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Dileptons and direct photons at SPS

Dileptons and direct photons at SPS. Motivation and milestones Excess in dilepton production f -puzzle Photons. QUARK MATTER 2009, March 30 – April 4, Knoxville, TN. Why do we measure them. γ. ℓ + ℓ -. g *. Convey dual information convoluted over the history of the collisions:

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Dileptons and direct photons at SPS

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  1. Dileptons and direct photons at SPS Motivation and milestones Excess in dilepton production f-puzzle Photons QUARK MATTER 2009, March 30 – April 4, Knoxville, TN R. Shahoyan, CERN

  2. Why do we measure them γ ℓ+ ℓ- g* • Convey dual information convoluted over the history of the collisions: • probe surrounding matter properties: T, density, flow… • probesthemselves are affected by hot and dense matter • Modification of Vector Meson spectral functions close to Chiral Symmetry Restoration point: VM broadening vs mass shift. • Thermal emission: both from QGP and Hadronic phase Rich variety of dilepton contributions in hadronic phase: Low Mass Range(LMR, m≤ m ) : resonant (VM, mostly r);Intermediate Mass Range(IMR, m<m< mJ/ ) :continuum-like; rates determined by T (parton–hadron duality?)  need to disentangle experimentally QQP and HG contributions. • Strangeness () enhancementfrom abundantly produced in QGP • Quarkonia suppression by colour screening in QGP (covered by R.Arnaldi talk) Caveats: experimental: small yields, strong backgrounds … interpretational: underlying ‘conventional’ sources need to be understood …

  3. Milestones I 200 A GeV/c Helios-3 NA38/NA50 pA 450 GeV/c Eur.Phys.C14(2000) 442 NA50Pb-Pb 158 A GeV/c<Npart> = 381 centralcollisions Nucl.Phys.A590 1995) 93c Eur.Phys.C13(2000) 433 Helios-3p-W, S-W @ 200 A GeV : LMR/IMR enhancement Thermal emission?Strong pa1 mm contribution at IMRLi, Gale,Phys.Rev.Lett. 81(1998) 1572 NA38p-W, S-U @ 200 A GeV NA50p-Al,Cu,Ag,W @ 450 GeV Pb-Pb @ 158 A GeV • IMR in p-A described by Drell-Yan and Open Charm • Yields in Pb-Pb exceed the extrapolation from the p-A • IMR excess resembles (m, pT) : nuclear modifications? • Also described by thermal emission from GQP and HG Rapp and Shuryak, Phys. Lett. B473 (2000) 13

  4. Milestones II / CERES measurements coils TPC1999/2000 only • Two RICH detectors separated by solenoidal field • 2 SiDC in vertex region • trigger on multiplicity p-Be,Au 450 GeV : good descripitonby hadronic ‘cocktail’ decays Central S-Au 200 A GeV excess: 5.0 ± 0.7(stat)± 2.0(syst) Eur.Phys.J.C 4 (1998) 231 Phys.Rev.Lett.75(1995)1272 >400citations

  5. LMR excess by CERES / S-Au 200 A GeV Brown/Rho Vacuum r Rapp/Wambach Not described by pp annihilation in vacuum r mass drop (Brown/Rho scaling): Li, Ko, Brown, Phys.Rev.Lett.75 (1995) 4007 r broadening: Chanfray, Rapp, Wambach, Phys.Rev.Lett. 76 (1996) 368 5

  6. LMR excess by CERES / Pb-Au 158 A GeV Eur.Phys.J. C 41 (2005) 475 Excess seen in S-Au confirmed in Pb-Au 158 A GeV 2.73 ±0.25(stat)±0.65(syst) ± 0.82(decays) Excess: 2.61 ± 0.31(stat)± 0.43(syst)± 0.76(decays) Rapp-Wambach Phys.Rev.Lett. 91 (2003) 042301 Phys. Lett. B666 (2008) 425 Brown-Rho scaling Vacuum-r from pp-annihilation 1999 data 2000 data RWBR The only dilepton data at 40 A GeV higher excess: 5.9±1.5(stat)±1.2(syst)±1.8(decays) Effect of higher baryonic density? RWBR Phys.Rev.Lett. 91 (2003) 042301 1999 TPC upgrade: sM/M 6%  4% at / region

