STAR low energy physics: results and plans Paul Sorensen for STAR Collaboration Meeting — July 12 th, 2006

1 clear physics motivation

2 location of the critical point

3

4 Ejiri, et.al Taylor Expansion Fodor, Katz 2001 Lattice Re-weighting Gavai, Gupta 2005 Taylor Expansion

5 location of the critical point Ejiri, et.al Taylor Expansion Fodor, Katz 2001 Lattice Re-weighting Gavai, Gupta 2005  B Lower Limit  B √s NN ——————————————————— 180 MeV25 GeV 420 MeV7.5 GeV 725 MeV4.5 GeV ———————————————————

6 location of the critical point

7 focusing C. Nonaka Focusing caused by the hydro evolution could lead many initial conditions to reach the critical point

8 experimental indications? larger k/  fluctuations could be due cluster formation at 1 st order p.t. (Koch, Majumder, Randrup)  critical point could be above √s NN ~15 GeV large increase in dv 2 /dp T seen between 17 and 62.4 GeV (but not between 62.4 and 200)

9 PHENIX |  |<0.35,  =  /2 CERES 2.0<  <2.9 STAR: 5% Central Au+Au C. Pruneau QM05 experimental indications? same shape for alternative variable dyn large increase in dv 2 /dp T seen between 17 and 62.4 GeV (but not between 62.4 and 200)

10 focus Some Key Measurements yields and particle ratios yields and particle ratios  T and  B elliptic flow v 2 elliptic flow v 2  collapse of proton flow? k/ , p/ ,  p T  fluctuations k/ , p/ ,  p T  fluctuations  the critical point signal scale dependence of fluctuations scale dependence of fluctuations  source of the signal v 2 fluctuations v 2 fluctuations  promising new frontier?

11 outline STAR’s Ability to Achieve yields: yields: triggering and centrality determination elliptic flow: elliptic flow: event-plane resolution k/ , p/  : systematics k/ , p/  : efficiency, pid, and systematics scale dep.: scale dep.: statistics v 2 fluct.: v 2 fluct.: statistics, statistics I don’t discuss machine performance or projected number of weeks etc.

12 acceptance for triggering Beam Rapidity BBC Inner  BBC Outer  UrQMD  dN/d  Potential problem: losing acceptance in trigger detectors Simulations show that particles will impinge on the Beam- Beam-Counters

13 triggering impact parameter GeV BBC InnerBBC OuterBBC InnerBBC Outer b< <b< <b< BBC Inner: 3.3 to 5.0 BBC Outer: 2.1 to 3.3 Number of particles striking the STAR Beam-Beam Counters (UrQMD Simulations). Simulations indicate that STAR’s BBCs will be adequate for triggering Centrality can be taken from reference multiplicity  expected number of particles is larger than what is used for p+p collisions (scintillator tiles)

14 particle identification log 10 (p) log 10 (dE/dx) PID capabilities over a broad p T range:  TPC dE/dx, ToF, Topology, EMC, etc. no anticipated obstacles to measuring particle spectra and ratios (T and  B ) but… some fluctuation analyses need track-by-track I.D.

15 v 2 motivation slide Hydrodynamic interpretation still evolving as analyses progress Energy dependence plays an important role in our interpretations

16 v 2 motivation slide In contradiction to PHENIX comparison: similar to v 2 at SPS may be similar to RHIC v 2 (when comparison is made with same centrality and same y/y beam interval) ~15% difference (when systematic errors are taken into account) but larger Scanning < 62.4 GeV with advanced analysis techniques more ideal detector potential for discovery has potential for discovery

17 v 2 motivation slide collapse of proton v 2 : signature of phase transition (H. Stöcker, E. Shuryak) but result depends on analysis technique difference between v 2 {4} and v 2 {2} depends on non-flow and fluctuations is it non-flow or fluctuations? A signature of the critical point? STAR can clarify with updated analysis techniques and a more ideal detector 40A GeV proton v 2 NA49 PRC

18 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 STAR will excel in these measurements Quark-number scaling and  v 2 accessible (requires 2x No. Events as 200 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

19 v 2 fluctuations simulations large  v 2 small  v 2  v 2 = 0 simulations precise v 2 measurements require knowledge of: non-flow g (non-event-plane correlations) and  v 2 (e-by-e v 2 fluctuations) Q distribution depends on  v 2 ,  v 2 and g (Q x =∑cos(2  ) and Q y =∑sin(2  )) potential for discovery: v 2 fluctuations near the critical point but measurement relies on the tail of the distribution and needs statistics

20 K/  fluctuations dynamical fluctuations: no clear signal seen at energy where k/  peaks hadron/string model matches the proton data but not the kaon data what do we make of the energy dependence?

21 K/  fluctuation: challenges 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% PID cuts reduce efficiency another 50% ToF will help kaon detection isn’t great: ToF will help (but a new start-time detector is needed) errors# of eventsdetector upgrades statistical and systematic errors depend on # of events and detector upgrades 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

22 K/  fluct. error estimate 100k central 40 AGeV Au+Au events: statistical errors only with ToF   5% (relative)without   11% (relative) but systematic errors are dominant particle mis-identification changes the width of the distribution 1%   K swapping: width  10%2% swapping: width  20% Counts Simulations (K + +K - )/(  + +  -)

23  p T  fluctuations PHENIX: Phys. Rev. Lett. 93, (2004) STAR Preliminary full acceptance is important: elliptic flow could enhance apparent  p T  fluctuations in measurements without 2  coverage (but it’s difficult to compare) differential analyses are often essential for correct interpretation all charged tracks (CI) like-sign - unlike-sign (CD)

24  p T  fluctuations

25 fluctuation inversion scale = full STAR acceptance fluctuationscorrelations variance excess residual  /  ref (GeV/c) 2 correlations lead to fluctuations variance excess can be inverted to p T angular correlations  /  ref elliptic flow, near/away-side and medium response components revealed reveals origins of the  p T  fluctuations signal what about at 7.6 GeV? at RHIC v 2, mini-jets, medium-response: what about at 7.6 GeV?

26 conclusion clear potential for discovery ,p v 2 more precise/accurate with ~half the events used by NA49  p T  and particle ratio fluctuations (address systematics) event-by-event v 2 fluctuations Part of the interesting energy range could be checked in Run 7: √s NN = 18, 25, 35, and perhaps even 9 GeV. ————————————————————————————— real potential exists with only 100k events but we should be sure we have enough data “after we do the scan we don’t want to end up with all the same uncertainties!” paraphrasing R. Stock (RBRC workshop)

27 thanks

28 BBC Acceptance lost regions some low p T particles can make it in inner BBC tiles may not be useful STAR detector acceptance TPC:  =1  =40.4  (0.705 rad) FTPC:  =2.5  =4.0  =9.39  =2.1  BBC inner:  =3.3  =5.0  =4.2  =0.77  BBC outer:  =2.1  =3.3  =14  =4.2 

29 acceptance √s NN =8.8 GeV STAR detector acceptance TPC:  =1  =40.4  (0.705 rad) FTPC:  =2.5  =4.0  =9.39   =2.1  BBC inner:  =3.3  =5.0  =4.2   =0.77  BBC outer:  =2.1  =3.3  =14   =4.2  For this energy the TPC will cover |y|<1.0 while NA49 covered -0.4<y<1.8 We should have similar multiplicities but the STAR acceptance will be more uniform

30 location of the critical point Gavai, Gupta 2005 Taylor Expansion

31 STAR Detector Designed and built for these 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