PParticle correlationsarticle correlations at STARat STAR

Jan Pluta

Heavy Ion Reactions Group (HIRG),

Faculty of Physics,

Warsaw University of Technology

Some results from the STAR HBT

group, presented recently by:

Z.Chajecki, A.Kisiel, M.Lisa,

M.Lopez-Noriega, S.Panitkin,

F.Retiere, P.Szarwas.

3-rd Budapest Winter School on Heavy Ion Physics, 10 XII 2003

Outline:

•The STAR experiment

•RHIC HBT Puzzle

•General analysis

•asHBT

•Two K0-short correlations

•p-Au, d-Au data

•Nonidentical particles - emission asymmetry

•Plans for future

Relativistic Heavy Ion Collider (RHIC)

2:00 o’clock

4:00 o’clock6:00 o’clock

8:00 o’clock

10:00 o’clock

STARPHENIX RHIC

AGS

LINACBOOSTER

TANDEMS

9 GeV/uQ = +79

1 MeV/uQ = +32

HEP/NP

g-2U-line

BAF (NASA)

PHOBOS12:00 o’clock BRAHM

S

Beam energy up to 100 GeV/A (250 GeV for p) Two independent rings (asymmetric beam collisions are

possible) Beam species: from p to Au Six interaction points Four experiments: STAR, PHENIX, PHOBOS and BRAHMS

=3.8 km1740 superconducting magnets

Solenoidal Tracker At RHIC

STAR Detector – side viewSTAR Detector – side view

and STAR Collaboration – face view

STAR Collaboration• 500 Collaborators

including– ~65 graduate students– ~60 postdocs

• 12 countries• 49 institutions• Spokesperson:

John Harris 1991 - 2002

Tim Hallman 2002 - now

USA, Brazil, China, Croatia Czech Republich, England, France, Germany, India, Netherlands, Poland, Rusia

HBT+FSI

Space-time

sizes anddynamics

Correlation function

Momenta andmomentum difference

The idea:

Quantum statistics and Final-State Interaction

Particle correlations

STAR Event 2

Central Event: AuAu 200GeV/A

Real-time track reconstruction Pictures from Level 3 Trigger, online display.

Typically 1000 to 2000 tracks per event into the

TPC

4m

Event and Particle SelectionAu+Au Collisions at Sqrt(SNN)=200GeV

• Particle identification via specific ionization (dE/dx)electron band removed by cuts

• Optimum performance for HBT: 0.150 < pT (GeV/c) < 0.550

for K0s:

0.100 < pT (GeV/c) < 3.500

• Centrality selection based on number of charged hadrons.

three different centralities

• Midrapidity-0.5 < y < 0.5

STAR PRELIMINARY

N ch

STAR PRELIMINARY

Minbias trigger

Two-particle kinematics

Main

(approximative)

relations:

Qout <--> Pt

Qside <-->

Qlong <-->

KT <--> Pt

Some base definitions - to be used for results presentation

LCMS: (P1+P2)z=0

HBT Excitation Function

Comparison with lower energies for

~ 10% most central events at

midrapidity

kT ~ 0.17 GeV/c

No significant increase in radii with energy

RO/RS ~ 1

Gap in energy that needs to be closed

RHIC HBT PuzzleMost “reasonable” models still do not

reproduce RHIC sqrt(SNN) = 130GeV

HBT radii

“Blast wave” parameterization (Sollfrank

model) can approximately describe data

…but emission duration must be small

= 0.6 (radial flow)

• T = 110 MeV

• R = 13.5 1fm (hard-sphere)

emission= 1.5 1 fm/c (Gaussian)

fromspectra, v2

√SNN = 130GeVPHENIX PRL 88 192302 (2002)

STAR 130 GeV

PHENIX 130 GeV

+

-

Hydro + RQMD

Statistical errors only!!

raw

Coulombcorrected

q* (GeV/c)

