STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar1 Emergence of a Consistent Picture from First...

31 Oct 2001 Mike Lisa - Kent State Seminar 1

STARHBT

Emergence of a Consistent Picture from First Results of STAR at RHIC?

Mike Lisa, Ohio State UniversitySTAR Collaboration

U.S. Labs: Argonne, Lawrence Berkeley National Lab, Brookhaven National Lab

U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton,Indiana,Kent State,Michigan State,CCNY, Ohio State,Penn State, Purdue, Rice,Texas A&M, UT Austin,Washington, Wayne State,Yale

Brazil: Universidade de Sao Paolo

China: IHEP - Beijing, IPP - Wuhan

England: University of Birmingham

France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes

Germany: Max Planck Institute – Munich, University of Frankfurt

Poland: Warsaw University, Warsaw University of Technology

Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino


STARHBT

Overview• ~ 1 year from initial data-taking in new energy regime• overall picture / underlying driving physics unclear

Outline• Ultrarelativistic Heavy Ion Collisions and STAR at RHIC• First data

Transverse momentum spectra Momentum-space anisotropy (elliptic flow)• Initial quantitative success of hydrodynamics• Two-pion correlations (HBT) STAR HBT and the “HBT Puzzle”

• Characterization of freeze-out from the data itself K- correlations particle-identified elliptical flow azimuthally-sensitive HBT: theory and first data

• Summary

Skipped

Skipped


STARHBT

Why heavy ion collisions?

• Study bulk properties of strongly-interacting matter far from ground state

• Extreme conditions (high density/temperature): expect a transition to new phase of matter…

• Quark-Gluon Plasma (QGP)• partons are relevant degrees of freedom over large

length scales (deconfined state)

• believed to define universe until ~ s

• Study of QGP crucial to understanding QCD• low-q (nonperturbative) behaviour

• confinement (defining property of QCD)

• nature of phase transition

• Heavy ion collisions ( “little bang”): the only way to experimentally probe the deconfined state

The “little bang”


STARHBT

The “little bang”Stages of the collision

• pre-equilibrium (deposition of initial energy density)• rapid (~1 fm/c) thermalization (?)

QGP formation (?)

hadronic rescattering

hadronization transition (very poorly understood)

freeze-out: cessation of hard scatterings

• low-pT hadronic observables probe this stage

“end result” looks very similar whether a QGP was formed or not!!!


STARHBT

Already producing QGP at lower energy?

J. Stachel, Quark Matter ‘99

Thermal model fits to particle yields (& strangeness enhancement, J/ suppression) approach QGP at CERN?

• is the system really thermal?• dynamical signatures? (no)• what was pressure generated?• what is Equation of State of strongly-interacting matter?

warning: e+e- yields fall on similar line!!

Must go beyond chemistry: study dynamics of system well into deconfined phase (RHIC)

lattice QCD applies


STARHBT

uRQMD simulation of Au+Au @ s=200 GeV

pure hadronic & stringdescription (cascade)

generally OK at lower energies

applicability in very high density (RHIC) situations unclear

produces too little collective flow at RHIC

freeze-out given by lasthard scattering


STARHBT

First RHIC spectra - an explosive source

data: STAR, PHENIX, QM01model: P. Kolb, U. Heinz

• various experiments agree well

• different spectral shapes for particles of differing mass strong collective radial flow

mT1/m

T d

N/d

mT

light

heavyT

purely thermalsource

explosivesource

T,mT1/

mT d

N/d

mT

light

heavy• very good agreement with hydrodynamic

prediction


STARHBT

Hydrodynamics: modeling high-density scenarios

• Assumes local thermal equilibrium (zero mean-free-path limit) and solves equations of motion for fluid elements (not particles)

• Equations given by continuity, conservation laws, and Equation of State (EOS)

• EOS: relates pressure, temperature, chemical potential, volume– direct access to underlying physics

• Works qualitatively at lower energybut always overpredicts collectiveeffects - infinite scattering limitnot valid there

lattice QCD input


STARHBT

Hydro time evolution of

non-central collisions

Equal energy density linesP. Kolb, J. Sollfrank,and U. Heinz

• correlating observations with respect to event-wise reaction plane allows much more detailed study of reaction dynamics

