ralf averbeck state university of new york at stony brook topics in heavy-ion collisions montreal,...

Ralf Averbeck State University of New York at Stony Brook

Topics in Heavy-Ion CollisionsMontreal, Canada, June 25-28, 2003

Leptons at RHIC:light messengers from heavy quarks

Ralf Averbeck, SUNY Stony BrookHIC03, Montreal

physics motivation: leptonic probes lepton measurements at RHIC

PHENIX low-mass dileptons charmonium (J/) open charm

summary outlook

Outline


deconfinement high temperature / high density dynamical screening of long

range QCD confining forces quarks and gluons free within

“large” & color neutral object thermalization quark gluon

plasma

one probe: heavy flavor (charm, beauty) production/propagation

measured in leptonic channels

chiral symmetry restoration CS spontaneously broken in nature:

<qq>~300 MeV3

<qq>0 at high T and/or B

constituent mass current mass CS (approximately) restored modification of meson properties? best candidate: decay also: e+e- vs. K+K-

Physics motivation fundamental issues in relativistic heavy ion collisions

leptons from heavy quarks leptons from light quarks


Lepton measurements at RHIC: PHENIX

only RHIC experiment optimized for lepton measurements

Two forward muon spectrometers

Two central electron/photon/hadron spectrometers

electrons: two central arms electron measurement

in range: 0.35 p 0.2 GeV/c

muons: two forward arms muon measurement

in range: 1.2 < || < 2.4 p 2 GeV/c


Low-mass dielectrons: lessons from SPS strong enhancement of low-mass

e+e- pairs in A+A collisions interpretation

thermal radiation from hadron gas (e+e-) not enough to reproduce data

in medium modifications of (CSR) dropping meson mass

(Brown at al.) broadening of the spectral shape

(Rapp and Wambach) high baryon density at mid rapidity

is the key factor prospects at RHIC

total baryon density is very large strong enhancement of low-mass pairs expected to persist contribution from open charm decays becomes significant


real and mixed e+e- distributionsreal - mixed = e+e- signal

net e+e-

e+e- from charm(PYTHIA)

e+e- from lighthadron decays

dielectrons from Au+Au @ sNN = 200 GeV combinatorial background from uncorrelated e± is huge

Leptons at RHIC: the landscape

what is expected? light hadron decays

(cocktail generator)

charm decays (PYTHIA)

data agree with expectation

background subtraction is under control uncertainties are large


512.28.4LMR 10(sys)7.2(stat)13.4PHX) N(into

10)()(60.238.0PHX) N(into 540.181.0IMR

sysstat

integrated dielectron yield in PHENIX expected from known sources low mass region LMR (0.3 -1.0 GeV): ~9.2 x 10-5

intermediate mass region IMR (1.1 -2.5 GeV): ~1.5 x 10-5

PHENIX preliminary data

reasonable agreement within huge uncertainties

Dielectron continuum and e+e-

e+e- in Au+Au at 200 GeV

1/20S/B sys)((stat) 47 101 Signal 56

20-

PHENIXpreliminary

Mass (GeV/c2)

PHENIXpreliminary

PHENIXpreliminaryY

ield PHENIX

preliminary

Mass (GeV/c2)

Yie

ld

Mass (GeV/c2)

minimum bias

Feasibility demonstrated

Statistics is

a severe problem

Improvement of S/B UPGRADE


cc: produced in early stage / embedded in medium can form bound state: J/ deconfinement & color screening J/suppression

(Matsui and Satz, PLB176(1986)416)

Heavy flavor: charmonium (J/l+l-)

Perturbative Vacuum

cc

Color Screening

cc

central Pb+Pb collisions at SPS J/suppression in excess of “normal”

nuclear suppression (NA50: PLB477(2000)28)

prospects at RHIC higher cc yield than at SPS possible J/ enhancement due to cc

coalescence as the medium cools important to measure J/ in p+p and d+A

to separate “normal” nuclear effects shadowing nuclear absorption in cold matter

PHENIX: R. Granier de Cassagnac’s talk


J: baseline from p+p at s = 200 GeV signal observed in e+e- and +- channel

kinematic distributions (pT, y) measured

reasonable agreement with Color Octet Model calcu-lations and extrapolations from lower s data


most probable value

190% C.L.

incl. systematic error

binary collisionscaling bandexpectation withabsorption ( =4.4 and 7.1 mb)NA50 pattern: PLB477(2000)28; normalized to p+p measurement

J e+e- in Au+Au collisions at sNN = 200 GeV

models that predict enhancement relative to binary collision scaling are disfavored

no discrimination between models that lead to suppression

PHENIX: nucl-ex/0305030

First J/ measurements a

t RHIC

Statistics is

a severe problem


BRAHMS preliminaryAu+Au->0+X

PHENIX PHOBOS

Open charm: why? charm production in HIC

gg fusion gluon density thermal temperature

observation at SPS (NA50) excess dimuon continuum yield

below J/ mass not explained charm enhancement thermal QGP …

high pT particle production in Au+Au suppressed relative to binary scaling

(large effect: suppression factor 3-5) not observed in d+Au final state effect (energy loss by gluon

radiation in deconfined medium?) what about charm?

NA50:

Eur. Phys. J.

C14(2000)443


ideal but very challenging direct reconstruction of charm decays

(e.g. )

Open charm: how?

