1. Heavy Ion Physics – A Brief Theory Overview Aleksi Vuorinen University of Helsinki Lepton-Photon 2015, Ljubljana, August 20, 2015

2. Tremendously successful theory with very nontrivial properties: Confinement → Nuclear physics Asymptotic freedom → Collider physics Collective behavior → Heavy ion physics

5. Theorists’ (naïve) hope: Do first principles calculations using lattice, pQCD,… Make predictions and compare to data Confirm expectations and claim victory

6. Theorists’ (naïve) hope: Do first principles calculations using lattice, pQCD,… Make predictions and compare to data Confirm expectations and claim victory First principles methods extremely tedious to apply to many interesting problems

7. Theorists’ (naïve) hope: Do first principles calculations using lattice, pQCD,… Make predictions and compare to data Confirm expectations and claim victory Several early theory expectations turned out qualitatively wrong

8. In practice: Need to expand toolbox: Apply effective theories and fundamentally new first principles machinery When necessary, use phenomenological models to make contact with experimental data

9. In practice: Need to expand toolbox: Apply effective theories and fundamentally new first principles machinery When necessary, use phenomenological models to make contact with experimental data This talk: Concentrate on first principles advances, even if it sometimes means making bold extrapolations or even modifying the theory

10. Four main branches of heavy ion theory: 1.Description of initial state and system’s approach to local thermal equilibrium 2.Equilibrium properties of the quark gluon plasma 3.Hydrodynamic expansion and hadronization 4.Hard probes of the plasma

11. 1. Initial state and thermalization

12. A. Kurkela

13. Recent successes/advances in small-x physics: NLO perturbative corrections to small-x evolution [Balitsky et al; Kovner et al; Iancu et al; Lappi et al; …] Quantitative description of the ridge correlation, also in pp and pA collis. [Dumitru et al; Kovner et al; …] New experimental idea: Do DIS at the LHC using ultraperipheral AA collisions

14. Key theory questions for the description of HICs: How to describe early dynamics and evolution towards thermalization/hydrodynamization? What are the correct initial conditions to be fed to hydro codes?

16. At weak coupling, power counting arguments → Bottom-up thermalization [Baier, Mueller, Schiff, Son], where Expansion makes system underoccupied before thermalization Soft gluons first create thermal bath, then hard modes undergo radiational breakup

18. Opposite limit: Collision of planar shock waves in AdS space – “HICs” in strongly coupled N = 4 SYM At high T, theory qualitatively similar to QCD: deconfinement, Debye screening, SUSY broken,… Very hard dynamical problems in field theory turned into calculations in classical (super)gravity

20. Key challenge for future: How to approach physical situation of QCD at intermediate energy/coupling? Derive and carry out simulations in NLO kinetic theory [Ghiglieri et al] Compute finite coupling corrections to holographic thermalization [Steineder, Stricker, AV; …] Merge weak and strong coupling descriptions with semi-holography [Iancu, Mukhopadhyay] From purely phenomenological point of view, not so clear if all of this important: Hydrodynamic simulations insensitive to many details of thermalization

21. 2. Quark gluon plasma in equilibrium

22. Two takes on equilibrium properties of QGP: Phenomenology: Need only few inputs (EoS, transport coeffs.) for hydro Theory: Many fundamental properties of theory (phase diagram,EoS,…) equilibrium quantities Historically very important problems; major motivator of heavy ion experiments!

26. Lattice results for QCD thermodynamics: Brief review 1.Cross-over deconfinement and chiral transitions around 160 and 155 MeV [HotQCD; Wuppertal-Budapest groups] Typically determined from Wilson line and chiral susceptibilities, respectively Realistic quark masses no longer a problem

27. Lattice results for QCD thermodynamics: Brief review

29. Minkowskian spectral function, needed for transport With transport properties, run into problem: Euclidean correlator, measurable on the lattice

32. 3. Hydrodynamic evolution

33. Nontrivial lesson from RHIC collisions: Hydrodynamic modeling of heavy ion collisions (predictions for particle spectra) works extremely well

34. What is hydrodynamics? What goes in and what comes out? How do we know hydro works, and what does it teach us? Where do we stand at the moment?

38. Main effect of hydrodynamic flow in HICs: Conversion of spatial anisotropy to momentum space H. Niemi

39. Main effect of hydrodynamic flow in HICs: Conversion of spatial anisotropy to momentum space [Heinz, Chen, Song]

41. New developments: Attempts to read off temperature dependence of shear viscosity from data [Eskola, Niemi, Paatelainen; …] Constraints on the EoS from comparison with data [Pratt et al,…] Incorporation of effects from magnetic fields and anomalies via Chiral MagnetoHydroDynamics [Kharzeev, Yee;…]

42. 4. Hard probes

44. Two examples

45. Jet quenching and broadening: Hard process → Back to back partons → Symmetric pair of jets in vacuum In dense medium, jets lose energy (asymmetrically) → `Jet quenching’ Related observation: Lots of soft hadrons at large angles Challenge for theory: Explain findings from 1 st principles! NB: Expect interplay between weak and strong coupling

46. Long history of energy loss calculations [Baier et al; Gyulassy et al; Arnold, Moore, Yaffe] : Distinction between collisional (heavy flavors) and radiative (light quarks) energy loss Nontrivial to turn this insight into quantitative jet structure calculations in HICs

47. Long history of energy loss calculations [Baier et al; Gyulassy et al; Arnold, Moore, Yaffe] : Distinction between collisional (heavy flavors) and radiative (light quarks) energy loss Nontrivial to turn this insight into quantitative jet structure calculations in HICs Two qualitative pictures: Vacuum: Ordered branching leads to coherent cascade Medium: Democr. branching, momentum broadening Y. Mehtar-Tani

49. EM probes (photons and dileptons) in HICs: Probe all stages of the collision Are sensitive to ICs, prethermal flow, as well as EoS and viscosities Interact weakly: Escape the plasma almost freely In particular, thermal photons and dileptons should be a good thermometer of the equilibrium plasma…

50. EM probes (photons and dileptons) in HICs: Probe all stages of the collision Are sensitive to ICs, prethermal flow, as well as EoS and viscosities Interact weakly: Escape the plasma almost freely In particular, thermal photons and dileptons should be a good thermometer of the equilibrium plasma… … if only we could separate them from prompt, jet- thermal, hadron gas thermal and decay photons

51. In fact, large excess of direct photons and their elliptic flow observed in AA collisions → “Direct photon puzzle”

52. [Chen, Heinz, Paquet, Kozlov, Gale]

53. Key development: Extension of thermal photon and dilept. production to NLO in pQCD [Ghiglieri et al; Ghisoiu, Laine; …] Also, NLO results in finite coupling expansion within strongly coupled N = 4 SYM [Hassanain, Schvellinger] : Consistent interpolation between weak and strong coupling limits In holography, even studies of off-equilibrium production possible [Baier, Stricker, Taanila, AV]

54. Conclusions

55. Quantitatively describing heavy ion collisions with first principles calculations is a daunting task… …but appears to be feasible with a combination of Hard work using old and fundamentally new tools Taking full advantage of effective theories Drawing insights from experimental data