Thermal Dimuon Emission in In-In at the CERN SPS Michele Floris for the NA60 Collaboration University and INFN, Cagliari, Italy

March, Rencontres de Moriond2 Outline  Thermal Dileptons in HI Collisions  The NA60 Experiment & the Data Sample  Results: LMR excess and the  spectral function p T distributions: a handle on the emission region

March, Rencontres de Moriond3 LMR: M<1 GeV IMR: M>1 GeV mainly:  →  * → ℓℓ probe of chiral symmetry restoration? (R. Pisarski, PLB ‘82) probe of deconfinement? (Kajantie, McLerran, al. ‘82 ff) Hadron-parton duality Thermal dileptons in High Energy Nuclear Collisions Dilepton emitted throughout the whole life-time of the fireball Production rate convoluted with space time evolution

March, Rencontres de Moriond4 statistical accuracy and resolution insufficient to unambiguously determine the in-medium spectral properties of the ρ LMR: mostly NA45/CERES IMR: NA34, NA38, NA50 no experimental capability to distinguish a prompt thermal dileptons from decay dileptons due to open charm Rapp-WambachBrown/Rhomeson cocktail 2000 data Previous SPS results on excess dileptons

March, Rencontres de Moriond5 2.5 T dipole magnet hadron absorber targets beam tracker vertex tracker muon trigger and tracking (NA50) magnetic field >10m<1m The NA60 Experiment Origin of muons can be accurately determined Improved dimuon mass resolution (~20 MeV/c 2 at  instead of 80 MeV/c 2 ) Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them (in both angles and momentum) to the tracks measured in the spectrometer High luminosity  experiment: possible with radiation tolerant detectors and high speed DAQ

March, Rencontres de Moriond6 net sample: events mass resolution: 20 MeV at the  position - combinatorial background subtraction of - fake matches between the two spectrometers for the first time, η, ω, ɸ clearly visible in dilepton channel in AA Low Mass data sample for 158 AGeV In-In - combinatorial background subtraction of

March, Rencontres de Moriond7 Phys. Rev. Lett. 96 (2006) Peripheral data (not shown): well described by meson decay cocktail (η, η’, ρ, ω, ɸ ) and DD existence of excess dimuons More central data (shown): isolation of excess by subtraction of the measured decay cocktail (without  ), based solely on local criteria for the major sources η Dalitz, ω and ɸ Excess dimuons

March, Rencontres de Moriond8 Eur.Phys.J.C 49 (2007) 235 clear excess above the cocktail ρ (bound to the ω with ρ/ω=1.0) excess centered at the nominal ρ pole rising with centrality no cocktail  subtracted DD subtracted all p T monotonic broadening with centrality “melting” of the  Excess mass spectra in 12 centrality bins

March, Rencontres de Moriond9 peak: R=C-1/2(L+U) continuum: 3/2(L+U)  - strong increase of continuum (by a factor of >10) - decrease of  peak (nearly a factor of 2) - rapid initial increase of total, reaching already 3 at dN ch /d  =N part =50 normalization to cocktail ρ, bound to ω with ρ  ω=1.0 Centrality dependence of excess yields

March, Rencontres de Moriond10 Predictions by Rapp (2003) for all scenarios Data and predictions as shown, after acceptance filtering, roughly mirror the  spectral function, averaged over space-time and momenta. (Eur.Phys.J.C 49 (2007) 235) Theoretical yields normalized to data for M<0.9 GeV Only broadening of  (RW) observed, no mass shift (BR) Phys. Rev. Lett. 96 (2006) Comparison of data to RW, BR and vacuum 

March, Rencontres de Moriond11 measurement of muon offsets  : distance between interaction vertex and track impact point charm not enhanced; excess: prompt, 2.4 × DY excess similar to open charm steeper than Drell-Yan isolation of excess by subtraction of measured open charm and Drell-Yan Extension to intermediate mass region

March, Rencontres de Moriond12 Renk/Ruppert, hep-ph/ Mass region above 1 GeV described in terms of hadronic processes, 4  … Hees/Rapp Phys.Rev.Lett. (2006) Mass region above 1 GeV described in terms of partonic processes, qq… How to distinguish? Hadron-Parton Duality for M >1 GeV

Transverse momentum spectra

March, Rencontres de Moriond14 reduce 4-dimensional acceptance correction in M-p T -y-cos  CS to 2-dimensional correction in M-p T, using measured y distributions and measured cos  CS distributions as an input ● requires separate treatment of the excess and the other sources, due to differences in the y and the cos  CS distributions acceptance vs. M, p T, y, and cosΘ understood to within <10%, based on a detailed study of the peripheral data Strategy of acceptance correction

March, Rencontres de Moriond15 steepening at low m T ; not observed for hadrons (like  fit m T spectra for p T >0.4 GeV with monotonic flattening of spectra with mass up to M=1 GeV, followed by a steepening above signs for mass-dependent radial flow? transverse mass: m T = (p T 2 + M 2 ) 1/2 Phys. Rev. Lett. 100 (2008) proper relative normalization Centrality-integrated excess m T spectra

