Probing a $Z'$ gauge boson via neutrino trident scattering in the Forward Physics Facility at the LHC and FCC

Abstract

The study of neutrino physics at the Large Hadron Collider is already a reality, and a broad neutrino physics program is expected to be developed in forthcoming years at the Forward Physics Facility (FPF). In particular, the neutrino trident scattering process, which is a rare Standard Model process, is expected to be observed for the first time with a statistical significance of $5σ$ using the FASER$ν$2 detector. Moreover, similar studies are expected to be performed in the proposed Future Circular Collider (FCC). Such perspectives motivate the investigation of the impact of New Physics on the predictions for the corresponding number of events. In this letter, we consider the $L_μ- L_τ$ model, which predicts an additional massive neutral gauge boson, $Z'$, that couples to neutrino and charged leptons of the second and third families, and estimate the production of a dimuon system in the neutrino trident scattering at the FASER$ν$2 assuming different models for the incoming neutrino flux at the LHC and FCC energies. We derive the associated sensitivity and demonstrate that the FPF@LHC is not able to improve the current bounds on the $L_μ- L_τ$ model, while a future measurement of the dimuons produced in neutrino trident events in the FPF@FCC will extend the coverage of the parameter space in comparison to previous experiments.

Figures (2)

• Figure 1: Feynman diagrams for the neutrino trident scattering off a nucleus target associated with a $W^\pm$ (left), $Z^0$ and $Z'$ (right) exchange. The lepton flavors $l$ and $k$ can be equal or different, and $A^{\prime} = A$ ($A^{\prime} \neq A$) in coherent (incoherent) interactions.
• Figure 2: Sensitivity of the far - forward detectors at the LHC (FASER$\nu$2) and FCC for the neutrino trident events associated with the $Z'$ gauge boson predicted by the $L_\mu - L_\tau$ model. Results, at the 2$\sigma$ level, derived considering different MC generators for the incoming neutrino flux. For comparison, the existing constraints from other processes and experiments are also presented. The green bands represent the parameter space in which the presence of a $Z'$ solves the muon magnetic moment anomaly at the $1\sigma$ and $2\sigma$ levels. The LHC band represents the region excluded in $pp$ collisions at the LHC, considering the analysis performed by the CMS CMS:2018yxg and ATLAS ATLAS:2023vxgATLAS:2024uvu collaborations of the production of $Z^{0}$ and $W^{\pm}$ bosons and subsequent decay in the $Z^{0} \rightarrow Z' \mu^{\pm} \mu^{\mp} \rightarrow \mu^{\pm} \mu^{\mp} \mu^{\pm} \mu^{\mp}$ and $W^{\pm} \rightarrow Z' \mu^{\pm} \nu_{\mu} \rightarrow \mu^{\pm} \mu^{\mp} \mu^{\pm} \nu_{\mu}$ channels. The BaBar band represents the associated excluded region in the study of the $Z^{0}$ boson decay process BaBar:2016sci. We also have in the figure the excluded regions by the Borexino experiment in interactions of neutrinos with electrons Bellini:2011rxHarnik:2012ni, which was constructed considering that the interaction rate should not be more than $8\%$ greater than the SM prediction. The magenta dotted line represents the recent results from the NA64$\mu$ experiment with 90% CL NA64:2024klw. The bounds from Big Bang Nucleosynthesis (BBN) derived in Escudero:2019gzq and from Planck 2018 Planck:2018vygGhosh:2024cxi, which implies that $m_{Z'} \gtrsim 5 \,\mathrm{MeV}$ and $m_{Z'} \gtrsim 10 \,\mathrm{MeV}$, respectively, are also presented. Finally, the excluded region derived by the CCFR experiment in the neutrino trident process with two muons in the final state CCFR:1991lplAltmannshofer:2014pba is also presented.