# The Reliability on the Direction of the Incident Neutrino for the Fully Contained Events and Partially Contained Events due to QEL in the Super-Kamiokande

## TL;DR

This paper critically examines the assumption used in Super-Kamiokande neutrino analysis that the incident neutrino's direction matches that of the detected muon, revealing this assumption to be unreliable through detailed simulations.

## Contribution

The study develops a comprehensive time sequential Monte Carlo simulation to test the validity of the SK directional assumption for neutrino events, providing a more accurate analysis method.

## Key findings

- The SK assumption on neutrino direction is unreliable.
- Simulation shows significant deviation between neutrino and muon directions.
- Results hold regardless of neutrino oscillation presence.

## Abstract

In the SK analysis of the neutrino events for [Fully Contained Events] and [Partially Contained Events] on their zenith angle distribution, it is assumed that the zenith angle of the incident neutrino is the same as that of the detected charged lepton. In the present paper, we examine the validity of [the SK assumption on the direction] of the incident neutrinos. Concretely speaking, we analyze muon-like events due to QEL. For the purpose, we develop [Time Sequential Monte Carlo Simulation] to extract the conclusion on the validity of the SK assumption. In our [Time Sequential Simulation], we simulate every physical process concerned as exactly as possible without any approximation. From the comparison between the zenith angle distributon of the emitted muons under [the SK assumption on the direction] and the corresponding one obtained under our [Time Sequential Simulation], it is concluded that the measurement of the direction of the incident neutrino for the neutrino events occurring inside the detector in the SK analysis turns out to be unreliable, which holds irrespective of the existence and/or non-existence of the neutrino oscillation.