• entrance-channel aniostropy in x-space pressure gradients (system response) p-space anisotropy (collective elliptic flow)

self-quenching effect - sensitive to early pressure


STARHBT

Azimuthal-angle distribution versus reaction plane

• v2 increases from central to peripheral collisions– natural space-momentum

connection

φ= 2cosv2

particle-reaction plane

( )φ+φ

2cosv21~d

dN2or


STARHBT

Measurements at AGS; E895 and E877 (Protons)

• At low beam energies negative v2 (“squeeze-out”)

• Balancing energy around 4 AGeV, sensitive to EOS

Elab (AGeV)

0.04

-0.08

-0.04

0

1 10

v 2E895, Phys. Rev. Lett. 83 (1999) 1295

P. Danielewicz, Phys. Rev. Lett. 81 (1998) 2438


STARHBT

Local thermal equilibrium versus Low Density Limit

SPS; Low-Density-Limit and Hydro miss pt dependence

RHIC; pt dependence quantitatively described by Hydro

pt dependence sensitive to early thermalization?

p

Charged particles


STARHBT

• Momentum-space characteristics of freeze-out appear well understood

• Coordinate-space ?• Probe with two-particle intensity interferometry (“HBT”)

The other half of the story…


STARHBT

“HBT 101” - probing source geometry

12 ppqrrr −=

2

21

2121 )q(~1

)p(P)p(P)p,p(P

)p,p(C ρ+== C (Qinv)

Qinv (GeV/c)

1

2

0.05 0.10

Width ~ 1/R

Measurable! F.T. of pion source

( ))xx(iq2

*21

*1T

*T

21e1UUUU −⋅+⋅⋅=ψψ

Creation probability ρ(x,p) = U*U

222111 p)xr(i22

p)xr(i11T e)p,x(Ue)p,x(U

rrrrrr rrrr ⋅−⋅−=

5 fm

1 m sourceρ(x)

r1

r2

x1

x2

{2

1

}e)p,x(Ue)p,x(U 212121 p)xr(i21

p)xr(i12

rrrrrr rrrr ⋅−⋅−+

p1

p2


STARHBT

“HBT 101” - probing the timescale of emission

K

( ) ( ) ( )( ) ( )( ) ( ) ( )Kt~x~KR

Kx~KR

Kt~x~KR

2llong

2l

2side

2s

2out

2o

rr

rr

rr

β−=

=

β−= ⊥

xxx~ −≡

∫∫

⋅⋅⋅

≡)K,x(Sxd

)x(f)K,x(Sxdf

4

4

RoutRside ( ) ( )y,xx,x sideout ≠

Decompose q into components:qLong : in beam directionqOut : in direction of transverse momentumqSide : qLong & qOut

(beam is into board)( )22

s2o RR τ⋅β+=

beware this “helpful” mnemonic!

( )2l2l

2s

2s

2o

2o RqRqRq

lso e1)q,q,q(C ++−⋅λ+=


STARHBT

Large lifetime - a favorite signal of “new” physics at RHIC

• hadronization time (burning log) will increase emission timescale (“lifetime”)

• magnitude of predicted effect depends strongly on nature of transition

• measurements at lower energies (SPS, AGS) observe <~3 fm/c

“”

withtransition

c

Rischke & GyulassyNPA 608, 479 (1996)

3D 1-fluid Hydrodynamics

~

…but lifetime determination is complicated by other factors…


STARHBT

First HBT data at RHIC

STAR Collab., PRL 87 082301 (2001)

( )2l2l

2s

2s

2o

2o RqRqRq

lso e1)q,q,q(C ++−⋅λ+=

Data well-fit by Gaussian parametrization

Coulomb-corrected(5 fm full Coulomb-wave)

“raw” correlation function projection

1D projections of 3D correlation functionintegrated over 35 MeV/cin unplotted components


STARHBT

HBT excitation function

STAR Collab., PRL 87 082301 (2001)

•decreasing parameter partially due to resonances

•saturation in radii

•geometric or dynamic (thermal/flow) saturation

•the “action” is ~ 10 GeV (!)

