Nuclear and Particle Physics 4a Electromagnetic Probes

Transcription

1 Nuclear and Particle Physics 4a Electromagnetic Probes Goethe University Frankfurt GSI Helmholtzzentrum für Schwerionenforschung Lectures and Exercise Winter Semester

2 Organization Language: English Lecture: Wednesday 09:00 (c.t.) - 11:00 Phys Marks / examination only if required / desired Seminar presentation schein Oral Exam grade Office hours: tbd on demand 2

3 Info: and Website Website: 3

4 Content 1) Introduction: Heavy Ion Physics OCTOBER 2) Detectors 3) Dielectrons: low energy 4) Dielectrons: intermediate energy 5) Dielectrons: high energy *** Dielectrons Theory (H. van Hees) *** 6) Photons: intermediate energy 7) Photons: high energy 8) Dark Matter 9) ElectroWeak Probes NOVEMBER DECEMBER JANUARY 4

5 Dielectrons: intermediate energy Motivation Dileptons and the Hadron Gas: Chirality, chiral symmetry breaking and chiral symmetry restoration Experimental challenges Dileptons and the QGP combinatorial background Dileptons in heavy-ion collisions experiments Intermediate-energy CERES NA60 5

6 The little bang in the lab High energy nucleus-nucleus collisions: fixed target colliders QGP formed in a tiny region (10-14m) for very short time (10-23s) Existence of a mixed phase? Later freeze-out Collision dynamics: different observables sensitive to different reaction stages 6

7 Probing the QGP atom discovery of nucleus SLAC electron scatteringe proton discovery of quarks Rutherford experiment probe penetrating beam (jets or heavy particles) QGP bulk absorption or scattering pattern Penetrating beams created by parton scattering before QGP is formed High transverse momentum particles jets Heavy particles open and hidden charm or bottom Probe QGP created in Au+Au collisions Calculable in pqcd Calibrated in control experiments: p+p (QCD vacuum), p(d)+a (cold medium) Produced hadrons lose energy by (gluon) radiation in the traversed medium QCD Energy loss medium properties Gluon density Transport coefficient 7

8 Electromagnetic Radiation Thermal black body radiation Real photons Virtual photons * which appear as dileptons e e or No strong final state interaction Leave reaction volume undisturbed and reach detector Emitted at all stages of the space time development Information must be deconvoluted Time time Jet p K e cc freeze-out hadronization formation and thermalization of quark-gluon matter? hard parton scattering Space Au Au 8

9 What can we learn from dilepton emission? arxiv: Emission rate of dilepton per volume ee decay Photon self-energy EM correlator Medium property Hadronic contribution Vector Meson Dominance Boltzmann factor temperature Medium modification of meson Chiral restoration + - e+ q q ee+ Hadron Gas QGP e- Thermal radiation from partonic phase (QGP) qq annihilation From emission rate of dilepton, one can decode medium effect on the EM correlator temperature of the medium 9

10 Theory predictions for dilepton emission rate arxiv: dn ee pt dpt dmdy 1/M * ee at y=0, pt=1.025 GeV/c Usually the dilepton emission is measured and compared as dn/dptdm Vacuum EM correlator Hadronic Many Body theory Dropping Mass Scenario q+q ee (HTL improved) (q+g q+ qee not shown) The mass spectrum at low pt is distorted by the virtual photon ee decay factor 1/M, which causes a steep rise near M=0 qq annihilation contribution is negligible in the low mass region due to the M2 factor of the EM correlator In the caluculation, partonic photon emission process q+g q+ qe+e- is not included qq * e+e (M2e-E/T) 1/M Theory calculation by Ralf Rapp 10

11 The mass of composite systems atom atomic nucleus nucleon m M mi binding energy effect m m M mi binding energy effect 10-3 M» mi the role of chiral symmetry breaking mass given by energy stored in motion of quarks and by energy in colour gluon fields chiral symmetry = fundamental symmetry of QCD for massless quarks chiral symmetry broken on hadron level 11

