Search for QCD Critical Point at RHIC Bedanga Mohanty National Institute of Science Education and Research (NISER) Outline:  Phase diagram of QCD  Observables for CP search  Experimental results on CP search at RHIC  Summary 1

Phase Diagram and Basic Interactions Phase diagram of Water Electromagnetic interaction Precisely known Phase diagram of strong interactions. Largely still a conjecture arXiv: [h ep-ph] 2

QCD Phase Diagram - Theory 3 At  B = 0 Lattice QCD  Quark – hadron Transition – Cross-over  Transition temperature ~ 150 MeV (Observable dependent)  Robust continuum limit results: EOS and Thermodynamics At  B non-zero  Efforts on to draw the transition line – large uncertainties  Efforts on to get EOS and thermodynamics  Critical point search progressing – positive results indicate CP region below beam energy 30 GeV

Search for Critical Point - Theory Numerical QCD calculations at large  B – sign problem Techniques: Reweighting, Taylor expansion & Im. potential Issues (not common to all) : lattice spacing, physical quark mass, continuum limit, Volume Theory: still some more work to be done …… need more CPU Acta Phys.Polon.Supp. 5 (2012) Phys. Rev. D 78, (2008 ) JHEP 0404, 50 (2004) Phys.Rev.D 71, (2005) 4 PoS LATTICE2013 (2013) 202 More : In Talk by R. Gavai

QCD Phase Diagram - Experimental Vary beam energy to change Temperature & Baryon Chemical Potential Vary: T,  B,  S,  Q Conservation in strong interactions -- Charge -- Baryon number -- Strangeness Nature 448 (2007) Search for CP helps establish the QCD phase diagram

RHIC Beam Energy Scan-Phase I √s (GeV)Statistics(Millions) (0-80%) Yearμ B (MeV)T (MeV)μ B /T 7.7~ ~ ~ ~ ~ ~ ~ ~ μ B, T : J. Cleymans et al., Phys. Rev. C 73, (2006). SN0598 6

STAR Detector System TPC MTD Magnet BEMC BBC EEMC TOF HFT 7

Particle Identification at STAR TPC TOF EMC HFT Neutral particles e, μ π K p d TPC TOF TPC Log 10 (p) Wide acceptance with excellent particle identification Multi-fold correlations for identified particles! Hyperons & Hyper-nuclei Jets Heavy-flavor hadrons MTD High p T muonsJets & Correlations Charged hadrons 8

Uniform Acceptance at Mid-rapidity Au+Au at 7.7 to Au+Au at 200 GeV 9

Freeze-out Results 10 STAR Preliminary Chemical Freeze- out: (GCE) - Central collisions - BES-I: 20 < μ B < 420 MeV T ch variation is small

11 Critical Point Observable Necessity: Observable sensitive to correlation length and susceptibilities Challenges: Finite system size effects,  < 6 fm Finite time effects,  ~ fm ~  2 ~  ~  7 S  ~               Phys.Rev.Lett. 107 (2011) Phys. Rev. Lett. 91, (2003) Phys. Rev. Lett. 102, (2009) Phys. Rev. D 61, (2000) Phys.Lett. B696 (2011) 459 Phys.Rev.Lett. 105 (2010) Observable:

12 QCD based Model: CP Region and Kurtosis J. Deng M. Stephanov: Phys.Rev.Lett. 107 (2011) M. Asakawa et al., Phys. Rev. Lett. 103 (2009) Poisson  Oscillation  Poisson Collisions Energy  increasing Potential Prob. Distribution

13 Critical Point Observable ~  2 ~  ~  7 S  ~               Observable:

14 QCD CP Results : Smaller p T acceptance Critical Region: below 27 GeV ? Is there a Oscillation ? Phys.Rev.Lett. 112 (2014) ; SN0598 Net-proton  Net-baryon Phys.Rev. C87 (2013) Phys.Rev. C86 (2012)

15 QCD CP Results: Smaller p T acceptance Data: Net-proton Distribution shape ~ protons Poisson: Net-proton and proton distributions shows deviation from Poisson Binomial: Net-proton and proton distributions deviate at 19.6 Hadron Resonance Gas Model: Deviations for net-proton and proton distributions at 19.6 and 27 GeV. UrQMD: Net-proton and proton distributions deviate at 19.6 and 27 GeV. Phys. Lett. B 724, 51 (2013). arXiv:

Net-proton results: All data show deviations below Poisson for κσ 2 at all energies. Larger deviation at √s NN ~ 20 GeV Comparing to LGT calculations T f = 146 ± 6 MeV, for √s NN > 39 GeV STAR: PRL112, 32302(14) / arXiv: STAR: PRL113,092301(14) / arXiv: Higher Moments Results 16 More : In Talk by F. Karsch

Directed Flow Results 17 Phys.Rev.Lett. 112 (2014) ; SN0598 Observations consistent with general theoretical expectations from a First Order Phase Transition D.H. Rischke et al. HIP1, 309(1995) H. Stoecker, NPA750, 121(2005) J. Steinheimer et al., arXiv: P. Konchakovski et al., arXiv:

18 QCD Phase Structure: 1 st order Phase Transition and CP  Non-monotonic variations of v 1 slope for net- protons – signature of softening of equation of state/1 st Order Phase transition ?  Hints of non-monotonic variation of  2 for net- protons --- Is there an osscilation ? – BES- II / larger phase space acceptance needed.

New CP results: Larger p T Acceptance Error bars are statistical only. Systematic errors estimation underway. Dominant contributors: a) efficiency corrections b) PID. STAR Preliminary X.F.-Luo:CPOD2014 Non-monotonic variation New method of efficiency Correction, X. Luo, arXiv:

Higher Moments: Rapidity Acceptance Approaches Poisson values for smaller rapidity range STAR Preliminary X.F.-Luo:CPOD

Higher Moments : Acceptance in p T X.F.-Luo:CPOD2014 Values depend on p T acceptance 21

Net-proton Higher Moments Net-proton results: All data show deviations below Poisson for κσ 2 at all energies. Larger deviation at √s NN ~ 20 GeV. Non-monotonic behavior in central collision data. X.F. Luo, CPOD2014 STAR Preliminary

Summary A. At high energy-nuclear collisions liquid-like quark-gluon plasma formed with small η/s. B.RHIC BES-I results show properties of matter changes around √s NN = 20 GeV i.e. μ B ~ 250 MeV. Higher statistics data are need at high baryon region 2000 – 2015: QGP at μ B ~ 0 discovered 2015 – 2020: 1 st order phase boundary QCD Critical Point 2000 – 2015: QGP at μ B ~ 0 discovered 2015 – 2020: 1 st order phase boundary QCD Critical Point Emergent properties of QCD Emergent properties of QCD

√S NN ( GeV )  B (MeV) BES I (MEvts) Rate(MEvts/ day) BES I L ( 1×10 25 /cm 2 sec ) BES II (MEvts) eCooling (Factor) Beam Time (weeks) BES Phase II Proposal BES Phase II is planned for two 22 cryo-week runs in 2018 and 2019 STAR Upgrades: iTPC, EndCap ToF and Event Plane Detector 24 SN0598