The quark-gluon plasma - UiOtransition to a deconfined state of matter: quark-gluon plasma (QGP)...

The quark-gluon plasma

Ionut-Cristian ArseneUniversity of OsloDepartment of Physics

FYS3500 – spring 2020

Ionut Arsene | UiO 2

Outline

● Introduction and motivation● The “standard model” of relativistic heavy-ion collisions● Colliders and experiments● Control parameters● Soft probes● Hard/penetrating probes● Summary

Part 1: Introduction and motivation

Quantum chromo-dynamics (QCD)

● 6 quarks, 3 colours (RGB)

and 8 gluons (coloured!)● ...difficult to calculate

● No analytical solutions (except 1+1)

LQCD=ψi(i (γμDμ)ij−mδ ij)ψ j−

a Gaμν

● High-Q: asymptotic freedomPhysics Nobel prize 2004 (Wilczek, Gross, Politzer)

● Typically solvable using perturbative theory ● Tested extensively at modern colliders

a Gaμν

● Low-Q: confinement / chiral symmetry breakingPhysics Nobel Prize 2008 (Y.Nambu)

● Non-perturbative, largely unknown● Most of the visible matter in the Universe

● also one of the Millennium Prize problems...

High energy nucleus-nucleus collisions: the scope

● What happens if “normal” nuclear matter is compressed and heated ?

in other words: how does the phase diagram of nuclear matter looks like?

● What happens if “normal” nuclear matter is compressed and heated ? ● What are the degrees of freedom ?● Are there any phase transitions ?

Nuclear matter

Pressure and low temperature

Double star system with one neutron starSource: astronomie.nl

● Increasing nuclear matter density while keeping temperature low leads to phase transitions in color superconducting phases● e.g. neutron stars

J.M.Lattimer, arXiv:1305.3510

“Canonical” mass at ~1.4 MSun

How can the outliers exist?→ requires stiff equation of state

High energy nucleus-nucleus collisions: the scopeTe

Neutron star mergerSource: NASA

● Increasing both nuclear density and temperature, more phases appear and possibly a transition to a QGP phase ● e.g. neutron star merger events (recent gravitational wave measurements

suggest a possible phase transition occuring during the final stages of the merger)

Similar conditions reached at nuclear colliders

● Create in the laboratory a chunk of deconfined matter (also called Quark-Gluon Plasma, QGP) and study its properties and phase diagram

Part 2: The “standard model” of relativistic heavy-ion collisions

The “Standard Model” of relativistic heavy ion collisions

Initial state Hard partonic Fireball Chemical freeze-out Kinetic freeze-out collisions expansion

0 10-26-10-24 10-24-10-23 ~10-23 10-23-10-22 t (s) 0 0.01-1 1-10 ~10 10-100 (fm/c)

Image: S.A.Bass (Duke Univ.)

Initial state

Lorentz contracted nuclei: 2 thin “pancakes” of nucleons

√1−v2/c2≃

at the LHC, γ is of the order of thousands

Initial state: Parton distribution functions

Parton Distribution Functions(PDF) in “free” nucleons

● Measurements done tipically using collisions of different projectiles (ν, e, μ, π, p, d) on nuclear targets

Eskola et al., EPJC77 (2017) 163

gluonsvalence quarks

● At small x-values, the nuclear PDFs are smaller wrt free nucleons: nuclear “shadowing”

gluons

“shadowing”

valence quarks

“shadowing”

● At small x-values, the nuclear PDFs are smaller wrt free nucleons: nuclear “shadowing”● At intermediate x-values, nPDFs are larger: anti-”shadowing”

gluons

“shadowing”

valence quarks

“shadowing”anti -

“shadowing”

anti - “shadowing”

● At small x-values, the nuclear PDFs are smaller wrt free nucleons: nuclear “shadowing”● At intermediate x-values, nPDFs are larger: anti-”shadowing”● At large x-values: the EMC effect; EMC = European Muon Collaboration

● Poorly understood; thought to originate in the Fermi motion of the nucleons inside nucleus

gluons

“shadowing”

valence quarks

“shadowing”anti -

“shadowing”

anti - “shadowing”

EMC effect

Initial state: evolution of proton structure

Q2 dependence of PDFs for different values of x-value

Increase of the gluon densities with Q2: more gluons in ultra-relativistic collisions at LHC

Initial state: evolution of proton strucure

Increase of the gluon densities with Q2: more gluons in ultra-relativistic collisions at LHC

Initial state: gluon (re)interactions

At sufficiently low-x, 2→1 processes may counterbalance 1→2 processes leading to a saturation effect

