There are many different ways people are testing the validity of the current theoretical model we have for particle physics, called the Standard Model. The LHCb experiment, one of the four large experiments operating at the Large Hadron Collider (LHC) at CERN, just released today at the Moriond conference in La Thuile, Italy, the most precise measurement to-date on an extremely rare phenomenon.
The team sifted through about 10 billion events looking for a particle called a Bs meson – a particle made of a b quark and antiquark s – that would have decayed into two muons.
The Bs meson is a very heavy particle, making it unstable and prone to decay into smaller, more stable particles. It can “break apart” in many different ways called decay channels. These are just like the many ways a machine can give change. Some decay channels occur more often, others only rarely. Of all the possible options, how often a particle will decay into a particular decay channel is called the branching ratio.
The Standard Model predicts that about three Bs mesons should disintegrate into two muons out of a billion decaying Bs mesons, that is, the branching ratio is 3 x 10-9.
LHCb found a few possible Bs mesons decaying into two muons, including the one shown below. There are not enough to establish a measurement of the branching ratio, but they managed to set the most precise limit on its value, namely that it has to be less than 4.5 x 10-9.
This means LHCb is now putting the Standard Model through the highest scrutiny in an area where many people expected to detect a deviation from the Standard Model prediction. As it is, the Standard Model still stands tall and strong even when pushed to such limits.
Display of one of the candidates for a Bs meson decaying into two muons in the LHCb detector. The muons are penetrating particles (shown in pink) that pass all the way through the detector. A zoom on the vertex region shown below indicates that they are offset from the ‘primary vertex’ where the protons collided, as would be expected if they come from a Bs meson that will have time to travel a little before decaying.
Finding a deviation from what is expected from the Standard Model would be a way to see if the current theoretical model is not just the tip of the iceberg. Many theorists suspect a more complex theory lies beyond the Standard Model but nobody has been able to crack the model open so far.Last Summer, LHCb had reported on another attempt at revealing small flaws in the Standard Model. Today, they released even more precise measurements on charge-parity or CP-violation in Bs mesons.
CP violation is a way to quantify why more matter than anti-matter remained when the Universe slowly cooled down after the Big Bang, leaving us with a world predominantly composed of matter. This is quite puzzling since in laboratory experiments, the measured preference for the creation of matter over antimatter is too small to explain why we mostly see matter around us.
Even with the increase in precision in measuring a parameter called Φs which is predicted by the Standard Model to be very small, the new LHCb measurement sees no deviation from this prediction and falls very close to the predicted value.
The red and green areas show the results from D0 and CDF, two experiments from the Tevatron, an accelerator near Chicago. In blue is the LHCb result of last summer, when an ambiguity on the sign of the parameter ∆Γs left two possibilities. This ambiguity is now gone such that only one small yellow area remains, in good agreement with the predicted value indicated by the black dot.
This increase in precision will help limit the range of possibilities for new models, making it increasingly easier to zoom onto the right solution among all the models proposed.
Pauline Gagnon
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