Particle Physics Bookkeeping

The mass media are reporting today that an experiment at the Fermi National Accelerator Laboratory (FNAL or “Fermilab” for short) has just announced some results that could be of great significance, and may be of relevance to how matter—and therefore how we ourselves—came to be. This is exciting stuff, especially for physicists, and most especially for particle physicists. Over the years I have done a considerable amount of research in the areas of particle physics most relevant to this experiment, namely “CP violation” and “baryogenesis”, and therefore am extremely interested in this result.

I am going to explain in non-technical language what this is all about, and why it is interesting to physicists. Before I do so, however, I should note that the result is not yet a discovery. Some of the results announced are still only at the “2 sigma” or “3 sigma” level, which means that they could be statistical aberrations. It will take further experiments to nail down what is going on.

The almost-discovery at FNAL has to do with whether the laws of nature treat matter and antimatter in a perfectly symmetric way. For every kind of fundamental particle there is a corresponding kind of antiparticle. The antiparticle of a proton is an antiproton, for example. (The antiparticle of an electron would be called an antielectron if nomenclature were consistent, but it is almost always called a “positron”, because that is what it was called a long time ago, and the name stuck.) A particle and its antiparticle are in some ways exactly the same (e.g. they are believed to have exactly the same mass). In some ways they are opposite (e.g. they have opposite electrical charge). In the terminology of physics, a particle and antiparticle can “annihilate” each other. That is really an abuse of language, however. “Annihilate” literally means to reduce to “nothing”. Actually, when a particle and antiparticle meet, they disappear, but something of equal energy is produced—often another kind of particle-antiparticle pair. For example, a proton and antiproton could annihilate into an electron and positron. Some particles are their own antiparticle: for example the antiparticle of a photon (particle of light) is a photon.

There is strong reason to believe that soon after the Big Bang explosion the universe had exactly, or almost exactly, the same amount of matter and antimatter. If that had remained the case, it would been terrible for us, because eventually the protons and antiprotons would have (almost) all annihilated, as would the electrons and positrons, and so forth, leaving things like photons out of which it is impossible to make interesting structures, like you and me.

So, it must have been the case that some processes happened very early in the history of the universe to generate an excess of particles over antiparticles. Then, when the antiparticles eventually annihilated with particles, the excess particles would have remained—and that is what stars, planets, and living things are made of. The process by which an excess of matter over antimatter was produced early in the history of the universe is called “baryogenesis”. (“baryo-“ here refers to a kind of particle called baryons, of which protons and neutrons are examples.)

In order for processes to create an excess of matter (also called a “matter-antimatter asymmetry” or “baryon asymmetry”) the laws of physics must not treat matter and antimatter in a perfectly symmetric or even-handed way. That is, a certain putative symmetry of the laws of physics called “C” must be “violated.” (“C” is a mathematical operation that interchanges—“conjugates”—particles and antiparticles.) More importantly, a putative symmetry called “CP” must be violated by the laws of physics. (CP interchanges particles and antiparticles, and at the same time interchanges “left” and “right”.)

It has been known since 1964 that CP is, in fact, violated by a certain force of nature called the “weak interaction.” For a quarter of a century, however, this effect (“CP violation”) was seen in the laboratory only in processes involving particles called “neutral kaons”, and then it was seen also in similar processes involving neutral “B mesons”. (The processes being studied in this Fermilab experiment also involve B mesons.) To this day, CP violation has only been observed in process involving these two kinds of particles. The CP violation so far observed is very well accounted for by our current theory of particle physics, called “the Standard Model”. It is all traceable to the fact that a certain quantity appearing in the laws of physics is a “complex number” rather than a “real number”. It has been shown, however, by theoretical calculations, that the CP violation that is built into the Standard Model and the kinds of processes that the Standard Model allows are not adequate to account for the matter-antimatter asymmetry of the universe. By themselves they would have produced such an asymmetry, but one far smaller than actually exists.

How then did the asymmetry between matter and antimatter come about? This is not a mystery in the sense of something very difficult to explain. If anything, it is too easy to explain. There are hundreds of theories of “baryogenesis” on the theoretical-particle-physics market. (I have proposed several of them myself.) All of them postulate “new” particles and forces, i.e. ones not contained in the Standard Model. Which, if any of these theories is correct remains to be seen.

What is exciting about the new results from Fermilab is that (if they pan out) they will be the first time any CP violation that is not accounted for by the Standard Model has ever been seen in the laboratory. That would raise the possibility that these could be the kinds of processes that generated the matter-antimatter asymmetry in the early universe. I stress the word “possibility”, because it is also quite possible that the CP-violating processes that (perhaps) have been seen at Fermilab are not the ones responsible for the matter-antimatter asymmetry.

In any event, these are the kinds of theoretical issues involved in these new results. Are there any philosophical or theological implications in any of this? No, at least none of any great significance.

This isn’t about “explaining existence” as the headlines say. To explain “existence” is to explain why there is anything at all—why there is a universe at all and why there are laws of physics at all. This is merely about stuff turning into other kinds of stuff. More specifically, these experiments are about how certain kinds of particles turn into other kinds of particles—how antiparticles can (in effect) turn into particles. Really, it is about bookkeeping. It is about how a ledger that contained equal numbers in its particle and antiparticle columns turned into one that doesn’t. That is no more metaphysically significant than understanding how the balance in some financial ledger goes from being in the black to being in the red. Except that money, it seems, really can be “annihilated”, as so many have learned recently to their great sorrow.