Search for Electron Neutrino Appearance in a 250 km Long-baseline Experiment

TL;DR

This paper reports a search for electron neutrino appearance in a long-baseline experiment, setting new limits on neutrino oscillation parameters and providing evidence consistent with neutrino oscillations.

Contribution

It presents the first search for electron neutrino appearance in the K2K experiment and establishes new constraints on oscillation parameters at 90% confidence level.

Findings

One candidate event observed against an expected background of 2.4 events.

Excluded certain oscillation parameters at 90% confidence level.

Set the most stringent limit on sin^2(2θ_μe) at 0.09.

Abstract

We present a search for electron neutrino appearance from accelerator produced muon neutrinos in the K2K long baseline neutrino experiment. One candidate event is found in the data corresponding to an exposure of 4.8*10^19 protons on target. The expected background in the absence of neutrino oscillations is estimated to be 2.4+-0.6 events and is dominated by mis-identification of events from neutral current pi^0 production. We exclude the \nu_\mu to \nu_e oscillations at 90% C.L. for the effective mixing angle in 2-flavor approximation of sin^2(2theta_\mu_e) (~= 1/2 sin^2 2 th_13) > 0.15 at Delta m^2_\mu_e = 2.8*10^{-3} eV^2, the best fit value of the \nu_\mu disappearance analysis in K2K. The most stringent limit of sin^2(2theta_\mu_e) < 0.09 is obtained at Delta m^2_\mu_e = 6*10^{-3} eV^2.