A Plasma of Quarks and Gluons

ISCO 2023 at OIST
Barbara JacakUC Berkeley & LBNL
March 3, 2023

Quarks and gluons?
quarks bind together byexchanging gluons
Heat nuclei to T > 1012 K(> trillion degrees!)

Liberate the quarks
What is this stuff?
How does it work????? 2

Quark gluon plasma in early universe
Director:
H. Murayama

QGP likely also in core of massive neutron stars G.Baym! Electromagnetic interactions: photons are exchanged
electrically uncharged! Interaction between quarks = strong interaction
Force is carried by exchanged particles: gluons
3
Both quarks and gluons have a “color charge”

The force between quarks is funny

Potential orforce between
two quarks

Distance between two quarks

At small distances, very little
force. Quarks are essentially
free of each other.

At large distances the force getsstronger, confines quarks inside
4
the particles we observe

The theory of quarks & gluons:
Quantum Chromodynamics (QCD)
Calculating is a challenge
enchange 1, 2 … many gluons
Use a lattice of quarks & gluons
T. Hatsuda

At high temperature:
Many new particles produced
E=mc2
The force is screened
so it gets weaker
Need T > 150 MeV(> 1012 degrees)
To enter plasma phase

5

The heaters
Large Hadron Collider

Relativistic Heavy Ion Collider

CERN in Geneva
Pb+Pb @ 2.76 TeV/A

Brookhaven in New York
Au+Au @ 200 MeV/A

Collide heavy nuclei for max temperature & volume
p+p and p+A for comparison

6

In the LHC tunnel

78How much energy in one collision?

9Experiments at RHIC

10

11

Inside the Time Projection Chamber

12

Nuclear collision timeline
plasma lives ~3x10-23 seconds, ~10-12 cm across

Study with particles coming out! radiated from bulk medium: “internal” plasma probes! large Etot (high p or M) set scale other than T(plasma)autogenerated “external” probe
Can calculate w/perturbative QCD
control probe: photons - EM, not strong interaction

13

Is the collision hot enough?

14

Hottest Science Experiment on the Planet*
plasma lives for 3x10-23 s
droplet is 10-12 cm across
can’t use a thermometer!
So we look at radiation* According to Discover Magazine (2010)

Thermal radiation
e+

g

e-

g*

Low mass, high pT e+e- ®
nearly real photons
Large enhancement above
p+p in the thermal region
PRL 104, 132301 (2010)

A. Monnai

pQCD g spectrum(Compton scattering @ NLO)
agrees with p+p data16

17

direct photons: Tinit > Tc !

• Exponential fit in pT:
Tavg = 221 ±23 ±18 MeV
But, a hot system expands!
• Use hydrodynamics
models to reproduce data
Tinit ≥ 300 MeV

NB: Tc ~ 150 MeV@ LHC Tavg = 304±51 MeV
T ~ 30% higher than at
RHIC

17

Collective flow: a common velocity boost

Model expansion of the system with fluid dynamics
A. Monnai18

Watch QGP blow up → collective flow (v2)zyAlmond shapeoverlap regionin coordinate
space

x

momentumspace

dN/df ~ 1 + 2 v2(pT) cos (2f) + …
“elliptic flow”
19Plasma properties

20

Compare v2 to hydrodynamics“perfect liquid”

Reproduce the data when:system has deconfined quarks and gluons
viscosity is approximately zero (freely flowing liquid)21

Discovery: Hot QCD matter is strongly coupled! viscosity is near limit predicted for equilibrium

systems from the uncertainty principle (& string𝜼⁄𝒔 ≥ ℏ⁄ 𝟒𝝅
theorists!)
Use duality of quantum field theory to black holes! Low viscosity = low resistance to transverse waves-> many interactions, large cross section

h/s *2p

22

Many types of strongly coupled matter
Quark gluon plasma is like other systems with strong
coupling - all flow and exhibit phase transitions
Cold atoms:
coldest & hottest
matter on earth
are alike!
Dusty plasmas &warm, dense
plasmasIn allthese cases have a competition:
Strongly correlated
have liquid and evencondensed
matter:Û
Attractive
forcesrepulsive
force
orkinetic
energycrystalline phases
liquid crystal
High Tc superconductors: magnetic vs. potential energy
phases andResult: many-body interactions, not pairwise!
superconductors
23How does the plasma transportenergy?

