Neutrinophilic nonstandard interactions

We construct UV-complete models for nonstandard neutrino interactions mediated by a sub-GeV gauge boson $Z^{\prime }$ coupled to baryon number $B$ or $B-L$. A flavor-dependent $Z^{\prime }$ coupling to neutrinos is induced by mixing a $U(1{)}^{\prime }$-charged Dirac fermion with the active neutrinos, naturally suppressing flavor violation or nonuniversality of the charged leptons to the loop level. We show that these models can give rise to large flavor-conserving as well as flavor-violating nonstandard neutral-current neutrino interactions potentially observable in current or future oscillation experiments such as DUNE without being in conflict with other constraints such as neutrino scattering or lepton-flavor-violating decays. In particular, the LMA-Dark solution to the solar-neutrino anomaly can be obtained for $U(1{)}_B$, but not for $U(1{)}_{B-L}$.

Figure 1

Parameter space of a gauge boson $Z^{\prime }$ coupled to $B-L$ (left) or $B$ (right). The shaded areas are excluded at 90% C.L. A $Z^{\prime }$ coupled to $L$ essentially yields the $B-L$ constraints.

Figure 2

Approximate 90% C.L. bounds on $\sqrt{|g_B{g}_{\nu }|}$ from solar-neutrino nuclear recoils in CDMSlite and optimistic projections for second-generation xenon (e.g., LUX–ZEPLIN) and germanium experiments (e.g., SuperCDMS SNOLAB), adapted from Ref. [60].

Figure 3

Loop-induced CLFV decay ${\ell }_{\alpha }\to {\ell }_{\beta }\gamma$.

Figure 4

Left: Loop-induced CLFV decay ${\ell }_{\alpha }\to {\ell }_{\beta }{Z}^{\prime }$. The $Z^{\prime }$ can be attached to both the $H^{-}$ and $\mathrm{\Psi }$ lines. Right: Resulting branching ratio $\mathrm{Br}(\tau \to e{Z}^{\prime })$, which also holds for $\mathrm{Br}(\tau \to \mu Z^{\prime })$ with $y_e\to y_{\mu }$, because we have neglected the mass of the final fermion.

Figure 5

Contours of diagonal NSI (top left and right) and off-diagonal NSI (bottom left and right) for $U(1{)}^{\prime }=U(1{)}_B$. Relevant experimental bounds are also shown (see the text for details). The inequality of Eq. (9) is satisfied in the allowed parameter space. The current (solid black lines) and projected (dot-dashed black line marked with $\mathrm{DUNE}+\mathrm{T}2\mathrm{HK}$, obtained by including a prior for current constraints) limits on $ϵ\simeq {ϵ}^e+3{ϵ}^u+3{ϵ}^d$ are taken from Ref. [4]. For ${ϵ}_{ee}$ (top left) we also indicate the preferred region for the LMA-dark solution.

Figure 6

Contours of diagonal NSI (${ϵ}_{\tau \tau }$, left) and off-diagonal NSI (${ϵ}_{\mu \tau }$, right) and additional constraints for a set of parameters of $U(1{)}_{B-L}$. The inequality of Eq. (9) is satisfied in the allowed parameter space. The constraint from $\tau \to \mu ee$ is extremely conservative.