Recent RHIC Results Spencer Klein, LBNL

SLAC Orange Room Seminar, Sept. 19, 2006

RHIC Collider & DetectorsOther Physics photoproduction & polarized protonsCold nuclear matter: pp/dAHot nuclear matter: AAFuture PlansConclusions

RHIC has published ~ 200 papersI can only hit the biggest highlights

RHIC RHIC was built to explore the

properties of nuclear matter under extreme conditions Polarized parton distributions Cold nuclear matter - nuclei Hot nuclear matter – the QGP?

Study ‘typical’ collisions, not rare events.

The Relativistic Heavy Ion Collider

h

STAR

PHENIX

PHOBOSBRAHMS/pp2pp

2 concentric rings of 1740 superconducting magnets 3.8 km circumference counter-rotating beams of ions from p to Au

BrookhavenNationalLaboratory

RHIC datasets from Au to p

System size scan AuAu & CuCu

Collision energy scan 200, 130, 62, 22 GeV per nucleon

dAu pp

Polarization up to 65%

STAR: full acceptance Detector for global eventreconstruction

PHENIX: 2 arm spectrometerGood PID for rare probes

PHOBOS: full acceptance

BRAHMS: 2 precision

spectrometers

pp2pp: elastic scattering

‘Other’ Physics

Polarized Protons and polarized parton distributions

Photoproduction Search for strangelets in AuAu

collisions

Polarized parton studies Quark, gluon polarization Compare cross sections for proton

polarizations in the same vs. in opposite directions

Longitudinally and transversely polarized beams Many different structure functions

LLA

Longitudinal AsymmetryPHENIX, 2006

ALL

Photonuclear and two-photon interactions at RHIC

Ions support a large Weizsäcker-Williams photon flux Photon flux ~ Z2

Copious photonuclear interactions Coherent vector meson production has a

large cross section RHIC has studied 0,0 and J/

photoproduction in AuAu, 0 in dA RHIC has studied ‘two-photon’ production

of e+e-

Rate ~ Z4

The LHC is the only place to study photoproduction at energies above HERA

Au

Au

“Pomeron

J

PHENIX J/ Mee (GeV)

Unique Reactions Can’t tell if nucleus 1 or nucleus 2

emitted the photon 2-source interferometer Negative parity --> subtract amplitudes

‘+’ sign at pp colliders ~ |A1 - A2eip·b|2

At y=0=0[1 - cos(pb)]

Large Z --> multiple reactions Au + Au --> Au* + Au* + 0

3 & 4 photon exchangedN

/dt

STAR Preliminary

Data (w/ fit)MC – no InterferenceMC - interference

t ~ pT2 (GeV2)

0.1 < |y| < 0.5

-

pp, dA and cold nuclear matter

pp collisions tests of pQCD calculations

dA collisions Use deuteron to probe gold nucleus

Cold nuclear matter Parton distributions at low Feynman-x

Benchmark for studies of ion-ion collisions

pp collisions & pQCD Jet cross sections and high pT particle spectra are both

well fit by pQCD + fragmentation functions Thanks to recent theoretical advances

0 pT(Dat

a-th

eory

)/the

ory

Hadron production at large pT:

data vs. QCD

Inclusive jets

Jet pT

(Dat

a-th

eory

)/the

ory

d/d

p T (m

b/G

eV)

dAu collisions Gluon density rises as x decreases

and/or Q2 increases At high densities gluons overlap &

densities saturate Saturation occurs at larger x in nuclei

Shadowing’ – reduced parton densities in ions, compared to nucleons

Many theoretical approaches Evolution equations(BFKL/DGLAP) Colored Glass Condensate – describe

gluons with a classical field

Decreasing x -->

Dec

reas

ing

Q2 -

->

Particle Production at Forward xB

RA

HM

S, P

RL 93, 242303

Sizable suppression in charged hadron production in d+Au collisions relative to p+p collisions at forward rapidity

BRAHMS(

dA)/

(pp)

(ce

ntra

l dA

)/(

perip

hera

l dA

)Increasing Rapidity (decreasing x)--->

Central Forward

Is this a Colored Glass Condensate?