24

Opacity via “external probe” of plasma
p-p PRL 91 (2003) 241803

p0

CYM & LGT
PCM & clust. hadronizationN

PCM & hadronic TM

Good agreementwith pQCD

head-on Au+Au
Ncoll = 975 ± 94

25

colored objects lose energy, photons don’tCentral Au + Au

d 2 N AA / dpT dηRAA ( pT ) =TAA d 2σ NN / dpT dη

26

Energy loss even by very energetic q & g! LHC experiments reach to 300 GeV!
27

Interactions inside the plasma28Jet formation not instantaneous
Y. Mehtar-Tani:
1602.01047
Blaizot, et al,
PRL114, 222002(2015)High energy q, g “hadronize” outside the plasma
collide in the plasma & radiate extra gluons
produce secondary particles at larger angle
Lower energy jets hadronize inside the plasma
29
Splittings are calculable by QCD

How much energy is lost?
! Quantify using a parameter called q-hat<

q= √µ2/l l=mean free path; µ=typical pT transfer per scatter~ kT2/l! We find:⌃q = 1.9 +/- 0.7 GeV2/fm @ T=470 MeV PRC90, 014909 (2014)
1.2 +/-0.3 GeV2/fm @ T=370 MeV
0.38 GeV2/fm @ 150 MeV (not plasma) arXiv:1910.07027! In cold nuclear matter⌃q ~ 0.02 GeV2/fm
arXiv:1907.11808

100 x more energy loss in hot plasma!
30

Shower evolution (how jets form)

Calculable in pQCD

Rey Cruz Torres

Not calculable in pQCD
31

How energy is lost: look inside jets
arXiv: 1908.05264
See also: CMS
arXiv: 1803.00042

Pb+Pb/
p+pr! Excess soft particles at large jet radius! Fewer particles at higher pT! Energy loss!

quantify: compare theory to the differential dataStructure within a jet*

Does the plasmachange the firstgluon splitting?

pT 2
zg =
pT1 + pT 2

Yes!
Now: study
other jetstructure
variables
33

So, what do we now know?
! Quark gluon plasma flows with almost no friction

strong coupling & efficient momentum transportmany-body interactions dominate the properties! qhat = 1.2 - 2 GeV2/fm in QGP: 100x larger than in nuclei

QGP induces gluon radiation and collisions
QGP is opaque to quarks and gluons! Cascade of gluon splittings is well modeled by QCD, until

they reach low energy (& many body interactions matter)
Modified by QGP - make softer & more particles
Narrowed jet core-> gluons lose more energy than quarks! What about cold, very dense QCD matter???
34

Inside nucleons and nuclei! Discovery at RHIC:

Spin of the nucleon is spread out among thequarks and gluons!
quarks & gluons in polarized proton also polarized! Implications:
Cold nuclear matter also strongly interacting whendensity of partons is large! Initial state of colliding Pb nuclei already has many-

body interactions
35

Study at future electron-ion collider
Scatter electrons
from nuclei!
e

e’, ν

γ, Z,Wp/Aqp remnant• √s = 30 to 140 GeV

Project:
Add electrons to
the ions in RHIC
at Brookhaven

Hadron Storage Ring
Electron Storage Ring
Electron Injector Synchrotron
Possible on-energy Hadron
injector ring
Hadron injector complex
36

! Backup

37

Other interesting substructure variables
Jet axis difference

angularity

Jet core narrows (q vs. g?)
Theory agrees

38

39

Thank you to my group!

40Quantum ChromoDynamics (QCD)! Field theory of strong interaction

Chromo: color charge of quarks
and gluons
q: 3 colors, g: c & anti-c (8 types)! Works well: key for Standard Model! Quantum Electrodynamics (QED)

for electromagnetic interactions
exchanged particles are photons
electrically uncharged! Can see the photons (fortunately!)

But particles with color are unstable
Cannot “see” a single quark or gluon-> measure color neutral, composite: hadrons

42

Open questions! How can the QGP be hydrodynamic so quickly?

Lots of theoretical work ongoing - How to verify?
! Are there (composite) quasiparticles in QGP?
Evidence that partons bounce off them??
! What size QGP droplet can affect jets?
Systematically measure small systems! Quantify temperature dependence of QGP opacity
Jet substructure at multiple energies! Is there a coherence length in QGP, below which
splittings are not resolved?
Scale dependence of jet substructure in data andtheory43Quantum Chromodynamics
Theory of quarks, gluons, and their interactions

44

Small viscosity/entropy
Viscosity: inability to transport
momentum & sustain a wave
internal friction damps waves
low h ® large s, transports
momentum across fluid
normal QCD: s not so large
large s in QGP® strongly
coupled!
large s ® many-body interactions
when the density is highSo dense QCD matter should alsostrongly affect quarks & gluons
transiting it1η ≈ n pλ f
3

λ

p T

4π p 3→σ≤

Example: milk.
Liquids with higher
viscosities will not
splash as highwhen poured at thesame velocity.

