High-resolution and low-background ^{163}Ho spectrum: interpretation of the resonance tails

C Velte,C Enss,S Rothe,Yu N Novikov,J Jochum,A Barth,K Johnston,T Stora,M Braß,A Fleischmann,M W Haverkort,Ch E Düllmann,R X Schüssler,L Gastaldo,U Köster,Menno Door ,Federica Mantegazzini ,Pavel Filianin ,Alexander Rischka ,D Hengstler,Charlotte König ,Sergey Eliseev ,Marius Keller ,Klaus Blaum ,M Wegner,M Zampaolo,Holger Dorrer ,Daniel Richter ,Christoph Schweiger ,T Day Goodacre,A Goeggelmann,Felix Ahrens ,B A Marsh ,F Piquemal,T Kieck,Sebastian Kempf ,Κ Wendt ,C Riccio,Kathrin Kromer ,Κ Zuber

doi:10.1140/epjc/s10052-019-7513-x

Abstract

The determination of the effective electron neutrino mass via kinematic analysis of beta and electron capture spectra is considered to be model-independent since it relies on energy and momentum conservation. At the same time the precise description of the expected spectrum goes beyond the simple phase space term. In particular for electron capture processes, many-body electron-electron interactions lead to additional structures besides the main resonances in calorimetrically measured spectra. A precise description of the ^{163}Ho spectrum is fundamental for understanding the impact of low intensity structures at the endpoint region where a finite neutrino mass affects the shape most strongly. We present a low-background and high-energy resolution measurement of the ^{163}Ho spectrum obtained in the framework of the ECHo experiment. We study the line shape of the main resonances and multiplets with intensities spanning three orders of magnitude. We discuss the need to introduce an asymmetric line shape contribution due to Auger–Meitner decay of states above the auto-ionisation threshold. With this we determine an enhancement of count rate at the endpoint region of about a factor of 2, which in turn leads to an equal reduction in the required exposure of the experiment to achieve a given sensitivity on the effective electron neutrino mass.

Highlights

The knowledge of the effective neutrino mass scale is one of the fundamental open questions in particle physics
The analysis of the endpoint region of the calorimetric measured 163Ho spectrum is considered to be the best approach to determine the value of the effective electron neutrino mass studying electron capture processes
The 163Ho spectrum we have presented has been acquired in underground laboratories in Modane

Summary

Introduction

The knowledge of the effective neutrino mass scale is one of the fundamental open questions in particle physics. The most sensitive approaches to define the neutrino mass scale are the study of the distribution of mass in the universe [2], the observation of neutrinoless double beta decay [3,4] and the analysis of the endpoint region in beta and electron capture (EC) spectra [5] Only this latter approach is considered to be model independent because it is based on energy and momentum conservation. While the analysis of beta and EC spectra is model-independent it still requires a precise understanding of the spectrum [7], since uncertainties in this respect represent a systematic error in the determination of the effective electron neutrino mass. The main idea behind this experiment was already proposed by DeRujula [13] and comprises of performing calorimetric measurements of the 163Ho spectrum This can be achieved if the 163Ho source is fully contained in detectors providing a 100% detection efficiency for all radiation emitted in the EC process besides the electron neutrino. The understanding of the processes required for the description of the low intensity part of the spectrum allows us to predict the shape of the spectrum in the higher-energy part below the end point region

Experimental methods

Analysis

Line-shape asymmetry

Conclusions