Search for Nucleon Decay with Super-K Hide-Kazu TANAKA (University - PowerPoint PPT Presentation

Search for Nucleon Decay 
 with Super-K Hide-Kazu TANAKA (University of Tokyo, ICRR) for the Super-Kamiokande collaboration TAUP2019, September 9, 2019

Nucleon Decay なぜ核子崩壊？ 核子崩壊実験の目的 標準理論では理解できない根源的な問 陽電子 陽子 核子が崩壊する！ 強い力 電磁気力 弱い力 大統一？ 力と素粒子の 3 新たなパラダイムの開拓を目指す レプトン・ クォーク間の直接遷移を ニュートリノの微小質量 での一致 い なぜクォークとレプトンが必要？ な ぜ つの力？ ？ なぜ 電子 ＝ クォーク ？ 素粒子と力の統一（大統一）が問い に答える可能性 大統一の間接的証拠 力の強さの • Nucleon decay can occur via a direct transition from quark into lepton p → e + π 0 decay mode 見る → 大統一の直接検証 • Baryon numbers ( B ) not conserved Positron Positron • Standard Model violates B at an Proton extremely small level • ➜ Observation of nucleon decay clear evidence of beyond the standard model Unification of running couplings • Grand Unified Theory (GUT) 60 • Attempt to unify forces and particles (at α 1 α 2 10 15-16 GeV) 50 α 3 • → Imply nucleon decay 40 -1 (Q) • Many GUT models and variety of 30 α i predictions on nucleon lifetime, decay 20 PDG 2018 modes and branching ratio 10 • Nucleon decay search an unique probe for SOFTSUSY 3.6.2 0 0 5 10 15 GUT and physics in very high energy log 10 (Q/GeV) � 2

Nucleon Decay 核子崩壊実験の目的 陽電子 陽子 核子が崩壊する！ 強い力 電磁気力 弱い力 大統一？ 力と素粒子の 3 新たなパラダイムの開拓を目指す レプトン・ クォーク間の直接遷移を なぜ核子崩壊？ 標準理論では理解できない根源的な問 での一致 力の強さの 大統一の間接的証拠 に答える可能性 素粒子と力の統一（大統一）が問い クォーク ？ 電子 ＝ なぜ ？ ぜ つの力？ なぜクォークとレプトンが必要？ な い ニュートリノの微小質量 • Nucleon decay can occur via a direct transition from quark into lepton p → e + π 0 decay mode 見る → 大統一の直接検証 • Baryon numbers ( B ) not conserved Positron Positron • Standard Model violates B at an ∅ Proton extremely small level • ➜ Observation of nucleon decay clear evidence of beyond the standard model Unification of running couplings • Grand Unified Theory (GUT) ∅ 60 • Attempt to unify forces and particles (at α 1 α 2 10 15-16 GeV) 50 α 3 • → Imply nucleon decay 40 -1 (Q) • Many GUT models and variety of 30 α i predictions on nucleon lifetime, decay 20 PDG 2018 modes and branching ratio 10 • Nucleon decay search an unique probe for SOFTSUSY 3.6.2 0 0 5 10 15 GUT and physics in very high energy log 10 (Q/GeV) � 2

Nucleon Decay なぜ核子崩壊？ 核子崩壊実験の目的 標準理論では理解できない根源的な問 陽電子 陽子 核子が崩壊する！ 強い力 電磁気力 弱い力 大統一？ 力と素粒子の 3 新たなパラダイムの開拓を目指す レプトン・ クォーク間の直接遷移を ニュートリノの微小質量 での一致 い なぜクォークとレプトンが必要？ な ぜ つの力？ ？ なぜ 電子 ＝ クォーク ？ 素粒子と力の統一（大統一）が問い に答える可能性 大統一の間接的証拠 力の強さの • Nucleon decay can occur via a direct transition from quark into lepton p → e + π 0 decay mode 見る → 大統一の直接検証 • Baryon numbers ( B ) not conserved Positron Positron • Standard Model violates B at an Proton extremely small level • ➜ Observation of nucleon decay clear evidence of beyond the standard model Unification of running couplings • Grand Unified Theory (GUT) 60 • Attempt to unify forces and particles (at α 1 α 2 10 15-16 GeV) 50 α 3 • → Imply nucleon decay 40 -1 (Q) • Many GUT models and variety of 30 α i predictions on nucleon lifetime, decay 20 PDG 2018 modes and branching ratio 10 • Nucleon decay search an unique probe for SOFTSUSY 3.6.2 0 0 5 10 15 GUT and physics in very high energy log 10 (Q/GeV) � 2

Super-Kamiokande SK atmospheric ν e μ μ -like e-like e-like 3985 muon-like 3915 300 250 CCQE electron CCQE muon 200 150 100 (MC) 50 (MC) 0 -10 -8 -6 -4 -2 0 2 4 6 8 10 • 42m (height) x 39m (diameter) large water Č PID likelihood PID likelihood sub-GeV 1ring (FC) ν detector filled with 50kton ultra-pure water SK cosmic-ray μ • Fiducial mass: 22.5kton (conventional) � � • Excellent particle identification ( μ and e) � � � • Separate EM-shower type (e-like) and � muon type ( μ -like) with Č ring pattern PID likelihood • Mis-PID rate below 1% at ~1GeV μ � � mis-PID: • Good energy resolution: ~3% at ~1GeV Data: 0.00±0.16(stat.)% MC : 0.10±0.10(stat.)% � 3 24

