Observation of the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ decay and measurement of its branching ratio
E. Cortina, J. Jerhot, N. Lurkin, T. Numao, B. Velghe, V. W. S. Wong, D. Bryman, Z. Hives, T. Husek, Karol Kampf, Marián Kolesár, M. Koval, B. De Martino, M. Perrin-Terrin, Laëticia Petit, Babette Döbrich, S. Lezki, J. Schubert, Atakan Tugberk Akmete, R. Aliberti, M. Ceoletta, L. Di Lella, N. Doble, L. Peruzzo, C. Polivka, S. Schuchmann, H. Wahl, Riccardo Wanke, P. Dalpiaz, R. Negrello, I. Neri, F. Petrucci, M. Soldani, L. Bandiera, N. Canale, A. Cotta Ramusino, A. Gianoli, M. Romagnoni, A. Sytov, M. Lenti, P. Lo Chiatto, I. Panichi, G. Ruggiero, A. Bizzeti, F. Bucci, A. Antonelli, V. Kozhuharov, G. Lanfranchi, S. Martellotti, M. Moulson, T. Spadaro, G. Tinti, F. Ambrosino, M. D’Errico, R. Fiorenza, M. Francesconi, R. Giordano, P. Massarotti, M. Mirra, M. Napolitano, Isadora Rosa, G. Saracino, G. Anzivino, P Cenci, V. Duk, R. Lollini, P. Lubrano, Moises Luiz Pepe, M. Piccini, F. Costantini, M. A. Giorgi, S. Giudici, G. Lamanna, E. Lari, E. Pedreschi, J. Pinzino, M. Sozzi, R. Fantechi, F. Spinella, I. Mannelli, M. Raggi, A. Biagioni, Paolo Cretaro, O. Frezza, A. Lonardo, M. Turisini, P. Vicini, R. Ammendola, V. Bonaiuto, A. Fucci, A. Salamon, F. Sargeni, R. Arcidiacono, B. Bloch-Devaux, E. Menichetti, E. Migliore, C. Biino, A. Filippi, F. Marchetto, D. Soldi
Abstract
A bstract A measurement of the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016–2018 data taking. Combining NA62 data collected in 2016–2022, a measurement of $$ \mathcal{B}\left({K}^{+}\to {\pi}^{+}\nu \overline{\nu}\right)=\left({13.0}_{-3.0}^{+3.3}\right)\times {10}^{-11} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>B</mml:mi> <mml:mfenced> <mml:mrow> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:mfenced> <mml:msubsup> <mml:mn>13.0</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3.3</mml:mn> </mml:mrow> </mml:msubsup> </mml:mfenced> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:math> is reported. With 51 signal candidates observed and an expected background of $$ {18}_{-2}^{+3} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mn>18</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> events, $$ \mathcal{B}\left({K}^{+}\to {\pi}^{+}\nu \overline{\nu}\right) $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>B</mml:mi> <mml:mfenced> <mml:mrow> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:mrow> </mml:mfenced> </mml:math> becomes the smallest branching ratio measured with a signal significance above 5 σ .