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 the collisions: nucleus-nucleus energy High Early Universe Ionut Arsene| UiO Ionut Similar conditions reached at nuclear colliders atnuclear reached conditions Similar 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” ● 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” Ionut Arsene | UiO 20 Initial state: Parton distribution functions Eskola et al., EPJC77 (2017) 163 valence quarks gluons anti - anti - “shadowing” “shadowing” “shadowing” “shadowing” ● 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” 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