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

Measuring unknown 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

We expect asymmetry in “out” direction, but not in “side”, which is used as cross-check

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 probabilty (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

We observe Lambda peaks at k*~minv of Λ

Good agreement for identical and non-identical charge combinations

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 limited by the knowledge of pi-p interaction

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

Fair comparison: same fitting for RQMD and data

Phys. Rev.Lett.

91 (2003) 262302

Proton – Lambda at 200 AGeV

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

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

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

Proc. CORINNE 90 Nantes, France, 1990 (ed. Ardouin, World Scientific) p.42

Phase shifts from:

F. Wang, S. Pratt, Phys. Rev. Lett. 83 (1999) 3138-3141

1.2

0.8

1.0

0.0

0.2

k* [GeV/c]

CF

Not purity corrected

Average pair purity: 15 %

Fully corrected

for purity and

momentum

resolution

STAR Preliminary

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 parameters now unknown Rinv = 1.5 ± 0.07 (+0.5 -0.9) fm

Early drop of correlation fuction signals significant annihilation

A new way to extract scattering lenghts

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-lamdba 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

πΞ function promises new information in future

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

Extra slides

Final State Interaction

Coulomb dominated interaction

Strong interaction addition small for pion-kaon, significant for kaon-proton

No HBT effect – non identical particles

CF Asymmetry

- Pair relative momentum

- Pair separation at freeze-out

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]

simulation

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

Purity correction

Average pair

purity = 0.75

Momentum resolution

Momentum smearing

and low pT shift

No change until resolution

4x worse than STAR

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]

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

Development in fitting methods

Account for the “real” pairs that are not correlated

K+ p

K- anti-p

Best Fit

STAR

preliminary

Fitting procedure

No analytic form of correlation function

Need to generate correlation functions from experimental momentum distributions, Monte-Carlo methods and pair weights (two codes: from R. Lednicky and S.Pratt)

Best fit parameters are taken at the minimum c2 value

c2 map

c2 map

Do we see time shifts, spatial shifts, or both?

The general formula:

r* =γpairΔr–βpairΔt

Δrout is a measure of flow

Turning off rescattering shuts down flow and decreases time shifts

We measure r*out, which combines time shifts and spatial shifts (from space-momentum φ correlations)

Δt

Δrout

r*out

RQMD simulations; πK

Rescattering models

Both models produce significant flow

Flow intensity scales with centrality

RMQD w/o rescattering produced no flow

pp collisions produce space-momentum correlations – jets?

RQMD

UrQMD

STAR data

Model: R=13.5 fm,	τ=1.5 fm/c

T=0.11 GeV, ρ = 0.6

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)

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%

Static source - time shift can be the cause for asymmetry

Like-sign

out ratio

Like-sign

corr.fctn

Unlike-sign

corr.fctn

Unlike-sign

out ratio

Best fit:

6 fm/c

Best fit:

7.5 fm

Hydro + RQMD pure and scrambled
can we disentangle time shift and flow

Scrambling the positions

removes some of the

Out asymmetry

Out asymmetry comes from the

combination of time shift and

position-momentum correlations

phi scrambling does not

remove all momentum-

position correlation

Hydro inspired parametrization -
radial flow produces asymmetry

Unlike sign

corr. Fctn

Like sign

out ratio

Unlike sign

out ratio

like sign

corr. fctn

T = 110 MeV – spectra

trans. flow v = 0.6c - flow

radius = 13 fm

How does it work

The flow of the program:

Read parameters

Read input data (correlation functions and pairs)

Construct a c2 map with source parameters on the axes

For each bin in c2	map calculate the theoretical CF

Calculate c2 between experimental and theoretical CF and put it into the bin

Get the minimum from the c2 map as the best fit

Sample results
fitting RQMD simulation

Fitting of the CF simulated by RQMD and Lednicky's weights

Model used is gaussian in r*Out

The values of the fit:

sigma: 12.4 fm

mu: 6.3 fm

Source comparisons

The resulting distribution can be compared to the input distribution from RQMD

Input distribution can only be approximated by a gaussian – we need new ideas

Other source types can be easily tested in a similar way

Adam Kisiel

2002-01-08T22:19:12