Low-mass dimuons in Indium collisions

1. Low-mass dimuonsin Indium collisions central CERESPb-Au 158 GeV peripheral mee (GeV/c2) Physics motivation Phase-space coverage The QDC Phase Diagram ϕ ω The dipole field in the target regionleads tomuch better pT coverage than previous dimuon experiments Monte-Carlo simulation No dipole field With 2.5 T field A(%) A(%) Chiral symmetry is spontaneously broken in hadronic matter… …and is restored in the deconfined phase Acceptance improvesin all M and pT windows by a factor 50 forM ~ 500 MeV andpT ~ 500 MeV/c after muontrack matching Dimuons now competitivewith respect todielectrons! Critical behaviour of the Chiral Condensate Yields and mass resolution <1 % of total statistics From a very preliminary analysis of a very small event sample… Nocentrality selection S/B ~ 1/1.3 Mass resolution20–25 MeV at Mµµ ~ 1 GeV Chiral susceptibility H reflects critical behaviour Directly related to hadronic spectral function • With respect to CERES: • Higher statistics by factor ~200 • Signal/background improved by factor ~10 • Higher effective statistics by factor 2000 • Mass resolution ~2%better by a factor 2 • Full information on associated track multiplicity • Completely different systematic uncertainties opposite-sign signal combinatorial background The combinatorial background from π and K decays is estimated through a mixed-event technique using like-sign muon pairs. The normalization is preliminary and fake matches are not yet included. Restoration of Chiral Symmetry Charged track multiplicity dependence The analysis of the dimuon mass distributions can be done as a function of the collision centrality 180 < Nch < 320 90 < Nch < 180 3 ω ϕ Strongly interacting matter under extreme conditions: Restructuring of QCD vacuum towards chiral symmetry restoration Disappearance of <qq> → change of spectral properties of light hadrons like masses and widths Degenerate parity doublets Most sensitive hadron: ρ(lifetime only 1.3 fm/c) The dilepton spectrum may provide a signal for the existence of a chirally restored phase created in heavy ion collision Energy dependence may elucidate the relative importance of T and µB Needsgood mass resolution,high statistics andassociated multiplicity Nch < 90 dN/dM 1 2 3 2 ω ϕ ω Charged particle multiplicity for reconstructed dimuon events 1 ϕ ω and ϕ peaks still visible in central Indium-Indium collisions NA60 M (GeV) pT spectra Previous results on low-mass electron pairs from CERES ϕ→µµ events Good pT coverage down to the lowest dimuon masses after applying a first orderacceptance correction no centrality selection Combined 95/96 data Effective number of electron pairsfor mee > 0.2 GeV/c2 : 215±15 Mass resolution at the ω: ~ 6% Expectation for 2000 data About the same effective number of pairs Mass resolution at the ω: ~ 4% raw data First hints for the ‘melting’ of the ρ: Chiral symmetry restoration? Much more statistics, better signal to backgroundratio and better mass resolution required for a convincing case • NA60 will solve the long standing ϕ→ µµ puzzle between NA49 and NA50 • 100 000 ϕ→ µµ decays in the full data sample • ϕ→K+K- decays also under analysis NA60