The Quark-Gluon Plasma

The quark-gluon plasma

Ionut-Cristian Arsene University of Oslo Department of Physics

FYS3500 – spring 2020 Outline

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

Ionut Arsene | UiO 2 Part 1: Introduction and motivation

Ionut Arsene | UiO 3 Quantum chromo-dynamics (QCD)

● 6 quarks, 3 colours (RGB) and 8 gluons (coloured!) μ 1 a μ ν LQCD=ψi(i (γ Dμ )ij−mδij)ψj − Gμ ν Ga ● ...difficult to calculate 4 ● No analytical solutions (except 1+1)

Ionut Arsene | UiO 4 Quantum chromo-dynamics (QCD)

● 6 quarks, 3 colours (RGB) and 8 gluons (coloured!) μ 1 a μ ν LQCD=ψi(i (γ Dμ )ij−mδij)ψj − Gμ ν Ga ● ...difficult to calculate 4 ● No analytical solutions (except 1+1)

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

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

Ionut Arsene | UiO 5 Quantum chromo-dynamics (QCD)

● 6 quarks, 3 colours (RGB) and 8 gluons (coloured!) μ 1 a μ ν LQCD=ψi(i (γ Dμ )ij−mδij)ψj − Gμ ν Ga ● ...difficult to calculate 4 ● No analytical solutions (except 1+1)

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

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

● also one of the Millennium Prize problems...

Ionut Arsene | UiO 6 High energy nucleus-nucleus collisions: the scope

Water

Ionut Arsene | UiO 7 High energy nucleus-nucleus collisions: the scope

Water

● What happens if “normal” nuclear matter is compressed and heated ? in other words: how does the phase diagram of nuclear matter looks like?

Ionut Arsene | UiO 8 High energy nucleus-nucleus collisions: the scope

Water Nuclear matter

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

Ionut Arsene | UiO 9 High energy nucleus-nucleus collisions: the scope

Double star system with one neutron star Source: astronomie.nl

Pressure and low temperature

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

Ionut Arsene | UiO 10 High energy nucleus-nucleus collisions: the scope

J.M.Lattimer, arXiv:1305.3510

Pressure and low temperature

● Increasing nuclear matter density while “Canonical” mass at ~1.4 M keeping temperature low leads to phase Sun transitions in color superconducting phases How can the outliers exist? → requires stiff equation of state ● e.g. neutron stars

Ionut Arsene | UiO 11 High energy nucleus-nucleus collisions: the scope e r u t a r e p m e T

Neutron star merger Source: NASA

Pressure and low temperature

● 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) Ionut Arsene | UiO 12 High energy nucleus-nucleus collisions: the scope e s r e v i n U

y l r a E

Similar conditions reached at nuclear colliders

Ionut Arsene | UiO 13 High energy nucleus-nucleus collisions: the scope

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

Ionut Arsene | UiO 14 Part 2: The “standard model” of relativistic heavy-ion collisions

Ionut Arsene | UiO 15 The “Standard Model” of relativistic heavy ion collisions

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

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

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)

Ionut Arsene | UiO 16 Initial state

Lorentz contracted nuclei: 2 thin “pancakes” of nucleons 1 E at the LHC, γ is of the order of thousands γ= 2 2 ≃ √1−v /c m0

Ionut Arsene | UiO 17 Initial state: Parton distribution functions

Parton Distribution Functions(PDF) in “free” nucleons

Ionut Arsene | UiO 18 Initial state: Parton distribution functions

Eskola et al., EPJC77 (2017) 163

valence quarks gluons

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

Ionut Arsene | UiO 19 Initial state: Parton distribution functions

Eskola et al., EPJC77 (2017) 163

valence quarks gluons

“shadowing” “shadowing”

Ionut Arsene | UiO 20 Initial state: Parton distribution functions

Eskola et al., EPJC77 (2017) 163

valence quarks gluons

anti - anti - “shadowing” “shadowing” “shadowing” “shadowing”

Ionut Arsene | UiO 21 Initial state: Parton distribution functions

Eskola et al., EPJC77 (2017) 163

valence quarks gluons

anti - EMC anti - EMC “shadowing” “shadowing” effect “shadowing” “shadowing” effect

● Measurements done tipically using collisions of different projectiles (ν, e, μ, π, p, d) on nuclear targets ● 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 Ionut Arsene | UiO 22 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

