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STAR Plans for Low Energy Running

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location of the critical point

Ejiri, et.al.

Taylor Expansion

Gavai, Gupta

Taylor Expansion

Fodor, Katz

Lattice Re-weighting

M. Stephanov: hep-ph/0402115

location of the critical pointEjiri, et.al.

Taylor Expansion

Fodor, Katz

Lattice Re-weighting

M. Stephanov: hep-ph/0402115

Gavai, Gupta

B Lower Limit

B √sNN

———————————————————

180 MeV 25 GeV

420 MeV 7.5 GeV

725 MeV 4.5 GeV

———————————————————

Cleymans, et.al.

location of the critical pointFocusing 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 pT<0.5 GeV/c (Berdnikov, Rajagopal and Asakawa, Nonaka): reduces signal amplitude

We shouldn’t count on sharp discontinuities

focusinglarger k/ fluctuations could be due cluster formation at 1st order p.t. (Koch, Majumder, Randrup) critical point could be above √sNN~15 GeV

large possible increase in dv2/dpT seen between 17 and 62.4 GeV (but not between 62.4 and 200)

experimental indications?QM05

STAR: 5% Central Au+Au

PHENIX ||<0.35, =/2

CERES 2.0< <2.9

large increase in dv2/dpT seen between 17 and 62.4 GeV (but not between 62.4 and 200)

same shape for alternative variable dyn

experimental indications?• yields and particle ratios

T and B

•elliptic flow v2

collapse of proton flow?

•k/, p/, pT fluctuations

the critical point signal

•scale dependence of fluctuations

source of the signal

•v2 fluctuations

promising new frontier?

focusyields: triggering and centrality determination

elliptic flow: event-plane resolution

k/, p/:efficiency, pid, andsystematics

scale dep.: statistics

v2 fluct.: statistics, statistics

I don’t discuss machine performance or projected number of weeks etc.

outlineBBC

- 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)

Beam Rapidity

BBC Inner

dN/d

BBC Outer

T. Nayak

Potential problem: losing acceptance in trigger detectors

Simulations show that particles will impinge on the Beam-Beam-Counters

acceptance for triggeringBBC Outer: 2.1 to 3.3

Number of particles striking the STAR Beam-Beam Counters (UrQMD Simulations).

(scintillator tiles)

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

triggeringlog10(dE/dx)

log10(p)

PID capabilities over a broad pT 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.

particle identification- Surprisingly:
- v2 at SPS may be similar to RHIC v2 (when comparison is made with same centrality and within same y/ybeam interval)
- ~15% difference (when systematic errors are taken into account)
- but larger <pT>

- Scanning < 62.4 GeV with
- advanced analysis techniques
- more ideal detector

- has potential for discovery

40A GeV

proton v2

- collapse of proton v2: signature of phase transition (H. Stöcker, E. Shuryak)
- but resultdepends on analysis technique

- difference between v2{4} and v2{2} depends on non-flow andfluctuations
- is itnon-flow or fluctuations?A signature of the critical point?

- STAR can clarify with updated analysis techniques and a more ideal detector

- 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 v2 accessible
- (requires 2x No. Events as 200 GeV)
- Estimates made using:
- v2 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

event-plane resolution- precise v2 measurements require knowledge of:
- non-flow g (non-event-plane correlations) and v2(e-by-e v2 fluctuations)
- Q distribution depends on v2, v2 and g(Qx=∑cos(2) and Qy=∑sin(2))

- potential for discovery: v2 fluctuations near the critical point
- but measurement relies on the tail of the distribution and needs statistics

simulations

simulations

large v2

small v2

v2 = 0

v2 = 0

v2 fluctuationsno clear fluctuation signal seen at k/ horn

UrQMD: matches p/ but not k/

what to make of the energy dependence?

K/ fluctuationsz = ln{dE/dx} - ln{Bethe-Bloch}

pion

protons

kaon

kaons

efficiency

z for kaons

pions

electrons

momentum p (GeV/c)

transverse momentum pT (GeV/c)

O. Barannikova, J. Ulery

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%

kaon detection isn’t great: ToF will help(but a new start-time detector is needed)

statistical and systematic errors depend on # of events and detector upgrades

K/ fluctuation: challengesCounts

(K++K-)/(++-)

- 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%

PHENIX: Phys. Rev. Lett. 93, 092301 (2004)

STAR Preliminary

all charged tracks (CI)

like-sign - unlike-sign (CD)

- full acceptance is important: elliptic flow could enhance apparent pT fluctuations in measurements without 2 coverage (but it’s difficult to compare)
- differential analyses are often essential for correct interpretation

pT fluctuations

correlations

residual /ref (GeV/c)2

variance excess

scale = full STAR acceptance

D. Prindle, L. Ray, T. Trainor

- correlations lead to fluctuations
- variance excess can be inverted to pT angular correlations /ref
- elliptic flow, near/away-side and medium response components revealed
- reveals origins of the pT fluctuations signal
- at RHIC v2, mini-jets, medium-response: what about at 7.6 GeV?

- clear potential for discovery
- ,p v2 more precise/accurate with ~half the events used by NA49
- pT and particle ratio fluctuations (address systematics)
- event-by-event v2 fluctuations

- Part of the interesting range may lie above RHIC injection energy
- RHIC covers a unique B range
- —————————————————————————————
- 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)

- lost regions
- some low pT 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

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

acceptance √sNN=8.8 GeVHydrodynamic interpretation still evolving as analyses progress

Energy dependence plays an important role in our interpretations

v2 motivation slidelocation of the critical point

Questions remain regarding validity/stability of all lattice results for non-zero B

location of the critical point
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