Search for K+ decays to a muon and invisible particles

E Cortina Gil,L Bician,C Lazzeroni,F Gonnella,E Iacopini,M Lupi,M Noy,R Wanke,M Misheva,Y Petrov,M Zamkovsky,M Mirra,F Marchetto,T Capussela,L Litov,E Migliore,S Schuchmann,A Bizzeti,A Mapelli,V Obraztsov,A Cotta Ramusino,J Kunze,M Sozzi,L Pontisso,E Gamberini,N De Simone,T Husek,E Maurice,M Piccini,V Kekelidze,V Bonaiuto,V Semenov,N Estrada-Tristan,P Dalpiaz,E Goudzovski,J Fu,H Wahl,A Korotkova,R Volpe,B Wrona,S Venditti,S Giudici,A Fucci,A Ostankov,T Spadaro,M Koval,L Fulton,G Latino,M Raggi,I Polenkevich,A Gianoli,A Filippi,A Mefodev,G Lamanna,E Menichetti,G Ruggiero,A Ceccucci,D Bryman,F Hahn,C Graham,P Petrov,M Moulson,D Lomidze,B Checcucci,S Fedotov,L Di Lella,D Protopopescu,F Ambrosino,J Engelfried,E Imbergamo,V Palladino,S Movchan,V Kurshetsov,Y Kudenko,F Spinella,O Yushchenko,P Rubin,N Lurkin,D Coward,R Arcidiacono,B Jenninger,G Lehmann Miotto,P Laycock,R Piandani,S Padolski,M Perrin-Terrin,M Corvino,G Lanfranchi,Yu Potrebenikov,G Saracino,R Fantechi,V Duk,M Lenti,P Massarotti,T Bache,P Vicini,B Bloch-Devaux,F Bucci,A Antonelli,S Shkarovskiy,E Minucci,B Velghe,I Mannelli,L Peruzzo,N Doble,R Marchevski,V Falaleev,F Petrucci,F Duval,C Cerri,A Kleimenova,A Lonardo,E Pedreschi,F Newson,V Kurochka,L Federici,A Norton,G Anzivino,D Soldi,T Numao,D Emelyanov,B Angelucci,N Molokanova,V Ryjov,L Iacobuzio,A Sturgess,J Pinzino,A Sergi,A Biagioni,R Guida,M Napolitano,G Khoriauli,M Vormstein,P Valente,E Gushchin,J Bernhard,V Kozhuharov,D Britton,L Gatignon,S Kholodenko,D Di Filippo,F Sargeni,R Lollini,E Leonardi,C Santoni,D Hutchcroft,A Khotyantsev,M Boretto,T Enik,M Barbanera,R Aliberti,P Lichard,A Conovaloff,C Biino,A Salamon,D Madigozhin,A Parenti,V Fascianelli,A Shaikhiev,A Zinchenko,F Costantini,S Martellotti,M Medvedeva,A Romano,P Lubrano,G Georgiev,P Cenci,M Fiorini,Ilaria Neri ,Roberto Ammendola ,Stefan Alexandru Ghinescu ,H F Heath ,M Pepé ,Joseph Carmignani ,Karim Massri ,Ovidiu Emanuel Hutanu ,Aigul Baeva ,Jan Jerhot ,Karol Kampf ,Francesco Brizioli ,P S Cooper ,T Blažek ,Jack Henshaw ,M A Giorgi ,V Černý ,C J Parkinson ,Ryan Page ,Maria Brigida Brunetti ,Joel Swallow ,J B Dainton ,G D’agostini ,R W L Jones ,J R Fry ,Babette Döbrich ,H O Danielsson ,Stoyan Miroslavov Trilov ,Z Kučerová ,Dosbol Baigarashev ,Enrico Lari ,V.p Sugonyaev ,A M Bragadireanu ,E B Holzer

doi:10.1016/j.physletb.2021.136259

Abstract

The NA62 experiment at CERN reports searches for K+→μ+N and K+→μ+νX decays, where N and X are massive invisible particles, using the 2016–2018 data set. The N particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of O(10−8) of the neutrino mixing parameter |Uμ4|2 for N masses in the range 200–384 MeV/c2 and lifetime exceeding 50 ns. The X particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for X masses in the range 10–370 MeV/c2 are reported for the first time, ranging from O(10−5) to O(10−7). An improved upper limit of 1.0×10−6 is established at 90% CL on the K+→μ+ννν¯ branching fraction.

Highlights

All Standard Model (SM) fermions except neutrinos are known to exhibit both chiralities
A search for heavy neutral leptons (HNLs) production in K+ → μ+N decays has been performed using the data set collected by the NA62 experiment in 2016–2018
Upper limits of the HNL mixing parameter |Uμ4|2 are established at the level of O(10−8) over the HNL mass range of 200–384 MeV/c2 with the assumption of mean lifetime exceeding 50 ns, improving on the previous HNL production searches

Summary

Introduction

All Standard Model (SM) fermions except neutrinos are known to exhibit both chiralities. A particular scenario, which accommodates dark matter (DM) freeze-out, involves a scalar or vector hidden sector mediator X coupling preferentially to the muon This mediator is expected to be produced in K+ → μ+νX decays with an estimated branching fraction of O(10−8) in case mX < mK − mμ, and is expected to decay promptly with a sizeable invisible branching fraction [7]. The data sample used for this analysis is obtained from 0.92 × 106 SPS spills recorded during 410 days of operation in 2016–2018, with the typical beam intensity increasing over time from 1.3 × 1012 to 2.2 × 1012 protons per spill The latter value corresponds to a mean instantaneous beam particle rate at the FV entrance of 500 MHz, and a mean K+ decay rate in the FV of 3.7 MHz. Data recorded with a minimum-bias trigger based on CHOD signals [11], downscaled by a factor of 400, is used for the analysis. This trigger is 99% efficient for single charged particles in the CHOD acceptance

Measurement principles and event selection

Background evaluation with simulations

Background source

Findings

Summary