23 The Origin of Matter and Neutrinos
173
Fig. 23.1 Andrei Sakharov. Source: Wikipedia.org
way to objects that do not carry a baryon number. An example of such
a process is p → e
+
+ π
0 as well as other processes such as n → ¯
n.
The first class of processes occur in the SU (5) grand unified theory of
Georgi and Glashow [80] and the second kind of process appears in the
left–right symmetric theories of neutrino masses described above [77]
once the quarks and leptons are unified in that theory. There could be
other less known processes that also violate the baryon number. The
breakdown of the baryon number has not yet been discovered.
(ii) There must be forces that are asymmetric between matter and antimatter (also known as CP violation). Such a force was already discovered
to exist in 1964, in the decay of K mesons to two pions. What was found
is that a particle and its anti-particle known as K
0 and ¯
K
0 combine to
give one state which is odd under CP and another which is even. The
even (symmetric) one is allowed by CP to go into two pions but the odd
one is not. So the discovery by the team of Christenson, Turlay, Fitch,
and Cronin found that very rarely but surely the CP odd combination
leads also to two pions. That proved the existence of CP asymmetric
forces in the universe for the first time in 1964.
(iii) The last is a more technical condition that the baryon number violating
processes must not be too rapid or go out of thermal equilibrium for
baryogenesis to start to occur. In plain language what it means is that
the reverse process to baryon violation (i.e. if the original process is p →
e
+ π
0 , the reverse process is e
+
+ π
0
→ p ) must be slow enough so
173
Fig. 23.1 Andrei Sakharov. Source: Wikipedia.org
way to objects that do not carry a baryon number. An example of such
a process is p → e
+
+ π
0 as well as other processes such as n → ¯
n.
The first class of processes occur in the SU (5) grand unified theory of
Georgi and Glashow [80] and the second kind of process appears in the
left–right symmetric theories of neutrino masses described above [77]
once the quarks and leptons are unified in that theory. There could be
other less known processes that also violate the baryon number. The
breakdown of the baryon number has not yet been discovered.
(ii) There must be forces that are asymmetric between matter and antimatter (also known as CP violation). Such a force was already discovered
to exist in 1964, in the decay of K mesons to two pions. What was found
is that a particle and its anti-particle known as K
0 and ¯
K
0 combine to
give one state which is odd under CP and another which is even. The
even (symmetric) one is allowed by CP to go into two pions but the odd
one is not. So the discovery by the team of Christenson, Turlay, Fitch,
and Cronin found that very rarely but surely the CP odd combination
leads also to two pions. That proved the existence of CP asymmetric
forces in the universe for the first time in 1964.
(iii) The last is a more technical condition that the baryon number violating
processes must not be too rapid or go out of thermal equilibrium for
baryogenesis to start to occur. In plain language what it means is that
the reverse process to baryon violation (i.e. if the original process is p →
e
+ π
0 , the reverse process is e
+
+ π
0
→ p ) must be slow enough so