Recent* Results from A+A Collisions at RHIC

Paul Stankus, ORNL

First colliding nuclear beams

First accelerator designed for high-energy heavy ions

* Recent = July ‘02, “Quark Matter” in Nantes, France

Goal

To persuade (some of) you that pQCD in A+A collisionsat RHIC could be a very rich and interesting field:

Exotic initial state effectsExotic collision effectsExotic final state effects

Many channels, processes and probes

Dramatic data available now! with much more to follow

Exotic (p & non-p)QCD effects

Initial State:High density

of partons at lowand modest x;

Unusual shadowing?“Colored Glass Condensate”?

Collision Overlap:Multiple scattering

of partons; largekT smearing?

Very high densityof gluons; Classical

color fields?

Final State:Hard-scatteredproducts exit

through excitedmedium;

Medium Effects?Is fragmentation

modified?

Our Original Motivation

• To use hard scattering products as probes to measure the properties of dense, highly excited QCD matter (what you would call final-state-effects)

• We originally conceived of hard-scattering products as “calibrated sources” created within HI collisions (we have since learned better!)

p p

We can measure a fullpalette of hard-scattering

products:

q: fast color triplet

g: fast color octet

Q: slow color triplet

QQbar: slow color singlet/octet

Virtual photon: colorless

Real photon: colorless

Unknown Medium

Inducedgluon radiation?

EnergyLoss?

Dissociation?

Controls

Jet Prelude: Why is this hard?• Cannot look at “true”, traditional calorimetric jets; soft particle energy

density dET/dd~ 100 GeV/unit-radian

• Next best thing: leading particles = high-PT hadrons, and also high-PT pairs, either same side (leading and next-leading) or opposite side (leading and opposite leading)

• Ambiguity between hadrons from jet fragmentation source and hadrons from multi-collisional/”thermal” source, even out to several GeV/c (and beyond?)

• Model “thermal” source: hadron gas with temperature, chemical (ie flavor) equilibration, baryon density and overall flow velocity in radial direction

Thermal Model can fit hadron spectra out to at least 4 GeV/c -- do you believe it?

• description by hydrodynamical source– perfect description possible

Tch = 172 ± 2 MeV

B = 37 ± 4 MeV

Tkin = 123 ± 6 MeV

< T > = 0.45 ± 0.02

• spectra of pions and (anti)protons

T.P., nucl-th/ 0207012

T. Peitzman

History of High-Energy A+A Beams

• BNL-AGS: mid 80’s, early 90’s

O+A, Si+A 15 AGeV/c s1/2NN ~ 6 GeV

Au+A 11 AGeV/c s1/2NN ~ 5 GeV

• CERN-SPS: mid 80’s, early 90’s

O+A, S+A 200 AGeV/c s1/2NN ~ 20 GeV

Pb+A 160 AGeV/c s1/2NN ~ 17 GeV

• BNL-RHIC: early 00’s

Au+Au s1/2NN ~ 130 GeV

Au+Au, p+p s1/2NN ~ 200 GeV

Finally: enough energy for copious hard scattering processes!

Nomenclature: Centrality

Characterize A+A collisionintuitively in Glauber model:Here NParticipant = 4 NCollision = 3

<NColl> = <TAB>N+N inel

40 mbarn

Describe classes of events by percentile of impact parameter

distribution:

Peripheral; 60%-80%<NCollisions> = 20 +- 5

Central; 0%-10%<NCollisions> = 850 +- 20

Quantifying Nuclear Effects

R = eA(x,Q2)/A ep(x,Q2) General DIS

RA = pA(PT)/A pp(PT) Hadron PT spectra

pA(xF) = A pp(xF) eg DY, J/

=⟩ ⟨

=)

) (inelastic

ppT pp

2binary

TAA 2

eventsT AA

/ d /dp (d N

d /dp N d 1/Np R

σ η σ

η A+A hadrons

1

1

1

R

RA

Shadowing, EMC, etc.

Cronin effect

?

x

PT

xFAbsorption, initial state energy loss

(This space available!)

