Microscopio electrónico de transmisión (MET)

Cosmic ray muons entering the detector can produce unstable isotopes via the interaction µ+¹⁶O→ ^µ+X or via capture on¹⁶O to produce¹⁶N. The resulting nuclei will then decay by emitting beta or gamma particles with an energy of up to about 20 MeV, making them an important background for neutrino detection at these energies.

The flux and average energy of cosmic ray muons in the Tochibora mine, where Hyper-Kamiokande will be located, was simulated with the muon simulation code MUSIC [221] using a topological map with 5 m mesh resolution and assuming a rock density of about 2.7 g/cm³. Figure 2.16 shows the directional dependence of the simulated muon flux in Super- and Hyper-Kamiokande as well as the measured muon flux in Super-Kamiokande, which shows good agreement with the simulation.

The expected muon flux in Hyper-Kamiokande is J_µ=7.55×^{10−7cm−2s−1with} an average energy of h^Eµi = 203 GeV, compared to Jµ = 1.54×^{10−7cm−2s−1} and h^Eµi = 258 GeV in Kamiokande. The higher overburden of Super-Kamiokande leads to a lower total flux than in Hyper-Super-Kamiokande and has a relatively stronger shielding effect for low-energy muons, which leads to an increase in the average energy of observed muons. Thus, while the muon flux in Hyper-Kamiokande is increased by a factor of five, the average spallation yield per muon calculated with the FLUKA code [222] is reduced by about 20 % due to the lower muon energy so that the total rate of spallation events per unit volume is about four times as high as in Super-Kamiokande.

The rate of downward-going muons is about 2 Hz in Super-Kamiokande [223]

and will increase to about 50 Hz in Hyper-Kamiokande due to the lower overburden and larger detector size. In Super-Kamiokande, the yield of unstable isotopes

Chapter 2 The Hyper-Kamiokande Detector

cosθ

0 0.2 0.4 0.6 0.8 1

)-1 s-2 Muon Flux (cm

0 0.5 1 1.5 2

10-6

×

Data MC

Hyper-K

Super-K

(deg) φ

0 100 200 300

)-1 rad-1 s-2 Muon Flux (cm

0 0.5 1 1.5 2 2.5

10-7

×

Data

MC Hyper-K

Super-K

Figure 2.16: Muon flux as a function of zenith angle θ (top) and azimuth angle φ (bottom). Red lines show the result of simulation with MUSIC in Super- and Hyper-Kamiokande, while blue lines show the measured fluxes in Super-Kamiokande.

Figure from reference [169].

72

2.4 Backgrounds

with decay energies of more than 3.5 MeV for a single muon was calculated to be 5×^{10−6g−1cm2}[224]. Assuming the density of water in the detector to be 1 g/cm³ and a track length of 32.2 m (corresponding to the height of the fiducial volume), this gives about 0.016 spallation events per muon. In Hyper-Kamiokande, a 20 % lower spallation yield per unit length combined with a 1.6 times taller fiducial volume yields about 0.02 spallation events per muon, As a result, the rate of spallation events in Hyper-Kamiokande is expected to be approximately 1 Hz.

Since the half-life of spallation products is between several milliseconds and a few seconds, these events can in principle be identified through spatial and temporal coincidence with cosmic ray muons that pass through the detector. The first modern search for supernova relic neutrinos with Super-Kamiokande employed a two-step process to reduce spallation backgrounds [225, 226], starting with a time correlation cut which removed all events within 0.15 s after a muon event. Remaining events were then subject to a likelihood function cut. In addition to the time delay after the muon event, this took into account the distance between the reconstructed event vertex and the preceding muon track as well as the residual charge Qres, which was defined as the detected charge (measured in photoelectrons) that is above the typical ionization loss of 2300 PE per metre track length. A large and positive value of Q_res indicates energy loss through showers, which could produce spallation products.

This likelihood cut reduces the spallation background by an order of magnitude while introducing a detector dead time of about 20 %.

A later analysis discovered that spallation events are correlated with peaks in the energy loss rate dE/dx along the muon track, which can be used for improved background rejection [227]. In a range of papers over the following years, Li and Beacom provided a theoretical description of spallation processes, which put this empirical observation onto a theoretical foundation, and suggested a number of significant improvements to muon reconstruction and spallation cuts [224, 228, 229].

While the lower overburden and increased rate of downward-going muons in Hyper-Kamiokande will require some modifications to these cuts, it is clear that the spallation event rate can be reduced to much less than 1 Hz, which enables an effectively background-free observation of supernova burst neutrinos.

True heroism is minutes, hours, weeks, year upon year of the quiet, precise, judicious exercise of probity and care—with no one there to see or cheer.

David Foster Wallace

Chapter 3

A Software Toolchain for Supernova Neutrino Events in Hyper-Kamiokande

In addition to the detector described in the previous chapter, the Hyper-Kamiokande collaboration is developing a software toolchain for generating and simulating events as well as reconstructing simulated or observed events to enable physics analyses. A schematic overview of this toolchain is given in figure 3.1.

In this chapter, I will describe the toolchain insofar as it applies to supernova burst neutrinos. It starts with neutrino fluxes as a function of neutrino flavour, time and energy that are produced by computer simulations of supernovae. Section 3.1 de-scribes the supernova models I use in this thesis. I then use a custom software called sntools, introduced in section 3.2, to generate neutrino interactions in the detector volume from these neutrino fluxes. Section 3.3 describes the detector simulation software, WCSim, which simulates propagation of particles and Cherenkov light in

!

Generators sntools

NEUT etc.

Physics Models

WCSim

BONSAI FiTQun etc.

Detector Simulation

further analysis

observed events

Event Reconstruction

Figure 3.1: Overview over the software toolchain for Hyper-Kamiokande. Software used for supernova burst neutrinos is highlighted in bold.

Chapter 3 A Software Toolchain for Supernova Neutrino Events in Hyper-Kamiokande

the detector and applies detector effects, including digitization and triggering. The output of this detector simulation should be equivalent to the output of the DAQ system once Hyper-Kamiokande is operational and observes actual neutrino inter-actions. Finally, reconstruction of the vertex, direction and energy of each simulated event is described in section 3.4. Further analysis of the simulated data sets, which builds on the results of the event reconstruction, is described in chapter 4.