D0 K- +

alternative but indirect charm semi leptonic decays contribute to

single lepton and lepton pair spectra:

|y|<1, pT < 4 GeV/c

KDD 00 d+Au at 200 GeVSTAR Preliminary !

c c

0DK

0D

K

0eD K

0D K

0 0e eD D e e K K

0 0eD D e K K

0 0D D K K


inclusive e± spectra from Au+Au at 130 GeV

use available data to establish “cocktail” of e± sources dominated by measured 0

and photon conversions

excess above cocktail increasing with pT

expected from charm decays

subtract cocktail from data

Inferring charm production: cocktail method

conversion

0 ee

ee, 30

ee, 0ee

ee, ee

ee

’ ee

PHENIX: PRL 88(2002)192303


compare excess e± spectra with PYTHIA open charm calculations PYTHIA tuned to fit SPS,

FNAL, ISR data (s<63 GeV)

scale to Au+Au using the number of binary collisions

reasonable agreement in min. bias AND central collisions between data and PYTHIA

Electron spectra from Au+Au at 130 GeV

neglect contributions from alternative sources (direct , beauty)corresponding charm cross section per binary collision from data

PYTHIA

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

b

c

PHENIX: PRL 88(2002)192303

0 10%cc 380 60 200 b and 0 92%

cc 420 33 250 b


Systematic trends with collision energy

PHENIX

PYTHIA ISR

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

PHENIX: PRL 88(2002)192303

assuming binary collision scaling, PHENIX data are consistent with the s systematics (within large uncertainties)


larger than at 130 GeV consistent with PYTHIA calculation, assuming binary scaling: cc(130 GeV) = 330 b and cc(200 GeV) = 650 b

large systematic uncertainty due to material thickness without converter (to be reduced in final result)

Inferring charm production: converter method Au+Au at sNN = 200 GeV

measure the e± spectrum from photonic sources (…) by adding a photon converter to PHENIX

subtract the photonic spectrum from the total to produce e± spectrum from non-photonic sources

non-photonic e± yield at 200 GeV


Centrality dependence

PHENIX data are consistent with the PYTHIA charm spectrum scaled by the number of binary collisions in all centrality bins!

Reasonable agreement with “simple” binary scaling!

Where is the energy loss e

ffect on the charm quark?


“Dead Cone” effect? “Dead Cone” effect

gluon radiation from massive partons suppressed at angles Mq/Eq (Y.L. Dokshitzer, D.E. Kharzeev PLB 519(2001)199)

also:– heavy (light) quark slow (fast) moving– nuclear medium expands– more dilute density profile sampled by heavy quark

may lead to reduced energy loss E in addition (M. Djordjevic, M. Gyulassy nucl-th/0302069)

polarization of QCD medium dispersion relation for radiated gluons can be approximated by an effective gluon mass suppresses the radiation of soft gluons


Hydrodynamic flow of charm? scenarios leading to thermalization / hydrodynamic flow of charm

D meson rescattering with other hadrons– cross sections are small– many hadrons present

charm quark rescattering in partonic medium followed by– fragmentation into D mesons or– coalescence with comoving spectators of low relative momentum

PHENIX e± consistent with medium transparent to heavy

quarks which then fragment into D/B mesons outside the system (scaled PYTHIA)

highly opaque medium with charm/beauty boosted via rescattering and hadronizing in the system

S. Batsouli et al. PLB557(2003)26


Quark coalescence?

formation of hadrons not via fragmentation but via recombination of quarks/antiquarks in densely populated phase space (R.J. Fries et al.: nucl-th/0301087)

hadron emission from thermal parton ensemble may be dominated by parton recombination (hadronization inside the medium)

medium pT (<5 GeV) suppression due to radiation may be counteracted by recombination

high pT fragmentation dominates hadron production (partons fast enough

to escape medium)


heavy flavor measurements at RHIC / PHENIX open charm

– (indirectly) measured in semileptonic decay channels in Au+Au at 130 and 200 GeV

– yields consistent with binary collision scaling– no large enhancement of yields– no large suppression of e± from charm at high pT

J/ – baseline established in p+p collisions at 200 GeV– strong enhancement scenarios disfavored in Au+Au– statistics is the limiting factor

low-mass continuum and e+e- at RHIC / PHENIX feasibility demonstrated statistics is one limiting factor S/B is poor (not unexpected)

Summary


open charm/beauty significant reduction of sys. errors possible

in e± analysis replace PYTHIA reference by

measurement from p+p d+Au measurement done to establish “cold

matter” reference independent cross checks: ± and lepton-

pair data STAR: D0/D0 in hadronic channels inclusive e± spectra: contribution from B

decays at high pT (> 4 GeV)? J/

d+Au measurement done to study “normal” nuclear effects

measurement from STAR? low-mass continuum / e+e-

increase S/B by removing material from acceptance

high statistics Au+Au data are needed!

Outlook

J/ in d+AuNorth arm


a possible solution identify e± from 0 Dalitz decays

and conversions and reject them: detector upgrade compact hadron-blind

detector (HBD) electron identification

complemented by miniTPC tracking

very high resolution vertex tracking to identify e± from charm/beauty decays via their displaced secondary vertex silicon vertex tracker

the problem

How to improve the low-mass dielectron measurement?

e+ e -

e+ e -

“combinatorial pairs”

total background

Irreducible charm backgroundall signal

charm signal

S/B~500

ralf averbeck state university of new york at stony brook topics in heavy-ion collisions montreal,...

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