The Standard Model describes all known particles and their interactions and explains a vast amount of experimental observation from all the known subatomic collisions to the microscopic workings of the universe. The Standard Model became complete with the discovery of the Higgs boson. However, the presence of the Higgs particle also creates a serious problem.

According to the Standard Model, the mass of the Higgs boson is affected by physics that is separated from it by 15 orders of magnitude, thus creating a ‘naturalness’ problem. Supersymmetry is an important theory in particle physics which is being pursued in the hope of explaining such theoretical dilemmas.

Supersymmetry offers a simple solution to the naturalness problem by pairing all standard particles with super-partners. This mechanism, in turn, relies on known symmetries of the Standard Model to naturally shield the Higgs mass from highenergy corrections. Supersymmetry also provides a dark matter candidate, lacking in the Standard Model.

It is a unified framework for all matter and forces, including gravity, which is not contained in the Standard Model. The 125 GeV Higgs boson is consistent with the requirements of Supersymmetry. The simplest supersymmetric extension of the Standard Model— the Minimal Supersymmetric Standard Model (MSSM)— requires the mass of the lightest Higgs boson to be below 130–140 GeV. Supersymmetric particles are being actively sought with no candidates yet being identified. 

One of the most important questions of supersymmetry phenomenology is: which supersymmetric model, if any, is realised in nature? Based on mathematical and physical arguments it is expected that the more natural the model the higher might be the chance that it appears. CoEPP researchers are analysing the naturalness of various supersymmetric models.

There are two parameter regions where the MSSM may hide at the Large Hadron Collider. One of these regions features several light Higgs-states and light superpartners. The discovery prospects of this region were analysed in the context of experimental searches at the LHC and the planned International Linear Collider. In the other parameter region the only light Higgs state is the one discovered at 125 GeV with the rest of the Higgs and super-partner masses lying in the TeV range. CoEPP members have performed a comprehensive scan of this parameter region and updated the constraints on the decoupled MSSM Higgs sector in light of the 8 TeV data from the LHC.

CoEPP researchers have examined the constraints imposed by the Large Hadron Collider on the super-partner of the heaviest quark, the top quark. Due to naturalness, and an inverted mass hierarchy, this super-partner, called the top squark is expected to be one of the lightest. The results show that a top squark lighter than 600 GeV can be already excluded in the natural MSSM.


Event with Supersymmetric Particles of the ATLAS Experiment. Photo: CERN