Breakdown of the impulse approximation and its consequences: the low-Q^2 problem

TL;DR

The paper investigates the breakdown of the impulse approximation in low-Q^2 neutrino scattering, revealing that standard models are unreliable below |q|<400 MeV/c and contribute to observed discrepancies in experimental data.

Contribution

It demonstrates that low-momentum interactions significantly affect neutrino cross sections and are inadequately modeled in current simulations, explaining the low-Q^2 discrepancy.

Findings

Low-|q| interactions contribute at least 19% to the quasielastic cross section.

Discrepancies at low-Q^2 are due to improper modeling of low-|q| interactions.

Standard models are unreliable for |q|<400 MeV/c.

Abstract

Neutrino scattering data and the standard calculations of the cross section show a discrepancy in the low-Q^2 (four-momentum transfer squared) region. The calculations rely on the assumption, called the impulse approximation, that the nucleus the neutrino scatters off can be described as a collection of independent nucleons and therefore only one nucleon takes part in the interaction. It is known from electron scattering that such picture is valid only when transferred momentum |q|>400 MeV/c. For lower |q|'s, the nucleus is probed with a lower spatial resolution and a few nucleons are involved in the scattering, so the use of the impulse approximation is unjustified. It means that the standard calculations of the cross sections are unreliable for |q|<400 MeV/c. I show that the contribution of low-momentum interactions to the quasielastic neutrino cross section cannot be reduced below ~19% by any selection of the beam energy in the few-GeV region. Moreover, low-Q^2 neutrino events happen only when transferred momentum is low too. Hence the discrepancy in the low-Q^2 region is caused by the inappropriate description of low-|q| interactions in the models commonly used in Monte Carlo simulations.