12 Chirality Chirality (from the Greek word for hand: ) when an object differs from its mirror image simplification of chirality: helicity (projection of a particle s spin on its momentum direction) massive particles P left and right handed components must exist m>0 particle moves w/ v<c P looks left handed in the laboratory P will look right handed in a rest frame moving faster than P but in the same direction chirality is NOT a conserved quantity in a massless word right-handed mu = md = ms = 0 chirality is conserved left-handed 12

13 QCD and chiral symmetry breaking the QCD Lagrangian: free gluon field interaction of quarks with gluon explicit chiral symmetry breaking free quarks of mass mn mass term mn n n in the QCD Lagrangian chiral limit: mu = md = ms = 0 chirality would be conserved left handed u,d,s, quarks remain left-handed forever all states have a chiral partner (opposite parity and equal mass) real life: mu and md are so small (mu 4 MeV md 7 MeV) that our world should be very close to chiral limit a1 (JP=1+) is chiral partner of (JP=1-): m 500 MeV even worse for the nucleon: N* (½-) and N (½+): m 600 MeV (small) current quark masses don t explain this chiral symmetry is also spontaneously broken spontaneously = dynamically 13

14 Origin of mass constituent quark mass ~95% generated by spontaneous chiral symmetry breaking (QCD mass) current quark mass generated by spontaneous symmetry breaking (Higgs mass) contributes ~5% to the visible (our) mass 14

15 Chiral symmetry restoration spontaneous symmetry breaking gives rise to a nonzero order parameter QCD: quark condensate <qq> -250 MeV3 many models (!): hadron mass and quark condensate are linked numerical QCD calculations at high temperature and/or high baryon density deconfinement and <qq> 0 approximate chiral symmetry restoration (CSR) constituent mass approaches current mass Chiral Symmetry Restoration expect modification of hadron spectral properties (mass m, width ) explicit relation between (m, ) and <qq>? QCD Lagrangian parity doublets are degenerate in mass how is the degeneracy of chiral partners realized? do the masses drop to zero? do the widths increase (melting resonances)? 15

16 Hadron masses G. Brown & M. Rho, PRL (1991) 2720 Brown-Rho scaling Vacuum: Vector and Axial spectral functions well separated (ALEPH data) At Tc: Chiral symmetry restoration 16

17 CSR and low mass dileptons what are the best probes for CSR? requirement: carry hadron spectral properties from (T, B) to detectors relate to hadrons in medium leave medium without final state interaction dileptons from vector meson decays m [MeV] 770 tot [MeV] [fm/c] BR e+e x x x 10-4 best candidate: meson short lived decay (and regeneration) in medium properties of in-medium and of medium itself not well known meson a special probe for CSR, long lifetime but m(ф) 2 m(k) simultaneous measurement of φ ee and φ KK could be a effects powerful tool to evidence in-medium 17

18 Dilepton Signal What is its temperature? measure thermal photons Does it restore chiral symmetry? modification of the vector mesons How does it affect heavy quarks? modification of the intermediate mass region All these questions can be answered by measuring dileptons (e+e or μ+μ ) no strong final state interactions: leave collision system unperturbed emitted at all stages: need to disentangle contributions 18

19 Dilepton Signal What is its temperature? measure thermal photons Does it restore chiral symmetry? modification of the vector mesons How does it affect heavy quarks? modification of the intermediate mass region All these questions can be answered by measuring dileptons (e+e or μ+μ ) no strong final state interactions: leave collision system unperturbed emitted at all stages: need to disentangle contributions 19

20 Dilepton Signal Dileptons characterized by 2 variables: M, pt M: spectral functions and phase space factors pt: three contributions to pt spectra pt - dependence of spectral function (dispersion relation) T - dependence of thermal distribution of mother hadron/parton M - dependent radial flow ( ) of mother hadron/parton hadron-like spectra at fixed T early emission: high T, low T late emission: T low T, high T final spectra from space-time folding over T- T history from Ti Tfo explosive source handle on emission region, i.e. nature of emitting source T, 1/mT dn/dmt dilepton pt spectra superposition of purely thermal source 1/mT dn/dmt Note I: M Lorentz-invariant, not changed by flow Note II: final-state lepton pairs themselves only weakly coupled light heavy mt light heavy mt 20