•no jump in effective lifetime

•NO predicted Ro/Rs increase(theorists: data must be wrong)

•Lower energy running needed!?

midrapidity, low pT -

from central AuAu/PbPb


STARHBT

First STAR HBT data - systematics

STAR Collab., PRL 87 082301 (2001)

• +, - HBT parameters similar• Grossly similar to AGS/SPS

• all radii increase with multiplicity• Ro, Rs - geometric effect

• Rl - increase not seen at AGS/SPS

• With increasing mT

• increases fewer resonances

• radii decrease x-p correlations

• stronger effect in Ro than at AGS/SPS

systematic errors


STARHBT

mT dependence at ycm for 2 AGeV central collisions

• collective flow dynamical correlation between position and momentum R(mT)

• R’s are “lengths of homegeity”

• - from decays (mT)

x (fm)

y (f

m)


STARHBT

Hydro attempts to reproduce data

out

side

long

KT dependence approximately reproduced correct amount of collective flow

Rs too small, Ro & Rl too big source is geometrically too small and lives too long in model

Right dynamic effect / wrong space-time evolution? the “RHIC HBT Puzzle”

generichydro


STARHBT

“Realistic” afterburner makes things worse

pure hydro

hydro + uRQMD

STAR data

1.0

0.8

Currently, no physical modelreproduces explosive space-timescenario indicated by observation

RO/R

S


STARHBT

Now what?

• No dynamical model adequately describes freeze-out distribution• Seriously threatens hope of understanding pre-freeze-out dynamics• Raises several doubts

– is the data consistent with itself ? (can any scenario describe it?)– analysis tools understood?

• Attempt to use data itself to parameterize freeze-out distribution• Identify dominant characteristics• Examine interplay between observables• Isolate features generating discrepancy with “real” physics models


STARHBT

Characterizing the freezeout: An analogous

situation


STARHBT

Probing f(x,p) from different angles

∫ ∫ ∫

⋅⋅⋅φφ=2

0

2

0

R

0Tps2

T

)p,x(fmdrrdddm

dN

Transverse spectra: number distribution in mT

∫ ∫ ∫∫ ∫ ∫

⋅⋅φφ

⋅φ⋅⋅φφ=φ≡

20

20

R0sp

20

20

R0 psp

pT2)p,x(fdrrdd

)p,x(f)2cos(drrdd)2cos()m,p(v

Elliptic flow: anisotropy as function of mT

HBT: homogeneity lengths vs mT, p

( )

( ) ν

ν

ν

−⋅⋅φ

⋅⋅⋅φ=φ

⋅⋅φ

⋅⋅⋅φ=φ

∫ ∫∫ ∫

∫ ∫∫ ∫

xx)p,x(fdrrd

)p,x(fxxdrrd,px~x~

)p,x(fdrrd

)p,x(fxdrrd,px

20

R0s

20

R0s

pT

20

R0s

20

R0s

pT


STARHBT

mT distribution from Hydrodynamics-inspired model

)r(tanh 1=ρ −

E.Schnedermann et al, PRC48 (1993) 2462

R

s

( ) ( )rRcosT

sinhpexp

T

coshmK)p,x(f pb

TT1 −Θ⋅⎥⎦

⎤⎢⎣⎡ φ−φ⋅

ρ⋅⎟⎠⎞

⎜⎝⎛ ρ

=

Infinitely longsolid cylinder

b = direction of flow boost (= s here)

2-parameter (T,) fit to mT distribution

)r(g)r( s ⋅=


STARHBT

• 2 contour maps for 95.5%CL

T th [

GeV

]

s [c]

- K-p

T th [

GeV

]

s [c]

T th [

GeV

]

s [c]

Tth =120+40-30MeV

<r >=0.52 ±0.06[c]

tanh-1(<r >) = 0.6

<r >= 0.8s

Fits to STAR spectra; r=s(r/R)0.5

-

K-

p

1/m

T d

N/d

mT

(a

.u.)

mT - m [GeV/c2]thanks to M. Kaneta

preliminary

STAR preliminary


STARHBT

Excitation function of spectral parameters

• Kinetic “temperature” saturates ~ 140 MeV already at AGS

• Explosive radial flow significantly stronger than at lower energy

• System responds more “stiffly”?