Suppression curves may be fit by multiple models

x=10-2 -10-3 is moderate, not small In lowest order pQCD g + g --> g + g (or q

+ q) produces back-to-back jets Moderate higher order corrections

In a CGC, the target reacts coherently g + target --> g + target Heavy target absorbs the impact with little

recoil ‘Monojets’

g g

jet

jet

g CGC

jet

No jet

pQCD

Colored Glass Condensate

2 particle correlations

Azimuthal angle correlations between forward 0 and central h± correlations in pp and dAu If a CGC is present in Au,

correlation should be smaller in dA

Correlations smaller in dA than pp More suppression at small pT

--> smaller x Consistent with CGC

nucl-ex/0602011

π0: |<η>| = 4.0h±: |η| < 0.75; pT > 0.5 GeV/c

STARSTAR

Forward0 trigger

Mid-rapidity h±

– h

– h

Heavy ion collisions and hot nuclear matter

Hot nuclear matter is produced by colliding heavy ions

Study the properties of hot nuclear matter Search for the Quark Gluon Plasma

Interacting quarks and gluons, in equilibrium Individual nucleons disappear

Quark Gluon Plasmaquarks+ gluons

Normal Nucleusprotons + neutrons

or

Nuclear matter phase diagram

Phase boundaries calculated with lattice QCD Transition temperature Tc ~ 170 MeV at zero baryon densityAt low baryon densities, no phase transition expected – gradual change At higher baryon densities, 2nd order phase change may be present

Plasma

Space-Time evolution

Chemical freezeout: inelastic scattering stopsChemical Composition

Initial Collisions: hard probes producedhigh pT particlescharm

Thermal freezeout: elastic scattering stopsGlobal characteristicsParticle SpectraElliptic FlowHBT

Outgoing baryons

CentralRegion

Observables in ion collisions Impact parameter (b) and centrality

Npart and Nbin

System composition and thermal equilibrium Expanding fireball (blast wave) model Nuclear flow & hydrodynamics System size – quantum (HBT) interferometry High pT particles and nuclear energy loss Particle Correlations J/ suppression

Impact parameter and centrality determination Impact parameter (b) is not directly

observable Charged particle multiplicity or ET

Spectator neutrons Common variables

Npart – number of nucleons involved in collision

Nbin – number of nucleon-nucleon collisions

Important for hard probes %age of collision

e.g. 0-10% most central

Nh-

d/d

Nh-

Log scale!

b=0Npart=2ANbin ~ A2

b ~ RAspectators

Charged particle Distributions Pseudorapidity related to

longitudinal velocity Neglecting particle mass

Rapidity plateau dN/d ~ constant for | | <2 Boost invariance Radial expansion

dN/d scales with Npart

Independent of incident nuclei Total charged multiplicity in central

AuAu collisions is 4200 +/- 470

dNch

/d

dN/d @ 200 GeV

=-6.3 =6.3

=0=1 =2

PHOBOS

Au+Au35-40%, Npart = 99

Cu+CuPreliminary

3-6%, Npart = 100

PHOBOS

Flavor Composition Particles of different mass, etc. are in in

thermal equilibrium N ~ exp(-m/kT)

Strangeness fully equilibrated Suppression s rises with Npart

Particles are produced individually, rather than as ss pairs

‘Bath’ conserves strangeness ‘Grand canonical ensemble

Possible exception – short-lived resonances e.,g *, K*

Tch ≈ TC ≈ 165 ± 10 MeV

shor

t liv

edre

sona

nces

Particle Ratios

Points – dataLines – thermal model

s – strangeness suppression

s

Npart

“Blast wave” model -explosive expansion

System is a fireball collective (hydrodynamic) expansion

Particle <pT> rises with particle mass Main parameters are fit from particle

spectra Temperature T ~ 106 MeV Expansion velocity <> ~ 0.55 c

Different velocity profiles can be used

Retiere & Lisa, 2003

pT (GeV/c)

