The ρ spectral function and evidence for radial flow of thermal dileptons in high-energy nuclear collisions

by Sanja Damjanovic (CERN)

Monday 22 Oct 2007, 16:30 → 17:30 Europe/Zurich

500/1-001 - Main Auditorium (CERN)

The NA60 experiment has studied low-mass muon pairs in 158 AGeV Indium-Indium collisions at the CERN SPS. A strong excess of pairs is observed above the yield expected from neutral meson decays. Its mass spectrum is consistent with a dominant contribution from π⁺π^- -> ρ -> μ⁺μ^- annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. The results are discussed in the frame of different theoretical approaches addressing hadron properties close to the QCD phase boundary. They rule out models linking hadron masses directly to the value of the chiral condensate. Beyond the mass spectra of the excess, we have now a precision measurement of the associated transverse momentum spectra up to masses of 2.6 GeV. The slope parameters T_eff extracted from the spectra rise with dimuon mass up to the ρ, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.

* Tea & coffee will be served at 16:00.

Slides EP-Seminar-SDamjanovic.pdf EP-Seminar-SDamjanovic.ppt