2 research outputs found
Dileptons and Direct Photons at SPS
The study of dilepton and direct photon emission was one of the main topics
of the experimental program at the SPS devoted to the search of signals for QGP
formation. Three generations of experiments, Helios-3, NA38/NA50, CERES and
NA60 measured e+e- or mu+mu- production in various colliding systems and at
different energies. While lepton pair production in p-A collisions was found to
be reasonably well described by the expected sources, all experiments observed
in nuclear collisions an excess of the yield above the extrapolation from p-A.
As a result of this joint experimental effort we have currently a large amount
of information characterizing this excess: its mass spectrum over the full
range from 0.2 GeV/c^2 up to the J/psi, its transverse momentum spectra
including their mass dependence, its angular distributions, its dependence on
collision centrality over the complete range etc. Putting together all this
information leads to the conclusion that what we observe is the long-sought
thermal radiation from the fireball.Comment: 8 pages, 6 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse
Study of the electromagnetic transition form-factors in \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0 decays with NA60
The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A
GeV In-In collisions. The mass and pT spectra associated with peripheral
collisions can quantitatively be described by the known neutral meson decays.
The high data quality has allowed to remeasure the electromagnetic transition
form factors of the Dalitz decays \eta -> \mu^+\mu^-\gamma and \omega ->
\mu^+\mu^-\pi^0. Using the usual pole approximation F =
(1-M_{\mu\mu}^{2}/\Lambda^{2})^{-1} for the form factors, we find \Lambda^{-2}
(in GeV^{-2}) to be 1.95+-0.17(stat.)+-0.05(syst.) for the \eta and
2.24+-0.06(stat.)+-0.02(syst.) for the \omega. While the values agree with
previous results from the Lepton-G experiment, the errors are greatly improved,
confirming now on the level of 10\sigma the strong enhancement of the \omega
form factor beyond the expectation from vector meson dominance. An improved
value of the branching ratio BR(\omega -> \mu^+\mu^-\pi^0) =
[1.73+-0.25(stat.)+-0.14(syst.)]*10^{-4} has been obtained as a byproduct.Comment: Submitted to Phys. Lett.