[Paper Review] Searching for Novel Neutrino Interactions at NOvA and Beyond in Light of Large theta_13

Alexander Friedland, Ian M. Shoemaker|arXiv (Cornell University)|Jul 27, 2012

Neutrino Physics Research1 references20 citations

TL;DR

This paper investigates the sensitivity of long-baseline neutrino experiments, particularly NOvA and LBNE, to nonstandard neutrino interactions (NSI) in the $e$-$\tau$ sector, leveraging the large $\theta_{13}$ angle to enhance interference effects. It shows that NOvA can detect NSI at $\sim3\sigma$ significance, but NSI phases can mimic $CP$ violation, necessitating longer baselines like LBNE’s $1300$ km to resolve degeneracies and confirm the mass hierarchy and true $CP$-violation.

ABSTRACT

We examine the prospects of probing nonstandard interactions (NSI) of neutrinos in the e-tau sector with upcoming long-baseline nu_mu -> nu_e oscillation experiments. First conjectured decades ago, neutrino NSI remain of great interest, especially in light of the recent 8B solar neutrino measurements by SNO, Super-Kamiokande, and Borexino. We observe that the recent discovery of large theta_13 implies that long-baseline experiments have considerable NSI sensitivity, thanks to the interference of the standard and new physics conversion amplitudes. In particular, in some parts of NSI parameter space, the upcoming NOvA experiment will be sensitive enough to see ~ 3sigma deviations from the SM-only hypothesis. On the flip side, NSI introduce important ambiguities in interpreting NOvA results as measurements of CP-violation, the mass hierarchy and the octant of theta_23. In particular, observed CP violation could be due to a phase coming from NSI, rather than the vacuum Hamiltonian. The proposed LBNE experiment, with its longer ~ 1300 km baseline, may break many of these interpretative degeneracies.

Motivation & Objective

• To assess the sensitivity of upcoming long-baseline experiments like NOvA and LBNE to nonstandard neutrino interactions (NSI), particularly in the $e$-$\tau$ sector.
• To evaluate how the recently measured large $\theta_{13}$ enhances NSI detection via quantum interference between standard and new physics amplitudes.
• To identify and quantify the ambiguities introduced by NSI phases in interpreting $CP$ violation, mass hierarchy, and $\theta_{23}$ octant measurements in NOvA.
• To demonstrate that longer baselines, such as LBNE’s $\sim1300$ km, can break key degeneracies in NSI interpretation.
• To advocate for combined analysis of data from multiple baselines (T2K, NOvA, LBNE, Deep Core) to maximize NSI sensitivity and resolve parameter ambiguities.

Proposed method

• Uses an effective field theory framework with a single off-diagonal $e$-$\tau$ NSI coupling $\varepsilon_{e\tau}^{f}$, parameterizing NSI via $\mathcal{L} \supset -2\sqrt{2} \varepsilon_{e\tau}^{f} G_F (\bar{f}\gamma^\mu f)(\bar{\nu}_e \gamma_\mu \nu_\tau) + \text{h.c.}$
• Analyzes neutrino oscillation probabilities $P(\nu_\mu \to \nu_e)$ in matter, incorporating interference between standard vacuum oscillations and NSI amplitudes.
• Applies spectral analysis of $\nu_\mu \to \nu_e$ appearance rates at different baselines (NOvA at $\sim810$ km, LBNE at $\sim1300$ km) to probe NSI sensitivity.
• Compares predicted $\nu_e$ survival probabilities in solar neutrino data (SNO, Borexino) with and without NSI to assess consistency with $\varepsilon_{e\tau} \sim 0.2-0.4$.
• Evaluates degeneracy breaking using both $\nu$ and $\bar{\nu}$ mode data across multiple baselines, focusing on phase and hierarchy discrimination.
• Performs statistical analysis to estimate $3\sigma$ and $5\sigma$ sensitivity thresholds for NSI detection in NOvA and LBNE.

Experimental results

Research questions

• RQ1 Can long-baseline experiments like NOvA and LBNE detect nonstandard neutrino interactions (NSI) in the $e$-$\tau$ sector, given the large $\theta_{13}$?
• RQ2 To what extent do NSI phases mimic or obscure genuine $CP$-violating effects in $\nu_\mu \to \nu_e$ oscillation experiments?
• RQ3 How do NSI effects interfere with the determination of the neutrino mass hierarchy and the octant of $\theta_{23}$ in NOvA?
• RQ4 Can LBNE’s longer baseline ($\sim1300$ km) resolve the degeneracies in NSI interpretation that plague shorter-baseline experiments?
• RQ5 What is the combined sensitivity of multiple long-baseline experiments (T2K, NOvA, LBNE, Deep Core) to NSI, and how does it improve over single-experiment analyses?

Key findings

• The large $\theta_{13}$ enhances NSI sensitivity via quantum interference, enabling NOvA to detect $\sim3\sigma$ deviations from the SM-only hypothesis for $|\varepsilon_{e\tau}| \sim 0.2-0.4$.
• LBNE’s $\sim1300$ km baseline provides sufficient sensitivity to detect NSI at $5\sigma$ for $\varepsilon_{e\tau} \sim 0.1$, and at $3\sigma$ even for $\varepsilon_{e\tau} \sim 0.1$.
• NSI introduce a $CP$-violating phase $\delta_\nu$ that can mimic the vacuum $CP$-violating phase $\delta$, creating a fundamental ambiguity in interpreting $CP$ violation in NOvA.
• The degeneracy between different combinations of $\delta$, $\delta_\nu$, and mass hierarchy in NOvA can be broken by combining data from multiple baselines, especially with LBNE’s long baseline.
• Spectral information, particularly at low energies and near the second oscillation maximum, significantly improves the discrimination of NSI phases and standard model parameters.
• Combined analysis of data from NOvA, LBNE, T2K, and Deep Core is expected to resolve degeneracies and provide a powerful, multi-faceted probe of NSI.