1. RHIC Low Energy Scan APS Division of Nuclear Physics 2007 Long Range Plan: Phases of QCD Matter Paul Sorensen

2. 1 outline why is a low energy scan interesting  explore the phase diagram of nuclear matter and  discover the critical point: a landmark study  turn off signatures of deconfinement why at RHIC  large energy range accessible  collider geometry provides great advantages  RHIC detectors: commissioned and well suited for the search what indications for a critical point do we have what can RHIC experiments accomplish what does the CBM detector at FAIR add conclusions

3. 2 physics motivation

4. 3

5. 4 *why does a critical point exist? pro con/maybe  B =0 transition is a crossover at T C =155-205 MeV Lattice: F.R. Brown et al., Phys. Rev. Lett. 65 (1990) 2491 for T=0, the  B transition seems to be first order Model calculations: J. Berges and K. Rajagopal hep-ph/9804233; M. Halasz, A. Jackson … hep-ph/9804290; O. Scavenius, Á. Mocsy … nucl- th/0007030; N. Antoniou, A. Kapoyannis hep-ph/0211392; “fluctuations on the crossover line increase with increasing  B, strongly suggesting the existence of a critical point” (F. Karsch) Lattice: Bielefeld-Swansea, Phys. Rev. D68 (2003) 014507 existence depends on curvature of the critical surface: “critical endpoint is extremely quark mass sensitive” (O. Philipsen) Lattice: P. de Forcrand and O. Philipsen hep-lat/0607017

6. 5 why at RHIC? RHIC collisions cover a broad region of interest

7. 6 advantages of a collider √s NN 6.27 GeV √s NN 17.3 GeV for fixed target geometry: detector acceptance changes with energy  not nice for energy scans track density at midrapidity increases rapidly with √s NN  changes in hit-sharing and track merging  changes in dE/dx and p T resolution for a collider: acceptance does not change and track density only varies slowly point-to-point systematic errors will be better under control acceptance track density

8. 7 RHIC detectors BBC PHENIXSTAR commissioned, proven performers: 1) 2  coverage at  2) P.I.D. across a broad p T range

9. 8 some of the key measurements yields and particle ratios yields and particle ratios  T and  B elliptic flow v 2 elliptic flow v 2   and  v 2 (deconfinement?)  quark number scaling (deconfinement?)  collapse of proton flow? (phase trans?) v 2 fluctuations v 2 fluctuations  enhancement near critical point k/ , p/ ,  p T  fluctuations k/ , p/ ,  p T  fluctuations  enhancement near critical point D mesons, di-leptons D mesons, di-leptons  chiral phase transition look for non-monotonic behavior as correlation lengths increase near the  N.B.finite system size and finite lifetime: correlation lengths are limited ~2 fm hydrodynamic focusing can spread the signals over a broad √s NN range we can’t necessarily count on sharp signatures

10. what we already know at lower √s NN

11. 10 particle ratios and fluctuations dynamical fluctuations:  no clear signature seen at energy where k/  peaks  hadron/string model matches the proton but not the kaon data  what do we make of the energy dependence? evidence still inconclusive  energy scans at FAIR, SPS, and RHIC under consideration the horn  non-monotonic signature, but…

12. 11 proton v 2 collapse of proton v 2 : signature of phase transition (H. Stöcker, E. Shuryak) but result depends on analysis technique: uncertain and inconclusive difference between v 2 {4} and v 2 {2} depends on non-flow and fluctuations is it non-flow or fluctuations? A signature for a phase transition? measurement needs to be repeated: uncertainty can be removed by measuring v 2 fluctuations 40A GeV proton v 2 NA49 PRC √s NN =8.77 GeV

13. 12 what can RHIC detectors do event rates without electron cooling: ~5 Hz at 4.6 GeV no. days to record 10 6 events 6 daysat 4.6 GeV 6 daysat 4.6 GeV 1.5 daysat 8.0 GeV 1.5 daysat 8.0 GeV 0.25 daysat 16 GeV 0.25 daysat 16 GeV electron cooling will improve rates by > an order of magnitude can we trigger on events at such low energies?!  simulations indicate no problems 2  tracking and particle identification  full barrel TOF expected in 2009  full barrel TOF expected in 2009 elliptic flow measurements  good reaction-plane resolution  good reaction-plane resolution multi-strange hadron v 2 within reach fluctuation measurements v 2 fluctuations: an important new capability at RHIC particle ratio fluctuations particle ratio fluctuations (k/  fluctuations aren’t trivial at RHIC: kaon decays reduce efficiency and purity is poor without a TOF p T fluctuations

14. 13 triggering at low √s NN impact parameter AuAu @ 5 GeVAuAu @ 8.75 GeV BBC InnerBBC OuterBBC InnerBBC Outer 0<b<3 5271254 3<b<6 11302157 6<b<9 22353940 BBC Inner: 3.3 to 5.0 BBC Outer: 2.1 to 3.3 Number of particles striking Beam-Beam Counters (UrQMD Simulations). simulations indicate BBCs will be adequate for triggering  expected no. of particles is larger than what is used for p+p collisions what will the background rates be? (scintillator tiles)

15. 14 event-plane resolution better resolution means smaller errors than NA49 (given the same number of events) NA49 flow PRC used less than 500k events per energy a big improvement on v 2 measurements Quark-number scaling and  v 2 (deconfinement) with several million events (several days at √s NN = 9 GeV) Estimates made using: v 2 from NA49 measurements estimate the dN/dy using 1.5*Npart/2 use tracks with |y|<0.5 (should be able to do better) simulate events STAR NA49 1/

