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Searches for nucleon decay and n-n oscillation in Super-Kamiokande

Searches for nucleon decay and n-n oscillation in Super-Kamiokande. Jun Kameda (ICRR, Univ. of Tokyo) for Super-Kamiokande collaboration Sep. 14 th , 2007 TAUP 2007, Sendai, Japan. Outline. Introduction of the Super-Kamiokande Searches for Nucleon decay (p e + + p 0 , p m + +p 0 )

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Searches for nucleon decay and n-n oscillation in Super-Kamiokande

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  1. Searches for nucleon decay and n-n oscillation in Super-Kamiokande Jun Kameda (ICRR, Univ. of Tokyo) for Super-Kamiokande collaboration Sep. 14th, 2007 TAUP 2007, Sendai, Japan TAUP2007

  2. Outline • Introduction of the Super-Kamiokande • Searches for Nucleon decay • (pe++p0, pm++p0) • Search for Neutron-antineutron oscillation • Summary TAUP2007

  3. Super-Kamiokande Super-Kamioka Nucleon Decay Experiment • Water Cherenkov detector • 50kton (22.5 kton fid.), • = 6.02x1033neutrons+ • 7.53x1033protons • Deep underground • (1000m, 2700 m.w.e), • Kamioka-mine, Japan. SK-I ID 11146 20inch PMTs OD 1885 8-inch PMTs SK-II ID 5182 20inch PMTs 39.3m 41.4m 5 countries, 35 institutes ~120 physicist Data period: SK-I : 91.6ktyr (Apr.1996-Jul.2001) SK-II: 49.5ktyr (Dec.2002-Oct.2005) SK-III had started from Jul.2006. TAUP2007

  4. Nucleon decay Searchesin Super-Kamiokande • Nucleon decay is the direct evidence of the Grand Unification Theories (GUTs). • Several GUTs predicts several decay modes. (pe+p0,m+p0,nK+,..) • No positive signal had been observed. TAUP2007 Experimental Lifetime lower limits (yr)

  5. Nucleon decay (I) p e+ + p0 Event selection: Super-K I 45.0% (was 42.9%) Efficiency : Super-K II 43.6% (was 42.2%) Improved efficiencies are due to an improved algorithm of separation of the overlapping Cherenkov rings. All visible particles contained 2 or 3 Cherenkov rings All rings are e-like no decay electron 85 < Mp[MeV/c2] <185 (for 3ring) 800 < Mtot [MeV/c2] < 1050 & Ptot [MeV/c] < 250 TAUP2007

  6. Total P vs Invariant Mass plotsp e+ + p0 atmn MC Data p  e++p0 MC SK-I Eff. 45.0% 0.4 event 0 event 1489.2 days Total P(MeV/c) SK-II Eff. 43.6% 0.04 event 0 event 798.6 days Invariant Mass (MeV/c2) No signal was observed in SK-I & SK-II. TAUP2007

  7. Result of the search forp e+ + p0(preliminary) • No Candidate was observed. • Estimated B.G. (n osc. considered) SK-I: 0.4, SK-II: 0.04 • tP/Bep0 > 5.5 x 1033 years (90 % C.L.) (SK-I) • tP/Bep0 > 2.9 x 1033 years (90 % C.L.) (SK-II) tP/Bep0 > 8.4 x 1033 years (90 % C.L) (SK-I + SK-II) TAUP2007

  8. Nucleon decay (II) p m+ + p0 Event selection: m+ Super-K I 35.6% (was 30.6%) Efficiency : Super-K II 35.5% (was 32.1%) All visible particles contained 2 or 3 Cherenkov rings 1 m-like 1 decay electron 85 < Mp[MeV/c2] <185 (for 3ring) 800 < Mtot [MeV/c2] < 1050 & Ptot [MeV/c] < 250 TAUP2007

