32
R. N. Mohapatra
that in any weak force or any known force-mediated reaction process, a lepton
never gets destroyed. In other words, if there is a lepton in the initial state, there
will be the same or another lepton in the final state; if there is no net lepton
number in the beginning state e.g. when there is a lepton and anti-lepton in
the initial state, in the final state, either there will be no leptons or a lepton
must appear with an anti-lepton so that the total lepton number remains zero.
The idea of the lepton number was suggested by E. J. Konopinski and H. J.
Mahmoud [68]. If the lepton number was not a good symmetry, the process
¯
ν e + n → e
−
+ p should be allowed. This process has been searched for and
has not been found. The lepton number conservation is the second reason that
the planets, stars, and animals are stable since they all contain elements with
many electrons and are therefore stable by virtue of their lepton number.
As we will discuss later, there is a strong suspicion that lepton number
may not be a good symmetry in nature if the neutrino mass has to be small.
Similarly, the proton could also be unstable. These violations of the lepton
number (and the baryon number) have to be very weak to explain the stability
of the universe. The rate for any such breaking reaction must be much longer
than the age of the universe. There are lots of experiments in progress and
planning right now to discover the violation of the lepton number. They are
looking for a process in which a nucleus decays to two electrons together with a
“daughter” nucleus but no anti-neutrinos. Since there are two electrons in the
final state and no electrons in the initial state, the lepton number in the final
state is two, since the final state nucleus (not the atom) has zero lepton number.
If this process is experimentally discovered, that will be a major breakthrough
in physics, revealing new forces and new phenomena. Searches to date have
revealed that such lepton number violating processes must do so with a chance
of less than one in 10
26 years, much longer than the age of the universe (few
times 10
9 years) and baryon number violating processes can occur only once
in 10
34 years.
R. N. Mohapatra
that in any weak force or any known force-mediated reaction process, a lepton
never gets destroyed. In other words, if there is a lepton in the initial state, there
will be the same or another lepton in the final state; if there is no net lepton
number in the beginning state e.g. when there is a lepton and anti-lepton in
the initial state, in the final state, either there will be no leptons or a lepton
must appear with an anti-lepton so that the total lepton number remains zero.
The idea of the lepton number was suggested by E. J. Konopinski and H. J.
Mahmoud [68]. If the lepton number was not a good symmetry, the process
¯
ν e + n → e
−
+ p should be allowed. This process has been searched for and
has not been found. The lepton number conservation is the second reason that
the planets, stars, and animals are stable since they all contain elements with
many electrons and are therefore stable by virtue of their lepton number.
As we will discuss later, there is a strong suspicion that lepton number
may not be a good symmetry in nature if the neutrino mass has to be small.
Similarly, the proton could also be unstable. These violations of the lepton
number (and the baryon number) have to be very weak to explain the stability
of the universe. The rate for any such breaking reaction must be much longer
than the age of the universe. There are lots of experiments in progress and
planning right now to discover the violation of the lepton number. They are
looking for a process in which a nucleus decays to two electrons together with a
“daughter” nucleus but no anti-neutrinos. Since there are two electrons in the
final state and no electrons in the initial state, the lepton number in the final
state is two, since the final state nucleus (not the atom) has zero lepton number.
If this process is experimentally discovered, that will be a major breakthrough
in physics, revealing new forces and new phenomena. Searches to date have
revealed that such lepton number violating processes must do so with a chance
of less than one in 10
26 years, much longer than the age of the universe (few
times 10
9 years) and baryon number violating processes can occur only once
in 10
34 years.