Abstract
A search for heavy neutral lepton production in K+ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to 10−6 level are established on the elements of the extended neutrino mixing matrix |Ue4|2 and |Uμ4|2 for heavy neutral lepton mass in the ranges 170–448 MeV/c2 and 250–373 MeV/c2, respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |Ue4|2, and above 300 MeV/c2 for |Uμ4|2.
Highlights
Non-zero masses and mixing of the Standard Model (SM) neutrinos are firmly established
To achieve hermetic acceptance for photons emitted in K + decays in the fiducial decay volume (FV) at angles up to 50 mrad, the liquid krypton (LKr) calorimeter is supplemented by annular lead glass large-angle veto (LAV) detectors installed in 12 positions along and downstream of the FV, and two lead/scintillator sampling calorimeters located close to the beam axis
Inputs to the computation of the numbers N K of kaon decays in the FV: numbers of selected data events in the SM signal region, acceptances evaluated with Monte Carlo (MC) simulations and their statistical errors, and K + →
Summary
Non-zero masses and mixing of the Standard Model (SM) neutrinos are firmly established. The beam is accompanied by a flux of muons produced by K + and π + decays upstream of the vacuum tank (the beam halo), with 3 MHz nominal rate in the detector acceptance. The momenta of charged K + decay products are measured by a magnetic spectrometer (STRAW) located in the vacuum tank downstream of the FV. To achieve hermetic acceptance for photons emitted in K + decays in the FV at angles up to 50 mrad, the LKr calorimeter is supplemented by annular lead glass large-angle veto (LAV) detectors installed in 12 positions along and downstream of the FV, and two lead/scintillator sampling calorimeters (intermediate-ring calorimeter, IRC, and smallangle calorimeter, SAC) located close to the beam axis. Loose timing conditions are used in this analysis because the accidental rates are small, due to the low beam intensity
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