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
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
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