Heavy-Quark and
Electromagnetic Probes
Ralf Averbeck
Stony Brook University
Probing strongly interacting matter
 formation of hot and dense sQGP is established
 various probes available to access properties
 landscape of heavy-
schematic dilepton mass distribution
quark and EM probes

heavy quarks
– hard probe with pQCD
baseline for all pT
– medium modification of
quarkonia and open
heavy flavor

light vector mesons and
low-mass continuum
– mass shifts, broadening,
excess yield?

‘thermal’ radiation from
the medium
– real and virtual photons
2
Ralf Averbeck,
Quarkonia
 20 years ago: Matsui & Satz
 color screening in deconfined matter
→ J/y suppression = “smoking gun”
 experimental & theoretical progress
since then
→ story is much more complicated

cold nuclear matter / initial state effects
–“normal” absorption in cold matter
–(anti)shadowing
–saturation, color glass condensate
suppression via comovers
 feed down from cc, y’
 sequential screening (first: cc, y’, J/y only well above Tc)
 regeneration via statistical hadronization or charm
coalescence

–relevant for “large” charm yield, i.e. RHIC and LHC
3
Ralf Averbeck,
J/y at SPS: pA reference measurement
E. Scomparin
 absorption with respect to what?
 need production cross section and nuclear absorption
cross section sabsJ/y from proton data
 before: no pA data at 158 GeV
– production cross section
measured at 450(400) GeV
and rescaled to 158 GeV
– sabsJ/y measured with pA
NA50 data at 450(400) GeV

(J/y)/DY = 29.2  2.3
L = 3.4 fm
preliminary NA60 pA data
at 158 GeV
– currently averaged over various
nuclear targets → no sabsJ/y yet
– rescaling of production cross
section to 158 GeV is correct!
Preliminary!
4
Ralf Averbeck,
J/y at SPS: suppression in In+In
 NA60: In+In @ 158 GeV/nucleon
E. Scomparin
 anomalous
J/y suppression
with respect to pure nuclear
absorption
– precision measurement of
suppression pattern
– qualitatively consistent with
Pb+Pb
– quantitative description of In+In:
challenge for models tuned to
describe Pb+Pb
wish:
Pb+Pb data from ‘NA60’
 evolution
of J/y momentum distribution with centrality
– pT broadening consistent with initial state gluon scattering
 no
J/y polarization in kinematical range probed by NA60
5
Ralf Averbeck,
J/y suppression: SPS vs. RHIC
many talks/posters
 RAA versus Npart
NA50 at SPS (0<y<1)
PHENIX at RHIC (|y|<0.35)
 stunning
similarity of J/y
suppression pattern for
– Pb+Pb from NA50 (0<y<1)
– Au+Au from PHENIX (|y|<0.35)
 an
accident?
– cold nuclear matter effects:
(slightly) larger at SPS than at RHIC
– more d+Au data needed at RHIC for
more quantitative statement
– plus recombination at RHIC?
 ‘sequential
dissociation’ at
SPS and RHIC?
Bar: uncorrelated error
Bracket : correlated error
Global error = 12% is not shown
– J/y survives well above Tc
– dissociation of y’ and cc
– feed down not well constrained
(~40 %)
6
Ralf Averbeck,
J/y at RHIC: rapidity dependence
 RAA: rapidity dependence
nucl-ex/0611020
 models
T. Gunji
 p+p
reference
→ rapidity and pT spectra
challenge for production
models ( A. Bickley )
 final Au+Au data
 more suppression at
forward rapidity!
– opposite to trend from
comover or CNM absorption
– suppression not only driven
by local particle density
– more regeneration at y~0?
– gluon saturation at forward y?
( many talks )
– no clear picture yet, but important new constraints
– two (or more) ingredients needed to describe suppression pattern
– suppression + regeneration
– sequential dissociation + saturation
7
the devil is in the details
Ralf Averbeck,
J/y at RHIC: hints for recombination?
A. Glenn
 y and pT distributions ↔ recombination
 rapidity: narrowing towards central collisions
 <pT2>: reduced relative to pT broadening from initial gluon scattering
 PHENIX: indications for both effects
 decisive: J/y flow at RHIC (answer from next RHIC Run?)
8
Ralf Averbeck,
Open heavy-flavor at RHIC
 reference for J/y suppression
 medium probe in its own right
 experimental approaches at RHIC
 direct reconstruction of charm
decays (e.g. D0→K+p-): STAR
K– difficult in HI environment
– remaining residual background
after subtraction of
combinatorial BG
p+
D0
c c
K
D0



