itzhak tserruya, qcd@work, conversano, june 14-18, 20031 penetrating probes: from sps to rhic itzhak...

Itzhak Tserruya, QCD@Work, Conversano, June 14-18, 2003

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Penetrating Probes: from SPS to RHIC

Itzhak TserruyaWeizmann Institute

QCD@Work, Conversano, June 14-18, 2003


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Outline• Introduction

- Motivation: penetrating probes are directly sensitive to the two fundamental issues of RHI collisions: deconfinement and chiral symmetry restoration.

• RHIC and SPS Highlights

- High pT phenomena

- J/ suppression

- Low-mass e+e- pairs

- Direct photons• Summary

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Penetrating probes Relativistic Heavy Ion collisions aim at producing and studying high

density matter. “Penetrating probes” provide sensitive diagnostic tools. Two types of penetrating probes:

a) probes created at early stages which propagate through

the medium and are modified by the medium.

* QCD hard scattering probes:

jet quenching suppression of high pT hadrons

J/ suppression b) e.m. probes (real or virtual photons) created inside the medium

* Large mfp no final state interaction

carry information from place of creation to detectors.

low-mass e+e- pairs

real photons


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The RHIC Experiments @ BNL

STAR

•Begun operation June 2000Outstanding start-up of machine and experiments


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Jets: A New Probe For High Density Matterschematic view of jet production

ppAA

• In the colored medium, quarks radiate energy (energy loss ~GeV/fm) modify jet shape.

• Jets from hard scattered quarks:

- produced very early in the collision (τ <1fm/c)

- expected to be significant at RHIC

q

q

leading particle

leading particle


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PHENIX STAR

RHIC events Au-Au central collision at √sNN = 200 GeV p-p collision at √s = 200 GeV

STAR


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Jets: A New Probe For High Density Matterschematic view of jet production

pp

• Not possible to observe jets directly in

RHIC due to the large particle multiplicty.

AA

• In the colored medium quarks radiate energy (energy loss ~GeV/fm) modify jet shape.

• Jets from hard scattered quarks:

- produced very early in the collision (τ <1fm/c)

- expected to be significant at RHIC

• Decrease their momentum Suppression of high pT particles

“Jet Quenching”

Identify jet and its possible modifications through leading particles

q

q

leading particle

leading particle


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Nuclear modification factor• Zero hypothesis: scale pp to AA with the number of NN collisions Ncoll:

d2NAA/dpTd (b) = pp TAA(b) = Ncoll d2Npp /dpTd?

d/dpNd N

d/dpNd )(pR

Tpp2

coll

TAA2

tAA

• Quantify “effect” with nuclear modification factor:

• If no “effect”: RAA < 1 at low pT in regime of soft physics RAA = 1 at high-pT where hard scattering dominates• If “jet quenching”: RAA < 1 at high-pT

AA

AA

AA


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High pT Suppression in Au-Au collisions !!

Same behavior observed in the ratio of central to peripheral collisions

Central/peripheral ratioAA / pp ratio

Peripheral collisions look like pp.

Central collisions are strongly suppressed


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Discovery of High pT Hadron Suppression at RHIC…

• At CERN: all previous measurements see

enhancement, not suppression:

Low pt : soft processes Npart R Npart / Ncoll ~ 0.3

High pt : broadening due to rescattering (Cronin effect)

R > 1.

PHENIX Preliminary

CERN WA98: Understood enhancement from Cronin

effect

• At RHIC: qualitatively new

physics made accessible by RHIC’s higher energy and ability to produce (copious)

perturbative probes


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…Made the cover of PRL Jan. 2002

PHENIX


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Suppression increases gradually with increasing collision centrality

Nuclear modification factor RAA for charged

particles in different centrality ranges in Au+Au collisions at

130GeV (result for most central collisions shown on

all panels).

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protons

Suppression is particle dependent

The proton puzzle:• protons and antiprotons are not suppressed• different production mechanism for protons and antiprotons?

PHENIX Preliminary

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Unusual Particle Mix at pT > 1.5 GeV

Peripheral collisions:

p/ ~ 0.4

as in pp collisions.

Central collisions:

p/ ~1

higher than in pp or

jets in e+e- collisions

In-medium modification of fragmentation function?


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Origin of the Suppression?Final state effect

• Energy loss of partons in dense matterGyulassy, Wang, Vitev, Baier….

