The electromagnetic production of p+p-pairs in charged-current neutrino scattering is calculated for a scaling parton model. This process gives trimuons at 0.4 X of the single-muon rate at 200 GeV. Energy and invariant-mass distributions for radiative trimuons are presented and characteristic features of azimuthal correlations are discussed.The trimuon events'-3 reported in deep-inelastic neutrino scattering m,ay signal important new physical effects such a s heavy-lepton, heavyquark, o r Higgs-boson p r o d~c t i o n .~ However, t h e r e a r e mechanisms in "old" physics that can produce trimuons, and these must b e evaluated before any new physics can be fully separated. One such mechanism is the associated production of c h a r m plus anticharm in the recoiling hadron jet, with muonic c h a r m decays; quantum-chromodynamic calculations indicate that this gives few trimuons, of o r d e r of the single-muon r a t e a t 200 GeV.' A m o r e direct mechanism i s tKe electromagnetic production of muon p a i r s via virtual photons radiated f r o m the final muon and the interacting parton; in the present paper we calculate and discuss the contributions f r o m this process . 6 We find non-negligible r a t e s f o r the radiative contributions to trimuon production. At E =200 GeV, the r a t e f o r vN-p -p+ p -X is 0.4 x of the single-muon r a t e . Averaged over the F e r m ilab quadrupole triplet spectrum, f o r E > 100 GeV, the predicted r a t e f o r vN including the E, > 4 GeV acceptance cuts i s tributions of the three graphs in F i g . 1 a r e x u (fi,)y,(i -y5)u(lrl) , where E, = i~( k , )~,~~( k , ) /~~ and q = k 3 + k,. The cornplete matrix element f o r distinguishable trileptons is We note that 31130 for E = q , a s required f o r gauge invariance. F o r a r e a l photon with a polarization vector E , Eq. (3) is identical to the bremsstrahlung calculation of Kiskis .7 F o r p-p+ p-production we must antisymmetrize the matrix element, (%??) radiative = 0 . 2 5 x lo-, .(1) %L(P-p + p-1 = W k 2 , kq) -314k4, 4 ) , (4) and divide the phase space by a factor of 2 . This i s a non-negligible background to the r eTo evaluate 1311(2 we have used the computer ported experimental rate2 reduction program REDUCE .8 After calculating experimental the differential c r o s s section d a ( v, d -(3pju) f o r (2) the parton process, we s e t Dl = x P l , iii, = nt, =xrMN, where P, is the target-nucleon momentum, and The quoted r a t e s in Refs. 1 and 3 a r e lower. In 134, is the nucleon m a s s . The observed c r o s s secthis article we present characteristics of radiative tion is then obtained by integrating over x , trimuon production that can be used in identifying weighting with the parton distribution function d(x), events of this origin.O u r calculations a r e based on the scaling parton model with afour-fermion weak-interaction vertex.d o ( v U P -( 3 p ) X ) = do(v,d-(3p)u)d(x)dx, F o r gauge invariance, the virtual photon must b e attached in a l l possible ways to the charged par-ticles in the vd-p-u ...