Rare kaon decays at NA62 Evgueni Goudzovski (University of Birmingham) on behalf of the CERN-NA62 collaboration Outline: 1) The NA62 experiment at CERN 2) Measurement of the K + + decay 3) Searches for lepton number/flavour violating K + decays 4) Searches for HNL production in K + decays 5) Summary CLFV 2019 conference Fukuoka, Japan 18 June 2019 0
Kaon programme at CERN Earlier: NA31 1997: ’/ : K L +K S 1998: K L +K S Jura mountains France 1999: K L +K S K S HI NA48 NA48/NA62: discovery 2000: K L only K S HI ECN3 hall Switzerland SPS of direct CPV 2001: K L +K S K S HI LHC NA48/1 2002: K S /hyperons 2003: K + /K − NA48/2 2004: K + /K − N Geneva airport 2007: K e2 /K 2 tests NA62 R K run 2008: K e2 /K 2 tests Main NA62 goal: K + + measurement to 10% precision 2015: commissioning NA62 with a novel decay-in-flight technique. 2016 18: physics run Currently ~200 participants from 31 institutions. 1 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
NA62 collaboration, Beamline & detector JINST 12 (2017) P05025 Hadronic Un-separated hadron (p/ + /K + ) beam. LAV: large-angle Calorimeter Muon SPS protons: 400 GeV , nominally 3.3 × 10 12 /spill. photon veto (12 stations) (HAC) detector K + : 75 GeV/c ( ± 1%), divergence < 100 rad. t =70ps (MUV) Nominal beam rate: 750 MHz, K + rate 45 MHz; ~5 MHz K + decays in fiducial volume Dump GTK: beam tracker 300 m 3 O(10 6 ) mbar KTAG: Cherenkov kaon tagger, t =70ps Small-angle Spectrometer: Anti-counters photon veto straw chambers LKr EM calorimeter Z [m] Currently, 1 year of operation 2× 10 18 protons on target; 3×10 12 K + decays. Single event sensitivities for K + decays: down to BR~10 12 . Kinematic rejection factors: 1×10 3 for K + + 0 , 3×10 4 for K + . Hermetic photon veto: 0 decay suppression (for E 0 >40 GeV ) = 3×10 8 . Particle ID (RICH+LKr+HAC+MUV): ~10 8 muon suppression. 2 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
NA62 data collection Commissioning run 2015 : minimum bias data ( ~3×10 10 protons/pulse). Physics run 2016 (30 days, ~1.3×10 12 ppp ): 10 11 useful K + decays. Physics run 2017 (161 days, ~2.0×10 12 ppp ): ~2×10 12 useful K + decays. Physics run 2018 (217 days, ~2.3×10 12 ppp ): expect 3×10 12 useful K + decays. Resuming data taking after Long Shutdown 2 in 2021 . 3 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
K + + measurement: first result and prospects Result based on the 2016 data: Phys. Lett. B791 (2019) 156 4 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Rare kaon decays: K SM: box and penguin diagrams SM branching ratios Ultra-rare decays with Buras et al., JHEP 1511 (2015) 033 the highest CKM suppression: BR SM 10 11 Mode A ~ (m t /m W ) 2 |V ts V td | ~ 5 * K + + ( ) 8.4 1.0 K L 0 3.4 0.6 SM precision surpasses any other FCNC process involving quarks. The uncertainties are largely parametric (CKM) Measurement of |V td | complementary to those from e.g. B B mixing. Theoretically clean, Main focus of kaon physics: measurement almost unexplored, of both K + + and K L 0 decays. sensitive to new physics. 5 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Status of theory and experiment BR(K L 0 ) vs BR(K + + ) CKM unitarity triangle with kaons Current experimental uncertainty ( 1 ) NP with CKM-like flavour structure Kaon measurements alone can fully constrain the unitarity triangle. Complementary to B physics in the description of NP flavour dynamics. 6 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
K signal region definition 2 =(P K P ) 2 vs track momentum Main K + decay modes m miss DATA 2016 ( >90% of BR) rejected K + + + kinematically. K + + 0 0 Resolution on m miss 2 : =1.0 10 3 GeV 4 /c 2 . Measured kinematical Region II K + + 0 background suppression: K + + 0 : 1×10 − 3 ; K + + : 3×10 − 4 . Region I Further background K + + suppression: PID (calorimeters & Cherenkov detectors): suppression 10 8 . Hermetic photon veto: suppression of 0 decays 3×10 8 . 