Femtoscopy with STAR

Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 20051

Femtoscopy with STARFemtoscopy with STAR

Zbigniew Chajęcki

for the Collaboration


OutlineOutline

Probing space and time in Heavy Ion Collisions with Femtoscopy Evolution and emission time Size and shape of the source

Do we understand the transverse mass dependence of HBT

radii? Underlying physics in AA: dynamically-driven geometric substructure AuAu relative to pp (and dAu)

Problem with baseline of correlation function for p+p and d+Au

collisions

Decomposition of correlation function in terms of spherical

harmonics –new approach of studying particle correlations

Summary


The sketch of Heavy Ion The sketch of Heavy Ion CollisionCollision

PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

initial state QGP andhydrodynamic expansion

hadronic phaseand freeze-out

hadronization

time

dN/dt Emission time

pre-equilibrium

“temperature”

Evolution time

Chemical freeze outChemical freeze outThermal freeze outThermal freeze out

FemtoscopyFemtoscopy probes the space probes the space and time in HICand time in HIC


Hydro predictions: Hydro predictions: Evolution and Emission timeEvolution and Emission time

Hydrodynamic predictionsHeinz & Kolb, hep-ph/0204061

Long lifetime of the sourceas signal of the QGP formation

Ro - Rs → Emission time Rl → Evolution time

Hydrodynamic calculations reproduce momentum observables but fail to reproduce HBT results → RHIC HBT puzzle


Origin of HBT puzzleOrigin of HBT puzzle

Modified Sinyukov fit to RL

M. Herrmann and G.F. Bertsch, PRC 51 (1995) 328

TmK

TmK

m

TtmR

T

T

T

foTL /

/

1

2

Hydro prediction: ~ 15 fm/c Heinz & Kolb, hep-ph/0204061

Evolution time changes from ~9 fm/c for central collisions to ~5 fm/c for peripheral colisions

Evolution time

Nparticipants

Nparticipants

Blast Wave fit to spectra, v2 and HBT

Au+Au@200GeV

Short emission time (~2fm/c) consistent with RO/RS ratio

Emission time1.2

1.0

STAR - AuAu@200GeV, PRC 71 (2005) 044906

* in the absence of transverse flow


Properties of the source at freeze-outProperties of the source at freeze-out

Size of homogeneity region scales with initial source size (increase with increase of centrality of the collision - b→0 )

Oscillation phases: out-of-plane extended source

22

22

xy

xy

RR

RR

2

0,

2

2,2S

S

FO R

R

estimate initial from Glauber

εfinal from asHBT:

PRC66 (2002) 044903

eccentricity

Source remains out-of-plane at freeze out

STAR, PRL93 012301 (2004) STAR, PRL93 012301 (2004)

system expands

system changes the shape

Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 200570.3 0.30.4 0.40.5 0.50.2 0.2 0.6

Au+Au@200GeV

.2

0.

0.

0.

22

TT kmm

Why do the HBT radii fall Why do the HBT radii fall with increasing pwith increasing pTT ? ?

Decrease of HBT radii with pT

usually attributed to collective flow

RL → longitudinal flow

RT → transverse flow

Non-flow possibilities

termally (not collectively) expanding source

hot core surrounded by cool shell

important ingredient of Buda-Lund hydro picture (Csorgo et al.)

t


HBT in HBT in Au+Au vs p+p (d+Au)Au+Au vs p+p (d+Au)

Similar pT (mT) dependence as in p+p observed in d+Au collisions – do they have the same origin?

Rout

Rside

Rlong

p+p+X

pT0.25 0.5

2

1

STAR preliminary

HBT radii from p+p collisions fall with pT – as observed in the past, and usually attributed to string fragmentation In Au+Au pT (mT) dependence

attributed to collective expansion of the source

0.3 0.30.4 0.40.5 0.50.2 0.2 0.6

STAR, Au+Au@200GeV

Ratio of (AuAu, CuCu, dAu) HBT radii by pp

STAR preliminary

mTmT

All pT(mT) dependences of HBT radii observed by STAR scale with pp although it’s expected that different origins drive these dependence

Is it only a coincidence or something deeper?


Au+Au like superposition of p+p?Au+Au like superposition of p+p?

AuAu = N*(strings in pp) ?

AuAu = N*(„blast-waves”) ?

