Search for proton decay into three charged leptons in Super-Kamiokande
2019176 pages
Supervisor:
Thesis: PhD - Masahiro Kuze
- Tokyo Inst. Tech.
- Published: 2019
Experiments:
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Abstract:
In nature, there are four interactions: strong, electromagnetic, weak and gravity and they
are considered to be one unified interaction at the very beginning of the universe. One of the
final goals in particle physics is explaining these four interactions by one unified theory. Now
we are on the way of unifying the three interactions of strong, electromagnetic and weak.
One of the candidates for this goal is grand unified theory (GUT). Basically the energy scale
of GUT is considered to be 1015 GeV. In such significantly high energy scale, the direct
search for GUT is impossible by any current high energy experiments. On the other hand,
in GUT, the proton which is a stable particle in Standard Model (SM) is predicted to decay.
A search for such proton decay events is a strong way to prove the GUT. Since the life time
of proton is predicted to be much longer than the age of the universe, we need to prepare
a tremendous number of proton source for the discovery. Water is often used as the proton
source in pure water Cherenkov detectors like IMB, KAMIOKANDE and Super-Kamiokande
(SK). Many decay modes of proton decay have been searched by these experiments but it has
never been observed yet.
In this thesis, a search for proton decay into three charged leptons has been performed by
using 0.37 Mtonyears of data collected in Super-Kamiokande(SK). All combinations of elec-
trons, muons and their anti-particles were considered as decay modes. The modes are com-
plementary to decays to a lepton and a meson, which are already extensively searched for.
The lifetime of proton is predicted to be 1033 years for these modes at the energy scale of
100 TeV according to the theory suggested by T. Hambye and J. Heeck[1]. This is a reachable
lifetime scale in SK. Indeed, the last search performed for these modes was by IMB-3 detector
20 years ago, thus a huge improvement in sensitivity is expected with the exposure of SK,
possibly leading to a discovery.
After dedicated selection criteria for each of six decay modes, the data are compared with
background expectations derived from atmospheric neutrino Monte Carlo. Since no signifi-
cant excess of events has been found over the background, lower limits on the proton lilfetime
divided by the branching ratio have been obtained for each mode. The limits range between
9:2 1033 to 3:4 1034 years at 90% confidence level, largely improving upon previous ex-
periments. The improvement factors range between 15 to 1,800, and in one of the modes first
limit has been set.- thesis
- p: decay modes
- p: lifetime
- lifetime: lower limit
- KAMIOKANDE
- numerical calculations: Monte Carlo
- experimental results
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