16. 15 v 2 fluctuations v 2 fluctuations at the crit. point: new potential for discovery analysis relies on central limit theorem: needs multiplicity and full acceptance also reduces uncertainty on mean v 2  v2 /  v 2  √  s NN,  B critical region *critical point signal size still to be investigated a new technique possible at RHIC to improve all v 2 measurements a new technique possible at RHIC to improve all v 2 measurements an additional robust critical point signature an additional robust critical point signature

17. 16 K/  fluct. error estimate 100k central 40 AGeV Au+Au events: statistical errors only with ToF   5% (relative)without   11% (relative) but systematic errors may be dominant particle mis-identification changes the width of the distribution TOF is important 0.5%   K swapping: width  5% and the signal is only 4%! TOF is important Counts Simulations (K + +K - )/(  + +  -) √s NN =8.77 GeV

18. 17  p T  fluctuations STAR Preliminary acceptance is important: elliptic flow can enhance apparent  p T  fluctuations in measurements without 2  coverage differential analyses are often essential for correct interpretation: full acceptance matters RHIC has the tools needed to best understand  p T  fluctuations Out-of-plane collision overlap zone In-plane

19. 18 CBM detector @FAIR: rare probes + RHIC scan can sweep a broad energy and  B range in upcoming runs + large acceptance commissioned detector already available + at a collider: acceptance won’t change with √s NN  but rare probes may be out of reach: lower luminosity  CBM@FAIR will study chiral symmetry restoration and hadrons in medium 1) low-mass di-leptons (feasibility at RHIC is under study) 2) open charm and deconfinement using 3) multi-strange hadrons (also accessible at RHIC) 4) charmonium suppression

20. 19 conclusions compelling physics motivation: mapping the phase diagram locating the critical point turning off signatures of deconfinement current SPS data are suggestive but inconclusive RHIC detectors are proven and important upgrades are under way: large acceptance available stable acceptance with √s NN  smaller systematic errors full STAR TOF ~2009 accelerator capabilities have been studied down to √s NN = 4.5 GeV: no “show stoppers” complementary international efforts being pursued good potential for discovery and within reach

21. Thanks

22. 21 K/  fluct: challenges at RHIC mis-identification  K K/   (K+1)/(  -1) or (K-1)/(  +1) K/  fluctuations can be distorted electron contamination pions  leptons that look like kaons mixed events can’t compensate kaon decays: K +   +  (c  =3.7 m) tracking efficiency < 50% for colliders PID cuts reduce efficiency another 50% kaon decays reduce efficiency at a collider p.i.d. purity without TOF will help be limited efficiency transverse momentum p T (GeV/c) kaon proton pion z for kaons momentum p (GeV/c) z = ln{dE/dx} - ln{Bethe-Bloch} kaons pions protons electrons STAR acceptance and efficiency

23. 22 what to expect chiral and confinement critical points may be different  experimental searches for chiral symmetry restoration and deconfinement are complimentary heavy-ion collisions may not probe the critical region  T 0 could drop below T C before we hit critical  B  still interesting to search for disappearance of QGP can we turn it off? heavy ion collisions won’t provide sharp signatures  limited correlation lengths (~1-2 fm)  focusing may broaden √  s NN range of signatures a RHIC energy scan may yield 1) critical point signatures in a wide √  s NN range disappearance of QGP 2) and/or disappearance of QGP signatures

24. 23 N.B. some expected limitations C. Nonaka Focusing by the hydro evolution could cause many initial conditions to cross the critical point region: broadening the signal region Correlation lengths expected to reach at most 2 fm  p T <0.5 GeV/c (Berdnikov, Rajagopal and Asakawa, Nonaka) : reduces signal amplitude We can’t count on sharp discontinuities

25. 24  p T  fluctuations

26. 25  p T  fluctuations scale = full acceptance fluctuationscorrelations variance excess acceptance is important: elliptic flow can enhance apparent  p T  fluctuations in measurements without 2  coverage differential analyses are often essential for correct interpretation: full acceptance and statistics matter RHIC has the tools needed to better understand  p T  fluctuations

27. 26 v 2 motivation slide Hydrodynamic interpretation still evolving as analyses progress Energy dependence plays an important role in our interpretations S. Voloshin

28. 27 location of the critical point Gavai, Gupta 2005 Taylor Expansion

29. 28 STAR Detector Designed for these kinds of measurements “The Solenoidal Tracker at RHIC (STAR) will search for signatures of quark- gluon plasma (QGP) formation and investigate the behavior of strongly interacting matter at high energy density. The emphasis will be on the correlation of many observables on an event-by-event basis… This requires a flexible detection system that can simultaneously measure many experimental observables.” STAR Conceptual Design Report (July 1992) BBC

30. 29 particle identification log 10 (p) log 10 (dE/dx) PID capabilities at RHIC over a broad p T range:  TPC dE/dx, ToF, Aerogel, Topology, EMC, etc. no anticipated obstacles to measuring particle spectra and ratios (T and  B ) fluctuation analyses prefer track-by-track I.D.

31. 30 detector capabilities Star: TOF, full acceptance, HFT Phenix: low-mass dileptons? HBD CBM: rare probes Jpsi, Dmesons, low mass dileptons RHIC (STAR PHENIX): Wide energy range, collider configuration, makes v2 and fluctuation measurements easier, critical point location FAIR (CBM): chiral symmetry restoration, rare probes, studies of first order phase transition? TOF+dE/dx+rdE/dx ( ,p) 0.3~12 GeV/c M. Shao et al., NIMA 558, (419) 2006

32. 31 v 2 and deconfinement PRL 92 (2004) 052302; PRL 91 (2003) 182301 large  and  v 2 and quark number scaling  deconfined valence quark stage? can we turn these signatures off ? can we prove they are not from a hadronic stage ? these are questions addressed with a low energy scan