3Dimensional Pion HBT

Pratt-Bertsch Parameterization

LCMS frame: (p1+p2)z=0

Central Events pT = 0.15-0.25 GeV/c

Coulomb correction→ spherical Gaussian source of 5fm

momentum resolution corrected(~1% effect at 200GeV, due to higher B-field)

)( 222222

1),,( LLSSOO RqRqRq

LSOeqqqC

Rout (fm) Rside (fm) Rlong (fm)

0.66 ± 0.01 6.41 ± 0.14 6.03 ± 0.09 6.65 ± 0.11

STAR PRELIMINARY

Sqrt(SNN) = 200 GeV– –

Centrality and mT dependence at 200 GeV

RL varies similar to RO, RS with centrality

HBT radii decrease with mT (flow)

Roughly parallel mT dependence for different centralities

RO/RS ~ 1 (short emission time)

Central

Midcentral

Peripheral

200GeV

STAR PRELIMINARY

Longitudinal radius:at 200GeV identical to 130 GeV

(fit to STAR Y2 data only)

STAR PRELIMINARY

Central

Midcentral

Peripheral

PHENIX Central

200GeV - 130 GeV

Comparison: 200 to 130 GeV. Longitudinal radius

Evolution timescale from RL

(fit to STAR Y2 data only)

Simple Mahklin/Sinyukov fit (assuming boost-invariant longitudinal flow)

T

KfoL m

TtR

Assuming TK=110 MeV(from spectra at 130 GeV)

fm/c 6.7t

fm/c 10t

periphfo

centralfo

Makhlin and Sinyukov,

Z. Phys. C 39 (1988) 69

STAR PRELIMINARY

Central

Midcentral

Peripheral

PHENIX Central

200GeV - 130 GeV

Comparison: 200 to 130 GeV. Transverse radii

*

Central

Midcentral

Peripheral

PHENIX Central

200GeV - 130 GeV

Higher B-field higher pT

Transverse radii:

• similar but not identical• low-pT RO, RS larger at 200 GeV• steeper falloff in mT

(PHENIX 130GeV)• Ro falls steeper with mT

STAR PRELIMINARY

Azimuthally sensitive HBT (asHBT)

• sensitive to interplay b/t anisotropic geometry & dynamics/evolution

• “broken symmetry” for b0 => more detailed, important physics information

• another handle on dynamical timescales – likely impt in HBT puzzle

P. Kolb and U. Heinz, hep-ph/0204061

HBT respect Reaction Plane

out

p

b

K

side

x

y

2ijji Rqq

e1),q(C

Lines: projections of 3D Gaussian fit

1D projections, =45° √SNN = 130GeV

HBT(φ) Results – 130 GeV

Star preliminary

Minbias events @130GeV

Bolstered statistics by summing results of p- and p+ analyses

Blast-wave calculation (lines) indicates out-of-plane extended source

Data corrected for both event plane resolution and merging systematic

T=100 MeV 0aR=11.7 fm,

=2.2 fm/c

RY

RX

A model of the freezeout - BlastWave

BW: hydro-inspired parameterization of freezeout• longitudinal direction

• infinite extent geometrically• boost-invariant longitudinal flow

• Momentum space• temperature T• transverse rapidity boost ~ r

)2cos(~),( 0 bas rr

• coordinate space• transverse extents RX, RY

00 r~R

r)r(

• freezeout in proper time • evolution duration 0

• emission duration

2

20

2exp~

ddN

00

RY

RX

A model of the freezeout- BlastWaveBW: hydro-inspired parameterization of freezeout• Longitudinal direction

