New limits on Heavy Neutral Lepton from Kaon experiments @ CERN Monica Pepe INFN Perugia On behalf of the NA62 and NA48/2 Collaborations International Workshop on Baryon & Lepton Number Violation 2017 May 15 18, 2017 Cleveland, USA
Outline v History: NA48 & NA62 v Theoretical motivations v Search for Heavy Neutrino in K + µ + N decays @ NA62-2007 v Search for Heavy Neutrino in K ± πµµ decays @ NA48/2 v Conclusions The NA62 Collaboration: Birmingham, Bratislava, Bristol, Bucharest, CERN, Dubna, Fairfax, Ferrara, Firenze, Frascati, Glasgow, Liverpool, Louvain, Mainz, Merced, Moskow, Napoli, Perugia, Pisa, Prague, Protvino, Roma I, Roma II, San Luis Potosi, Sofia, Torino, TRIUMF, Vancouver The NA48/2 Collaboration Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Wien 2
NA48 : K L + K S Search for direct CPV : Measurement of ε /ε NA48/1 : K S Rare K S / Hyperon decays, CPV tests NA48/NA62 NA48 & NA62 1997 1998 1999 2000 2001 2002 2003 2004 : 2007 2008 : 2014 Jura mountains Geneva airport SPS France NA48/NA62: at the heart of the LHC! LHC Switzerland NA48/2 : K + + K - Search for direct CPV : Charge asymmetry measurement NA62-2007 : K + + K - R K = K ± e2/k ± µ2 measurement NA62 : K + Measurement of the decay K + π + νν N 3
The NA48/2 and NA62-2007 detector Magnetic spectrometer: 4 DCHs + dipole magnet σ(p)/p = (1.02 0.044*p)% [p ingev/c] NA48/2 = (0.48 0.009*p)% [p ingev/c] NA62-2007 Liquid Krypton EM calorimeter (LKr) σ(e)/e = (3.2/ E 9.0/E 0.42)%[E in GeV] σ x = σ y ~ 1.5 mm @ E = 10 GeV Charged Hodoscope Fast trigger + Precise time measurement : σ = 150 ps PT KICK : 265 MeV/c in NA62-2007 125 MeV/c in NA48/2 Beam pipe Narrow K ± beams unseparated charged hadron beam K ± ~ 6% p K = 60 GeV/c, δp K/ p K ~ 4% rms NA48/2 p K = 74 GeV/c, δp K/ p K ~ 1% rms NA62-2007 nominal K ± decay rate ~ 100 khz Decay region expands over ~ 114 m in vacuum Pion decay products, from the hadronic beam, remain into the beam pipe Similar acceptance between K + and K - beams ensured reversing magnetic fields 4
Majorana Neutrinos Observation of neutrino oscillations à Neutrino mass needs to be accomodated in the SM. Ø Asaka-Shaposhnikov model (νmsm) [PLB 620 (2005) 17] Ø Dark Matter + Baryon Asymmetry of the Universe (BAU) + low mass of SM ν can be explained by adding 3 right-handed neutrinos Ni to the SM N 1 is the lightest O(keV) à Dark Matter candidate N 2 N 3 (mass ~ 100 MeV to few GeV) introduce extra CPV-phases to account for Baryon Asymmetry N 2 N 3 produce standard neutrino masses through see-saw mechanism with a Yukawa coupling of ~10-8 Active-sterile neutrino Ni mixing with SM is described by the U-matrix Effective vertices involving Ni, W ± /Z bosons and SM leptons N 2,3 production in K ± decays K ± l ± N, K ± π 0 l ± N This talk: l = µ N 2,3 decays for m 2,3 < m K - m l N π ± l, N π 0 ν N l 1± l 2 ν 2, N ν 1 l 1+ l 2 -, N ν l νν 5
NA48/2 & NA62-2007 strategies A complementary study: v Heavy Neutrino Production: K + µ + N 4 è Independent of HN decay modes è Sensitive to long-lived (or stable) HN è Scales linearly with kaon flux Search for a peak in the m miss = (P K P µ ) 2 spectrum [paper in preparation] NA62-2007 v Heavy Neutrino Production + Decay: K ± µ ± N 4 (N 4 πµ) [PLB 769 (2017) 67] è Model dependent (HN decay modes and lifetime) è Sensitive to short-lived (unstable) HN è Sensitive to Majorana/Dirac nature of HN Search for 3-tracks vertex topology events (K ± µ ± πµ) Main trigger streams Ø NA62-2007 : min-bias, 1-track Ø NA48/2 : 3-track vertex N48/2 6