What can we learn from p T spectra? Radial Flow Origin of dileptons

March, Rencontres de Moriond17 Two main contributions to p T spectra note: final-state lepton pairs themselves only weakly coupled → handle on emission region, i.e. nature of emitting source T - dependence of thermal distribution of “mother” hadrons/partons M - dependent radial flow (v  ) of “mother” hadrons/partons dilepton p T spectra superposition from all fireball stages early emission: high T, low v T late emission: low T, high v T final spectra from space-time folding over T-v T history from T i → T f (including low-flow partonic phase) hadron p T spectra determined at freeze-out T f (restricted information) Dilepton transverse momentum spectra

March, Rencontres de Moriond18 Strong rise of T eff with dimuon mass, followed by a sudden drop for M>1 GeV Rise reminiscent of radial flow of a hadronic source But: thermal dimuons emitted continuously during fireball expansion (reduced flow), while hadrons are emitted at final freeze-out (maximal flow); how can T eff be similar? Systematic errors studied in great detailed (CB, cocktail subtraction, acceptance, y and cos CS distributions, subtraction of DY and open charm)  on level <= statistical errors. Evolution of inverse slope parameter T eff with mass

Disentangling the m T spectra of the  peak and the continuum

March, Rencontres de Moriond20 peak: C-1/2(L+U) continuum: 3/2(L+U) m T spectra very different for the  peak and continuum: T eff of peak higher by MeV than that of the continuum ! all spectra pure exponential, no evidence for hard contributions use side-window subtraction method identify the  peak with the freeze-out  in the dilute final stage, when it does not experience further in-medium influences. Shape analysis and p T spectra

March, Rencontres de Moriond21 for a given hadron M, the measuredT eff defines a line in the T fo -v T plane crossing of hadrons with  defines T f, v T max reached at respective hadron freeze-out different hadrons have different coupling to pions (  maximal)  clear hierarchy of freeze-out (also for light-flavored hadrons) use of Blast wave code large difference between  and  (same mass) Hierarchy in hadron freeze-out

March, Rencontres de Moriond22 Strong rise of T eff with dimuon mass, followed by a sudden drop for M>1 GeV Rise consistent with radial flow of a hadronic source (here  →  →  ), taking the freeze-out ρ as the reference Drop signals sudden transition to low-flow source, i.e. source of partonic origin (here qq→  ) Combining M and p T of dileptons seems to overcome hadron-parton duality Phys. Rev. Lett. 100 (2008) The rise and fall of radial flow of thermal dimuons

March, Rencontres de Moriond23 Conclusions  Pion annihilation major contribution to the lepton pair excess in heavy-ion collisions at SPS energies in the region M<1 GeV  In-medium  spectral function identified; no significant mass shift of the intermediate , only broadening; connection to chiral restoration?  First observation of radial flow of thermal dileptons; mass dependence tool to identify the nature of the emitting source; mostly partonic radiation for M>1 GeV?

March, Rencontres de Moriond24 Lisbon CERN Bern Torino Yerevan Cagliari Lyon Clermont Riken Stony Brook Palaiseau Heidelberg BNL ~ 60 people 13 institutes 8 countries R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen, B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanović, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, 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

March, Rencontres de Moriond25 use measured mass and p T spectrum as input to the acceptance correction in y (iteration procedure) average rapidity density in NA60 acceptance smaller by only ~15% than at mid-rapidity results close to  rapidity distribution of pions, independent of p T NA60  (  =1.5) In-In Rapidity distribution of the excess

March, Rencontres de Moriond26 For the first time, the polarization of thermal radiation is measured and found to be zero (different from DY), as anticipated since decades. p projectile p target z axis CS p µ+ y x Viewed from dimuon  rest frame Collins Soper frame integration over azimuth angle errors purely statistical Polarization also found to be zero for the ω and ɸ Experimental results on cos  CS distributions: excess

March, Rencontres de Moriond27 Acceptance-corrected mass spectra

March, Rencontres de Moriond28 One common absolute normalization factor, fixed for 0.6<M<0.9 GeV 0<p T <2.4 GeV Differences at low mass mostly reflect differences in the low-mass tail of the spectral functions Data in comparison to theory : 0<p T <0.8 GeV

March, Rencontres de Moriond29 One common absolute normalization factor fixed for 0.6<M<0.9 GeV 0<p T <2.4 GeV Differences at higher masses and higher p T mostly reflect differences in the flow Data in comparison to theory : 1.6<p T <2.4 GeV

March, Rencontres de Moriond30 - very steep increase of high mass yield (DD and DY subtracted) - even steeper than that of total low-mass yield  increase of ratio - decrease of  peak (nearly a factor of 2) - strong increase of low-mass continuum - peak/continuum flat for 30<dNch/dh<100, decreasing above - increase mostly above dN ch /d  =100, roughly quadratic in dN ch /d  Centrality dependence of the individual yields