Remember: gluons interact with each other

Initial state: Color-glass condensate

Increasing Q2

● Phenomenon called Color Glass Condensate● Characterized by an x-dependent saturation scale Q

s● May occur in both pp or nuclear collisions, but should

be enhanced in nuclear collisions

Early collision stage: excited vacuum

● Nuclei pass through each other (partly transparent at high energies) leaving behind a highly excited gluon field → rapid production of additional gluons and qq pairs

● Most of the system entropy is created at this stage

Excited vacuum

Early collision stage: hard partonic scatterings

● Hard partonic collisions (large-Q2) take place leading to the creation of ● High-p

T partons (jets)

● Heavy quarks (c, b, t)● Weak bosons (W, Z)

Fireball expansion and creation of QGP

● In heavy-ion collisions at relativistic energies the fireball undergoes a phase transition to a deconfined state of matter: quark-gluon plasma (QGP)

● Fast medium expansion and cooling well described by hydrodynamics● Main objective of the heavy-ion research field

Chemical freeze-out

● System temperature and density decrease● Inelastic collisions cease● Hadronization (quarks and gluons

become bound in hadrons)

● Yields of various particle species are frozen

● Non-perturbative process

Kinetic freeze-out

● System cools further and at a given density, it decouples

● Elastic collisions also stop● Kinetic distributions are frozen

ALICE, PRL 109 (2012) 252301

● At the LHC (√sNN

=2.76 TeV), spectra are

harder than at RHIC (√sNN

=200GeV)● Stronger particle flow at high energy

● Hydrodynamical models reproduce the data → the fireball expands hydrodynamically nearly as a perfect fluid (very low viscosity)

Part 3: Colliders and experiments

An early picture of a heavy-ion collision

A Pb-Pb collision as seen by ALICE

● A 3D picture (with 500 million voxels) of a central collision (about 3000 primary tracks)● Billions of such pictures are taken to be analyzed offline

Heavy-ion accelerators

● Past● Bevalac @ LBL, Berkeley (1980-1990): √sNN=2.4 GeV● AGS @ BNL, Brookhaven (1985-1995): √sNN=4.8 GeV● SPS @ CERN, Geneva (1987-2004): √sNN=17.3 GeV

● Present:● SIS @ GSI, Darmstadt: √sNN=2.5 GeV● RHIC @ BNL, Brookhaven: √sNN=200 GeV (*)● LHC @ CERN, Geneva: √sNN=2760, 5020 GeV (*)

● Future:● FAIR @ GSI, Darmstadt (~2025): √sNN=2-5 GeV● NICA @ JINR, Dubna-Moscow: √sNN=5-11 GeV● J-PARC-HI @ J-PARC, Tsukuba: √sNN=2-6 GeV

(*) colliders

Heavy-ion accelerators

● The physics programme at the existing heavy-ion accelerators is centered around two main aspects:

● Study the quark-gluon plasma properties at an energy scale where this is well established● LHC, RHIC

● Scan of the phase diagram, i.e. searching for the tri-critical point● Lower energy experiments: SPS, RHIC-BES, FAIR, NICA, J-PARC

The ALICE detector at the LHC

Particle identification with ALICE

● Particle identification and tracking over a large kinematic window

Other heavy-ion detectors at the LHC

Image: https://cms.cern/detectorImage: https://atlas.cern/discover/detector

ATLAS CMS

● Although their primary objective is the study of Higgs and physics beyond the Standard Model, ATLAS and CMS detectors make significant contributions to the study of QGP, e.g.● High-p

T measurements (jets)

● Heavy-quark and weak bosons production

Part 4: Control parameters

Collision energy

● Usually expressed in the center-of-mass system and per nucleon pair: √sNN

● How much of the collision energy is actually used for creating NEW particles ?

Collision energy

● Initial state: all nucleons at beam rapidity ybeam

● Final state: most nucleons found in the “fragmentation peak”● At RHIC (√s

NN=200 GeV): <δy> ≈ 2

~70% of the available energy used for generating NEW particles

Net-protons: N(p) – N(p)Measure of the initial nucleons in the nuclei

Average shift in rapidity:<δy> = <y

beam> - <y

net-p>

BRAHMS, PRL93 (2004) 102301

Collision centrality

● Nuclei are extended objects → properties of the collision will be very different between central (head-on) and peripheral collisions● Impact parameter (b) needs to be known, but it cannot be measured directly...