Heavy Neutrino search @ NA62-2007 7
The NA62-2007 single-muon sample Only K + beam data (43% of NA62-2007 sample)! higher muon halo rejection Ø Event selection One well reconstructed µ + track No extra clusters in LKr with E > 2 GeV Five-dimensional (z vertex, θ, p, CDA, ϕ) kinematic suppression of muon halo Ø Data driven study of: Halo background Spectrometer resolution tails Trigger and µ-id efficiencies Ø HN detailed MC simulation for: Acceptance vs HN mass m miss peak resolution vs HN mass 1 MeV/c 2 mass intervals Ø No HN decay in the detector Signal region : m miss (µ + ) in range 300-375 MeV/c 2 Ø Strong limit exists below Ø No acceptance above Kaon decays in the fiducial volume N K ~ 6 10 7 (from reconstructed K + µ + ν) (Downscale D=150 of min-bias trigger) 8
HN : search for K + µ + N 4 Uncertainty on N (exp) Upper Limits on Signal Events UL(N sig ) Ø Statistical uncertainty on muon halo background dominates for HN masses > 300 MeV/c 2 No excess above 3σ observed Ø 1 MeV/c2 steps Ø Mass window size : ±(σ HN = 12 MeV/c 2 0.03 m HN ) Ø Rolke-Lopez method to get UL(N sig ) 9
BR UL (K + µ + Ν 4 ) = HN : NA62-2007 result (I) From N sig to BR From BR to U µ4 2 UL(N sig ) Ν κ Acceptance 1 BR(K + µ + Ν 4 ) U µ4 2 = f(m HN ) BR(K + µ + ν µ ) Phase Space + helicity suppression BR UL (K + µ + Ν 4 ) U µ4 2 UL 10
HN : NA62-2007 result (II) Comparison to existing peak search measurements U µ4 2 UL 1 BR(K + µ + Ν 4 ) U µ4 2 = f(m HN ) BR(K + µ + ν µ ) NA62-2007 result extends the mass range for UL on U µ4 2 in HN production search experiments Most stringent limit on HN production in the mass region 300 < m HN < 375 MeV/c 2 11
Heavy Neutrino search @ NA48/2 12
LNV: NA48/2 same-sign µ sample M(π µ ± µ ± ) Ø K ± π µ ± µ ± à ΔL =2 transitions mediated by Majorana neutrino exchange Ø Blind analysis Selection based on MC simulation of K ± π µ ± µ ± and K ± π ± π + π - Additional K ± π ± π + π - MC for background estimation Control region M(π µ ± µ ± ) < 480 MeV/c 2 for Data/MC Ø Event selection One well reconstructed 3-track vertex 2 same-sign muons, 1odd sign pion Total P t consistent with zero Signal region M(π µ ± µ ± ) - M k < 5 MeV/c 2 Kaon decays in the fiducial volume N K ~ 2 10 11 (from reconstructed K ± π ± π + π - ) Ø Events in Signal Region observed after K ± π µ ± µ ± selection: N obs = 1 Ø Expected background (from MC): N exp = 1.163 ± 0.867 stat ± 0.021 ext ± 0.116 syst Ø Rolke-Lopez method to get UL(N sig ) BR(K ± π µ ± µ ± ) < 8.6 10-11 @ 90% CL 13
LNC: NA48/2 opposite-sign µ sample M(π ± µ + µ - ) Ø Event selection Similar to same-sign muon sample One well reconstructed 3-track vertex 2 opposite-sign muons, 1same-sign pion Total P t consistent with zero Signal region M(π ± µ + µ - ) - M k < 8 MeV/c 2 M(π ± µ ) Improved selection wrt previous NA48/2 K ± π ± µ + µ - analysis [PLB 697(2011)107] Ø 3489 K ± π ± µ + µ - candidates in Signal Region Ø K ± π ± π + π - background: (0.32 ± 0.09)% Search for 2-body resonances in the M(π ± µ ) invariant mass 14
