23 The Origin of Matter and Neutrinos
175
conservation (it is a sacred principle, like conservation of energy). Similarly,
since the proton has one unit of electric charge, when it decays, the electric
charge must also be carried by one of the decay products or a combination
of them. This leads to the following typical possibilities for proton decay:
p → e
+ π
0 , e
− π
+ π
+ , K
+ ν, K
+
¯
ν, ρ
+ ν, ρ
+
¯
ν, e
+
¯
νν, etc. There are a few
other decay modes as well that satisfy same conditions.
Since the world around us is pretty stable, and the universe has been around
for 13.8 billion years, it implies that proton lifetime is longer than 13 billion
years. Since there are a lot of protons, and as far as we know all protons are
intact, that actually means that protons are stable with a lot longer of a lifetime
than 13 billion years. In fact, Maurice Goldhaber gave an argument to have a
lower bound on the proton lifetime without doing any experiment. He argued
that the human body has about 10
27 protons. If a proton inside a human
body decayed, that would lead to radioactive products, which would emit
intense gamma rays and cause cancer. The fact that humans (and animals) are
surviving without getting sick implies that the lifetime of the proton must be
at least 10,000 trillion years. Goldhaber called it “feeling in the bone” bound.
There are many other sophisticated methods that have been used in dedicated
facilities to search for proton decay during the last 40 years.
All the models of proton decay listed above have been searched for by various
experiments. The most extensive of them is the Super-Kamiokande experiment that discovered neutrino oscillation. Another experiments is located in
a salt mine near Cleveland (called IMB, short form for Irvine–Michigan–
Brookhaven experiment) and the Frejus experiment in the Frejus tunnel in the
Alps, Europe. The Super-Kamiokande experiment started out as Kamiokande
(where the last three letters NDE stand for nucleon decay experiment). The
efforts started in 1980s, with these experiments deep underground, using a
large body of water. The idea was to look for decaying protons in the water, so
that if it decays as p → e
+
+π
0 , both the final state positron and neutral pion
would leave a spectacular light signal in water. They used photomultipliers,
which are devices that magnify this signal to make observations possible.
Locating the experiment underground was important since that helps to
shield spurious events looking like proton decay but actually coming from
atmospheric neutrinos. The current best limit on the proton lifetime is bigger
than 3 × 10
34 years from the Super-Kamiokande experiment [3]. Right now,
there are two searches which are being planned—one in a gold mine in
Lead, South Dakota, where Ray Davis did his solar neutrino experiment,
and another in Japan. At the end of these experiments, the current limits are
expected to be surpassed by at least a factor of ten. The first experiment is
175
conservation (it is a sacred principle, like conservation of energy). Similarly,
since the proton has one unit of electric charge, when it decays, the electric
charge must also be carried by one of the decay products or a combination
of them. This leads to the following typical possibilities for proton decay:
p → e
+ π
0 , e
− π
+ π
+ , K
+ ν, K
+
¯
ν, ρ
+ ν, ρ
+
¯
ν, e
+
¯
νν, etc. There are a few
other decay modes as well that satisfy same conditions.
Since the world around us is pretty stable, and the universe has been around
for 13.8 billion years, it implies that proton lifetime is longer than 13 billion
years. Since there are a lot of protons, and as far as we know all protons are
intact, that actually means that protons are stable with a lot longer of a lifetime
than 13 billion years. In fact, Maurice Goldhaber gave an argument to have a
lower bound on the proton lifetime without doing any experiment. He argued
that the human body has about 10
27 protons. If a proton inside a human
body decayed, that would lead to radioactive products, which would emit
intense gamma rays and cause cancer. The fact that humans (and animals) are
surviving without getting sick implies that the lifetime of the proton must be
at least 10,000 trillion years. Goldhaber called it “feeling in the bone” bound.
There are many other sophisticated methods that have been used in dedicated
facilities to search for proton decay during the last 40 years.
All the models of proton decay listed above have been searched for by various
experiments. The most extensive of them is the Super-Kamiokande experiment that discovered neutrino oscillation. Another experiments is located in
a salt mine near Cleveland (called IMB, short form for Irvine–Michigan–
Brookhaven experiment) and the Frejus experiment in the Frejus tunnel in the
Alps, Europe. The Super-Kamiokande experiment started out as Kamiokande
(where the last three letters NDE stand for nucleon decay experiment). The
efforts started in 1980s, with these experiments deep underground, using a
large body of water. The idea was to look for decaying protons in the water, so
that if it decays as p → e
+
+π
0 , both the final state positron and neutral pion
would leave a spectacular light signal in water. They used photomultipliers,
which are devices that magnify this signal to make observations possible.
Locating the experiment underground was important since that helps to
shield spurious events looking like proton decay but actually coming from
atmospheric neutrinos. The current best limit on the proton lifetime is bigger
than 3 × 10
34 years from the Super-Kamiokande experiment [3]. Right now,
there are two searches which are being planned—one in a gold mine in
Lead, South Dakota, where Ray Davis did his solar neutrino experiment,
and another in Japan. At the end of these experiments, the current limits are
expected to be surpassed by at least a factor of ten. The first experiment is