INT 10-2A, July 13, 2010 INT 10-2A, July 13, 2010 Itzhak Tserruya Dileptons: outstanding issues and prospects

Itzhak Tserruya INT 10-2A, July 13, 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

Introduction  The Quark Gluon Plasma created in relativistic heavy ion collisions is characterized by two fundamental properties:  Deconfinement  Chiral 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?

PHENIX + HBD STAR? Dileptons in A+A at a Glance: Itzhak Tserruya CERES DLS NA60 HADES CBM PHENIX Time Scale MPD = Period of data taking 4 INT 10-2A, July 13, 2010 Energy Scale DLS HADES 10158[A GeV] 17[GeV]√s NN 200 // CBM CERES NA60 PHENIXMPD 1

SPS Low-masses SPS Low-masses (m  1GeV/c 2 ) Itzhak Tserruya5 INT 10-2A, July 13, 2010  Consistent story between CERES and NA60 results

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/c 2 ): 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

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) 475PRL 91 (2003) 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 Itzhak Tserruya7 INT 10-2A, July 13, 2010

p T and Multiplicity Dependencies Enhancement is mainly at low p T Increases faster than linearly with multiplicity Itzhak Tserruya

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 data * vacuum ρ not enough to reproduce data Itzhak Tserruya9 INT 10-2A, July 13, 2010

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 data * vacuum ρ not enough to reproduce data Data favor the broadening scenario. Itzhak Tserruya10 INT 10-2A, July 13, 2010

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 !    Superb data! 11Itzhak Tserruya INT 10-2A, July 13, 2010

Dimuon Excess PRL 96 (2006) 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 p T

13 NA60 low mass: comparison with models All calculations normalized to data at m  = 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) Itzhak Tserruya INT 10-2A, July 13, 2010

SPS SPS Intermediate masses (m = 1-3 GeV/c 2 ) Itzhak Tserruya14 INT 10-2A, July 13, 2010  Thermal radiation from the partonic phase?

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 p T shapes similar to the contribution of the Open Charm (DY + 3.6OC nicely reproduces the data). Drell Yan + Open charm Drell Yan x Open charm charm enhancement? Itzhak Tserruya

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 <    0.17 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

Origin of the IMR Excess 17Itzhak Tserruya Hees/Rapp, PRL 97, (2006)Renk/Ruppert, PRL 100, (2008) Dominant process in mass region m > 1 GeV/c 2 : INT 10-2A, July 13, 2010 hadronic processes, 4  … partonic processes, qq annihilation Quark-Hadron duality?

NA60 excess: absolutely normalized mass spectrum 18Itzhak Tserruya INT 10-2A, July 13, 2010

NA60 excess: absolutely normalized mass spectrum 19Itzhak Tserruya INT 10-2A, July 13, 2010

p T distributions Low-mass region Intermediate mass region Fit in 0.5<P T <2 GeV/c (as in LMR analysis) The m T spectra are exponential, the inverse slopes do not depend on mass. The m T spectra are exponential, the inverse slopes depend on mass.  Radial Flow Thermal radiation from partonic phase? Itzhak Tserruya

RHIC results RHIC results Itzhak Tserruya21 INT 10-2A, July 13, 2010

Dileptons in PHENIX: p+p collisions 22Itzhak Tserruya  Mass spectrum measured from m = 0 up to m = 8 GeV/c 2  Very well understood in terms of:  hadron cocktail at low masses  heavy flavor + DY at high masses INT 10-2A, July 13, 2010

Dileptons in PHENIX: Au+Au collisions  Low masses:  strong enhancement in the mass range m = 0.2 – 0.7 GeV/c 2.  Enhancement extends down to very low masses  Enhancement concentrated at central collisions  No enhancement in the IMR ?

Low mass region: evolution with p T  Excess present at all pair p T but more pronounced at low pair p T

m T distribution of low-mass excess PHENIX  The excess m T distribution exhibits two clear components  It is well described by the sum of two exponential distributions with inverse slope parameters:  T1 = 92  11.4 stat  8.4 syst MeV  T1 =  37.3 stat  9.6 syst MeV Itzhak Tserruya25 INT 10-2A, July 13, 2010 All this is very different from the SPS results

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

Itzhak Tserruya INT 10-2A, July 13, Low-mass pair excess at RHIC  The low-mass pair enhancement observed in Au+Au at √s NN = 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?

28 Au+Au vs Cu+Cu N part = 98 Is there enhancement in the IMR also?

Cu+Cu Centrality Spectra

Itzhak Tserruya INT 10-2A, July 13, 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?

Prospects at RHIC Prospects at RHIC Itzhak Tserruya31 INT 10-2A, July 13, 2010

Dileptons in PHENIX: Au+Au collisions 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 Integral:180,000 above  0 :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 √s NN = 200 GeV arXiv: [nucl-ex]

Matching resolution in z and  HBD Installed and fully operational in Run9 and Run10 Single vs double e separation Hadron blindness h in F and R bias e-h separation h rejection

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: √s NN = 200 GeV √s NN = 200 GeV Better quality data over the entire mass range 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, R AA of  and , in particular comparison of   KK and   ee.   KK and   ee. IMR: confirm whether or not the yield is enhanced Additional measurement of charm cross section using high p T electrons with less background, different systematic and smaller errors √s NN = 62.4 GeV (and 39 GeV?) √s NN = 62.4 GeV (and 39 GeV?) Onset of the second source?

Thermal Radiation at RHIC Thermal Radiation at RHIC Itzhak Tserruya35 INT 10-2A, July 13, 2010

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 m  0, 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 36 INT 10-2A, July 13, 2010

Enhancement of (almost real photons) low-mass dileptons Restricted kinematic window: Low mass e + e - pairs m<300MeV & 1<p T <5 GeV/c p+p: Good agreement of p+p data and hadronic decay cocktail Au+Au: Clear enhancement visible above m  =135 MeV for all p T 1 < p T < 2 GeV 2 < p T < 3 GeV 3 < p T < 4 GeV 4 < p T < 5 GeV Excess  Emission of almost real photons Itzhak Tserruya37 INT 10-2A, July 13, 2010

Thermal radiation from the QGP at RHIC e + 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 (p T >4GeV) NLO pQCD (W. Vogelsang) Good agreement in range of overlap  pQCD consistent with p+p down to p T =1GeV/c  Au+Au data are above N coll scaled p+p for p T < 2.5 GeV/c  Fit Au+Au excess with exponential function + n coll scaled p+p T ave = 221  19 stat  19 syst MeV corresponds to T ini = 300 to 600 MeV  0 = 0.15 to 0.6 fm/c exp + n coll scaled pp

Itzhak Tserruya INT 10-2A, July 13, Low-energies: Low-energies: DLS and HADES

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

HADES confirms the DLS results 41Itzhak Tserruya INT 10-2A, July 13, 2010 Mass distributionp T distribution

Putting the puzzle together (I) 42Itzhak Tserruya INT 10-2A, July 13, 2010  Spectra normalized to  0 measured in C+C and NN 1 AGeV: /A part = 0.06 ± GeV (using pp and pd measurements) /A part = 1/4(pp+2pn+nn)/2 = 1/2(pp+pn) =  AGeV – pp & 1.25 GeV Dielectron spectrum from C+C consistent with superposition of NN collisions! No compelling evidence for in-medium effects in C+C

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 (2008) The DLS puzzle seems to be reduced to an understanting of the elementary contributions to NN reactions.

The  meson The  meson   l + l - and   K + K -   l + l - and   K + K - Itzhak Tserruya44 INT 10-2A, July 13, 2010  Inconclusive results

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.

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