Muons, matter and mystery

The new measurement uses the DZero detector like a set of scales, weighing the amount of matter and antimatter. However, the scales are themselves asymmetric, and the main challenge is to understand and quantify the effect of this behavior.

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Experiments at particle colliders are often described as "recreating the big bang": making new particles out of energy, and then watching to see what happens. In almost all such experiments, we find that matter and antimatter are produced in equal amounts, and this is consistent with our current model. However, if the big bang followed the same rules, all the matter and antimatter would have mutually annihilated, with nothing left to form stars, planets and life. The very fact that we exist, and observe a matter-dominated universe, shows that our picture of the particle interactions is not complete.

At the DZero experiment, scientists have spent a decade studying this matter-antimatter asymmetry using muons produced in their detector, and last week they released their final results. The measurement counts the number of observed muons (negatively charged) and antimuons (positively charged) to compare the amount of matter and antimatter resulting from the originally symmetric proton-antiproton collisions.

As an analogy, this is much like weighing the matter and antimatter with a set of scales. The challenge is that the scales themselves introduce their own asymmetry into the measurement. Because the detector is built out of matter, it responds slightly differently to matter and antimatter particles as they are detected. These "detector effects" must be precisely determined before the measurement can be made. Luckily, the DZero detector has some clever attributes that make it uniquely suited for this kind of analysis.

Intriguingly, after correcting for the detector effects, the results indicate a statistically significant asymmetry in the number of same-charge muon pairs, with around one part in 400 more pairs of negative muons than positive muons. This is much larger than can be accounted for by current theories, suggesting the presence of additional as-yet unknown processes that favor the production of matter over antimatter. Now, assuming that this isn’t a very unlikely statistical fluctuation, the big question is: What could be causing this asymmetry, and could it be the same process that helped shape the early universe? Future precision measurements of specific asymmetries, as well as theoretical developments, are needed to help understand this puzzle.

Mark Williams

These physicists both made major contributions to this analysis.
These physicists have served as guides for many of the recent tours of the DZero detector. More than 1,300 guests have toured the detector since 2012. Please contact if you are interested in a tour. Top row, from left: Stefan Gruenendahl (tour coordinator), Mike Cooke, George Ginther, Julie Hogan. Second row: Andy Jung, Ashish Kumar, Bill Lee, Carrie McGivern. Third row: Jesus Orduña, Bjoern Penning, Mandy Rominsky, Hang Yin. Bottom row: Sung Woo Youn, Yunhe Xie.