms

η ALICE, NSD, charged particles, |

Saturation (CGC), rcBK-MC Saturation (CGC), rcBK Saturation (CGC), IP-Sat

2 4 6 8 10 12 14 16 18 20

pPb

R

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

) π Shadowing, EPS09s ( LO pQCD + cold nuclear matter

(GeV/c)

T

p

2 4 6 8 10 12 14 16 18 20 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

HIJING 2.1

=0.28

g

s =0.28

g

DHC, s DHC, no shad. DHC, no shad., indep. frag.

2 4 6 8 10 12 0.5 1 1.5 2 0.5 1 1.5 2

RpPb(η=4)

rcBK-MC, min bias rcBK-MC, LO+inelastic term α=0.1

pt (GeV/c)

rcBK-MC, Npart >10

ch

cme= 5 TeV EPS09 nPDF 2 4 6 8 10 12 0.5 1 1.5 2 0.5 1 1.5 2

RpPb(η=6)

rcBK-MC, min bias rcBK-MC, LO+inelastic term α=0.1

pt (GeV/c)

rcBK-MC, Npart >10

ch

cme= 5 TeV EPS09 nPDF

−5 −4 −3 −2

0.5 1 1.5

Data Theory

r

σ

2

=0.85 GeV

2

Q

−5 −3

0.5 1 1.5

r

σ

2

=4.5 GeV

2

Q

−5 −3

0.5 1 1.5

r

σ

2

=10.0 GeV

2

Q

−5 −4 −3 −2

0.5 1 1.5

r

σ

2

=15.0 GeV

2

Q

−5

10

−4

10

−3

10

−2

10

0.5 1 1.5

r

σ

2

=35 GeV

2

Q x

−5 −4 −3 −2

2

=2.0 GeV

2

Q

−5 −3

2

=8.5 GeV

2

Q

−5 −3

2

=12.0 GeV

2

Q

−5 −4 −3 −2

2

=28.0 GeV

2

Q

−4

10

−3

10

−2

10

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

2

=45 GeV

2

Q

x

Checks of validity of the formalism with proton-nucleus data

[Albacete, Dumitru, Marquet 2013] [AAMQS - 2010]

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Multiparticle production and the CGC

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arXiv:1209.2001 arXiv:1209.2001

[Albacete, Dumitru, Fujii, Nara 2013] [From Dumitru at IS2014]

Gluon distributions obtained in the fits with BK reproduce multiplicities Multiplicities are reproduced in a QCD-based approach QCD evolution equations with initial conditions from DIS experiments Uncertainties in geometry, kinematics, etc First results at NLO available [Chirilli, Xiao, Yuan 2012; Stasto, Xiao, Zaslavsky 2013] ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Mutatis mutandis...

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Mutatis mutandis...

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Checks of hydrodynamics

[degree of thermalization of the medium]

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∂µT µν = 0 T µν = (ε + p)uµuν − pgµν + viscosity corrections

+ Equation of state

Does not address the question on how thermal equilibrium is reached Far from equilibrium initial state needs to equilibrate fast (less than 1fm) Most of the theoretical progress in the last years: Viscosity corrections Fluctuations in initial conditions ESHEP - Garderen - June 2014 Heavy Ion Collisions

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The essential measurement for hydro

x y

φ

Outgoing particle d dt = c2 ⇥ + P ⇥P Recall the Euler equation transverse plane

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The essential measurement for hydro

x y

φ

Outgoing particle d dt = c2 ⇥ + P ⇥P Recall the Euler equation transverse plane More momentum in these directions dN dφ ∝ 1 + 2 v2 cos(2φ) Elliptic flow normally measured by the second term in the Fourier expansion

= 3P =

  • ⇥xP < ⇥yP

Initial anisotropies in spacial distributions translate into final (measurable) anisotropies in momentum

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Fluid behavior from hydro: viscosity of the QGP

) c (GeV/

t

p

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

{4}

2

v

0.05 0.1 0.15 0.2 0.25 0.3

10-20% 20-30% 30-40% 10-20% (STAR) 20-30% (STAR) 30-40% (STAR)

[ALICE 2010]

10 20 30 40 0.5 1 1.5 2 2.5 3 pT [GeV] 30-40% central 5 10 15 20 25 10-20% central h+/- v2 [%] ideal, avg ideal, e-b-e /s=0.08, e-b-e /s=0.16, e-b-e 2 4 6 8 10 12 0-5% central STAR PHENIX

[Schenke, Jeon, Gale 2010]

Lowest viscosity known “perfect liquid”: sQGP AdS/CFT bound LHC similar to RHIC

η s ≥ 1 4π

[Policastro, Son, Starinets, 2001]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Hydro: description of the data

[Paatelainen, Eskola, Niemi, Tuominen 2013] ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Event-by-event fluctuations

  • 6
  • 4
  • 2

2 4 6 -6

  • 4
  • 2

2 4 6 τ=0.01 fm/c x [fm] y [fm] 400 800 1200 ε [GeV/fm3]