NO! – that would generate local space-momentum correlations but in AuAu we see global correlations – although both scenarios would lead to the identical p-space observablesp+p like a little Au+Au?p+p like a little Au+Au?

flow not expected in such a small system as pp

Shuryak: a hydro approach cannot describe data in pp at RHIC – hep-ph/0405066

Csorgo: mT dependence of HBT radii in pp is not generated by the transverse flow, but by the transverse temperature inhomegeneities of hadron-hadron collisions due to the freezing scale - hep-ph/0406042


Initial vs Final Size : pp(dAu) vs Initial vs Final Size : pp(dAu) vs AuAuAuAu

• AuAu mostly “understood” in terms of collective flow

• pp (dAu) show apparent global space momentum correlations, but no (or little) expansion

Model-dependent approach of

veryfication of expansion scenario

• RMS of initial size of the source calculated with Glauber model vs.

experimental RSIDE for

<kT>=0.2 GeV/c

STAR preliminary


Intriguing results ....Intriguing results ....

Let’s take a closer lookLet’s take a closer look


Projection of 3D CF in Projection of 3D CF in dd++AuAu

• Gaussian source

vs non-Gaussianess of CF

• It looks like fit describes the shape of

correlation function

d+Au central, kt: [0.25-0.35] GeV/c

STAR preliminary


Worst-case picture of dWorst-case picture of d++AuAu

• Not great fit

• One imagines one could do a better

job, but fit must work in 3D; we

see tiny portion of space

• What is going on in the

normalization region?

(independent of Gaussian ansatz)

• Problem with projections– cannot see systematics in 3D

– experiments w/same CF will have different projections

d+Au peripheral, kt: [0.15-0.60] GeV/c

STAR preliminary


binsall

iiiiimlml QCYQA

.

,

cos

, ),cos|,(|),(|)(|

4

Decomposition of CF onto spherical Decomposition of CF onto spherical harmonicsharmonics

QOUT

QSIDE

QLONG Q

• Cartesian-space (out-side-long) naturally encodes physics, but is poor/inefficient representation

• Recognize symmetries of Q-space -- decompose by spherical harmonics!

• Direct connection to source shapes [Danielewicz,Pratt: nucl-th/0501003] – decomposition of CF on cartesian harmonics

• ~immune to acceptance

• full information content at a glance[thanks to symmetries]

: [0,2] : [0,]

OUT

SIDE

TOT

LONG

LONGSIDEOUT

Q

Q

Q

Q

QQQQ

arctan

)cos(

222

Z.Ch., Gutierrez, Lisa, Lopez-Noriega, nucl-ex/0505009


Some properties of ASome properties of Almlm coefficientscoefficients

Alm = 0 for l or m odd – identical particle correlations (for non-id particles, odd l encodes shift information)

A00(Q) ≈ one-dimensional “CF(Qinv)” (bump ~ 1/R)

Alm(Q) = l,0 where correlations vanish

Al≠0,m(Q) ≠ 0 anisotropy in Q space

Im[Alm] = 0


Simulations with Gaussian Simulations with Gaussian modelmodel

Simple, Gaussian source calculations

~acceptance freeRL < RT

RL > RT

RO < RS

RO > RS


Decomposition of CF for Decomposition of CF for centralcentral d+Au collisiond+Au collision

Decomposition of CF for Decomposition of CF for peripheralperipheral d+Au collisiond+Au collision


Baseline in p+p, d+Au and Baseline in p+p, d+Au and Au+AuAu+Au

Some coefficients don’t vanish for large Q

Bias from zero is larger for more peripheral collisions

This problem is getting more significant for small systems (pp & dAu)

It can be due to

an experimental artifact

long range correlations (e.g. Jets)

problem with momentum conservation ?

or ?


SSummaryummary Femtoscopy indicates system evolution due to dynamic bulk behaviour

mT dependence of HBT radii for AuAu, dAu and pp seems to be the same

although it’s expected (?) to have different origins

a new puzzle?

Looking in more details

baseline problem

Spherical harmonics allow efficent diagnostic of the correlation function

Use symmetry of the q-space

All information at a glance

Pratt and Danielewicz [nucl-th/0501003] : connection of one-to-one CF and spatial anisotropies

Investigation into the source of non-femtoscopic correlations under way

Femtoscopy with STAR

Documents

Transcript of Femtoscopy with STAR