Perspectives

is being done by the Duke University T2K/SK group. The other detector calibrations should not be affected directly by this bug.

3.7 Perspectives

We have successfully created an external device that emits single ring-shaped light inside the Super Kamiokande detector and measured its properties in an external setup. The single cone simulation based on this measurement and the single cone data taken at SK agree well at the ring region, even though at the backwards region there is still a slight difference between data and Monte-Carlo even after the correction of a bug found in skdetsim.

The next step for the Cone Generator project will be to either calibrate the Super Kamiokande detector or to improve the understanding of Super Kamiokande reconstruc-tion. There are currently three different groups that are using the Cone Generator data for different purposes :

• The TRIUMF T2K/SK group are using the CG data to test a new reconstruction algorithm they are developing for Super Kamiokande. In this algorithm the shape and timing informations of all PMTs are used to perform the reconstruction and the CG is therefore an useful control sample where verifications can be made.

• The Imperial College T2K group are using the CG data taken at about Z = 16 m to evaluate the vertex reconstruction at the fiducial volume boundary with the intention of increasing the fiducial volume.

• The LLR T2K group will continue the development of the CG Monte-Carlo and plan to measure the Z dependency of the scattering of light in water using the different CG measurements. Furthermore we plan to take two cone data at SK at different configurations reproducing theπ⁰ kinematics, specially when the SK recon-struction fails to measure two rings, and use the understanding of the SK detector reconstruction imparted by such measurements to improve its reconstruction and further reduce the π⁰ background at SK, given that the control sample currently used to estimate such error, which is presented in the next chapter, is limited by its statistics.

Chapter 4

Systematic error estimation for the

reconstruction efficiency of π ⁰ events in Super Kamiokande

Contents

4.1 Introduction . . . 104 4.2 The hybrid-π⁰ sample . . . 105 4.2.1 Overview . . . 105 4.2.2 Construction of the electron from the atmospheric νe sample . 107 4.2.3 Construction of the decay-electron sample . . . 107 4.2.4 Construction of the hybrid-π⁰ sample . . . 110 4.2.5 Effect of reuse of electrons from atmosphericν_e sample . . . 111 4.2.6 Extension of the hybrid-π⁰ sample to study non single π⁰ final

states . . . 113 4.3 Difference between hybrid-π⁰samples andπ⁰decays from the

T2K Monte-Carlo . . . 114 4.3.1 Vertex distribution between NC 1π⁰ and hybrid-π⁰ samples . . 115 4.3.2 Direction betweenγ fromπ⁰ decay of the NC1π⁰ sample andγ

or efrom the hybrid-π⁰ sample . . . 119 4.3.3 Difference between the γ momentum from the NC 1π⁰ sample

and the momentum of the electron from the hybrid-π⁰ sample . 122 4.3.4 Difference betweenγ ande in the Super Kamiokande detector . 122 4.3.5 Motivation to use the reconstructed information for einstead of

its Monte-Carlo information . . . 122 4.4 Results . . . 124 4.4.1 NC 1π⁰ background . . . 124 4.4.2 other NC events withπ⁰ background . . . 128 4.4.3 ν_µ CC withπ⁰ background . . . 135 4.4.4 Summary . . . 137 4.5 Impact of these results and perspectives . . . 137

4.1 Introduction

In order to compare the T2K neutrino data with what would be expected from the T2K beam it is essential to understand to which precision we know the Super Kamiokande reconstruction algorithms for each type of particle. The dominant background sources at Super Kamiokande for T2K are the νe CCQE beam contamination (∼49%) and mis-reconstructed π⁰ events¹ generated by neutral current (∼34%). Table 4.1 shows the breakdown by “final state” of expected signal and background for the T2K Run I+Run II.

Table 4.1: Expected number of events in SK after the ν_e CCQE selection (see sec-tion 2.5.3) and in parenthesis the fracsec-tion of νe selected events categorized by final state for signal and background samples. The number of events correspond to the Run I+Run II (1.431·10²⁰ POT) expected statistics using the 10d v3.1 beam flux, near detector nor-malization of 1.036. In order to estimate number of events we assume sin²2θ13 = 0.1, δCP = 0^◦ and normal mass hierarchy. The samples for which we estimate systematic error in this chapter are highlighted.

Final state Signal Background

νe CC 1e 3.986 (97.0%) 0.659 (49.1%) other νe CC 0.122 ( 3.0%) 0.042 ( 3.1%) νµ CC without π⁰ 0.000 ( 0.0%) 0.024 ( 1.8%) νµ CC with π⁰ 0.000 ( 0.0%) 0.005 ( 0.4%) NC 1π⁰ 0.000 ( 0.0%) 0.457 (34.1%) other NC with π⁰ 0.000 ( 0.0%) 0.049 ( 3.7%) NC 1γ 0.000 ( 0.0%) 0.036 ( 2.7%) NC 1π^± 0.000 ( 0.0%) 0.039 ( 2.9%) other NC 0.000 ( 0.0%) 0.031 ( 2.3%)

At Super Kamiokande there is no pureπ⁰ control sample available independent of the event selection therefore in order to understand the Super Kamiokande reconstruction for π⁰ events it was necessary to create a control sample.

The hybrid-π⁰ method was originally developed to estimate the uncertainty of the rejection efficiency of NC 1π⁰ events using the T2K selection cuts. Afterwards the use of the hybrid-π⁰ sample was extended to estimate the uncertainty of the “other NC with π⁰” sample and the “νµ CC with π⁰” sample. In this chapter we will discuss in more details the hybrid-π⁰ technique and the estimated systematic uncertainty of the reconstruction efficiency of events with a π⁰ in the final state at Super Kamiokande.

The hybrid-π⁰ method consists of the following steps which will be explained further : 1. construct the hybrid-π⁰ samples following the π⁰ kinematics of the T2K

Monte-Carlo;

2. apply the usual Super Kamiokande reconstruction for the hybrid-π⁰ samples events;

3. estimate systematic errors of the reconstruction efficiency by comparing the recon-struction efficiency of hybrid-π⁰ data and Monte-Carlo samples.

1Mis-reconstructedπ⁰ events in this case are theπ⁰ events that passes allνeCCQE event selections, that is, events where only one ring was reconstructed.