21 Dilepton signal Low Mass Region: mee < 1.2 GeV/c2 Dalitz decays of pseudo-scalar mesons Direct decays of vector mesons In-medium decay of mesons in the hadronic gas phase Intermediate Mass Region: 1.2 < mee < 2.9 GeV/c2 correlated semi-leptonic decays of charm quark pairs Dileptons from the QGP High Mass Region: mee> 2.9 GeV/c2 LMR: mee < 1.2 GeV/c 2 o LMR I (pt >> mee) quasi-real virtual photon region. Low mass pairs produced by higher order QED correction to the real photon emission Dileptons from hard processes Drell-Yan process correlated semi-leptonic decays of heavy quark pairs Charmonia Upsilons HMR probe the initial stage o LMR II (pt<1gev) Little contribution from thermal radiation Enhancement of dilepton discovered at SPS (CERES, NA60) 21

22 HI low mass dileptons at a glance Time scale of experiments CBM NA60 PHENIX HADES (KEK E235) CERES DLS Energy scale of experiments HADES CBM NA60 PHENIX ALICE [A TeV] DLS (KEK E235) 1 // // 10 CERES // // // // [A GeV] 200 snn [GeV] 22

23 Dilepton Analysis Challenges Experimental Challenge Need to detect a very weak source of pairs ~ 10-6 /π0 in the presence of hundreds of charged particles in central AA collision and several pairs per event from trivial origin π0 Dalitz decays + γ conversions (assume 1% radiation length) huge combinatorial background (dnch/dy)2 ~ 10-2/π0 2x10-2 /π0 Analysis Challenge Electron pairs are emitted through the whole history of the collision (from the QGP phase, mixed phase, HG phase and after freeze-out) need to disentangle the different sources. need excellent reference pp and da data. need independent information about the known sources in nuclear collisions 23

24 Dilepton Analysis Steps Tracking + Momentum reconstruction Resolution (position, momentum) Particle Identification Purity Rejection close pairs Significance, Signal/Background Pairing: mee = [2 p1p2 (1-cosθ)]1/2 Subtraction of Background (mixed events, like-sign) Efficiency Correction Mass Spectrum 24

25 Remarks on S/B how is the signal obtained? depends on magnitude of B, not S S 2B (for S<<B) S2 = F2 + B2 = S2 + B2 + B2 2 B2 = 2B2 B = B background free equivalent signal Seq combinatorial background: B (like-sign pairs or event mixing) S=F B statistical error of S unlike-sign pairs: F signal with same relative error in a situation with zero background Seq = S * S/2B example: S = 104 pairs with S/B = 1/250 Seq = 20 Seq / Seq = 2B / S Seq = Seq / Seq = 2B / S / Seq = 2B / S2 systematic uncertainty of S dominated by systematic uncertainty of B S/S = B/B * B/S example: B/B = 0.25% precision, S/B = 1/250 S/S = ~60% systematic uncertainty of S 25

26 Separating Signal from Background 26

27 CERN SuperProtonSynchrotron (since 1976) parameters circumference: 6.9 km beams for fixed target experiments protons up to 450 GeV/c lead up to 158 GeV/c past SppS proton-antiproton collider discovery of vector bosons W ±, Z now injector for LHC experiments Switzerland: west area (WA) France: north area (NA) dileptons speak french! 27

28 Experimental Setup: CERES setup minimal configuration, no particle tracking Optimized for minimum response to hadrons and photons of hadronic origin double RICH spectrometer ( thr=32), for eid and hadron rejection (hadron blind) Magnetic field azimuthal deflection momentum measurement between the two radiators conversion pairs: double rings in RICH1, open up in RICH2 28

