1. A Search for n n Oscillations in Super-Kamiokande I
Motivation
SK-I (1996~2001)
50000 tons of water
~11,146 of 20inch PMTs
Fid. vol. 22.5kt
Photo-coverage 40%
SK-II (2002~2005)
5,182 20in. PMTs
Photo-coverage 19%
SK-III (Jul. 2006~)
40% Photo-coverage
OD Segmentation
LINAC
Electronics hut
Water and air purification system
Minimal GUT theories such as minimal SU (5), Minimal Super-Symmetric Models, and the Simplest B-L conserving models have already 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.
The Baryon Asymmetry of the Universe (BAU) and by the observation first put forward by A. Sakharov in 1967 that Baryon Number Violation, CP (and thus C) Violation, and non-equilibrium thermodynamics are needed to explain BAU.
Some Theories with extra dimensions also predict neutron oscillations.
There have been no new results published on neutron oscillations in water Cherenkov counters in over 20 years.
Control room
Atotsu
entrance
Kenneth S. Ganezer1, Brandon Hartfiel1, JeeSeung Jang2, and Jun Kameda3: for the Super- Kamiokande Collaboration.
Department of Physics, California State University Dominguez Hills, Carson CA,
Department of Physics, Chonnam National University, Kwangju , Korea.
Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu, , Japan.
41.4m
Ikeno-yama
Kamioka-cho, Gifu
1km
(2700mwe)
The 30th International Cosmic Ray Conference, ICRC 2007, Mérida Yucatán, México. July 3rd – 11th, 2007.
3km
2km SK
Mozumi
39.3m
Atotsu
Outer Detector (OD)
inch PMTs
(SK-I)
Inner Detector (ID)
11, inch PMTs
(SK-I)
50,000 ton
stainless steel tank
The Phases of Bound Neutron Oscillations
4. Pion propagation phase: Products of the annihilation travel through and interact with nucleons in the residual excited nucleus with a time scale of ~ 2 x s.
2. The annihilation phase: the antineutron annihilates with a nucleon, time scale s
3. The pionization phase: Pions, omegas, gammas, etc. emerge from the site of the annihilation.
5. Nuclear Fragmentation phase: the residual nucleus de-excites, decays, and/or breaks up .
1.Oscillation phase: a neutron oscillates into an antineutron in 16O8 : Possible time scale 1033 yrs. p+ P p0 p0 → P p+
4 He p0 p0 p+ Or p0
3 H p0
absorbed p
annihilation ω0 p- p0
Oxygen 16
16O8 p+ p-
5Be ω0 p- n
time scale <<1 x10-8 seconds
Data Analysis
On the right is a neutron oscillation Monte Carlo (signal) event including MC rings and rings that were reconstructed using the data analysis programs.
Our final data analysis cuts were chosen to optimize the efficiency for neutron oscillation events divided by the square root of the number of atmospheric neutrino (background) events and involve the number of reconstructed rings, visible energy, invariant mass, and total momentum.
Systematic Errors on the Signal Monte Carlo
Systematic Errors on the Atmospheric Neutrino Background
error
Uncertainty
Detection efficiency %
Fermi momentum
20% of Pf
<4.2%
Annihilation branching ratio
(model dependence)
Baltay(’66), Bettini(’67)
5.2%
Non-uniformity of detector gain
+/-1.2% of Ptot
4.0%
Energy scale
+/-2.5% of evis
1.7%
Ring counting
0.6%
Nuclear propagation (model dependence)
NEUT
Nuclear propagation (cross section)
Elastic 20%, Charge ex 30%, Abs 25%, Pi prod 30%
12.5%
Exposure < 3.2%
Detector live-time
< 0.1%
Fiducial volume
3.2%
TOTAL < 15.2%
error
Uncertainty
Un-uniformity of detector gain
+/-1.2% of Ptot
9.0%
Energy scale
+/-2.5% of evis
12.0%
Ring counting
4.3%
Neutrino flux %
flux absolute normalization
20%
flavor ratios (En<5GeV,>5GeV)
3%, 3-10%
-, 0.1%
nbar/n ratio for ne (En<10GeV, >10GeV)
5%, 5-10%
0.9, -%
nbar/n ratio for nm (En<10GeV, >10GeV)
0.8, -%
Up/down ratio %
- %
Hor./vertical ratio
% (3D calc.)
K/p ratio
20.0%
5.2%
Energy spectrum
0.05 for Ep>100GeV
5.8%
Neutrino cross section %
MA in quasi-elastic and single-pi
10% in MA
4.4%
Quasi elastic scattering (model dependence)
1s = Fermi-gas vs. Oset
- %
Quasi elastic scattering (cross section)
10%
0.4%
single-pion production (cross section)
2.8%
multi-pion production (model dependence)
1s = 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)
2.7%
TOTAL %
efficiency = 10.4% ± 1.6%
number of neutrons = 6.02 x 10 33
SK-I live-time = years
observed candidates = 20
expected background = ± Bayesian statistics is used.
Official Results at the 90% CL: For neutrons in O-16 t/B = 1.77 x 1032 years.
For free Neutrons
In order to compare with previous results (and the PDG) the nuclear suppression factor for O-16 is taken to be R=1.0 x 1023 s-1 (Dover et. al.) This corresponds to tfree = 2.36 x 108 s and can be compared to the limit of .87 x 108 s for the ILL experiment at Grenoble. If we assume a more conservative value of R=2.15 x1023 s-1 (from Alberico et al.) then tfree = x 108 s.
Results from previous
bound and free neutron
oscillation experiments:
Bound Neutron lifetimes can be converted to free neutron oscillation times using R*, the nuclear suppression factor which depends upon atomic number and is outlined in the log-log plot of τbound versus τfree to the right*.
Results
tbound years
Conclusions
Our result is t/B=1.77x1032 years in O-16 (Bayesian statistics, including systematic errors).
This is an improvement of the limit for tnuclei by a significant factor over previous measurements (see the table to the left)
Our result for tfree including systematic errors is an improvement by a factor of 2.71 over the previous best limit (from ILL Grenoble).
This is the first neutron oscillation experiment to incorporate a detailed error analysis in its limits and the first water Cherenkov neutron oscillation measurement in over 20 years, which included much improved statistics and data analysis than earlier water Cherenkov experiments.
Possible constraints on theoretical models including those with 1. Left-right symmetry 2. The see-saw mechanism and 3. extra dimensions, as well as guidance for future neutron oscillation experiments.
Official Results
* The nuclear suppression factor used by experiments involving Iron (Soudan II and Frejus) is
R=1.4 x 1023 s-1 (Dover et. al.) while experiments involving H2O (Super-K I, Kamiokande, and IMB) have used a (40%) smaller value of R=1.4 x 1023 s-1 computed by the same authors (Dover et. al.).
Density2
Dover
.17
2.3
.14
3.2
1983
IMB
Linear
.43
1.2
.33 3
1986
Kamiokande
Volume
.65
2.1
.30 5
1990
Frejus*
Vol. (Dov.)
.72 (.84)
5.5
2.5
.18
2.15
2002
Soudan II*
1.77**
8.1
21.3 20
.104
245
2006
SuperK I
Absorption
cross section
Start
point
Limit
(1032yr)
Signal BG
Data
Efficiency
Exposure
(1032 neutron-yr)
Year
Experiment*
** Including systematic errors and using Bayesian rather than Frequentist statistics has lowered our
limit by more than a factor of two compared to other experiments.