[Paper Review] Searches for sterile neutrinos using the T2K off-axis near detector

Debra Dewhurst|arXiv (Cornell University)|Apr 30, 2015

Neutrino Physics Research1 references3 citations

TL;DR

This study searches for sterile neutrinos via short-baseline oscillations using the T2K off-axis near detector (ND280), analyzing 5.9×10²⁰ protons on target. It finds a p-value of 0.085 for the null hypothesis, excluding parts of the gallium and reactor anomalies in the 3+1 sterile neutrino model with effective mass-squared difference Δm²ₑₑ > 7 eV²/c⁴ and sin²2θₑₑ > 0.3 at 95% CL.

ABSTRACT

In the last decades, the results from a number of short-baseline neutrino experiments and the reanalysis of previous reactor experiments with updated antineutrino fluxes suggest some incompatibility with the standard three-neutrino model. A possible solution to these anomalies is the existence of neutrinos that do not interact via the electroweak force, known as sterile neutrinos. The existence of sterile neutrinos can be investigated with the T2K Experiment. Here we present a search for the disappearance of electron neutrinos over a baseline of 280 m using the T2K off-axis near detector. The data analysed corresponds to an exposure of $5.9 imes 10^{20}$ protons on target. The region $\sin^2 2 θ> 0.2$ and $Δm^2 > 8~ extrm{eV}^2$ is excluded at 95% CL. We also present an introduction to a new analysis looking for muon neutrino disappearance with the T2K near detector, which is in its early stages but promises to have interesting results in 2015.

Motivation & Objective

• Investigate the existence of sterile neutrinos as a solution to the gallium and reactor anomalies in short-baseline neutrino experiments.
• Test the 3+1 sterile neutrino model using the T2K off-axis near detector (ND280) with a focus on νₑ disappearance.
• Constrain the parameter space of sterile neutrino mixing using data from 5.9×10²⁰ protons on target.
• Develop a new analysis for νₘu disappearance to improve sensitivity to sterile neutrino oscillations.
• Reduce systematic uncertainties through dedicated control samples and νₘu CC interaction measurements.

Proposed method

• Construct binned Monte Carlo templates of reconstructed neutrino energy distributions assuming charged-current quasi-elastic (CCQE) interactions.
• Weight event-by-event templates with oscillation probabilities using the 3+1 model: P(νₑ → νₑ) = 1 − sin²2θₑₑ × (1.27Δm²₄₁Lν / E) with Lν in km and E in GeV.
• Perform a binned likelihood ratio fit with systematic uncertainties as Gaussian-constrained nuisance parameters.
• Use a control sample enriched in νₘu → π⁰ interactions to measure backgrounds from neutral current and CCDIS processes.
• Subdivide νₘu CC interactions into CC0π, CCπ⁺, and CCOth to constrain flux and cross-section uncertainties.
• Apply the same likelihood framework to a preliminary νₘu disappearance analysis using CCQE-dominated events.

Experimental results

Research questions

• RQ1 Does the observed νₑ event distribution at ND280 show evidence for short-baseline νₑ disappearance consistent with sterile neutrino oscillations?
• RQ2 To what extent can the T2K off-axis near detector exclude regions of the 3+1 sterile neutrino parameter space?
• RQ3 How do systematic uncertainties from flux and cross-sections affect the sensitivity to νₑ and νₘu disappearance?
• RQ4 What is the expected sensitivity of the T2K ND280 detector to νₘu disappearance in the 3+1 model with full systematics included?
• RQ5 Can the data exclude parts of the parameter space associated with the gallium and reactor anomalies?

Key findings

• The best-fit oscillation parameters for νₑ disappearance are sin²2θₑₑ = 1 and Δm²ₑₑ = 2.05 eV²/c⁴.
• The 95% confidence level exclusion region excludes sin²2θₑₑ > 0.3 and Δm²ₑₑ > 7 eV²/c⁴.
• The p-value for the null oscillation hypothesis is 0.085, indicating a 1 in 12 chance of observing the data under the standard model.
• The results exclude parts of the parameter space favored by the gallium and reactor anomalies.
• The analysis is limited by statistical uncertainties, indicating a need for more data to improve sensitivity.
• Preliminary sensitivity studies for νₘu disappearance show promising results, with expected 90% CL sensitivity when systematics are fully included.