Heavy Ion Physics
at LHC
Tatsuya Chujo 

Univ. of Tsukuba

TCHoU workshop, 2022, March 24 (online)Temperature
1019 TeV

Phase transitionin the universe

2cInflation

1016 TeV≈

0.1 TeV

EW phase transition≈≈

elementary particleʼsmass =0

Quark Gluon

Plasma(QGP)
QCD phase trans.

100 GeV
0.1 GeV(Chiral phase trans.)

Tc ~ 160 MeV

hadronNeutrino decoupling
Photon reheating
Nucleosynthesis

0.1 MeV

Higgs mechanism

2% of mass
Chiral Symmetry
Breaking

98% of mass

0.1 keV

Recombination
Photon decopling

0.1 eVt = 10-6 sec≈

0.1 meV

Present

10-36 10-34

10-10

1

1010

1020

Time (sec)QCD phase diagram 3

T

Quark Gluon Plasma(QGP)

Chiral Symmetry

High Energy

Nucleus-Nucleus Collisions

Crossover Phase
Transition

TPC

CERN (Switzerland)
LHC (2009-), 27 km
sNN = 2.76, 5.02 TeV Pb-Pb•

Creation of QGP in thelaboratory•

Properties of QGP, Restorationof Chiral Symmetry, Origin of
nucleon mass

Chiral SB

Neutron Star Merger
Interior of Neutron StarChiral SB

Hadron⟨q̄q⟩ ≠ 0
Normal Nucleus

Baryon density* Neutron star image: https://phys.org/news/2018-09-neutron-star-jets-theory.html

Lattice QCD prediction

4

Crossover phase transition fromhadronic phase to partonic phase
Pseudo Critical Temp.
TPC=156 ± 1.5 MeV

Ideal Stephan-Boltzmann Eq.
ε: energy density
T: temperature
g: degrees of freedom(3: hadrons, 37: u, d quarks & gluon(spin, color, flavor))

To produce QGP, we need:Tpc ~ 160 MeV
ε ~ 1 GeV/fm3
HotQCD Collaboration, PRD 90 (2014)
094503, arXiv:1407.6387 [hep-lat]

Manifestation of dof for quarks and gluons

5Part 1. 

Experimental results from LHC
heavy ion experiments

Nov. 23, 2009First p-p collisions at√s= 900 GeV (ALICE)First Collisions (2009 - 2010)

Jan. 2010
First ALICE paper

Mar. 30, 2010
First p-p collisionsat √s = 7 TeV

Nov. 8, 2010
First Pb-Pb collisions at√sNN = 2.76 TeV

67ALICE data collection
System

Pb-Pb

Xe-Xe

p-Pb

2018

Beam energysNN (TeV)
5.02

0.9 nb-1

2015

5.02

250 μb-1

2010-20112.76

75 μb-1

2017

5.44

0.3 μb-1

8.16

25 nb-1

5.02

3 nb-1

5.02

15 nb-1

13

59 pb-1

5.02

1.3 pb-1

8

2.5 pb-1

7

1.5 pb-1

2.76

100 μb-1

0.9

200 μb-1

Year

2016
2013
2015-2018pp
2009-2013

Lint

Run-1: 2009 - 2013
Run-2: 2015 - 2018
ALICE publications (2010-2021): 363 papers!

Year
2010

2022

8

Space-time evolution of Heavy Ion Collisions
5. Kinetic freeze-out(momenta are fixed)

54. Chemical freeze-out(Hadronization)

4

3
2
1

3. Local thermal
equilibrium and QGP
2. Glasma
Collision!
1. Initial Condition (CGC?)
arXiv:1804.06469v1, Jonah E. Bernhard

Tinit.

Pb-Pb

9Initial temperature• Thermal radiation• Observed an excess at low pT over the pQCD
expectation• Teff = 304 ± 11(stat.) ± 40 (syst.) MeV• 3.4 trillion K

Tinit. ~ 300 MeV

link
ALICE, PLB 754 (2016) 235, arXiv: 1509.07324 [nucl-ex]

ε10Energy Density

Bjorken Energy Density

Volume, Duration time3
V ~ 300 fm

τ ~ 10 fm/cxz

estimate!