  • 6
  • 4
  • 2

2 4 6 -6

  • 4
  • 2

2 4 6 τ=0.2 fm/c x [fm] y [fm] 100 200 300 ε [GeV/fm3]

  • 12
  • 9
  • 6
  • 3

3 6 9 12 -12

  • 9
  • 6
  • 3

3 6 9 12 τ=5.2 fm/c x [fm] y [fm] 0.2 0.4 0.6 0.8 ε [GeV/fm3]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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CGC as initial conditions for hydro

0.02 0.04 0.06 0.08 0.1 0.12 0.14 10 20 30 40 50 〈vn

2〉1/2

centrality percentile η/s = 0.2

Data: ALICE vn{2}, pT>0.2 GeV

v2 v3 v4 v5

0.05 0.1 0.15 0.2 0.5 1 1.5 2 〈vn

2〉1/2

pT [GeV] ATLAS 20-30%, EP η/s=0.2 v2 v3 v4 v5

[Gale, Jeon, Schenke, Tribedy, Venugopalan 2013] Initial conditions from MV model (IPsat) Hydro evolution with viscosity (made event-by-event) ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Towards isotropization...

  • 1

1/3 1/2 +1 0.1 1.0 10.0 1 Qs [fm/c] 0.01 0.1 10.0 20.0 30.0 40.0 2 3 4 PT / PL / LO

  • 1

1/3 1/2 +1 0.1 1.0 10.0 1 Qs [fm/c] 0.01 0.1 10.0 20.0 30.0 40.0 2 3 4 PT / PL / LO

αs = 0.02 αs = 0.0008

The CGC picture provides a framework to study the evolution to equilibrium State just after the collision has a very strong anisotropy (MV model) Solving Color Yan Mills equations to larger times with NLO corrections Anisotropy greatly reduced with still tiny coupling constants [Epelbaum, Gelis 2013] A lot of activity not quote here - both weak and strong (AdS/CFT) coupling ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Structure needs to be formed very early by causality requirements Observed in pp, pA (LHC) and AA (RHIC+LHC)

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ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Jet quenching

What are the effects of a medium in the jet evolution?

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Suppression in one plot

n

) [GeV]

T

(m

T

p

1 10

2

10

AA

R

0.5 1 1.5 2 2.5

  • 1

b µ L dt = 7-150

∫

= 2.76 TeV

NN

s CMS *PRELIMINARY PbPb

*Z (0-100%) |y| < 2 | < 2.1

µ

η , | > 25 GeV/c

µ T

W (0-100%) p | < 1.44 η Isolated photon (0-10%) | | < 1 η Charged particles (0-5%) | | < 2.4 η (0-100%) | ψ J/ → *B | < 2 η *Inclusive jet (0-5%) | | < 2 η *b-jet (0-10%) |

[ A . F l

  • r

e n t

  • H

a r d P r

  • b

e s 2 1 3 ]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

RAA = dN AA/dpt hNcollidN pp/dpt

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ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Di-jet asymmetry at the LHC

Energy imbalance indicates strong energy loss

Aj = ET 1 − ET 2 ET 1 + ET 2

Reconstructed jet measurements sensitive to broadening Jets are suppressed: Studied sample is a subset of the total

Dijets in PbPb - asymmetry in central collisions

PLB 712 (2012) 176

[talk Krofcheck]

ET1 ET2

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Reconstructed jets at the LHC - jet shapes

[GeV]

T

p

0.2 CP

R /

R CP

R

0.8 1 1.2 1.4 1.6 1.8 2

40 50 60 70 100 200

= 0.3 R = 0.4 R = 0.5 R 0 - 10 % ATLAS

= 2.76 TeV

NN

s Pb+Pb

  • 1

b µ = 7 L dt

∫

0.1 0.2 0.3

(r) ρ

  • 1

10 1 10

CMS Preliminary

  • 1

b µ L dt = 129.0

∫

radius (r)

0.1 0.2 0.3

pp_reference

(r) ρ /

PbPb

(r) ρ

0.5 1 1.5 2

50-100%

0.1 0.2 0.3

  • 1

10 1 10

=2.76 TeV s PbPb pp reference

radius (r)

0.1 0.2 0.3 0.5 1 1.5 2

30-50%

0.1 0.2 0.3

  • 1

10 1 10

Ak PF, R=0.3 >1 GeV/c

trk T

p

radius (r)

0.1 0.2 0.3 0.5 1 1.5 2

10-30%

0.1 0.2 0.3

  • 1

10 1 10

>100 GeV/c

jet T

p < 2

jet

| η |

radius (r)

0.1 0.2 0.3 0.5 1 1.5 2

0-10%

What is the distribution in energy for different jet angles?

