Prospect of the NUCLEUS experiment at Chooz for coherent elastic neutrino-nucleus scattering and new physics searches

The NUCLEUS experiment aims to measure coherent elastic neutrino–nucleus scattering ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>CE</a:mi> <a:mi>ν</a:mi> <a:mi>NS</a:mi> </a:mrow> </a:math> ) at unprecedentedly low nuclear recoil energies using gram-scale cryogenic calorimeters operated at the Chooz nuclear power plant in France. Access to recoil energies at the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi mathvariant="script">O</c:mi> <c:mo stretchy="false">(</c:mo> <c:mn>10</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>eV</c:mi> <c:mo stretchy="false">)</c:mo> </c:math> scale enables <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mrow> <h:mi>CE</h:mi> <h:mi>ν</h:mi> <h:mi>NS</h:mi> </h:mrow> </h:math> studies at extremely low momentum transfer and provides enhanced sensitivity to new physics. In this work, we present sensitivity projections for the upcoming NUCLEUS technical and physics runs, incorporating a data-driven treatment of the low-energy excess (LEE) observed during commissioning. We develop a likelihood framework that exploits reactor-power variation to disentangle signal and background in a low signal-to-background regime and to assess the impact of the dominant systematic uncertainties. For the with a 7 g <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msub> <j:mi>CaWO</j:mi> <j:mn>4</j:mn> </j:msub> </j:math> target, we find competitive sensitivity to several scenarios beyond the Standard Model, which do not require a <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mrow> <l:mi>CE</l:mi> <l:mi>ν</l:mi> <l:mi>NS</l:mi> </l:mrow> </l:math> observation. For the , assuming complete suppression of the LEE, we project a <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mn>4.7</n:mn> <n:mi>σ</n:mi> </n:math> observation of <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:mrow> <p:mi>CE</p:mi> <p:mi>ν</p:mi> <p:mi>NS</p:mi> </p:mrow> </p:math> with a statistical precision of about 20% in 1 year, enabling a determination of the weak mixing angle at the lowest momentum transfer probed to date with <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mrow> <r:mi>CE</r:mi> <r:mi>ν</r:mi> <r:mi>NS</r:mi> </r:mrow> </r:math> and leading <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"> <t:mrow> <t:mi>CE</t:mi> <t:mi>ν</t:mi> <t:mi>NS</t:mi> </t:mrow> </t:math> -based constraints on the neutrino charge radius and new mediator models.