Post on 31-Dec-2015
description
Vitaly Okorokov
XXXII International Symposium on Multiparticle Dynamics
September 7-13, 2002, Alushta, Crimea, Ukraine
Two-particle correlation measurements with STAR detector
at RHIC
Vitaly A. Okorokov(for the STAR Collaboration)
Moscow Engineering Physics Institute (State University), Kashirskoe Ave.31, Moscow, 115409, Russian Federation
Vitaly Okorokov
Outline
Introduction– STAR detector– Physics motivation– HBT methods and techniques
STAR HBT experimental results– HBT correlations of identical particles
• Various dependences of HBT parameters• Status of the “RHIC HBT puzzle”
– Correlations of non-identical particles• K correlations• Kp and Kp HBT results and model predictions
Conclusions
Vitaly Okorokov
STAR experiment at RHIC
First Au-Au collision events at RHIC @ 100+100 GeV/c per beam recorded by STAR
Vitaly Okorokov
Physics motivation
Ultra-relativistic heavy ion physics is entering the new era of colliderexperiments with the start-up of RHIC at BNL. The basic questions and central in the goals of RHIC today are,
“What is the nature of nuclear matter at energy densities comparable to those of the early Universe?”
“What are the new phenomena and physics associated with the simultaneous collisions of hundreds of nucleons at relativistic energies?”
The study of small relative momentum correlations, a technique also known as HBT interferometry, is one of the most powerful tools at ourdisposal to study complicated space-time dynamics of heavy ion collisi-ons. It provides crucial information which helps to improve our under-standing of the reaction mechanisms and to constrain theoretical mo-dels of heavy ion collisions. It is also considered to be a promising signature of the Quark Gluon Plasma (QGP).
Vitaly Okorokov
Two-particle correlations
Single particle spectrum is sensitive to momentum distribution only
Relative momentum distribution of particle pairs is sensitive to space-time information
Intensity interferometry, HBT technique, etc….
),(4 pp
xSdxddNE
),(|),(|)/)(/(
/)( 2
2111
2122 qrqrr
pppp
q SdddNddN
dddNC
Source functionFSI
Vitaly Okorokov
Information:
•geometrical source size: Rside
•lifetime
(for simple sources!)
kRqkRqkRq
LongSideOut
LongLongSideSideOutOute
qqqkC222222
1
),,,(
Decomposition of the pair relative momentum
(measured in the longitudinal co-moving source –LCMS- frame; (p1+ p2)z=0)
Pratt-Bertsch parameterization
Rout2=Rside
2+(pair)2
beam direction
p2p1
QT
QS
QO
beam direction
p1 p2
QT
Q
QL
)T2PT1P(21
TK
Vitaly Okorokov
In search of the QGP. Naïve expectations
QGP has more degrees of freedom than pion gas
Entropy should be conservedduring fireball evolution
Hence: Look in hadronic phasefor signs of: Large size, Large lifetime, Expansion……
Vitaly Okorokov
In search of the QGP: Expectations
“Naïve” picture (no space-momentum correlations):
– Rout2=Rside
2+(pair)2
One step further:– Hydro calculation of Rischke
& Gyulassy expects Rout/Rside ~ 2->4 @ kt = 350 MeV.
– Looking for a “soft spot”
Rout
Rside
Vitaly Okorokov
Centrality and transverse mass dependences
Centrality dependence:
Larger initial size->Larger final size.
Significant expansion !
STAR Final
mT dependence:
Generally behavior isconsistent with flow andobservations at AGS, SPS
PRL 87 (8) 2001
s=130 GeV/A
Vitaly Okorokov
HBT Parameters: mT dependence
HBT radii increase with centrality (including RL).
HBT radii decrease with increasing <mT>
Experimental data at 130 GeV/A consistent with results at 200 GeV/A
According to Sinyukov fits, evolution duration:
t (central) = 10.08 fm
t (midcentral) = 9.30 fm
t (peripheral) = 7.59 fm
Au+Au (data @ S=200 GeV are preliminary) 200 GeV
Vitaly Okorokov
Excitation function of the HBT parameters
• ~10% Central AuAu(PbPb) events
• y ~ 0
• kT 0.17 GeV/c
no significant rise in spatio-temporal size of the emitting source at RHIC
RO/RS ~ 1
Note ~100 GeV gap betweenSPS and RHIC !
Vitaly Okorokov
Pion HBT: STAR, PHENIX Results @ 130 GeV
D.H.Rischke, Nucl.Phys. A610 (1996) 88c; D.H.Rischker, M.Gyulassy, Nucl.Phys. A608 (1996) 479.S.A. Bass, A. Dumitry, S. Soff PRL 86, nucl-th/0012085,December 2000 (Hydro+UrQMD).
STAR and PHENIX agree
All theoretical calculations show only Rout/Rside ratio to be greater than unity due to system lifetime effects which cause Rout to be larger than Rside. They also predict that the ratio
increases with kT. Such an increase seems to be a generic feature of the models based on the Bjorken-type, boost-invariant expansion
scenario.
