RHIC Low Energy Scan

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RHIC Low Energy Scan

APS Division of Nuclear Physics2007 Long Range Plan:

Phases of QCD Matter

Paul Sorensen



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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



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physics motivation



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physics motivation



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*why does a critical point exist?

pro con/maybeB=0 transition is a crossover at TC=155-205 MeVLattice: F.R. Brown et al., Phys. Rev. Lett. 65 (1990) 2491

for T=0, the B transition seems to be first orderModel 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



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why at RHIC?RHIC collisions cover a broad region of interest RHIC collisions cover a broad region of interest



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advantages of a collider√sNN 6.27 GeV

√sNN 17.3 GeV

for fixed target geometry:detector acceptance changes with energy

not nice for energy scanstrack density at midrapidity increases rapidly with √sNN

changes in hit-sharing and track merging changes in dE/dx and pT resolution

for a collider:acceptance does not change and track density only varies slowly

point-to-point systematic errors will be better under controlpoint-to-point systematic errors will be better under control

acceptanceacceptance track densitytrack density



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RHIC detectors

BBCBBC



PHENIXSTAR



commissioned, proven performers:commissioned, proven performers: 1) 2 1) 2 coverage coverage at at 2) P.I.D. across a broad p 2) P.I.D. across a broad pTT range range



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some of the key measurements

• • yields and particle ratiosyields and particle ratios T and B

•• elliptic flow velliptic flow v22 and v2 (deconfinement?) quark number scaling

(deconfinement?) collapse of proton flow? (phase trans?)

•• vv22 fluctuations fluctuations enhancement near critical point

•• k/k/, p/, p/, , ppTT fluctuations fluctuations enhancement near critical point

•• D mesons, di-leptonsD mesons, di-leptons chiral phase transitionlook for non-monotonic behavior as correlation lengths increase near the look for non-monotonic behavior as correlation lengths increase near the

N.B. finite system size and finite lifetime: correlation lengths are limited ~2 fmhydrodynamic focusing can spread the signals over a broad √sNN range

we can’t necessarily count on sharp signatures



what we already know at lower √sNN



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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…



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proton v2

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

• but result depends on analysis technique: uncertain and inconclusive

difference between v2{4} and v2{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 v2 fluctuations

40A GeV

prot

on v

2

NA49 PRC√sNN=8.77 GeV



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what can RHIC detectors do

event rates without electron cooling: event rates without electron cooling: ~5 Hz at 4.6 GeV~5 Hz at 4.6 GeVno. days to record 10no. days to record 1066 events events 6 days6 days at 4.6 GeVat 4.6 GeV 1.5 days1.5 days at 8.0 GeVat 8.0 GeV 0.25 days0.25 days at 16 GeVat 16 GeVelectron cooling will improve rates by > an order of magnitudeelectron cooling will improve rates by > an order of magnitude

can we trigger on events at such low energies?!can we trigger on events at such low energies?! simulations indicate no problemssimulations indicate no problems

22 tracking and particle identification tracking and particle identification full barrel TOF expected in 2009full barrel TOF expected in 2009

elliptic flow measurementselliptic flow measurements good reaction-plane resolutiongood reaction-plane resolutionmulti-strange hadron vmulti-strange hadron v22 within reach within reach

fluctuation measurementsfluctuation measurementsvv22 fluctuations: fluctuations: an important new capability at RHICan important new capability at RHICparticle ratio fluctuations particle ratio fluctuations (k/ fluctuations aren’t trivial

at RHIC:kaon decays reduce efficiency and purity is poor

without a TOFppTT fluctuations fluctuations



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triggering at low √sNN

impact paramete

r

AuAu @ 5 GeV AuAu @ 8.75 GeV

BBC Inner BBC Outer BBC Inner BBC Outer

0<b<3

5 27 12 54

3<b<6

11 30 21 57

6<b<9

22 35 39 40

BBC Inner: 3.3 to 5.0BBC 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 collisionswhat will the background rates be?

(scintillator tiles)



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event-plane resolutionbetter 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 v2 measurements

Quark-number scaling and v2 (deconfinement) with several million events

(several days at √sNN = 9 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

1/



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v2 fluctuationsv2 fluctuations at the crit. point: new potential for discovery

analysis relies on central limit theorem: needs multiplicity and full acceptancealso reduces uncertainty on mean v2

v2/

v 2

√sNN, B

critical region

*critical point signal size still to be investigated

• a new technique possible at RHIC to improve all va new technique possible at RHIC to improve all v22 measurements measurements• an additional an additional robustrobust critical point signature critical point signature



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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 may be dominant• particle mis-identification changes the width of the distribution• 0.5% K swapping: width 5% and the signal is only 4%! TOF is TOF is importantimportant

Coun

ts

Simulations

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

√sNN=8.77 GeV



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pT fluctuationsSTAR Preliminary

• acceptance is important: elliptic flow can enhance apparent pT 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 RHIC has the tools needed to best understand ppTT fluctuationsfluctuations

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collision overlap zone

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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 √sNN

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 using3) multi-strange hadrons (also accessible at RHIC)4) charmonium suppression



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conclusionscompelling 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 √sNN smaller systematic errors• full STAR TOF ~2009

accelerator capabilities have been studied down to √sNN = 4.5 GeV:

no “show stoppers”

complementary international efforts being pursued

good potential for discovery and within reach



Thanks



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K/ fluct: challenges at RHIC

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+ + (c=3.7 m)tracking efficiency < 50% for collidersPID cuts reduce efficiency another 50%

kaon decays reduce efficiency at a colliderp.i.d. purity without TOF will help be limitedp.i.d. purity without TOF will help be limited

efficie

ncy

transverse momentum pT (GeV/c)

kaon

protonpion

z for

kao

ns

momentum p (GeV/c)

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

kaons

pions

protons

electr

ons

STAR acceptance and efficiency



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what to expectchiral 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

T0 could drop below TC 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 √sNN range of signatures

a RHIC energy scan may yielda RHIC energy scan may yield1) critical point signatures in a wide √sNN range2) and/or disappearance of QGPdisappearance of QGP signatures



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

We can’t count on sharp discontinuities



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pT fluctuations





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pT fluctuations

scale = full acceptance

fluctuations correlations

variance

excess

• acceptance is important: elliptic flow can enhance apparent pT 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 pT fluctuations



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v2 motivation slide

Hydrodynamic interpretation still evolving as analyses progressEnergy dependence plays an important role in our interpretations

S. Voloshin



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location of the critical point

Gavai, Gupta 2005Taylor Expansion



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STAR DetectorDesigned 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)

BBCBBC



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particle identification

log10(p)

log 1

0(dE/

dx)

PID capabilities at RHIC over a broad pT range:TPC dE/dx, ToF, Aerogel, 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))fluctuation analyses prefer track-by-track I.D.



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detector capabilitiesStar: TOF, full acceptance, HFT

Phenix: low-mass dileptons? HBDCBM: 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



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v2 and deconfinementPRL 92 (2004) 052302; PRL 91 (2003) 182301

large and v2 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

Documents

RHIC Low Energy Scan