A new source of CP violation?

[n2doc]

Roy A. Briere
Department of Physics, Carnegie Mellon University, Pittsburgh, PA USA
Published August 16, 2010

Scientific results are sometimes noteworthy because of the “wow” factor that captures the imagination: even David Letterman noticed the news that the expansion of the universe was accelerating. Other times, it may be the process of doing science that makes people take note: how sure are we that man-made global warming is really proven, given those stolen emails? Unfortunately, the process stories we encounter are too often sensationalist, as with the climate change example, and fail to illustrate how research is really conducted. While not without fits and starts, the real story of science is in the “how” that is behind the “wow.”

A recent high-energy physics result published in Physical Review Letters and Physical Review D from the D0 collaboration working at the Fermilab Tevatron accelerator may offer us a chance to watch both of these facets unfold together <1, 2>. It may indeed be a “wow” discovery, heralding new physics, and is also a chance to watch the machinery of science confront a new result and vet it. The result in question? Abazov et al. report an unexpectedly large value of the same-sign dimuon charge asymmetry. This means that they see pairs of positive muons, μ+μ+, among the debris of their proton-antiproton collisions more often than they see pairs of negative muons, μ-μ- (Fig. 1). The key point is that their measurement violates CP symmetry, which relates the behavior of matter and antimatter particles.

CP violation is one of the three “Sakharov conditions” <3> required to explain the baryon asymmetry of the universe—that is, the puzzle of why there is so much matter and so little antimatter. While CP violation has been observed in several different ways since its surprising debut in 1964 <4>, all current observations are consistent with a single root cause in the current standard model of particle physics. But detailed considerations lead experts to conclude that this known CP violation is not sufficient to explain the cosmic overabundance of matter. This is one of many reasons that high-energy physicists are pursuing signs of “new physics” beyond the standard model: what new source of CP violation is capable of producing the puzzling imbalance of matter that countenances our very existence?

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http://physics.aps.org/articles/v3/69