Relativistic Heavy Ion Relativistic Heavy Ion Collisions: Collisions:
The Past Through the The Past Through the FutureFuture
(and vice versa)(and vice versa)W.A. Zajc
Columbia University
Thanks to:R. Averbeck, B. Cole, A. Drees, T.
Csorgo, M. Gyulassy, H. Hiejima, F. Muehlbacher,
J. Nagle, S. Sorensen, X. Yang,
OutlineOutline
Q. How to review 15 years of heavy ion data from two
programs preview 4 new experiments at a new collider
in 40 minutes ?!?
Answer: I won’t I will provide a
prejudiced partial selective
view of recent developments in the field.
Relevant Heavy Ion Relevant Heavy Ion PhysicsPhysics
Q1: How to (re)-create this deconfined state?
Q2: How to (re)-create energy densities 10-20 x normal nuclear density?
A: Relativistic Heavy Ion Collisions (Collide “large” nuclei at “large” energies)
b
Event characterization (geometry is destiny) Impact parameter b
is well-defined in heavy ion collisions Event multiplicity predominantly
determined by collision geometry Characterize this by global measures
of multiplicity and/or “transverse energy”
A Tale of Two LabsA Tale of Two Labs BNL
AGS: ECM ~5 GeV (1986-1998)
RHIC: ECM ~200 GeV (beginning 2000)
CERN SPS: ECM ~20 GeV (1986-1998?)
LHC: ECM ~5500 GeV (beginning 2005++)
Note: The program at each laboratory has benefited (and will continue to do so) from developments at and insights from the other lab.
CERN CERN AnnouncementAnnouncement
Available at http://press.web.cern.ch/Press/Releases00/PR01.00EQuarkGluonMatter.html
The Key The Key StatementsStatements
The evidence for this new state of matter is based on a multitude of different observations.
Many hadronic observables show a strong nonlinear dependence on the number of nucleons which participate in the collision.
Models based on hadronic interaction mechanisms have consistently failed to simultaneously explain the wealth of accumulated data.
On the other hand, the data exhibit many of the predicted signatures for a quark-gluon plasma.
Even if a full characterization of the initial collision stage is presently not yet possible, the data provide strong evidence that it consists of deconfined quarks and gluons.
Formation of Dense Matter at Formation of Dense Matter at CERNCERN
A combined analysis of their momentum distributions and two-particle correlations shows that, at the point where they stop interacting and "freeze out", the fireball is in a state of tremendous explosion, with expansion velocities exceeding half the speed of light, and very close to local thermal equilibrium at a temperature of about 100-120 MeV. This characteristic feature gave rise to the name "Little Bang".
NA44NA49
Formation of Dense Matter at Formation of Dense Matter at the AGSthe AGS
~same analysis for AGS data givesT ~ 93 MeV
vT ~ 0.5 (!)
B ~ 540 MeV(Dobler, Sollfrank, Heinz)
E866
Summary (1)Summary (1)
BNL AGS CERN SPSTemperature (MeV) 90-95 100-120Expansion Velocity ~0.5 ~0.55
Energy Density (GeV/fm3) 1-2 2-3
Non-Linear Non-Linear Dependences?Dependences?
There is no a priori reason to expect “a strong non-nonlinear dependence on the
number of nucleons which participate in the collision”
That is, a linear dependence on the number of participants is one of many physically plausible scaling behaviors:~ Number of participants (W, Npart, NWOUNDED)
~ Number of binary collisions (Nbin ~ A*B )
~ Number of constituent quark interactions
~ Number of absorbers ( ~ A * B )
Determining NDetermining NPARTPART
Best approach: Directly measure in a “zero degree
calorimeter”
(for A+A collisions)
Strongly (anti)-correlated with produced transverse energy:
PerNucleon
ZDCPART E
EAN 2
ET
ET
EZDC
NA50
Non-linear Non-linear Dependences?Dependences?