Collision centrality: the Glauber model

● A probabilistic model of the collision

T A(b)=∫−∞

dzρ( z , b)

∫d sT A (b)T B (b−s )σ NNinel≡T AB (b)σ NN

N collAB (b)=A B T AB (b )σ NN

N partA (b)=∫ d sB T B (s)exp(−A T A(b−s)σNN

Nuclear thickness function:

Optical approximation and overlap function:

Number of binary collisions:

Number of participant (colliding) nucleons:

Collision centrality: the Glauber model

● Collision centrality can be determined using measurements of produced particles● Main assumption: more produced particles → more central events● The number of participants N

part and number of binary collisions N

coll can be

inferred using a Glauber fit of the measured particle multiplicity

(central)(semi-central)(peripheral)

What are the conditions that can be achieved?

● Temperature: T=100-1000 MeV (1MeV ≈ 10 billion degrees Kelvin)

up to 106 x temperature of the Sun core

● Pressure: P=100-300 MeV/fm3 (1MeV/fm3 ≈ 1028 atmospheres)

Earth center: 3.6*106 atmospheres

● Density: ρ=1-10ρ0 (ρ0: density of a Au nucleus = 2.7*1014 g/cm3)

Density of Au = 19 g/cm3

● Volume: about 2000 fm3 (1 fm = 10-15 m)

● Duration: about 10 fm/c (or about 3*10-23 sec.)

(extracted from data and models)

Part 5: Soft probes

Total particle multiplicity

● Measure of the energy density in the fireball● Higher particle density at mid-rapidity (direction perpendicular to the beam

direction)● Higher particle density in central wrt more peripheral collisions● Particle density grows with energy

ln(|p|+ pL

|p|−pL

)=−ln [ tan( θ2)]

Energy density estimate: from Bjorken

ln(|p|+ pL

|p|−pL

)=−ln [ tan( θ2)]

At a given rapidity, all the matter is created in the same space-time volume

ϵBjorken≃1

Δ EΔ z≃

π R2τ0

Δ EΔ η

Using τ0=1 fm and R=6fm: ϵBjorken∼10 GeV / fm3

ϵcold=0.15 GeV / fm3

Nearly 100x higher energy density than normal nuclear matter!

Identified hadron yields

Many other particle species measured, e.g. γ, e, μ, π0, η, K0, ρ, ω, φ, ψ, Υ, Λ, Σ, Ξ, Ω, Z0, W, d, t, 3He, 4He

Identified hadron yields: chemical freeze-out

● Yields of identified particles can be explained assuming thermal equilibration of the QGP

● Only 3 parameters needed: temperature Tchem

, baryon chemical potential μB and volume V

μ i=μB Bi+μI I i+μS S i+μC Ci

Tchem 156 MeV, ≃156 MeV, μB 0≃156 MeV, E

Anisotropic flow

● Strong spatial anisotropy of the initial system● Large energy density gradients

● High density in the system core and zero at the boundary● Large density fluctuations in the core

Schenke et al, PRC82 (2010) 014903

Anisotropic flow

● If the system is dense enough, the evolution follows the laws of hydrodynamics ● Initial spatial anisotropy converted to final state momentum anisotropy via the

equation of state● Sensitivity to

● initial state fluctuations ● QGP properties: equation of state and viscosity

Tμ ν=(ε+ p)uμ uν−pgμ ν+πμν

Anisotropic flow

● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles

dNd ϕ≈(1+2∑

vn cos [n(ϕ−Ψn)])

vn – flow coefficients

Anisotropic flow

● Second order harmonic (v2 – elliptic flow)

is dominant due to the ellitic shape of the nuclei overlap

● Higher order harmonic flow originate mainly from initial state energy density fluctuations

dNd ϕ≈(1+2∑

Anisotropic flow

● Second order harmonic (v2 – elliptic flow)

is dominant due to the ellitic shape of the nuclei overlap

● Higher order harmonic flow originate mainly from initial state energy density fluctuations

● Hydrodynamics calculations in good agreement to data

dNd ϕ≈(1+2∑

Anisotropic flow

● Shear viscosity to entropy ratio, η/s = 0.2● Lower bound for any substance, conjectured from AdS/CFT: η/s = 1/4π ≈ 0.08

● ~400 times less viscous than water → closest to perfect liquid

Kovtun, Son, Starinets hep-th/0405231

Direct photonsPHENIX ALICE

T = 304 ± 51 MeV

● Direct photons originate from● Initial hard scaterings● quark anti-quark annihilations in the QGP phase: thermal radiation

