Sergey Panitkin

Femtoscopy at RHIC

Sergey PanitkinBrookhaven National Lab

Sergey Panitkin

Importance of soft physics at RHIC

99.5%

• We want to create/study a new type of matter (= bulk system)

• large-scale (soft??) deconfinement• Collective properties (T,p, flow?)

• Jets/hard probes are probes of this system

• Crucial to understand bulk properties and dynamics in their own right

• Soft sector gives information about EoS

Sergey Panitkin

Collective behavior at RHIC

P. Kolb, J. Sollfrank, U. Heinz

Heinz & Kolb, hep-th/0204061

• Hydrodynamics seems to reproduce p-space aspects (spectra and elliptical flow) of particle emission up to pT~2GeV/c

Bulk system?! Perfect Liquid?Note: Hydro provides complete space-time evolution. Can and should be tested!

Sergey Panitkin

General Comment

Single particle spectrum is sensitive to momentum distribution only

Relative momentum distribution of particle pairs is sensitive to space-time information

Basis for Identical and Non-identical particle femtoscopy

•EoS determines S(r,q)

),(4 pp

xSdxddNE

),(|),(|)/)(/(

/)( 2

2111

2122 qrqrr

pppp

q SdddNddN

dddNC

Source functionFSI

Sergey Panitkin

Two Particle Interferometry: Idealized Case

y

X

1

2

Source

24

2xiq4

21)K,x(Sxd

e)K,x(Sxd1)p,p(C

21 ppq 2121 ppK

emission function

For non-interacting identical bosons:

C (Q

inv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

Sergey Panitkin

Correlation functions for different colliding systems

C2(

Qin

v)

Qinv (GeV/c)

STAR preliminary p+pR ~ 1 fm

d+AuR ~ 2 fm

Au+AuR ~ 6 fm

Correlations have more informationOne can use more sophisticated analysis to extract it

Sergey Panitkin

beam direction

p1 p2

Q T

Q

Q L

beam direction

p2p1

Q T

Q S

Q O

“Standard” Pratt-Bertsch coordinate system

)T2

PT1

P(2

1T

K

2long

2long

2side

2side

2out

2out)(1),(

RqRqRqekkqC

Sergey Panitkin

Pratt-Bertsch parameterization

Kt~x~KR

Kx~KR

Kt~x~KR

2llong

2l

2side

2s

2out

2o

xxx~

)K,x(Sxd

)x(f)K,x(Sxdf

4

4

Decompose q into components:qLong : in beam directionqOut : in direction of transverse momentum KT

qSide : qLong & qOut

222222

1),,( llssoo RqRqRqlso eqqqC

Radii are related to source variances:

221

21

ppk

ppq

In Longitudinally Co-Moving System (LCMS) l =0

Sensitive to emission time

Sensitive to transverse extent

Sensitive to longitudinal extent

Sergey Panitkin

In Search of the QGP. Generic Expectations.

“Energy density”

Hydro calculation of Rischke & Gyulassy expects Rout/Rside ~ 2->4 @ Kt = 350 MeV.

•Looking for a softest point in EoS

Sergey Panitkin

RHIC Energy Scan

•Measurements at 200, 130, 62 GeV

•No significant change with energy from AGS to RHIC

•Ro/Rs ~1

PHOBOS nucl-ex/0409001

Where are signs of softest point?!Lower energies?Smaller systems?

See talk by D. d’Enterria

Is HBT sensitive to geometry at all?!

Puzzle #1

Sergey Panitkin

More Experimental Systematics

STAR PRL 93:12301 (2004)PHENIX nucl-ex/0401003

Clear sensitivity to source geometry !

b≠0

Centrality dependence asHBT

Sergey Panitkin

Source expansion at RHIC

initi

al =

final

22

22

xy

xy

RR

RR

Rx

Ry

STAR preliminary

STAR Collaboration, nucl-ex/0312009

Expansion at low Pt Change in eccentricity of the source

Sergey Panitkin

Model Comparisons. Puzzle #2

Good agreement between experiments

Subset of models shown

Broad range of physics scenarios explored

Good description of p-space (Pt, V2)

Poor description of HBT data the puzzle

Sergey Panitkin

General observations

Each theory paper has explored systematic changes in at least one important variable– Tf = Freeze-out Temp/profile

– Tc = Critical Temp.

