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Leptons at RHIC: light messengers from heavy quarks

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Presentation on theme: "Leptons at RHIC: light messengers from heavy quarks"— Presentation transcript:

1 Leptons at RHIC: light messengers from heavy quarks

2 Outline physics motivation: leptonic probes
lepton measurements at RHIC PHENIX low-mass dileptons charmonium (J/Y) open charm summary outlook

3 Physics motivation fundamental issues in relativistic heavy ion collisions chiral symmetry restoration CS spontaneously broken in nature: <qq>~300 MeV3 <qq>0 at high T and/or rB constituent mass  current mass CS (approximately) restored modification of meson properties? best candidate: r decay also: fe+e- vs. fK+K- 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 leptons from light quarks leptons from heavy quarks

4 Lepton measurements at RHIC: PHENIX
only RHIC experiment optimized for lepton measurements electrons: two central arms electron measurement in range: |h|  p  0.2 GeV/c Two central electron/photon/hadron spectrometers Two forward muon spectrometers muons: two forward arms muon measurement in range: < |h| < p  2 GeV/c

5 Low-mass dielectrons: lessons from SPS
strong enhancement of low-mass e+e- pairs in A+A collisions interpretation thermal radiation from hadron gas (p+p-rg*e+e-) not enough to reproduce data in medium modifications of r (CSR) dropping r meson mass (Brown at al.) broadening of the r 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

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

7 Dielectron continuum and f  e+e-
integrated dielectron yield in PHENIX expected from known sources low mass region LMR ( GeV): ~9.2 x 10-5 intermediate mass region IMR ( GeV): ~1.5 x 10-5 PHENIX preliminary data reasonable agreement within huge uncertainties Feasibility demonstrated Statistics is a severe problem Improvement of S/B  UPGRADE f  e+e- in Au+Au at 200 GeV PHENIX preliminary Mass (GeV/c2) Yield minimum bias

8 Heavy flavor: charmonium (J/Y  l+l-)
cc: produced in early stage / embedded in medium can form bound state: J/Y deconfinement & color screening  J/Y suppression (Matsui and Satz, PLB176(1986)416) central Pb+Pb collisions at SPS J/Y suppression in excess of “normal” nuclear suppression (NA50: PLB477(2000)28) prospects at RHIC higher cc yield than at SPS possible J/Y enhancement due to cc coalescence as the medium cools important to measure J/Y in p+p and d+A to separate “normal” nuclear effects shadowing nuclear absorption in cold matter PHENIX: R. Granier de Cassagnac’s talk

9 J/Y: baseline from p+p at s = 200 GeV
signal observed in e+e- and m+m- channel kinematic distributions (pT, y) measured reasonable agreement with Color Octet Model calcu-lations and extrapolations from lower s data

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

11 Open charm: why? charm production in HIC observation at SPS (NA50)
gg fusion  gluon density thermal  temperature observation at SPS (NA50) excess dimuon continuum yield below J/Y mass not explained charm enhancement thermal QGP NA50: Eur. Phys. J. C14(2000)443 PHOBOS Au+Au->p0+X PHENIX 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? BRAHMS preliminary

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

13 Inferring charm production: cocktail method
g conversion p0  gee h  gee, 3p0 w  ee, p0ee f  ee, hee r  ee h’  gee inclusive e± spectra from Au+Au at 130 GeV use available data to establish “cocktail” of e± sources dominated by measured p0 and photon conversions excess above cocktail increasing with pT expected from charm decays subtract cocktail from data PHENIX: PRL 88(2002)192303

14 Electron spectra from Au+Au at 130 GeV
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 PYTHIA direct g (J. Alam et al. PRC 63(2001)021901) b c PHENIX: PRL 88(2002)192303 neglect contributions from alternative sources (direct g, beauty) corresponding charm cross section per binary collision from data

15 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)

16 Inferring charm production: converter method
Au+Au at sNN = 200 GeV measure the e± spectrum from photonic sources (g, p0, h, …) 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 larger than at 130 GeV consistent with PYTHIA calculation, assuming binary scaling: scc(130 GeV) = 330 mb and scc(200 GeV) = 650 mb large systematic uncertainty due to material thickness without converter (to be reduced in final result)

17 Centrality dependence
Reasonable agreement with “simple” binary scaling! Where is the energy loss effect on the charm quark? PHENIX data are consistent with the PYTHIA charm spectrum scaled by the number of binary collisions in all centrality bins!

18 “Dead Cone” effect? “Dead Cone” effect
gluon radiation from massive partons suppressed at angles q < 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 DE in addition (M. Djordjevic, M. Gyulassy nucl-th/ ) polarization of QCD medium  dispersion relation for radiated gluons can be approximated by an effective gluon mass suppresses the radiation of soft gluons

19 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 S. Batsouli et al. PLB557(2003)26 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

20 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/ ) 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)

21 Summary 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/Y 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 fe+e- at RHIC / PHENIX feasibility demonstrated statistics is one limiting factor S/B is poor (not unexpected)

22 Outlook open charm/beauty J/Y low-mass continuum / f  e+e-
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: m± and lepton-pair data STAR: D0/D0 in hadronic channels inclusive e± spectra: contribution from B decays at high pT (> 4 GeV)? J/Y d+Au measurement done to study “normal” nuclear effects measurement from STAR? low-mass continuum / f  e+e- increase S/B by removing material from acceptance high statistics Au+Au data are needed! J/Y in d+Au North m arm

23 How to improve the low-mass dielectron measurement?
the problem a possible solution identify e± from p0 Dalitz decays and g 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  e+ e - po   e+ e - “combinatorial pairs” total background Irreducible charm background all signal charm signal S/B~500

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