• Hadronic absorption of fragments: (Absorption with comovers)

Gallmeister, et al. PRC67,044905(2003)

• Parton recombination

(coalescence)Fries, Muller, Nonaka, Bass nuclth/0301078Lin & Ko, PRL89,202302(2002)

• Gluon Saturation RdA ~ √AA ~ 0.5(Color Glass Condensate)

(McLerran, Kharzeev …)

• Multiple elastic scatterings (Cronin effect) RdA > 1

• Nuclear shadowing RdA decreases

Initial state effect

No final state expected in d+Au collisions!d+Au is the “control” experiment


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d – Au Results (I): Spectra

Final spectra for charged hadron and identified pions.Data span 7 orders of magnitude.

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d - Au Results (II): Identified 0

Two independent measurements ! Suppression in AuAu is a final state effect !!

CGC ruled out as possible explanation of Au-Au results

d-Au: Initial state effects only

d-Au: Initial state effects only

Au-Au: Initial + final states effects

Au-Au: Initial + final states effects

π0

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Charged hadrons

See “Cronin” effect in d-Au?

Enhancement more pronounced in the charged hadron than in the 0 measurement ?

d - Au Results (III): Charged Particles

Third independent measurement !!

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pTpT

RR R

R

d-Au results (IV): Centrality Dependence

Charged hadron spectra show centrality evolution with

opposite trend to Au-Au collisions

PHENIX preliminaryPHENIX preliminary


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Suppression of high pT hadrons in central Au-Au collisions at RHIC energies

• Observed in the first RHIC run at √sNN = 130 GeV

• Confirmed in the second run at √sNN = 200 GeV

• No suppression observed in d-Au collisions at √sNN = 200 GeV, the third RHIC run, which ended a couple of months ago.

• The most significant RHIC result so far:

•The basis for the BNL press release issued on June

11:“Exciting first results from deuteron gold collisions at Brookhaven. Findings intensify search for new form of matter”


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J/ Suppression

Perturbative Vacuum

cc

Color Screening

cc

Suppression Mechanism

• An “old” signature of QGP formation: (Matsui and Satz PL B178, (1986) 416).

• One of the first observations at CERN:* J/ suppression in 200 A GeV S-Au collisions explained by

absorption in nuclear medium J/ + N DD abs ~ 6mb

• Anomalous suppression in Pb-Pb collisions at CERN

At high enough color density, the J finds itself

enveloped by the medium. When screening radius < binding radius J/ will dissolve (Debye screening) The small cc production cross section makes it

unlikely that they find each other at the hadronization stage


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Anomalous J/ suppression in Pb-Pb collisions

Normal nuclear absorption: abs = 6.4 ± 0.8 mb

NA50NA50

Anomalous absorption in Pb-Pb forET > 40 GeV or Npart >100 or b < 8fm

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J/ Suppression at SPS: Evidence of QGP?

Conventional models ruled out

Two-step pattern: successive melting of charmonium states c (b.e. 250 MeV) and J/ (650 MeV)

NA50NA50

Also: Capella et al.

Hadronic models: cold nuclear + “comover” dissociation

QGP models: energy density thresholds + ET fluctuations

“thresholds” and high ET behavior favor QGP models ….


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J/ at RHIC: Prospects Suppression or enhancement?

• suppressed: because of Debye screening of the attractive potential between c and c in the deconfined medium.• enhanced: charm cross section at RHIC is much larger than at SPS. The J/ melting mechanism could be compensated by recombination orcoalescence of cc as the medium cools down.

Energy loss of charm quarks in the high density medium

J/ is becoming a complex observable. Will require precise measurements of pp, pA and AA The PHENIX experiment was specifically designed to measure J/ e+e- at mid-rapidity and J/ + - at forward rapidities

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J/ @ RHIC: Establishing pp baseline

Clear J/ signals seen in both central and muon arms. Resolutions in agreement with expectations.

Integrated cross-section : 3.98 ± 0.62 (stat) ± 0.56 (sys) ± 0.41(abs) b In very good agreement with Color Evaporation Model calculations

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Poor statistics N=10.8 3.2 (stat) 3.8 (sys)

J/ e+e- in Au-Au @ RHIC

Need much higher luminosity runs Au-Au expected in run 2003-4

Ncoll scaling band

Most probable value

90 % C.L.

Incl. systematic errors

Expectation with abs =4.4 and 7.1 mb

p-p

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• Dileptons (e+e -, + -): best probes to look for thermal radiation from

QGP: q q * l + l -

HG: + - * l + l -

• Photons

* Same underlying physics but much higher background less sensitivity

Chiral symmetry spontaneously broken in nature.

Quark condensate is non-zero:

< qbarq > 300 MeV3 0 at high T and/or high Constituent mass current mass

Chiral Symmetry (approximately) restored.

Physics accessible through e.m. probes (I)

Meson properties (m,) expected to be modified (?)

