TAUP2007 1

Searches for nucleon decay and n-n oscillation

in Super-Kamiokande

Jun Kameda (ICRR, Univ. of Tokyo)

for Super-Kamiokande collaborationSep. 14th, 2007

TAUP 2007, Sendai, Japan

TAUP2007 2

Outline

Introduction of the Super-Kamiokande Searches for Nucleon decay

(pe++0, p0) Search for Neutron-antineutron oscillatio

n Summary

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Super-Kamiokande

• Water Cherenkov detector• 50kton (22.5 kton fid.), 　 = 6.02x1033 neutrons 　 + 7.53x1033 protons• Deep underground (1000m, 2700 m.w.e), Kamioka-mine, Japan.

SK-I : 91.6ktyr (Apr.1996-Jul.2001)SK-II: 49.5ktyr (Dec.2002-Oct.2005)SK-III had started from Jul.2006.

39.3m

41.4m

SK-IID 11146 20inch PMTsOD 1885 8-inch PMTsSK-IIID 5182 20inch PMTs

Data period:

Super-Kamioka Nucleon Decay Experiment

5 countries, 35 institutes~120 physicist

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Nucleon decay Searchesin Super-Kamiokande

Nucleon decay is the direct evidence of the Grand Unification Theories (GUTs).

Several GUTs predicts several decay modes. (pe+0,0,K+,..)

No positive signal had been observed.

Experimental Lifetime lower limits (yr)

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Nucleon decay (I) p e+ + 0

All visible particles contained 2 or 3 Cherenkov rings All rings are e-like no decay electron 85 < M[MeV/c2] <185 (for 3ring) 800 < Mtot [MeV/c2] < 1050

& Ptot [MeV/c] < 250

Event selection:

Efficiency : Super-K I 45.0% (was 42.9%)Super-K II 43.6% (was 42.2%)

Improved efficiencies are due to an improvedalgorithm of separation of the overlapping Cherenkov rings.

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Total P vs Invariant Mass plots p e+ + 0

SK-ISK-I

p e++0 MC

SK-IISK-II

No signal was observed in SK-I & SK-II.

atm MC Data

Eff. 45.0% 0.4 event 0 event

Eff. 43.6% 0.04 event 0 event

1489.2days

Invariant Mass (MeV/c2)

Total P(MeV/c)

798.6days

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• No Candidate was observed.• Estimated B.G. ( osc. considered) SK-I: 0.4, SK-II: 0.04• P/Be0 > 5.5 x 1033 years (90 % C.L.) (SK-I) • P/Be0 > 2.9 x 1033 years (90 % C.L.) (SK-II)

Result of the search for p e+ + 0 (preliminary)

P/Be0 > 8.4 x 1033 years (90 % C.L)

(SK-I + SK-II)

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Nucleon decay (II) p + + 0

All visible particles contained 2 or 3 Cherenkov rings 1 -like 1 decay electron 85 < M[MeV/c2] <185 (for 3ring) 800 < Mtot [MeV/c2] < 1050

& Ptot [MeV/c] < 250

Event selection:

Efficiency : Super-K I 35.6% (was 30.6%)Super-K II 35.5% (was 32.1%)

+

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Result of p + + 0 (preliminary)

SK-ISK-I

p ++0 MC

SK-IISK-II

atm MC Data

Eff. 35.6% 0.4 event 0 event

Eff. 35.5% 0.2 event 0 event

1489.2days

Total P(MeV/c)

798.6days

•P/Be0 > 4.4 x 1033 yrs (90 % C.L.) (SK-I)

• P/Be0 > 2.3 x 1033 yrs (90 % C.L.) (SK-II)

No candidates. Invariant Mass(MeV/c2)

P/Be0 > 6.7 x 1033 years (SK-I + SK-II, 90 % C.L)

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Summary of nucleon decay search

No positive signals have been observed in any mode.

