A joint Fermilab/SLAC publication

New Tevatron collider result may help explain the matter-antimatter asymmetry in the universe

06/30/11

About a year ago, the DZero collaboration at Fermilab published  a tantalizing result in which the universe unexpectedly showed a preference for matter over antimatter. Now the collaboration has more data, and the evidence for this effect has grown stronger.

The result is extremely exciting: The question of why our universe should exist solely of matter is one of the burning scientific questions of our time. Theory predicts that matter and antimatter was made in equal quantities. If something hadn’t slightly favored matter over antimatter, our universe would consist of a bath of photons and little else. Matter wouldn’t exist.

The Standard Model predicts a value near zero for one of the parameters that is associated with the difference between the production of muons and antimuons in B meson decays. The DZero results from 2010 and 2011 differ from zero and are consistent with each other. The vertical bars of the measurements indicate their uncertainty.

The 2010 measurement looked at muons and antimuons emerging from the decays of neutral mesons containing bottom quarks, which is a source that scientists have long expected to be a fruitful place to study the behavior of matter and antimatter under high-energy conditions. DZero scientists found a 1 percent difference between the production of pairs of muons and pairs of antimuons in B meson decays at Fermilab’s Tevatron collider. Like all measurements, that measurement had an uncertainty associated with it. Specifically, there was about a 0.07 percent chance that the measurement could come from a random fluctuation of the data recorded. That’s a tiny probability, but since DZero makes thousands of measurements, scientists expect to see the occasional rare fluctuation that turns out to be nothing.

During the last year, the DZero collaboration has taken more data and refined its analysis techniques. In addition, other scientists have raised questions and requested additional cross-checks. One concern was whether the muons and antimuons are actually coming from the decay of B mesons, rather than some other source.

Now, after incorporating almost 50 percent more data and dozens of cross-checks, DZero scientists are even more confident in the strength of their result. The probability that the observed effect is from a random fluctuation has dropped quite a bit and now is only 0.005 percent. DZero scientists will present the details of their analysis in a seminar geared toward particle physicists later today.

Scientists are a cautious bunch and require a high level of certainty to claim a discovery. For a measurement of the level of certainty achieved in the summer of 2010, particle physicists claim that they have evidence for an unexpected phenomenon. A claim of discovery requires a higher level of certainty.

If the earlier measurement were a fluctuation, scientists would expect the uncertainty of the new result to grow, not get smaller. Instead, the improvement is exactly what scientists expect if the effect is real. But the uncertainty associated with the new result is still too high to claim a discovery. For a discovery, particle physicists require an uncertainty of less than 0.00005 percent.

The new result suggests that DZero is hot on the trail of a crucial clue in one of the defining questions of all time: Why are we here at all?

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