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Fermi's Tevatron finds another bias against antimatter
05-19-2010, 04:41 PM
We tend to view antimatter as exotic and unstable, prone to annihilation when it combines with the vast excess of normal matter present in our Universe. But it didn't have to be that way; most behavior of subatomic particles shows no preference for matter over antimatter, and calculations suggest the two should have been produced in roughly equal proportions during the Big Bang. Figuring out why we live in a matter-filled Universe has been one of the nagging questions facing physicists.
Over the last couple of decades, a few cases of what are called C-P violations have been identified. These are cases where a particle decay that should, in theory, produce equal amounts of antimatter and matter, doesn't. These few instances, however, don't occur with sufficient frequency to explain why the Universe has its current abundance of regular matter. That has kept physicists looking and, this morning, Fermilab announced that research performed in its Tevatron accelerator has provided strong evidence for another C-P violation.
Fermi has posted a copy of a paper that has been submitted for publication in Physical Review D, which means that the paper hasn't been through peer review yet. The huge number of authors (it takes nearly three pages to list them all and their affiliations) suggests that there's a reasonable chance that one of them might have caught any errors. In addition, the analysis simply involves performing a new analysis of several years' worth of data obtained by the DZero collaboration at Fermi.
The outline of the work is about the only thing that's simple about it. It takes 19 pages and 67 equations to describe the numbers that were crunched to produce the new result—and that's not including the seven pages of appendix and references. The plan was to follow up on early hints of an asymmetry in the decay of a set of particles called B-mesons that are formed by a bottom quark (or antiquark) and one of any of a number of additional quarks (or antiquarks).
The neutral forms of these B-mesons undergo a process called "flavor oscillation," in which they rapidly shift between their matter and antimatter forms. We can't directly observe these changes given their short lifespan, but we can detect their impact in the decay particles. B-mesons can decay into pairs of muons or antimuons depending on their current state. By looking at the relative numbers of paired muons and antimuons that are produced from individual decays, the authors can calculate whether there is an excess of the matter version of B-mesons around. (As the paper puts it, the authors searched the data for "like-sign dimuon events, with one muon arising from direct semileptonic b-hadron decay.")
Right about now would be a good time to refer to our guide to particle colliders if you're not sure how these detectors work.
The challenge isn't one of spotting muons, so much as it is that we've spotted way too many muons, along with a host of other particles, like pions, that sometimes look a lot like a muon from the detector's perspective. So, a huge chunk of the paper's body is devoted to describing how to focus in specifically on the events that are likely to be informative.
Some of these were controlled at the hardware level. For example, the polarity of the magnets in the detector were reversed every few months to ensure that any bias in the equipment ended up balanced out. Specific energy levels and tracks were selected, meaning that the vast majority of the data picked up by the detector was thrown out before the analysis took place. Different approaches were used to pick out cases where another particle like a pion or kaon created a track through the detector that looked like a muon's. In the end, the filtering process left them with 3.73 million di-muon events with identical signs.
Each one of these steps introduced a degree of error into the calculations, however. Fortunately, the calculations produced two different measures of uncertainty that were distinct but related; that relationship enabled the authors to combine them in a way that significantly lowered the overall uncertainty. In the end, they came up with an asymmetry measurement of −0.00957 ± 0.00251 (statistical) ± 0.00146 (systemic). That may not look very exciting, but it's over three standard deviations away from the value predicted by the Standard Model, which means that the probability of this occurring by chance is less than one-tenth of one percent. Symmetry is apparently being broken in these decays.
Just to make sure you're convinced, the authors go on to describe 16 different consistency checks they performed on the results. They passed.
So, we can add another situation where basic physics appears to favor the production of matter over antimatter. It may take a little while for cosmologists to tell us whether that's enough to balance the books on the Universe, but the results already tell us something about particle physics.
For starters, the B-meson doesn't require especially high energies to produce; it's been within the range of particle detectors for a while. But detecting this sort of tenuous bias requires producing lots and lots of them. For Fermi, that required running the accelerator for years. Thanks to the LHC, we should be able to get an independent confirmation much sooner, simply because the luminosity—the number of collisions per unit time—is much, much higher.
From that, we can also conclude that the excitement over the chance to see new particles at higher energies is only part of the allure of the LHC. There may be entirely new physics lurking among the particles we're already aware of, just waiting for us to look at enough of them.