Rencontres de Moriond EW 2012: New physics further constrained by LHCb results

Today, at the Rencontres de Moriond EW conference, the LHCb Collaboration presented a few important results. They confirm the Standard Model predictions with an unprecedented precision and put limits on possible deviations due to new physics.


LHCb is a precision experiment that looks for new physics by studying in particular some rare decays: yet undiscovered particles could leave their footprint by modifying the decay rate and other physics parameters. Deviations from values predicted by the Standard Model – the established set of physics theories that describes and encompasses a large variety of natural phenomena – could point scientists to the existence of new territories.

An example is the decay of the so-called B0s meson – a particle made of a bottom anti-quark bound to a strange quark – into two muons. Theorists have calculated that, in the Standard Model, this type of decay should occur about 3 times in every billion (10-9) total decays of the particle. However, if new particles predicted by theories such as supersymmetry exist, the decay could occur much more often. LHCb has shown that the frequency with which a Bs particle decays into a pair of oppositely charged muons is not larger than 4.5 times out of one billion decays, thus very close to the value predicted by the Standard Model. As a consequence, the measurement constrains more severely the existence of new particles outside the Standard Model, at least in the ways predicted so far by theorists.

The B0s mesons were also holding onto another secret that the LHCb scientists have recently uncovered: evidence that the B0s mesons violate CP symmetry. This phenomenon is considered of primary importance by physicists as it describes differences between matter and antimatter. The CP violation has previously been observed in the K0 and B0 particles. Recently, LHCb also published the first evidence that such a phenomenon occurs also in the D0 particles.

At the same conference, LHCb scientists have also put limits on the decay rate of the neutral D meson into two muons. This decay pattern is predicted to be very rare in the Standard Model, occurring only once in every million million (10-12) decays. The new result shows that this decay occurs fewer than ten times in a billion, improving the previous limit by more than an order of magnitude.

Another parameter sensitive to the existence of new particles is the so-called “φs parameter”, which is predicted to be small in the Standard Model. LHCb scientists have exploited sophisticated quantum mechanical effects to make this measurement. Effects of new particles not predicted by the Standard Model could make the measured value quite large. However, the new result shows that indeed the parameter is small. The measurement comes with an uncertainty that also defines the region where further studies are needed in order to keep on searching for possible non-Standard Model effects.


More detailed information about the new LHCb results can be found on the LHCb Public Website and the CERN Press Release. The 2012 data-taking period will start soon and the LHCb Collaboration expects to improve all the above measurements by the end of this year.

by Antonella Del Rosso