Fermilab contributes another piece to the matter-antimatter puzzle

Antimatter, made famous in the movie “Angels and Demons” as well as in the warp drive of Star Trek’s Enterprise, is conceptually simple. A piece of antimatter has exactly the same properties as its matter counterpart except that it has opposite charge. For example, the antimatter equivalent of an electron is the positron; they have exactly the same mass and energy properties but have opposite electrical charge. An antiproton is the same as a proton, except that it has negative rather than positive electric charge. An anti-hydrogen atom is made of a negatively charged antiproton with a positively charge positron in an orbital.

The weird thing about antimatter is that our universe has so little of it. In every reaction that we know of, when matter is produced from energy, an equal amount of antimatter is also produced. It might sound like an esoteric process but it’s an everyday phenomenon high in earth’s atmosphere, at particle physics labs, not to mention within stars and at the borders of black holes. The obvious assumption is that during the first instants of the big bang equal amounts of matter and antimatter must have formed from the original energy. So what happened that rid our universe of almost all of its antimatter?

In 1964, Cronin and Fitch discovered a tiny asymmetry in the decays of short lived particles called “strange mesons” (a.k.a., long lived neutral Kaons); the particles decayed 0.33% more often into decay chains with positrons than with electrons. The asymmetry was reproduced in many similar systems over the years but it is still too small to explain the difference.

A couple of weeks ago, The D-Zero (or D0) Experiment at Fermilab (Fermi National Accelerator Laboratory in the Chicago suburbs) may have found a new piece to the puzzle. Something utterly unexpected that would violate the existing theory and, therefore the most delicious type of discovery!

At Fermilab, beams of protons and antiprotons are accelerated in a huge machine called the Tevatron. The D-Zero experiment is located at one of the points where those beams collide head-on. When they collide, energy is released and among many other things B and anti-B mesons are produced. The Bs and anti-Bs are identified in the D-Zero detector, a huge device composed of specialized equipment that allows physicists to analyze the properties of things like B mesons.

The new piece of the puzzle is that when the B and anti-B mesons decay into things called muons and antimuons (try not to get caught up in the details) there are 1% more pairs of muons than pairs of antimuons. The reported result is the fractional difference in the number of pairs of muons over pairs of antimuons: 0.00957 +/- 0.00251 +/- 0.00146, where the two latter numbers are the statistical and systematic uncertainties, respectively. Statistical uncertainties are those that limit experimental accuracy due to to the number of observed events and systematic uncertainties are the limits of accuracy from the equipment and analysis techniques.

It’s a terrific example of how science works. The observed asymmetry is small and even with eight years of data – which has been combed through many times in search of these types of effects – the result is not conclusive. Big experiments like D-Zero (which has over 500 collaborating physicists) are very careful in reporting new results. After working through the results over and over again, they’ve determined the “statistical significance”: evidence for the asymmetry is consistent with a random fluctuation (i.e., nothing is really there) at the level of 1 in a 1000. This is called significant deviation from a random process – “evidence” for an asymmetry but not quite at the level of “discovery.”

This is the usual path to discovery, especially in an avenue that people have been looking down for over 40 years. If the effect were large, it would have already have been observed. It’s a small effect and D-Zero was uniquely situated to see the first tendrils of evidence. You can bet that every other experiment that might confirm or deny the results is gearing up!
 

(You are welcome to republish the text of this article, but not the images, without needing further permission, provided that you attribute the work to its author, Ransom Stephens, Ph.D. In a fit of disclosure, Ransom is happy to admit that he worked on the D-Zero experiment for the better part of the ‘90s. He is also the author of The God Patent, where matter and antimatter play a role in the collision of sex, drugs, and quantum physics with artificial intelligence, faith and free will.)