● Measurements are sensitive to the time dependence of the emission power and to medium temperature

● The observed effective temperature is the highest recorded for any object

Part 6: Hard/penetrating probes

Nuclear suppression

● Z0, W±, high momentum photons● No direct information on the QGP, but they act as standard candles for the

nuclear modification effects: RAA

N coll

– yield in AA collisions

– yield in pp collisions

– number of binary collisions

● Strong nuclear effects in Pb-Pb collisions, not observed in p-Pb

Jet quenching

● Large imbalance in back-to-back jets● Typically assumed to be due to different path length traversed in QGP

Jet quenching

Gluon multiplicity:

Energy loss:

● Energy loss depends on the path length in the medium and on the QGP transport coefficient q

Extracting q from data

● Transport coefficient can be extracted from data using pQCD calculations together with hydro models for the evolution of the system

● q(√sNN

=0.2 TeV) ≈ 1.2 GeV2/fm● q(√s

NN=5 TeV) ≈ 2.2 GeV2/fm

● Higher energy density at the LHC lead to larger energy losses per unit length

Heavy quarkonia

● Bound states of heavy quark pairs: cc and bb● Typical masses:

● > 2.9 GeV/c2 for charmonium (mc ~ 1.27 GeV/c2)● > 9.3 GeV/c2 for bottomonium (mb ~ 4.6 GeV/c2)● Non-relativistic quark – antiquark system !

Heavy quarkonia

● Lets assume a quark – antiquark pair● Potential seen by the two quarks will have two components:

a Coulomb term (quark electric charge) :

and a linear term (strong force):

● The potential energy of the pair is then

● The Hamiltonian can be written as

● Surprisingly good quantitative description of the heavy quarkonia spectroscopy can be obtained with just this simple Hamiltonian

V Coulomb (r )=q

4 π r

V linear (r )=k r

V total=(−q)q

4π r+kr

2μ−αeff

αeff=q2 /4 π

μ=m c/2

Heavy quarkonia and the QGP

V total=(−q)q

4π r+kr V medium (r )=

e−r / λD

r(vacuum) (Yukawa-like short range

potential)

λ D≃1T

Debye screening length

● In a deconfined medium two main phenomena affect quarkonia:● String tension disappearing● Coulomb potential is screened by other charges (Debye-

screening)

● Quarkonium states will be melted if their size is larger than the screening length● Melting point of each state is temperature dependent → QGP

thermometer Matsui and Satz, PLB 178 (1986) 416

J/ψ suppression in Au-Au collisions at RHIC

PHENIX arXiv:1208.2251

● Au-Au collisions at 200 GeV

● Strong suppression observed in central Au-Au collisions at RHIC energies

● Confirms the proposed idea of color screening in hot and dense deconfined nuclear matter

Heavy quarkonium at the LHC (re-generation)

Nature 448 (2007) 302-309

● Many charm quark-pairs created in one single collision (~100)● Possible to create charmonium states on a statistical basis when system

temperature is low enough ● enhancement of charmonium states at LHC

Braun-Munzinger and Stachel, PLB 490 (2000) 196Thews et al., PRC 63 (2001) 054905

J/ψ measurements with ALICE

● ALICE results shows less suppression compared to lower energies (PHENIX) in central collisions, despite larger energy density

● Charmonium regeneration plays an important role in the production of charmonium● Important probe of deconfinement

Light-by-light scattering

● Light-by-Light scattering in EM fields of nuclei ● Test for Standard Model, sensitive to BSM physics

● Photoproduction of vector mesons in ultra-peripheral AA collisions● Test for Cold Nuclear Matter and gluon saturation effects

d’Enterria, Silveira PRL111(2013)080405

ATLAS, PRL 123 (2019) 052001

X(3872) structure

● Exotics: X(3872)● Production in AA collisions impose strong constraints on X(3872) structure

Summary

● The aim of studying the high energy heavy-ion collisions is to better understand QCD at low-Q and finite temperature, i.e. confinement/deconfinement phase transitions

● Conditions reachable are similar to the ones during the early Universe (few microseconds) and in the core of neutron stars

● This field incorporates knowledge from many other areas of physics:● Thermodynamics, hydrodynamics, string theory

● … and technology● Detector, Electronics, High Performance Computing

● and provides input for fields like:● cosmology, astrophysics, solid-state physics, etc.

● A relatively young and very challenging field of study with a rich phenomenology, the manifestation of many-body QCD

The quark-gluon plasma - UiOtransition to a deconfined state of matter: quark-gluon plasma (QGP)...

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