This week the LHCb collaboration has reported publicly (http://arxiv.org/abs/1211.2674) a new measurement on the rare decay $B^0_s \to \mu^+ \mu^-$.
As I am part of this collaboration and have given my little contribution to this result, I feel obliged (and honoured) to give a quick explanation of why this result is important.
Note: what follows is more pedagogical than usual, so I will not try to be too precise.
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| Figure 1 An event with two muons at LHCb |
Why we want to see it
The $B^0_s$ is one of the various type of particles produced when smashing protons at the Large Hadron Collider (LHC) in Geneva.
This particle is composed of two quarks ($s$ called strange and $\bar b$ called beauty) with same but opposite charge but different type (or flavour). If they had also same type they could annihilate one with each other and transform in a photon or something similar. The photon could then transform into two electrons with opposite charge, or into two muons, which are the fatter brothers of electrons.
But $s$ and $\bar b$ are of different type and in order to transform into something neutral and then into two muons something "in between" is needed.
Only few and complicated ways are present to make this transition possible and involve many smaller transitions, with various particles in them. This can be predicted with quite some accuracy and gives a theoretical probability for this decay to occur of 3.5 times every billion decays of $B^0_s$.
That is why we are talking about rare decays!
What said holds for the presently most accepted theory of particle physics, called Standard Model.
Now, if the Standard Model is not the best theory to explain nature, then maybe we are missing other pieces, other particles which can give contributions to this rare decay allowing an easier transition between the $s$ and $\bar b$, inside the $B^0_s$, to the two muons.