RHIC Year-1 High-PT Hadrons

Charged and neutral hadron spectra out to pT~4-5 GeV/c

Nominally expect production through hard scattering, scale spectra from N+N by number of binary collisions

Peripheral reasonably well reproduced; but central significantly below binary scaling

Last Year’s Big News

Observe:

RHIC spectra fall below binary scaling at all pT for central events

Previous highest energy A+A collisions exceed binary scaling (Cronin expectation)

Suspect: scattered parton interaction in dense medium; but must keep an open mind

“The cover of the Rolling Stone”

(Almost) No one reads PRL on paper these days.

Cover artists thought the graph looked better without numbers on the axes.

(We were pleased nonetheless.)

Charged Hadron Spectra

Preliminary sNN = 200 GeV

Preliminary sNN = 200 GeV

C. Jorgensen, BRAHMS Parallel Saturday

C. Roland, PHOBOS Parallel Saturday

J. Jia, PHENIX Parallel Saturday

J. Klay, STAR Parallel Saturday

200 GeV results from all experiments

T.Peitzman

130 GeV nucl-ex/0206011

Preliminary sNN = 200 GeV

Preliminary sNN = 200 GeV

RAA Comparison to pT = 6 GeV/c

Similar Suppression in all centralities at 200 GeV

J.Klay

Central/Peripheral Comparison

200 GeV

130 GeV

•

At 130 GeV, the suppression increases up to pT = 6 GeV/c.

Preliminary sNN = 200 GeV

With higher pT data from 200 GeV, we see that the suppression has saturated at pT ~6 GeV/c

0.5

0.5

J.Klay

High pt suppression at 130 GeV

130GeV

PHENIX (nucl-ex/0207010)

Consistent with STAR(nucl-ex/0206011)

• Detailed pT and centrality dependence– Peripheral RAA 1

– Central RAA saturates ~ 0.6 at pt >2GeV/C

J.Jia

Ratio central/peripheral

• Lower ratio for 200 GeV– more suppression

or change in proton yield?

• Similar shape for 130 and 200 GeV– increase to 2

GeV/c– decrease to 4

GeV/c

colored bracket represent the systematic error. thick black line is uncertainty of the scaling factor from N. collisions

J.Jia

Charged particle pT spectra from 200 GeV

pT <2 GeV/c, increase of inverse slope flow

pT >2 GeV/c, decrease of inverse slope suppression

h+ + h-

J.Jia

Comparison with NN references I

• RAA for peripheral collisions – ~ 0.75 ± 0.3 for pT>2GeV/c– consistent with 1

– similar to 130 GeV

Shaded band is syst. error from NN Common syst. Scaling error

• RAA for central collisions

– significantly below 1– 200 GeV below 130 GeV data– ~ 0.2 ± 0.08 from 4 to 8 GeV/c

Calculate RAA: divide data by NN references

J.Jia

PHENIX Overview

• Most of central arms used to measure the pion spectrum

• Powerful cross-checks of results GeV200AuAu =+ NNs

S.Mioduszewski

Comparison with UA1 Fitting

• UA1 data are only up to 6GeV/c and extrapolated to higher pT

• The extrapolation is below our data at high pT

Now have pp data to use as important reference for Au+Au collision and jet quenching measurement.

PHENIX Preliminary

UA1 data

extrapolation

pT dependent systematic errorNormalization systematic error 30% is not included here.

H. Torii

• NLO pQCD calculation– CTEQ5M pdf

– Potter-Kniehl-Kramer fragmentation function

= pT/2, pT, 2pT

• Consistent with data within the scale dependence.

Comparison with QCD Calculation PHENIX Preliminary

pT dependent systematic errorNormalization systematic error 30% is not included here.

H. Torii

Nuclear Modification Factor

SPS – “Cronin” effect

RHIC - suppression

Our own measure of the p+p spectrum reduces the uncertainty!