• infinite extent geometrically• boost-invariant longitudinal flow

• Momentum space• temperature T• transverse rapidity boost ~ r

)2cos(~),( 0 bas rr

• Coordinate space• transverse extents RX, RY

00 r~R

r)r(

• freezeout in proper time • evolution duration 0

• emission duration

2

20

2exp~

ddN

7 parameters describing freezeout

BlastWave fits to published RHIC data

• reasonable centrality evolution

• OOP extended source in non-central collisions

central midcentral peripheral

74.3 / 68153.7 / 9280.5 / 1012 / ndf

0.8 1.90.8 3.20.0 1.4 (fm/c)

6.5 0.87.4 1.28.9 0.30 (fm/c)

10.1 0.411.8 0.612.8 0.3RY (fm)

8.0 0.410.2 0.512.9 0.3RX (fm)

0.04 0.010.05 0.010.06 0.01a

0.81 0.020.87 0.020.88 0.010

95 4106 3108 3T (MeV)

PeripheralMidcentralCentral

Estimate of initial vs F.O. source shape

2x

2y

2x

2y

RR

RR

20,S

22,S

FO R

R2

• estimate INIT from Glauber

• from asHBT:

FO =

INIT

FO < INIT → dynamic expansion

FO > 1 → source always OOP-extended

• constraint on evolution time

asHBT at 200 GeV in STAR – R() vs centrality

12 (!) -bins b/t 0-180 (kT-integrated)

• 72 independent CF’s

• clear oscillations observed in

transverse radii of symmetry-

allowed* type

• Ro2, Rs

2, Rl2 ~ cos(2)

• Ros2 ~ sin(2)

• centrality dependence reasonable

• oscillation amps higher than 2nd-

order ~ 0→

(*) Heinz, Hummel, MAL, Wiedemann, Phys. Rev. C66 044903 (2002)

Pion correlation in d – Au : data selection

p-Au selection

1D correlation function

3D correlation function

d-Au vs p-Au

KT dependence

Centrality dependence

Pion Correlations d-Au and p-AuPion Correlations d-Au and p-Au

p-Au selection:p-Au selection:

Using information from ZDC-d STAR can separate events with neutron spectator from deuteron

ZDC-dAu d

ZDC-Au

FTPC E -Au

All trigger events

1D Correlation Function:1D Correlation Function:

Gaussian fit:

➢ CF is very wide (rel Au-Au)➢ Coulomb/merging less important➢ CF looks reasonable➢ 1D Gaussian fit is not good➢

needed more deeply study of fit

method

STAR preliminarytheoretical CF: R

inv=6 fm, = 0.5

d*-Au : d-Au without p-Au

collision 1.89 +- 0.01 0.364 +- 0.003 4672 / 33

d – Au 1.85 +- 0.01 0.362 +- 0.003 5359 / 33

Rinv [fm] NDF

d* – Au

only statistical error included !

3D Correlation Function:3D Correlation Function:

Gaussian parametrization is not perfect but HBT radii characterize the width of CF

cut on the others Q's

components < 30 MeV/c

3D Gaussian fit:

STAR preliminary

d – Au p – Au

1.58 +- 0.02 1.21 +- 0.03

1.51 +- 0.01 1.21 +- 0.02

1.71 +- 0.02 1.67 +- 0.05

0.354 +- 0.003 0.372 +- 0.008

Rout

Rside

Rlong

Fit results:

Rout

, Rside

sensitive to the number of participants

[GeV/c]

KKTT dependence: dependence:

p – Au d – Au

● clear KT dependence

●Rout

and Rside

- sensitive to the number of participants●R

long – the same K

T

dependence for dAu and pAu

STAR preliminary

KKTT dependence: d-Au & Au-Au divided by p-p dependence: d-Au & Au-Au divided by p-p

● the same trend of KT

dependence for d-Au

and Au-Au as for p-p ● HBT radii are scaled by constant factors

STAR preliminary

for different collisions

MMTT dependence of R dependence of Rlonglong: :

STAR preliminary

Rlong

= const (mT)-

p-p d-Au Au-Au Au-Au peripheral midcentral

STAR preliminary

mTk

T2 + mass

Sinyukov fit:

Centrality definition in d-Au:Centrality definition in d-Au:

centrality bin FTPC multiplicity percent of events

1 [0 , 9] 100 – 40

2 [10 , 16] 40 – 20

3 [17 , 99] 20 – 0

FTPC-Au: charged primary particle multiplicity in -3.8<<-2.8

ZDC-dAu d

ZDC-Au

FTPC E -Au

321

most peripheral

most central

Centrality dependence:Centrality dependence:

● clear centrality dependence

● similar to AuAu

● connection to geometry

p – Au d – Au

centrality

minbias

STAR preliminary

1 2 3

4.3 +- 0.1 10.4 +- 0.4 16.3 +- 0.7<Npart> [*]

<Nch

> TPC . 7.9 . . 12.1 . . 17.1 .

<Nch

> FTPC E . 5.2 . . 12.8 . . 24.3 .

[*] - Glauber calculations (Mike Miller)

K0sK0

s Correlations

mt scaling violation?

Next RHIC HBT puzzle ?

inv

The asymmetry analysis

Catching up•Interaction time larger•Stronger correlation

Moving away•Interaction time smaller•Weaker correlation

“Double” ratio•Sensitive to the space-time asymmetry in the emission Kinematics selection

on any variablee.g. kOut, kSide, cos(v,k) R.Lednicky, V. L.Lyuboshitz,

B.Erazmus, D.Nouais,Phys.Lett. B373 (1996) 30.

Non-identical particle correlations:

Double ratio definitions

p1

p1

p2

p2

2k* = p1 – p

2 P = p1 + p

2

kside

< 0

kside

> 0

kout

> 0

kout

> 0

kksideside signselectionarbitrary

kkoutout signselection determined by the directionof the pair momentum P

Correlationfunctions

Double ratios

kklong long is the z componentof the momentumof first particle in LCMS

2k* [GeV/c]

simulation

What to expect from double ratios

• Initial separation in Pair Rest Frame (measured) can come from time shift and/or space shift in Source Frame (what we want to obtain)

• We are directly sensitive to time shift, the space shift arises from radial flow – possibility of a new radial flow measurement

r

T

F

x

yobservedtransversevelocity

thermal velocity

Flow velocity Out direction

Side direction

What do we probe?

Source ofparticle 1

Source ofparticle 2

Boost to pair rest frame

• Mean shift (<r*>) seen in double ratio

• Sigma (r*) seen in height of CF

r* =pairr–pairt

Separation between source 1 and 2 in pair rest frame

r

r (fm) r* (fm)

<r*>

r*

Separation due to space and/or time

shift

t

Correlation functions and ratios

Good agreement for like-sign and unlike-sign pairs points to similar emission process for K+ and K- Out

Side

Long

CF

Clear sign of emission asymmetry

Two other ratios done as a double check – expectedto be flat

Preliminary

STARpreliminary

Results for Pion-Proton 130 AGeV

• Similar preliminary analysis done for pion-proton

• We observe Lambda peaks at k*~m

inv of Λ

• Good agreement for identical and non-identical charge combinations

Λ peaks

Preliminary results for Kaon-Proton

• Using data from Year2 (200 AGeV) – sufficient statistics

• No corrections for momentum resolution done

• No error estimation yet – fit indicates theoretical expectations

K+ pK- anti-pBest Fit

STARpreliminary

Modeling the emission asymmetry

• Need models producing strong transverse radial flow:– Blast-wave as a

baseline– RQMD– UrQMD– T. Humanic's

rescattering model

• What do we measure and how to compare it to the models?

• Is our fitting method working? And if yes, what does it tell us?

