Precision study of the ! + and !! + 0 electromagnetic transition form-factors and of the ! + line shape in NA60

The NA60 experiment studied low-mass muon pair production in proton-nucleus (p-A) collisions using a 400 GeV proton beam at the CERN SPS. The low-mass dimuon spectrum is well described by the superposition of the twobody and Dalitz decays of the light neutral mesons , , !, 0 and , and no evidence of in-medium e ects is found. A new high-precision measurement of the electromagnetic transition form factors of the and ! was performed, profiting from a 10 times larger data sample than the peripheral In-In sample previously collected by NA60. Using the pole-parameterisationjF(M)j 2 = (1 M 2 = 2 ) 2 we find 2 = 1:934 0:067 (stat.) 0:050 (syst.) (GeV/c 2 ) 2 and 2 ! = 2:223 0:026 (stat.) 0:037 (syst.) (GeV/c 2 ) 2 . An improved value of the branching ratio of the Dalitz decay !! + 0 is also obtained, with BR(!! + 0 ) = [1:41 0:09 (stat:) 0:15 (syst:)] 10 4 . Further results refer to the line shape and a new limit on =! interference in hadron interactions.