=⟩ ⟨

=)

) (inelastic

ppT pp

2binary

TAA 2

eventsT AA

/ d /dp (d N

d /dp N d 1/Np R

σ η σ

η

pp

centralbinarycentral

Yield

NYield / ⟩⟨

Effect of nuclear medium on yields

PHENIX Preliminary

D. d’Enterria talk

binary scaling

S.Mioduszewski

Suppression in Inclusive Photons

Photons (primarily from 0 decays) also show suppression

Not an artifact of extraction of 0 peak yield

Klaus Reygers talk

peripheralbinaryperipheral

centralbinarycentral

NYield

NYield

//

⟩⟨⟩⟨S.Mioduszewski

Hadron Species Ratios in Run-1

(Anti)Baryon/pion ratios rise well above values in p+p

Suspect radial hydrodynamical flow boosting baryons while mesons are suppressed; but

Similar effect seen in p+A/p+p (Cronin);

Could potentially be a modification to the fragmentation process

pbar/ and p/

By Takao Sakaguchi at Quark Matter 2002, July 18-24 at Nantes, France

• pbar/ , p/ ratios– pT<2GeV, pbar/-, p/+

– pT>1GeV, use 0 with -, +

• Point-by-Point Errors include point-by-point statistics+systematic errors

• Bands: pT independent systematic errors

• Decreasing at much more high pT?

pbar/pi

p/pi

T.Sakaguchi

Comparison with Year-1 Data• Data Compared to Year-1• Both Year-1 and Year-2 are consistent within systematic errors

By Takao Sakaguchi at Quark Matter 2002, July 18-24 at Nantes, France

•Another hint.–More rather than protons?

T.Sakaguchi

Particle Composition at high pT

0/(h++h-)/2 ratio ~ 0.5 up to 9 GeV/c

do protons continue to make up a large fraction of charged hadron yield?

S.Mioduszewski

Interlude: “Elliptic Flow”

b

+-

dN/dv2 cos(2)

The impact parameter vector defines the “reaction plane direction” in non-central collisions. Low PT particles “feel” this geometry and show a quadrupole distribution relative to the event plane direction.

Event plane directions were first measured with recoiling beam fragments, but can also be derived from low-PT distributions

How to sense geometry

Pressure gradients leadto collective motion

High-Pthadrons

Fragmentation

Heavy flavor quark endures;Is there medium interaction?Thermal production?

Hard-scattered partons travelthrough early medium; modification?

Difference in pressure gradientscan lead to anisotropic motion

Identical pair correlations reveal

space-time geometryBr?

Comparison v2 (pT) with models (130 GeV)

• qualitative agreement with “jet-quenching” scenario

Adler et al., nucl-ex/0206006K.Filimonov

v2(pT) up to 12 GeV/c

• Statistical errors only

• Finite v2 up to 12 GeV/c in mid-peripheral bin

K.Filimonov

Sources of azimuthal correlations• Au+Au

– flow

• p+p and Au+Au collisions:– dijets– momentum

conservation– jets – resonances Small

All

STAR Preliminary Au+Au @ 200 GeV/c

0-5% most central4<pT(trig)<6 GeV/c

2<pT(assoc.)<pT(trig)

D.Hardtke

Relative Charge DependenceStrong dynamical charge correlations in jet fragmentation

Compare ++ and -- charged azimuthal correlations to +- azimuthal correlations

STAR Preliminary @ 200 GeV/c0-10% most central Au+Au

p+p minimum bias4<pT(trig)<6 GeV/c

2<pT(assoc.)<pT(trig)

System (+-)/(++ & --)

p+p 2.7+-0.6

0-10% Au+Au 2.4+-0.6

Jetset 2.6+-0.7

||<0.5 - ||>0.5 (scaled)

0<||<1.4

Same particle production mechanism for pT>4 GeV/c in pp

and central Au+Au

Au+Au

p+p

D.Hardtke

Excess Above Flat Background, p-p

1/N

trig d

N/d 0.6-1 GeV 1- 2 GeV 2- 4 GeV

PHENIX Preliminary PHENIX Preliminary PHENIX Preliminary

1/N

trig d

N/d

0.6-1 GeV 1- 2 GeV 2- 4 GeVPHENIX PreliminaryPHENIX PreliminaryPHENIX Preliminary

•Data points (black) are background subtracted and acceptance corrected.•Blue is the PYTHIA curve * apythia * <>

M.Chiu

Excess from Flat Bkg, Au-Au

2-4 GeV0-10% Central

PHENIX Preliminary2-4 GeV40-60% Central

PHENIX Preliminary

black = associated charged particles, green = mixed, purple = subtracted

•In AuAu collisions there is a statistically significant excess from a flat distribution at all centralities and all pt bins.•So what is that excess? Try both PYTHIA only and also PYTHIA + elliptic flow contribution.

