First measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasielasticlike interactions using an accelerator neutrino beam

We report the first measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasielasticlike interactions at the gadolinium-loaded Super-Kamiokande detector using the T2K neutrino beam, which has a peak energy of about 0.6 GeV. A total of 30 neutral-current quasielasticlike event candidates were selected from T2K data corresponding to an exposure of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mn>1.76</a:mn><a:mo>×</a:mo><a:msup><a:mn>10</a:mn><a:mn>20</a:mn></a:msup></a:math> protons on target. The <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>γ</c:mi></c:math> ray signals resulting from neutron captures were identified using a neural network. The flux-averaged mean neutron capture multiplicity was measured to be <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msubsup><e:mrow><e:mn>1.37</e:mn><e:mo>±</e:mo><e:mn>0.33</e:mn><e:mtext> </e:mtext><e:mo stretchy="false">(</e:mo><e:mi>stat</e:mi><e:mo>.</e:mo><e:mo stretchy="false">)</e:mo></e:mrow><e:mrow><e:mo>−</e:mo><e:mn>0.27</e:mn></e:mrow><e:mrow><e:mo>+</e:mo><e:mn>0.17</e:mn></e:mrow></e:msubsup><e:mtext> </e:mtext><e:mo stretchy="false">(</e:mo><e:mi>syst</e:mi><e:mo>.</e:mo><e:mo stretchy="false">)</e:mo></e:mrow></e:math>, which is compatible within 2.3 sigma than predictions obtained using our nominal simulation. We discuss potential sources of systematic uncertainty in the prediction and demonstrate that a significant portion of this discrepancy arises from the modeling of hadron-nucleus interactions in the detector medium.