NICA – Round Table IV JINR, Dubna, September 9-12, 2009 Dileptons in Heavy Ion Collisions NICA – Round Table IV JINR, Dubna, September 9-12, 2009 Itzhak Tserruya

Outline Introduction Low energies: DLS and HADES SPS results CERES - Low-mass region NA60 - Low and intermediate mass region RHIC: first results from PHENIX Elementary collisions: search for cold nuclear matter effects Summary and outlook Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Introduction Electromagnetic probes (real or virtual photons) are sensitive probes of the two fundamental properties of the QGP: Deconfinement Chiral symmetry restoration Lepton pairs are unique probes of CSR The therrmal radiation emitted in the form of dileptons (virtual photons) provides a direct fingerprint of the matter created: first the QGP (qqbar  e+e-) and subsequently a dense HG (+-  e+e-) What have we learned in about 20 years of dilepton measurements? What is the NICA potential? Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Low-mass dilepton experiments Nuclear Collisions CERES DLS HADES HELIOS NA38/50 NA60 PHENIX Elementary Reactions CLAS CBELSA/TAPS KEK E235 TAGX Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Dileptons in A+A at a Glance: 1 Energy Scale DLS HADES PHENIX 10 158 [A GeV] 17 [GeV] √sNN 200 // CERES NA60 2nd generation 1st generation Time Scale CERES DLS NA60 HADES NICA 90 95 10 00 05 85 PHENIX CBM NICA CBM Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Low-energies: DLS and HADES Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

DLS “puzzle” DLS data: Porter et al., PRL 79, 1229 (1997) Calculations: Bratkovskaya et al., NP A634, 168 (1998) put side by side instead of overlay? Enhancement not described by in-medium  spectral function All other attempts to reproduce the DLS results failed Main motivation for the HADES experiment Strong enhancement over hadronic cocktail with “free”  spectral function Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

HADES confirms the DLS results C+C 1 AGeV Mass distribution pT distribution Good agreement between DLS and HADES in di-electron mass and pT distributions Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

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 * Assume A_part = 2 and sigma_pp = sigma_nn * HADES measured dileptons in pp and pd collisions at 1.25 GeV. The selected np reactions by measurement of spectator proton in Forward Wall Dielectron spectrum from C+C consistent with superposition of NN collisions! No compelling evidence for in-medium effects in C+C Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Putting the puzzle together (II) Recent transport calculations: enhanced NN bremsstrahlung , in line with recent OBE calculations) HSD: Bratkovskaya et al. NPA 807214 (2008) Heavier systems? The DLS puzzle seems to be reduced to an understanding of the elementary contributions to NN reactions. Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

SPS Low-masses Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

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 NICA - RT, JINR, Sept. 9-12, 2009

CERES Pioneering Results (II) First CERES result PRL 75, (1995) 1272 Last CERES result PLB 666 (2008) 425 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 Strong enhancement of low-mass e+e- pairs in all A-A systems studied Eur. Phys J. C41 (2005) 475 PRL 91 (2003) 042301 Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

pT and Multiplicity Dependencies Enhancement is at low pT Increases faster than linearly with multiplicity

Dropping Mass or Broadening (I) ? Interpretations invoke: * +-  *  e+e- thermal radiation from HG CERES Pb-Au 158 A GeV 2000 data * 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. Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

NA60 Low-mass dimuons in In-In at 158 AGeV Real data ! h w f Superb data! Mass resolution: 23 MeV at the  position S/B = 1/7 ,  and even  peaks clearly visible in dimuon channel Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Dimuon Excess Excess centered at the nominal ρ pole Eur.Phys.J.C 49 (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 NICA - RT, JINR, Sept. 9-12, 2009

NA60 low mass: comparison with models PRL 96 (2006) 162302 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 * Data and predictions as shown, after acceptance filtering, roughly mirror the rho spectral function, averaged over space-time and momenta. (Eur.Phys.J.C 49 (2007) 235) * Discrepancy at masses m>0.9 will be addressed in a couple of slides Is this telling us something about CSR? All calculations normalized to data at m < 0.9 GeV performed by Rapp et al., for <dNch/d> = 140 Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

NA60 low mass: comparison with models Subtract the cocktail from the data (without the ) NP A806, 339 (2008) Excess shape consistent with broadening of the  (Rapp-Wambach) Mass shift of the  (Brown-Rho) is ruled out * Data and predictions as shown, after acceptance filtering, roughly mirror the rho spectral function, averaged over space-time and momenta. (Eur.Phys.J.C 49 (2007) 235) * Discrepancy at masses m>0.9 will be addressed in a couple of slides Is this telling us something about CSR? Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

RHIC look at YA QM099 Alberica QM08 Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Dileptons in PHENIX: p+p collisions 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 Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

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 NICA - RT, JINR, Sept. 9-12, 2009

Comparison to theoretical model (Au+Au) PHENIX From yoshihide nakamiya All models and groups that successfully described the SPS data fail in describing the PHENIX results

Dileptons in PHENIX: Au+Au collisions Min bias Au+Au √sNN = 200 GeV arXiv: 0706.3034 [nucl-ex] Integral:180,000 above p0:15,000 All pairs Combinatorial BG Signal 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

h in F and R bias e-h separation h rejection 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 h rejection

LVM in Elementary Collisions * The preficted LVM modifications are large enough that even at normal nuclear density one can expect to see them * LVM produced in nuclei with reactions that leave the nucleus in well controlled conditions (normal nuclear density, T=0) Rho decay inside; omega and phi require very low p Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

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 NICA - RT, JINR, Sept. 9-12, 2009

Raw spectra Toy model confirms that this is due to dropping rho mass (see slides 13) Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

Cannot fit the  with m and  from PDG  yield consistent with zero Raw spectra fitted with known sources. Toy model confirms that this is due to dropping rho mass (see slides 13) Hadronic sources: , , Ф -> e+e-,  -> e+e- , η ->  e+e- Width: Breit-Wigner shape convoluted with experimental resolution. Position: PDG values Relative abundances determined by fit Combinatorial background : event mixing method Cannot fit the  with m and  from PDG  yield consistent with zero Itzhak Tserruya NICA - RT, JINR, Sept. 9-12, 2009

 and  masses drop by 9.2% at normal nuclear matter density Dropping  and  masses KEK E325 PRL 96, 092301 (2006) The model predicts 61% rho decays and 9% omega decays, inside the Cu nucleus. 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%  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 NICA - RT, JINR, Sept. 9-12, 2009

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 NICA - RT, JINR, Sept. 9-12, 2009

Summary and outlook 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? Consistent picture from the SPS: Low-mass pair enhancement: thermal radiation from the HG Approach to CSR proceeds through broadening (melting) of the resonances Enhancement persists down to 40 AGeV 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 Elementary collisions: no coherent picture and no compelling evidence of in- medium modification effects of LVM in cold nuclear matter NICA’s energy range very well suited to fill an important niche: unveil the onset of the low-mass pair enhancement. Systematic studies of pA colliisions Study pair enhancement under highest baryon density conditions