Zero-th step: Study systematics of
transverse energy production d/dET
A basic measure of Nuclear overlap “Thermalization” of initial
directed energy Calculate “transverse energy
per participant” Non-linear?
WA98 (Preliminary)
Lesson in Non-Lesson in Non-linearitylinearity
Same data (plot of dET/d maximum
versus number of participants)
is either Non-linear
or Linear
Surprising strong dependence on inclusion of errors in determining number of participants
NB: This is (presumably) not the non-linearity referred to in the press release.
WA98 (Preliminary)
Strangeness as a QGP Strangeness as a QGP SignalSignal
An old prediction: J. Rafelski and B. Müller, Phys. Rev. Lett. 48,
1066 (1982). Based on
High rate for ggss relative to hadronic processes
Or Reduced threshold effects from
reduced mass in deconfined stateand/or Fermi energy of u,d quarks
u du d s
Ene
rgy
Lev
el
Strange Quark Mass
Quark Matter Strange Quark Matter
Strangeness Enhancement Seen at Strangeness Enhancement Seen at CERNCERN
Clear evidence for increase in K/ ratio with NPARTICIPANTS Collision centrality
(As presented by R. Stock at 10-Feb-00 CERN Announcement)
Small “Problem”Small “Problem” Same (actually larger)
enhancement also seen in heavy ion collisions at the AGS(at much lower energy)
QGP at the AGS? QGP everywhere??
See “extra” enhancement for the multiply-strange baryons: (Kudos to CERN for this unique measurement!)
Assertion: Yield doesn’t “scale” from p-p, p-A
New physics!
But CERN is Different But CERN is Different (?)(?)
WA97
The Precise The Precise StatementStatement
It’s not describable by a “superposition of independent nucleon-nucleon collisions”
Therefore it must signal
“a new process … involving intense rescattering among quarks and gluons.”
A particularly striking aspect of this apparent "chemical equilibrium" at the quark-hadron transition temperature is the observed enhancement, relative to proton-induced collisions, of hadrons containing strange quarks…Lead-lead collisions are thus qualitatively different from a superposition of independent nucleon-nucleon collisions. That the relative enhancement is found to increase with the strange quark content of the produced hadrons contradicts predictions from hadronic rescattering models where secondary production of multi-strange (anti)baryons is hindered by high mass thresholds and low cross sections. Since the hadron abundances appear to be frozen in at the point of hadron formation, this enhancement signals a new and faster strangeness-producing process before or during hadronization, involving intense rescattering among quarks and gluons.
Naïve QuestionNaïve QuestionQ. When is a nucleus-nucleus collision
describable as a “superposition of independent nucleon-nucleon collisions”?
A1. ~Never.A2. Not even in proton-nucleus collisions:
Q64K$: In a nucleus-nucleus collision, how to scale effect of + collisions??
A1SF: Scale as NPARTICIPANTS
(number of “wounded” nucleons) (??)
In this cartoon are there
5 N-N collisions 5 x Npp?
OR
6 “wounded” N’s 3 x Npp?
Inspired AnswerInspired Answer Let’s measure proton-nucleus as
completely as possible Measure ~ all charged particles in final state Infer = number of N-N sub-collisions event-
by-event Characterize particle yields versus
Done in E910 at BNL AGS B. Cole, Spokesperson Based on
TPC Downstream
detectors for Particle ID Further tracking
E910 Strangeness E910 Strangeness ProductionProduction
BCWN
starts to saturate
p-p data
CQM
WA97 data
Systematic study of productionversus indicates Initial scaling intermediate between
NBINARY and NPARTICIPANTS Saturation for > ~3 hits Suggestive of “ Constituent Quark Model”
Applying CQM to CERN production data Gives good parameter-free description of data Strong hints towards explaining S>1 data
Summary (2)Summary (2)
BNL AGS CERN SPSTemperature (MeV) 90-95 100-120Expansion Velocity ~0.5 ~0.55
Energy Density (GeV/fm3) 1-2 2-3Strangeness Increased Increased Multiply Strange Hyperons Hint (Only) Increased (CQM?)