  7. Measuring dimuons in NA60 2.5 T dipole magnet muon trigger and tracking (NA50) beam tracker Si-pixel tracker magnetic field targets hadron absorber >10m <1m Phys.Rev.Lett. 96 (2006) 162302 • Radiation-hard silicon pixel vertex tracker • Muons from the NA50 spectrometer matched to tracks in the vertex region: • Improved mass resolution • offset wrt the vertex ( @ ~20 GeV)  promt vs open charm (ct=123,312 mm) separation • Dipole improves low-mass and low-pT acceptance • Matching rejects decay kinks  improved S/B • Selective trigger, high luminosity

  8. NA60 LMR data: peripheral (Nch<30) In-In collisions Eur.Phys.J.C 49 (2007) 235 Well described by meson decay ‘cocktail’: η, η’, ρ, ω, f and contributions(Genesis generator developed within CERESand adapted for dimuons by NA60). Eur.Phys.J.C 43 (2005) 407 Similar cocktail describes NA60 p-Be,In,Pb 400 GeV data

  9. EM transition form-factors for and peripheral NA60 InIn data (hep-ph/0902.2547, submitted to PLB) pole approximation: In-In, peripheral Acceptance-corrected data (after subtraction of , and  peaks) fitted by three contributions: • Confirmed anomaly ofF wrt the VDM prediction. • Improved errors wrt the Lepton-G results. • Removes FF ambiguity in the ‘cocktail’

  10. More central In-In data Clear excess of data above decay ‘cocktail’ describing peripheral events Phys. Rev. Lett. 96 (2006) 162302 • Excess isolated subtracting the measured decay cocktail (without r), independently for each centrality bin • Based solely on localcriteria for the major sources: η, ω and f 2-3% accuracy. •  No need of reference data (pA, peripheral data, models) Less uncertainties (e.g. strangeness enhancement: ,)

  11. NA60 LMR Excess dimuons Eur.Phys.J.C 49 (2007) 235 • Excess above the cocktail ρ (bound by ρ/ω=1.0), centered at nominal ρ pole • Monotonically rises and broadens with centrality • By coincidence, NA60 acceptance roughly removes the phase-space factor  r spectral function convoluted over the fireball evolution is directly measured

  12. Centrality dependence of LMR excess CERES, Pb-Au 158A GeV data/cocktail 95/96 data combined r NA60, In-In 158A GeV Eur.Phys.J. C41 (2005) 475 peak: R=C-1/2(L+U) continuum: 3/2(L+U)cocktail ρ is fixed by ρ/ω=1.0 Excess rises faster than linear with multiplicity: compatible with emission from annihilation processes Total excess wrt “cocktail” r: roughly indicative of the number of rho generations: r – clock? Question to theory: can the fireball life-time be inferred? How does the modified r life-time evolve? Eur.Phys.J.C 49 (2007) 235

  13. LMR Excess: r dropping mass vs broadening NA60, InIn 158A GeV Phys. Lett. B666 (2008) 425 Phys. Rev. Lett. 96 (2006) 162302 CERES,Pb-Au 158A GeV cocktail subtractedusing stat.model Calculations by R.Rapp for both scenarios Only broadening of r observed; interactions with baryons is very importantMass shift (Brown/Rho scaling) is ruled out. (CERES data also described by flat spectral function: Kämpfer et all,Nucl. Phys. A 688 (2001) 939)

  14. NA60 IMR data (1.16 < M < 2.56 GeV/c2) Data Prompt: 2.290.08 Charm: 1.160.16 Fit 2/NDF: 0.6 Prompt 1.120.17 ~1mm ~50mm Fit range Prompt Eur.Phys.J. C59 (2009) 607 Mass spectrum is similar to NA50: Good description by Drell-Yan + ~2Open Charm (extrapolated from pA data) Such explanation is rejected by the spectra of dimuon offsets wrt the interaction vertex! Offset fit shows that the enhancement is not due to Open Charm  the excess is prompt 14

  15. NA60 IMR excess (1.16 < M < 2.56 GeV/c2) Mass (GeV/c2) Mass shape and yield close to Open Charmcontribution measured agrees within ~20% with fromNA50 pA data (same kinematical domain |cosCS|<0.5) no strong modifications. Scales with centrality faster than Drell-Yan (~Nbin.coll), but less faster than Eur.Phys.J. C59 (2009) 607 Much softer in pT than Drell-Yan: rules out higher-twist DY? [Qiu, Zhang, Phys. Lett. B 525, (2002) 265]

