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.