Search for p → e + π 0 Signal MC • Positron and π 0 run back-to-back • Event selection: • Momentum 459 MeV/c • All particles are fully contained in FV • 2 or 3 rings (two of them from π 0) • All particles in the final stable are • All particles are e-like, w/o Michel-e • 85 < M π 0 < 185 MeV/c 2 visible with Super-K • 800 < M p < 1050 MeV/c 2 • Able to reconstruct p mass • 100 < P tot < 250 or P tot < 100MeV/c • Neutron-tagging (SK-IV) and momentum • Further reduce bkg by ~50% � 4

Signal & background Free!proton→e + π 0 ! • Signal : p → e + π 0 !e +! ! π 0! • One of major causes of signal e ffi ciency loss p→e + π 0 !in! 16 O! is due to final state interaction (FSI) of π in the recoiling nucleus ! π +! !e +! • An advantage of water Č detector is to have ! π 0! ‘free proton’ target • cf. p → e + π 0 signal selection e ffi ciency: 
 ex. π 0 from PDK interacts with nucleons in the target in oxygen: ~40%, 
 nucleus and loose original in hydrogen: 80+% (SK-IV) kinematics (ex. momentum) and/or modified charge • Background for proton decay search K2K beam data • Atmospheric neutrino; CC- π production 10 3 • Background rate prediction confirmed 10 2 number of events Total with data from K2K-1KT Č detector 10 invariant • PRD77, 032003 (2008) mass 1 PRD77, 032003 (2008) • Background under control -1 10 � 5 200 400 600 800 1000 1200 total invariant mass (MeV/c 2 )

p → e + π 0 : Signal & Bkg Signal (MC) Oxygen • Signal selection e ffi ciency: ~40% Hydrogen • cf. ~80% for free proton decay Super-Kamiokande I-IV atm ν MC • Expected bkg contamination in signal Background (MC) region for entire SK period (SK-I~IV): • Lower P tot : 0.05 events • Upper P tot : 0.58 events � 6

p → e + π 0 : Results ● : Data - : Bkg MC - : Signal (oxygen) MC - : Signal (hydrogen) MC SK-I~IV Data 10 3 10 2 Number of events 10 1 π -1 10 -2 10 -3 10 -4 10 0 10 500 1000 0 Total mass (MeV/c 2 ) • Found no event in the signal box • Lifetime limit at 90% C.L. with 365 kton ∙ yrs (SK-I~IV) • τ /Br > 2.0 × 10 34 years [preliminary] • Most stringent constraint � 7

Search for p → μ + π 0 • Spirit of the event selection is similar to p → e + π 0 mode but SK-I~IV Data requires 1 μ -like ring + Michel-e • Signal selection e ff : ~40% • Expected bkg contamination for entire SK observation period (SK-I~IV): • Lower P tot : 0.07 events • Upper P tot : 0.65 events • Found 1 evt in upper signal box • It’s not obvious data excess • Lifetime limit at 90% C.L. with 
 compared to expected bkg 365 kt ∙ years (SK-I~IV) exposure 
 • See PRD95, 012004 (2017) τ /Br>1.2 × 10 34 years [preliminary] � 8

Search for p →ν̅ K + • K + has momentum of 340 MeV/c • Below Cherenkov threshold (560 MeV/c) • Identify K + by finding its decay products K + → µ + ν µ K + → π + π 0 (K+ leptonic decay) (K+ hadronic decay) 236 MeV/c 205 MeV/c Search Methods Search Method � Nuclear de-exitation γ , µ , and decay e+ � π + and two γ from π 0 decay � Monochromatic µ from K+ decay ( π + Č threshold 156MeV/c) � 9

Search for p →ν̅ +K + Super-Kamiokande I-IV Number of Events 160 【 K+ leptonic decay 】 140 120 (a) Search for 100 mono-energetic 80 ● : Data 60 (236MeV/c) μ - : Bkg MC 40 - : Signal MC 20 0 200 225 250 275 300 P µ (MeV/c) (b) De-excitation γ (6.3MeV) + μ decay Super-Kamiokande I, III, IV Number of Events 10 2 Dot: Data γ" 16 O →ν K +15 N γ , K + → µ + ν" ν" Box: ATM MC µ + ! Hist: PDK MC 10 µ + ! (arbitrary norm.) e + " γ" 1 t ! 16 O ! K + ! SK-I/III/IV ν" -1 10 Proton decays in 16 O → Excited nucleus ( 15 N*) emits -2 10 6.3 MeV γ -ray (~40% probability) 2 10 10 Number of γ hits ➜ γ , μ and Michel-e from μ -decay triple coincidence Number of Events largely reduce the bkg contamination 10

Search for p →ν̅ K + 【 K+ hadronic decay 】 Signal MC π 0 →γγ π + (backward) K+ → π + π 0 : π + and π 0 run back-to-back with 205 MeV/c • Found no evidence of p → ν̅ K+ • Lifetime limit combining all three search methods: 
 τ /Br > 8.2 × 10 33 years [preliminary] • at 90% C.L. with 365 kt ∙ years (SK-I~IV) � 11