Ionut Arsene | UiO 23 Initial state: evolution of proton strucure

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

Ionut Arsene | UiO 24 Initial state: gluon (re)interactions

Remember: gluons interact with each other

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

Ionut Arsene | UiO 25 Initial state: Color-glass condensate x

g n i s a e r c e D

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

Ionut Arsene | UiO 26 Early collision stage: excited vacuum

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

Ionut Arsene | UiO 27 Early collision stage: hard partonic scatterings

● Hard partonic collisions (large-Q2) take place leading to the creation of ● High-p partons (jets) T ● Heavy quarks (c, b, t) ● Weak bosons (W, Z) Ionut Arsene | UiO 28 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

Ionut Arsene | UiO 29 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

Ionut Arsene | UiO 30 Kinetic freeze-out

● System cools further and at a given density, it decouples ● Elastic collisions also stop ● Kinetic distributions are frozen

● At the LHC (√s =2.76 TeV), spectra are NN harder than at RHIC (√s =200GeV) NN ● Stronger particle flow at high energy

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

ALICE, PRL 109 (2012) 252301

Ionut Arsene | UiO 31 Part 3: Colliders and experiments

Ionut Arsene | UiO 32 An early picture of a heavy-ion collision

Ionut Arsene | UiO 33 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 Ionut Arsene | UiO 34 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

Ionut Arsene | UiO 35 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

Ionut Arsene | UiO 36 The ALICE detector at the LHC

Ionut Arsene | UiO 37 Particle identification with ALICE

● Particle identification and tracking over a large kinematic window

Ionut Arsene | UiO 38 Other heavy-ion detectors at the LHC

ATLAS CMS

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

● 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 measurements (jets) T ● Heavy-quark and weak bosons production

Ionut Arsene | UiO 39 Part 4: Control parameters

Ionut Arsene | UiO 40 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 ?

Ionut Arsene | UiO 41 Collision energy

BRAHMS, PRL93 (2004) 102301 Net-protons: N(p) – N(p) Average shift in rapidity: Measure of the initial nucleons in the nuclei <δy> = - beam net-p

● Initial state: all nucleons at beam rapidity y beam ● Final state: most nucleons found in the “fragmentation peak” ● At RHIC (√s =200 GeV): <δy> ≈ 2 NN ~70% of the available energy used for generating NEW particles Ionut Arsene | UiO 42 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...

Ionut Arsene | UiO 43 Collision centrality: the Glauber model

● A probabilistic model of the collision ∞ Nuclear thickness function: T A(b)=∫ dzρ( z, b) −∞ Optical approximation and overlap function: inel inel ∫d sT A (b)T B (b−s)σ NN≡T AB (b)σ NN

AB inel Number of binary collisions: N coll (b)= A B T AB (b)σ NN

A inel Number of participant (colliding) nucleons: N part (b)=∫ d s B T B (s)exp(− A T A(b−s)σ NN )

Ionut Arsene | UiO 44 Collision centrality: the Glauber model

(peripheral) (semi-central) (central)

● Collision centrality can be determined using measurements of produced particles ● Main assumption: more produced particles → more central events ● The number of participants N and number of binary collisions N can be part coll inferred using a Glauber fit of the measured particle multiplicity

Ionut Arsene | UiO 45 What are the conditions that can be achieved? (extracted from data and models)

● 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

● 14 3 Density: ρ=1-10ρ0 (ρ0: density of a Au nucleus = 2.7*10 g/cm ) 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.)

Ionut Arsene | UiO 46 Part 5: Soft probes

Ionut Arsene | UiO 47 Total particle multiplicity

1 |p|+ pL η= ln( )=−ln[ tan( θ )] 2 |p|− pL 2

● 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

Ionut Arsene | UiO 48 Energy density estimate: from Bjorken

1 |p|+ pL η= ln( )=−ln[ tan( θ )] 2 |p|− pL 2

At a given rapidity, all the matter is created in the same space-time volume 1 Δ E 1 Δ E ϵ ≃ ≃ Bjorken 2 2 Δ η π R Δ z π R τ0 Using τ =1 fm and R=6fm: ϵ ∼10GeV / fm3 0 Bjorken Nearly 100x higher energy density 3 than normal nuclear matter! ϵcold =0.15GeV /fm

Ionut Arsene | UiO 49 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

Ionut Arsene | UiO 50 Identified hadron yields: chemical freeze-out e s r e v i n U

y l r a

Tchem ≃156 156MeV, MeV, μ ≃156 MeV, 0 E B

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

μi=μB Bi +μI I i+μS Si+μC Ci

● Only 3 parameters needed: temperature T , baryon chemical potential μ and volume V chem B