– el = parton scattering cross-section– E = Latent Heat/transition order

– Tch = chem. Freeze-out temp

– n = viscoscity (-20% Rout, -60% Rlong)D. Teany, nucl-th/0301099

• Femtoscopic observables are sensitive to a variety of parameters, at least in the models!

•Sensitivity to physics at early stages of reaction.•Still no compelling solution from dynamical models•Open questions about late stages effects•Hydro inspired parameterization can fit the data, but not hydro•More theoretical work is needed

Sergey Panitkin

Non identical particle correlations

No symmetrization or antisymmetrization requirement

Pair wave function is of general form Existence of odd terms in w.f. provides sensitivity

to space-time asymmetries f(x) ≠ f(-x) Now it matters (for example) what was emitted

earlier and what was emitted later Asymmetries can exist due to a variety of physics

phenomena: flow, sequence of emission, early decoupling, strangeness distillation, etc

Sergey Panitkin

Effect of space-momentum correlation

Evidence of a space – time asymmetry

– Qualitatively consistent with “default” blast wave calculation

• T=110 MeV, • <> = 0.6,

t=9,R=13fm– No need for extra time

shift

Kaon <pt> = 0.42 GeV/c

Pion <pt> = 0.12 GeV/c

STAR PRL 91 (2003)

Never compared with hydro, AFAIKMore measurements to come!

Sergey Panitkin

STAR Femtoscopy Matrix

+ - + - 0 p p ++ +

0 -

+

-

Sergei's HBT matrix 0

Y1 p

Y1 ? p

Y2

“traditional”femtoscopy axis

Analysisin progress

published

3 CorrelationsasHBTPhase space densityCorrelations with CascadesdAu, ppGamma-Gamma HBT

submittedNot shown:

Sergey Panitkin

RHIC as a QCD Machine

Strong interaction for many particle combinations that we want to study is poorly known

Non-identical correlations can shed light on that Sometimes the only way to study certain systems Fixing space time parameters from well known

correlations and models Determine scattering lengths Not the main goal of two particle correlations, but

a nice additional topic Interesting for theory of hadron interactions, hypernuclear community, exotic hadrons

H0,ppK-,..

Sergey Panitkin

What about RHIC II ?

Is there any soft physics left to do at RHIC II ? “RHIC is a worst enemy of RHIC II” - Anonymous

– Many things are and will be done at RHIC What is RHIC II in general terms ? –

– High luminosity, high rates– New detector capabilities (extended/improved pid to

higher Pt…)– Rare probes

Femtoscopy with rare probes– Photons, (electrons?)– HBT in Jets– Non-identical correlations with hyperons– Non-identical correlations with charm ?! Charm

thermalization and flow

Sergey Panitkin

Photon HBT

• Prospects• Photons are penetrating probes • Photons are emitted at all stages of collision• Can carry information from the earliest stages of

reaction (QGP?,PC)• Photons from different Pt can come from different

stages of collision ( QGP, Hadronic gas, resonances,…)• Very few experimental results

• Challenges• Relatively small yields• Large background (pi0 decays, eta decays, etc)

• Statistics is dominated by background

Same applies for electrons !

Sergey Panitkin

First measurements at SPS

WA98 PRL

Sergey Panitkin

Photon HBT at RHIC

Lots of theoretical activity: Different approaches (just to mention a few): VNI/BMS+hydro+HG

– S. Bass, B. Muller, D. Srivastava PRL 94(2005) Hydro inspired fireball

– T.Renk, hep-ph/050382 ; 0408218 HG

– J. Alam, et al - Phys. Rev. C67 (2003) Still large uncertainties about contributions from

different sources Pt>2 GeV may be a window to early stages

Experimental studies under way (see talk by J. Sandweiss today)

Sergey Panitkin

Photon correlation function predictions

Bass et al -PRL 94(2005)

VNI/BMS

No background. Realistic calculations are needed

Sergey Panitkin

Conclusions

Rich set of femtoscopic results already exists at RHIC Evidence that matter at RHIC exhibits collective bulk

properties Most models still can not reproduce simultaneously

experimental observables at low Pt: v2, spectra, correlations We are gaining better understanding of model failures Some promising solutions are offered Ball is in theory corner More detailed correlation measurements will be available soon:

non-identical correlations, HBT Pt>1 GeV/c, Kaons, protons, etc Lower energy scan? RHIC II will offer new opportunities for femtoscopy with rear

probes Will shed light on detailed properties of bulk matter at RHIC

Sergey Panitkin

Femtoscopy at RHIC Workshop

At RHIC/AGS Users Meeting- June 21, 2005