Probing High-Temperature QCD Matter at the Relativistic Heavy-Ion Collider (RHIC) Saskia...
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Probing High-Temperature QCD Matter
at the Relativistic Heavy-Ion Collider (RHIC)
Saskia Mioduszewski
18 September 2008
Group Members
Postdocs:Rory F. ClarkeAhmed Hamed
Graduate Students:Matthew CervantesMartin Codrington (Chemistry)
Supported by D.O.E. and Sloan Foundation
Goal of RHIC: To Study Fundamental Puzzles of Hadrons
• Confinement– Quarks do not exist as free particles
• Generation of mass– Free quark mass ~ 5-7 MeV – Quarks become “fat” in hadrons,
constituent mass ~ 300-400 MeV
• Complex structure of hadrons– Sea anti-/quarks– Gluons– Origin of Spin of the nucleon
These phenomena must have occurred with formation of hadrons
nuclear matter p, n
~ 10 s after Big Bang
Hadron Synthesisstrong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV/c2
~ 100 s after Big Bang
Nuclear Synthesisstrong force binds protons and neutrons in nuclei
Expectation from Numerical Simulations of Finite-Temperature QCD
Stefan-Boltzmann limit
Expectation: create a “weakly coupled gas of quarks and gluons” by reaching Tc in high-energy heavy-ion collisions
New State of Matter created at CERN
At a special seminar on 10 February, spokespersons from the experiments on CERN's Heavy Ion programme presented compelling evidence for the existence of a new state of matter in which quarks, instead of being bound up into more complex particles such as protons and neutrons, are liberated to roam freely.
(Year 2000)
Pb+Pb collisions at √sNN = 17 GeV at the SPS
“Travel” Back in Time
Quest of heavy-ion collisions: heat and compress nuclear matter
– create Quark Gluon Plasma (QGP) as transient state in heavy ion collisions (e.g. Au+Au collisions)
– verify existence of QGP– study properties of QGP– study QCD confinement and how hadrons get their masses
neutron stars
Quark Matter
Hadron Resonance Gas
Nuclear Matter
Color Superconductor
SIS
AGS
SPS
RHIC& LHC
early universe
B
T
TC~170 MeV
940 MeV 1200-1700 MeVbaryon chemical potential
tem
per
atu
re
RHIC & SPS
Relativistic Heavy Ion Collider
• RHIC was proposed in 1983
• RHIC began providing collisions in 2000
• √sNN = 200 GeV = 10 x Collision-Energy at SPS
New probe available
High-pT particles from “hard” scattering
RHIC Specifications• 3.83 km circumference• Two independent rings
– 120 bunches/ring– 106 ns crossing time
• Capable of colliding ~any nuclear species on ~any other species
• Energy: 22-500 GeV for p-p 5-200 GeV for Au-Au
(per N-N collision)• Luminosity
– Au-Au: 5 x 1027 cm-2 s-1
– p-p : 1.5x1032 cm-2 s-1 (polarized)
The RHIC Experiments
STAR
PHENIX
STAR
Characterizing the collisions
• Different centralities, i.e. size of overlap region
• Asymmetry of reaction zones
• How does the matter behave?
• Can we probe the matter that exists only for a short time?
15 fm b 0 fm0 Npart 394
Spectators
Participants
For a given b, “billiard ball” model predicts Npart (No. participants)and Nbinary (No. binary collisions)
Not all A+A collisions are the same -- “Centrality”
0 Nbinary ~1000
Kinematics for colliders
2tanln Pseudo-rapidity:
Transverse momentum (pT) and pseudorapidity ()provide a convenient description
Mid-rapidity: η = 0, perpendicular to the incident beamsη = 4: Scattering at θ = 2.1o in the CM (or lab) frame
Radial Flow– Collective Expansion of system due to
pressure
– Heavier particles shifted to higher pT
– Observable: <T> from slopes as a function of mass and/or centrality
– Spectra can be described by hydrodynamic models for pT< 2-3 GeV/c and mid-peripheral to central events
Single Particle Spectra (low pT)
• Decreasing slope for increasing particle mass and centrality
peripheral
central
< T
>
Elliptic Flow in Non-central Collisions
Early state manifestation of collective behavior: • Asymmetry generated early in collision, quenched by expansion observed asymmetry emphasizes early time
Fourier Expansion: dN/d ~ 1 + 2 v2(pT) cos (2) + ...
x
y
p
patan2cos2 vSecond Fourier coefficient v2:
Coordinate space: initial asymmetry
Momentum space: final asymmetry
multiple collisions (pressure)
py
px
Data compared to Hydro
Reaction Plane (Angle 2)
Hydrodynamics with 0 viscosity
Thermalization in < 1 fm/c
pT [GeV/c]
v2
How does the expected “Quark Gluon Plasma” compare with the “Perfect Fluid” that we have found at RHIC?