NA45NA45 Study physics in
e+e- channel After heroic efforts to
Suppress Dalitz pairs Suppress conversions Understand background
Then: Form M(e+e-) spectrum Divide by charged yield Compare to known sources
Excess seen for 0.3 GeV < M(e+e-) < 0.7 GeV
from annihilation?collision-broadening?density dependent masses?Chiral symmetry restoration?
Mixing the CocktailMixing the Cocktail
Requires detailed understanding of Resonance yields PT spectra
Form factors Decay kinematics Detector Resolution
Two versions: GENESIS
G. Agakichiev at al.: Eur.Phys.Jour. C4(98)231
EXODUSR. Averbeck, A. Drees
Screening by the Screening by the QGPQGP
In pictures:
QCD potential at T=0
r -->
V(r
)
QCD potential at high T
r -->
V(r
)
QCD potential at high T and
high density
r -->
V(r
)
Non-perturbative Vacuum
Perturbative Vacuum
cc
Perturbative Vacuum
cc
Color Screening
cc
Screening by the Screening by the QGPQGP
In first-order finger physics: Follow usual derivation of Debye screening
Now put in QGP scales and assumptions:
Hadrons with radii greater than ~ D will be dissolved
Study “onium” bound states
oD
Do
kTekTeo
ne
kTkTne
een
2
2
22
//2
42 with
1/2 4
44
fm 0.41
2
1
MeV 200
QGP)for Boltzman -(Stefan 6.3
1~433
22
gT
T
TTn
ge
D
o
Di-Muon Di-Muon MeasurementsMeasurements
Physics: Look at J/ via decay to
Experiment: Absorb “all” hadrons
before they make muons!
Analysis: Form spectrum of Extract J/ and Drell-Yan yields by
fitting and removing background and open charm
Plot J/ to Drell-Yan ratio versus measured ET in calorimeter
Compare to theory calculations of same
2))()(( ppM
Absorber
Calorimeter
Calorimeter
SpectrometerIncident Beam
Target(s)
+
EmphasisEmphasis
Suppression pattern vs. L is different for Pb-Pb
What the L is L? “the mean length of the
path of the (cc) system through nuclear matter of mean density 0”
A way to combine different beams, targets and energies
“Anomalous J/ suppression in Pb-Pb interactions at 158 GeV/c per nucleon”, Phys. Lett. B410, 337 (1997).
It’s AnomalousIt’s Anomalous
dzzEbL T ),(1
0
It’s LumpyIt’s Lumpy
More data more wiggles!
“Observation of a threshold effect in the anomalous J/ suppression”, Phys. Lett. B450, 456 (1999).
The sudden change of behaviour observed in our data suggest that the observed abnormal suppression results from a discontinuity in the state of nuclear matter. … A clear onset of the anomaly is observed as a function of transverse energy. It excludes models based on hadronic scenarios since only smooth behaviours with monotonic derivatives can be inferred from such calculations.
“A clear onset of the anomaly is observed. It excludes models based on hadronic scenarios since only smooth behavior with monotonic derivatives can be inferred from such calculations” Phys. Lett. B 450, 456 (1999).
The second suppression is preliminary and contradicts the published results shown here in the above paper.
1st Derivatives
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0 20 40 60 80 100 120 140
E_T (GeV)
d(R
ati
o)/
dE
_T
2nd Derivatives
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 20 40 60 80 100 120
E_T (GeV)
d^
2(R
ati
o)/
dE
_T
^2
Discontinuous?Discontinuous?
