# Presentation: 603190

## The Search for          Oscillations at Super-Kamiokande I

**The Search for          Oscillations at Super-Kamiokande I**

**Kenneth S. Ganezer  for the Super-Kamiokande Collaboration**
** ****Thursday September 20, 2007**

**International Workshop on the Search for Baryon and Lepton Number**
**Violation**
**Building 50 Auditorium**** **
**Lawrence Berkeley National Laboratory**
**One Cyclotron Road, Berkeley , CA ****94720-8158**** **
**September 20-22, 2007**

**Physics Department California State University Dominguez Hills, Carson, CA **
**90740**

**With much help from the other Members of the SK nnbar group**
**Brandon Hartfiel, J. S. Jang, and Jun Kameda**

## Outline

**Introduction and Motivation (theoretical and experimental)**
**A very brief description of the Super-Kamiokande Detector**
**Analysis for (a) the n-nbar signal Monte Carlo (b) the atmospheric neutrino background Monte Carlo, and (c) the Super-K data. **
**Analysis of Systematic Uncertainties (errors).**
**Calculation of limits and comparison with previous results**
**Conclusions**

## Introduction and Motivation

##  Some GUTs such as  minimal SU (5), minimal Super-Symmetric Models, and some B-L conserving models have been ruled out by nucleon decay experiments. 

-  Neutrino Oscillations and relatively small neutrino masses are best explained by the see-saw mechanism which is included in B-L violating left-right symmetric models that also predict neutron oscillations as well as neutrino-less double beta decay.

-  Baryogenesis and the Baryon Asymmetry of the Universe (BAU) may be explained by theories that allow neutron oscillations.

-  Some Theories with extra dimensions allow neutron oscillations.

-  There have been no new results published on neutron oscillations in water Cherenkov counters in over 20 years.

## Proton decay in  minimal SU(5)

-  Ex. ; New low quantum gravity scale models – N.Arkani Hamed(1998)
-     p→e+: The intermediate mass scale is 1015GeV
-     *n**↔ **n* is observable in the mass range of 105~106GeV
-     (In between the SM and GUT scales)

*n**→** n* transition in SO(10)
-  SU(2)L×SU(2)R×SU(4)C

- R. N. Mohapatra, hep-ph/9604414

## Experimental methods for n↔ n searches 

- 						and previous limits.
- Use free neutrons from reactors or neutron spallation sources
- or neutrons bound in nuclei
- Experimental lower limits at  the 90% *CL*

- boundR×(Tfree)2

**1.0**

**1.4**

**1.4**

**-**

**Suppression Factor [R]**
**(10****23****s****-1****)**

**0.43**

**0.65**

**0.72**

**-**

- Limit [
- (1032yr)

**0.86**

**-**

**Reactor beam**

**1990**

**ILL**

**1986**

**1990**

**2002**

- Year

**16****O**

**56****Fe**

**56****Fe**

- Source of neutrons

**1.2**

**1.2**

**1.3**

- Limit [Tfree]
- (108s)

**Kamokande**

**Frejus**

**Soudan 2**

- Experiment

**3.0**

**5.0**

**21.9**

- Exposure [NnT]
- (1032 neutron-yr)

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

## The Super-Kamiokande Detector

## Super-Kamiokande (Super-K)

-   The world’s largest water Cherenkov detector, observing neutrinos from the Sun, atmosphere, astrophysical sources, and accelerator beams as well as searching for nucleon decay.
-  Collaborators ; Japan(17 institutes), USA(14), Poland(1), Korea(3), China(1)

- 50ktons water, diameter: 40m, height: 40m
- Inner PMTs:11,146(50 cm) Inner Detector PMTs:1,885(20cm) Outer Detector

## The Super-K Detector

- 22.5 Kton fiducial volume
- 2,700 m water equivalent overburden
- 11,146 inner detector PMTs
- 1,885 outer detector PMTs
- SK1  4.1 yr (this analysis)
- SK2  ~ 2 yr (1/2 PMTs)
- SK3 ~ 1 yr
- 2.2 nsec timing
- Can see neutrino interactions
- at energies as low as
- 4.5 or 5 MeV

