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R. N. Mohapatra
that any baryon asymmetry generated by the above two conditions must
not be undone by the reverse process. In the early universe, if the baryon
asymmetry is being produced by the decay of a heavy particle, then when
the temperature of the universe is below the mass of the heavy particle,
the inverse reaction cannot happen since the final particles just do not
have enough energy to produce the particle that is decaying. This will
satisfy Sakharov’s third condition.
Since the SU (5) grand unified theory of Georgi and Glashow apparently
satisfied all these conditions, using Sakharov’s conditions, Japanese physicist
M. Yoshimura [106] wrote a paper showing how the Sakharov conditions
can generate the desired matter–anti-matter excess in the universe. It was
however later discovered that this model does not work due to symmetries of
the standard model that erase any baryon asymmetry produced in the SU(5)
model at very high temperature of the universe. However, soon many other
models which do not share the problem of the SU (5) model and produce
baryon asymmetry were discovered. A key element of all these models is that
the sacred principle baryon number must be broken.
23.2 The Search for Baryon Number Violation
As we just saw, baryon number violation is one of the key ingredients for
understanding why the universe has only matter and no anti-matter. How
can we experimentally test this idea? Since the lightest baryons are protons
and neutrons, there must be processes in nature where a proton disappears
into lighter particles or a neutron transmutes into an anti-neutron. Both
these processes would break baryon number conservation and would provide
key support for Sakharov’s idea. Of course, as we have been discussing, a
free neutron is unstable and undergoes beta decay with a lifetime of under
about 15 min, without breaking baryon number. The process of neutron–antineutron oscillation therefore must occur before the neutron has had a chance
to beta decay.
So how do we know what a proton decays to and how long is its lifetime?
Energy conservation dictates that a proton can decay to particles whose
total mass is less than its own mass. The particles of lower mass than the
proton, which do not have a baryon number, are K
±,0 , π
± , π
0 , ρ
±,0 , e
± ,
neutrinos. Also the spin of the original particle must be conserved in the
baryon number violating process by the principle of angular momentum
R. N. Mohapatra
that any baryon asymmetry generated by the above two conditions must
not be undone by the reverse process. In the early universe, if the baryon
asymmetry is being produced by the decay of a heavy particle, then when
the temperature of the universe is below the mass of the heavy particle,
the inverse reaction cannot happen since the final particles just do not
have enough energy to produce the particle that is decaying. This will
satisfy Sakharov’s third condition.
Since the SU (5) grand unified theory of Georgi and Glashow apparently
satisfied all these conditions, using Sakharov’s conditions, Japanese physicist
M. Yoshimura [106] wrote a paper showing how the Sakharov conditions
can generate the desired matter–anti-matter excess in the universe. It was
however later discovered that this model does not work due to symmetries of
the standard model that erase any baryon asymmetry produced in the SU(5)
model at very high temperature of the universe. However, soon many other
models which do not share the problem of the SU (5) model and produce
baryon asymmetry were discovered. A key element of all these models is that
the sacred principle baryon number must be broken.
23.2 The Search for Baryon Number Violation
As we just saw, baryon number violation is one of the key ingredients for
understanding why the universe has only matter and no anti-matter. How
can we experimentally test this idea? Since the lightest baryons are protons
and neutrons, there must be processes in nature where a proton disappears
into lighter particles or a neutron transmutes into an anti-neutron. Both
these processes would break baryon number conservation and would provide
key support for Sakharov’s idea. Of course, as we have been discussing, a
free neutron is unstable and undergoes beta decay with a lifetime of under
about 15 min, without breaking baryon number. The process of neutron–antineutron oscillation therefore must occur before the neutron has had a chance
to beta decay.
So how do we know what a proton decays to and how long is its lifetime?
Energy conservation dictates that a proton can decay to particles whose
total mass is less than its own mass. The particles of lower mass than the
proton, which do not have a baryon number, are K
±,0 , π
± , π
0 , ρ
±,0 , e
± ,
neutrinos. Also the spin of the original particle must be conserved in the
baryon number violating process by the principle of angular momentum