The Models Catch The Models Catch UpUp
More sophisticated calculations than the simple “co-movers” ansatz describe the qualitative features of the (pre-98) data
But…
The Data RecedesThe Data Recedes
New data set disagrees substantially with All models (previous data)
Leading to …
Latest TheoryLatest Theory Recent work
(Capella, Ferreiro and Kaidolov, hep-ph/0002300)
has dramatically improved description of data
As before: Two dissociation mechanisms
Nuclear absorption ABS
Break-up by co-movers CO
New: Now account for
fluctuations in b ET mapping( b 0 while ET continues to increase)
Smaller value of ABS (as implied by E866 data nucl-exp/9909007)
Summary (3)Summary (3)
BNL AGS CERN SPSTemperature (MeV) 90-95 100-120Expansion Velocity ~0.5 ~0.55
Energy Density (GeV/fm3) 1-2 2-3Strangeness Increased Increased Multiply Strange Hyperons Hint (Only) Increased (CQM?)Electron Pairs No Medium Modifications(?)J/ No SuppressedDirect Photons No LimitHard Scattering No HintCharm No Hint
Lessons LearnedLessons Learned Beware of false dichotomies:
Failure of “all” conventional models Success of “any” QGP model All QGP’s increase strangeness All strangeness increasea are
QGP
Beware of inclusive averages:
Beware of simple models: Models should be as simple as possible– but no simpler
Beware of wording: Press is insensitive to “evidence for” vs. “discovery of” Press is sensitive to anything combining
New state of matter Early Universe Big Bang
)()( xfxf
Life on the EdgeLife on the Edge CERN has done an admirable job
of extracting maximal information from phenomena on threshold of Phase transition Excitation function Energy distribution
RHIC will transcend these “boundaries” by factors of 4-50
RHIC
CERN
RHIC = Relativistic Heavy Ion Collider
Located at Brookhaven National Laboratory
Schedule: Commissioning
machine as we speak Will run through end of August(?)
RHICRHIC
RHIC RHIC SpecificationsSpecifications
3.83 km circumference Two independent rings Capable of colliding
~any nuclear species on ~any other species
Energy:
500 GeV for p-p 200 GeV for Au-Au
(per N-N collision) Luminosity
Au-Au: 2 x 1026 cm-2 s-1
p-p : 2 x 1032 cm-2 s-1 (polarized)
11
3344
1’1’
22
66
55
)GeV 500(A
Zs
How is RHIC How is RHIC Different?Different?
It’s a collider Detector systematics independent of ECM (No thick targets!)
It’s dedicated Heavy ions will run 20-30 weeks/year
It’s high energy Access to non-perturbative phenomena
Jets Non-linear dE/dx
Its detectors are comprehensive ~All final state species measured with a suite
of detectors that nonetheless have significant overlap for comparisons
Uniqueness of RHICUniqueness of RHIC Substantial increase in ECM
Access to high Q2 probes Dominance of mini-jets
Highest physics priority should be development of sufficient luminosity to access this new regime at RHIC
(Argument by V. Pantuev,
see also K. Eskola, hep-ph/9610365)
Njets
pT > 2 GeV/c
(GeV)s
-4.8, 0.66, 2.86, 9.39, 18.48, 35.96
In PicturesIn Pictures
(PID) Acceptances(PID) Acceptances
STAR AcceptanceSTAR Acceptance
PHOBOS AcceptancePHOBOS AcceptanceBRAHMS AcceptanceBRAHMS Acceptance
PHOBOSPHOBOS
An experiment with a philosophy: Global phenomena
large spatial sizes small momenta
Minimize the number of technologies: All Si-strip tracking Si multiplicity
detection PMT-based TOF
Unbiased global look at very large number of collisions (~109)
PHOBOS “Results”PHOBOS “Results”
BRAHMSBRAHMSAn experiment with an
emphasis: Quality PID spectra over a broad
range of rapidity and pT
Special emphasis: Where do the baryons go? How is directed energy
transferred to the reaction products?