indirect measurement via semileptonic decays: PHENIX & STAR
– requires good control of lepton
background from other sources
9
Ralf Averbeck,
Charm cross section at RHIC: PHENIX vs. STAR
 PHENIX e± agree with FONLL pQCD calculation
 STAR e±, m±, D mesons disagree with FONLL
F. Kajihara
C. Zhong
rescaled
hep-ex/0609010
 charm cross section: STAR ~ 2 x PHENIX
 advantage PHENIX: superior S/B for electrons
 advantage STAR: direct D measurement
 how to address this issue
Theoretical Uncertainty Band
 PHENIX: direct D measurement
 STAR: low material Run
 future: silicon vertex trackers in PHENIX & STAR
10
Ralf Averbeck,
RAA of electrons from heavy flavor decays
 PHENIX & STAR: rough agreement
→ disagreement is common to p+p & Au+Au,
cancels in the nuclear modification factor RAA
A. Suaide

describing the suppression is
difficult for models
– radiative energy loss with typical
gluon densities is not enough
(Djordjevic et al., PLB 632(2006)81)
– models involving a very opaque
medium agree better
(Armesto et al., PLB 637(2006)362)
– collisional energy loss / resonant
elastic scattering
(Wicks et al., nucl-th/0512076,
van Hees & Rapp, PRC 73(2006)034913)
– heavy quark fragmentation and
dissociation in the medium
→ strong suppression for charm
and bottom
(Adil & Vitev, hep-ph/0611109)
11
Ralf Averbeck,
Does charm flow?
 strong elliptic flow of electrons
S. Sakai
from D meson decays → v2D > 0
 v2c of charm quarks?
 recombination Ansatz:
(Lin & Molnar, PRC 68 (2003) 044901)
v2D ( pT )  av2q (
mu
m
pT )  bv2q ( c pT )  v2e
mD
mD
 universal v2(pT) for all quarks
 simultaneous fit to p, K, e v2(pT)
a=1
2σ
b = 0.96
c2/ndf: 21.85/27
χ2 minimum result
D->e
1σ
4σ
 within recombination model:
charm flows as light quarks!
12
Ralf Averbeck,
Constraining medium properties
F. Kajihara, S. Sakai
 large suppression and
large v2 of electrons
→ charm thermalization
 transport models require



small heavy quark
relaxation time
small diffusion coefficient
DHQ x (2pT) ~ 4-6
this value constrains the
ratio viscosity/entropy
– h/s ~ (1.5 – 3) / 4p
– within a factor 2-3 of
conjectured lower
quantum bound
– consistent with light hadron
v2 analysis (A. Taranenko et al.)
 electron RAA ~ p0 RAA at high pT
 is bottom suppressed as well?
13
Ralf Averbeck,
Bottom production at RHIC?
 electron-hadron azimuthal correlation
X. Lin
 decay kinematics: heavy flavor e-h correlation is the sum
of D and B decay contributions
 relative bottom contribution consistent with FONLL
 caveats
– subtraction of (large) background
– model dependent (PYTHIA)
– would imply excess of bottom production relative to FONLL
14
Ralf Averbeck,
Light vector mesons and dilepton continuum
 chiral condensate changes as
function of baryon density r
and temperature T
SIS18
heavy ion reactions:
SPS
A+AV+X
mV(r>>r0;T>>0)
RHIC
LHC
SIS300
/(FAIR)
, p. p - beams
elementary reaction:
, p  V+X
mV(r=r0;T=0)
C. Ratti, M. Thaler, W. Weise,
PRD73 (2006) 014019
 study in-medium modifications of hadrons
15
Ralf Averbeck,
Light vector mesons in elementary reactions
 TAPS @ ELSA: w mesons photoproduction off nuclei

low momentum w mesons decay in medium: w→p0

V. Metag
after background subtraction
difference in w line shape for
proton and nuclear target
consistent with w mass shift
– mw = m0 (1 – a r/r0); a = 0.13
sm
m
 3.0 %
 r in cold medium

