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Proton decay sensitivity in future water cherenkov detector

Proton decay sensitivity in future water cherenkov detector. M.Miura Kamioka Observatory, ICRR NNN 2010, Toyama. Contents. Introduction p -> e + + p 0 mode p -> K + + n mode Summary. 1. Introduction. Proton decay is a breakthrough toward GUTs.

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Proton decay sensitivity in future water cherenkov detector

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  1. Proton decay sensitivity in future water cherenkov detector M.Miura Kamioka Observatory, ICRR NNN 2010, Toyama

  2. Contents • Introduction • p -> e++p0 mode • p -> K++n mode • Summary

  3. 1. Introduction • Proton decay is a breakthrough toward GUTs. • pe+p0, nK+ are regarded as dominant mode. • The latest results of proton lifetime limit; • e+p0: 1.2x1034 years @ 206 kton・year • nK+: 3.9x1033 years (14 years since SK start) • Reach 1Megaton・year => 56 years more by SK? • or construct larger detector. • Cost is bottle neck to construct larger detector. One of dominant is cost for photo sensor. • Physics results : as much as possible • Cost : as much as low •  Need optimize photo coverage .

  4. In this talk, I made proton decay MC (PDK) and atmospheric n MC (ATM) with; Detector size, PMT(20inch), Electronics, water quality => Same as Super-Kamiokande(SK) photo coverage 40 %: Current configuration of SK. 20 %: same as SK2 10 %: new one to study sensitivity for pe+p0, nK+ .

  5. 2. p->e++p0 mode 2-1: Event features and selection • Event features; • e+ and p0 are back-to-back (459 MeV/c) in nucleon rest frame. • p0 decays into two g s (one g may be missed if direction of the other g is close to p0). • =>2 or 3 e-like ring should be observed. • p0 mass should be reconstructed by two rings (3-ring case). • =>Proton mass should be reconstructed by all ring and total momentum should be small. g P p0 e+ g • Selection; • Fully contained, VTX in fiducail volume. • 2 or 3 ring and all e-like, w/o decay-electron. • 85 < Mp0 < 185 MeV (for 3-ring event) . • 800 < MP < 1050 MeV & Ptot < 250 MeV/c

  6. 2-2) Current results (SK1~SK4) PDK MC ATM MC Data Total momentum (MeV/c) Total momeutum (MeV/c) Total Mass (MeV) Total Mass Total Momentum No candidate. Data and Atmospheric n MC agree well.

  7. Photo coverage 40% 20% 40% 40% (±sys.error) In this mode, 40% and 20% are almost same in efficiency and BKG rate. Total exposure: 205.7 kton・year Lifetime limit: > 1.2x1034 years (90% C.L.)

  8. 2-3) Photo coverage study 40% 20 % 10 % Electron ring p0 rings

  9. Zoom in: p0 rings 40% 20 % 10 %

  10. Apply Standard Cuts BKG rate 1.62±0.39 2.55±0.57 (stat. only) (/Mton・yr)

  11. Loose efficiency for 10 % photo coverage Increase fake ring Poor energy resolution 40 % 40 % 20 % 20 % 10 % 10 % Tune cuts for 10 % photo coverage

  12. Cut tuning for e+p0 Visible energy for 4th ring p0mass PDK PDK Ring counting thr = 40 MeV &>5% of Eall. ATMn ATMn Cut: 2≤ Nring≤3 => 2≤ Nring≤3 .or. (Nring=4&Evis(4)<100 MeV) Cut: 85<Mp0<185 MeV => 85< Mp0≤230 MeV After tune: Eff = 35.1 %, BKG = 3.47 evts/Mton・year

  13. Sensitivity curve for e+p0 search (90% CL) 40 % (~20%) 10 % Preliminary 1035 year  ~4 Megaton・year for 40(20) % photo coverage ~9 Megaton・year for 10 % photo coverage

  14. 2. p-> n + K+ mode K+: below cherenkov threshold.  Use decay products of K+. visible Method A) K+ -> m++nm invisible • Event features; • Proton => K+(below Č thrs.)+n. • K+ mostｌｙ stops and decays into m+ (236 MeV/c)+n (Br.64%). • => Monochromatic m n ne e+ P nm m+ • Selection: • 1 m-like ring with decay-e (except method-B). K+: t=12ns nm