Two magnetic dipole spectrometers in “classic” fixed-target configuration
BRAHMS “Results”BRAHMS “Results”
STARSTAR An experiment with a challenge:
Track ~ 2000 charged particles in || < 1
ZCal
Silicon Vertex Tracker
Central Trigger Barrel or TOF
FTPCs
Time Projection Chamber
Barrel EM Calorimeter
Vertex Position Detectors
Endcap Calorimeter
Magnet
Coils
TPC Endcap & MWPC
ZCal
RICH
STAR “Results”STAR “Results”Demonstrate large
hadronic rates from:
Large acceptance
coupled with Large
multiplicities
(Assuming centraltriggers )
yield from ~12 minutes of
running
count per hour limit
STAR ChallengeSTAR Challenge
South muon Arm
North muon Arm
West Arm
East ArmCentral ArmsCoverage (E&W) -0.35< y < 0.35 30o <||< 120o
M(J/)= 20MeVM() =160MeV
Muon ArmsCoverage (N&S) -1.2< |y| <2.3 - < <M(J/)=105MeVM() =180MeV
3 station CSC5 layer MuID (10X0)p()>3GeV/c
GlobalMVD/BB/ZDC
PHENIXPHENIX An
experiment with something for everybody
A complex apparatus to measure Hadrons Muons Electrons Photons
Executive summary: High
resolution High
granularity
PHENIX DesignPHENIX Design
PHENIX Approach to QGP PHENIX Approach to QGP DetectionDetection
1. DeconfinementR() ~ 0.13 fm < R(J/) ~ 0.29 fm < R(’ ) ~
0.56 fm Electrons, Muons
2. Chiral Symmetry RestorationMass, width, branching ratio of to e+e-, K+K- with
M < 5 Mev:
Electrons, Muons, Charged HadronsBaryon susceptibility, color fluctuations, anti-baryon
production:
Charged hadronsDCC’s, Isospin fluctuations:
Photons, Charged Hadrons
3. Thermal Radiation of Hot GasPrompt , Prompt * to e+e-, +- :
Photons, Electrons, Muons
4. Strangeness and Charm ProductionProduction of K+, K- mesons: HadronsProduction of , J/, D mesons:
Electrons, Muons
5. Jet QuenchingHigh pT jet via leading particle spectra:
Hadrons, Photons
6. Space-Time EvolutionHBT Correlations of ± ±, K± K± :
Hadrons
Summary: Electrons, Muons, Photons, Charged Hadrons
PHENIX “Results”PHENIX “Results”
Vector mesons: Superb e/ rejection Excellent momentum
resolution
High pT hadrons: Very fine
segmentation High rate capability
RHIC ZDC’sRHIC ZDC’s ZDC Zero Degree Calorimeter Goals:
Uniform luminosity monitoring at all 4 intersections
Uniform event characterization by all 4 experiments
Process: Correlated Forward-Backward Dissociation
tot = 11.0 Barns (+/- few %)
Summary (4)Summary (4)
BNL AGS CERN SPS BNL RHICTemperature (MeV) 90-95 100-120 ALLExpansion Velocity ~0.5 ~0.55 ALL
Energy Density (GeV/fm3) 1-2 2-3 ALLStrangeness Increased Increased ALLMultiply Strange Hyperons Hint (Only) Increased (CQM?) STAR, PHOBOS(?)Electron Pairs No Medium Modifications(?) PHENIXJ/ No Suppressed PHENIX, (STAR)Direct Photons No Limit PHENIXHard Scattering No Hint PHENIX, STARCharm No Hint PHENIXBeauty No No PHENIX(?)
Summary (5)Summary (5)
The Past: Wealth of data obtained on essentially all
created particle species since inception of programs at BNL and CERN in 1986.
Many appropriate analysis tools and techniques have been developed in direct response to these data
Current data clearly challenge present state ofmodel-building
The Future RHIC will
Provide 10-100 times as much data Open up new channels and signals Remove many of the ambiguities from
“life on the edge”
SummarySummary
The CERN program has Created nuclear matter at unprecedented
densities Explored its properties in unprecedented
detail Provided unprecedented challenges to the
theoretical community
The RHIC heavy ion community is ready to begin experiments with a set of detectors designed for the first dedicated heavy ion collider The varierty, energy, uniqueness, promise
and challenge of this program exceeds even that of the very impressive CERN era.