• Expect dominant space-momentum correlations from flow field


STARHBT

Implications for HBT: radii vs pT

Assuming , T obtained from spectra fits strong x-p correlations, affecting RO, RS differently

pT=0.2

pT=0.4

y (f

m)

y (f

m)

x (fm)

x (fm)

( )22S

2O RR τ⋅β+=


STARHBT

Implications for HBT: radii vs pT

STAR data

model: R=13.5 fm, =1.5 fm/c T=0.11 GeV, ρ0 = 0.6

Magnitude of flow and temperature from spectra can account for observed drop in HBT radii via x-p correlations, and Ro<Rs

…but emission duration must be small

pT=0.2

pT=0.4

y (f

m)

y (f

m)

x (fm)

x (fm)

Four parameters affect HBT radii


STARHBT

Kaon – pion correlation:dominated by Coulomb interaction

• Static sphere :– R= 7 fm ± 2 fm (syst+stat)

• Blast wave – T = 110 MeV (fixed)

– <r> = 0.62 (fixed)

– R = 13 fm ± 4 fm (syst+stat)

• Consistent with other measurements

STAR preliminary


STARHBT

Initial idea: probing emission-time ordering

• Catching up: cos0• long interaction time• strong correlation

• Ratio of both scenarios allow quantitative study of the emission asymmetry

• Moving away: cos0• short interaction time• weak correlation

Crucial point: time-ordering meanskaon begins farther in “out” direction

purple emitted firstgreen is faster

purple emitted firstgreen is slower


STARHBT

Space-time asymmetry

• Evidence of a space – time asymmetry– -K ~ 4fm/c ± 2 fm/c, static

sphere

– Consistent with “default” blast wave calculation

pT = 0.12 GeV/c

STAR preliminary

KpT = 0.42 GeV/c


STARHBT

Non-central collisions: coordinate- and momentum-space anisotropies

Equal energy density lines

P. Kolb, J. Sollfrank, and U. Heinz


STARHBT

More detail: identified particle elliptic flow

soliddashed

0.04 0.010.09 0.02a (c)

0.04 0.01 0.0S2

0.54 0.030.52 0.020(c)

100 24135 20T (MeV)

STAR, in press PRL (2001)

( ) ( ) ( ) ( )( ) ( )∫

∫ ρρ

ρρ

φ

φφ=

20 T

coshm1T

sinhp0b

20 T

coshm1T

sinhp2bb

T2TT

TT

KId

KI2cosdpv

( )ba0 2cos φρ+ρ=ρFlow boost:

b = boost direction

Meaning of ρa is clear how to interpret s2?

hydro-inspiredblast-wave modelHouvinen et al (2001)


STARHBT

Ambiguity in nature of the spatial anisotroy

b = direction of the boost s2 > 0 means more source elements emitting in plane

( )( )

( ) ( )rR2cosR

rs21ecosh

T

mKp,xf s2

cossinhT

pT

1ps

T

−θ⎟⎠⎞

⎜⎝⎛ φ+⎟

⎠⎞

⎜⎝⎛ ρ=

φ−φρrr

case 1: circular source with modulating density

RMSx > RMSy

RMSx < RMSy

( )( ) ( )y222cossinh

T

pT

1 R/xy1ecoshT

mKp,xf

psT

η+−θ⎟⎠⎞

⎜⎝⎛ ρ=

φ−φρrr

case 2: elliptical source with uniform density

x

y

R

R≡η

1

1

2

1s

3

3

2 +η−η

≅


STARHBT

Azimuthal HBT: (transverse) spatial anisotropy

•Source in b-fixed system: (x,y,z)•Space/time entangled in

pair system (xO,xS,xL)

