star low energy physics: results and plans paul sorensen for star collaboration meeting — july 12...

STAR low energy physics: STAR low energy physics: results and plansresults and plans

Paul Sorensen forPaul Sorensen for

STAR Collaboration Meeting — July 12th, 2006

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clear physics motivation

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location of the critical point

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Ejiri, et.al. 2003Taylor Expansion

Fodor, Katz 2001Lattice Re-weighting

Gavai, Gupta 2005Taylor Expansion

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Ejiri, et.al. 2003Taylor Expansion

Fodor, Katz 2001Lattice Re-weighting

Gavai, Gupta 2005B Lower Limit

B √sNN

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180 MeV 25 GeV420 MeV 7.5 GeV725 MeV 4.5 GeV

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focusingC. Nonaka

Focusing caused by the hydro evolution could lead many initial conditions to reach the

critical point

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experimental indications?

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

and 200)

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%νdyn

PHENIX ||<0.35, =/2CERES 2.0< <2.9

STAR: 5% Central Au+Au

C. PruneauQM05

experimental indications?

same shape for alternative variable νdyn

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

and 200)

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focus

Some Key MeasurementsSome Key Measurements• • yields and particle ratiosyields and particle ratios

T and B

•• elliptic flow velliptic flow v22 collapse of proton flow?

•• k/k/, p/, p/, , ppTT fluctuations fluctuations the critical point signal

• • scale dependence of fluctuationsscale dependence of fluctuations source of the signal

•• vv22 fluctuations fluctuations promising new frontier?

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outline

STAR’s Ability to AchieveSTAR’s Ability to Achieveyields: yields: triggering and centrality

determination

elliptic flow:elliptic flow: event-plane resolution

k/k/, p/, p/:: efficiency, pid, and systematicssystematics

scale dep.:scale dep.: statistics

vv22 fluct.: fluct.: statistics, statistics

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

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acceptance for triggering

Beam Ra

pidity

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

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triggering

impact paramete

r

AuAu @ 5 GeV AuAu @ 8.75 GeV

BBC Inner BBC Outer BBC Inner BBC Outer

b<0 5 27 12 543<b<6 11 30 21 576<b<9 22 35 39 40

BBC Inner: 3.3 to 5.0BBC 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)

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particle identification

log10(p)

log 1

0(dE/dx)

PID capabilities over a broad pT range:TPC dE/dx, ToF, Topology,

EMC, etc.

no anticipated obstacles to measuring no anticipated obstacles to measuring particle spectra and ratios (T and particle spectra and ratios (T and BB))

but… some fluctuation analyses need track-by-track I.D.

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v2 motivation slide

Hydrodynamic interpretation still evolving as analyses progress

Energy dependence plays an important role in our interpretations

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v2 motivation slide

In contradiction to PHENIX comparison:

• v2 at SPS may be similar tosimilar to RHIC v2 (when comparison is made with same centrality and 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 potential for discoverydiscovery

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v2 motivation slide

collapse of proton v2: signature of phase transition (H. Stöcker, E. Shuryak)

• but result depends on analysis technique

difference between v2{4} and v2{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

prot

on v

2

NA49 PRC

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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

STAR

NA49

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v2 fluctuations

simulations

large v2small v2

v2 = 0 v2 = 0

simulations

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 pointbut measurement relies on the tail of the distribution and needs

statistics€

1qdNdq

∝ 12πδvσ g

2 dv exp −M2v − vδv

⎛ ⎝ ⎜

⎞ ⎠ ⎟2 ⎧

⎨ ⎪

⎩ ⎪

⎫ ⎬ ⎪

⎭ ⎪exp − 1

2v M − qσ g

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

2 ⎧ ⎨ ⎪

⎩ ⎪

⎫ ⎬ ⎪

⎭ ⎪exp − v Mq

σ g2

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟I0v Mqσ g

2

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟∫

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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?

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K/ fluctuation: challenges

mis-identification KK/ (K+1)/(-1) or (K-1)/(+1) K/ fluctuations can be distorted

electron contaminationpions leptons that look like kaonsmixed events can’t compensate

kaon decays: K+ +ν (c=3.7 m)tracking efficiency < 50%PID cuts reduce efficiency another 50%

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

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

efficiency

transverse momentum pT (GeV/c)

kaon

protonpion

z for kaons

momentum p (GeV/c)

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

kaons

pions

protons

electrons

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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-)/(++-)

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pT fluctuations

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

• 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

all charged tracks (CI)

like-sign - unlike-sign (CD)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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pT fluctuations

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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fluctuation inversion

scale = full STAR acceptance

fluctuations correlations

variance

excess

residual

/ r

ef (GeV/c)

2

• 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 reveals originsorigins of the of the ppTT fluctuations fluctuations signalsignal

at RHIC v2, mini-jets, medium-response: what about at what about at 7.6 GeV?7.6 GeV?

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conclusion

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 energy range could be checked in Run 7: √sNN = 18, 25, 35, and perhaps even 9 GeV.

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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)

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thanks

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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

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acceptance √sNN=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

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Gavai, Gupta 2005Taylor Expansion

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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)

BBCBBC

star low energy physics: results and plans paul sorensen for star collaboration meeting — july 12...

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