Model does not reproduce the data
The top panel shows the measured Rside from identical pions for STAR and PHENIX. Lines arefits of analytical equations (1) and (2) for boost-invariant, hydrodynamically expanding source to the STAR and PHENIX data.The bottom panel shows the ratio Rout/Rside as a function of kT overlaid with theoretical predictions for a phase transition for two critical temperatures.
)1(1 2
22
Tm
RR
Tf
geomside
)2(
21
1 2
22
Tm
RR
Tf
geomside
f=0.69-the boost velocity, f=0.85-transverse rapidity boost, T=125 MeV is the temperature.
fmRPHENIXgeom 2.07.6
Fit (1) - dashed line
fmRPHENIXgeom 3.01.8
Fit (2) - solid line
fmRSTARgeom 1.04.9
Fit (2) - dot-dashed line
C. Adler et al. (STAR Col.), PRL 87 (2001) 082301.K. Adcox et al. (PHENIX Col.), nucl-ex/0201008, January 2002.S.C. Johnson, nucl-ex/0205001, May 2002.
Vitaly Okorokov
Possible solutions for RHIC HBT puzzle
One of the most dramatic results from RHIC are the STAR results for interferometry. There are at least two outstanding issues in two-particle HBT pion results that have avoided easy physical descriptions:
(i) the absolute magnitude of the radii and their shape in kT at RHIC is strikingly similar
to lower energy measurements; (ii) the ratio of the transverse radii Rout/Rside is close to 1 over all kT.
It is not yet clear what physics leads to the kind of dependences reported by STAR Collaboration above.
Something exotic– Freeze out at critical point ?!– Sudden hadronization ?!– Non-Bjorken expansion scenario (Landau?)– ………– 3d hydro seems to be needed
In any case solution will require some sort of paradigm shift
Vitaly Okorokov
Correlations of non-identical particles
Correlations due to the final state interactions– Coulomb or strong
No symmetrization requirement ! => Pair wave function has odd terms => Sensitivity to source asymmetries Source asymmetries can be due to:
– different emmision times– Collective flow
Since particles, in general, have different mass => Correlations in small relative velocities not
momentum !
Vitaly Okorokov
Non-identical particle correlations How to reveal the asymmetries?
• Catching up: cos0• small mean separation• strong correlation
• Ratio of both scenarios allow quantitative study of the emission asymmetry
• Moving away: cos0• large mean separation• weak correlation
Crucial point:kaon begins farther in “out” direction(in this case due to time-ordering)
purple K emitted firstgreen is faster
purple K emitted firstgreen is slower
See talk by R. Lednicky
Vitaly Okorokov
1D relativistic view. What can be probed?
Source ofparticle 1 (pion)
Source ofparticle 2 (kaon)
Separation between particle 1 and particle 2 and boost to pair rest frame
2 parameters- Mean shift (<r*>)- Sigma (r*)
Separation between source 1 and 2 in pair rest frame
r
r (fm)
r* =pairr–pairtr* separation in pair rest frameFunction of pair(pair) which depend on the pair acceptance
We can assume that pions, kaons, and protons sources have different size and may be their are shifted in source frame
We measure only size and shift in pair rest frame
Vitaly Okorokov
Correlation functions and ratios K @ 130 GeV/A
Good agreement for like-sign and unlike-sign pairs points to similar emission process for K+ and K-
Clear sign of emission asymmetry
Two other ratios done as a double check – expected to be flat
CF
Out
Side
Long
a), b) pion kaon correlation functions
c), d) ratio of correlation function C-/C+
with respect to the sign of k*out
e), f) ratio of correlation function C-/C+
with respect to the sign of k*side
g), h) ratio of correlation function C-/C+
with respect to the sign of k*long
•Positive correlations forunlike sign pairs•Negative correlations for like sign pairs
Shape of correlation functionsdifferent for different cuts!
Vitaly Okorokov
K, p and Kp HBT results combined
Blast wave consistent with data
However, systematic errors need to be reduced to conclude
STAR Preliminary218.6 3
28.6 3
58.2
Parametersof source
Correlations
K @ 130 GeV/A
p @130 GeV/A
Kp @ 200 GeV/A
Fit in pair rest
frame
<r1* - r2
*>,
fm(t1 - t2),
fm/c6.4 - -
(r1 - r2), fm <4.6 - -
Table 1. Fit parameters for HBT correlations of non-identical particle pairs (Preliminary STAR data)
Vitaly Okorokov
Conclusions
The HBT interferometry measurements have been performed at RHIC energies and hereby extended the HBT excitation function into the new energy domain. A various dependences of the particle-emitting source parameters can be measured with high statistics in STAR.
Pion HBT results from Au-Au interactions at 130 GeV/A and 200 Gev/A are presented. The anomalously large source size or source lifetimes predicted for a long-lived mixed phase have not been observed in this study. The one of the most intriguing feature of the (preliminary) HBT results from RHIC is the KT dependence of the ROut/RSide ratio observed by the STAR Collaboration.
Evidence of space-time shift between , K, and p sources are obtained. Protons are ahead of kaons, kaons are ahead of pions (in the radial direction). Qualitative agreement is observed between experimental results and a Blast wave scenario which describes other STAR data.