## Data Analysis

##  Monte Carlo simulations of n↔ n 

**Oxygen 16**
- 16O8

*n*

- or

- annihilation

********

********

********

********

********

**ω******

********

********

**ω******

********

*n*

********

********

********

********

*n*

********

********

*2**H*

*p*

*p*

*p*

*p*

*4**He*

*3**He*

- 1. The Oscillation phase

- 2. The Annihilation phase
- With Fermi momentum

- 3. Pionization phase Branching ratios
-      Horowitz et Al., Phys Rev. 115 (2), 1959
-      and Armenteros, 1983

- 4. The pion propagation phase
- with Nuclear effects
- D. Ashery, Nucl. Phys. A354, 555 (1981).

- 5. Fragmentation phase

-  Monte Carlo simulations of *n↔ n*

********

********

## Branching Ratios

_**more recent p-pbar**_
_**branching ratios in**_
- astro-ph/0005419
- kaons 7%
- eta 3%
- won’t change our results

- p pbar annihilations (use isospin invariance)

- n pbar annihilations (from pbar deuterium data)

- Decays are according to
- phase space (Genbod)

**Leads to 5.2% uncertainty in final efficiency**

## Pion (and Omega) Propagation

- Pions propagate through the residual nucleus in .2 fm steps.
- Total pion-nucleus cross section is taken from scattering experiments and distributed throughout the residual nucleus according to the nuclear density
- Cross section peak due to resonance

- 25% of pions absorbed 25%scattered
- 2.2 charged 1.3 neutral pions escape
- About  5 pions (7.4 rings) before and 3.5 pions
- (5.1 rings) after propagation  (preliminarily).
- Reconstruction yields 3.5 rings on average (will show this).

**Leads to 12.5% uncertainty in final efficiency**

## NNBAR Data Analysis in Super-K I (Apr.1996-Jul.2001) 

- Atmospheric MC (36525days)
*n**↔** n* MC (5000events)

- Detector Simulation & Reconstruction

- Data selection

- Systematic uncertainty estimation

- Oscillation time limit calculation
- including the systematic uncertainties

- Using Bayesian statistics
- First estimation

- Optimization of
- selection criteria

- Uncertainties in efficiency
- and background rate

-  Fully contained events
- are used.

- NNBAR Data Analysis in Super-K I (Apr.1996-Jul.2001)

- Detection efficiency,
- number of candidates,
- and background

- Atmospheric  MC is
- used for background studies.

## Reconstructed events

- Number of rings (Nring)

- Visible energy (Evis)

- Total momentum (Ptot)

- Total invariant mass (Mtot)

## Results from event reconstruction

## Selection of candidate events

- Selection criteria is chosen
- to optimize the efficiency
- of *n**↔ **n *events divided by
- the number of background (atmospheric events)

**eliminates 64% **
**	****background events**

- Chosen to optimize ε/b
- (a) Nring > 1
- (b) 700< Evis (MeV) <1300
- (c) 0< Ptot (MeV/C) <450
- (d) 750< Mtot (MeV/C2) <1800

## Event selection criteria (cuts)

- (a) Nring > 1
- (b) 700< Evis (MeV) <1300
- (c) 0< Ptot (MeV/C) <450
- (d) 750< Mtot (MeV/C2) <1800

## Detection efficiency

- 21.3056
- (0.16)

**P****tot**** & M****tot**

- 907.628
- (7.0)

**Evis**

- 4668.66
- (36.0)

**Nring**

- 12968

**N****initial**** in FV**

**atm. ******** ****MC (%)**

**Criteria**

- 100years atm.  MC is normalized by detector livetime (1489.2days)

- 10.4 % detection efficiency

- 20 *n**↔ n* candidates

- 21.3 background events

- 20
- (0.16)

- 731
- (6.0)

- 3865
- (31.7)

- 12180

**Data**
**(%)**

- 521
- (10.4)

- 1673
- (33.5)

- 4275
- (85.5)

- 5000

## The Detection efficiency

- 21.3056
- (0.16)

**P****tot**** & M****tot**

- 907.628
- (7.0)

**Evis**

- 4668.66
- (36.0)