45

p

2

→

“Gauge gravity duality”

Field theoryof quarks
and gluons

situation near a
black hole (butwith an extra
dimension)46

Also explain heavy quark energy loss

Chesler & Yaffe, 0706.0368(hep-th)

String theory: yes

Theory:.Quark gluon plasma stops
heavy charm quarks.Even the heavier b quarks!!
Experiment:.Charm stopped.Beauty less so47

arXiv: 1910.09110

Data + theory = transport
Heavy quark energy loss dominatedby collisions at low pT , radiation athigh pT. Light + heavy: precision

S. Cao, et al.
arXiv:1809.07
894

RHIC and LHC data analyzed by
JETSCAPE collaboration:
S. Cao et al., arXiv: 2102.11337

48

Heavy quark RAA fitsIs there a mass effect on g radiation?
! Soft gluon radiation spectrum

HF/ inclusive

Large M suppresses small angle
radiation (phase space effect)
Known as “dead cone effect”

Dokshitzer, et al. J.Phys.G17,1602 (1991)
Dokshitzer & Kharzeev, PL B519, 199 (2001)

ALICE D-tagged vs.
inclusive jets in p+p

qg

Yes!

49
arXiv:2106.05713

Eeek! Hydrodynamics in small systems!
Not big & dense

PHENIX
Collaboration
Nature Physics(2018)

See collective flow!
Seeded by initialgeometry
Droplet of QGP?

Measured v2(pT) in the 0–5% most central p+Au collisions and 20–40% central d+Au
collisions compared with SONIC predictions and MSTV postdictions. Each point
represents an average over pT bins of width 0.2 GeV c–1 to 0.5 GeV c–1. The vertical lines(boxes) represent one standard deviation statistical (systematic) uncertainties. The
quoted dNch/dη values are taken from ref. 38. Blue and red curves correspond to SONIC
predictions for d+Au and p+Au, respectively. The green curve corresponds to MSTV
calculations for 0–5% central p+Au collisions, which the authors state yield an identical

How can pressurebuild up before thistiny systemevaporates??
50

! Quantify by q-hat

Pb+Pb/p+p

Parton energy loss<

q= √µ2/l l=mean free path;µ=typical pT transfer per scatter~ kT2/lJ/Y Alice 0- J/Y Atlas
0-80%
90%
! Constrain theory by measured RAA
Parton interactions in expanding QGP
Many body q & g! For a 10 GeV parton
interactions!
⌃q = 1.9 +/- 0.7 GeV2/fm @ T=470 MeV
1.2 +/-0.3 GeV2/fm @ T=370 MeV
JET Collaboration PRC90, 014909 (2014)0.38 GeV2/fm @ 150 MeV(hot hadron gas)
arXiv:1910.07027: Dorau, Rose, Pablos & Elfner

0.02 GeV2/fm in cold nuclear matter
arXiv:1907.11808: Ru, Kang, Wang, Xing & Zhang

51

For early splitting: “groom” the jets

! Grooming works on hadrons or partons! Calculate hard parton splittings with perturbative

QCD! Important to benchmark the jets in p+p collisions if
we want to look for modification by the plasma!
52

HOW ABOUT COLD, DENSE
QCD MATTER?
Is it strongly coupled too?

53

Look deep inside nuclei! 3 valence quarks + gluons + sea quarks

Gluons split into quarks + antiquarks
Measured by scattering electrons (g transfer)! Count quark and gluon densities vs. momentum

fraction they carryx is momentum fraction in ∞ momentum
frame (i.e. in proton beam frame)
Q2 is photon virtuality (effective mass)! For p, n inside nucleus:

densities are different!
q, g from different nucleons interact

Higher density –> gluons can fuse
@ higher x values than in isolated protons– Scale goes as xs~ A 1/3

54

Electron-ion collider at Brookhaven
Scatter electrons
from nuclei!
e

e’, ν

γ, Z,Wp/Aqp remnant• √s = 30 to 140 GeV

Add electrons to
the ions at RHIC

Hadron Storage Ring
Electron Storage Ring
Electron Injector Synchrotron
Possible on-energy Hadron
injector ring
Hadron injector complex
55

See quarks & gluons with electron beam?
! Deep inelastic scattering

scatter virtual g off the q charge! “See” gluons when quark
distributions don’t scale withenergy transfer from the electron! Seeing gluons more directly:

in p+p: QCD Compton scattering

q
g

g
q

in e+p: g + g -> ccphoton-gluon fusion

56

Jet’s fate in cold, dense QCD matter! Energy & angle balance

via lepton-jet correlations
compare energy loss tohot, denser QCD matter! Jet broadening?
! Jet substructure

Energy flow/shower development
Quantum # correlation in jets
Hadron formation in jets
Jet angularities57§ At high density, what?
gluon # saturates?

