A substandard candle: the low-$\nu$ method at few-GeV neutrino energies

TL;DR

This paper critically examines the effectiveness of the low-$ u$ method for constraining neutrino fluxes at few-GeV energies, revealing significant model dependence and limited potential for improvement in flux uncertainty reduction.

Contribution

The study provides an in-depth analysis of the low-$ u$ method's applicability at low energies using multiple event generators, highlighting its limitations due to nuclear and form-factor effects.

Findings

Flux constraints are highly model-dependent below 5 GeV.

The low-$ u$ method offers limited improvement in flux uncertainties.

Model dependence increases as energy transfer decreases in the few-GeV region.

Abstract

As accelerator-neutrino oscillation experiments improve oscillation parameter constraints, control over systematic uncertainties on the incoming neutrino flux and interaction models is increasingly important. The intense beams offered by modern experiments permit a variety of options to constrain the flux using in situ "standard candle" measurements. These standard candles must use very well understood processes to avoid introducing bias. One option discussed in this context is the "low-$\nu$" method, designed to isolate neutrino interactions where there is low energy-transfer to the nucleus, such that the cross section is expected to be approximately constant as a function of neutrino energy. The shape of the low-energy transfer event sample can then be used to extract the flux shape. Applications of the method at high neutrino energies (many tens of GeV) are well understood. However, the applicability of the method at the few-GeV energies of current and future accelerator neutrino experiments remains unclear due to the presence of nuclear and form-factor effects. In this analysis we examine the prospects for improving constraints on accelerator neutrino fluxes with the low-$\nu$ method in an experiment-independent way, using (anti)neutrino interactions on argon and hydrocarbon targets from the GENIE, NEUT, NuWro and GiBUU event generators. The results show that flux constraints from the low-$\nu$ method would be severely dependent on the specific interaction model assumptions used in an analysis for neutrino energies less than 5 GeV. The spread of model predictions show that a low-$\nu$ analysis is unlikely to offer much improvement on typical neutrino flux uncertainties, even with a perfect detector. Notably -- running counter to the assumption inherent to the low-$\nu$ method -- the model-dependence increases with decreasing energy transfer for experiments in the few-GeV region.