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Ralf Averbeck State University of New York at Stony Brook Topics in Heavy-Ion Collisions Montreal, Canada, June 25-28, 2003 Leptons at RHIC: light messengers.

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Presentation on theme: "Ralf Averbeck State University of New York at Stony Brook Topics in Heavy-Ion Collisions Montreal, Canada, June 25-28, 2003 Leptons at RHIC: light messengers."— Presentation transcript:

1 Ralf Averbeck State University of New York at Stony Brook Topics in Heavy-Ion Collisions Montreal, Canada, June 25-28, 2003 Leptons at RHIC: light messengers from heavy quarks

2 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l physics motivation: leptonic probes l lepton measurements at RHIC l PHENIX l low-mass dileptons charmonium (J/  ) l open charm l summary l outlook Outline

3 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l deconfinement l high temperature / high density l dynamical screening of long range QCD confining forces l quarks and gluons free within “large” & color neutral object l thermalization  quark gluon plasma l one probe: heavy flavor (charm, beauty) production/propagation l measured in leptonic channels l chiral symmetry restoration l CS spontaneously broken in nature: ~300 MeV 3  0 at high T and/or  B l constituent mass  current mass l CS (approximately) restored l modification of meson properties? best candidate:  decay also:  e + e - vs.  K + K - Physics motivation l fundamental issues in relativistic heavy ion collisions l leptons from heavy quarks l leptons from light quarks

4 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal Lepton measurements at RHIC: PHENIX l only RHIC experiment optimized for lepton measurements Two forward muon spectrometers Two central electron/photon/hadron spectrometers l electrons: two central arms electron measurement in range:  0.35 p  0.2 GeV/c l muons: two forward arms muon measurement in range: 1.2 < |  | < 2.4 p  2 GeV/c

5 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal Low-mass dielectrons: lessons from SPS l strong enhancement of low-mass e + e - pairs in A+A collisions l 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) l high baryon density at mid rapidity is the key factor l prospects at RHIC l total baryon density is very large l strong enhancement of low-mass pairs expected to persist l contribution from open charm decays becomes significant

6 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal real and mixed e + e - distributions real - mixed = e + e - signal net e + e - e + e - from charm (PYTHIA) e + e - from light hadron decays l dielectrons from  s NN = 200 GeV l combinatorial background from uncorrelated e ± is huge Leptons at RHIC: the landscape l what is expected ? l light hadron decays (cocktail generator) l charm decays (PYTHIA) l data agree with expectation l background subtraction is under control l uncertainties are large

7 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l integrated dielectron yield in PHENIX expected from known sources l low mass region LMR ( GeV): ~9.2 x l intermediate mass region IMR ( GeV): ~1.5 x l PHENIX preliminary data l reasonable agreement within huge uncertainties Dielectron continuum and   e + e -   e + e - in Au+Au at 200 GeV PHENIX preliminary Mass (GeV/c 2 ) PHENIX preliminary PHENIX preliminary Yield PHENIX preliminary Mass (GeV/c 2 ) Yield Mass (GeV/c 2 ) minimum bias Feasibility demonstrated Statistics is a severe problem Improvement of S/B  UPGRADE

8 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l 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 - ) l central Pb+Pb collisions at SPS J/  suppression in excess of “normal” nuclear suppression (NA50: PLB477(2000)28) l prospects at RHIC l 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 l shadowing l nuclear absorption in cold matter l PHENIX: R. Granier de Cassagnac’s talk

9 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal J  : baseline from p+p at  s = 200 GeV signal observed in e + e - and  +  - channel l kinematic distributions (p T, y) measured l reasonable agreement with Color Octet Model calcu- lations and extrapolations from lower  s data

10 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal most probable value 11 90% C.L. incl. systematic error binary collision scaling band expectation with absorption (  = 4.4 and 7.1 mb) NA50 pattern: PLB477(2000)28; normalized to p+p measurement J   e + e - in Au+Au collisions at  s NN = 200 GeV l models that predict enhancement relative to binary collision scaling are disfavored l no discrimination between models that lead to suppression PHENIX: nucl-ex/ First J/  measurements at RHIC Statistics is a severe problem

11 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal BRAHMS preliminary Au+Au->  0 +X lPHENIX lPHOBOS Open charm: why? l charm production in HIC l gg fusion  gluon density l thermal  temperature l observation at SPS (NA50) excess dimuon continuum yield below J/  mass not explained l charm enhancement l thermal l QGP l … l high p T particle production in Au+Au l suppressed relative to binary scaling (large effect: suppression factor 3-5) l not observed in d+Au l final state effect (energy loss by gluon radiation in deconfined medium?) l what about charm? NA50: Eur. Phys. J. C14(2000)443

12 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l ideal but very challenging l direct reconstruction of charm decays (e.g. ) Open charm: how? D0  K- +D0  K- + l alternative but indirect l charm semi leptonic decays contribute to single lepton and lepton pair spectra: |y|<1, p T < 4 GeV/c d+Au at 200 GeV STAR Preliminary !

