The LHCb experiment at CERN recently announced results that put the theory of Supersymmetry into ever growing doubt.
Our current picture of the Universe at the smallest scale is wrapped up in the mathematics of the Standard Model of particle physics, with 12 building blocks (6 quarks and 6 leptons), four force carrying particles and the Higgs boson (see image, right). It can be used to predict the ways in which the twelve building blocks of Nature interact through the exchange of the four force carrying particles. Then there is the Higgs boson, which gives mass to all of these particles. It is known that this model isn't the final word in our understanding of Nature and there are a number of theories which try to answer the questions the Standard Model can't.
Supersymmetry is the poster boy of these "new physics" theories. In brief it states that every building block and force carrying particle has a supersymmetric partner called a sparticle. These sparticles have not been seen yet because they are believed to have a large mass, so you need large energies to create them because, as Einstein told us, E=mc2. It is hoped that the record-breaking energies of the Large Hadron Collider (LHC) will be enough to create sparticles and confirm that supersymmetry can go from theory to fact.
The LHCb experiment is designed to look for rare decay of heavy particles called B-mesons. B-mesons are pairs of quarks and anti-quarks where at least one is a bottom quark. The latest results are interested in the decay of Bs mesons; an anti-beauty and a strange quark. The result published by the experiment last week talks about the rare decay where a Bs forms a two particles called Muons (μ).
Following the rules of the Standard Model there are a limited number of ways in which a Bs can decay into two Muons; we can draw these as Feynman diagrams (image right). When the numbers are plugged into the maths it is calculated that if we have just the standard model routes available, those in black, then a decay of Bs -> μμ should happen about 3 times for every billion deaths of a Bs. If, however, supersymmetry were to exist then this number would be higher because with sparticles (marked in red/green) around there are more routes to take to get from a Bs to two Muons.
The result published by LHCb shows a high level of agreement with the standard model result of 3 parts per billion. This suggests it is unlikely that there are "new physics" routes to get from a Bs to two. This could be because LHCb have been unlucky and through nothing but pure chance seen fewer Bs -> μμ than it should have; more time and data will be the test of this. Another possible reason for the result is the current 8TeV energy of the LHC machine is not high enough to create sparticles; the good news here is the LHC will be increasing its energy to around 13TeV in 2014. Or it could be that supersymmetry is not the right route to explaining the shortcomings of the Standard Model. Either way supersymmetry still remains a theory and the standard model stands strong but time and energy may yet change all that.
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Ben Still is a particle physicist at Queen Mary, University of London, UK.