**Nring**

- 12968

**N****initial**** in FV**

**atm. ******** ****MC (%)**

**Criteria**

- 20
- (0.16)

- 731
- (6.0)

- 3865
- (31.7)

- 12180

**Data**
**(%)**

- 521
- (10.4)

- 1673
- (33.5)

- 4275
- (85.5)

- 5000

- The atmospheric  MC considered the effects of neutrino oscillations normalized by detector livetime (1489.2 days) as follows:

**Notes:**
The number of events after passing through each cut criteria are like this. 

## Systematic Uncertainties (errors)

##  Systematic uncertainties 

|  | error | Uncertainty |
| --- | --- | --- |
| Detection efficiency                                                                                                                                                                      14.9 % |  |  |
| Fermi momentum | 20% of Pf | 4.2% |
| Annihilation branching ratio
(model dependence) | Baltay(’66), Bettini(’67) | 5.2% |
| Nuclear propagation (model dependence) | NEUT | 1.7% |
| Nuclear propagation (cross section) | Elastic 20%, Charge ex 30%, Abs 25%, Pi prod 30% | 12.5% |
| Non-uniformity of detector gain | +/-1.2% of Ptot | 4.0% |
| Energy scale | +/-2.5% of Evis | 1.7% |
| Ring counting |  | 0.6%  |
| Exposure                                                                                                                                                                                       < 3.2% |  |  |
| Detector livetime |  | < 0.1% |
| Fiducial volume	 |  |   3.2% |
| TOTAL                                                                                                                                                                              < 15.2% |  |  |

**  ****Systematic uncertainties in the efficiency and exposure **

-  Systematic uncertainties

**  ****Systematic uncertainties in the background rate ****	****       **- : negligible (less than 0.1%)

|  | error | Uncertainty |
| --- | --- | --- |
| Non-uniformity of  the detector gain | +/-1.2% of Ptot | 9.0% |
| Energy scale | +/-2.5% of evis | 12.0% |
| Ring counting |  | 4.3% |

## (cont’d)

|  | error | Uncertainty |
| --- | --- | --- |
| Neutrino flux                                                                                                                                                                                21.5% |  |  |
| flux absolute normalization | 20% | 20% |
| flavor ratios (En<5GeV,>5GeV)  | 3%, 3-10% | -, 0.1% |
| bar/ ratio for e (E<10GeV, >10GeV) | 5%, 5-10% | 0.9,  -%  |
| bar/ ratio for  (E<10GeV, >10GeV) | 5%, 5-10% | 0.8,  -% |
| Up/down ratio | 0.4-2.1% | - % |
| Hor./vertical ratio | 0.3-2.8% (3D calc.) | - % |
| K/ ratio | 20.0% | 5.2% |
| Energy spectrum | 0.05 for Ep>100GeV | 5.8% |

| Neutrino cross section                                                                                                                                                                    18% |  |  |
| --- | --- | --- |
| MA in quasi-elastic and single-pi | 10% in MA | 4.4% |
| Quasi elastic scattering (model dependence) | 1 = Fermi-gas vs. Oset |  - % |
| Quasi elastic scattering (cross section) | 10% | 0.4% |
| single-pion production (cross section) | 10% | 2.8% |
| multi-pion production (model dependence) | 1 = w/, vs.  w/o Bodek | 15.5% |
| multi-pion production (cross section) | 5% | 3.4% |
| coherent pion production (cross section) | 30% | 0.1% |
| NC/(CC) ratio | 20% | 6.2% |
| Nuclear effect in 16O (mean free path)	 | 30% | 2.7% |
| TOTAL                                                                                                                                                                              32.1% |  |  |

**(cont’d)**

## Calculation of limits and comparison with previous 

- results

## The oscillation limits including  systematic uncertainties

- : decay rate
**: detector exposure
- : detection efficiency
*b*: # of background (BG) event

-  *n n *oscillation probability , *P*(|*n*) is calculated by Bayesian statistics.

