Overview of RHIC Experiments:

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Overview of RHIC Experiments What have we learnt
after 4 years? (Part 2)
Joakim Nystrand Institutt for Fysikk og
Teknologi, Universitetet i Bergen
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The goal of relativistic heavy-ion collisions is
to study hot and dense nuclear matter
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What is the nature of the phase transition? First
or Second Order? Crossover? Is there a critical
point?
Results from Lattice QCD
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Another remarkable result
2. Collective Elliptic Flow
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Elliptic Flow
A Nucleus-Nucleus Collision at intermediate
impact parameter
Reaction Plane Plane defined by beam axis and
b (impact parameter, 2-D vector)
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How are particles distributed in the transverse
plane?
No collective effects ? flat distribution in ?
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Definition of v2
For 180 symmetry
v2 measures the strength of the flow
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v2 gt 0 Flow in the reaction plane
v2 lt 0 Flow out of the reaction plane
v2gt0
v2lt0
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Experimentally, v2 is calculated as the average
of cos(2?) relative to the reaction plane.
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v2 vs collision energy
beam kinetic energy in lab frame
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v2 vs collision energy
Low Energy The spectators block flow in reaction
plane, squeeze-out. High Energy
Hydrodynamic pressure leads to flow of
particles in the reaction plane.
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A few years ago
Predictions before RHIC
?The collective (elliptic) flow is much stronger
at RHIC than anticipated!
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An apparent paradox
v2 is calculated from the angular distributions
of pions, kaons, protons, emitted at the late
stages of the collision, yet v2 is sensitive to
the equation of state early in the collision.
Explanation Self-quenching
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Initial Configuration
Time evolution
P. Kolb, J. Sollfrank, and U. Heinz
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Time evolution
?x Spatial eccentricity ?p Momentum eccentricity
Kolb, Heinz 2003
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If locally equilibrated hydrodynamics is taken as
the mechanism for generating elliptic flow, then
the observation of any substantial amount of
elliptic flow can be taken as evidence that local
thermal equilibrium is achieved on a time scale
before the spatial anisotropy would be completely
erased.
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Centrality dependence of v2
Stronger in peripheral than in central
collisions.
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Centrality dependence of v2
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? Eccentricity S Area of overlap
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Is there a hydro limit? Or is v2/? simply ?
dnch/dy?
One argument for accelerating 238U at RHIC!
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Data from ?s 62 GeV
Somewhat conflicting conclusions from PHENIX and
PHOBOS
PHENIX nucl-ex/0411040
PHOBOS nucl-ex/0406021
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v1 and v4
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v1 and v4
v1 significant only away from mid-rapidity
v4 ltlt v2
J. Adams et al. (STAR collaboration), PRL 92
(2004), 062301.
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Some more basic observations
Nuclear Stopping. How much energy do the
nucleon lose? How does the charged particle
multiplicity scale? What is the size of the
produced system and how can it be measured?
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Nuclear Stopping
Definition of rapidity, y
p// 100 GeV/c, mn0.939 GeV/c2
p// 100 GeV/c, mn0.939 GeV/c2
ybeam 5.4
ybeam -5.4
mid-rapidity ? y0 ? p// 0 particles emitted
with ?90?
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p dN/dy spectra single Gaussian fits from 2 to
200 GeV
indicate ybeam for the various energies
AuAu or PbPb
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Nuclear Stopping
AGS ?snn 4.9 GeV, ybeam 1.6 SPS ?snn
17.3 GeV, ybeam 2.9
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Nuclear Stopping
BRAHMS
Net-baryon after feed-down neutron corrections
Gaussians in pz
6 order polynomial
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Nuclear Stopping

• Upper limit to rapidity loss?
• Energy loss

?E 25.7 ? 2.1 TeV ?E/nucleon 72 ? 6 GeV
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Stopping at RHIC - summary

• Net-baryon poor midrapidity region
• dN(net-protons)/dy 7
• Total-baryon rich midrapidity region
• dN(all baryons)/dy ? 65
• Largest observed rapidity loss
• lt?ygt 2
• as large as in pA
• Stopping power
• central PbPb at RHIC 72
• central SS at SPS 58
• pp collisions ? 50

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Particle Production
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Particle Production - width of y distributions

• Landau hydrodynamics
• Gaussian rapidity distribution
• Observed in hadron - hadron collisions
• Width ? depends only on c.m. energy

L.D. Landau, Izv. Akad. Nauk SSSR 17 (1953)
52 P.Carruthers, M.Duong-van, PRD 8 (1973) 859
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Alternative to rapidity - pseudo-rapidity
? ? y when p// ? E (m ? 0)
pseudo-rapidity
rapidity
200 GeV AuAu
200 GeV AuAu
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Particle Production
Note difference between symmetric and asymmetric
systems
PHOBOS
PHOBOS
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Particle density, dn/dy, at mid-rapidity