D. Trnka et al.,
PRL 94 (2005) 192203

JLAB-CLAS: G7 C. Djalali
– A → e+e- X
– no mass shift
– some broadening
KEK-E325 M. Naruki
– pA → e+e- X
– mass shift (9%), no broadening
 no consistent picture yet!
16
Ralf Averbeck,
Dielectrons in low energy HI collisions
J. Markert
 HADES @ GSI
 scaling of excess yield
 DLS: C+C @ 1.04 AGeV
G. Agakichiev et al, subm. to PRL
– total/h = 6.5 ± 0.5 ± 2.1
C+C
excess
x6
 dielectron excess beyond
expectation from decays of
long lived mesons

total/h = 2.07 ± 0.21 ± 0.38
for 0.17 < mee < 0.55 GeV/c2
17

excess scales with p0,
which are produced via
baryon resonances (!)
Ralf Averbeck,
Low-mass dimuons at SPS: NA60
J. Seixas
 precision data: In+In @ 158 GeV
 excess beyond cocktail r studies in 12 centrality bins
– excess rises with centrality and is more pronounced at low pT

models predicting strong broadening & no mass shift
are in fair agreement with data
hep-ph/0604269

coupling to baryons important at low mass
(also indicated by CERES: D. Adamova et al., submitted to PRL)
18
Ralf Averbeck,
Intermediate-mass dimuons at SPS: NA60
R. Shahoyan
 excess of yield in IMR
observed by NA50

NA60 vertex spectrometer
– separation of prompt dimuons
from off-vertex (charm) dimuons

characteristics of the excess
– prompt, not charm
– slightly increasing with centrality
– increasing towards low pT
M (GeV/c2)
very preliminary
very preliminary
Corrected for
acceptance
central
collisions
Corrected for
acceptance
All data
19
All data
Ralf Averbeck,
Dielectrons at RHIC: PHENIX
 dielectrons in centrality bins
S. Campbell
60-100%
PHENIX Preliminary

PHENIX Preliminary
intermediate mass region, dominated by charm decays
– suppression towards central collisions, compatible with
suppression pattern observed for HF electrons and J/y

low mass region
A. Milov
– hint of enhancement but uncertainties are large
→ HadronBlindDetector to reduce BG by a large factor (Run-7)
20
Ralf Averbeck,
Thermal radiation at RHIC
 thermal radiation: another holy grail
 Quark Matter 2005
 first measurement of virtual photons
beyond hadronic decays
 data consistent with sum of

NLO pQCD (uncertain @ low pT)
– Gordon, Vogelsang, PRD 48(1993)3136

thermal radiation
– d’Enterria, Perresounko, EPJ C46(2006)451
 required for firm conclusions


reference data from p+p and d+Au
real photon measurement to
consolidate virtual photon excess
21
Ralf Averbeck,
Thermal radiation at RHIC
cold matter reference (d+Au)

D. Perresounko
double ratio: (all )/(decay )
– photons & internal conversion
– consistent with 1 within errors
– sys. uncertainty for internal
conversion analysis
– smaller than for calorimeter data
– larger than achieved for
Au+Au (larger background)
consolidation of
Au+Au analysis

measurement of external
photon conversions
– in agreement within errors with
previous results (calorimeter
and internal conversion
22
Ralf Averbeck,
Summary and outlook
 heavy quark and electromagnetic probes
 are an experimental and theoretical challenge
 help to quantitatively constrain the properties
of strongly interacting matter
 remain a hot topic in this field
 the future is bright
 PHENIX and STAR upgrade programs at RHIC
 RHIC-II
 further exploration of the QCD phase diagram
– LHC: towards maximum energy density
– FAIR: towards maximum baryon density
23
Ralf Averbeck,