Improved Limit on the Effective Electron Neutrino Mass with the ECHo-1k Experiment

The effective electron neutrino mass can be determined by analyzing the end-point region of the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mmultiscripts> <a:mrow> <a:mi>Ho</a:mi> </a:mrow> <a:mprescripts/> <a:none/> <a:mrow> <a:mn>163</a:mn> </a:mrow> </a:mmultiscripts> </a:mrow> </a:math> electron capture spectrum, provided a measurement with high-energy resolution and high statistics using calorimetric techniques. Here, the Electron Capture in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mmultiscripts> <c:mrow> <c:mi>Ho</c:mi> </c:mrow> <c:mprescripts/> <c:none/> <c:mrow> <c:mn>163</c:mn> </c:mrow> </c:mmultiscripts> </c:mrow> </c:math> Collaboration (ECHo) presents an analysis of the most precise <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:mmultiscripts> <e:mrow> <e:mi>Ho</e:mi> </e:mrow> <e:mprescripts/> <e:none/> <e:mrow> <e:mn>163</e:mn> </e:mrow> </e:mmultiscripts> </e:mrow> </e:math> spectrum currently available, obtained with the ECHo-1k experiment and comprising about 200 million events. A very low background rate of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mi>b</g:mi> <g:mtext>const</g:mtext> </g:msub> <g:mo>=</g:mo> <g:mn>9.1</g:mn> <g:mo stretchy="false">(</g:mo> <g:mn>1.3</g:mn> <g:mo stretchy="false">)</g:mo> <g:mo>×</g:mo> <g:msup> <g:mn>10</g:mn> <g:mrow> <g:mo>−</g:mo> <g:mn>6</g:mn> </g:mrow> </g:msup> </g:math> eV/pixel/day was achieved allowing for a reliable analysis of the end-point region. The derived end-point energy <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>Q</k:mi> <k:mo>=</k:mo> <k:mn>2862</k:mn> <k:mo stretchy="false">(</k:mo> <k:mn>4</k:mn> <k:mo stretchy="false">)</k:mo> <k:mtext> </k:mtext> <k:mtext> </k:mtext> <k:mi>eV</k:mi> </k:math> is in excellent agreement with the one independently determined via Penning-trap mass spectrometry of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:mi>Q</o:mi> <o:mo>=</o:mo> <o:mn>2863.2</o:mn> <o:mo stretchy="false">(</o:mo> <o:mn>6</o:mn> <o:mo stretchy="false">)</o:mo> <o:mtext> </o:mtext> <o:mtext> </o:mtext> <o:mi>eV</o:mi> </o:math> [Ch. Schweiger , ]. The upper limit of the effective electron neutrino mass is improved by almost a factor of 2 compared to the lowest current value [B. K. Alpert , ], reaching <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:msub> <s:mi>m</s:mi> <s:msub> <s:mi>ν</s:mi> <s:mi mathvariant="normal">e</s:mi> </s:msub> </s:msub> <s:mo>&lt;</s:mo> <s:mn>15</s:mn> <s:mtext> </s:mtext> <s:mtext> </s:mtext> <s:mrow> <s:mi>eV</s:mi> <s:mo stretchy="false">/</s:mo> <s:msup> <s:mrow> <s:mi>c</s:mi> </s:mrow> <s:mn>2</s:mn> </s:msup> </s:mrow> </s:math> (90% credible interval).