Proton Decays. -- motivation, status, and future prospect -- Univ. of Tokyo, Kamioka Observatory Masato Shiozawa

Transcription

1 Proton Decays -- motivation, status, and future prospect -- Univ. of Tokyo, Kamioka Observatory Masato Shiozawa

2 Look for Baryon number violation B number conservation is experimental subject B number conservation is probably accidental Different from electric charge conservation assured by local gauge symmetry (EM force) indirect evidence for B by B asymmetry in the universe

3 GUTs or Plank scale physics Grand Unification G SU(2) U (1) SU(3) Quark+lepton Proton decays

4 Motivation for nucleon decay searches directly confirm or exclude GUTs or Plank scale physics Successful unification of 15(+1?) fermions in one or two representation(s). 3 coupling constants seem to meet at ~10 16 GeV small neutrino mass indicates heavy n R

5 Lifetime prediction p p Dimension=6 (2 fermion 2 fermion) + e 4 5 u u d g m Γ = p M 4 X : τ( p e+ π 0 ) = 10 ~35? years Dimension=5 (2 fermion - 2 sfermion) q~ ν h 4 m 5 u u d w ~ X q~ H ~ C d d s d p 0 K + Γ = p M 2 M 2 Hc X : τ( p νk+ ) = 10 29~39? years minimal SU(5) minimal SUSY SU(5) Super-K SUSY SO(10) partial nucleon life time (year)

6 The Super- Kamiokande detector C Scientific American Ring Imaging Water Cherenkov detector Charged particle 42m 39m 1000 m underground 50,000 ton (22,500 ton fid.) 11, inch PMTs 1,885 anti-counter PMTs

7 Super- K as a proton decay detector Source H 2 O 2/10 free proton no nuclear effect, accurate high efficiency no Fermi motion, good momentum valance 22.5 kton protons Good detector performance Vertex resolution: 30 cm (1-ring) :15 cm(p e + π 0 ) Trigger threshold: 5 MeV e trigger ε=100% for most of nucleon decay modes Energy resolution: 3~4% for e, µ Particle ID : 99% 1-ring 1 µ,, e : >97% p e + π 0,p µ + π 0

8 Proton decay kinematics Nucleon decay kinematics are calculated taking into account; Fermi motion of parent nucleons in 16 O Nuclear potential Nucleon-nucleon correlation in 16 O 10% decays in 16 O are correlated decay (p+n e + +p 0 +N) (Yamazaki and Akaishi, PLB453(1999)1-6) Uncorrelated (normal) decay

9 Meson interactions Meson interactions in 16 O p 0, p +, p - h r w scattering, absorption, charge exchange in 16 O Nh Np, Nh, Npp short lifetime, 100% decay without interactions. conversion to p, r, s, w 16 O e + p 0 p + ~50% p 0 escape w/o any interactions

10 energy reconstruction Full SK-I period cosmic ray mu p0 invariant mass +-1% decay electrons +-1% Corrected for light attenuation length in water Time variation of E scale ~ 0.9% E scale difference < 1.8% (decaye, pi0, cosmic mu) energy scale uncertainty of neutrino detection < 2.0%

11 p e + p p e + p 0 MC e + g g

12 p e + p p e + p 0 MC Criteria for p e + p 0 2 or 3 Cherenkov rings All rings are showering 85 < Mπ0 M < 185MeV/c 2 (3-ring) No decay electron 800 < M proton < 1050 MeV/c 2 P total < 250 MeV/c ε = 40 %

13 p e + p AtmnBG MC data 0.3 exp d BG 0 candidate t p/b(p e + p 0 ) > years (90% CL)

14 Comparison of data and MC in p e + p 0 search

15 p e + h 0, h gg p e + h 0 MC AtmnBG MC data ε = 17% 0.2 exp d BG 0 candidate t p/b(p e + h 0 ) > years (90% CL)

16 p e + r 0, r p + p - p e + r 0 MC ε = 4.6% AtmnBG MC 0.4 exp d BG data 0 candidate t p/b(p e + r 0 ) > years (90% CL)

17 p e + w 0, w p 0 g p e + w 0 MC ε = 3.0% AtmnBG MC 0.5 exp d BG data 0 candidate t p/b(p e + w 0 ) > years (90% CL)

18 p nk +, K + m + Method I (spectrum fit) 1 µ and 1 decay electron 215 < Pµ P < 260 MeV/c no proton ν p K + m + e + Br(K + + )=63% Number of Events m momentum (MeV/c) tp/b(p nk + ) > 3.8 x10 32 years (90% CL)

