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