[Paper Review] First Oscillation Results for the T2K Experiment

Mark Hartz|arXiv (Cornell University)|Jan 9, 2012

Neutrino Physics Research3 citations

TL;DR

This paper presents the first search for electron neutrino appearance in the T2K long-baseline neutrino oscillation experiment using 3.23×10¹⁹ protons on target. With one observed candidate event against a background expectation of 0.30±0.07 events, T2K sets a 90% confidence level upper limit of sin²(2θ₁₃) < 0.5, marking a key step toward measuring the last unknown neutrino mixing angle and probing CP violation in the lepton sector.

ABSTRACT

T2K is a long baseline high intensity neutrino oscillation experiment employing an off-axis design to search for the as yet unobserved appearance of ν_e neutrinos in a ν_μ beam. The neutrino beam originates at the J-PARC facility in Tokai, Japan and the Super-Kamiokande (SK) detector, located 295 km away, measures the composition of the oscillated beam. The SK data are searched for an excess of ν_e, constraining the allowed parameter space of sin^2(2θ_{13}), the parameter governing the amplitude of oscillations from ν_μ to ν_e. This amplitude is of particular interest since it also modulates the amplitude of CP violating terms in the lepton mixing matrix. This paper presents results from the first T2K physics run in 2010 with 3.23 imes10^19 protons on target.

Motivation & Objective

• To search for the appearance of electron neutrinos (νₑ) in a muon neutrino (νₘ) beam, a key signature for measuring the mixing angle θ₁₃.
• To constrain the value of sin²(2θ₁₃), which governs the amplitude of νₘ → νₑ oscillations and modulates CP-violating effects in the lepton mixing matrix.
• To test the sensitivity of the T2K experiment to νₑ appearance with the first physics run data, establishing a baseline for future measurements.
• To reduce systematic uncertainties in neutrino flux and interaction modeling using near detector (ND280) data to improve far detector (Super-K) predictions.

Proposed method

• The T2K experiment uses a neutrino beam produced at J-PARC, with 30 GeV protons hitting a graphite target and decaying pions generating a νₘ beam focused via magnetic horns.
• The beam is directed 295 km to the Super-Kamiokande detector, located off-axis to produce a narrow-band beam peaking at ~0.6 GeV, optimal for νₘ → νₑ oscillation measurements.
• Neutrino interactions are reconstructed via Cherenkov light in Super-K, with νₑ candidates identified by electron-like ring topology and kinematic cuts.
• Backgrounds from intrinsic νₑ in the beam and misreconstructed neutral-current π⁰ events are modeled using NEUT and GENIE generators, with uncertainties evaluated via data comparisons and parameter variations.
• The near detector (ND280) measures the inclusive νₘ rate to constrain the neutrino flux, and this data is used to renormalize the far detector prediction via N^exp_SK = (N^data_ND280 / N^MC_ND280) × N^MC_SK.
• A Feldman-Cousins statistical method is applied to set a 90% confidence level upper limit on sin²(2θ₁₃) based on the observed one νₑ candidate event and background prediction.

Experimental results

Research questions

• RQ1 What is the upper limit on sin²(2θ₁₃) based on the first T2K data run with 3.23×10¹⁹ protons on target?
• RQ2 How well can the T2K experiment constrain νₑ appearance given the measured background rate and signal prediction?
• RQ3 To what extent do systematic uncertainties from neutrino flux, interaction modeling, and detector response affect the sin²(2θ₁₃) limit?
• RQ4 How does the inclusion of ND280 data improve the precision of the far detector prediction compared to simulation alone?
• RQ5 What is the sensitivity of the T2K experiment to sin²(2θ₁₃) for different values of δ_CP and Δm²₂₃?

Key findings

• T2K observes one νₑ candidate event in the Super-Kamiokande detector after applying all selection cuts.
• The expected background rate for νₑ candidates is 0.30±0.07 events, dominated by beam-induced νₑ and NC π⁰ interactions.
• The 90% confidence level upper limit on sin²(2θ₁₃) is found to be less than 0.5, assuming Δm²₂₃ = 2.4×10⁻³ eV² and sin²(2θ₂₃) = 1.0.
• The dominant systematic uncertainties in the background prediction stem from neutrino flux (9.2%) and neutrino interaction modeling (14.2%), particularly from final state interactions and NC π⁰ cross sections.
• The near detector measurement (ND280) yields a data-to-MC ratio of 1.061±0.028(stat.)⁺⁰.⁰⁴⁴₋₀.⁰³⁸(syst.)±0.039(phys.model), which is used to renormalize the SK prediction and reduce flux-related uncertainties.
• The sensitivity of the T2K experiment for this data set is shown to be competitive, with the potential to improve significantly with future data, including the fourfold increase already available.