First Results From the STAR Experiment at RHIC : II

1
Physics at RHIC Results from the RHIC STAR
Experiment
2
Outline

• Motivation for studying Relativistic Heavy Ion
  Collisions
• RHIC and the STAR experiment
• Soft Physics from STAR
• Hard Physics from STAR
• Summary

3
Why heavy ion collisions?

• Study bulk properties of nuclear matter

• 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 ms

• 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 deconfined
  state

4
Stages of the collision
Does end result look about the same whether a
QGP was formed or not???
time
temperature
5
The Phase Space Diagram
TWO different phase transitions at work!
Quarks and gluons roam freely over a large
volume Quarks behave as though they are
massless Calculations show that these occur at
approximately the same point Two sets of
conditions High Temperature High Baryon
Density Lattice QCD calc. Predict
Deconfinement transition
Quark-Gluon Plasma
Chiral transition
Hadrons
Tc 150-170 MeV ec 0.5-0.7 GeV/fm
6
(No Transcript)
7

• Beam energy up to 100 GeV/A (250 GeV for p)
• Two independent rings (asymmetric beam collisions
  are possible)
• Beam species from proton to Au
• Six interaction points STAR, PHENIX, PHOBOS and
  BRAHMS

8
RHIC Data-Taking
Year 2000 Au Au _at_ 130 GeV 2 weeks Year
2001 Au Au _at_ 200 GeV 15 weeks Au Au _at_ 20
GeV 1 day p p _at_ 200 GeV 5 weeks Year
2003 1st of January d Au _at_ 200 GeV 10
weeks p p _at_ 200 GeV (5) 3 weeks
? ?
? ?
9
The STAR Collaboration
The Ohio State U. Group Profs
PostDocs Students T.Humanic D.Majestro
S.Bekele M.Lisa B. Nilsen
M.Lopez- Noriega E.Sugarbaker I. Kotov
R.Wells R.Willson
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 JINRDubna,
  IHEP-Protvino
• U.S. Labs Argonne, Berkeley, Brookhaven
  National Labs
• U.S. Universities Arkansas, UC Berkeley, UC
  Davis, UCLA, Carnegie Mellon, Creighton,
  Indiana, Kent State, MSU, CCNY, Ohio State,
  Penn State, Purdue,Rice, Texas AM, UT
  Austin, Washington, Wayne State, Yale

Institutions 36 Collaborators 415
10
The STAR Detector
11
The STAR Detector
12
STAR Time Projection Chamber (TPC)

• Active volume Cylinder r2 m, l4 m
• 139,000 electronics channels sampling drift in
  512 time buckets
• active volume divided into 70M 3D pixels

13
Triggering/Centrality
Spectators Definitely going down the beam
line Participants Definitely created moving
away from beamline
Several meters
Spectators
Zero-Degree Calorimeter
Participants
Impact Parameter
Spectators
14
Triggering/Centrality
Spectators Definitely going down the beam
line Participants Definitely created moving
away from beamline
Several meters
Spectators
Zero-Degree Calorimeter
Participants
Impact Parameter
Spectators
15
Au-Au Event at 130 A-GeV
Peripheral Event From real-time Level 3 display.
16
Au- Au Event 130 A-GeV
Mid-Central Event From real-time Level 3 display.
17
Au -Au Event 130 A-GeV
Central Event From real-time Level 3 display.
18
STAR Pertinent Facts (130 GeV)
Field 0.25 T (Half Nominal value) ?
worse resolution at higher p ? lower pt
acceptance TPC Inner Radius 50cm
(ptgt75 MeV/c) Length 200cm
( -1.5lt h lt 1.5) Events 300,000 Central
Events top 8 multiplicity 160,000 Min-bias
Events
19
Needle in the Hay-Stack!
How do you do tracking in this regime? Solution
Build a detector so
you can zoom in close and see individual tracks
high resolution
Clearly identify individual tracks
Pt (GeV/c)
Good tracking efficiency
20
Particle ID Techniques - dE/dx
21
Particle ID Techniques - dE/dx
22
Particle ID Techniques - dE/dx
23
Particle ID Techniques - Topology
Decay vertices Ks ? p p - L ? p
p - ?L ? ?p p X- ? L p - X
??L p W ? L K -
?L
Vo
kinks K?? ?? ?
24
Physics Measurements(ones in red will be shown)

• dN/dh for h- (hlt 1.5)
  particle density, entropy
• Elliptic flow early
  dynamics, pressure
• p/p, L/L stopping
• Particle spectra temperature, radial flow
• Particle ratios chemistry
• Particle correlations
  geometry, collective flow
• High Pt jet
  quenching

_
_

• Neutral particle decays L,K0s, X
  strangeness production

25
Transverse Energy
Phenix Electromagnetic Calorimeter measures
transverse energy in collisions Central
Events Lattice predicts transition at
PHENIX Preliminary
5.0 GeV/fm3
ecritical 0.5-0.7 GeV/fm3
Have the Energy Density!!
26
Soft Physics (pT lt 2 GeV/c)
27
Soft Physics (pT lt 2 GeV/c)
The majority of produced particles are low
pT. Do they interact and exhibt collective
behaviour? What are the bulk dynamics ?
28
Is there Thermalization?
Look at Elliptic Flow
Origin spatial anisotropy of the system when
created and rescattering of evolving
system ?probe of the early stage of the
collision
Almond shape overlap region in coordinate space
29
Elliptic Flow of Pions and Protonsfrom STAR (130
GeV)