U. Wiedemann, PRC 57, 266 (1998)

( )( )( ) ( ) p

2221

ppT2os

pp22

p22

pT2s

22pp

22p

22pT

2o

2sinx~y~2cosy~x~,pR

2siny~x~cosy~sinx~,pR

t~2siny~x~siny~cosx~,pR

φ−+φ⋅=φ

φ⋅−φ+φ=φ

β+φ⋅+φ+φ=φ ⊥

large flow @ RHIC induces space-momentum

correlations

p-dependent homogeneity lengths

sensitive to more than “just” anisotropic geometry

( )pT ,px~x~ φνμ

out

b

K

x

yside


STARHBT

Reminder: observations for Au(2 AGeV)Au

p (°) 0 180

0

0 180 0 180

10

-10

20

40

R2 (

fm2 ) out side long

ol os sl

E895 Collab., PLB 496 1 (2000)

p=0°

p=90°

out-of-planeextended source

interesting physics, but not currenly accessible in STARwith 2nd-order reaction plane

Lines are global fitOscillation magnitude eccentricityOscillation phases orientation


STARHBT

xout

xside

K

Meaning of Ro2() and Rs

2() are clearWhat about Ros

2()

p (°) 0 180

0

0 180 0 180

10

-10

20

40

R2 (

fm2 ) out side long

ol os sl

E895 Collab., PLB 496 1 (2000)

• Ros2() quantifies correlation between xout and xside

• No correlation (tilt) b/t between xout and xside at p=0° (or 90°)

K

x out x sid

e K x out x sid

e

K x out x side

K xout

x side

K xout

xside

K xout

xside

p = 0°p ~45°

• Strong (positive) correlation when p=45°


STARHBT

STAR HBT

“Out”

“Side”

“Long”

1.0

1.3

1.0

1.3

1.0

1.3

0 0.1 0.2

C(Q

)

Q (GeV/c)

Correlation function: p=45º

RO

2 (fm

2 )R

S2 (

fm2 )

RO

S2 (

fm2 )

- from semi-peripheral events

raw

corrected forreactionplane resolution

data fit

• only mix events with “same” RP

• retain relative sign between q-components• HBT radii oscillations similar to AGS• curves are not a global fit• RS almost flat

STAR preliminary


STARHBT

Out-of-plane elliptical shape indicated

case 1

using (approximate) values ofs2 and ρa from elliptical flow

case 2

opposite R() oscillations would lead to opposite conclusion STAR preliminary


STARHBT

s2 dependence dominates HBT signal

error contour fromelliptic flow data

color: 2 levelsfrom HBT data

STAR preliminary

s2=0.033, T=100 MeV, ρ0ρaR=10 fm, =2 fm/c


STARHBT

A consistent picture

parameter spectra elliptic flow HBT K-

Temperature T≈11MeV √ √ √ √

Radialflowvelocity

ρ≈. √ √ √ √

Oscillationinradialflow

ρa≈.4 √ √

Spatialanisotropy

s2≈.4 √ √

Radiusiny Ry≈1-1fm(dependsonb)

√ √

Natureofxanisotropy

* √

Emissionduration

≈2fm/c √ √

( )( ) ( ) 22ps

T

2/ty

222cossinhT

pT

1 eR/xy1ecoshT

mKp,xf τφ−φρ

⋅η+−θ⎟⎠⎞

⎜⎝⎛ ρ=

rr


STARHBT

SummarySpectra• Very strong radial flow field superimposed on thermal motion• T saturates rapidly ~ 140 MeV higher at RHIC

•space-momentum correlations important•“stiffer” system response?

• consistent with hydro expectation

Momentum-space anisotropy• sensitive to EoS and early pressure and thermalization• significantly stronger elliptical flow at RHIC, compared to lower

energy• indication of coordinate-space anisotropy as well as flow-field

anisotropy (v2 cannot distinguish its nature, however)• for the first time, consistent with hydro expectation


STARHBT

Summary (cont’)HBT• radii grow with collision centrality R(mult)• evidence of strong space-momentum correlations R(mT)• non-central collisions spatially extended out-of-plane R()• The spoiler - expected increase in radii not observed• presently no dynamical model reproduces data

Combined data-driven analysis of freeze-out distribution• Single parameterization simultaneously describes

•spectra•elliptic flow•HBT•K- correlations

• most likely cause of discrepancy is extremely rapid emission timescale suggested by data - more work needed!