13 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l inclusive e ± spectra from Au+Au at 130 GeV l use available data to establish “cocktail” of e ± sources dominated by measured  0 and photon conversions l excess above cocktail l increasing with p T l expected from charm decays l subtract cocktail from data Inferring charm production: cocktail method  conversion  0   ee    ee, 3  0   ee,  0 ee   ee,  ee   ee  ’   ee PHENIX: PRL 88(2002)192303

14 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l compare excess e ± spectra with PYTHIA open charm calculations l PYTHIA tuned to fit SPS, FNAL, ISR data (  s<63 GeV) l scale to Au+Au using the number of binary collisions l 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) l 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

15 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal Systematic trends with collision energy PHENIX PYTHIA ISR NLO pQCD (M. Mangano et al., NPB405(1993)507) PHENIX: PRL 88(2002) l assuming binary collision scaling, PHENIX data are consistent with the  s systematics (within large uncertainties)

16 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l larger than at 130 GeV consistent with PYTHIA calculation, assuming binary scaling:  cc (130 GeV) = 330  b and  cc (200 GeV) = 650  b l large systematic uncertainty due to material thickness without converter (to be reduced in final result) Inferring charm production: converter method l Au+Au at  s NN = 200 GeV measure the e ± spectrum from photonic sources (    …) by adding a photon converter to PHENIX l subtract the photonic spectrum from the total to produce e ± spectrum from non-photonic sources l non-photonic e ± yield at 200 GeV

17 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal Centrality dependence l 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 effect on the charm quark?

18 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal “Dead Cone” effect? l “Dead Cone” effect gluon radiation from massive partons suppressed at angles  M q /E q ( Y.L. Dokshitzer, D.E. Kharzeev PLB 519(2001)199 ) l 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 l in addition ( M. Djordjevic, M. Gyulassy nucl-th/ ) l polarization of QCD medium  dispersion relation for radiated gluons l can be approximated by an effective gluon mass l suppresses the radiation of soft gluons

19 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal Hydrodynamic flow of charm? l scenarios leading to thermalization / hydrodynamic flow of charm l D meson rescattering with other hadrons –cross sections are small –many hadrons present l charm quark rescattering in partonic medium followed by –fragmentation into D mesons or –coalescence with comoving spectators of low relative momentum l PHENIX e ± consistent with l medium transparent to heavy quarks which then fragment into D/B mesons outside the system (scaled PYTHIA) l highly opaque medium with charm/beauty boosted via rescattering and hadronizing in the system S. Batsouli et al. PLB557(2003)26

20 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal Quark coalescence? l formation of hadrons not via fragmentation but via recombination of quarks/antiquarks in densely populated phase space ( R.J. Fries et al.: nucl-th/ ) l hadron emission from thermal parton ensemble may be dominated by parton recombination (hadronization inside the medium) l medium p T (<5 GeV) l suppression due to radiation may be counteracted by recombination l high p T l fragmentation dominates hadron production (partons fast enough to escape medium)

21 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l heavy flavor measurements at RHIC / PHENIX l 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 p T 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 l feasibility demonstrated l statistics is one limiting factor l S/B is poor (not unexpected) Summary

22 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l open charm/beauty l significant reduction of sys. errors possible in e ± analysis l replace PYTHIA reference by measurement from p+p l d+Au measurement done to establish “cold matter” reference l independent cross checks:  ± and lepton- pair data l STAR: D 0 /D 0 in hadronic channels l inclusive e ± spectra: contribution from B decays at high p T (> 4 GeV)? J/  l d+Au measurement done to study “normal” nuclear effects l measurement from STAR? low-mass continuum /  e + e - l increase S/B by removing material from acceptance l high statistics Au+Au data are needed! Outlook J/  in d+Au North  arm

23 Ralf Averbeck, SUNY Stony Brook HIC03, Montreal l a possible solution identify e ± from  0 Dalitz decays and  conversions and reject them: detector upgrade l compact hadron-blind detector (HBD)  electron identification l complemented by miniTPC  tracking l very high resolution vertex tracking to identify e ± from charm/beauty decays via their displaced secondary vertex l silicon vertex tracker l the problem How to improve the low-mass dielectron measurement?  e + e -    e + e - “combinatorial pairs” total background Irreducible charm background all signal charm signal S/B~500


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