19 p nk +, K + m + Method II (prompt g tag) ν 16 O 15 N * K + m + g e + 1 µ and 1 decay electron 215 < Pµ P < 260 MeV/c no proton (non oscillated) µ + e + γ TOF subtracted time (nsec) g s number of hits ε x B= 8.6 %, 0.7 exp d BG, 0 candidate t p /B(p nk + ) > 11.4 x10 32 years (90% CL)

20 Method III ν p nk +, K + p + p Br(K + p + p 0 )=21% p K + µ + p + π 0 e + g g 2 showering Cherenkov rings 1 decay electron 85 < Mπ0 M < 185 MeV/c < Pπ0 P < 250 MeV/c tag π + (40 < Qback < 100 p.e.).) e x B= 6.0 %, 0.6 exp d BG, 0 candidate tp/b(p nk + ) > 7.9 x years (90% CL) combined limit tp/b(p nk + ) > 2.2 x years (90% CL)

21 SK nucleon decay search summary

22 Soudan 2 detector Fine-grained tracking calorimeter x224 (0.96kton) Active component 2.7m Track resolution : 0.18 cm (X,Y) : 1.0 cm (Z) Trigger threshold (50 %) : 75MeV : 100MeV e Energy resolution : E/E = 7.0/E 1/ % e, = 8 % Partile ID : 95% : 95% >400MeV e drift tube 11mm 1m 1m 14.7mm

23 p K +, K + X TOP VIEW K charged tracks (no proton) K range < 50 cm 28 < range < 58 cm decay electron ( 60 %) e + B(K + + ) = 9.0% Y SIDE VIEW Z Y FRONT VIEW 3.56 kt yr exposure observed candidate = 1 exp d BG = 0.21 (atm )+0.19(rock) Z X

24 X + TOP VIEW 0 2 p K +, K + + K + 2 tracks and 2 showers K range < 50 cm 100 < M K < 660 MeV/c 2 80 < P + < 400 MeV/c 40 < P < 390 MeV/c 10 < M < 290 MeV/c 2 B(K ) = 5.5% SIDE VIEW Z FRONT VIEW 3.56 kt yr exposure observed candidate = 0 exp d BG = 1.05 (atm )+0.09(rock) Y Z Y X combined limit p /B(p K + ) > years (90% CL)

25 pfink + for Soudan2 and Super-K Exposure (Kton?year ) Efficiency for K + µ + ν (K + π + π 0 ) #Observed Events for K + µ + ν (K + π + π 0 ) Expected Background 90%CL limit (years) Soudan (5.5) 1 (0) 0.4 (1.14) Super-K (6.0) 0 (0) 0.7 (0.6)

26 Summary of nucleon decay searches in Soudan2

27 Searches for decays into invisible particles n nnn (Kamiokande) tagging nuclear g (19MeV) from the residual nucleus /B(n nnn) > years (90% CL) p invisible(sno, PLB 553 (2003) ) tagging residual neutron /B(p invisible) > years (90% CL)

28 Future prospects

29 Next generation Mton size Water Cherenkov detector M.Koshiba Phys. Rep. 220 (1992) Megaton underground neutrino detector Supernova neutrino burst from the Local Group, within 1 Mpc - modern SN as well as ancient SN (relic neutrinos) long baseline neutrino oscillation experiment - nm ne oscillation, CP violation in neutrino oscillations proton decays atomospheric neutrinos - sign of Dm 2 by measuring earth s matter effect high energy neutrino point sources WINPs annihilating in the sun and/or in the earth mass composition of the primary cosmic rays at energies >10 15 GeV/nucleon

30 Hyper-Kamiokande Detector D design 48m x 50m x 500m, Total mass = 1 Mton (0.54Mton fid. mass)

31 Candidate site Shallower than SK (1000m ~700m) OK for >100MeV particles maybe more BG for relic neutrino observation (need study)

32 e + p 0 sensitivity SK cut Tight momentum cut HK 20 yr HK 20 yr /B > yr (Hyper-K 20yrs, 90%CL)

33 nk+ sensitivity SK cut (old ones) HK 20 yr /B > yr (Hyper-K 20yrs, 90%CL)

34 Photo sensor R&D

35 Conclusion Proton decay is the direct test of unification of quarks and leptons. some GUT models have been excluded. we may be close to proton decay signals. No proton decay evidence so far. 5.4 x %CL (ep 0 ) 2.2 x %CL (nk( + ) Low BG level (O(10-1 )) in many modes enable us to further improve lifetime limits (or to find proton decays) Megaton size water Cherenkov detector is a multi- purpose detector and has rich physics topics.