Signal selection and model-independent extraction of the neutrino neutral-current single <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> </mml:math> cross section with the T2K experiment

This article presents a study of single <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msup> <a:mi>π</a:mi> <a:mo>+</a:mo> </a:msup> </a:math> production in neutrino neutral-current interactions ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi>NC</c:mi> <c:mn>1</c:mn> <c:msup> <c:mrow> <c:mi>π</c:mi> </c:mrow> <c:mrow> <c:mo>+</c:mo> </c:mrow> </c:msup> </c:mrow> </c:math> ) using the FGD1 hydrocarbon target of the ND280 detector of the T2K experiment. We report the largest sample of such events selected by any experiment, providing the first new data for this channel in over four decades and the first using a sub-GeV neutrino flux. The signal selection strategy and its performance are detailed together with validations of a robust cross section extraction methodology. The measured flux-averaged integrated cross-section is <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:mi>σ</e:mi> <e:mo>=</e:mo> <e:mo stretchy="false">(</e:mo> <e:mn>6.07</e:mn> <e:mo>±</e:mo> <e:mn>1.22</e:mn> <e:mo stretchy="false">)</e:mo> <e:mo>×</e:mo> <e:msup> <e:mrow> <e:mn>10</e:mn> </e:mrow> <e:mrow> <e:mo>−</e:mo> <e:mn>41</e:mn> </e:mrow> </e:msup> <e:mtext> </e:mtext> <e:mtext> </e:mtext> <e:msup> <e:mrow> <e:mi>cm</e:mi> </e:mrow> <e:mrow> <e:mn>2</e:mn> </e:mrow> </e:msup> <e:mo>/</e:mo> <e:mi>nucleon</e:mi> </e:mrow> </e:math> , <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:mn>1.3</i:mn> <i:mi>σ</i:mi> </i:mrow> </i:math> above the NEUT v5.4.0 expectation.