• Systematic error 10
• dN/dy -gt dN/d? Jacobian

G. Roland, QM 2004
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Particle density, dn/dy, at mid-rapidity

• Logarithmic rise with collision energy

G. Roland, QM 2004
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Particle Production
Better scaling with Npart for global (4?)
multiplicity than for dnch/d? at ?0.
PHOBOS
PHENIX
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Particle Production
Surprisingly, the ltNchgt/Npart in AA is in better
agreement with ee- than pp!
Energy loss pp ?50 AuAu ?72
?
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Particle Production
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Multiplicity at RHIC - summary

• dnch/d? well described by gaussian with width
• The total multiplicity scales with the number of
  participating nucleons.
• ltNchgt/Npart in AuAu in better agreement with
  ee than pp.

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System Size
How can we measure the size of the interaction
volume?

• Hanbury-Brown Twiss (HBT) Correlations
• proton-neutron coalescence

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System Size - HBT
Two-particle correlations of bosons at small
relative momenta, Qp1 p2 , provide a measure
of the size of the emitting source A
consequence of the symmetric boson wave function.
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System Size - Coalescence
Results from PHENIX nucl-ex/0406004, 0409006
Central arm detectors Drift Chamber, Pad
Chambers (2 layers), Time-of-Flight.
Combining the momentum information (from the
deflection in the magnetic field) with the
flight-time (from ToF)
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Anti-deuteron m2 spectra
The yield is extracted by fitting the m2
spectrum to a function for the signal (gaussian)
background (1/x or e-x)
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Correction for acceptance and efficiency ?
normalized d and d pT spectra
deuterons
anti-deuterons
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System Size - Coalescence
Imagine a number of neutrons and protons
enclosed in a volume V
A deuteron will be formed when a proton and a
neutron are within a certain distance in
momentum and configuration space.
This leads to
where pd2pp and B2 is the coalescence
parameter, B2 ? 1/V. Assuming that n and p have
similar d3N/dp3
The proton yield must be corrected for weak decays
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The reality is more complicated B2 depends on pT
? not a direct measure of the volume
Possible explanation Radial flow.
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System Size
Energy dependence of B2 similar to R(HBT)
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System size - summary

• Despite clear indiactions of strong collective
  flow (expansion), the measured system radius is ?
  R(Au)
• The RHIC HBT-puzzle
• (Anti-)deuteron formation through pn coalescence
  provide the same information as HBT correlations
  (but has attracted much less attention).

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The Final(?) Verdict
In the words of the experiments (White Papers)
STAR The bottom line is that in the absence of
a direct smoking gun signal of deconfinement
revealed by experiment alone, a QGP discovery
claim must rest on the comparison with a
promising, but still not yet mature, theoretical
framework We judge that a QGP discovery claim
based on RHIC measurements to date would be
premature (as was the claim made in 2000 5 on
the basis of SPS results).
PHENIX In conclusion, there is compelling
experimental evidence that heavy-ion collisions
at RHIC produce a state of matter characterized
by very high energy densities, There is not
yet irrefutable evidence that this state of
matter is characterized by quark deconfinement or
chiral symmetry restoration
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Large Hadron Collider (LHC) First collisions
2007/2008 pp at 7 TeV PbPb at 2.75 A TeV
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(No Transcript)
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Experiments at the LHC
ATLAS
CMS
ALICE
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ALICE
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STAR The bottom line is that in the absence of
a direct smoking gun signal of deconfinement
revealed by experiment alone, a QGP discovery
claim must rest on the comparison with a
promising, but still not yet mature, theoretical
framework We judge that a QGP discovery claim
based on RHIC measurements to date would be
premature (as was the claim made in 2000 5 on
the basis of SPS results).
BRAHMS There is no doubt that the experiments at
RHIC have revealed a plethora of new phenomena
that for the most part have come as a surprise.
In this sense it is clear that the matter that is
created at RHIC differs from anything that has
been seen before. What name to give it must
await our deeper understanding of this matter.
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PHOBOS In the most central AuAu collisions at
the highest beam energy, evidence is found for
the formation of a very high energy density
system whose description in terms of simple
hadronic degrees of freedom is inappropriate.
PHENIX In conclusion, there is compelling
experimental evidence that heavy-ion collisions
at RHIC produce a state of matter characterized
by very high energy densities, This state of
matter is not describable in terms of ordinary
color-neutral hadrons, because there is no known
self-consistent theory of matter composed of
ordinary hadrons at the measured
densities There is not yet irrefutable evidence
that this state of matter is characterized by
quark deconfinement or chiral symmetry
restoration