Thermal dileptons from NA60 and possible future opportunities at SPS Introduction “thermal” radiation Experimental Challenges State of the art experiment NA60 Experimental method Thermal radiation r spectral function Angular distribution Remarks on future perspectives l Drees, Stony Brook University --- for Gianluca Usai who could not des mostly derived from talks given by Sanja Damjanovic and Hans Spe
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Thermal dileptons from NA60 and possible future opportunities at SPS Introduction “thermal” radiation Experimental Challenges State of the art experiment.
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Thermal dileptons from NA60 and possible future opportunities at SPS
Acceptance correction with separate treatment of individual sources
in 4-dimensional M-pT -y-cosQCS
Project to 2-dimensional corrections Example M-pT
Data driven iterative procedure
acceptance vs. M, pT, y, and cosΘ understood to within <10%, based on a detailed study of the peripheral
Example dimuon excess
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Phys. Lett. B 677 (2009) 260
anomaly of wtransition formfactor
confirmed
data corrected for acceptance
Electromagnetic Transition Form Factors
Large statistics data setExcellent agreement with Lepton G experimentDeviation from Vector Dominance Model 10 sp-Au data still higher accuracy
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Transition Formfactor in PDG since 2010
First result from a heavy-ion experiment in the PDG ever
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Acceptance-Corrected M-pT Matrix of Excess
Continuum Excess Measured by NA60
Axel Drees13
Planck-like mass spectrum m > 1 GeV1.1-2.0 GeV: T=205±12 MeV1.1-2.4 GeV: T=230±10 MeV
Model comparisonMain Sources m < 1 GeV
p+p- r m+m-
broadening spectral functionMain sources m > 1 GeV
qq m+m- p a1 m+m- (consistent up to 1.5 GeV)
Systematic uncertaintiesFits to theory (RR and DZ)
Global fit: T = 215 MeVLocal fit 1.2 GeV: T = 205 MeV
2.5 GeV: T = 225 MeV
20-30% Drell Yan oversubtraction10-20 MeV reduction in T
Eur. Phys. J. C 59 (2009) 607; CERN Courier 11/2009
Thermal dilepton radiationTeff ~ 220 MeV > TC
)/exp(/ 2/3 TMMdMdN
Dileptons for M >1 GeV dominantly of partonic origin
Transverse Mass Distributions of Excess Dimuon
All mT spectra exponential for mT-m > 0.1 GeV
Fit with exponential in 1/mT dN/dmT ~ exp(-mT/Teff)
Soft component for <0.1 GeV ??Only in dileptons not in hadrons
Axel Drees14
transverse mass: mT = (pT2 + m2)1/2
Phys. Rev. Lett. 100 (2008) 022302 Eur. Phys. J. C 59 (2009) 607
hadronicp+p-→r→m+m-
partonicqq→m+m-
Eur. Phys. J. C 59 (2009) 607
“thermal” dimuons
Unfolding Time-Evolution Using m-pT Dependence
Inverse slope verses mass for thermal radiation
Mass < 1 GeV from hadronic phase
<Tth> = 130-140 MeV < Tc
Mass > 1 GeV from partonic phase <Tth> = 200 MeV >Tc
Schematic hydrodynamic evolutionPartonic phase
early emission: high T, low vT
Hadronic phaselate emission: low T, high vT
Axel Drees15
Teff ~ <Tth> + M <vT>2
Dileptons for M >1 GeV dominantly of partonic origin
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Combined Conclusions from Mass and pT Spectra
mass spectrum: T = 205±12 MeV
pT spectra: <Teff> = 190±12 MeV
M >1 GeV
T = 205 MeV > Tc = 170 (MeV)
- Teff independent of mass within errors
- same values within errors
negligible flow soft EoS above Tc
all consistent with partonic phase
SPS RHIC LHC rapid rise of energy density ε, slow rise of pressure p (not ideal gas)
EoS above Tc very soft initially (cS minimal)
Lattice QCD
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Choice of reference frame: Collins-Soper (CS)
In rest frame of virtual photon:
θ : angle between the positive muon pμ+ and the z-axis.
z axis : bisector between pproj and - ptarget
Angular Distributions
ϕ
pprojectile ptarget
z axis CS
pµ+
yx
Viewed from dimuon rest frame
, , l m n : structure functions related to helicity structure functions and the spin density matrix elements of the virtual photon
Expectation: completely random orientation of annihilating particles (pions or quarks) in 3 dimensions would lead to , , l m n = 0
2cossin
2cos2sincos1
cos d
dσ1 22
d
17 H.J.Specht, Erice 2012
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Results on structure coefficients , l m, n
μ = 0.05 ± 0.03 (~0 as expected) set m = 0 and fit projections
fit function for polar angle
fit function for azimuth angle
n=0.00±0.12
l=-0.13±0.12
example:excess 0.6<M<0.9 GeV
2cos1| cos|d
dN
2cos
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||d
dN
18 H.J.Specht, Erice 2012
Phys. Rev. Lett. 102 (2009) 222301
Zero polarization within errors
Axel Drees
Sensitivity to Spectral Function
Models for contributions from hot medium (mostly pp from hadronic phase)
Vacuum spectral functions Dropping mass scenariosBroadening of spectral function
Broadening of spectral functions clearly favored!
pp annihilation with medium modified r
works very well at SPS energies!