29 Experimental Setup: CERES-1bis 1995 setup tracking: doublet of SiDC RICH1 RICH2 - PC Radial drift Silicon Detector: high resolution vertex and tracking RICH2 RICH1 29

30 Target region 13 segmented target 13 Au disks (thickness: 25 m; diameter: 600 m) Silicon drift chambers: provide vertex: = 216 m z provide event multiplicity ( = ) powerful tool to recognize conversions at the target 30

31 Electron Identification: RICH main tool for electron ID use the number of hits per ring (and their analog sum) to recognize single and double rings 31

32 pa results dielectron mass spectra and expectation from a cocktail of known sources Dalitz decays of neutral mesons ( e+e- and dielectron decays of vector mesons ( e+e-) semileptonic decays of particles carrying charm quarks dielectron production in p+p and p+a collisions at SPS understood in terms of known hadronic sources well 32

33 AA results CERES PRL 92 (95) 1272 discovery of low mass e+e- enhancement in 1995 significant excess in S-Au (factor ~5 for m>200 MeV) 33

34 AA results CERES Eur.Phys.Jour. C41(2005)475 dielectron excess at low and intermediate masses in HI collisions is well established onset at ~2 m - annihilation? maximum below meson near 400 MeV hint for modified meson in dense matter e e+ 34

35 CERES-1 CERES-2 addition of a TPC to CERES improved momentum resolution improved mass resolution de/dx hadron identification and improved electron ID particle ID and momentum measurement are separated inhomogeneous magnetic field a nightmare to calibrate 35

36 CERES-2 results the CERES-1 results persists strong enhancement in the low-mass region enhancement factor (0.2 <m < 1.1 GeV/c2 ) 3.1 ± 0.3 (stat.) but the improvement in mass resolution isn t outrageous 36

37 CERES at low-energy PRL 91 (2003) data taking in 1999 and 2000 improved mass resolution improved background rejection results remain statistics limited Pb-Au at 40 AGeV enhancement for mee> 0.2 GeV/c2 5.9±1.5(stat)±1.2(sys) ±1.8(decay) strong enhancement at lower s or larger baryon density vacuum Brown-Rho scaling broadening of 37

38 pt dependence mee<0.2 GeV/c2 0.2<mee<0.7 GeV/c2 mee>0.7 GeV/c2 hadron cocktail Brown-Rho scaling broadening of low mass e+e- enhancement at low pt qualitatively in a agreement with annihilation pt distribution has little discriminative power 38

39 Thermal radiation from HG e- low mass enhancement due to annihilation? Thermal radiation from HG spectral shape dominated meson e+ vacuum vacuum values of width and mass in-medium Brown-Rho scaling m ρ mρ qq ρ 1/3 qq 0 dropping masses as chiral symmetry is restored conjecture that links hadron masses to the quark condensate. Effective QCD Lagrangian, quarks are the relevant d.o.f. ρ ρ0 Rapp-Wambach melting resonances ρ-meson scatters off particles in the high density medium collision broadening of spectral function Pure hadronic model, only indirectly related to CSR medium modifications driven by baryon density model space-time evolution of collision 39

40 Theory comparison attempt to attribute the observed excess to vacuum meson ( ) inconsistent with data overshoot in region low mass modification meson needed to describe data data do not distinguish between broadening or melting of -meson (Rapp-Wambach) dropping masses (BrownRho) indication for medium modifications, but data are not accurate enough to distinguish models largest discrimination between and need mass resolution! 40

41 Theory comparison Interpretation invoke + - * e+ethermal radiation from hadron gas Vacuum not enough to reproduce the data In medium-modifications of : Broadening spectral fc. (Rapp-Wambach) Dropping mass (Brown-Rho) Thermal radiation: e+e- yield from qq annihilation in pqcd (Kaempfer) Data favor broadening scenario Uncertainties are large for a firm conclusion 41

