Heavy neutral lepton (HNL) Heavy neutral lepton (HNL) production searches at NA62 production searches at NA62 XXVIII International Conference on Neutrino Physics and Astrophysics, Heidelberg, June 2018 Lorenza Iacobuzio, University of Birmingham - lorenza.iacobuzio@cern.ch Neutrino Minimal Standard Model Previous measurements HNL production searches at NA62 Event selection Data-MonteCarlo comparison SM extension accounting for baryon asymmetry of universe (BAU), dark matter (DM), neutrino masses and oscillations 3 additional right-handed, singlet, Majorana HNLs (not observed yet) N1 mass O(10 keV/c2), good DM candidate N2,3 mass O(1 GeV/c) [2] B(K+ → l+N) depends on BSM(K+ → l+ν), HNL-lepton coupling U 2 l4, and kinematic factor [2] Fixed-target experiment at CERN SPS: SPS 400 GeV/c p beam onto Be target → 75 GeV/c K+ In-flight K+ decays in 60 m long fiducial volume (FV) Data taking (2015-2018) with possibility to extend beyond 2021 Measure B(K+ → π+νν) with 20% precision to study CKM matrix element Vtd Perform hidden sector searches in presence of K+ beam (axions, dark photons, HNLs) Minimum bias run (2015) [1]: Data collected during 1.2·104 SPS spills (about 3 s effective spill) 0.4% - 1.3% of nominal beam intensity (3.3·1012 protons on target per spill) HNL production searches in kaon decays [1]: Kaons tagged with upstream Cherenkov detector (70 ps time resolution) Lepton tracks reconstructed in downstream spectrometer (130 μm spatial resolution) Particle ID with downstream Cherenkov detector, spectrometer and calorimeters One positive track with momentum in (5, 70) GeV/c, in time with kaon No extra activity in photon veto systems Positrons and muons identified through energy-momentum ratio [1] Squared missing mass: m2 miss = (PK - Pl)2 (SM K+ → l+ν peak at m2 miss = 0) HNL signal regions: 170 < mmiss < 448 MeV/c2 for K+ → e+N and 250 < mmiss < 373 MeV/c2 for K+ → μ+N [1] [2] JHEP0710:015 (2007) [3] Physical Review D 91, 052001 (2015) [4] Physical Review Letters 49, 18 (1982) [5] Physics Letters B 772, 712–718 (2017) ν M S M p a r t i c l e z o o l o g y NA62 schematic layout. HNL signal topology is visible ● PMNS mixing matrix between ● HNL flavour and mass eigenstates ● Expected and observed UL (at 90% CL) from NA62 2007 data-taking [4] Production searches: Look for peaks in missing mass distribution Decay-model independent Sensitive to long-lived HNLs Previous measurements in K + → μ+N 90% CL upper limits (UL) on HNL-muon coupling U 2 μ: KEK E089 (1982) [3]: (10 6 −, 10 4 −) for mN in (70, 300) MeV/c2 BNL E949 (2015) [4]: (10 9 −, 10 7 −) for mN in (175, 300) MeV/c2 NA62 (2017) [5]: (2·10 6 −, 10 5 −) for mN in (300, 375) MeV/c2 ● Expected and observed number of events: K+ → e+N (left) and K+ → μ+N (right) [2] ● Squared missing mass distributions for data and simulated events passing the e+ (left) and μ+ (right) selections [2] Technique, results and future prospects ● Squared missing mass distributions for data and simulated events passing the e+ (left) and μ+ (right) selections [2] Mass scans performed in HNL signal regions with step size of 1 MeV/c2 For each HNL mass hypothesis, background evaluated from sidebands of data mmiss distribution No statistically significant HNL production signal observed UL on B(K+ → l+N) established for each HNL mass hypothesis UL on coupling computed from UL on BR UL on |U2 e| in (10 7 −, 10 6 −) for mN in (170, 448) MeV/c2 and on |U2 μ| in (10 7 −, 10 6 −) for mN in (250, 373) MeV/c2 [1] Results improve world existing limits on HNL production searches on |U2 e| (over whole mass range) and on |U2 μ| (for masses above 300 MeV/c2) 2016-2018 data collected by NA62 much larger than 2015 sample Opportunity to further improve existing limits on |U2 e| and |U2 μ| Published in Physics Letters B 778, 137–145 (2018) [1]