  15. Current results (SK1-SK4) Black: Data Red: ATMn MC Blue: PDK MC No excess in Data. Agree with ATM n.

  16. B) K+ -> m++nm with prompt g • Event features; • Proton in 16O decays and excited nucleus emits 6 MeV g (Prob. 41%, not clear ring). • => Tag g to eliminate BKG. visible ne g 6.3MeV invisible e+ 16O->15N nm m+ • Selection: • 1 m-like ring with decay-e. • 215 < Pm < 260 MeV/c • Search Max hit cluster by sliding time window (12ns width); • - 8 < Ng < 60 hits for SK-1,3,4 • 4 < Ng < 30 hits for SK-2 • & • - Tm-Tg < 75 nsec K+ nm n Tm(dN/dt=max) Hits m e g t 12ns window Tstart (dN/dt=0)

  17. Current results (SK1-SK4) SK1,SK3,SK4 merged plot No candidate.

  18. C) K+ -> p++p0 • Event features; • Br. 21 %. • p0and p+ are back-to-back and have 205 MeV/c. • Pp+ is just above Č thres. • (not clear ring). ne nm p+ Eres e+ visible K+ Ebk invisible m+ nm g => Search for monochromatic p0 with backward activities. p0 205 MeV/c g • Selection: • 2 e-like rings with decay-e. • 85 < Mp0 < 185 MeV. • 175 < Pp0 < 250 MeV/c. • Ebk: visible energy sum in 140-180 deg. of p0 dir, • Eres: in 90-140 deg. • 7< Ebk < 17 MeV& Eres < 12 MeV

  19. Current results (SK1-SK4) SK1,SK3,SK4 merged plot No candidate.

  20. Current results (SK1-SK4) K+ -> m++nm with prompt g (±sys.error) K+ -> p++p0 Decay-e tagging improved by new electronics. Total exposure: 205.7 kton・year Lifetime limit: > 3.9x1033 years (90% C.L.)

  21. 3-2) Photo coverage study 40 % 20 % 10 % m ring (236 MeV/c, Evis~50 MeV)

  22. Apply Standard Cuts:K+ -> m++nm with prompt g * 2<Ng<15 is applied to 10% MC BKG 1.5 1.6 12.1 /Megaton ・yr 8 times larger !

  23. Comparison some distributions |VTXrec-VTXtrue| (1R events) Ng 40 % 40 % 44 cm 20 % 20 % 66 cm 10 % 91 cm 10 %

  24. Cut tuning for Ng PDK MC ATMn MC After tuning Cut 2 < Ng < 15  3 < Ng <15 Eff. 3.6 %  2.7 % BKG 12.1  2.1 events/Megaton・year

  25. Apply Standard Cuts:K+ -> p++p0 BKG 6.5 6.7 8.4 /Megaton・yr

  26. Comparison some distributions Ebk Eres 40 % 40 % 20 % 20 % 10 % 10 %

  27. Cut parameter tuning for Ebk After tuning Cut 7< Ebk < 17 MeV 7 < Ebk <20 MeV Eff. 3.0 %  3.4 % BKG 8.4  8.6events/Megaton・year

  28. Sensitivity curve for nK+ search (90% CL) 40% 20% 10% Preliminary 1034 year  ~1 Megaton・year for 40 % photo coverage ~1.2 Megaton・year for 20 % photo coverage ~7 Megaton・year for 10 % photo coverage

  29. Sensitivity curve with each method Red: Merged Blue: prompt g Green: p+p0 Black: Pm spectrum 40 % 20 % 10 % Prompt g method is dominant in each case.

  30. Improve g tagging with better PMT Crude estimation of QE effect No g, or g overlap with m tail. Ng Eff. for Ng cut Should have g hit (by vector cuts) 20 % coverage g tagging eff (%) 10% coverage Red hist x 2 Factor Better QE can recover efficiency. Should study PMT performance (QE, dark rate, T-resolution, e.t.c.)

  31. 4. Summary • Study for proton decay sensitivity by changing photo coverage (tank size, 20inch PMT are same as SK). • Rough estimate required exposure e+p0 1035year: ~9Mt・year exposure for 10 % coverage ~4Mt・year for 40 (20) %

  32. nK+ 1034year: ~7Mt・year exposure for 10 % coverage ~1Mt・year for 40 (20) % Next Step • Improve reconstruction tools. • Investigate PMT performance. - QE - Dark rate - PMT size - T resolution - …….

  33. Backup

  34. K+ -> m++nm with prompt g Decay-e PID 40 % 40 % 19 % 19 % 10 % 10 %

  35. K+ -> m++nm with prompt g Pm Lm-e 40 % 40 % 19 % 19 % 10 % 10 %

  36. Example 2: decrease dark rate Simple MC: m 236 MeV/c Black: 5.3kHz default) Red: 3.0 kHz Ng Should study PMT performance (QE, dark rate, T-resolution, PMT size…)

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