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INT 10-2A, July 13, 2010
Itzhak Tserruya
Dileptons: outstanding issues and prospects
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Itzhak Tserruya INT 10-2A, July 13, 2010 2
Outline Introduction
SPS results Low-mass region (CERES and NA60) Intermediate mass region (NA50, NA60)
RHIC results first results from PHENIX
Prospects with the HBD
Low energy (DLS and HADES)
meson Summary
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Introduction The Quark Gluon Plasma created in relativistic heavy ion
collisions is characterized by two fundamental properties:
DeconfinementChiral Symmetry Restoration
Electromagnetic probes (real or virtual photons) are sensitive probes of both properties and in particular lepton pairs are unique probes of CSR.
Thermal radiation emitted in the form of dileptons (virtual photons) provides a direct fingerprint of the matter formed: QGP (qqbar annihilation) and dense HG (+- annihilation)
What have we learned in almost 20 years of dilepton measurements?
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PHENIX + HBD STAR?
Dileptons in A+A at a Glance:
Itzhak Tserruya
CERES
DLS
NA60
HADES
CBM
90 95 1000 0585
PHENIX
Time Scale
MPD
= Period of data taking
4INT 10-2A, July 13, 2010
Energy Scale
DLS
HADES
10 158 [A GeV]
17 [GeV]√sNN200
// // //
// // //
CBM
CERES
NA60
PHENIXMPD
1
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SPS Low-masses(m 1GeV/c2)
Itzhak Tserruya 5INT 10-2A, July 13, 2010
Consistent story between CERES and NA60 results
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6
CERES Pioneering Results (I) Strong enhancement of low-mass e+e- pairs
(wrt to expected yield from known sources)
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
Last CERES result (2000 Pb run PLB 666(2008) 425)
Itzhak Tserruya INT 10-2A, July 13, 2010
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CERES Pioneering Results (II)
Strong enhancement of low-mass e+e- pairs in all A-A
systems studied
First CERES result PRL 75, (1995) 1272
Last CERES result PLB 666 (2008) 425
Eur. Phys J. C41 (2005) 475 PRL 91 (2003) 042301
Better tracking and better mass resolution (m/m = 3.8%) due to: Doublet of silicon drift chambers close to the vertex Radial TPC upgrade downstream of the double RICH spectrometer
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pT and Multiplicity Dependencies
Enhancement is mainly at low pT
Increases faster than linearly with multiplicity
Itzhak Tserruya
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Dropping Mass or Broadening (I) ? Interpretations invoke:
* +- * e+e-
thermal radiation from HG
dropping meson mass (Brown et al)
* in-medium modifications of : broadening spectral shape (Rapp and Wambach)
CERES Pb-Au 158 A GeV 95/96 dataCERES Pb-Au 158 A GeV 95/96 data
* vacuum ρ not enough to reproduce data
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Dropping Mass or Broadening (I) ? Interpretations invoke:
* +- * e+e-
thermal radiation from HG
* in-medium modifications of :
broadening spectral shape (Rapp and Wambach)
dropping meson mass (Brown et al)
CERES Pb-Au 158 A GeV 2000 dataCERES Pb-Au 158 A GeV 2000 data
* vacuum ρ not enough to reproduce data
Data favor the broadening scenario.
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NA60 Low-mass dimuons in In-In at 158 AGeV
, and even peaks clearly visible in dimuon channel
S/B = 1/7
Mass resolution:23 MeV at the position
Real data !
h
wf
Superb data!
11Itzhak Tserruya INT 10-2A, July 13, 2010
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Dimuon Excess PRL 96 (2006) 162302
Dimuon excess isolated by subtracting the hadron cocktail (without the )
Eur.Phys.J.C 49 (2007) 235
Excess centered at the nominal ρ pole
confirms & consistent with, the CERES results
Excess rises and broadens with centrality
More pronounced at low pT
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13
NA60 low mass: comparison with models
• All calculations normalized to data at m < 0.9 GeV performed by Rapp et al., for <dNch/d> = 140
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?
Subtract the cocktail from the data (without the )
PRL 96 (2006) 162302
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SPS
Intermediate masses(m = 1-3 GeV/c2)
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Thermal radiation from the partonic phase?
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NA50 IMR Results Drell-Yan and Open Charm are the main contributions in the IMR
p-A is well described by the sum of these two 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 + Open charm
Drell Yan + 3.6 x Open charm
charm enhancement?
Itzhak Tserruya
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NA60: IMR excess in agreement with NA50
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
2.90.14
2.750.14 1.120.17
Free prompt and open charm scaling factors
Full agreement with NA50
… But the offset distribution (displaced vertex) is not compatible with this assumption
Fixed prompt and free open charm
NA60: IMR excess is a prompt source
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Origin of the IMR Excess
17Itzhak Tserruya
Hees/Rapp, PRL 97, 102301 (2006) Renk/Ruppert, PRL 100,162301 (2008)
Dominant process in mass region m > 1 GeV/c2:
INT 10-2A, July 13, 2010
hadronic processes, 4 … partonic processes, qq annihilation
Quark-Hadron duality?
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NA60 excess: absolutely normalized mass spectrum
18Itzhak Tserruya INT 10-2A, July 13, 2010
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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 do not depend on mass.
The mT spectra are exponential, the inverse slopes depend on mass.
Radial Flow
Thermal radiation from partonic phase?
Itzhak Tserruya
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RHIC results
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Dileptons in PHENIX: p+p collisions
22Itzhak 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
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Dileptons in PHENIX: Au+Au collisions
Low masses: strong enhancement in the mass range
m = 0.2 – 0.7 GeV/c2. Enhancement extends down to very low masses Enhancement concentrated at central collisions
No enhancement in the IMR ?
