What is CP violation and how does it explain the fact that matter outnumbers antimatter in the universe?
Asked by: Robyn Soares
Answer
During experimentation within physics one begins
to notice several quantities which remain constant
after repeated measurement. The most basic are
the so-called 'integrals of motion': the
Conservation of Energy (or perhaps more exactly
Total Mass-Energy), the Conservation of Linear
Momentum, and the Conservation of Angular
Momentum*. It turns out that physicists love
conservation laws, trying to break them, and
finding out that they can't be broken (in that
order). Because of this physicists have discovered
two new types of conservation laws in the realm
of particle physics: Number conservation laws
(which we will not discuss here) and Discrete
Space-Time Symmetries (which brings us to the
beginning of your question).
There exist 3 Space-Time Symmetries: Charge
Conjugation (C), Parity (P), and Time Reversal
(T) which are defined in the following manner:
Charge Conjugation (C) -- It is believed that
every particle discovered has (or will be
discovered will) have its own anti-particle. The
positron is the anti-particle of an electron and
certain neutral particles (like the neutral pion)
are their own anti-particle. The question asked
to determine whether Charge Conjugation is
conserved is: What if every particle of a system
(usually taken to be the universe) was exchanged
with its anti-particle, would the laws of physics
remain the same? It turns out that the answer is
'NO'. During weak interactions (so-called because
of involving the nuclear weak force) neutrinos
only trace a right-handed spiral. (That is, only
right-handed neutrinos have ever been observed in
nature). So if we lived in this 'anti-universe'
where all the particles were swapped with there
anti-particles the laws of physics would be
DIFFERENT.
Parity (P) -- Parity is the exact reversal of
spatial coordinates the x-coordinate goes to -x,
y to -y and so forth**. A simple way to look at
it is ask whether the laws of physics be the same
if we looked in a 'mirror' (not an actual mirror
but conceptually works in the same way) in which
the coordinates are reversed. It turns out that
the answer for this is 'NO' as well. The beta
decay of a cobalt 60 nucleus prefers a direction
dependent on whether the interaction is left or
right 'handed'
Time Reversal (T) -- can be thought of as running
time backward. The laws of physics hold for
water flowing out of a hose and back into it
(even if one never notices the water flowing
into the hose)***. It turns out that Time
Reversal is also violated****.
Lastly we have combinations of the above: CP --
anti-particle universe COMBINED with the mirror
image universe -- and CPT -- CP with time running
backwards. It turns out that CPT is the only
quantity shown to be conserved. CP was
'indirectly' shown to be violated (violation
occurring during particle mixing) back in 1964
and was shown 'directly' (violation due to
particle decay) in 1999/2000 by observation
of the decay of a particle called a Kaon.
CP violation in general implies a bias in favor
of matter over anti-matter. If a certain decay,
say a matter particle M decays into various matter
particles more readily than the anti-particle
A decays into anti-particles then this would be
the cause of the matter dominance -- there is
more matter because creation of matter (by decay)
happens more often than antimatter does.
Furthermore because anti-matter and some of our
matter (remember we have an excess of matter)
annihilate to form energy all we have in the end
is our left over matter.
For the second part of your question: while CP
violation may HELP explain why matter dominance
(baryogenesis) occurs it DOES NOT explain the
DEGREE to which it does and cannot explain the
amount of matter that we have in our universe.
This is still a mystery that is unsolved.
*At its most fundamental level these integrals
of motion are derived respectively from the
homogeneity of time, the homogeneity of space,
and the isotropy of space. (See Landau
'Mechanics')
**This is with respect to some arbitrary origin
and in fact is more general than the standard
Cartesian coordinates.
*** For the hose case it should be noted that
Time Reversal should only be considered when all
initial-conditions are specified. So just
because one does not notice the water going back
into the hose does not mean Time Reversal has
been violated.
**** It turns out that if T if violated CP must
be violated.
Answered by: James Morris, B.S., Physics Senior, University of Arizona
'I have deep faith that the principle of the universe will be beautiful and simple.'