42 Centrality dependence mee<0.2 GeV/c2 0.2<mee<0.6GeV/c2 strong centrality dependence challenge for theory! CERES F=yield/cocktail mee>0.6 GeV/c2 pt > 200 MeV/c 1995/ Nch naïve expectation: quadratic multiplicity dependence medium radiation particle density squared more realistic: smaller than quadratic increase density profile in transverse plane life time of reaction volume 42

43 Summary of CERES first systematic study of e+e- production in elementary and HI collisions at SPS energies pp and pa collisions are consistent with the expectation from known hadronic sources a strong low-mass low-pt enhancement is observed in HI collisions consistent with in-medium modification of the meson data can t distinguish between two scenarios dropping mass as direct consequence of CSR collisional broadening of in dense medium WHAT IS NEEDED FOR PROGRESS? STATISTICS MASS RESOLUTION 43

44 How to overcome these limitations more statistics run forever not an option higher interaction rate higher beam intensity thicker target needed to tolerate this extremely selective hardware trigger reduced sensitivity to secondary interactions, e.g. in target can t be done with dielectrons as a probe, but dimuons are just fine! better mass resolution stronger magnetic field detectors with better position resolution silicon tracker embedded in strong magnetic field! 44

45 The NA60 Experiment a huge hadron absorber and muon spectrometer (and trigger!) and a tiny, high resolution, radiation hard vertex spectrometer 45

46 Standard detection: NA50 muon trigger and tracking Muon Other target beam magnetic field hadron absorber thick hadron absorber to reject hadronic background trigger system based on fast detectors to select muon candidates (1 in 104 PbPb collisions at SPS energy) muon tracks reconstructed by a spectrometer (tracking detectors+magnetic field) extrapolate muon tracks back to the target taking into account multiple scattering and energy loss, but poor reconstruction of interaction vertex ( ~10 cm) z poor mass resolution (80 MeV at the ) 46

47 The NA60 experiment Based on the NA50 spectrometer with the addition of a Si tracker Additional bend by the dipole field Track matching in coordinate and momentum space Improved dimuon mass resolution Distinguish prompt from decay dimuons Dimuon coverage extended to low pt 47

48 The NA60 pixel vertex detector DIPOLE MAGNET 2.5 T HADRON ABSORBER TARGETS 12 tracking points with good acceptance ~40 cm 8 small 4-chip planes 8 large 8-chip planes in 4 tracking stations ~3% X0 per plane 750 m Si readout chip 300 m Si sensor ceramic hybrid readout channels in 96 pixel assemblies 1 cm 48

49 Vertexing in NA60 X Resolution ~ m in the transverse plane Beam Tracker sensors windows z ~ 200 m along the beam direction Good vertex identification with 4 tracks Extremely clean target identification (Log scale!) 49

50 Contribution to mass resolution two components multiple scattering in the hadron absorber dominant at low momentum tracking accuracy dominant at high momentum high mass dimuons (~3 GeV/c2) absorber doesn t matter low mass dimuons (~1 GeV/c2) absorber is crucial momentum measurement before the absorber promises huge improvement mass resolution track matching is critical for high resolution in low mass dimuon measurements! 50

51 Muon Track Matching Muon spectrometer Absorber Pixel telescope Measured points Measured points p1 p2(1) p2 z1 z2 track matching has to be done in position space momentum space to be most effective the pixel telescope has to be a spectrometer! 51

52 In+In: LMR Min. Bias unlike sign dimuon mass distribution before quality cuts and without muon track matching S/B ~ 1/7, and even peak clearly visible Mass resolution: 23 MeV at the peak BR = 5.8x10-6! drastic improvement in mass resolution still a large unphysical background 52

53 Fake matches fake match: matched to wrong track in pixel telescope important in high multiplicity events muon trigger and Muon spectrometer tracking fake target correct hadron absorber Hadron absorber how to deal with fake matches keep track with best 2 (but is it right?) embedding of muon tracks into other event identify fake matches and determine the fraction of these relative to correct matches as function of centrality transverse momentum 53