Can we quantify the properties of this new form of matter?
Same behavior as observed in gases of
strongly coupled Li atoms
The matter we have created at RHIC behaves like a strongly coupled fluid, not like “weakly coupled gas of quarks and gluons”
K. M. O’Hara et al, Science 298, 2179
/S
[1/
4]
AdS/CFT for calculating properties of strongly-coupled gauge theories
RHIC “fluid” mightbe at ~2-3 on this scale (at T~1012 K)
How small can viscosity be?
Conjectured lower bound on viscosity/entropy = 1/4
P.K. Kovtun, D.T. Son, and A.O. Starinets, Phys. Rev. Lett. 94:111601, 2005.
Probing the MediumThe QCD analogue of x-ray tomography
• Need an external calibrated source
• Calculate absorption cross sections
Interpret the results
“Hard” processes to probe the matter • Large momentum transfer – or close distance• Can resolve partons: valence quarks, sea quarks
and gluons – scattered parton fragments into a “jet”• Coupling is weak - pQCD applicable
dtd
Aaf /A
a
Abf /B
b
dDh2
d
2h
cDh1
c
1h
quark or
gluon
JetFragmentation Function
cone of hadrons “jet”
p p
hard-scattered
parton in p+p
hadron distributionsoftened, jets broadened?
hard-scatteredparton during Au+Au
increased gluon-radiationwithin plasma?
Jets in heavy ion collisions
Hard scattering
Thermally-shaped Soft Production
Hard Scattering
• Good agreement with NLO perturbative QCD calculations
• High pT particle yields serve as a calibrated probe of the nuclear medium in nucleus+nucleus (A+A) and deuteron+nucleus (d+A) collisions
Production cross section of 0
p+p collisions = “baseline”
Systematizing Our Expectations• Describe in terms of
scaled ratio RAA
= 1 for “baseline expectations”
• Will present most of Au+Au and d+Au data in terms of this ratio
“no effect”
ppAuAubinary
AuAuAA YieldN
YieldR
centralN
binary = 975 94
Scaling of calibrated probe works in peripheral Au+Au, but strong suppression in central Au+Au
peripheralN
binary = 12.3 4.0
Discovery of Strong Suppression
Nuclear Modification Factor
RHIC 200 GeV central -
Suppressionperipheral –
Nbinary scaling
ppperipheralbinary
peripheral
YieldN
Yield
Comparison of peripheral to central
binary scaling
ppcentralbinary
central
YieldN
Yield
Theoretical Understanding?
Understood in an approach that calculates energy loss of hard-scattered parton through gluon radiation in a dense partonic medium (15 GeV/fm3 ~100 x normal nuclear matter)
Au+Au suppression (I. Vitev and M. Gyulassy, hep-ph/0208108)d+Au enhancement (I. Vitev, nucl-th/0302002 )
Our high pT probeshave been calibratedand are now being used to explore propertiesof the medium
Au-Au
d-Au
* Note deuteron-gold control experiment with no suppression
What have we learned?
• Nuclear matter created at RHIC is very opaque and dense (estimates of 100 x normal nuclear matter density)
• Strong collective behavior
• Coupling must be strong for v2 to be so large
Now we want to characterize this new matter more quantitatively (viscosity, transport coefficients, color charge density)
Jet Reconstruction in Au+Au Collisionsee q q
(OPAL@LEP)pp jet+jet
(STAR@RHIC)
Au+Au ??? (STAR@RHIC)
0-
dN/d
Jet Studies via Correlations
pT,trig – pT of the trigger particle
pT,assoc bin – range of pT selected to associate with the trigger particle
pT,trig > 4 GeV/c
pT,assoc = 2-4 GeV/c
Azimuthal distributions in Au+Au
Near-side: peripheral and central Au+Au similar to p+p
Strong suppression of back-to-back correlations in central Au+Au collisions
Au+Au peripheral Au+Au central
pedestal and flow subtracted
Phys Rev Lett 90, 082302
Escaping Jet -“Near Side”
Lost Jet -“Away Side”
“Reappearance of away-side jet”With increasing trigger pT, away-side jet correlation reappears
4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig Increasing pT,trig
Increasin
g pT
,assoc
Medium Modification to Fragmentation Function
Are we probing the medium? Or is it simply too opaque?
Punch-through Jet ?Or just tangential emission ?
Centrality8 < pT,trig< 15 GeV/c, pT,assoc > 6 GeV/c
Hard Scattering + jet
increased gluon-radiationwithin plasma?
• If is produced in hard scattering, instead of q or g, expect it to escape without interaction calibrated probe
• Then could study jet on opposite side as a function of the energy of photon
Is there any particle not affected by the opaque medium?
