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CERN | Geneva | Switzerland

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Huge impact from a tiny decay

The Hadron Collider Physics Symposium opened on November 12 in Kyoto on a grand note. For the first time, the LHCb collaboration operating at the Large Hadron Collider (LHC) at CERN showed evidence for an extremely rare type of events, namely the decay of a Bs meson into a pair of muons (a particle very similar to the electron but 200 times heavier). A meson is a composite class of particles formed from a quark and an antiquark. The Bs meson is made of a bottom quark b and a strange quark s. This particle is very unstable and decays in about a picosecond (a millionth of a millionth of a second) into lighter particles.

Decays into two muons are predicted by the theory, the Standard Model of particle physics, that states it should occur only about 3 times in a billionth of decays. In scientific notation, we write (3.54±0.30)x10-9 where the value of 0.30 represents the error margin on this theoretical calculation. Now, the LHCb collaboration proudly announced that they observed it at a rate of (3.2+1.5-1.2)x10-9 , a value very close to the theoretically predicted value, at least within the experimental error.

Here is the plot shown by the LHCb collaboration for the number of events found in data as a function of the combined mass of the two muons. The solid blue line represents the sum of all types of events from known phenomena containing two muons. The dashed curve in red shows the number of events coming from a Bs meson. With the current error margin on the measurement (shown by the

vertical and horizontal bars on the data points), the data seem to agree with all expected contributions from known sources, leaving little room for new phenomena.

This represents a great achievement, not only because this is the rarest process ever observed, but because it puts stringent limits on new theories. Here is why.

Theorists are convinced that a theory far more encompassing than the Standard Model exists even though we have not detected its presence yet. As if the Standard Model is to particle physics what the four basic operations (addition, multiplication, division and subtraction) are to mathematics. They are sufficient to tackle daily problems but one needs algebra, geometry and calculus to solve more complex problems. And in particle physics, we do have problems we cannot solve with the Standard Model, such as explaining the nature of dark matter and dark energy.

A good place to catch the first signs of “new physics” is where the Standard Model predicts very faint signals such as in Bs mesons decaying into two muons. These decays occur extremely rarely because the Standard Model only has limited ways to produce them. But if an additional mechanism comes into play due to some new theory, we would observe these decays at a rate different from what is expected within the Standard Model.

This is a bit like using the surface of a lake to detect the presence of an invisible creature, hoping its breath would create a ripple on the water surface. It would only work if the lake were extremely calm or disturbed only by an occasional tiny fish.  Here the Standard Model acts like all known little animals creating ripples on the water surface.  The hope was to detect other ripples in the absence of known causes (fish, frogs or mosquitoes). The LHCb result reveals no extra ripples yet. So either the new creature does not breathe as expected or we need to find another method to see it. It will be easier to know once the error margin is reduced with more data.

This new result pushes the reach for new physics even further. Nevertheless, it will help theorists eliminate faulty models like on the plot below and eventually zoom on the right solution. Meanwhile, experimentalists will have to devise yet more stringent tests to be able to discover the way to this new physics.

This plot shows how this measurement (horizontal axis) shown earlier this year reduced the space where new physics could be seen. With this new result, the constraints will even be stronger.

(For more details, see LHCb website)

Pauline Gagnon

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