Pb-Pb

ε ~ 16 GeV/fm3

ALICE, Phys. Rev. Lett. 107 (2011) 032301

Re

ac

tio
n

Pla

ne

Collectivity of the system

11

Geometrical AnisotropyΔϕMomentum Anisotropy

Particleʼs
direction
Reaction PlanedNdϕ

Δϕ (rad)dN∝ (1 + 2v2 cos(2Δϕ) + . . . )dϕ
2nd coefficient of Fourier expansion : v2 （elliptic flow）

STAR PRL86,402 (2001)

12• Large v at RHIC and LHC• To produce large v , it needs two conditions in Hydro cal.
2

Au-Au atsNN = 200 GeV

2(1)

Early thermalization 〜 0.6 fm/c

(2)

Very small η/s

Because at early stage of collisions:
１）Reaction zone is elliptic→ Different pressure gradient between short and long axis

Extraction ofproperties from thefinal state→ Elliptic flow (v2) generation
２）Hydrodynamic equation works for QGP at a very early time(~0.6 fm/c) and also needs a small η/s (= strong coupling)“strongly” coupled QGP (sQGP) withearly thermalization

η/s ζ/sdN/dy

Viscosity< vn >

13

η/sBayesianestimation using
RHIC and LHC
data< pT >< pT > fluctuation

J. S. Moreland, J. E. Bernhard, S. A. Bass, PRC 101 (2020) 024911, arXiv:1808.02106 [nucl-th]
Trajectum [PRL 126 (2021) 202301, arXiv:2010.15130], JETSCAPE [PRL 126 (2021) 242301,
arXiv:2010.03928]

ζ/s

η/s

Extracting QGP properties with flow
Shear viscosity (η/s) is near
the quantum lower bound(1/4π)→ QGP is the “perfect liquid”

J. E. Bernhard et al. Nature Phys. 15 (2019) 1113

η/s ~ 0.1 ~ 1/4π

14

15

Parton stopping power in QGP
AA collisions =QGP production

pp collisions＝vacuumq

q
q- Hard probes = Jet heady quarks (c, b, (t))
2
Producedinearlypartonscattering
at
early
stage
with
large
Q
-

- Calculable by pQCD

q

q̂

16

Energy loss in QGP
Jet

λ

qt

１）Gluon radiation (Radiative)

fragmentationq

ΔE ∝ L

R

L
Stopping Power

pQCD

q̂ =

2
⟨qt ⟩

λ

• What is the mechanism of energy loss in QGP?
• How much the stopping power q̂ ？
• Any difference between quark and gluon?
• Where the energy goes?

̂ΔE ∝ αsCR⟨q⟩L22２）Elastic scattering (Collisional)
ΔE ∝ L1

q̂

17

Energy loss in QGP

Hot Dense QGP in Pb − Pb
RAA =Vacumme in pp• Significant suppression of jet in AA• Large energy loss is possible by QGP only
• Extract stopping power from model comparisonq̂ ~ 1-2 GeV2/fm

ALICE, PLB 746 (2015) 1
arXiv:1502.01689 [nucl-ex]
ATLAS, PRL 114 (2015)072302arXiv:1411.2357 [hep-ex]

Burke et al.,arXiv:1312.5003v2(JET collaboration)

CMS HIN-12-004-PAS

Hard probes: Heavy quarks
Mass-dependent suppression of D (c-hadron) and non-prompt J/ψ (from b-hadron)→ dead cone effectHard probes also flow, mass-dependent v2 of muons from c and b decays

ALICE, JHEP 11 (2015) 205, arXiv:1506.06604 [nucl-ex]
CMS, EPJC 77 (2017) 252, arXiv:2003.03565 [nucl-ex]

ATLAS, PLB 807 (2020) 135595,
arXiv:1610.00613 [nucl-ex]

18Melting and regeneration of J/ψ
Quarkonia dissociate at high temperatures → suppression

suppression

T. Matsui, H. Satz, PLB 178 (1986) 416
ALICE, PLB 766 (2017) 212, arXiv:1606.08197 [nucl-ex]

19Melting and regeneration of J/ψ
Quarkonia dissociate at high temperatures → suppression
Clear evidence for recombination as new production mechanism
More charm quarks available at
LHC compared to those at RHIC
→ regeneration→ an evidence of de-confined state
and thermalization

regeneration

R. Thews, M. Schroedter, J. Rafelski, PRC 63(2001) 054905, arXiv:hep-ph/0007323
P. Braun-Munzinger and J. Stachel, Nature 448 (2007) 302
ALICE, PLB 766 (2017) 212, arXiv:1606.08197 [nucl-ex]

20

Thermometer: Melting of quarkonia: ϒMelting excited ϒ states• Suppression of ground state Y(1s), and excited states Y(2S) and Y(3S).

