An investigation of the very rare $K^+\rightarrow\pi^+\nu\bar{\nu}$ decay

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">     <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en" xmlns:og="http://ogp.me/ns#" xmlns:fb="http://ogp.me/ns/fb#"> <head> <title>An investigation of the very rare $K^+\rightarrow\pi^+\nu\bar{\nu}$ decay - CERN Document Server</title> <link href='https://framework.web.cern.ch/framework/2.0/fonts/PTSansWeb/PTSansWeb.css' rel='stylesheet' type='text/css' /> <link rel="stylesheet" href="https://cds.cern.ch/img/invenio.css?v=20141127" type="text/css" /> <link rel="stylesheet" href="https://cds.cern.ch/img/cern_theme/css/cern_theme.css?v=20141127" type="text/css" /> <link rel="stylesheet"href="/css/font-awesome.min.css"> <meta http-equiv="X-UA-Compatible" content="IE=Edge"/> <link rel="stylesheet" href="https://cds.cern.ch/img/cern_toolbar/css/toolbar.css" type="text/css" />    <link rel="canonical" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002" /> <link rel="alternate" hreflang="el" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=el" /> <link rel="alternate" hreflang="fr" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=fr" /> <link rel="alternate" hreflang="bg" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=bg" /> <link rel="alternate" hreflang="zh-TW" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=zh_TW" /> <link rel="alternate" hreflang="pt" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=pt" /> <link rel="alternate" hreflang="no" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=no" /> <link rel="alternate" hreflang="hr" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=hr" /> <link rel="alternate" hreflang="ca" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=ca" /> <link rel="alternate" hreflang="de" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=de" /> <link rel="alternate" hreflang="it" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=it" /> <link rel="alternate" hreflang="zh-CN" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=zh_CN" /> <link rel="alternate" hreflang="sv" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=sv" /> <link rel="alternate" hreflang="sk" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=sk" /> <link rel="alternate" hreflang="en" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=en" /> <link rel="alternate" hreflang="pl" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=pl" /> <link rel="alternate" hreflang="ru" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=ru" /> <link rel="alternate" hreflang="ka" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=ka" /> <link rel="alternate" hreflang="ja" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=ja" /> <link rel="alternate" hreflang="es" href="https://cds.cern.ch/record/2717647/files/CERN-EP-DRAFT-NA62-2020-002?ln=es" /> <link rel="alternate" type="application/rss+xml" title="CERN Document Server RSS" href="https://cds.cern.ch/rss" /> <link rel="search" type="application/opensearchdescription+xml" href="https://cds.cern.ch/opensearchdescription" title="CERN Document Server" /> <link rel="unapi-server" type="application/xml" title="unAPI" href="https://cds.cern.ch/unapi" /> <link rel="apple-touch-icon" href="/apple-touch-icon.png"/> <link rel="apple-touch-icon-precomposed" href="/apple-touch-icon-precomposed.png"/> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta http-equiv="Content-Language" content="en" /> <meta name="description" content="A search for heavy neutral lepton (N) production in $K^+\rightarrow\pi^+\nu\bar{\nu}$ decays using the data sample collected by the NA62 experiment at CERN in 2017–2018 is reported. Upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{−9}$ over most of the accessible heavy neutral lepton mass range 144–462 MeV/$c^2$, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The $|U_{e4}|^2$ range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/$c^2$. The NA62 experiment reports an investigation of the $ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $ mode from a sample of K$^{+}$ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0.389 ± 0.024) × 10$^{−10}$, corresponding to 2.2 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10$^{−11}$. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1.78 × 10$^{−10}$ for the $ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $ branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of ($ {0.48}_{-0.48}^{+0.72} $) × 10$^{−10}$, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements. The NA62 experiment reports an investigation of the $K^+\rightarrow\pi^+\nu\bar{\nu}$ mode from a sample of $K^+$ decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of $(0.389\pm 0.024)\times10^{-10}$, corresponding to 2.2 events assuming the Standard Model branching ratio of $(8.4\pm1.0)\times10^{-11}$. Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of $1.78\times10^{-10}$ for the $K^+\rightarrow\pi^+\nu\bar{\nu}$ branching ratio at 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of $(0.48^{+0.72 }_{-0.48})\times10^{-10}$, are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements. Cortina Gil, Eduardo; Minucci, Elisa; Padolski, Sergey; Petrov, Plamen; Velghe, Bob; Bryman, Douglas; Fu, Ji; Numao, Toshio; Husek, Tomas; Kampf, Karol; Zamkovsky, Michal; Jerhot, Jan; Aliberti, Riccardo; Khoriauli, Gia; Kunze, Jonas; Lomidze, David; Marchevski, Radoslav; Peruzzo, Letizia; Vormstein, Mario; Wanke, Rainer; Cotta Ramusino, Angelo; Dalpiaz, Pietro; Fiorini, Massimiliano; Gamberini, Enrico; Gianoli, Alberto; Neri, Ilaria; Norton, Alan; Petrucci, Ferruccio; Wahl, Heinrich; Bizzeti, Andrea; Bucci, Francesca; Iacopini, Enrico; Latino, Giuseppe; Lenti, Massimo; Parenti, Andrea; Volpe, Roberta; Antonelli, Antonella; Gonnella, Francesco; Lamanna, Gianluca; Lanfranchi, Gaia; Martellotti, Silvia; Moulson, Matthew; Kozhuharov, Venelin; Litov, Leandar; Georgiev, Georgi; Raggi, Mauro; Spadaro, Tommaso; Ambrosino, Fabio; Capussela, Tiziana; Corvino, Michele; Di Filippo, Domenico; Massarotti, Paolo; Mirra, Marco; Napolitano, Marco; Saracino, Giulio; Anzivino, Giuseppina; Imbergamo, 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class="portalboxheader"> <font size="-2">version 2<br />(see <a href="https://cds.cern.ch/record/2717647/files/?docname=2007.08218&version=all">previous</a>)</font> </td> <td> <table> <tr> <td valign="top"> <small><a href="https://cds.cern.ch/record/2717647/files/2007.08218.pdf?version=2">2007.08218.pdf</a></small> </td> <td valign="top"> <font size="-2" color="green">[2.78 MB]</font> <font size="-2"><em>22 Oct 2020, 04:10</em> </td> <td valign="top"><em>Fulltext</em></td> </tr></table></td></tr></table><table border="0" cellspacing="1" class="searchbox"> <tr> <td align="left" colspan="2" class="portalboxheader"> <img src='https://cds.cern.ch/img/smallfiles.gif' border="0" />  CERN-EP-2020-132 </td> </tr><tr> <td class="portalboxheader"> <font size="-2">version 2<br />(see <a href="https://cds.cern.ch/record/2717647/files/?docname=CERN-EP-2020-132&version=all">previous</a>)</font> </td> <td> <table> <tr> <td valign="top"> <small><a 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An investigation of the very rare $K^+\rightarrow\pi^+\nu\bar{\nu}$ decay - CERN Document Server