Chapter 4
Baryogenesis
4.1 Introduction
The success of the standard model in describing the fundamental interactions has
the consequence, among many others, of verifying the TCP invariance of nature.
This requires that. for each particle X having mass m x, decay width r x and
quantum numbers Q X etc, there is an antiparticle i with the same mass and
width, mx = mx, rx = rx but with opposite quantum numbers Qx = -Qx
etc. One might, therefore, suppose that the world we inhabit would share this
symmetry and contain equal numbers N x of particles and antiparticles N x = N x .
This is clearly not the case. We know that the solar system is made of matter
(protons, neutrons, electrons) and not antimatter, and the experimental bound on
antihelium is [1]
at 95% CL.
(4.1)
Any region of antimatter must be well separated from regions of matter, since,
in any region where protons and anti protons coexisted, their annihilation into
pions with the subsequent tr° -+ 2y decays would significantly distort the
cosmic microwave background. The data require that such domains of matter
and antimatter are separated by a length scale 18 with, conservatively,
18 ~ 3 kpc
(4.2)
the radius of our galaxy, and probably [2,3]
18~lOkpc
(4.3)
the scale of the Virgo cluster.
The asymmetry between baryons (b) and antibaryons (b) may be quantified
by the difference in their number densities n8 == nb -nJj. However, the expansion
of the universe dilutes both nb and nfj and, hence, their difference, since, as
DOl: 10.1201/9780367806637-4
91
Baryogenesis
4.1 Introduction
The success of the standard model in describing the fundamental interactions has
the consequence, among many others, of verifying the TCP invariance of nature.
This requires that. for each particle X having mass m x, decay width r x and
quantum numbers Q X etc, there is an antiparticle i with the same mass and
width, mx = mx, rx = rx but with opposite quantum numbers Qx = -Qx
etc. One might, therefore, suppose that the world we inhabit would share this
symmetry and contain equal numbers N x of particles and antiparticles N x = N x .
This is clearly not the case. We know that the solar system is made of matter
(protons, neutrons, electrons) and not antimatter, and the experimental bound on
antihelium is [1]
at 95% CL.
(4.1)
Any region of antimatter must be well separated from regions of matter, since,
in any region where protons and anti protons coexisted, their annihilation into
pions with the subsequent tr° -+ 2y decays would significantly distort the
cosmic microwave background. The data require that such domains of matter
and antimatter are separated by a length scale 18 with, conservatively,
18 ~ 3 kpc
(4.2)
the radius of our galaxy, and probably [2,3]
18~lOkpc
(4.3)
the scale of the Virgo cluster.
The asymmetry between baryons (b) and antibaryons (b) may be quantified
by the difference in their number densities n8 == nb -nJj. However, the expansion
of the universe dilutes both nb and nfj and, hence, their difference, since, as
DOl: 10.1201/9780367806637-4
91