Abstract

The goal of the HIBEAM/NNBAR program is to search for baryon number violation via the conversion or oscillation of neutrons into sterile neutrons and/or antineutrons at the European Spallation Source. A key experimental component of the program is the construction of an annihilation detector to directly observe the production of an antineutron following the oscillation. Design studies for the annihilation detector are presented. The predicted response of the detector models are studied using Geant4 simulations made with Monte Carlo simulations of the annihilation signal topology and cosmic ray backgrounds. Particle identification and sensitive discriminating observables, such as invariant mass and sphericity, are shown.

Highlights

  • The HIBEAM/NNBAR program [1] is a proposed two-stage experiment at the European Spallation Source (ESS) designed to search for neutrons converting, or oscillating, into antineutrons (n → n) and/or sterile neutrons (n → n0 )

  • Neutron conversions feature in theories which extend the Standard Model (SM), including supersymmetry [8,9] and extra dimensions [10,11], and can accommodate dark sectors of feebly interacting particles [12]

  • This paper focuses on design studies for a key component of the HIBEAM/NNBAR experimental program: the detector which would observe antineutronnucleon annihilation interactions following the hypothetical baryon violating process

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Summary

Introduction

The HIBEAM/NNBAR program [1] is a proposed two-stage experiment at the European Spallation Source (ESS) designed to search for neutrons converting, or oscillating, into antineutrons (n → n) and/or sterile neutrons (n → n0 ). Such an observation would indicate baryon number violation, a fundamental Sakharov condition for baryogenesis [2], or act as a sign of a potential dark sector [3]. Particle identification is detailed, followed by a study of quantities based on the observed final state in the detector, which can be used to discriminate signal and background (Section 6).

Free Neutron Oscillation Experiments
Annihilation Detector
Guiding Principles behind the Design of an Annihilation Detector
Design of HIBEAM and NNBAR Annihilation Detectors
Signal and Background Simulations
Particle Identification
Energy Loss in the TPC
Neutral Pion Reconstruction
Cosmic Muon Identification with Timing
Charged Pion and Proton Definitions
Event Level Distributions
Neutral and Charged Pion Multiplicity
Invariant Mass Reconstruction
Sphericity
Findings
Discussion and Summary
Full Text
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