Matter effects in long baseline experiments, the flavor content of the heaviest (or lightest) neutrino and the sign of Delta m^2

TL;DR

This paper analyzes matter effects on neutrino oscillations in long baseline experiments, showing how the sign of Delta m^2 influences oscillation probabilities and proposing measurements to determine this sign.

Contribution

It provides a detailed analysis of matter effects on neutrino oscillations in Earth's crust and suggests experimental strategies to determine the sign of Delta m^2.

Findings

Matter effects can enhance or suppress oscillation probabilities by up to 25%.

The sign of Delta m^2 determines whether neutrinos or antineutrinos experience resonance.

Measuring both neutrino and antineutrino oscillations can resolve the sign ambiguity.

Abstract

The neutrinos of long baseline beams travel inside the Earth's crust where the density is approximately rho = 2.8 g cm^-3. If electron neutrinos participate in the oscillations, matter effects will modify the oscillation probabilities with respect to the vacuum case. Depending on the sign of Delta m^2 an MSW resonance will exist for neutrinos or anti-neutrinos with energy approximately E_nu(res) = 4.7 |\Delta m^2|/(10^-3 eV^2) GeV. For Delta m^2 in the interval indicated by the Super-Kamiokande experiment this energy range is important for the proposed long baseline experiments. For positive Delta m^2 the most important effects of matter are a 9% (25%) enhancement of the transition probability P(nu_mu -> nu_e) for the KEK to Kamioka (Fermilab to Minos and CERN to Gran Sasso) beam(s) in the energy region where the probability has its first maximum, and an approximately equal suppression of P(antinu_mu -> antinu_e). For negative Delta m^2 the effects for neutrinos and anti-neutrinos are interchanged. Producing beams of neutrinos and antineutrinos and measuring the oscillation probabilities for both (nu_mu -> nu_e) and (antinu_mu -> antinu_e) transitions can solve the sign ambiguity in the determination of Delta m^2.