• Sequential melting scenario, Y(3S) > Y (2S) > Y (1S), which are more
weakly bound in this order

CMS, PLB 790 (2019) 270, arXiv:1805.09215 [hep-ex]

21

22

Identified Hadron pT spectra in Pb-Pbd23

Tch, μB

Hadron Chemistry (with 22 species by Tch, μB)

Hadron production:Bose, Fermi distributions
Statistical thermal model
2

∞

g
p dp
ni =
,
E
=
i
∫
2
( E ( p )− µ ) / T
2π 0 e
±1
i

i

p

2

2
+ mi

Particle production assuming thermal
equilibrium with a common chemical
freeze-out temperature→ excellent agreement with the data overnine orders of magnitude!
Tch : 153 MeV, μB ~ 0
A. Andronic et al., Nature 561 (2018) 321

Laboratory for strong interaction•Unveiling strong-interaction potentials among hadrons via femtoscopy•Important test for lattice QCD, input for EOS of neutron stars

ALICE Nature 588 (2020) 232
ALICE Phys. Rev. Lett. 127 (2021) 172301
ALICE Phys. Lett. B822 (2021) 136708

24

25

Small systems
pp

p-Pb

Pb-Pb

Highest p-Pb multiplicity ~ 55-60% Pb-Pb

Sep. 13, 2012 (ALICE)
p+Pb test run

Collective behavior in small systems

26• Two particle correlationsin η and φ spaces• In high multiplicity p-p

and p-Pb, “ridge”
structure in Δη direction
are seenpp
CMS, JHEP 1009 (2010) 91,
arXiv: 1009.4122 [hep-ex]

p-Pb
LHCb, PLB 762 (2016) 473,
arXiv:1512.00439 [nucl-ex]

Collective behavior in small systems

pp

p-Pb

27

• Two particle correlationsin η and φ spaces• In high multiplicity p-p

and p-Pb, “ridge”
structure in Δη direction
are seen
CMS, JHEP 1009 (2010) 91,
arXiv: 1009.4122 [hep-ex]

LHCb, PLB 762 (2016) 473,
arXiv:1512.00439 [nucl-ex]+e eno ridge
ALEPH Archived Data
Phys. Rev. Lett. 123,
212002 (2019)• No ridge in e+e- data

•Origin of the ridge

structure is still unclear• Initial stage (e.g. CGC) or
small QGP?

Strangeness enhancement in small systems
The smooth increase of strange particle yields (w.r.t.
pions) as a function of multiplicity was observed fromp+p to p+Pb to Pb+Pb
Data reproduce by Color reconnection and multiparton interaction (MPI)
Q) At the same multiplicity, physics is same for pp vs.
p-Pb and p-Pb and Pb-Pb?

MPI

Color Reconnection

Figure by H. Menjo (Paolo Gunnellini, MPI@LHC 2017)

Figure by H. Menjo

ALICE, Nature Physics 13 (2017)
535, arXiv: 1606.07424 [nucl-ex]

pp

p-Pb

Pb-Pb

28

29

Part 2.
Open questions

30

List of (my) questions• What is the origin of “collectivity” or ridge in small systems? small system• Color reconnection with multi-parton interaction vs. small QGP• Same physics in pp, p-Pb and Pb-Pb if the all multiplicities are same?
• Why there is no jet quenching in high multiplicity in pp and p-Pb, but Pb-Pb does?
Initial condition• What is the initial condition, CGC exists?
• Why QGP thermalizes so quickly?
• How QGP is created in heavy ion collisions?
Phase diagram
Is
there
a
critical
end
point
in
QCD
phase
diagram?
•• Thermal photons are coming from an early stage or late hadronic stage or both?
Matterproperties
What
is
the
diffusioncoefficientD?
•
Mediumresponce
Mediumresponse:
is
there
a
Mach
cone
shock
wave?
•
Chiralsymmetry
Chiralsymmetry
restoration
at
µ
=0
and
high
temperature?
•
Early
universe
QGP
in
heavy
ion
collisions
is
same
as
QGP
in
early
universe?
•
s

B

31

Part 3.
Future of Heavy Ion
Physics @ LHC (ALICE)

ALICE detecter in Run-2 (2025-2018)32

ALICE upgrades and physics program

33

Run 3 (2022-2025)

Run 4 (2029-2032)

Run 5 (2035-2038)

New TPC (GEM), ITS, muontracker (MFT), Fast online
trigger (FIT) O2 DAQ

Forward Calorimeter (FoCal),
new silicon tracker (ITS3)