+

?

n
o
i
t
a
r
u
t
Sa
BFKL

1/QS2

Smaller gluon momentumè

Deep in a nucleus: gluons are numerous

DGLAP
Increasing probe energy èNuclear PDF’s! Inside nucleus: densities modified

q, g from different nucleons interact
arXiv:1708.01527

Inclusive DIS off
nuclear beams

59

The measurements

arXiv:1708.01527

sred = F2 – y2/Y+ FL Y+ = 1 + (1 - y)2 F2 = Seq2(xq2 + xq2)

DIS with associated charm for F2charm and FLcharm to
extract gluon distributions

60

Big detectors measure emitted particles

61

Mystery of rapid hydrodynamics*! One of the key questions for hot QCD

But it’s very hard to measure directly!
! Coherent scattering?
Ø Parton interactions in hot, dense QCD andcold, dense QCD should be similar! Rapid expansion driven loss of initial state
memory (hydrodynamic attractor) causing outof-equilibrium hydrodynamic behavior?
! Pre-existing many-body interactions?
* Full thermalization is not required for hydrodynamics
62

arXiv: 1910.09110

Heavy quarks
Lose energy too!
Dominated by collisions at low pT
Radiation at high pT<q= √µ2/l

Phys. Rev. C 90, 014909 (2014)

S. Cao, et al.
arXiv:1809.07
894

Light quark RAA fits: JET collaboration

63

Heavy quark RAA fitsFig. 3: Measuredvn(pcollision
T) in
Hydro
for
p,threed, 3He+ Ausystems
compared with models.
From: Creation of quark–gluon plasma droplets with three distinct geometries

a, Measuredvn(pT) in the 0–5% most central p+Au collisions compared with models. b,
PHENIX
Collaboration
NatureMeasured
Physics (2018)vn(pT) in the 0–5% most central d+Au collisions compared with models. c,
Measured vn(pT) in the 0–5% most central 3He+Au compared with models. Each point in
a–c represents an average over pT bins of width 0.2 GeV c–1 to 0.5 GeV c–1; black circles
are v2(pT), black diamonds are v3(pT). The vertical lines (boxes) represent one standard
deviation statistical (systematic) uncertainties. The solid red (dashed blue) curves

64Probe cold, dense matter: Collide e + A
Electron beam: deep
inelastic scattering

Hadron-Hadronpp/A• Probe is a gluon• Probe has structure!
• Dynamics of the probe

mixed up with structure of
the nucleus• RHIC & LHC

p/A• Point-like probe• No strong interaction before

high momentum transfer
process• Control probe kinematics bymeasuring scattered electron• Needs Electron-Ion Collider

We’ll also find out: will there be hydrodynamic flow if
we excite a hot spot with a point particle??!
65

Quantify viscosity: Fourier coefficients
Nucleons move aroundinside the nucleus-> locations of NN
scattering fluctuate! Hydro gets it right if NN

fluctuations included! v3, etc. more sensitive tothe viscosity/entropy
ratio (viscosity increases
dissipation)! h/s (QGP) = 0.085 – 0.11

vn(h/s=0.08)/vn(ideal)vn(h/s=0.16)/vn(ideal)66

Look at parton “fragmentation”! Can’t see a single quark or gluon in the detector

They don’t exist alone in nature for very long
! Colored objects radiate gluons (bremsstrahlung!)
Angle ordered process in vacuum. In plasma - not.
! In final state these gluons become normal hadrons
Measure a correlated bunch of hadrons – a “jet”
Process of becoming a jet is called fragmentationz = pT hadron / pT parton

67

Transport properties of QGP
weak coupling limit
perturbative QCD
kinetic theory, cascadesinteraction of particles∞ strong coupling limit
not easy! Try a pure field…
gravity « supersym 4-d(AdS/CFT)2®3,3®2,n®2… Lattice
QCD

68Look at parton “fragmentation”! Can’t see a single quark or gluon in the detector

They don’t exist alone in nature for very long
! Colored objects radiate gluons (bremsstrahlung!)
Angle ordered process in vacuum. In plasma???
! In final state: the gluons become normal hadrons
Ø Measure a correlated bunch of hadrons – a “jet”
Ø Look at particle distribution insidez = pT hadron / pT parton
Process of becoming a jet iscalled fragmentation69