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Not acceptance corrected
Phys. Rev. Lett. 96 (2006) 162302
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van Hees and Rapp, Phys.Rev.Lett. 97 (2006) 102301
In this model, low-mass tail requires baryon interactions
Role of Baryons in Broadening the ρ
Whole spectrum reasonably well described, even in absolute terms
Role of baryons important at SPS energies!
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SPS Energy Range
80160
SPS
RHIC11
SIS-100
SIS-300
Prime physics goal:
systematic measurement of EM radiation and charm over the full energy range from SIS-100 (11 AGeV) to top SPS (160 AGeV)
1.0 ρmax0.9
SPS covers full range above and below maximum
baryon density
Assessing Performance of Experiment
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B S
Decisive parameter for data quality: Sample size N and Signal/Background
Choose data for B and cocktail at 0.6 GeV for S Independent of excessUnambiguous scaling between experiments by dNch/dy
effective signal: Neff ~ N × S/B
H.J.Specht, Erice 2012 24
Experiment Centrality Lepton flavor
B/S as meas. or simul.
B/S rescaled to dNch/dy=300
B/S field data ×1/3
B-field at vertex
HADES-SIS100 semicentr e+e- 20 60 60 −
CERES DR semicentr e+e- 80 100 100 −
CERES SR/TPC central e+e- 110 100 100 −
PHENIX with HBD central e+e- 250 100 100 −
PHENIX w/o HBD central e+e- 1300 600 200 +
STAR central e+e- 400 200 70 +
ALICE Upg ITS central e+e- 1200 200 70 +
CBM-SIS100 central e+e- 200 300 100 +
CBM-SIS300 central e+e- 200 200 70 +
NA60 semicentr μ+μ- 55 80 80
CBM-SIS300 central μ+μ- 600 600 600
Combinatorial Background/Signal in Dilepton Experiments
‘penalty’ factor 3 (4) for B-field, hindering optimal rejection of low-mass pairs
‘reduced’ values 80±20 (w/o red) only small influences of experimental details
Reference: hadron cocktail at masses of 0.5-0.6 GeV
NA60 setup already optimized and proven concept
Need scalability for beam energy
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G. Usai, CERN Town Meeting 2012
Setup under intensive MC investigationwatch out for future presentations by Gianluca
H.J.Specht, Erice 2012 26
Beam conditions: CERN vs. GSI/FAIR
Luminosity at the SPS comparable to that of SIS100/300
No losses of beam quality at lower energies except for emittance growth
RP limits at CERN in EHN1, not in (former) NA60 cave
< 11 – 35 (45)
SPS SIS100/300
beam target interaction intensity thickness rate
2.5×106 5×105
[λi ] [Hz] [Hz]
20% NA60 (2003)
new injection scheme
108 10% 107
108 1% 106
interaction rate [Hz]
105 - 107
Energy range: 10 – 158 [AGeV]
LHC AA 5×104
Pb beams scheduled for the SPS in 2016-2017, 2019-2021
SPS beam intensity increased by factor 40Increase of sample size by >>10 possible
4/6 years from now
Ideas about LHC fixed target under discussionwould reach RHIC energies
Conclusion from NA60 at SPS and OutlookInterpretation of excess dimuons as thermal radiation
Planck-like exponential mass spectraexponential mT spectrazero polarizationgeneral agreement with thermal models
Excess directly connected to deconfinement M < 1 GeV consistent with emission from p+p- annihilationM > 1 GeV consistent with mostly partonic emission associated temperatures quantifiedhints at soft EoS close to Tc
Excess (indirectly) connected to chiral symmetry restorationIn-medium r spectral function identified no significant mass shift of the intermediate , only broadening
Future opportunities at CERN SPSNA60 like experiment at 40x beam intensity Scan beam energies baryon densityMap out phase boundary
Axel Drees27
H.J.Specht, Erice 2012
http://cern.ch/na60
Lisbon
CERN
Bern
Torino
Yerevan
CagliariLyon
Clermont
Riken
Stony Brook
Palaiseau
Heidelberg
BNL
~ 60 people13 institutes8 countries
R. Arnaldi, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen,B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanovic, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees,
L. Ducroux, H. En’yo, A. Ferretti, M. Floris, A. Förster, P. Force, A. Grigorian, J.Y. Grossiord,N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço,
J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri
The NA60 experiment
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Axel Drees29
Backup
Centrality Dependence of Spectral Shape
peak: R=C-1/2(L+U) continuum: 3/2(L+U)
- near divergence of the width
reflects the number of r‘s regenerated in p+p- → r* → m+m-
- rapid increase of relative yield
‘ρ clock’
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‘melting’ of the r
Experimental Issue: Combinatorial BackgroundPerformance in terms of Background/Signal