• Need to disentangle flow and time shift

Understanding modelsBlast wave = Flow baseline

• Blast wave– Parameterizes source size

(source radius) radial flow (average flow rapidity) and momentum distribution (temperature):

– No time shift– Only spatial shift due to flow

– Infinitely long cyllinder (neglects long contribution)

R

t

RsideRout

Kt = pair Pt

Parameterizationof the final state

Blast wave: how does the flow workBlast wave: how does the flow work

Pionp

t = 0.15 GeV/c

t = 0.73

Kaonp

t = 0.5 GeV/c

t = 0.71

Protonp

t = 1. GeV/c

t = 0.73

Average emission points

Spatial shifts (r) Particle momentum

Fitting and quantitative comparisons

• Fits assume gaussian source in PRF

• r*out

distributions have non-

gaussian tails• Use the same fitting

procedure for models and data - correlation functions constructed with “Lednicky's weights”

Example of r*out

distribution from RQMD

Comparing models to data

• Rescattering models and blast-wave are consistent with data

• Blast wave parameters constrained by STAR measurements

• In models flow is required to reproduce the data

• More points in βt needed to map and discriminate the flow profile – needs

STAR upgrades in PID capability (TOF barrel)

STAR HBT Matrix (circa 2003)

+ - + - 0 p p ++ +

0 -

+

-

Sergei's HBT matrix 0

Y1 p

Y1 ? p

Y2

“traditional”HBT axis

Analysisin progress

published

3 Correlations (accepted PRL)asHBTPhase space densityCorrelations with CascadesdAu, ppCascades

submittedNot shown:

What have we learned so far?

• RHIC HBT puzzle– Break down of theoretical description of correlations at RHIC– Indication of short source lifetime and freeze-out duration at RHIC– Short lived hadronic phase?

• Out of plane extended pion source in non-central collisions– Also points to short emission times

• Weak energy dependence of the HBT radii– Where is the phase transition?

• Large pion phase space densities (non-universal)– Small entropy per pion?

• Chaotic pion source from 3p correlations– No multiparticle effects above Pt~200 MeV/c

• Source asymmetries from non-identical correlations– Consistent with collective flow and short time scales

• Only systematics measurements may provide answers!

What will affect STAR HBT analysis?

• RHIC upgrades progress• STAR upgrades• Various other measurements (e.g. spectra,

high Pt,

strangeness, flow, etc)• New theoretical ideas

Consequences for STAR HBT• Large statistics AuAu datasets • Plans for 2004: 14 weeks of AuAu “physics” running:

– ~30M central, ~50M peripheral events• What can be done? Many analysis which were statistics limited!

– Rare particle correlations W, X,L, etc (identical, non-identical)• Early freeze-out, sequence of emission, flow, FSI, etc

– Correlations relative to reaction plane• Kaons• Non-identical

– Baryon correlations: ppbar, LLbar, pL, etc– Coalescence, light nuclei and anti-nuclei

• Large statistics pp (~100M events) datasets @200, 500 GeV– STAR HBT matrix (e.g. non identical correlations)– HBT in Jets?– spin dependent HBT? (with polarized beams)

• Different energies • Different beams

Add dependencies on centrality, Kt, reaction planeEvent by Event HBT

New analyses ideas (S.Pratt, imaging, etc)

Consequences for STAR HBT

• Better particle identification• Extension of HBT systematics to higher Kt: 1-3

GeV/c• Region of transition from Hydro to pQCD• What is space-time picture in this region?

– Correlations of identical particles– Scan in Pt for Non-identical correlations

• Sensitivity to flow profile, model details– asHBT

• Higher efficiency of hyperons reconstruction– ~x10 for W compare to TPC alone– High statistics correlations with hyperons

Summary

• RHIC and STAR future seems to be certain for next 5-10 years

• Upgrade path is visible• The number of available datasets and possible

analysis topics will be rapidly increasing• Data volumes will be unprecedented (at least for

us)– Can we do analysis in a reasonable time?

• Analyses will be “moving” to rare particles• Shall we continue with systematic approach?

– Probably yes• If new results or theoretical predictions will suggest

promising measurement - we will concentrate on it