7 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Data 2016 K + + + region PLB791 (2019) 156 Blinded Region 2 Data sample: 30 days at 1.3×10 12 ppp . K + + Number of kaon decays: region K + + 0 Blinded N K = (1.21 0.02 syst )×10 11 . region 2016 data Region 1 Background estimates are mostly data-driven. Signal acceptance: A = (4.0 0.1)% . Single-event sensitivity: SES = (3.15 0.24)×10 10 . 8 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Result 2016 K + + + region The RICH ring for the track y [mm] K + + region K + + 0 region 2016 data e One K + + candidate observed: BR(K + + ) < 11×10 10 at 90% CL. x [mm] PLB791 (2019) 156 +1.15 BNL-E949 ( K + decay at rest): BR(K + + ) = (1.73 ) 10 10 1.05 SM prediction: BR(K + + ) = (0.84 0.10)×10 10 The NA62 decay-in-flight technique works. A non-trivial result obtained with 2% of the total statistics collected. 9 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
K + + : next steps (CERN SPSC open session, 2 April 2019) Analysis of the 2017 data Data sample: 161 days at 2.0×10 12 ppp . Analysis procedure is similar to the 2016 one. Number of kaon decays: N K = (1.3 0.1)×10 12 . Single-event sensitivity: SES = (3.4 0.4)×10 11 . Expected signal: 2.5 0.4 SM K + + events. Expected background : 0.76 0.10 events, excluding upstream decays. Result expected later this year, surpassing present best sensitivity. Analysis of the 2018 data The largest sample collected so far: 217 days at 2.3×10 12 ppp . Analysis procedure being optimized to improve acceptance. Further data collection necessary to reach the 10% precision Data taking will resume after LS2 (in 2021 ). Developing a strategy to collect 100 SM events by 2024 . Possible beam dump operation ( 3 months of data taking = 10 18 pot ): competitive searches for hidden sector (long-lived HNL, DP , ALP). 10 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
K L 0 vs K + + : prospects KOTO 2015 data: BR<3.0×10 9 @90% CL PRL122 (2019) 021802 KOTO sensitivity with 2015 18 data. Expect result in 2019. KOTO sensitivity goal following beam & detector upgrades (~2025?) NA62 precision by 2025 (with 100 SM events) 11 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Searches for lepton flavour and lepton number violation Downscaled di-lepton trigger chains are in operation. First results based on 80% of the 2017 data: CERN-EP-2019-104, arXiv:1905.07770, submitted to PLB. Analyses completed so far: searches for K + ℓ + ℓ + decays. 12 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Backgrounds and PID Pion mis-ID probability vs momentum Major background for 3-track decays: the K + + + decay ( BR=5.6% ). Studied with data-driven methods and dedicated simulations. Background to decays into leptons: 1) via decays in flight; 2) via misidentification as e . Pion/electron identification: 1) by energy deposit in LKr ( E/p ); 2) by the RICH signal pattern. 13 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
K + e + e + : auxiliary selection LNV selection: m( e + e + ) SM selection: m( + e + e ) K + + + K + + + Signal region K + + e + K + + e + K + + 0 D Auxiliary selection: LKr only is used for pion/electron ID Validation of the background estimates using control mass regions. Sensitivity is limited by K + + 0 D background. Therefore RICH is used for positron ID ( 10% loss of SES, 6 times lower background)… E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
K + e + e + : main selection LNV selection: m( e + e + ) SM selection: m( + e + e ) Signal region K + + e + K + [e + e ] 0 e + SES = (0.94 0.03)×10 10 Candidates observed: 2484 BR(K + + e + e ) = (3.00 0.09)×10 7 Expected background: 0.16 0.03 evt K + decays in FV: (2.14 0.07)×10 11 Candidates observed: 0 Result: BR(K + e + e + )<2.2×10 10 at 90% CL 15 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
Search for K + + + decay LNV selection: m( + + ) SM selection: m( + + ) Signal region K + + + K + + + Signal region K + + + K + + + Candidates observed: 8357 SES = (1.28 0.04)×10 11 Background: 0.07% Expected background: 0.91 0.41 evt BR(K + + + ) = (0.962 0.025)×10 7 Candidates observed: 1 K + decays in FV: (7.94 0.23)×10 11 16 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018
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