  9. Result of p m+ + p0(preliminary) atmn MC Data p  m++p0 MC SK-I Eff. 35.6% 0.4 event 0 event 1489.2 days Total P(MeV/c) SK-II Eff. 35.5% 0.2 event 0 event 798.6 days No candidates. Invariant Mass(MeV/c2) • tP/Bep0 > 4.4 x 1033 yrs (90 % C.L.) (SK-I) • tP/Bep0 > 2.3 x 1033 yrs (90 % C.L.) (SK-II)  tP/Bep0 > 6.7 x 1033 years (SK-I + SK-II, 90 % C.L) TAUP2007

  10. Summary of nucleon decay search • No positive signals have been observed in any mode. • Obtained lifetime limits: • pe++p0 : t/B > 8.4 x1033 yr (SK-I+SK-II, preliminary) • pm++p0 : t/B > 6.7 x1033 yr (SK-I +SK-II, preliminary) • pn+K+ : t/B > 2.3 x1033yr (SK-I), PRD 72, 052007 (2005), • Many other modes (B-L violating modes, etc.) have been searched for in Super-K. TAUP2007

  11. Search for neutron - anti-neutron oscillation in Oxygen in Super-K I • Several type of (B-L)-violating Gauge theories predicts that neutron spontaneously converts to anti-neutron, and vise versa.(ex. K.S.Babu et al., Phys.Lett.B518(2001) 269-275) • |DB| =2 process. Good test for new physics including GUTs. TAUP2007

  12. Branching ratio derived from Bubble Chamber p + d data p p p p n n n p p p Expected signals in Water Cherenkov detector • Anti-neutrons quickly pair-annihilates with the surrounding nucleon. • About 2 x nucleon mass energy will be released in the residual nuclei. anti-n + n anti-n + p Pions (<np>~4 ) of several 100MeV/c’s are emitted isotropically, and propagate inside the nuclein TAUP2007

  13. Typical Event (anti-n+p p++p++p0+p-) n-nbar MC p0 g p- g g p0 g p+ p+ g p0 g Many pions and their secondaries + low energy nuclear fragments Multi-Cherenkov ring events are expected. TAUP2007

  14. Event Selection 0) all visible particles contained a) #of rings >=2 b) 700< visible energy (MeV) <1300 c) 0< Ptot (MeV/c) <450 d) 750< Mtot (MeV/c2) <1800  10.4% efficiency Invariant Mass is smaller than 2xnucleon due to p absorption in & p interaction in nuclear & water # of rings Invariant Mass (MeV/c2) Total Momentum (MeV/c) Visible Energy(MeV) TAUP2007

  15. Results ofn-n oscillation n-n MC 10.4 % detection efficiency 21.3 background events (n osc. effects are included) 20 candidates atm. n MC Ptot (MeV/c) = 1.77×1032 yrs ( 90% CL ) (SK-1) SK-I data (systematic errors are included by Baysian statistics) Not a spontaneous decay of neutron, but we call the inverse of the event rate is the “lifetime” of the neutron in Oxygen. Inv. Mass (MeV/c2) TAUP2007

  16. Systematic uncertainties Possible sources of the systematic uncertainty in experimental and modeling (Pion interactions, n+anti-n annihilation, etc.) are investigated. B.G rate Detection efficiency & Exposure TAUP2007

  17. Comparison with other experimental limits Bound neutron lifetime can be interpreted to the oscillation time of the free neutron by Tbound = R*(tfree)2 * C.B.Dover et al. Phys. Rev. D 27 (1983) 1090 Super-K I results includes systematic uncertainty. TAUP2007

  18. Summary • Nucleon decay searches and neutron-antineutron oscillation are reported. • No evidence of nucleon decay,no evidence of n-n oscillation were found. • pe++p0 : t/B > 8.4 x1033 yr (SK-I+SK-II, preliminary) • pm++p0 : t/B > 6.7 x1033 yr (SK-I +SK-II, preliminary) • n-n oscillation: Tbound> 1.77x1032 yr (SK-I, preliminary) tfree> 2.36x108 sec TAUP2007

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