Baryon enhancementat moderate pT

For 1 < pT < ~ 5 GeV, baryon production is enhanced over pp/dA… Not a mass effect - behaves ‘normally’

Not consistent with usual fragmentation picture One model – recombination

among already produced quarks

Rainer Fries

PHENIX data

pT (GeV/c) pT (GeV/c)

p/

ratio

pT (GeV/c)

Elliptic flow Pressure converts initial state spatial asymmetries into

density/momentum asymmetries Occurs very early in collision

dN/d ~ 1+2v2 cos(2) + … Directed flow - v1cos() ignored Small v4cos(4) ignored

v2 ~ pT for pT < 2.5 GeV v2 is at hydrodynamic limits Spatial anisotropy is completely converted into

particle asymmetry Hot nuclear matter acts like a nearly perfect fluid

v2 ~ constant for 4 < pT < 8 GeV

Do partons or hadrons flow? v2 per valence quark

pT quark = ½ pT (meson) pT(quark) = 1/3 pT (baryon) v2(quark) = 1/2 v2 (meson) v2(quark) = 1/3 v2 (baryon) Factor of 6 for deuterons

Evidence that partons flow, not hadrons Valence quark model works well

Expected from quark recombination

v 2 p

er q

uark

Fabrice Retiere, QM2005

pT per quark

Two-particle interferometry (HBT) Hanbury-Brown Twiss interferometry gives 3-d system size Enhancement of identical boson pairs with q = p1 - p2 <h/R

: Fourier transform of the density distribution

3-d Bertsch-Pratt parameterization

2

21

2121 )(1

)()(),(),( qpPpPppPppC

)exp(1),,( 22iilongsideout RqqqqC

qlong (z)

qside

qout

System Size at Thermal Freezout

System size not very sensitive to collision energy

Source (Gaussian) radii ~ 6 fm ~ 2X size of initial nuclei

Rout/Rside ~ 1 Short emission time explosive expansion

Modest variation with respect to reaction plane (elliptic flow)

Energy dependence, Rout/Rside ratio challenge most models

Collision Energy

High pT particles as probes of the medium

High pT hadron production should be the same in pp, peripheral AA and central AA collisions pQCD + universal fragmentation functions

Jet fragmentation occurs on a long time scale, outside the medium

• True at high enough pT

Small differences from nuclear shadowing, proton:neutron ratio and initial state scattering

Differences in high pT hadron yields in AA vs. dA, pp are due to parton energy loss in the medium

Energy Loss inMedium??

g q

q

Jet

High pT hadron

Compare AA, dA, pp using

QCD gives RAA=1 RAA ~>1 for dAu

Initial state scattering gives partons pT RAA ~ 1 for direct photons in AA

Photons do not lose energy RAA ~ 0.2 for 0 in AuAu

4 < pT < 20 GeV Energy loss seems very large

Energy loss >> calculations based on interactions with hadrons

In pQCD calculations, requires very large (unphysical) gluon densities or cross sections

dAu and AuAu

ppNAuAuR

binaryAA

STARSTAR

pT

pT

Energy Loss Scaling System size

AuAu, CuCu have similar energy loss for same Npart

Energy loss scales smoothly with system size

No clear ‘transition’ Beam energy

No large suppression seen at SPS energies

Species RAA larger for strange particles & baryons

Known strangeness, baryon enhancement

RA

ANPart

RAA for electrons from heavy quark decay

Semileptonic decay of charm and bottom

RAA ~ 0.2, same as for lighter hadrons

Lower quark velocity suppresses small angle gluon radiation ‘Dead cone’ Less energy loss than for light

quarks Cannot explain light and

heavy quark energy loss simultaneously

RAA(h)

