Non-identical particle correlations in 130 and 200 AGeV collisions at STAR

Adam Kisiel

Warsaw University of Technology

for the STAR experiment

Outline

Physics motivation

Emission asymmetry measurement

Measuring flow as space-momentum correlations

Determining interaction potentials

Current results

130 AGeV data

200 AGeV data

Flow in models

Blast-wave parameterization

RQMD – transport model

Exotic correlation functions

Proton-Lambda and antiproton-Lambda

Pion-Cascade

The asymmetry analysis

k*

1

Catching up

Effective interaction time larger

Stronger correlation

Moving away

Effective Interaction time smaller

Weaker correlation

“Double” ratio

Sensitive to the space-time asymmetry in the emission process

R.Lednicky, V. L.Lyuboshitz,

B.Erazmus, D.Nouais,

Phys.Lett. B373 (1996) 30.

C-

C+

C+

C-

Kinematics selection

along some direction

e.g. kOut, kSide, cos(v,k)

Heavier particle

faster

Lighter particle

faster

Flow in the transverse plane

S.Voloshin, R.Lednicky,

S. Panitkin, N.Xu,

Phys.Rev.Lett.79(1997)30

Average

emission

points

Pion

<t>= 0.7

pt = 0.15 GeV/c

Kaon

<t> = 0.7

pt = 0.5 GeV/c

Proton

<t> = 0.7

pt = 1. GeV/c

Spatial shifts (Dr)

Pair

momentum

(out direction)

Source

density

nucl-th/0312024

F. Retiere,

M. Lisa

Flow produces emission asymmetries in space Dr

Observed asymmetry r* can come from emission time difference Dt	too

We expect asymmetry in “out” direction, but not in “side”, due to symmetry

R. Lednicky,

nucl-th/0305027

side

direction

Data sample

Central AuAu collisions

130 AGeV – 0.7 Mevents

200 AGeV – 2.2 Mevents

Identification probability from dE/dx in STAR TPC

Purity cuts on particle level

γe+e- removal on pair level

Midrapidity data |y|< 0.5

Momentum range [GeV/c]

Pions (0.13, 0.5)

Kaons (0.3, 1.0)

Protons (0.4, 1.2)

Detector corrections

Two-track effects:

elimination of pairs possibly sharing hits in the TPC

Particle purity

PID probability (all), estimation of contamination from weak decay (pions, protons)

Momentum resolution

STAR

preliminary

Λ peaks

Mean: -7.4 ± 0.9 fm

Sigma: 15.1 ± 0.4 fm

Pion-Proton 130 AGeV

Good agreement for identical and opposite charge combinations

We observe Lambda peaks at k*~decay momentum of Λ

Out

double ratio

Side

double ratio

+ 1.0 syst.

- 1.5 syst.

- 3.4 syst.

+ 1.9 syst.

Fit assumes source is a

gaussian in r*out

proton faster

pion faster

Pion-Kaon at 200 AGeV

Good agreement for same-charge combinations

Clear emission asymmetry signal

Systematic error under study – influenced by purity and fits to all CFs separately

Out

double ratio

Side

double ratio

Sigma: 17.3 ± 0.8 fm

Mean: -7.0 ± 1.2 fm

STAR

preliminary

+ 0.9 syst.

- 1.6 syst.

+ 6.1 syst.

- 4.0 syst.

kaon faster

pion faster

Pion – Proton at 200 AGeV

Good agreement for same-charge combination pairs

Double ratios influenced by e+e-

Systematic error under study

STAR

preliminary

Out

double ratio

Side

double ratio

Sigma: 14.8 ± 0.5 fm

Mean: -6.0 ± 1.5 fm

+ 4.0 syst.

- 3.0 syst.

- 5.5 syst.

+ 3.0 syst.

proton faster

pion faster

Kaon-Proton at 200 AGeV

Only like-sign data well described by theory and fitted

Surprising correlation shape for unlike-sign – a question to theorists

Mean shift opposite to pi-K and pi-p

Out

double ratio

Side

double ratio

STAR

preliminary

CF different

from

expectation

Sigma: 10.6 ± 0.6	fm

Mean: 0.7 ± 1.5	fm

+ 1.4 syst.

- 2.1 syst.

- 0.6 syst.

+ 1.6 syst.

proton faster

kaon faster

Modeling the emission asymmetry

Need models producing strong transverse radial flow:

Blast-wave – hydro-like flow

RQMD – flow through interactions

Are we measuring time shifts, space shifts (flow) or both?

Is the flow hypothesis consistent with the data?

How do we compare theory and experiment?

Blast-wave: nucl-th/0312024

F. Retiere, M. Lisa

Comparing models to data

130 AGeV

200 AGeV

We do see space-momentum correlations:

Data and blast-wave consistent

RQMD needs flow to reproduce data

Time difference can explain K-p

Correct comparison - same fitting for RQMD and data

Phys. Rev.Lett.