54 Event mixing: like-sign pairs compare measured and mixed like-sign pairs accuracy in NA60: ~1% over the full mass range 54

55 In+In: LMR peripheral Well described by meson decay cocktail : η, η, ρ, ω, f and DD contributions (Genesis generator developed within CERES and adapted for dimuons by NA60). Eur.Phys.J.C 43 (2005) 407 Eur.Phys.J.C 49 (2007) 235 Similar cocktail describes NA60 data: p-be, p-pb, In-In peripheral 55

56 Digression: EM transition form factor 0 Acceptance-corrected data (after subtraction of, and peaks) fitted by three contributions: m d ( ) 2 ( ) dm 3 m m m 2 4m m m 2m 2 d ( ) ( ) dm 3 m m d R( ) m dm 3(2 ) m 2 M2 3/ 2 1 4m 2 M2 1/ m M m M 2 2 4m 2 2m 2 M2 1/ 2 1/ 2 2 F (m ) 2 m 1 2 m m pole approximation: 3/ 2 2 4m 2 m m 2 m F (m ) F (m ) 1 m / MT 3 / 2 e MT Confirmed anomaly of F wrt VDM prediction. Improved errors wrt the Lepton-G results. Removes FF ambiguity in the cocktail In-In, peripheral hep-ph/

57 Cocktail subtraction (w/o ) how to nail down an unknown source? try to find excess above cocktail without fit constraints ω and : fix yields such as to get, after subtraction, a smooth underlying continuum : set upper limit, defined by saturating the measured yield in the mass region close to 0.2 GeV (lower limit for excess). - - use yield measured for pt > 1.4 GeV/c 57

58 Excess vs centrality Data cocktail (all pt) No cocktail and DD subtracted Clear excess above cocktail, - centered at the nominal pole - rising with centrality Excess even more pronounced at low pt Confirm CERES data! 58

59 Excess shape vs centrality Quantify the peak and the broad symmetric continuum with a mass interval C around the peak (0.64 <M<0.84 GeV) and two equal side bins L, U continuum = 3/2(L+U) peak = C-1/2(L+U) Fine analysis in 12 centrality bins continuum/ peak/ Peak/cocktail drops by a factor 2 from peripheral to central: the peak seen is not the cocktail nontrivial changes of all three variables at dnch/dy>100? peak/continuum 59

60 Theory comparison Rapp & Wambach hadronic model with strong broadening but no mass shift Brown & Rho dropping mass due to dropping chiral condensate calculations for all scenarios in In-In for dnch/d = 140 (Rapp et al.) spectral functions after acceptance filtering, averaged over space-time and momenta Keeping original normalization data consistent with broadening of (RW), mass shift (BR) not needed 60

61 Acceptance corrected spectrum Mass spectrum corrected for acceptance in M-pT 61

62 In+In: IMR hadron-parton duality Rapp / van Hees dominant at high M hadronic processes 4 Ruppert / Renk dominant at high M partonic processes mainly qqbar annihilation 62

63 IMR: the NA50 measurement NA50: excess observed in IMR in central Pb-Pb collisions charm enhancement? thermal radiation? central collisions M (GeV/c2) answering this question was one of the main motivations for building NA60 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 + 3.6xC nicely reproduces the data). 63

64 IMR: disentangling the sources K + charm quark-antiquark pairs are mainly - produced in hard scattering processes in the earliest phase of the collisions D0 c K l D0 c D0 l charmed hadrons are long lived 100 m D0 K- + e identify the typical offset ( displaced vertex ) of D-meson decays (~100 m) need superb vertexing accuracy (20-30 m in the transverse plane) NA60 64

65 IMR: disentangling the sources measure for vertex displacement primary vertex resolution momentum dependence of secondary vertex resolutions dimuon weighted offset charm decays (D mesons) displaced J/ prompt vertex tracking is well under control! 65

