RIKEN ADVANCED INSTITUTE FOR COMPUTATIONAL SCIENCE Lattice QCD at non-zero Temperature and Density Akira Ukawa Deputy Director, RIKEN AICS, Japan 18 August 2015 XXVII International Symposium on Lepton Photon Interactions at High Energies Ljubljana, Slovenia

Lattice QCD/Lattice Field Theory 1 Nuclear physics Particle physics Astrophysics Hadron spectrum QCD parameters CKM elements Glueballs and exotica Quantum gravity Nuclear masses and properties condensed matter physics computer science … Theories beyond the Standard Model Hadron structure QCD at high temperature and density adapted from C. Davies, LP2013 Composite Higgs, Many-flavor theories, near-conformality Super-symmetric models, etc AdS/CFT, etc

Plan of talk  Non-zero temperature but Zero baryon density  Phase diagram and equation of state (EoS)  Loose ends?  Non-zero but small baryon density  Lattice QCD and comparison with phenomenology  Conserved charge fluctuations and experiment  Search for the critical end point?  Summary 2

Development since 80’s 3 Titan Sequoia K computer VAX 11/780 1 st hadron mass calculations by D. Weingarten, H. Hamber&G. Parisi 1 st Monte Carlo of lattice gauge theory by M. Creutz, L. Jacobs, C. Rebbi Deeper theoretical understandings Better numerical algorithms O(10 10 ) increase of FLOPS power

State of the art of lattice QCD includes all light quarks (u, d, s), and even charm or physical values for the quark masses results are extrapolated to the continuum limit A word of caution:thermodynamic calculations still use mostly “staggered quark action” with some unwanted properties (continuum limit mandatory) Full-fledged results with chirally symmetric actions (e.g., domain-wall) needs more FLOPS to catch up. 4

Non-zero temperature but Zero baryon density 5

“Standard” picture of the QCD phases 6 “Columbia Plot” F. R. Brown et al, PRL 65, 2491 (1990) : pure gluon theory

“Standard” picture of the QCD phases 7 : pure gluon theory Summarizes 3 decades of work in theory and simulations

Transition at the physical point (I) Continuous crossover as a function of temperature i.e., no jumps or divergences in the observables 8 Y. Aoki et al, Nature 443:675 (2006)

Transition at the physical point (II) 9 Checked with 3 types of staggered actions (stout, Asqtad, HISQ) Significant difference at finite lattice spacings, but consistent results in the continuum limit Pseudo critical temperature: Wuppertal-Butapest Collaboration Y. Aoki et al, Nature 443: 675 (2006); S. Borsanyi et al, JHEP 2010:73 (2010) HotQCD Collaboration A. Bazavov et al, PRD 85: (2012) Bazavov et al,PRD85, (2012) Continuum limit pseudo critical temperature (MeV)

Equation of State Wuppertal-Budapest: S. Borsanyi et al, Phys. Lett. B730, 99 (2014) HotQCD: A. Bazavov et al, PRD90, (2014) 10 velocity of sound energy density

QGP in heavy ion collisions? Direct photon transverse distribution as a probe of temperature in heavy ion collisions 11 energy density RHIC LHC PHENIX collaboration, PRL104, (2010) M. Wilde for ALICE collaboration, arXic: (2012)

Loose ends? 12

13 N f =2 limit N f =3 end point

U A (1) symmetry and N f =2 limit 14 U A (1) symmetry is broken by anomaly but its effect maybe negligible around the transition This may affect the order and universality class of the N f =2 transition at m u =m d =0 Long debate, not yet completely settled

My assessment 15 No strong evidence of 1 st order; 2 nd order likely Theoretically important to settle But, quantitative effects likely limited at the physical point, since O(4) and O(4)xO(2) critical exponents are similar (only about 10% difference) Finite quark mass effects further softens the difference

16 N f =3 end point 1 st order transition is expected to terminate at a 2 nd order end point

N f =3 end point Pion mass marking the end point is very uncertain: For the staggered action, small values, even consistent with zero? Karsch et al (p4) ‘03 Forcrand-Philipsen (unimproved) ’07 Endrodi et al (stout) ‘07 Ding et al (HISQ) ‘11 For the Wilson action, large scaling violation, and sizable difference from the staggered values Iwasaki et al ‘96 Nakamura et al ‘14 17 Continuum limit pion mass at the end point ? ?

Non-zero but small baryon density 18

How much do we really know? 19 From How does the pseudo critical temperature extend to non-zero density? How does it compare with phenomenological “freeze-out” temperatures? Is there a critical end point, and if yes, where?

Simulation for Standard Monte Carlo simulation methods not applicable Two alternative methods in use for small Taylor expansion in Analytic continuation from imaginary potential 20 expansion coefficients are calculable with simulations at calculable by Monte Carlo, hence analytically continued to real potential

Pseudo critical temperature Numerical methods for small chemical potential: Taylor expansion in Imaginary chemical potential 21 Recent continuum results sizably larger than the pioneering numbers O. Kacmarek et al, Phys.Rev.D83:014504(‘11) G. Endrodi et al, JHEP04, 001(‘11) L. Cosmai et al, Lattice 2015 R. Bellwied et al, Lattice 2015 C. Bonatti et al, Lattice 2015 My eyeball value

Lattice QCD 22 (MeV)

Heavy ion phenomenology Statistical hadronization model Statistical equilibrium at the time of hadronization (freeze out) Boltzman distribution for hadron yields Fits to experimental yields provides an estimate of 23 e.g., review by P. Braun-Munzinger et al, Nucl-th/ (2003)

Lattice QCD 24 (MeV)

Lattice QCD vs phenomenology 25 RICH, SPS Cleymans et al, PRC (2006) RICH Star collaboration Abelev et al, PRC79, (2009) LHC, SPS Bocattini etal, PRL 111, (2012) RHIC LHC SPS (MeV)

Qualitative agreement but higher value from phenomenology? “Precise” comparison rather difficult since Lattice QCD side Pseudo critical temperature has inherent ambiguity, e.g., definition, choice of observable etc Statistical and systematic errors of Monte Carlo calculation Heavy ion phenomenology side Inherent ambiguity in the extraction of i.e., at what stage do hadrons materialize? 26

Conserved charge fluctuations and experiment 27

Moments as thermometers Moments of conserved quantities, e.g., electric charge Q Measurable in experiments for given beam energy Measurable in lattice QCD as functions of Hence, provide a direct thermometer of QGP, e.g., 28 or A. Bazavov et al, PRL 109, (2012) S. Borsanyi et al, PRL 111, (2013)

29 RHIC STAR measurement L. Adamczyk et al, PRL 113, (2014) for given S. Mukherjee, PoS CPOD2014, 005 for S. Borsanyi et al, PRL113, (2014)

Search for the critical end point? 30

31 (MeV)

Critical end point? 32 No new results in spite of many attempts, e.g., reweighting, complex Langevin, Lefschez thimble, … Fodor-Katz JHEP 0404, 050 (2004) ? (MeV)

Summary 33

1 st review of lattice QCD thermodynamics at LP/ICHEP conference series in 10 years! Significant progress in both lattice QCD and experiment (RHIC, LHC) in the mean time. Direct comparison of theory and experiment is now beginning to be possible. The location of critical end point remains elusive; hope for development in near future. 34