Transcript DPP Annual Report - University of Wrocław

Dileptons in Heavy Ion
Collisions
EMMI Workshop and XXVI Max Born
Symposium
Wroclaw, July 9-11, 2009
Itzhak Tserruya
Outline

Introduction

SPS results


Low-mass region
Intermediate mass region

RHIC: first results from PHENIX

Low energies: DLS and HADES

 meson

Elementary collisions: search for cold nuclear
matter effects

Summary and outlook
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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Introduction

Electromagnetic probes (real or virtual photons) are
sensitive probes of the two fundamental properties of the
QGP:
 Chiral symmetry restoration
 Deconfinement

Lepton pairs are unique probes of CSR.

Thermal radiation emitted in the form of dileptons (virtual
photons) provide a direct fingerprint of the matter formed:
QGP and dense HG

What have we learned in about 20 years of dilepton
measurements?
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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Low-mass dilepton experiments
Nuclear Collisions
CBM
CERES
DLS
HADES
HELIOS
NA38/50
NA60
PHENIX
Itzhak Tserruya
Elementary Reactions
CLAS
CBELSA/TAPS
KEK E235
TAGX
EMMI Workshop, Wroclaw, July 9-11, 2009
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Dileptons in A+A at a Glance:
Energy
Time Scale
Scale
CBM
MPD
NA60
HADES
CBM
MPD
NA60
PHENIX
PHENIX
HADES
CERES
DLS
DLS
CERES
//
85
90
//
95
10
//
//
158
00
//
05
10
[A GeV]
//
17
200
√sNN [GeV]
= Period of data taking
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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SPS
Low-masses
CERES Pioneering Results (I)
Strong enhancement of low-mass e+e- pairs
(wrt to expected yield from known sources)
Last CERES result
(2000 Pb run PLB 666(2008) 425)
Enhancement factor (0.2 <m < 1.1 GeV/c2 ):
2.45 ± 0.21 (stat) ± 0.35 (syst) ± 0.58 (decays)
No enhancement in pp
nor in pA
Itzhak Tserruya
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CERES Pioneering Results (II)
First CERES result
Last CERES result
PRL 75, (1995) 1272
PLB 666 (2008) 425
Strong enhancement
of low-mass e+eBetter tracking and better mass resolution (m/m = 3.8%) due to:
pairs in all A-A
 Doublet of silicon drift chambers
systems close
studiedto the vertex
 Radial TPC upgrade downstream of the double RICH spectrometer
Eur. Phys J. C41 (2005) 475
PRL 91 (2003) 042301
pT and Multiplicity Dependencies
Enhancement is at low pT
Increases faster than linearly with
multiplicity
Itzhak Tserruya
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Dropping Mass or Broadening (I) ?
Interpretations invoke:
* +-  *  e+e-
CERES Pb-Au 158 A GeV 2000 data
thermal radiation from HG
* vacuum ρ not enough to reproduce data
* in-medium modifications of :
broadening  spectral shape

(Rapp and Wambach)
dropping  meson mass

(Brown et al)
Data favor the broadening
scenario.
NA60 Low-mass dimuons in In-In at 158 AGeV
Superb data!

Mass resolution:
23 MeV at the  position




S/B = 1/7
,  and even  peaks
clearly visible in dimuon
channel

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Dimuon Excess
Eur.Phys.J.C
PRL 96 (2006)
49162302
(2007) 235
Dimuon excess isolated by subtracting
the hadron cocktail (without the )
Excess centered at the
nominal ρ pole

Excess rises and broadens
with centrality


More pronounced at low pT
confirms & consistent with,
CERES results
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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NA60 low mass: comparison with models
PRL 96 (2006) 162302


All calculations normalized to data at m < 0.9 GeV
performed by Rapp et al., for <dNch/d> = 140
Subtract the cocktail from the data
(without the )

Excess shape consistent with
broadening of the 
(Rapp-Wambach)

Mass shift of the  (Brown-Rho)
is ruled out
Is this telling us something
about CSR?
•
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SPS
Intermediate masses
NA50 IMR Results
p-A is well described by the sum of Drell-Yan and Open Charm contributions (obtained
from Pythia)
The yield observed in heavy-ion collisions exceeds the sum of DY and OC decays,
extrapolated from the p-A data.
The excess has mass and pT shapes similar to the contribution of the Open Charm (DY +
3.6OC nicely reproduces the data).
Drell Yan + 3.6 x Open charm
Drell Yan + Open charm
charm enhancement?
Itzhak Tserruya
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NA60: IMR excess in agreement with NA50
2.90.14

IMR yield in In-In collisions enhanced
compared to expected yield from DY and OC

Can be fitted with fixed DY (within 10%) and
OC enhanced by a factor of ~3
Fit range
4000 A, 2
<1.5
Full agreement with NA50
… But the offset distribution is not compatible with this assumption
NA60:
Fixed prompt and free open charm
prompt and open charm scaling factors
IMR excess is Free
a prompt
source
1.120.17
2.750.14
4000 A, 2 <1.5
4000 A,2 <1.5
Origin of the IMR Excess
Hees/Rapp, PRL 97, 102301 (2006)
Renk/Ruppert, PRL 100,162301 (2008)
Dominant process in mass region m > 1 GeV/c2:
hadronic processes, 4 …
partonic processes, qq annihilation
Quark-Hadron duality?
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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pT distributions
Low-mass region
Intermediate mass region
Fit in 0.5<PT<2 GeV/c
(as in LMR analysis)
The mT spectra are
exponential, the inverse
slopes depend on
mass.
The mT spectra are
exponential, the inverse
slopes do not depend
on mass.
 Radial Flow
Partonic radiation?
Itzhak Tserruya
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RHIC
Dileptons in PHENIX: p+p collisions


