Present and Future Bounds on Non-Standard Neutrino Interactions

TL;DR

The paper studies non-standard neutrino interactions (NSI) described by $\mathcal{L}_{NSI} = - \varepsilon^{fP}_{αβ} 2 \sqrt{2} G_F (\bar{ν}_α γ_ρ L ν_β)(\bar{f} γ^ρ P f)$, generated at dimension $≥8$, which avoids tree-level charged-lepton couplings. It notes that these NSI induce one-loop SM dressings via $W$ exchange that generate charged-lepton operators, enabling model-independent bounds from charged-lepton processes. The authors compile current bounds from neutrino scattering, atmospheric neutrinos, LEP, and related charged-lepton operators, and project future sensitivity from KamLAND–SNO solar data and from near-detector measurements at a neutrino factory, with implications for oscillation-parameter determinations. For NSI involving $ν_e$ and $ν_μ$, short-baseline measurements constrain them without significantly biasing far-detector oscillation analyses at a ν-factory, while $ν_τ$-related NSI yield comparable near/far sensitivities, highlighting the complementary reach of solar, reactor, and accelerator data.

Abstract

We consider Non-Standard neutrino Interactions (NSI), described by four-fermion operators of the form $(\barν_α γν_β) (\bar{f} γf)$, where $f$ is an electron or first generation quark. We assume these operators are generated at dimension $\geq 8$, so the related vertices involving charged leptons, obtained by an SU(2) transformation $ν_δ \to e_δ$, do not appear at tree level. These related vertices necessarily arise at one loop, via $W$ exchange. We catalogue current constraints from $\sin^2 θ_W$ measurements in neutrino scattering, from atmospheric neutrino observations, from LEP, and from bounds on the related charged lepton operators. We estimate future bounds from comparing KamLAND and solar neutrino data, and from measuring $\sin^2 θ_W$ at the near detector of a neutrino factory. Operators constructed with $ν_μ$ and $ν_e$ should not confuse the determination of oscillation parameters at a $ν$factory, because the processes we consider are more sensitive than oscillations at the far detector. For operators involving $ν_τ$, we estimate similar sensitivities at the near and far detector.