Resonance search in LNV decays Same-sign µ sample: Search for K ± µ ± N 4 (N 4 π µ ± ) UL(N sig ) UL(N sig ) UL(BR(K ± µ ± N 4 ) BR(N 4 π µ ± )) = Νκ Acceptance z N Statistical significance obs - N z = exp never exceed 3σ: σ(nobs ) σ(n exp ) no signal observed 15
Resonance search in LNC decays Opposite-sign µ sample: Search for K ± µ ± N 4 (N 4 π ± µ ) UL(N sig ) UL(N sig ) UL(BR(K ± µ ± N 4 ) BR(N 4 π ± µ )) = Νκ Acceptance z N Statistical significance obs - N z = exp never exceed 3σ: σ(nobs ) σ(n exp ) no signal observed 16
NA48/2 Constraints on U µ4 2 From UL on BR to UL on U µ4 2 : Same sign µ sample (LNV) Opposite-sign µ sample (LNC) UL on U µ4 2 UL on U µ4 2 17
Conclusions Ø The NA48/2 and NA62-2007 experiments at CERN were exposed to ~ 2 10 11 and ~ 2 10 10 K ± decays respectively v NA48/2: HN Production + Decay [PLB 769 (2017) 67] è Search for LNV K ± π µ ± µ ± decay : BR(K ± π µ ± µ ± ) < 8.6 10-11 @ 90% CL (World Best Limit) Factor 10 improvement wrt previous best limit (1.1 10-9 @ 90% CL ) è Search for K ± µ ± N 4 (N 4 π ± µ ) (LNC Heavy neutrino) Limits on BR products of the order of 10-9 for HN lifetimes < 100 ps è Search for K ± µ ± N 4 (N 4 π µ ± ) (LNV Majorana neutrino) Limits on BR products of the order of 10-10 for HN lifetimes < 100 ps v NA62-2007: HN Production in K + µ + N 4 decays [Paper in preparation] è Limits on BR(K + µ + Ν 4 ) ~ 10-5 è Limits on U µ4 2 ~ 10-5 for m HN > 300 MeV/c 2 v NA62 perspectives: Ø 2015 preliminary analysis of 5 days MB data: ~23M K + µ + ν µ, 1/100 bkg wrt 2007 data Ø 2016A data set: event yield expected to be comparable to NA48/2 but much lower bkg! improved results from K + l + N and K + πll expected in the coming years 18
SPARES 19
The NA62 experiment @ CERN Primary beam: CERN SPS protons 3 10 12 ppp, 400 GeV/c (x3 NA48/2) Secondary beam: unseparated positive beam π/k/p K + ~ 6%, p K = 75 GeV/c (Δp/p ~ 1.1%) K + decays/year = 4.5 10 12 ( 45 NA48/2) integrated average rate = 750 MHz average K decay rate 10 MHz Goal: measurement of BR(K + π + ν ν) è O(50) SM events/year, BR @ 10% accuracy è 2014: detector commissioning è 2015: Trigger and high intensity beam line commissioning, detector quality studies è 2016: High level trigger and full beam tracker commissioning, physics analysis (ongoing) è Data samples 2015: Low intensity beam with minimum bias trigger 2016: Stable conditions up to 40% of nominal intensity 20
NA62: Search for HN production Ø In 2015 NA62 collected 5 days Minimum Bias data Ø Preliminary analysis of data shows: ~23M K ± µ ± ν µ decays (~1500 K ± e ± ν e ) with a background level 100 times lower wrt NA62-2007 à Can set worlds most stringent limits on heavy neutrino production 2015 data ~23M events 2015 data ~1500 events 21
NA62: 3-track sample Ø K + πµµ and K + πee three-track decays can be used to probe lepton universality, as well as new particles (HN and Inflatons) Ø Preliminary analysis of 2016A data set: à event yield expected to be comparable to that of NA48/2 (currently the world largest sample) but with much lower background level 2016A data ~1k K + π + e + e - 2016A data ~2k K + π + µ + µ - 22
Limits on U µ4 2 23