Neutrino Trident Production: A Powerful Probe of New Physics with Neutrino Beams

TL;DR

The production of a μ+ μ- pair from the scattering of a muon neutrino off the Coulomb field of a nucleus is shown to be an exquisitely sensitive probe in the search for new neutral currents among leptons, putting the strongest constraints on well-motivated and well-hidden extensions of the standard model gauge group.

Abstract

The production of a mu+mu- pair from the scattering of a muon-neutrino off the Coulomb field of a nucleus, known as neutrino trident production, is a sub-weak process that has been observed in only a couple of experiments. As such, we show that it constitutes an exquisitely sensitive probe in the search for new neutral currents among leptons, putting the strongest constraints on well-motivated and well-hidden extensions of the Standard Model gauge group, including the one coupled to the difference of the lepton number between the muon and tau flavor, L_mu-L_tau. The new gauge boson, Z', increases the rate of neutrino trident production by inducing additional $(\barμγ_αμ)(\barνγ^αν)$ interactions, which interfere constructively with the Standard Model contribution. Existing experimental results put significant restrictions on the parameter space of any model coupled to muon number L_mu, and disfavor a putative resolution to the muon g-2 discrepancy via the loop of Z' for any mass m_Z' > 400 MeV. The reach to the models' parameter space can be widened with future searches of the trident production at high-intensity neutrino facilities such as the LBNE.

Figures (4)

• Figure 1: The leading order contribution of the ${\rm Z^\prime}$ to neutrino trident production (another diagram with $\mu^+$ and $\mu^-$ reversed is not shown). Other contributions at the same order in $g'$ are further suppressed by the Fermi scale.
• Figure 2: Parameter space for the ${\rm Z^\prime}$ gauge boson. The light-grey area is excluded at 95% C.L. by the CCFR measurement of the neutrino trident cross-section. The grey region with the dotted contour is excluded by measurements of the SM $Z$ boson decay to four leptons at the LHC CMS:2012bwAad:2014wra. The purple (dark-grey) region is favored by the discrepancy in the muon g-2 and corresponds to an additional contribution of $\Delta a_\mu = (2.9 \pm 1.8) \times 10^{-9}$ to the theoretical value Jegerlehner:2009ry.
• Figure 3: Same as Fig. \ref{['fig:constraints']} but focusing on the low mass region. Constraints from CHARM-II and CCFR, Eqs. (\ref{['eq:CHARM']}) and (\ref{['eq:CCFR']}) are shown separately. We do not attempt a statistical combination of the results. The dashed lines show the expected limit if the trident cross-section could be measured with 10% or 30% accuracy using 5 GeV neutrinos scattering on Argon.
• Figure 4: Expected number of trident events per ton of Argon and per $10^{20}$ POT at the LBNE near detector for a neutrino energy of $E_\nu = 5$ GeV as a function of the ${\rm Z^\prime}$ mass. The horizontal line shows the SM prediction. The purple (dark grey) region corresponds to ${\rm Z^\prime}$ masses and couplings that yield a contribution to the muon g-2 in the range $\Delta a_\mu = (2.9 \pm 1.8) \times 10^{-9}$. The light grey region is excluded by CCFR.