Invisible neutrino decay: first vs second oscillation maximum

Abstract

We study the physics potential of the long-baseline experiments T2HK, T2HKK and ESSνSB in the context of invisible neutrino decay. We consider normal mass ordering and assume the state ν₃ as unstable, decaying into sterile states during the flight and obtain constraints on the neutrino decay lifetime (τ₃). We find that T2HK, T2HKK and ESSνSB are sensitive to the decay-rate of ν₃ for τ₃/m₃ ≤ 2.72 × 10⁻¹¹s/eV, τ₃/m₃ ≤ 4.36 × 10⁻¹¹s/eV and τ₃/m₃ ≤ 2.43 × 10⁻¹¹s/eV respectively at 3σ C.L. We compare and contrast the sensitivities of the three experiments and specially investigate the role played by the mixing angle θ₂₃. It is seen that for experiments with flux peak near the second oscillation maxima, the poorer sensitivity to θ₂₃ results in weaker constraints on the decay lifetime. Although, T2HKK has one detector close to the second oscillation maxima, having another detector at the first oscillation maxima results in superior sensitivity to decay. In addition, we find a synergy between the two baselines of the T2HKK experiment which helps in giving a better sensitivity to decay for θ₂₃ in the higher octant. We discuss the octant sensitivity in presence of decay and show that there is an enhancement in sensitivity which occurs due to the contribution from the survival probability P_μμ is more pronounced for the experiments at the second oscillation maxima. We also obtain the combined sensitivity of T2HK+ESSνSB and T2HKK+ESSνSB as τ₃/m₃ ≤ 4.36 × 10⁻¹¹s/eV and τ₃/m₃ ≤ 5.53 × 10⁻¹¹s/eV respectively at 3σ C.L.