Dr. Anthony Palladino (Laboratori Nazionali di Frascati dell'INFN)
In some models of physics beyond the Standard Model it is predicted the existence of light neutral vector particles (U bosons) mediator of new gauge interactions under which ordinary matter is uncharged. Motivated by astrophysical considerations, their mass, M_U, is expected to be of order 1 GeV or ligther. These new particles can be observed as sharp resonances at M_U in the invariant mass distribution of charged lepton or pion pairs in reactions of the type e+e- --> l(pi)+l(pi)- gamma, or in meson Dalitz decays. KLOE has formerly searched for U boson production using phi daitz decay events, setting limits in the mass region 50 < M_U < 520 MeV. We have now completed our analysis of e+e- --> mu+mu- gamma, which has allowed us setting the best limits to date in the region between 500 and 1000 MeV. Preliminary results for the electron-positron final state are also available.
Dr. Fabio Bossi (INFN-LNF)