M.Chiu

v chv tr

ig1

/Ntr

ig d

N/d

0.3-0.6 GeV 0.6-1 GeV 1- 2 GeV 2- 4 GeV

PHENIX Preliminary PHENIX Preliminary PHENIX PreliminaryPHENIX Preliminary

•For lower pt, ambiguity between the contribution from the elliptic flow component and the jet-like component.

•At higher pt (2 GeV and above), the jet-like component dominates over any elliptic flow component.

Fitting Pythia + 2vchvtrigcos(2),pt dependence

apythia

20-40% Cent

M.Chiu

Peripheral Au+Au data vs. pp+flow

€

C2(Au + Au) = C2(p + p) + A * (1+ 2v22 cos(2Δφ))

D.Hardtke

Central Au+Au data vs. pp+flow

€

C2(Au + Au) = C2(p + p) + A * (1+ 2v22 cos(2Δφ))

D.Hardtke

Ratio vs. # participants

D.Hardtke

Npart scaling describes data at pT 4.25 GeV/c

Normalized to yield at Npart = 65

Npart Scaling at high pT

Ncoll-scaling

PHOBOS Preliminary

C.Roland

• Indicates suppression of high pT pions at y2.• Sets in at lower pT (compared to y=0)?

y=2.2

Central/Semi-peripheral collision at y2C.Jorgensen

Separation of non-photonic e±: cocktail method

conversion

0 ee

ee, 30

ee, 0ee

ee, ee

ee

’ ee

PYTHIA

direct (J. Alam et al. PRC 63(2001)021901)

R.Averbeck

Energy dependence of charm production

PHENIX

PYTHIA ISR

NLO pQCD (M. Mangano et al., NPB405(1993)507)

PHENIX: PRL 88(2002)192303

R.Averbeck

Centrality dependence at 200 GeV R.Averbeck

Same plots as previously but now on a linear scale.p-p ee Invariant Mass SpectrumThis analysis All triggers

NJ/ = 24 + 6 (stat) + 4 (sys)

Bd/dy = 52 + 13 (stat) + 18 (sys) nb

A.Frawley

pp Invariant Mass Spectrum

1.2 < y < 1.7 NJ/ = 26 + 6 + 2.6 (sys) B d/dy = 49 + 22%

+ 29% (sys) nb1.7 < y < 2.2 N

J/ = 10 + 4 + 1.0 (sys) B d/dy = 23 + 37%

+ 29% (sys) nb

A.Frawley

(pp->J/) = 3.8 + 0.6 (stat) + 1.3 (sys) b

Gaussian and PYTHIA shape fits give essentially the same integral.

The quoted result is the average of the two fits.

* See J.F. Amundson et al., Phys. Lett. B 390 (1997) 323.

A.Frawley

Au-Au ee Invariant Mass Spectra

NJ/ = 10.8 + 3.2 (stat) + 3.8 - 2.8 (sys) N

J/ = 5.9 + 2.4 (stat) + 0.7 (sys)

A.Frawley

Are our data consistent with binary scaling?

A confidence level of 16% says that a truly flat distribution would produce a fit as poor as this in 16% of cases.

So it probably trends down with increasing N

part, but

don't bet the farm!

A.Frawley

Scorecard

• Jets/High-PT hadrons: Lots of action! Singles yields, spectra, composition do not agree with pQCD expectations, while high-PT pair correlations are a mixed bag. And this is only the beginning!

• Open charm and J/ seem to agree with Pythia predictions, but statistics are limited.

• Lots more to come! From existing data: direct photons, more detailed pair correlations; in later years ’, Y, DY, g+jet, double direct photons; plus p+A/d+A; etc.etc….

• Plenty for any theorist to work with! And many fundamental QCD issues potentially involved.