STARHBT

The End


STARHBT

Very large event anisotropies seen by STAR, PHENIX, PHOBOS

v2

centrality

• space-momentum connection clear in multiplicity dependence

• different experiments agree well

• finally, we reach regime of quantitative hydro validity evidence for early thermalization

• AGS: magnitude described by cascade models

• RHIC; Hydro description for central to mid-central collisions

– 26% more particles in-plane than out-of-plane (even more at high pT)!!


STARHBT

Experimental correlation functions

12 ppqrrr −=

B(q)

A(q)C(q)

)p(P)p(P

)p,p(P)p,p(C Practice In

21

2121 = →=

A(q

)

q (GeV/c)

# pairs fromsame event

B(q

)

q (GeV/c)

# pairs fromdifferent events

• most pairs at high q (need statistics!)

• shape of A(q), B(q) dominated by phasespace and single-particle acceptance (complicated in principle, especially in multiple dimensions)

q (GeV/c)0.1 0.2 0.3

C(q

)

00

2

1

• only correlated effects persist in ratio (including residual detector artifacts…)

• Correlation functions from different experiments (and from theory) can be compared


STARHBT

A consistent picture

parameter spectra elliptic flow HBTTemperature T ≈11MeV √ √ √

Radialflowvelocity

ρ≈. √ √ √

Oscillationinradialflow

ρa≈.4 √ √

Spatialanisotropy

s2≈.4 √ √

Radiusiny Ry≈1-1fm(dependsonb)

√

Natureofxanisotropy

* √

Emissionduration

≈2fm/c √main sourceof discrepancy?

( )( ) ( ) 22ps

T

2/ty

222cossinhT

pT

1 eR/xy1ecoshT

mKp,xf τφ−φρ

η+−θ⎟⎠⎞

⎜⎝⎛ ρ=

rr


STARHBT

Geometry of STAR

ZCal

Barrel EM Calorimeter

Endcap Calorimeter

Magnet

Coils

TPC Endcap & MWPC

ZCal

FTPCs

Vertex Position Detectors

Central Trigger Barrel or TOF

Time Projection Chamber

Silicon Vertex Tracker

RICH


STARHBT

Peripheral Au+Au Collision at 130 AGeV

Data Taken June 25, 2000.

Pictures from Level 3 online display.


STARHBT

Au on Au Event at CM Energy ~ 130 AGeV

Data Taken June 25, 2000.


STARHBT

Summary• Spectra, elliptic flow, and HBT measures consistent with a freeze-out

distribution including strong space-momentum correlations

• In non-central collisions, v2 measurements sensitive to existence of spatial anisotropy, while HBT measurement reveals its nature

• Systematics of HBT parameters:• flow gradients produce pT-dependence (consistent with spectra and v2(pT,m))

•anisotropic geometry (and anisotropic flow boost) produce -dependence

• (average) out-of-plane extension indicated• however, distribution almost “round,” --> more hydro-like evolution as

compared to AGS

While data tell consistent story within hydro-inspired parameterization, hydro itself tells a different story - likely point of conflict is timescale


STARHBT

STAR TPC • Active volume: Cylinder r=2 m, l=4 m

– 139,000 electronics channels sampling drift in 512 time buckets

– active volume divided into 70M3D pixels

On-board FEE Card:Amplifies, samples, digitizes 32 channels


STARHBT

Joint view of freezeout: HBT & spectra

• common model/parameterset describes different aspects of f(x,p) for central collisions

• Increasing T has similar effect on a spectrum as increasing

• But it has opposite effect on R(pT) opposite parameter correlations in

the two analyses tighter constraint on parameters

spectra ()

HBT

STAR preliminary


STARHBT

Time-averaged freezeout shape

3

2

2

x

y

s21

s21

R

R

−+

=≡η

• close to circular @ RHIC• info on evolution duration?

STAR preliminary

(E895)

STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar1 Emergence of a Consistent Picture from First...

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Transcript of STAR HBT 31 Oct 2001Mike Lisa - Kent State Seminar1 Emergence of a Consistent Picture from First...