Obtained lifetime limits: pe++0 : /B > 8.4 x1033 yr (SK-I+SK-II, preliminary) p++0 : /B > 6.7 x1033 yr (SK-I +SK-II, preliminary) p+K+ : /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.

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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)

|B| =2 process.Good test for new physicsincluding GUTs.

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Expected signals in Water Cherenkov detector

Anti-neutrons quickly pair-annihilates with the surrounding nucleon.

Pions (<n>~4 ) of several 100MeV/c’s are emitted isotropically, and propagate inside the nuclein

anti-n + p anti-n + n

np

np

np

About 2 x nucleon mass energy will be released in the residual nuclei.

Branching ratio derived fromBubble Chamber p + d data

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Typical Event (anti-n+p )

Multi-Cherenkov ring events are expected.

n-nbar MC

Many pions andtheir secondaries+ low energynuclear fragments

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0) all visible particles containeda) #of rings >= 2b) 700< visible energy (MeV) <1300c) 0< Ptot (MeV/c) <450d) 750< Mtot (MeV/c2) <1800

# of rings

Visible Energy(MeV) Total Momentum (MeV/c) Invariant Mass (MeV/c2)

Invariant Mass is smaller than 2xnucleondue to absorption in & interactionin nuclear & water

Event Selection

10.4% efficiency

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10.4 % detection efficiency

20 candidates

21.3 background events (osc. effects are included)

n-n MC

Results ofn-n oscillation

limit

1T

bound

= 1.77×1032 yrs ( 90% CL ) (SK-1)

(systematic errors are included by Baysian statistics)

atm. MC

SK-I data

Not a spontaneous decay of neutron,but we call the inverse of the event rateis the “lifetime” of the neutron in Oxygen.

Ptot

(MeV

/c)

Inv. Mass (MeV/c2)

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Systematic uncertaintiesPossible sources of the systematic uncertainty in experimental and modeling (Pion interactions, n+anti-n annihilation, etc.) are investigated.

-nucleon scattering in nuclear(cross sections, modeling of nuclear, etc)

12.6 %

nbar+N Annihilation branching ratio

5.2 %

Fermi motion of nucleons 4.2 %

Energy scale 1.7 %

Asymmetry of gain 4.0 %

Cherenkov ring finding 0.6 %

Fid.Vol. determination 3.2%

Total 15.2%

Detection efficiency & Exposureatmospheric flux(flavor ratio, zenith angle dist., spectrum, absolute norm, etc.)

21.5%

Neutrino interactions(absolute cross section uncertainties, differential uncertainties, etc.)

17.8%

-propagations in 16O(cross sections, 16O modelings,etc)

2.7%

Energy scale 12.0%

Asymmetry of gain 9.0 %

Cherenkov ring finding 4.3 %

Total 32.1%

B.G rate

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Comparison with other experimental limits

Experiment Source

Exposure(n yr)

Tbound

(1032yr)

free

(108sec)

ILL Beam --- --- 0.86Soudan2 56Fe 21.9 0.72 1.3Frejus 56Fe 5.0 0.65 1.2Kamiokande

16O 3.0 0.43 1.2

IMB 16O 3.0 0.24 0.88Super-K I 16O 254.5 1.77 2.36

R(1023sec-1)

---

1.4

1.4

1.0

1.0

1.0*

Super-K I results includes systematic uncertainty.

Bound neutron lifetime can be interpreted to the oscillation time of the free neutron by Tbound = R*(free)2

* C.B.Dover et al. Phys. Rev. D 27 (1983) 1090

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Summary

Nucleon decay searches and neutron-antineutron oscillation are reported.

No evidence of nucleon decay, no evidence of n-n oscillation were found.

pe++0 : /B > 8.4 x1033 yr (SK-I+SK-II, preliminary)p++0 : /B > 6.7 x1033 yr (SK-I +SK-II, preliminary) n-n oscillation: bound> 1.77x1032 yr (SK-I, preliminary)

free> 2.36x108 sec