### Question

What is CP violation and how does it explain the fact that matter outnumbers antimatter in the universe?

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

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