A New Approach to Probe Non-Standard Interactions in Atmospheric Neutrino Experiments

We propose a new approach to explore the neutral-current non-standard neutrino interactions (NSI) in atmospheric neutrino experiments using oscillation dips and valleys in reconstructed muon observables, at a detector like ICAL. We show that the non-zero value of NSI parameter $\varepsilon_{\mu\tau}$ shifts the oscillation dip locations in $L/E$ distributions of the up/down event ratios of reconstructed $\mu^-$ and $\mu^+$ in opposite directions. We introduce a new variable $\Delta d$ representing the difference of dip locations in $\mu^-$ and $\mu^+$, which is sensitive to $\varepsilon_{\mu\tau}$, and is independent of the value of $\Delta m^2_{32}$. We further note that the oscillation valley in the ($E$, $\cos \theta$) plane of the reconstructed muon observables bends in the presence of NSI, its curvature having opposite signs for $\mu^-$ and $\mu^+$. We illustrate how the measurement of contrast in the curvatures of valleys in $\mu^-$ and $\mu^+$ can be used to estimate $\epsmutau$. Using these proposed oscillation dip and valley measurements, the achievable precision on $|\varepsilon_{\mu\tau}|$ at 90\% C.L. is about 2\% with 500 kt$\cdot$yr exposure including the effects of statistical fluctuations, systematic errors, and uncertainties in oscillation parameters.