2-particle azimuthal angle () correlations

High pT ‘trigger’ + lower pT ‘associated’ particle

Single jets --> small correlations dA, AuAu data similar to pp Jets

Dijets --> back to back correlations ( = ) Peak in dAu & peripheral AuAu Suppressed in central AuAu

Peak disappears for smaller trigger pT

Surface emission?? Only partons produced near surface

escape

pTtrigger>8 GeV/c

Yie

ld p

er tr

igge

r

(radians)

Trigger

Associated

– A

Au+Au 0-10% 3<pT,trigger<4 GeV

pT,assoc.>2 GeV

Angular correlation widths

No pT cut on associated track Background is much larger Flow introduces correlated background

‘Near’ peak widths unchanged broadened

Interactions with longitudinally expanding system

Back-to-back peak Peak appears

2X wider than dAu Possible ‘wings’/Mach cone?

Energy conservation produces some back-to-back correlations

Look at low pT recoils…

STAR, Phys Rev Lett 95, 152301

3 particle correlations High pT partons in dense media might

radiate in a Mach (shock wave) cone Produces 3-particle correlations Background subtraction very tricky

PHENIX may see, STAR may not

PHENIX Preliminary

Ridges may indicate conic radiation

away

near

Medium

mach cone

Mediumaway

near

deflected jets

High pT trigger

J/ suppression Quarkonium should ‘melt’ in a QGP

J/, ’,. states,… melt at different temperatures

J/ melt at ~ 1.5 - 2.0 Tcritical

J/ production was suppressed at the CERN SPS More suppression than expected

due to absorption in cold nuclear matter

Mee (GeV)

J/ Summary RAA ~ 0.3 in central collisions

A bit larger than for other particles Similar to RAA(J/) at the SPS

‘Should’ be larger at RHIC Suppression scales smoothly with number of

collisions (system size) No break in spectrum, as was seen at SPS

Similar behavior at 62 and 200 GeV

AuAu data summary Initial energy density >> expected critical energy for a

QGP System is described by an expanding fireball, with <T>

=0.55, T=106 MeV Anisotropic flow is large, at hydrodynamic limit

System acts like a liquid Production of high pT particles is suppressed

Very small 2-particle back-to-back high pT correlations Heavy quarks behave similar to light

J/ production is suppressed by ~ 1/3, similar to SPS

Puzzles The observed high pT suppression and large flow

appear to require either very high gluon densities or very high cross sections.

Why is the system size (and duration) measured by HBT so small (and independent of collision energy)?

Why are jets elongated in rapidity in central collisions?

Surprise! Strong Coupling Tc < T < 4 Tc is a strongly coupled regime for partons

Duality arguments relate strong coupled QCD to weak coupled string theory Many colored bound states/resonances (qq, qg,ggg…)

Lightly bound -- > large radii Rescattering cross sections 10-100X larger than pQCD

• Huge cross sections & similar behavior seen for atomic Feshbach resonances and with ultra-cold 6Li

– Atoms tuned (with a B field) to be barely bound Extremely low viscosity produces large elliptic flow High pT hadrons interact with these bound states and lose energy.

J/ melting is gradual; survive up to at least 2Tc

Other mesons can survive at temperatures above Tc Strong coupling might explain many puzzles

Quantitative studies needed!!

E. Shuryak, hep-ph/0405066

?X.N. WangICHEP06

Viscosity

/s

Flow (v2) depends on shear viscosity/entropy (/s) Data shows/s < 0.1

RHIC nuclear matter is a much better fluid than water /s ~ 10 for water

Inconsistent with hadron gas and hot QGP calculations

Viscosity ~ 1/4 calculated for sQGP using duality arguments

T (MeV)

Has RHIC seen the Quark Gluon Plasma?

