Hard vs. Soft Physics at RHIC - Insights from PHENIX

Why hard vs. soft? Soft physics: thermal, flow effects Hard processes at RHIC Conclusion

Barbara JacakStony Brook

Why hard vs. soft?

Soft physics : thermalsystem with T ~ 200 MeVcollective flow boosts pt spectra

Hard processeshappen earlycreate useful probes of the system

J/, charm, jets QGP signals

calculable via pQCDsensitive to parton

distribution evolutioni.e. gluon saturation

Theoretical tools very differentGyulassy/Wang: boundary 1-2 GeV/c

from partonstructure functions

from Glaubermodel

Experimental challenge: constrain which to be used where!

)()()( sss softjetqq

PHENIX at RHIC

2 Central spectrometers2 Forward spectrometers3 Global detectors

Philosophy: optimize for signals / sample soft physics

PHENIX configuration

RUN1:DAQ recorded EACH of ~5M events

RUN2:full centralarmsS muonarm (

running now!

Centrality selection

In PHENIX “min bias” = 92% of geometriccross section

Use Glauber model to calculate Npart

Charged particle multiplicity:

with quenching

No jet quenchingSTARPHENIXBRAHMS

PRL86, 3500 (2001)Find A = 0.88 0.28B = 0.34 0.12 hard: scales with Ncollsoft: scales with Npart

hard processes arebecoming significant

ch binpart0dN d A N B N

Fit with

ET per charged particle

ET grows because of particle number!

4.6 GeV/fm3

PRL87, 052301 (2001)

NB. Does not include mass of baryons

Does include MB

>50% higher thanat CERN

TOF to identify the hadrons

Measure momentum & flight timecalculate particle mass

mT spectra

See mass dependence collective radial expansion

Integrate spectra, extrapolating to pT=0

antiproton dN/dy = 20!

hadron dN/dy at y=0

per participant pair:

K,p perparticipantrise withNpartmorethan !

K/ at y=0

PHENIX preliminary

s=17 GeV Pb+PbPhys.Lett.B 471, 6 (1999)

Both K+/ and K-/increase with NpartPeripheral collisions near pp valueK+/ and K-/do not diverge as at SPS,AGS

K/ ats=200 p+p

Z.Phys.C41,179(1988)(UA5)

Net baryons per participant

PHENIX preliminary

Mid-rapidity

Net baryon density increases somewhat with Npart more stopping in central collisions

Pb+Pb at SPS: net baryon density per participant = 0.18

(NB: in PHENIX ~50% of decays included)

Fit spectra with mt exponential

Teff = Tfo + m <radial>2

Tfo = 140 - 150 MeV radial = 0.5 - 0.6 (higher for central collisions)

was 0.4 at SPS

less flow in peripheral collisions!

range mT-m0 < 1 GeV/c

HBT in PHENIX

Identified pions in 15% central collisions :high resolution TOF in East armEMCAL TOF in West arm

See R parameters are rather small (5-6 fm)!very similar to those at SPSagreement with STAR

Qualitatively to be expected if radial flow is large ...

Many high pt baryons!

Not been seen before!

boosted by thecollective outwardexpansion

hydrodynamicalcalculation by Teaneyagrees with data

soft physics to ~ 3 GeV/c pt!

<pt> increases with centrality

Expect such trend from radial flowbut also from partonic multiple scatteringand gluon saturation

final or initial state effect???

Turn now to hard probes

beams of hard probes:jets, J/ ….

vacuum

QGP

1. dE/dx in QGP jet quenching2. Deconfinement J/suppression

hadrons

q

q

hadronsleadingparticle

leading particle

schematic view of jet productionJets: primarily from gluons at RHIC

produced early

Observed via fast leading particles orazimuthal correlations between them

PHENIX measures 0 in PbSc and PbGl calorimeters

0’spT >2 GeV, asym<0.8

in PbSc

excellentagreement!

Compare h and 0

Peripheral collisions (60-80% of geom):

~ p-p scaled by <N bin coll> = 20 6

central (0-10%):shape different (more exponential)below scaled p-p!(<N bin coll> = 905 96)

Compare central Au-Au to p-p

central coll central

pp

/Yield N

Yield

Comparing CERN-SPS Pb-Pb to p-p

RAA exhibits “Cronin effect” behavior

X.N.Wang

soft/hardtransition?

parton energy loss, if any, overwhelmedby initial state soft multiple scattering!

tppA ppA

Is SPS-RHIC comparison fair?

Same pt implies different x!

RHIC

if pT(had) / pT(jet) ~ 1 then xT ~ x(parton) at y=0

xT =

Shadowing at RHIC?Zheng Huang, Hung Jung Lu, Ina Sarcevic: Nucl.Phys.A637:79-106,1998 (hep-ph/9705250 )

Shadowingof structure functions small in RHIC x range!!

Gluon shadowingeven less (accordingto theory)

pt comparison is fair!deficit shadowing!

quark structure function

central coll central

peripheral coll peripheral

/

/

Yield N

Yield N

different systematics: <N binary coll> instead of pp extrapolation

depletion remains!/h smaller in central vs. peripheral!

Identified electrons in PHENIX

All tracks:

0.8>p>0.9 GeV/c

Electron enriched

sample (using RICH)p

Associate trackswith RICH andEMCAL

Inclusive electron pt spectrum

Coming this year

Statistics! Reach pT ~ 15 GeV/c

Fully instrumented central arms South muon arm ready to go

large acceptance for J/ Selective L1, L2 triggers Electron pairs

Conclusions

Soft physics at RHIC

4.6 GeV/fm3, driven by more Nch

K, p / part. increase in central collisionsdNpbar/dy ~ 20 but net baryon density is lowlarge radial flow

baryons cross p at pT 2 GeV/csoft physics extends to nearly 3 GeV/c!

Hard processescontribute to particle productionPHENIX observes a deficit at high pT

vs hard scattering expectationslikely NOT shadowingopposite direction from Cronin effectpA data will settle underlying physics

This run: spectra to pT ~ 15 GeV/c

back-to-back correlationsJ/, electron pairs, direct photons...

hadron dN/dy at y=0

statistical errors in table;

syst = 13%, 15%, 14% K p, pbar

Integrate spectra, extrapolate to pT=0PHENIX preliminary