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Low mass region: evolution with pT
Excess present at all pair pT but more pronounced at low pair pT
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mT distribution of low-mass excess
PHENIX
The excess mT distribution exhibits two clear components
It is well described by the sum of two exponential distributions with inverse slope parameters:
T1 = 92 11.4stat 8.4syst MeV
T1 = 258.3 37.3stat 9.6syst MeV
Itzhak Tserruya 25INT 10-2A, July 13, 2010
All this is very different from the SPS
results
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Comparison to theoretical model (Au+Au)PHENIX
All models that successfully described the SPS data fail in describing the PHENIX results
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Itzhak Tserruya INT 10-2A, July 13, 2010 27
Low-mass pair excess at RHIC The low-mass pair enhancement observed in Au+Au at
√sNN = 200 GeV implies at least two sources.
Source I: e+ e- (with intermediate modified in the medium mainly through scattering off baryons) as observed at CERN, must be present at RHIC also.
Pion annihilation (Rapp – Van Hees) is insufficient to describe the data
Source II - The remaining excess – Origin not at all clear
Obvious question: when does this second source appear?
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28
Au+Au vs Cu+Cu
Npart = 98
Is there enhancement in the IMR also?
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Cu+Cu Centrality Spectra
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Itzhak Tserruya INT 10-2A, July 13, 2010 30
Au+Au vs Cu+Cu: surprising results
In Cu+Cu like in Au+Au the enhancement is observed only in most central collisions.
But for all observables I know, there is no difference in the results from Cu+Cu and Au+Au when compared at the same number of participants (global observables, J/ suppression, …. )
Are low-mass electron pairs different?
IMR: no enhancement in Au+Au. Is there an enhancement in Cu+Cu?
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Prospects at RHIC
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Dileptons in PHENIX: Au+Au collisions
All pairsCombinatorial BGSignal • BG determined by event mixing
technique, normalized to like sign yield
• Green band: systematic error w/o error on CB
Integral:180,000 above p0:15,000
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
Min bias Au+Au √sNN = 200 GeVarXiv: [nucl-ex]
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Matching resolution in z and
HBD Installed and fully operational in Run9 and Run10
Single vs double e separation
Hadron blindnessh in F and R bias e-h separation h rejection
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34
What can we expect from Run-10
Itzhak Tserruya INT 10-2A, July 13, 2010
In Run-10 PHENIX accumulated a large sample of Au+Au collisions at:
√sNN = 200 GeV
Better quality data over the entire mass range
Significant improvement of S/B in the LMR
Further characterization (better centrality dependence) of the low mass excess
Good quality data on LVM, RAA of and , in particular comparison of
KK and ee.
IMR: confirm whether or not the yield is enhanced
Additional measurement of charm cross section using high pT electrons with less background, different systematic and smaller errors
√sNN = 62.4 GeV (and 39 GeV?)
Onset of the second source?
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Thermal Radiation at RHIC
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Itzhak Tserruya
Thermal radiation at RHIC (I) Search for the thermal radiation in the dilepton spectrum Avoid the huge physics background inherent to a real photon
measurement. Capitalize on the idea that every source of real photons should also
emit virtual photons. At m0, the yield of virtual photons is the same as real photon
Real photon yield can be measured from virtual photon yield, observed as low mass e+e- pairs
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Enhancement of (almost real photons) low-mass dileptons
Restricted kinematic window: Low mass e+e- pairs m<300MeV & 1<pT<5 GeV/c
p+p:• Good agreement of p+p data
and hadronic decay cocktail • Au+Au:• Clear enhancement visible
above mp =135 MeV for all pT
1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV
Excess Emission of almost real photons
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Thermal radiation from the QGP at RHICe+e- invariant mass excess: - transformed into a spectrum of real photons under the assumption that the excess is entirely due to internal conversion of photons.- compared to direct (real) photon measurement (pT>4GeV)
NLO pQCD (W. Vogelsang)
Good agreement in range of overlap
pQCD consistent with p+p down to pT=1GeV/c
Au+Au data are above Ncoll scaled p+p for pT < 2.5 GeV/c
Fit Au+Au excess with exponential function + ncoll scaled p+p
Tave = 221 19stat 19syst MeV corresponds to
Tini = 300 to 600 MeV t0 = 0.15 to 0.6 fm/c
exp + ncoll scaled pp
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Itzhak Tserruya INT 10-2A, July 13, 2010 39
Low-energies:
DLS and HADES
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DLS “puzzle”
Strong enhancement over hadronic cocktail with “free” spectral function
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 All other attempts to reproduce the DLS results failed Main motivation for the HADES experiment
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HADES confirms the DLS results
41Itzhak Tserruya INT 10-2A, July 13, 2010
Mass distribution pT distribution
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Putting the puzzle together (I)
42Itzhak Tserruya INT 10-2A, July 13, 2010
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) = 0.0760.015
C+C @ 1 AGeV – pp & pd @ 1.25 GeV
Dielectron spectrum from C+C consistent with superposition of NN collisions!
No compelling evidence for in-medium effects in C+C
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Putting the puzzle together (II)
43Itzhak Tserruya INT 10-2A, July 13, 2010
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.
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The meson l+l- and K+K-
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Inconclusive results
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Inconclusive results SPS
PHENIX
Uncertainties in the e+e- channel too large for a conclusive statement. Waiting for HBD improved results
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.
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Summary 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
RHIC results very intriguing: Strong enhancement of low-mass pairs down to very low masses Enhancement observed only in central Au+Au and Cu+Cu collisions No enhancement in the IMR ? Challenge for theoretical models Looking forward to more precise results with the HBD
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?
meson – elusive probe