Energy densities, temperature adequate, and partonic flow indicates equilibrium reached during partonic stage Seems to meet definition – quarks and gluons

interacting in equilibrium But… the very strong interactions were not

expected New name: sQGP – strongly interacting quark

gluon plasma

RHIC Future – Short Term Ion low-energy-scan

search for tri-critical point 500 GeV pp collisions, higher luminosity

Polarized structure functions STAR: TOF system + vertex detector

High-statistics heavy-flavor production PHENIX: Hadron blind tracker for

intermediate mass dileptons Vector meson mass shifts and chiral

symmetry breaking

200 GeV RHIC

Lower energy AuAu

Plasma

Longer Term plans RHIC II

10X luminosity upgrade Electron cooling of hadron beams

High current electron accelerator Technically challenging

eRHIC Polarized ep, eA collisions Study cold nuclear matter

Heavy Ion physics at the LHC Higher energy --> higher temperature, denser system Will the LHC reach high enough temperatures to see signs (lower

v2) of the weakly coupled QGP?

Conclusions In dAu collisions, forward particle production is

suppressed and back-to-back correlations are reduced, consistent with saturation models.

In heavy-ion collisions, the system thermalizes quickly, and has a very high interaction cross section. This is consistent with the expectations from a sQGP –

strongly interacting QGP – per recent theoretical studies. RHIC is awash in good data.

We need a comprehensive, quantitative theoretical framework.

Backups/spares/rejects

Direct Photons Hadrons measure

temperature at freeze-out Direct photons may measure

temperature earlier on Large background from 0

decays ‘Signal’ is QCD processes

and thermal radiation Data consistent with QCD +

Thermal radiation T ~590 MeV

Ion Collisions at RHIC

The beam energy is large enough that the incident baryons (mostly) do not stop (Mostly) baryon free high energy density central region Energy goes into copious particle production

Collision region ~ (10 fm)3, lifetime ~ few 10-23 s

Initial State

Final State

Baryon freeregion

Chiral Symmetry Restoration Expected in a Quark-Gluon Plasma Light quarks lose mass Meson masses, widths and

branching ratios will change -->e+e- is experimentally

accessible Narrow(ish), leptonic final state

In --> K+K-, kaons are also subject to medium effects

Br(-->e+e-) = 3*10-4

Rates are low No changes seen

Charm Production

Charm is too heavy for thermal production pQCD calculations should

apply

Direct Photons Hadrons measure

temperature at freeze-out Direct photons may measure

temperature earlier on Large background from 0

decays ‘Signal’ is QCD processes

and thermal radiation Data consistent with QCD +

Thermal radiation T ~590 MeV

Flavor, Baryon number dependence

KS, similar to h±

has slightly less suppression

RAA for baryons larger than for mesons

RAA increases with strangeness Lack of strangeness

suppression System size effect

0-5% Au+Au

p+p

STAR Preliminary

√sNN=200 GeV

2-d angularcorrelations

2 GeV/c > pT > 100 MeV/c 2 charged-particle angular correlations

vs Peak at Small & from jets

Jets are broadened in in central collisions

Seen by several analyses, not well understood

Cos(2) modulation from flow Contribution at from back-to-

back jets

Cen

tral

Mid

-cen

tral

Per

iphe

ral

STAR, 2006

pp2pp : small angle pp Elastic Scattering at RHIC

AN (t) (t ) (t) (t) (t)

Im[ 5

* ]d /dt

r5 e5 im5 m5

t Im

Re r5 = -0.033 ± 0.035, Im r5 = -0.43 ± 0.56

Phys. Lett. B 632, (2006) 167-172

The pp2pp Roman pots are now being moved to STAR, to study central production ofresonances in pp diffractive events.

pp2pp will measure:

1. Total and elastic cross sections in pp collisions;2. Single and double diffraction in pp collisions;

(important to modeling and interpretation of cosmic ray shower data.)

3. Spin dependence of elastic scattering;.