• Hydrodynamic calculations P. Huovinen, P. Kolb
  and U. Heinz

Mass dependence of v2(pt) shows a behavior in
agreement with hydro calculations, which assumes
a system in equilibrium
30
Charged particle elliptic flow 0lt ptlt 4.5 GeV/c
from STAR(130 GeV)
Around pt gt 2 GeV/c the data starts to deviate
from hydro. However, v2 stays large.
Only statistical errors Systematic error 10 -
20 for pt 2 4.5 GeV/c
31
Kinetic Freeze-out and Radial Flow
Want to look at how energy distributed in
system. Look in transverse direction so not
confused by longitudinal expansion
32
Kinetic Freeze-out and Radial Flow
Want to look at how energy distributed in
system. Look in transverse direction so not
confused by longitudinal expansion
33
Kinetic Freeze-out and Radial Flow
Want to look at how energy distributed in
system. Look in transverse direction so not
confused by longitudinal expansion
34
Kinetic Freeze-out and Radial Flow
Want to look at how energy distributed in
system. Look in transverse direction so not
confused by longitudinal expansion
35
First RHIC spectra - an explosive source

• various experiments agree well
• different spectral shapes for particles of
  differing mass? strong collective radial flow

data STAR, PHENIX, QM01 model P. Kolb, U. Heinz
36
First RHIC spectra - an explosive source

• various experiments agree well
• different spectral shapes for particles of
  differing mass? strong collective radial flow

data STAR, PHENIX, QM01 model P. Kolb, U. Heinz
37
First RHIC spectra - an explosive source

• various experiments agree well
• different spectral shapes for particles of
  differing mass? strong collective radial flow

data STAR, PHENIX, QM01 model P. Kolb, U. Heinz
38
First RHIC spectra - an explosive source

• various experiments agree well
• different spectral shapes for particles of
  differing mass? strong collective radial flow

• good agreement with hydrodynamiccalculations

data STAR, PHENIX, QM01 model P. Kolb, U. Heinz
39
mt slopes vs. Centrality

• Increase with collision centrality
• ? consistent with radial flow Tfreeze out0.12
  GeV, bflow0.6c

40

• Weve shown so far that for RHIC collisions
• Some evidence that source is thermalized
• Particles kinetically freeze-out with common T
• Large transverse flow -
• common to all particle species

41
HBT 101 - probing source geometry
p1
r1
x1
YT U(x1,p1)expi(r1-x1)p1U(x2,p2)expi(r2-x2)p2
U(x1,p2)expi(r2-x1)p2U(x2,p1)expi(r1-
x2)p1
p source r(x)
1 m
x2
r2
p2
5 fm
q p2 p1
Integrate YY over r(x)
e.g. r exp(-r2/2R2) ? C 1 lexp(-q2R2)
42
HBT 101 - probing source geometry
p1
r1
x1
YT U(x1,p1)expi(r1-x1)p1U(x2,p2)expi(r2-x2)p2
U(x1,p2)expi(r2-x1)p2U(x2,p1)expi(r1-
x2)p1
p source r(x)
1 m
x2
r2
p2
5 fm
q p2 p1
Integrate YY over r(x)
e.g. r exp(-r2/2R2) ? C 1 lexp(-q2R2)
43
HBT 101 - probing source geometry
p1
r1
x1
YT U(x1,p1)expi(r1-x1)p1U(x2,p2)expi(r2-x2)p2
U(x1,p2)expi(r2-x1)p2U(x2,p1)expi(r1-
x2)p1
p source r(x)
1 m
x2
r2
p2
5 fm
q p2 p1
Integrate YY over r(x)
e.g. r exp(-r2/2R2) ? C 1 lexp(-q2R2)
44
HBT 101 - probing the timescale of emission
Decompose q into components qLong in beam
direction qOut in direction of transverse
momentum qSide ? qLong qOut
RO2 lt(xOut - bTt)2gt RS2 lt xSide2 gt RL2
lt(xLong bLt)2gt
(beam is into board)
45
HBT and the Phase Transition
Generic prediction of 3D hydrodynamic models
emission timescale
Rischke Gyulassy NPA 608, 479 (1996)
Primary HBT signature of QGP
Phase transition ? longer lifetime Rout/Rside
1 (bt)/Rside
46
Two-pion interferometry (HBT)from STAR
qout

• Correlation function for identical bosons
• 1d projections of 3d Bertsch-Pratt
• 12 most central out of 170k events
• Coulomb corrected
• y lt 1, 0.125 lt pt lt 0.225

STAR preliminary
qlong
STAR preliminary
47
Radii dependence on centrality and kt
central collisions
low kT
p-
p
STAR preliminary

• Radii increase with multiplicity - Just geometry
  (?)
• Radii decrease with kt Evidence of flow (?)

multiplicity
48
Hydro attempts to reproduce data
generic hydro
Rlong model waits too long before emitting
Rout
model emission timescale too long

• KT dependence approximately reproduced? correct
  amount of collective radial flow
• Right dynamic effect / wrong space-time
  evolution???? the RHIC HBT Puzzle

Rside
49
HBT excitation function
midrapidity, low pT p- from central AuAu/PbPb

• decreasing l 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!?

