Figure 1 A Feynman diagram represents the decay of a neutral B-meson, B0s, to a pair of muons, u+u-. The green sphere on the left indicates the meson, which is a bound state of a strange quark and an antibottom quark. These radiate two W bosons and exchange a top quark in the process. The W bosons fuse to a Z boson, which then produces the two muons in the final state.

Viewpoint: Mixed Feelings About a Rare Event

Herber Dreiner, Physikalisches Institute, University of Bonn, Nußallee 12, 53115 Bonn, Germany
Published January 7, 2013 | Physics 6, 3 (2013) | DOI: 10.1103/Physics.6.3

Measurement of a rare-meson decay is an experimental achievement worth celebrating, but so far, no new physics beyond the standard model is indicated.

One of the most important missions of the Large Hadron Collider at CERN is to search for phenomena that cannot be explained by the standard model of particle physics. In this context, the latest result from the LHCb experiment, now reported in Physical Review Letters, is a bittersweet victory [1]. The LHCb collaboration has, for the first time, observed evidence for the very rare decay of a neutral meson into a pair of muons. Only about one in every 300 million of the meson’s decays happen this way, and it is no small feat that LHCb has been able to detect the few that do. The rate at which the decay occurs also agrees with the value calculated using the standard model, a theoretical success considering the intricacies involved in the calculations. But many particle physicists were hopeful that the agreement between theory and experiment wouldn’t be quite so good, since a deviation would have been a sign that new physics was at play. So far, no such signs are there, but in the future, the precision of the measurement will improve considerably, potentially allowing smaller deviations from the standard model predictions to be detected.

For all of its successes, the standard model of elementary particle physics has left us with a number of mysteries. In the model, there are six quarks. Three—the up, charm, and top quarks—have electric charge 2/3; and three—the down, strange, and bottom quarks—have electric charge -1/3. To this day we have no idea why we have six quarks and not, say, just the two (the up and down quarks) we need to make protons and neutrons, the building blocks of ordinary matter. Another puzzle is the so-called flavor problem: we do not understand why the masses of the six quarks are what they are, or why they vary over several orders of magnitude. The gaps in our knowledge are a bit like having the periodic table without knowing that atoms consist of electrons and nuclei. But this analogy only goes so far: to the best of our knowledge, the quarks are elementary down to a scale of 10^-20 meters.

Read more: Physics – Mixed Feelings About a Rare Event.

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