Ionut Arsene | UiO 51 Anisotropic flow

Schenke et al, PRC82 (2010) 014903

● 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

Ionut Arsene | UiO 52 Anisotropic flow

Schenke et al, PRC82 (2010) 014903

Tμ ν=(ε+ p)uμ uν− pgμ ν+πμ ν

● 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

Ionut Arsene | UiO 53 Anisotropic flow

● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles dN ≈(1+2∑ vn cos[n(ϕ−Ψ n)]) d ϕ n v – flow coefficients n

Ionut Arsene | UiO 54 Anisotropic flow

● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles dN ≈(1+2∑ vn cos[n(ϕ−Ψ n)]) d ϕ n v – flow coefficients n

● Second order harmonic (v – elliptic flow) 2 is dominant due to the ellitic shape of the nuclei overlap ● Higher order harmonic flow originate mainly from initial state energy density fluctuations

Ionut Arsene | UiO 55 Anisotropic flow

● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles dN ≈(1+2∑ vn cos[n(ϕ−Ψ n)]) d ϕ n v – flow coefficients n

● Hydrodynamics calculations in good agreement to data

Ionut Arsene | UiO 56 Anisotropic flow

Schenke et al, PRC82 (2010) 014903

● Shear viscosity to entropy ratio, η/s = 0.2 ● Lower bound for any substance, conjectured from AdS/CFT: η/s = 1/4π ≈ 0.08 Kovtun, Son, Starinets hep-th/0405231

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

Ionut Arsene | UiO 57 Direct photons

PHENIX 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 Ionut Arsene | UiO 58 Part 6: Hard/penetrating probes

Ionut Arsene | UiO 59 Nuclear suppression

1 Y AA R AA = N coll Y pp

Y – yield in AA collisions AA Y – yield in pp collisions pp N – number of binary collisions coll

● Z0, W±, high momentum photons ● No direct information on the QGP, but they act as standard candles for the nuclear modification effects: R =1 AA ● Strong nuclear effects in Pb-Pb collisions, not observed in p-Pb

Ionut Arsene | UiO 60 Jet quenching

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

Ionut Arsene | UiO 61 Jet quenching

Gluon multiplicity:

Energy loss:

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

Ionut Arsene | UiO 62 Extracting q from data

● Transport coefficient can be extracted from data using pQCD calculations together with hydro models for the evolution of the system 2 ● q(√s =0.2 TeV) ≈ 1.2 GeV /fm NN 2 ● q(√s =5 TeV) ≈ 2.2 GeV /fm NN ● Higher energy density at the LHC lead to larger energy losses per unit length

Ionut Arsene | UiO 63 Heavy quarkonia

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

● 2 2 > 2.9 GeV/c for charmonium (mc ~ 1.27 GeV/c ) ● 2 2 > 9.3 GeV/c for bottomonium (mb ~ 4.6 GeV/c ) ● Non-relativistic quark – antiquark system !

Ionut Arsene | UiO 64 Heavy quarkonia

● Lets assume a quark – antiquark pair ● Potential seen by the two quarks will have two components: q a Coulomb term (quark electric charge) : V (r)= Coulomb 4 π r

and a linear term (strong force): V linear (r)=k r q ● V =(−q) +kr The potential energy of the pair is then total 4π r

2 2 p αeff αeff =q /4 π ● The Hamiltonian can be written as H= − +kr 2μ r μ=m c/2

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

Ionut Arsene | UiO 65 Heavy quarkonia and the QGP

−r / λ q q e D 1 V =(−q) +kr V (r)= λ D≃ total 4π r medium 4 π r T (vacuum) (Yukawa-like short range Debye screening length potential)

● 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 Ionut Arsene | UiO 66 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

Ionut Arsene | UiO 67 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) 196 Thews et al., PRC 63 (2001) 054905 Ionut Arsene | UiO 68 J/ψ measurements with ALICE n o i s s e r p p u s

s s e L

● 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 Ionut Arsene | UiO 69 Light-by-light scattering

ATLAS, PRL 123 (2019) 052001

d’Enterria, Silveira PRL111(2013)080405

● 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

Ionut Arsene | UiO 70 X(3872) structure

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

Ionut Arsene | UiO 71 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

Ionut Arsene | UiO 72