  16. NA60 excess: comparison to theory Eur.Phys.J. C59 (2009) 607 • All known sources subtracted (‘cocktail’, Drell-Yan, Open Charm) • Corrected for acceptance • Integrated over pT • Absolute normalization: data and models. H. van Hees and R. Rapp, Nucl. Phys. A 806 (2008) 339 : mostly pp annihilation in LMR (collisional broadening of r, strong effect from baryons), 4p in IMR (full chiral mixing enhanced a1), QGP contribution 20 – 60% (fireball scenario A … C) T. Renk and J. Ruppert, Phys. Rev. C 77, (2008) 024907 : mostly pp annihilation in LMR (r spectral function by Eletsky et al, Phys.Rev.C 69 (2001) 035202), QGP dominates (~80%) in IMR K. Dusling, D. Teaney and I. Zahed, Phys. Rev. C 75, (2007) 024908 + PhD thesis of K.Dusling : hydrodynamic calculation with virial expansion of hadronic rates, QGP contribution dominates in IMR: 60 – 90%

  17. NA60 excess vs pT: comparison to theory J.Phys. G 35 (2008) 104036 Absolute normalization both for theory and data Differences at low masses reflect differences in the tail of r spectral function Differences at high masses, pT reflect differences in flow strength

  18. mT spectra of NA60: excess Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053 ‘Cocktail’ r and excess continuum are separated using side-window method: r is hotter M Teff0.2 – 0.4 192±40.4-0.6 223±50.6-0.9 256±4f 242±21.0-1.4 224±11 (DY not subtracted) Phys. Rev. Lett. 100 (2008) 022302 • Fit of excess by (‘cocktail’ r is not subtracted) • Teff rises up to the f mass, then drops • Spectra steepen at low mT (not for hadrons) Fit in 0.5<PT<2 GeV/c Confirmed by independent IMR excess analysis

  19. mT spectra from NA60Hierarchy of hadrons freeze-out Blast wave analysis of NA60 data: crossing of hadrons with p defines Tf, and bTmax reached at respective hadron freeze-out Suggests different freeze-out time for different hadrons: f first, r last (maximal coupling to pions) J.Phys. G 35 (2008) 104036 Large difference between r and w

  20. Teff evolution with mass <> Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053 • LMR • Thermal emission dominates in HG phase(RH, RR, DZ models agree) • Integration over the flow development  rise of Teff with mass(RH: relatively week flow compensated by primordial contributions, but insufficient for pure in-medium LMR emission) • Hadrons affected by different freeze-out times IMR RR, DZ: thermal emission dominates in the QGP phase, when flow has not yet built up  Teff are not affected by blue shift  no strong mass dependence RH : emission from HG dominates

  21. Polarization of dileptons Submitted to PRL, nucl-ex/0812.3100 NA60 also measured the polarization (in the Collins-Soper frame) for m≤ m Lack of any polarization in excess (and in hadrons) supports emission from thermalized source. Details in the presentation of G.Usai /session 4C/

  22. Evidence of ω in-medium effects? Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053 nucl-ex/0812.3053 Flattening of the pT distributions at low pT, developing very fast with centrality. Low-pT ω’s have more chances to decay inside the fireball? Appearance of that yield elsewhere in the spectrum, due to ω mass shift and/or broadening, unmeasurable due to masking by the much stronger ppmm contribution. Disappearance of yield out of narrow ω peak in nominal pole position  Can only measure disappearance

  23. NA60 results on omega yield suppression Determine suppression vs pT with respect to (extrapolated from pT>1GeV/c)Account for difference in flow effects using the results of the Blast Wave analysis Eur.Phys.J. C (2009), in press nucl-ex/0812.3053 Reference line: f/Npart = 0.0284 f.ph.s. (central coll.) Consistent with radial flow effects Reference line: ω/Npart = 0.131 f.ph.s. Strong centrality-dependent suppression at pT<0.8 GeV/c , beyond flow effects