66 Eur.Phys.J. C59 (2009) 607 IMR excess is prompt approach Dix Drell-Yan (within 10%) Fix prompt component Charm can't describe the small offset region Fix charm component Good description of offset IMR excess is a prompt DY Fit range DD component DD Prompt ~50 m DD Data Prompt: Charm: Fit 2/NDF: 0.6 Prompt ~1mm 66

67 Phys. Rev. Lett. 96 (2006) Analysis of mt decomposition of low mass region contributions of mesons (,, ) continuum plus meson extraction of vacuum hadron mt spectra for,, vacuum dilepton mt spectra for low mass excess intermediate mass excess 67

68 Interpretation of Teff interpretation of Teff from fitting to exp(-mt/teff) static source: Teff interpreted as the source temperature radially expanding source: Teff reflects temperature and flow velocity Teff depends on the mt range large pt limit: Teff T f 1 vt 1 vt 1 T T m vt low pt limit: eff f 2 pt m 2 pt m common to all hadrons mass ordering of hadrons final spectra: space-time history Ti Tfo & emission time hadrons interact strongly freeze out at different times depending on cross section with pions Teff temperature and flow velocity at thermal freeze out dileptons do not interact strongly decouple from medium after emission Teff temperature and velocity evolution averaged over emission time 68

69 Mass ordering of hadronic slopes separation of thermal and collective motion reminder blast wave fit to all hadrons simultaneously simplest approach 1 Teff T f m vt AGeV Central collisions Pb-Pb pt m In-In slope of <Teff> vs. m is related to radial expansion baseline is related to thermal motion Si-Si C-C works (at least pp qualitatively) at SPS 69

70 Example of hydrodynamic evolution (specific for In-In: Dusling et al.) monotonic vt = 0.4 decrease of T from 0 = vt.1 hadron phase parton phase monotonic dileptons may allow to disentangle emission times early emission (parton phase) large T, small vt early times to late times medium center to edge late emission (hadron phase) increase of vt from early times to late times medium center to edge small T, large vt 70

71 LMR mt distributions -region Extract -peak Same-side window: continuum = 3/2(L+U) peak = C-1/2(L+U) IMR Fit with: dn mt / Teff e mt dmt Extract Teff from each mass slice 71

72 Dilepton Teff systematics hadrons ( ) T eff depends on mass T eff smaller for decouples early T eff large for decouples late low mass excess clear flow effect visible follows trend set by hadrons possible late emission intermediate mass excess no mass dependence indication for early emission Close to T, critical c temperature where phase transition occurs Eur.Phys.J. C (2009), in press, nucl-ex/

73 Digression: Polarization Submitted to PRL, nucl-ex/ NA60 also measured the polarization (in the Collins-Soper frame) for m m 1 d 1 cos 2 sin 2 cos sin 2 cos 2 d 2 Lack of any polarization in excess (and in hadrons) supports emission from thermalized source

74 Digression: in medium effects? Eur.Phys.J. C (2009), in press, nucl-ex/ 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 contribution. Disappearance of yield out of narrow ω peak in nominal pole position Can only measure disappearance 74

75 Digression: in medium effects? Eur.Phys.J. C (2009), nucl-ex/ Determine suppression vs pt with respect to (extrapolated from pt>1gev/c) dn / dmt2 ~ exp mt Teff Account for difference in flow effects using the results of the Blast Wave analysis Reference line: /Npart = f.ph.s. (central Reference line: ω/npart = f.ph.s. coll.) Strong centrality-dependent suppression at pt<0.8 GeV/c, Consistent with radial flow effects beyond flow effects 75

76 Summary NA60 high statistics & high precision dimuon spectra decomposition of mass spectra into sources LMR: access to in-medium spectral function data consistent with broadening of the data do not require mass shift of the IMR: large prompt component at intermediate masses dimuon mt spectra promise to separate time scales low mass dimuons shows clear flow contribution indicating late emission intermediate mass dimuons show no flow contribution hinting toward early emission 76