the quark-gluon plasma - uiotransition to a deconfined state of matter: quark-gluon plasma (qgp)...
TRANSCRIPT
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
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Part 1: Introduction and motivation
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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−
14
Gμν
a Gaμν
Ionut Arsene | UiO 5
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−
14
Gμν
a Gaμν
● High-Q: asymptotic freedomPhysics Nobel prize 2004 (Wilczek, Gross, Politzer)
● Typically solvable using perturbative theory ● Tested extensively at modern colliders
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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−
14
Gμν
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...
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High energy nucleus-nucleus collisions: the scope
Water
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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?
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High energy nucleus-nucleus collisions: the scope
Water
● What happens if “normal” nuclear matter is compressed and heated ? ● What are the degrees of freedom ?● Are there any phase transitions ?
Nuclear matter
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High energy nucleus-nucleus collisions: the scope
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
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High energy nucleus-nucleus collisions: the scope
Pressure and low temperature
● 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
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High energy nucleus-nucleus collisions: the scopeTe
mpe
ratu
re
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)
Pressure and low temperature
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High energy nucleus-nucleus collisions: the scope
Similar conditions reached at nuclear colliders
Ear
ly U
nive
rse
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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
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Part 2: The “standard model” of relativistic heavy-ion collisions
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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.)
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Initial state
Lorentz contracted nuclei: 2 thin “pancakes” of nucleons
γ=1
√1−v2/c2≃
Em0
at the LHC, γ is of the order of thousands
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Initial state: Parton distribution functions
Parton Distribution Functions(PDF) in “free” nucleons
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Initial state: Parton distribution functions
● 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
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Initial state: Parton distribution functions
● 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”
Eskola et al., EPJC77 (2017) 163
gluons
“shadowing”
valence quarks
“shadowing”
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Initial state: Parton distribution functions
● 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”
Eskola et al., EPJC77 (2017) 163
gluons
“shadowing”
valence quarks
“shadowing”anti -
“shadowing”
anti - “shadowing”
Ionut Arsene | UiO 22
Initial state: Parton distribution functions
● 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
Eskola et al., EPJC77 (2017) 163
gluons
“shadowing”
valence quarks
“shadowing”anti -
“shadowing”
anti - “shadowing”
EMC effect
EMC effect
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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
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Initial state: evolution of proton strucure
Increase of the gluon densities with Q2: more gluons in ultra-relativistic collisions at LHC
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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
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Initial state: Color-glass condensate
Increasing Q2
Dec
reas
ing
x
● 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
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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
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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)
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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
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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
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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)
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Part 3: Colliders and experiments
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An early picture of a heavy-ion collision
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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
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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
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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
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The ALICE detector at the LHC
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Particle identification with ALICE
● Particle identification and tracking over a large kinematic window
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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
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Part 4: Control parameters
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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 ?
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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
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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...
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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
inel
N collAB (b)=A B T AB (b )σ NN
inel
N partA (b)=∫ d sB T B (s)exp(−A T A(b−s)σNN
inel)
Nuclear thickness function:
Optical approximation and overlap function:
Number of binary collisions:
Number of participant (colliding) nucleons:
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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)
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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)
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Part 5: Soft probes
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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
η=12
ln(|p|+ pL
|p|−pL
)=−ln [ tan( θ2)]
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Energy density estimate: from Bjorken
η=12
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
πR2
Δ EΔ z≃
1
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!
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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
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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
arly
Uni
vers
e
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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
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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
Schenke et al, PRC82 (2010) 014903
Tμ ν=(ε+ p)uμ uν−pgμ ν+πμν
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Anisotropic flow
● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles
dNd ϕ≈(1+2∑
n
vn cos [n(ϕ−Ψn)])
vn – flow coefficients
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Anisotropic flow
● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles
● 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∑
n
vn cos [n(ϕ−Ψn)])
vn – flow coefficients
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Anisotropic flow
● Momentum-space anisotropy measured using a Fourier decomposition of the azimuthal distributions of produced particles
● 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∑
n
vn cos [n(ϕ−Ψn)])
vn – flow coefficients
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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
Schenke et al, PRC82 (2010) 014903
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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
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Part 6: Hard/penetrating probes
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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
=1
RAA=1
N coll
Y AA
Y pp
YAA
– yield in AA collisions
Ypp
– yield in pp collisions
Ncoll
– number of binary collisions
● Strong nuclear effects in Pb-Pb collisions, not observed in p-Pb
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Jet quenching
● Large imbalance in back-to-back jets● Typically assumed to be due to different path length traversed in QGP
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Jet quenching
Gluon multiplicity:
Energy loss:
● Energy loss depends on the path length in the medium and on the QGP transport coefficient q
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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
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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 !
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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
H=p2
2μ−αeff
r+kr
αeff=q2 /4 π
μ=m c/2
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Heavy quarkonia and the QGP
V total=(−q)q
4π r+kr V medium (r )=
q4 π
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
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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
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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
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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
Less
sup
pres
sion
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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
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X(3872) structure
● Exotics: X(3872)● Production in AA collisions impose strong constraints on X(3872) structure
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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