Not strong changes with respect to pp Small broadening seen Jets with larger-R are less suppressed

[ C M S , Q u a r k M a t t e r 2 1 2 ] [ATLAS, 2012]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Fragmentation functions

Distribution of particles inside a jet: fragmentation functions

Fragmentation functions carry “longitudinal” information Modifications: enhancement in soft (expected) no suppression at hard (unexpected) Strong constraints to underlying dynamics

[ A T L A S , Q u a r k M a t t e r 2 1 2 ] [ C M S

  • s

i m i l a r r e s u l t s ]

z = phadron

T

pjet

T

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Di-jet asymmetry at the LHC

No strong change with respect to the vacuum jets

ET1 ET2

Azimuthal distribution of two most energetic jets

[CMS 2011; ATLAS similar results]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Di-jet asymmetry at the LHC

Energy taken by soft particles at large angles

[CMS 2011]

6 pk

T =

X

Tracks

pTrack

T

cos (φTrack φLeading Jet)

Where does the energy go?

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Summary of experimental data

Reconstructed jets Suppression similar to inclusive hadrons for similar pT Fragmentation functions are mildly modified - more in soft Jet shapes have mild modifications Azimuthal decorrelation of di-jets as in proton-proton Energy taken by soft particles at large angles Small modifications of the jet structure but energy loss

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Medium-induced gluon radiation

K(y0, u; y, y) = Z u(y0)

y(y)

Dr exp ( iω 2 Z dξ ✓dr(ξ) dξ ◆2) ˜ P(y0, y, r)

M M∗

tform

ω dI dωdk ∼ αsCR Z dy Z dy0 Z du eik·u ∂u · ∂yK(y0, u; y, y)

  • y=0

˜ P(L, y0; u)

[Zakharov, Baier, Dokshitzer, Mueller, Peigne, Schiff, Wiedemann, Gyulassy, Levai, Vitev, and many others...]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Medium-induced gluon radiation

K(y0, u; y, y) = Z u(y0)

y(y)

Dr exp ( iω 2 Z dξ ✓dr(ξ) dξ ◆2) ˜ P(y0, y, r)

M M∗

tform

ω dI dωdk ∼ αsCR Z dy Z dy0 Z du eik·u ∂u · ∂yK(y0, u; y, y)

  • y=0

˜ P(L, y0; u)

[Zakharov, Baier, Dokshitzer, Mueller, Peigne, Schiff, Wiedemann, Gyulassy, Levai, Vitev, and many others...]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Medium-induced gluon radiation

K(y0, u; y, y) = Z u(y0)

y(y)

Dr exp ( iω 2 Z dξ ✓dr(ξ) dξ ◆2) ˜ P(y0, y, r)

M M∗

tform

ω dI dωdk ∼ αsCR Z dy Z dy0 Z du eik·u ∂u · ∂yK(y0, u; y, y)

  • y=0

˜ P(L, y0; u)

[Zakharov, Baier, Dokshitzer, Mueller, Peigne, Schiff, Wiedemann, Gyulassy, Levai, Vitev, and many others...]

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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(z)

unresolved

P 0.2 0.4 0.6 0.8 1

z

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

(z)/E

  • ut

E Δ

0.1 0.2 0.3 0.4 0.5 0.6

43

A new picture of jet quenching

  • 0.4 -0.2

0.2 0.4

  • 0.4
  • 0.2

0.2 0.4 5 10 15 20 25

  • 0.4 -0.2

0.2 0.4

  • 0.4
  • 0.2

0.2 0.4 5 10 15 20 25

In the extreme case of only one subjet The whole jet radiates (medium-induced) as a single object The inner structure of the jet is (almost) unmodified ESHEP - Garderen - June 2014 Heavy Ion Collisions

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A new picture of jet quenching

The parton shower is composed of un-modified subjets (vacuum-like) With a typical radius given by the medium scale For medium-induced radiation each subject is one single emitter [Casalderrey-Solana, Mehtar-Tani, Salgado, Tywoniuk 2013] ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Observable consequences

1 2 3 4 5 6 7

dN

  • dl

ωBH = 0.5 GeV ωc = 70 GeV Qs = 2 GeV

0.5 1 1.5 2 2.5 1 2 3 4 5

Ratio

l

Q0 = 0.270 GeV Q = 30.00 GeV

Vacuum limiting spectrum Medium-modified shower wo/ AAO CMS Preliminary, 0-10%

i) intra-jet shape modified ii) branching+broadening depletes energy inside iii) AAO enhances soft gluons inside!

A simple model implementation Assume complete coherence Include both BDMPS and anti-angular ordering (modifying MLLA)

= log(1/x)

K Tywoniuk HP2013

ESHEP - Garderen - June 2014 Heavy Ion Collisions

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Summary

Remarkable progress in the last years Saturation at NLO and fixing IC event-by-event for hydro Hydrodynamic models reaching precision Jet quenching calculations Finite energy corrections; resummation; next orders in alphaS Computations of qhat in lattice, renormalization... A complete picture of LHC data still work in progress Proton-nucleus collisions showing unexpected behavior Is a QCD medium created in proton-nucleus as well?

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