Recent Nucleon Decay Results from Super-Kamiokande

Jennifer L. RaafBoston University

April 14, 2008

MotivationSuper-Kamiokanden → ν π0 decay searchResults

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Interesting candidate for grand unification:SUSY SO(10) model with B-L violation (by 126 Higgs field)

✦ predicts all neutrino mixings and ratio of in agreement with experimental observations

✦ automatic R-parity conservation ⇒ stable dark matter

✦ prediction of upper limits on some proton decay modesrequires some fine-tuning to make compatible with current experimental limits

Motivation

2

Model can be directly tested by searching for predicted modes!

H.S.Goh, R.N.Mohapatra, S.Nasri, S-P. Ng, Phys Lett B587:105-116 (2004)

∆m2!/∆m2atmos

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J.L.Raaf, Boston University APS Meeting, April 14, 2008

Super-Kamiokande

3

Gifu prefecture, Japan, Kamioka-Mozumi zinc mine1000 meters (2700 meters-water-equivalent)

beneath peak of Mt. Ikeno

Water Cherenkov detector

50 ktons (22.5 ktons fiducial) ultra-pure water7.5 x 1033 protons6.0 x 1033 neutrons

Instrumented with photomultiplier tubes (PMTs)11146 50-cm PMTs in inner region1885 20-cm PMTs in outer region

SK-1, SK-2, and beyond...SK-1: 1996-2001

40% photocathode coverageSK-2: Jan 2003 - Oct 2005

Recovery from accident, 20% photocathode coverageSK-3: May 2006 - present

Restore 40% coverageSK-4:

Replace DAQ electronics, Autumn 2008

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Identifying particle types in Super-K

4

Super-Kamiokande IRun 999999 Sub 0 Ev 4202-11-06:00:12:25

Inner: 3166 hits, 6293 pE

Outer: 0 hits, 0 pE (in-time)

Trigger ID: 0x03

D wall: 1240.0 cm

Fully-Contained Mode

Charge(pe) >26.723.3-26.720.2-23.317.3-20.214.7-17.312.2-14.710.0-12.2 8.0-10.0 6.2- 8.0 4.7- 6.2 3.3- 4.7 2.2- 3.3 1.3- 2.2 0.7- 1.3 0.2- 0.7 < 0.2

+1 subevt

0 500 1000 1500 20000

260

520

780

1040

1300

Times (ns)

2 ringsπ0 → γ γ

1 ringμ+

Simulated p → μ+π0 event

✤ Cherenkov ring IDmuon-like = crisp edgeelectron-like = fuzzy edge

✤ Outer detector (OD) activityclusters of hits in the OD may indicate event due to cosmic ray

✤ Decay electron countlook for electron from muon decay in delayed coincidence with muon ring

1 dot = 1 PMTlarger dot = more light collected

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Nucleon decay: n → ν π0

5

Mono-energetic π0 from n → ν π0 decay (signal) would sit atop a large background of π0’s from atmospheric neutrino interactions in Super-K.

Single π0 event selection:Fully contained in the fiducial volume

(no activity in outer detector)2 electron-like ringsNo decay electronsReconstructed mπ0 85-185 MeV/c2

n → ν π0(signal)

Atmospheric ν(background)

J.L.Raaf, Boston University APS Meeting, April 14, 2008

n → ν π0 Fitting Method

6

Atmospheric ν(background)

χ2 =nbins

∑i=1

(Nobsi −N

expi

(1+∑Nsyserrj=1 f ij · ε j

))2

σ2i+

Nsyserr

∑j=1

(ε jσ j

)2

Nexpi = α · Nνi +β · NNDKi

Minimize χ2 with systematic pull terms to find best fit parameters (α,β):

where

n → ν π0(signal)

Systematic error pull termsπ0 absorptionscatteringcharge exchangedetection efficiency

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Corresponding 90% CL lifetime lower limit:> 7.6 x 1032 years

SK-1 n → ν π0 Results

7

Fit prefers unphysical NDK: -7.5 events90% CL allowed: 15.6 events

χ2/DOF

Unphysical fitPhysical fit

3.90/84.10/8

J.L.Raaf, Boston University APS Meeting, April 14, 2008

SK-2 n → ν π0 Results

8

Fit prefers unphysical NDK: -2.5 events90% CL allowed: 11.7 events

χ2/DOF

Unphysical fitPhysical fit

6.41/86.43/8

Corresponding 90% CL lifetime lower limit:> 5.0 x 1032 years

J.L.Raaf, Boston University APS Meeting, April 14, 2008

SK-1 + SK-2 n → ν π0 Results

9

Fit prefers unphysical NDK: -5.0 events90% CL allowed: 20.2 events

χ2/DOF

Unphysical fitPhysical fit

9.90/1810.02/18

Corresponding 90% CL lifetime lower limit:> 8.8 x 1032 years

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Fitting via χ2 with systematic pull terms allows Super-K to set a lower limit on the decay mode n → ν π0:

SK-1+2 result:

Previous best experimental limit by IMB:

New Super-K limit begins to constrain SO(10) GUTs!