  24. f - puzzle • Disagreement between results for f in Pb-Pb 158A GeV • NA49fK+K-(pT<1.6 GeV/c) NA50 fm+m-(pT>1.1GeV/c) [Phys. Lett. B491, (2000) 59; J. Phys. G 27,(2001) 355] [Nucl. Phys. A661 (1999)534c; Phys. Lett. B 555 (2003) 147] • NA50 sees >2 times higher yield than NA49 • Large difference in Teff : Tmm =218±6 TKK = 305±15 In-medium effects on f and K + K absorption and rescattering  reduced yield and harder pT spectrum in hadronic channel? Recent developments:CERES mesured fK+K-andfe+e-channels in central Pb-Au 158A GeV [Phys.Rev.Lett. 96 (2006) 152301] Both channels agree with each other (large errors on e+e- channel) and with NA49 NA50 reanalysed its old + 2000 Pb-Pb data [J. Phys. G 35 (2008) 104163] Old results are confirmed within 8% New results from NA60in In-In 158 A GeV data: comparison of fm+m-andfK+K-

  25. f - puzzle • NA60 finds good agreement between the m+m-and K+K-channels  no f – puzzle in In-In • Teff in central In-In collisions is lower compared to cetral Pb-Pb of NA49 and CERES • <f>/Npartin central In-In collisions is lower than the NA50 results (f.ph.s), but slightly higher than measured by NA49 and CERES Details in the presentation of A.de Falco /session 5D/

  26. Direct photon (old) measurements at SPS Extremely difficult to measure: large background from p0 and ’  very few results (especially at low pT where QCD contribution is small) 15-20% upper limits (90%CL) wrt the hadronic sources for central S-Au 200 A GeV/c from CERES[Z.Phys.C71 (1996) 571] and WA80[Phys.Rev.Lett. 76 (1996) 3506] Most significant results from WA98 in central Pb-Pb 158 A GeV [Phys.Rev.Lett.85 (2000) 3595]:excess of up to 20±7% for pT>1.5 GeV/c Calculation by Turbide, Rapp and Gale within the same approach as for l+l-Phys.Rev.C69 (2004) 014903 • Even more difficult to interpret than in the case of dileptons: • same ambiguity: close to the Tc hadronic and partonic descriptions provide similar rates… • no extra handle like mass, only pT

  27. Direct photons from internal conversion ? Teff = 183  10 MeV Ti = 210–350 MeV ? PHENIX: interpret enhancement at pT>>mee as internally converted direct photons(‘almost’ real: 0.1<mee<0.3 GeV/c2, pT>1 GeV/c) Teff = 2212318 MeV interpret as TQGP(weighted over the QGP evolution)  Ti = 300 – 600 MeV NA60 observes an excess at same masses Smoothly continues to m>0.3 GeV/c2 (in PHENIX also) Exponential down to low pT’s (‘high’ virtuality) Teff = 183 MeV fits well to the smooth rise with M (flow)  Tth ~ 160 MeV No evidence of direct ‘real’ photons ?

  28. Prospects from LHC? Much higher Tinitial and life-time of the fireball  stronger thermal emission of , l+l-(Surprisingly, PHENIX sees excess only at M < 750 MeV/c2, see next presentation by A.Drees) Even higher background to cope with: Sll /B < 10-2 vs 10-1 on SPS ALICE, ATLAS, CMS: excellent capabilities to measure photons (ALICE and CMS from pT > ~1 GeV/c) See presentation of D. Peressounko on photons in Alice /session 5D/ Possibilities of l+l- measurements below J/ are limited by acceptance, tracking, PID Currently, only ALICE has some preliminary estimates for low mass dileptons: +-: no acceptance below mT ~ 1 GeV/c due to the 0.5 GeV/c cut on single m pT e+e- : good PID using TPC+TRD for pT > 1GeV/c Expectations from Alice Muon Arm 1 month of Pb-Pb

  29. Summary • Excess in dilepton emission at SPS: good agreement with models of thermal emission • ‘Planck’-like mass spectra (slightly modified by non-flat spectral function in LMR) • Teff rising with mass in LMR and, after sharp drop, flat in IMR • Lack of polarization: emission from isotropic source • Faster than linear scaling with Ncharged • LMR: major contribution from pp annihilation • IMR: naturally explained by mostly partonic radiation • First evidence of low-pTin-medium modifications in central In-In collisions • f-puzzle • Pb-Pb: impossible to reconcile results of mm from NA50 with KK from NA49ee and KK channels measured by CERES agree with each other and NA49 • In-In : mm and KK channels measured by NA60 agree (both yield and Teff) <f>/Npart exceeds Pb-Pb value, Teff is in-between of conflicting Pb-Pb values • Direct photons: no conclusive results. Excess seen by WA98 in Pb-Pb is compatible with thermal emission but the errors are very large

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