ALICE3 (~all silicon)→ x100 statistics in Pb-Pb than Run-2

Heavy quark, jet, heavy nuclei, exotic hadron

Color Glass Condensate, Gluon
density (nPDF), Heavy quark

Chiral symmetry, Heavy
quark, exotic hadron,thermal radiation

34

ALICE Run-3 upgrade during LS2 (2019-2021)

MFT
ITS2

35

ALICE Run-3 upgrade during LS2 (2019-2021)

Pilot beam test (October 2021) – first results
ITS-TPC-MFT tracks online→ 1 Time Frame = 11.5ms (~6 collisions)

TPC

MFT

36

ALICE Run-3 upgrade during LS2 (2019-2021)
FIT

2
O

•Interaction trigger•Event characterizations(e.g. charge particle
multiplicity, vertex position,
event plane, centrality)•Collision time (TOF PID)

• 250 FLPs receive data from detectors → ~3.5 TB/s
(TPC most relevant)• 1500 EPN nodes process sub-timeframes (→merged to complete Time Frames)• Synchronous reconstruction, calibration and datacompression → use of GPU mandatory• Asynchronous stage: reconstruction with final
calibration → Final Analysis Object Data (AOD)Run-3 Physics (1)Thermal radiation/dM eedy (GeV/c 2)-1

Di-electrons (less material, better tracking, low-B run)
1 ALICE Upgrade Simulation−1

Rapp Sum (broad ρ)

ITS2, B = 0.2 T

Rapp Sum (vac ρ)‘meas.’ - cc - cockt.

|η e| < 0.8
p > 0.2 GeV/ cRapp Sum (drop ρ)

Syst. uncert. sig. + bkg.

T,eSyst. uncert. cc + cocktail

10−2

1/N evt d2N

raw

10

Pb-Pb s NN = 5.5 TeV
0-10%, L int = 3 nb-1

10−3

tem

−4

pe

10

0
ALI−SIMUL−306847

0.5

1

1.5

rat

ure

2

fit

2.5
M ee (GeV/c 2)

Heavy flavor recombination
Recombination vs radial flow, also
crucial for diffusion coefficient37Run-3 Physics (2)Access to beauty at low pT- Not achievable with Run 2 data38

Run-3 Physics (3)Search for Chiral symmetry restoration at high T and μB = 0
Towards precision measurement of around ρ mass region

Before

After

39

Ds

40

Run-3 Physics (4)Transport coefficients (Ds)heavy-quark diffusion coefficients

ALI-SIMUL-308744

Open heavy-flavour RAA and v2 down to pT=0 →Precise determination of (2πT)Ds vs. T

ALI-SIMUL-308758

Run-3
Physics(5)
cs

41

Tachibana, Phys.Rev.C 95,
044909 (2017)

EMCal

DCal

ΔE

medium excitation
Quenched parton shower
Vacuum parton shower

ALICE PID capability (TOF)•Medium response, where the lost energy goes?

→ Large angle soft hadron emission• ALICE Run-3:•di-jet and γ-jet using DCal with PID• x100 MB statistics + EMCal trigger• Mach cone search, sound velocity in QGP, EOS (?)

ALICE Run-4 upgrade during LS3 (2026-2028)
ITS3

New ITS Inner Barrel → ITS3

LoI (2019):

https://cds.cern.ch/record/2703140

FoCal

New Foward Calorimeter (FoCal)
LoI (2020):
https://cds.cern.ch/record/2719928

42

Tinit Run-4 Physics (1)A new ultra-light inner barrel (ITS3)• Low-mass di-electrons:
Vertexing (better charm rejection)
Lower material thickness (less conversions)
Higher low-pT efficiency (better conv. rejection)-

1 ALICE Upgrade Simulation

10−1

Pb-Pb s NN = 5.5 TeV
0-10%, L int = 3 nb-1

Rapp Sum (broad ρ)

ITS2, B = 0.2 T

Rapp Sum (vac ρ)‘meas.’ - cc - cockt.

|η e| < 0.8
p > 0.2 GeV/ cRapp Sum (drop ρ)

raw

Syst. uncert. cc + cocktail

tem

pe

−4

10

0
ALI−SIMUL−306847

0.5

1

1.5

Pb-Pb s NN = 5.5 TeV
0-10%, L int = 3 nb-1

Rapp Sum (broad ρ)

ITS3, B = 0.2 T

Rapp Sum (vac ρ)‘meas.’ - cc - cockt.

|η e| < 0.8
p > 0.2 GeV/ c

Syst. uncert. cc + cocktail

10−2

10−3

rat

ure

Rapp Sum (drop ρ)

Syst. uncert. sig. + bkg.

