Simultaneous Measurement of the Half-Life and Spectral Shape of In115 β Decay with an Indium Iodide Cryogenic Calorimeter

Current bounds on the neutrino Majorana mass are affected by significant uncertainties in the nuclear calculations for neutrinoless double-beta decay. A key issue for a data-driven improvement of the nuclear theory is the actual value of the axial coupling constant <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>g</a:mi><a:mi mathvariant="normal">A</a:mi></a:msub></a:math>, which can be investigated through forbidden <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mi>β</d:mi></d:math> decays. We present the first measurement of the 4th-forbidden <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi>β</f:mi></f:math> decay of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mmultiscripts><h:mrow><h:mi>In</h:mi></h:mrow><h:mprescripts/><h:none/><h:mrow><h:mn>115</h:mn></h:mrow></h:mmultiscripts></h:mrow></h:math> with a cryogenic calorimeter based on indium iodide. Exploiting the enhanced spectrum-shape method for the first time to this isotope, our study accurately determines simultaneously spectral shape, <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:msub><j:mi>g</j:mi><j:mi mathvariant="normal">A</j:mi></j:msub></j:math>, and half-life. The interacting shell model, which best fits our data, indicates a half-life for this decay at <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mn>1</m:mn><m:mo>/</m:mo><m:mn>2</m:mn></m:mrow></m:msub><m:mo>=</m:mo><m:mo stretchy="false">(</m:mo><m:mn>5.26</m:mn><m:mo>±</m:mo><m:mn>0.06</m:mn><m:mo stretchy="false">)</m:mo><m:mo>×</m:mo><m:msup><m:mrow><m:mn>10</m:mn></m:mrow><m:mrow><m:mn>14</m:mn></m:mrow></m:msup><m:mtext> </m:mtext><m:mtext> </m:mtext><m:mi>yr</m:mi></m:mrow></m:math>.