- Each probability ,*P(****)*, *P(****), P(****),** *and *P(b) *was calculated considering systematic errors as follows ;

**: detector exposure
****: estimated detector exposure
****: uncertainty of  exposure
*** *:* *detection efficiency
****:* *estimated detection efficiency
****:* *uncertainty of efficiency
*b*:* *# of BG event
*C*:* *oversampling factor of BG
***b*:* *uncertainty of BG
*b**MC*:* *# of true mean BG MC events
*n**b*: # of BG MC events

## =  1.77×1032 yrs ( 90% CL ).

- =  *1.77**×10**32** yrs ( 90% CL ).*

- This result can be translated to the oscillation time for free neutrons using
*******bound*=R** ****×****(**T*free** *)2*  with *R=1.0x1023 s-1 : the nuclear suppression factor.
- 				 C. B. Dover et al. Phys. Rev. D 27 (1983) 1090
- 	T*free* > 2.36*×10**8** s ( 90% CL )**  *

-  Our Results

- The total Systematic uncertainty in the efficiency and exposure : 15.2 %.
- The total Systematic uncertainty in the background rate : 32.1%.

## Current Super-K I limits compared with previous results

**1.0**

**1.4**

**1.4**

**-**

**1.0**

**Suppression**
**Factor [R]**
**(10****23****s****-1****)**

**1.2**

**1.2**

**1.3**

**0.86**

**2.36***

**Limit ****[T****free****]**
**(10****8**** s)**

**-**

**-**

**-**

**-**

**-**

**-**

**Reactor beam**

**ILL**

**16****O**

**56****Fe**

**56****Fe**

**16****O**

**Source of **
**neutron**

**0.43**

**0.65**

**0.72**

**1.77***

**Limit [******
**(10****32 ****yr)**

**Kamokande**

**Frejus**

**Sudan 2**

**Super-K I**

**Experiment**

**3.0**

**5.0**

**21.9**

**245.4**

**Exposure **
**(10****32 ****neutron-yr)**

**33.0**

**30.0**

**18.0**

**10.4**

**Eff.**
**(%)**

**0**

**0**

**5**

**20**

**Cand.**

**0.9/yr**

**4.0**

**4.5**

**21.3**

**BG**

**5.5**

**8.1**

**Signal **
**Limit**

**2.3**

**2.3**

- * includes systematic uncertainties

## Conclusions

## A search for neutron-antineutron oscillation, a process with |(B-L)|=2, was undertaken using the  full 24.5×1033 neutron-yr exposure from the full Super-K I data set. 
This is  the first neutron oscillation measurement to incorporate most major sources of experimental uncertainties in its limit calculation and the first result from water a Cherenkov detector in more than 20 years. 
The lower limit on the oscillation lifetime for neutrons bound in 16O has been determined to be 1.77 × 1032 yrs at the 90% CL, using Bayesian statistics. This limit is higher than the previous best limit for bound neutrons, of 0.72 × 1032 yrs from Soudan 2, by a factor of 2.5.
At  the 90% CL, the lower limit for the oscillation time for free neutrons has been calculated to be 2.36×108 s using a suppression factor of 1.0 × 1023 s-1. This is an improvement over the previous best 1imits of  0.86 ×108 s and 1.3 ×108 s from ILL and Soudan 2 (in 56Fe) of factors of 2.7 and 1.8, respectively.
Even though the inclusion of systematic errors decreases lower limits, the limits we obtained are significantly higher than those of previous neutron oscillation experiments.

- This is  the first neutron oscillation measurement to incorporate most major sources of experimental uncertainties in its limit calculation and the first result from water a Cherenkov detector in more than 20 years.
- The lower limit on the oscillation lifetime for neutrons bound in 16O has been determined to be 1.77 × 1032 yrs at the 90% *CL, *using Bayesian statistics. This limit is higher than the previous best limit for bound neutrons, of 0.72 × 1032 yrs from Soudan 2, by a factor of 2.5.
- At  the 90% *CL*, the lower limit for the oscillation time for free neutrons has been calculated to be 2.36×108 s using a suppression factor of 1.0 × 1023 s-1. This is an improvement over the previous best 1imits of  0.86 ×108 s and 1.3 ×108 s from ILL and Soudan 2 (in 56Fe) of factors of 2.7 and 1.8, respectively.
- Even though the inclusion of systematic errors decreases lower limits, the limits we obtained are significantly higher than those of previous neutron oscillation experiments.