T,e

1/N evt d2N

10−2

10−3

1 ALICE Upgrade Simulation

10−1

Syst. uncert. sig. + bkg.

T,e

1/N evt d2N

raw

ITS3
/dM eedy (GeV/c 2)-1

/dM eedy (GeV/ c 2)-1

ITS2

tem

pe

fit

10−4

2.5
M ee (GeV/ c 2)

0

rat

ure

fit

2

ALI−SIMUL−306852

0.5

1

1.5

2

2.5
M ee (GeV/c 2)

43

ρg Run-4 Physics (2)44
FoCal-H

FoCal-E

FoCal = Forward Calorimeterγ• FoCal-E: High granularity Si-W sampling Electromagnetic
Calorimeter (direct photon, π0)• FoCal-H: Hadron calorimeter (photon isolation, jet)
• Purposes : Gluon density down to very low x (x~10-6), CGC

CERN EP newsletter (2022 March)

Color Glass
Condensate(CGC)“FoCal-E PIXEL”
photon position
Gluon density in
Pb nucleus

2 layers
cell size: 30x30 μm2

FoCal-E“FoCal-E PAD”
photon energy
18 layerscell size: 1x1 cm2

mini-FoCal (2018)

“ALICE3” in Run-5 (2035-2038)

Expression of Interest arXiv:1902.01211

ALICE3 LoI @ CERN research board

Recent workshop (Oct. 2021)

https://indico.cern.ch/event/1063724/

Novel measurements of electromagnetic andhadronic probes of the QGP at very low momenta

45

“ALICE3” in Run-5 (2035-2038)

Track

Si tracker inside thebeam pipe!

46

“ALICE3” in Run-5 (2035-2038)

47

Selection of key points of ALICE 3 physics
・Precision measurements of di-leptons- Characterization and evolution of the QGP- Chiral symmetry restorationTrack・(multi-)heavy-flavoured hadrons- Transport properties and diffusion in the QGP- Mechanisms of hadronization
・Hadron correlation- Interaction potentials- Fluctuations of conserved charges

ALICE3 : Chiral Symmetry Restoration
Chiral SB ⟨q̄q⟩ ≠ 0

⟨q̄q⟩
SPS
RHIC
LHC

48

Restoration of Chiral symmetry

KEK PS, J-PARK⟨q̄q⟩ ≈ 0

@ high T@ high ρ

Achieved highest temp. in ALICE
Vector mesons (ρ,ω,φ, a1, ..)Low mass region ( < 1 GeV/c2)
Highest precision measurement for
e+e- pairs

Increase of ρ meson width, ρ-a1 mixing

49

P.M Hohler and R. Rapp, Phys. Lett. B 731 (2014) 103

spectral function

ρ
ρ a

a1

1

T 増加

・Linked to a partial restoration of chiral symmetry
・Difficult to measure a1 meson experimentally. Needed precision measurement below < 1 GeV/c2
・At high temperature, in-medium modification of ρmeson and regeneration of a1 meson→Look at this change!

+High precision measurement of e e pairs

50

Thermal dielectron spectrum for different spectral function ρ function

・Mee ~ chage around 1 GeV/c2 ：ρ-a1 mixing!
・Needed an extremely high precision
・Cannot be achieved ALICE Run-3 (ITS2),
Run-4 (ITS3)
・Need ALICE Run-5 (“ALICE 3”)

R. Rapp, Adv. High Energy Phys. 2013 (2013) 148253
P.M Hohler and R. Rapp, Phys. Lett. B 731 (2014) 103
R. Rapp private communication

ALICE3 & FoCal+? (Run-5)

CGC, Glasma

QGP production, Chiral symmetry
restoration @ high T

51

52

EIC eA vs. LHC HI- At BNL, EIC will start to operate around 2030

- A high luminosity polarized e, p / ion collider at √s = 28-140 GeV- Factor 100 to 1000 higher luminosity as HERA
- ECCE has been recommended as “Detector-1” by DPAP (Mar.
2022)

Good synergies
with EIC

CGC, initial condition of
Heavy Ion

Detector
Technology

ZDC design for EIC, figure by
S. Shimizu

Summary✓ Since 2009, LHC HI program provides rich and new views on quark
gluon plasma produced at LHC.
✓ Still there are remaining big questions.

✓ To answer these, upgrades are planed.
✓ Another discoveries and surprises in next two decades !

53

54

New directions in science by new tools

Slide from K. Hanagaki (KEK), 2021

55

Thank you for your attention!