* Best candidate: -meson decay ( = 1.3fm/c)

• Low-mass dileptons: best probe of Chiral Symmetry Restoration


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Physics accessible through e.m. probes (II)

• meson

* simultaneous measurement of l+ l- and K+ K-

very powerful tool to evidence in-medium effects

* strangeness enhancement

• Charm production

* semileptonic decays of charmed mesons

accessible at RHIC through high pT single electrons


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Low-mass Dileptons: Main CERN Result

Strong enhancement of low-mass e+e- pairs in A-A collisions

(wrt to expected yield from known sources)

Enhancement factor (m > 0.2 GeV/c2 ): 2.6 ± 0.2 (stat) ± 0.6 (syst)

No enhancement in pp nor in pA

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Multiplicity DependenceCERES Pb-Au 158 A GeV 95+96 dataCERES Pb-Au 158 A GeV 95+96 data

• Enhancement factor rises linearly with dN/d pair yield (dN/d)2

• Data consistent with straight line passing through 1 at dN/d=0• Largest enhancement at 500 MeV/c2

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Pt Dependence

Enhancement much more pronounced at low pair pT :• reaches a factor of 10 ! at masses of 0.4 – 0.6 GeV/c2

CERES Pb-Au 158 A GeV 95+96 dataCERES Pb-Au 158 A GeV 95+96 data

• at high pair pT, mass spectrum is much closer to cocktail

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Onset of Chiral Symmetry Restoration?

What happens as CSR is approached? Dropping masses or line broadening?

Quark-hadron duality down to m ~ 0.5 GeV/c2 ?

Dropping -meson mass

(Rapp, Wambach et al)

In-medium -meson broadening

(G.E. Brown et al, using Brown-Rho scaling )

d.o.f. hadrons

quarks

Looking forward to high resolution CERES results

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Low-mass eLow-mass e++ee-- Pairs: Pairs: Prospects at RHICProspects at RHICR. Rapp nucl-th/0204003R. Rapp nucl-th/0204003

• Strong enhancement of low-mass pairs persists at RHIC• Contribution from open charm becomes significant

Possibility to observe in-medium effects on the ?

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Low and intermediate mass pairs at RHIC: first results

e+e- pairs (real)e+e- pairs (mixed)

Real and Mixed e+e- Distribution

net e+e-

e+e- from charm(PYTHIA)

e+e- from lighthadron decays

Real - Mixed e+e- Distribution

LMR (0.3 – 1.0 GeV): Predictions: = 9.2 x 10-5

Measurements: ]/[c 10)()(2.74.13 252.124.8 GeVsysstat

Problem: combinatorial background too high S/B 1/300

Need an upgrade. R&D already started to develop an HBD


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Meson * sensitive to strangeness production * simultaneous measurement of e+ e- and K+ K-

very powerful tool to evidence in-medium effects unique capability of the PHENIX experiment

mass [GeV/c2]

e+e-

mass [GeV/c2]

K+K-


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Intermediate Mass Region: CERN DataEnhancement of dimuons in the IMR (1 – 2.5 GeV/c2) seen by:

• HELIOS – 3

• NA38/50 – increasing with centrality

Charm enhancement or thermal radiation from HG?

HELIOS3- pW and SW 200 A GeV

HELIOS3- pW and SW 200 A GeV

NA50 PbPb 158 A GeV Peripheral Central

NA50 PbPb 158 A GeV Peripheral Central


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Open Charm at RHIC

Measure inclusive single electrons

Subtract hadronic sources and gamma conversions.

Attribute difference to open charm.

pT distribution (in minimum bias and central collisions) and total cross section in very good agreement with Pythia.

No charm enhancement?

No high pT suppression of charm quarks?

NOTE: Pythia comparison is on absolute scale, no free parameters.


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Direct Photons at CERN

• Evidence for direct photons in central Pb-Pb collisions? 10-20% excess but 1-2 effect only

WA98 WA98

• No direct photons in peripheral Pb-Pb collisions

• Previous attempts with O,S beams by CERES, HELIOS2 and WA80 resulted only in upper limits


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Direct photons at RHIC:

first results

No photon excess seen within errorsNeed better understanding of systematic errors


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Summary

• Real photons - no convincing evidence of thermal radiation at the SPS. - expect RHIC results from the 2003-4 run.

• J/ suppression - most direct evidence of deconfinement at SPS? - Situation at RHIC more complex. Expect significant results from next run

• Jet quenching: - The most spectacular RHIC result so far.

• Enhancement of low-mass e+ e-

- thermal radiation from HG. - evidence of chiral symmetry restoration? - very difficult measurement: PHENIX upgrade underway.

itzhak tserruya, qcd@work, conversano, june 14-18, 20031 penetrating probes: from sps to rhic itzhak...

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