Effect of Dense Medium on Direct Photons
Hadrons are suppressed, photons are not – photons serve as the “control” experiment
PHENIX, Phys. Rev. Lett. 96, 202301 (2006)
ppγ
centralbinary
centralγ
γAA YieldN
YieldR
0
Fragmentation Function
Fragmentation Function - Study the particle distribution in a jet
initial
Modified Jet
• Calculate yields as a function of pT,assoc/pT,trig from correlation function
• Compare distribution in vacuum to medium to look for medium modification
Integrate yields
-rich triggers00 triggers triggers
Direct Measure of Medium Modification to Fragmentation Function
initialDirect
0
Ass
ocia
ted
yie
lds
per
trig
ger
Modified Jet
A. Hamed, Hard Probes 2008
STAR Preliminary
Ratio of Central Au+Au to Peripheral (~ Medium/Vacuum) Jet Yields
Within the current uncertainty in the scaling the medium effect on jets associated to a direct trigger is similar to jets associated to 0 trigger.
Summary
• RHIC has been successfully operating since 2000• The expectation of QGP as a weakly coupled gas of
quarks and gluons has been challenged by data• Medium created is strongly interacting (liquid-like)
and very opaque• Currently experiments are trying to make
measurements that can characterize the medium properties more quantitatively
• +jet measurement holds promise to be one of such probes
• Higher luminosity needed for definitive +jet measurement
• Future at RHIC is exciting
Extra Slides
0
Extraction of direct away-side yields
R=Y-rich+h/Y0+h
near near
Y+h = (Y-rich+h - RY0+h )/(1-R)away away
Assume no near-side yield for direct
then the away-side yields per trigger obey
A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.A. Hamed STAR Experiment ICHEP08 Philadelphia, PA July 29th -August 5th.
Results: Method of extract direct associated yield
This procedure removes correlations due to contamination (asymmetric decay photons+fragmentation photons) with assumption that correlation is similar to
0 – triggered correlation at the same pT.
O(αs2α(1/αs+g))
This atomic system may also be near the bound.
T. Schafer, arXiv:0707.1540v1 (2007).
What do we learn from RAA?
Effect of collision medium on hadron pT spectra:
• Parton scattering with large momentum transfer Hard-scattered partons (jets) present in early stages of
collisions
• Hot and dense medium Hard-scattered partons sensitive to hot/dense medium
Theory predicts radiative energy loss of parton in QGP
• Emission of hadrons High pT hadrons (jet fragments)
Dense medium (QGP) would cause depletion in spectrum of leading hadron at high pT - “jet quenching”
High-pT Predictions X-N. Wang, Phys. Rev. C58 (1998) 2321
It has been predicted that jet production will be affected by medium effects due to the production of hot dense matter in high energy relativistic heavy ion collisions
Scaling from p+p to A+A
• For hard-scattering processes, expect point-like scaling. For inclusive cross sections :
• For semi-inclusive yields, expect :
2
pp
AA A sources like-point ofnumber the of ratio the σ
σ
class centralityA A for theN
collisionsbinary Nucleon -Nucleon ofnumber Yield
Yield
binary
pp
AA
0-
dN/d
Elliptic flow
Jet Studies via Correlations
pTtrig – pT of the trigger particle
pTassoc bin – range of pT selected to
associate with the trigger particle
pTtrig > 4 GeV/c
pTassoc = 2-4 GeV/c
An example of Nbinary ~ A*B scaling
• Small cross section processes scale as though scattering occurs incoherently off nucleons in nucleus
• scale as A1.0 in +A
• scale as Nbinary ~A*B in A+B
7.2 GeV muons on various targets. M. May et al., Phys. Rev. Lett. 35,
407, (1975)
“Binary-Scaling” and RAA
• Define Nuclear Modification Factor RAA
Effect of nuclear medium on yields
pp
centralbinarycentral
Yield
NYield /
peripheralbinaryperipheral
centralbinarycentral
NYield
NYield
//
pp
peripheralbinaryperipheral
Yield
NYield /
• The probability for a “hard” collision for any two nucleons is small
• The total probability in A+A collision is multiplied by the number of times we try, i.e. – the cross-section scales with the number of binary collisions - Nbinary
Yield of 0 measured by PHENIX
p+p collisions Au+Au collisions
Evolution of Jet Structure
At higher trigger pT (6 < pT,trig < 10 GeV/c), away-side yield varies with pT,assoc
For lower pT,assoc (1.3 < pT,assoc <1.8 GeV/c), away-side correlation has non-gaussian shape becomes doubly-peaked for lower pT,trig
pedestal and flow subtracted
4 < pT,trig< 6 GeV/c, 2 < pT,assoc< pT,trig