91 (2003) 262302

Interaction potential parameters from: F. Wang, S. Pratt, Phys. Rev. Lett. 83 (1999) 3138-3141

Main challenge is particle purity, feed-down from from Lambda-Lambda accounted for

Good agreement between proton-lambda and anti-proton anti-lambda correlation functions

Proton – Lambda at 200 AGeV

Confirm measurements from NA49 and E895.

Fit using analytical model of proton-lambda correlation: Rinv = 2.8 ± 0.3 (+1.0 -1.4) fm	(see poster HBT9 by S. Bekele)

Fully corrected

for purity and

momentum

resolution

STAR Preliminary

Anti-Proton Lambda at 200 AGeV

First time measurement of anti-proton lambda CF

Fit with the same analytical model as pΛ – interaction potentials extracted from fit Rinv = 1.5 ± 0.07 (+0.5 -0.9) fm

Early drop of correlation function signals significant annihilation

A new way to extract unknown interaction potential parameters

STAR Preliminary

1.2

0.8

1.0

0.0

0.2

k* [GeV/c]

CF

Fully corrected for purity and momentum resolution

Not purity corrected

STAR

preliminary

Pion – Xi at 200 AGeV

Also a first time measurement

Will enable to address the question of Ξ flow

Theoretical expectation with assumptions:

Source size as for pions

Significant Ξ flow

Coulomb + strong interaction

Gives input onto cross-sections similar to proton-lambda case

Calculation using S.Pratt’s code

Combined π+-Ξ- and π--Ξ+

Purity assumed = 50%±25%

No momentum resolution correction

STAR

preliminary

Theoretical expectation

(not a fit)

Summary and outlook

Measured correlations for πK, πp and Kp pairs at 130 and 200AGeV AuAu collisions

Clear asymmetry of emission observed for all pairs

Space-momentum correlations from transverse radial flows present a consistent description of emission asymmetries for all systems

Emission time differences found to be important, but not enough to explain the data

New way to measure system interactions: pΛ, anti-pΛ, Kp (unlike sign), πΞ

New, non-HBT size estimation method for pΛ, anti-pΛ

πΞ function promises new information in future

Extra slides

Pion-Kaon at 130 AGeV

Correlation functions consistent for same and opposite charge

Clear asymmetry signature

Side and Long “double ratio” flat as expected

nucl-ex/0307025

130 AGeV

Sigma: 12.5 ± 0.4	fm

Mean:	-5.6 ± 0.4	fm

+ 2.2

- 3

+ 1.9

- 1.3

1D relativistic view
What can be probed?

Source of

particle 1

Source of

particle 2

Boost to pair rest frame

2 parameters

Mean shift (<r*>)

Sigma (σr*)

r* =γpairΔr–βpairΔt

r* separation in pair rest frame

Function of γpair(βpair) which depend on the pair acceptance

Separation between source 1 and 2 in pair rest frame

Δr

r [fm]

r* [fm]

Double ratio definitions

p1

p1

p2

p2

2k* = p1 – p2

P = p1 + p2

kside < 0

kside > 0

kout > 0

kout > 0

kside sign

selection

arbitrary

kout sign

selection

determined by

the direction

of the pair

momentum P

Correlation

functions

Double

ratios

klong is the z component

of the momentum

of first particle in LCMS

2k* [GeV/c]

Purity correction

Average pair

purity = 0.75

Momentum resolution

Momentum smearing

and low pT shift

No change until resolution

4x worse than STAR

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

observed

transverse

velocity

thermal velocity

flow velocity

out

direction

side

direction

Fitting procedure

No analytic form of correlation function

Need to generate correlation functions using experimental momentum distributions, Monte-Carlo methods and Lednicky's pair weights

Best fit parameters are taken at the minimum c2 value

c2 map

Successes of the blast wave parameterization

-

K-

p

1/mT dN/dmT (a.u.)

mT - m [GeV/c2]

STAR

preliminary

Masashi Kaneta

Spectra

Pion source size

Elliptic flow

STAR Collaboration,

PRL 87 182301 (2001)

STAR data

Model: R=13.5 fm,	τ=1.5 fm/c

T=0.11 GeV, ρ = 0.6

Short emission duration

Blast wave fit including space and momentum anisotropy

Mike Lisa

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

nucl-ex/0307025

Hit sharing cut effect on Side ratio

Before the cut

After the cut

Should be at 1.0 as there

is no physics effect that

can cause the asymmetry

Maximum number of

shared hits between

two tracks = 10%

Proton-Lambda raw CF

1.2

0.8

1.0

0.0

0.2

k* [GeV/c]

CF

Not purity corrected

Average pair purity: 15 %

STAR

preliminary

Proton-lambda and anti-proton – anti-lambda in good agreement

Both show positive correlation

Adam Kisiel

2002-01-08T22:19:12