CP-Invariance Violation at Short-Baseline Experiments in 3+1 Neutrino   Scenarios
  New neutrino degrees of freedom allow for more sources of CP-invariance violation (CPV). We explore the requirements for accessing CP-odd mixing parameters in the so-called 3+1 scenario, where one assumes the existence of one extra, mostly sterile neutrino degree of freedom, heavier than the other three mass eigenstates. As a first step, we concentrate on the nu_e to nu_mu appearance channel in a hypothetical, upgraded version of the nuSTORM proposal. We establish that the optimal baseline for CPV studies depends strongly on the value of Delta m^2_14 -- the new mass-squared difference -- and that the ability to observe CPV depends significantly on whether the experiment is performed at the optimal baseline. Even at the optimal baseline, it is very challenging to see CPV in 3+1 scenarios if one considers only one appearance channel. Full exploration of CPV in short-baseline experiments will require precision measurements of tau-appearance, a challenge significantly beyond what is currently being explored by the experimental neutrino community. 
Andr\'e de Gouv\^ea, Kevin J. Kelly, and Andrew Kobach
hep-ph