Conclusions

10

τB.R.

> 1.12×1032 years

τB.R.

> 8.8×1032 years

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extras

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Standard Model (SM)Represented by product of symmetry groups SU(3) x SU(2) x U(1)

Local gauge symmetries responsible for forces that mediate EM, weak, and strong interactions

Finite but unobservably long proton lifetime due to baryon number (B) conservation Introduced empirically!Other conserved quantities (e.g., electrical charge) result from gauge symmetries

Nucleon Decay

12

Grand Unified Theories (GUTs)Motivated partly by desire to constrain quantities that are seemingly arbitrary in SM

Attempt to unify the 3 fundamental interactions: coupling constants that describe strong, weak, and EM forces unified at large energies⇒ fundamental forces are low energy manifestations of a single unified force

Many GUTs result in proton lifetime ranges accessible to current experiments

e.g., MSSM

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Super-KamiokandeRun 999999 Sub 0 Ev 1702-11-06:00:06:33

Inner: 3594 hits, 9239 pE

Outer: 1 hits, 1 pE (in-time)

Trigger ID: 0x03

D wall: 398.4 cm

Fully-Contained Mode

Charge(pe) >26.723.3-26.720.2-23.317.3-20.214.7-17.312.2-14.710.0-12.2 8.0-10.0 6.2- 8.0 4.7- 6.2 3.3- 4.7 2.2- 3.3 1.3- 2.2 0.7- 1.3 0.2- 0.7 < 0.2

0 500 1000 1500 20000

240

480

720

960

1200

Times (ns)

Simulated event

1 ringe+

2 ringsπ0 → γ γ

Decay mode: p → e+ π0

13

Selection Criteria3 e-like rings (1 from e+, 1 or 2 from π0 decay)no decay electrons85 < mπ < 185 MeV/c2 (3-ring events only)0 < pp < 250 MeV/c800 < mp < 1050 MeV/c2

SK-ISK-II

Selection criterion

Dete

ctio

n ef

ficie

ncy

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

nring e-like decay e ! mass mp pp

Efficiency (%)

SK-1 40.8

SK-2 42.2

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Super-KamiokandeRun 999999 Sub 0 Ev 1702-11-06:00:06:33

Inner: 3594 hits, 9239 pE

Outer: 1 hits, 1 pE (in-time)

Trigger ID: 0x03

D wall: 398.4 cm

Fully-Contained Mode

Charge(pe) >26.723.3-26.720.2-23.317.3-20.214.7-17.312.2-14.710.0-12.2 8.0-10.0 6.2- 8.0 4.7- 6.2 3.3- 4.7 2.2- 3.3 1.3- 2.2 0.7- 1.3 0.2- 0.7 < 0.2

0 500 1000 1500 20000

240

480

720

960

1200

Times (ns)

Simulated p → e+π0 event

1 ringe+

2 ringsπ0 → γ γ

Event Signatures

14

Super-Kamiokande IRun 999999 Sub 0 Ev 4202-11-06:00:12:25

Inner: 3166 hits, 6293 pE

Outer: 0 hits, 0 pE (in-time)

Trigger ID: 0x03

D wall: 1240.0 cm

Fully-Contained Mode

Charge(pe) >26.723.3-26.720.2-23.317.3-20.214.7-17.312.2-14.710.0-12.2 8.0-10.0 6.2- 8.0 4.7- 6.2 3.3- 4.7 2.2- 3.3 1.3- 2.2 0.7- 1.3 0.2- 0.7 < 0.2

+1 subevt

0 500 1000 1500 20000

260

520

780

1040

1300

Times (ns)

2 ringsπ0 → γ γ

1 ringμ+

Simulated p → μ+π0 event

μ+π0 selection2 or 3 rings, 1 μ-like

(1 from μ+, 1 or 2 from π0 decay)1 decay electron85 < mπ < 185 MeV/c2

(for 3-ring events only)pp < 250 MeV/c800 < mp < 1050 MeV/c2

e+π0 selection2 or 3 e-like rings

(1 from e+, 1 or 2 from π0 decay)no decay electrons85 < mπ < 185 MeV/c2

(for 3-ring events only)pp < 250 MeV/c800 < mp < 1050 MeV/c2

J.L.Raaf, Boston University APS Meeting, April 14, 2008

Decay mode: p → e+ π0

15

SK-I atmospheric !

Proton mass (MeV/c2)

Prot

on m

omen

tum

(MeV

/c)

0

200

400

600

800

1000

0 200 400 600 800 1000 1200

SK-I e+!0

Proton mass (MeV/c2)

Prot

on m

omen

tum

(MeV

/c)

0

200

400

600

800

1000

0 200 400 600 800 1000 1200

SK-I data

Proton mass (MeV/c2)

Prot

on m

omen

tum

(MeV

/c)

0

200

400

600

800

1000

0 200 400 600 800 1000 1200

SK-ISK-II

Selection criterion

Dete

ctio

n ef

ficie

ncy

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

nring e-like decay e ! mass mp pp