STAR Collab., PRL 87 082301 (2001)
50
time
Before collision (heavy nuclei)
Evolution of a heavy-ion collision
After collision QM formation?? Hadronization
In order to study QM/hadronization stage of
collision from freezeout hadrons, need to
understand rescattering stage first!
Strong hadronic rescattering
Freezeout (hadrons freely stream to detectors)
51
Hadronic rescattering model(T. J. Humanic,
Phys.Rev.C 57, 866, (1998))
1) Assume a simple hadronization picture to set
the initial geometry and momenta. 2) Put in
a bunch of hadrons whose multiplicities are
consistent with RHIC experiments (or
predictions). 3) Let hadrons undergo strong
binary collisions until the system gets so
dilute (since it is expanding) that all
collisions cease. 4) Record the time, mass,
position, and momentum of each hadron when it no
longer scatters. ? freezout condition. 5)
Calculate hadronic observables ? pT
distributions, elliptic flow, HBT,
? parameters initial temperature (T),
hadronization proper time (t)
r
1/mTdN/dmT mTexp(-mT/T) T 300 MeV
z t sinh y t t cosh y t 1 fm/c
z
(thermal)
(cylindrical)
? p, K, N, D , L , w, r, f, h, h..
? s(i,j)
52
Comparison of the Rescattering model with RHIC
data for pT distributions
As seen above, the qualitative shapes are the
same for pT lt 3 GeV/c !
53
Comparison of Recattering model with RHIC data
for mT distributions
The slopes are seen to agree !
54
Elliptic Flow vs. pT from rescattering model
compared with STAR
flattening at high pT as in data
55
STAR pp HBT vs Rescattering Model
Rescattering qualitatively describes the
centrality and momentum dependences of the pion
HBT data!!
56
( rescattering model )




Comparison of Rescattering model with SPS and
RHIC data for pion HBT




Model is seen to describe the beam energy
dependence of the HBT parameters well!




57
Conclusion for soft (i.e. low pT) RHIC
physics Hadronic rescattering with a
short hadronization time (t 1 fm/c) describes
dynamic features well!
58
Hard Physics pT gt 2GeV/c
Goal Use jets to probe properties of medium
STAR pp ? Di-Jet
Some Basic Observables - Inclusive Spectra and
RAA - Azimuthal Anisotropy, v2 - Statistical ??
?? Correlations
59
The Experimental Challenge
Find this .in here
Central AuAu Event
pp ?dijet
60
Inclusive Charged Hadron Production
?s 130 GeV
?s 200 GeV
STAR, PRL 89, 202301 (2002)
nucl-ex/0210026
61
Leading Particle Suppression Theory

• Wang and Gyulassy partonic energy loss
• proportional to gluon density, ?glue
• effective softening of fragmentation
• suppression of leading hadron yield

leading particle
Partonic Energy loss in high density matter
Nuclear Modification Factor
ltNbinarygt/?inelpp
(Nuclear Geometry)
62
Leading Hadron Suppression Data
RAA using UA1 NN Reference
?s 200 GeV Preliminary
RCP ? Central/Peripheral
Suppression similar at 130 GeV (PRL 89, 202301
(2002))
Suppression saturates at 35 for pT gt 6 GeV/c
STAR pp reference in the works
nucl-ex/0210026
63
Azimuthal Correlations

• ?? Correlation with respect to leading particle
  (pTgt4 GeV/c)
• Consider only particles above 2 GeV/c

• Small difference in relative pseudorapidity

Pt
?
64
High pT Azimuthal Correlations
Ansatz AuAu pp Elliptic Flow
nucl-ex/0210033

• Near-side correlation shows jet-like signal in
  central/peripheral AuAu
• Away-side correlation suppressed in central AuAu

65
Surface Emission of Jets ?

• This is in accordance with 1) the measured
  suppresion of the inclusive spectra with respect
  to binary collisions, and 2) high-pT azimuthal
  correlations.
• We only see jets emitted from the surface?

66
Suppression of away-side jet consistent with
strong absorption in the bulk, with emission
dominantly from the surface
67
Summary of AuAu Collisions at RHIC

• Soft physics
• System appears to be thermalized
• Rapid hadronization, strong rescattering
• Large radial flow, elliptic flow, and HBT
  results all explainable as resulting from
  hadronic rescattering

• Hard physics
• Strong suppression of inclusive yields
• Azimuthal anisotropy at high pT
• Suppression of back-to-back hadron pairs

Large parton energy loss with surface emission?
68
STAR STRANGENESS!
(Preliminary)
K
L
f
K0s
L
X-
X
K
69
The Collisions
The End Product