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STAR Plans for Low Energy Running. Paul Sorensen for. QCD at High Temperature — BNL — July, 2006. RBRC workshop. M. Stephanov: hep-ph/0402115. location of the critical point. Ejiri, Taylor Expansion. Gavai, Gupta Taylor Expansion. Fodor, Katz Lattice Re-weighting.

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STAR Plans for Low Energy Running

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Star plans for low energy running

STAR Plans for Low Energy Running

Paul Sorensen for

QCD at High Temperature — BNL — July, 2006

Rbrc workshop

RBRC workshop

Location of the critical point

M. Stephanov: hep-ph/0402115

location of the critical point

Location of the critical point1


Taylor Expansion

Gavai, Gupta

Taylor Expansion

Fodor, Katz

Lattice Re-weighting

M. Stephanov: hep-ph/0402115

location of the critical point

Location of the critical point2


Taylor Expansion

Fodor, Katz

Lattice Re-weighting

M. Stephanov: hep-ph/0402115

Gavai, Gupta

B Lower Limit



180 MeV25 GeV

420 MeV7.5 GeV

725 MeV4.5 GeV



location of the critical point


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  pT<0.5 GeV/c (Berdnikov, Rajagopal and Asakawa, Nonaka): reduces signal amplitude

We shouldn’t count on sharp discontinuities


Experimental indications

The Horn

STAR Preliminary

experimental indications?

Experimental indications1

STAR and NA49 Measurements

larger 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?

Experimental indications2

C. Pruneau


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?


Some Key Measurements

• 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?



STAR’s Ability to Achieve

yields: 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.


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)

STAR Detector

Acceptance for triggering


Beam Rapidity

BBC Inner 


BBC Outer 

T. Nayak

Potential problem: losing acceptance in trigger detectors

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

acceptance for triggering


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).

(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


Particle identification



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

V 2 motivation slide

  • 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

v2 motivation slide

V 2 motivation slide1


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

v2 motivation slide

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  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



event-plane resolution

V 2 fluctuations

  • 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



large v2

small v2

v2 = 0

v2 = 0

v2 fluctuations

K fluctuations

no clear fluctuation signal seen at k/ horn

UrQMD: matches p/ but not k/

what to make of the energy dependence?

K/ fluctuations

K fluctuation challenges


z = ln{dE/dx} - ln{Bethe-Bloch}






z for kaons



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: challenges

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%

K/ fluct. error estimate

P t fluctuations

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

P t fluctuations1

pT fluctuations

Fluctuation inversion



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?

fluctuation inversion


  • 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)




Bbc acceptance

  • 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

BBC Acceptance

Acceptance s nn 8 8 gev

STAR detector acceptance


=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 GeV

Location of the critical point3

Gavai, Gupta 2005

Taylor Expansion

location of the critical point

V 2 motivation slide2

S. Voloshin

Hydrodynamic interpretation still evolving as analyses progress

Energy dependence plays an important role in our interpretations

v2 motivation slide

Location of the critical point4

M. Stephanov: hep-ph/0402115

location of the critical point

Location of the critical point5

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

location of the critical point

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