Itzhak Tserruya
Mass spectrum measured from m=0 up to m=8 GeV/c2
Very well understood in terms of:
 hadron cocktail at low masses
 heavy flavor + DY at high masses
EMMI Workshop, Wroclaw, July 9-11, 2009
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Dileptons in PHENIX: Au+Au collisions
Strong enhancement of e+e- pairs at low masses:
m= 0.2 – 0.7 GeV/c2.
 Very different from SPS:
 Enhancement down to very low masses
 Enhancement concentrated at central collisions
 No enhancement in the IMR

Itzhak Tserruya
Comparison to theoretical model (Au+Au)
PHENIX
All models and groups that successfully described the
SPS data fail in describing the PHENIX results
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Dileptons in PHENIX: Au+Au collisions
Min bias Au+Au √sNN = 200 GeV
arXiv:
[nucl-ex]
All pairs
Combinatorial BG
Signal
Integral:180,000
above 0:15,000
• BG determined by event mixing technique,
normalized to like sign yield
• Green band: systematic error w/o error on CB
PHENIX has mastered the event mixing technique to
unprecedented precision (±0.25%). But with a S/B ≈ 1/200 the
statistical significance is largely reduced and the
systematic errors are large
Matching resolution in z and 
Single vs double e separation
HBD
Installed and fully operational in Run-9
Hadron blindness
h in F and R bias e-h separation
Itzhak Tserruya
h rejection
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Low-energies:
DLS and HADES
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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DLS “puzzle”
DLS data: Porter et al.,
PRL 79, 1229 (1997)
Calculations:
Bratkovskaya et al.,
NP A634, 168 (1998)
 Enhancement not described by in-medium  spectral function
 Allenhancement
other attempts
to reproduce
the DLS
results
Strong
over
hadronic cocktail
with
“free”failed
 spectral function
 Main motivation for the HADES experiment
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HADES confirms the DLS results
Mass distribution
Itzhak Tserruya
pT distribution
EMMI Workshop, Wroclaw, July 9-11, 2009
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Putting the puzzle together (I)
C+C @ 1 AGeV – pp & pd @ 1.25 GeV
Spectra normalized to 0 measured in
C+C and NN

C+C @ 1 AGeV:
<M>/Apart = 0.06 ± 0.07
N+N @ 1.25 GeV (using pp and pd
measurements)
<MNN>/Apart = 1/4(pp+2pn+nn)/2
= 1/2(pp+pn) = 0.0760.015
Dielectron spectrum from C+C consistent with
superposition of NN collisions!
EMMI Workshop, Wroclaw, July 9-11, 2009
No compelling evidence
for in-medium effects in C+C38
Itzhak Tserruya
Putting the puzzle together (II)
Recent transport calculations:
enhanced NN bremsstrahlung , in line with recent OBE calculations
HSD: Bratkovskaya et al. NPA 807214 (2008)
The DLS puzzle seems to be reduced to an understanting of the elementary
contributions to NN reactions.
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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The  meson

+
ll
and  
+
KK
Inconclusive results
SPS
The reanalyzed NA50 results in 
and the CERES results in the   ee are
compatible within 1-2σ and within errors
there is room for some effect.
PHENIX
Uncertainties in the   e+e- channel too large
for a conclusive statement. Waiting for HBD
improved results
Itzhak Tserruya
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LVM in Elementary
Collisions
KEK E235
p+C, Cu @ E=12 GeV
Cold nuclear matter
Excellent mass resolution:
m = 8.9+-0.2 MeV/c2 @ mФ=1017 MeV/c2
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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Raw spectra
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
45
Raw spectra fitted with known sources.
Hadronic sources: , , Ф -> e+e-,  -> e+e- , η ->  e+e-
Cannot fit the  with m and  from PDG
 yield consistent with zero
Width: Breit-Wigner shape convoluted with experimental resolution.
Position: PDG values
Relative abundances determined by fit
Combinatorial background : event mixing method
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Dropping  and  masses
KEK E325
PRL 96, 092301 (2006)
Model:  and  produced at nuclear surface, decay with modified mass if decay point is
inside nucleus:
mV() / mV(0) = 1 – k(/0)
common k parameter for C and Cu target and for  and .
k= 9.2%
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 and  masses drop by 9.2% at normal nuclear matter density
CBELSA / TAPS
Similar effect seen in 
0  photo-produced on Nb and LH2 targets:
At low momenta, clear excess in the low-mass side of the  meson for the Nb target
No effect at high momenta
k = 13%
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
48
CLAS
No effect seen in 
e+e- photo-produced on H2, C, Fe and Ti targets
Mass spectra look very similar to those measured by KEK .
However, CLAS results can be very well reproduced by a transport model using the
vacuum mass values of ,  and .
k=22%
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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Summary and outlook
Consistent and coherent picture from the SPS:
 Low-mass pair enhancement: thermal radiation from the HG
 Approach to CSR proceeds through broadening (melting) of the resonances
 IMR enhancement: thermal radiation from partonic phase
DLS puzzle solved in C+C. Dilepton spectrum understood as mere
superposition of NN collisions. Is that so also for heavier system? Onset of
low-mass pair enhancement?
RHIC results very intriguing:
 Strong enhancement of low-mass pairs down to very low masses
 No enhancement in the IMR
 Challenge for theoretical models
 Looking forward to more precise results with the HBD
 meson results inconclusive
Elementary collisions: no coherent picture and no compelling evidence of inmedium modification effects of LVM in cold nuclear matter
Itzhak Tserruya
EMMI Workshop, Wroclaw, July 9-11, 2009
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