The colour scale shows the fraction of supersymmetric models excluded by ATLAS searches as a function of the mass of the chargino (x-axis) and the mass of the neutralino dark matter candidate (y-axis).
Oxford researcher Ben Hodkinson, in collaboration with an international team at CERN, has released a comprehensive analysis of the supersymmetric landscape following the second round of data-taking at the Large Hadron Collider (LHC). Supersymmetry is a compelling theory of fundamental physics that could explain the origin of dark matter, the relationship between the fundamental forces of nature and the puzzlingly low mass of the Higgs boson.
Supersymmetry predicts that force-carrying particles (such as photons) and matter particles (such as electrons and quarks), should come in pairs. This implies the existence of a suite of new matter and force particles to partner those already discovered. Physicists have been scrutinising the data collected at the Large Hadron Collider for hints of these new particles, but as yet they have remained elusive.
This latest study focuses on the supersymmetric partners of the “weak” nuclear and electromagnetic force-carriers and the Higgs boson. These “weakly-interacting” supersymmetric particles would leave particularly subtle traces in the LHC dataset, making them challenging to observe. However, the lightest of them could be the source of the mysterious ‘dark matter’ which astrophysicists believe makes up 25% of the contents of the universe.
Twenty thousand models of supersymmetric dark matter, each with different properties, were examined by simulating more than 3 billion LHC collisions. Ten searches from the ATLAS experiment were performed on each model to determine which have been ruled out and highlight remaining gaps in coverage. The colours in Figure 1 indicate the fraction of ‘excluded’ models as a function of the masses of two supersymmetric particles, with darker colours indicating less exclusion. While there is coverage across a range of masses, the results reveal that this is far from exhaustive. Viable models remain all over this plane – these are ripe candidates to target with future ATLAS searches.
In addition to searches for ‘lab-made’ dark matter at the LHC, other experiments aim to directly detect ‘relic’ dark matter left over from the Big Bang through its scattering upon atomic nuclei. The Oxford team analysed the complementarity between the coverage of these two approaches. ATLAS is particularly sensitive to dark matter particles around half the Higgs boson mass – the ‘purple spikes’ in Figure 2. In such scenarios, dark matter particles would have annihilated with each other to produce Higgs bosons in the early universe, reducing the amount of dark matter left to the level observed today.
The third data-taking run of the LHC is now underway, and new searches are being designed to target the gaps highlighted by these results. Naturally, the scenarios that still remain viable are those that would be most difficult to observe. The Oxford supersymmetry team is working hard to develop new analysis techniques involving machine learning methods to scrutinise the areas where supersymmetry could still be hiding.