ATLAS has summarised 22 Run 1 searches, using more than 310,000 models to work out where the elusive SUSY particles might be hiding.
The first run of the LHC taught us at least two significant things. First, that there really is a Higgs boson, with properties broadly in line with those predicted by the Standard Model. Second, that the hotly anticipated supersymmetric (SUSY) particles – which were believed to be needed to keep the Higgs boson mass under control – have not been found.
If, as many believe, SUSY is the solution to the Higgs-mass problem, there should be a heavy partner particle for each of the familiar Standard Model fermions and bosons. So why have we missed the super partners? Are they not present at LHC energies? Or are they just around the corner, waiting to be found?
ATLAS has recently taken stock of its progress in addressing the question of the missing SUSY particles. This herculean task examined an astonishing 500 million different models, each representing a possible combination of SUSY-particle masses. The points were drawn from the 19 parameter “phenomenological Minimal Supersymmetric Standard Model (pMSSM)” and concentrated on those models that can contribute to the cosmological dark matter.
The ambitious project involved the detailed simulation of more than 600 million high-energetic proton–proton collisions, using the power of the LHC computing grid. Teams from 22 individual ATLAS SUSY searches examined whether they had sensitivity to each of the 310,000 most promising models. This told them which combinations of SUSY masses have been ruled out by the ATLAS Run 1 searches and which masses would have evaded detection so far.
The results are illuminating. They show that in Run 1, ATLAS had particular sensitivity to SUSY particles with sub-TeV masses and with strong interactions. Their best constraints are on the fermionic SUSY partner of the gluon and, to a lesser extent, on the scalar partners of the quarks. Weakly interacting SUSY particles have been much harder to pin down, because those particles are produced more rarely. The conclusions are broadly consistent with those obtained using simplified models, which are being used to guide Run 2 SUSY searches.
The paper goes on to examine the knock-on effects of the ATLAS searches for other experiments. The ATLAS searches constrain the SUSY models that are being hunted by underground searches for dark-matter relics, and by indirect searches, including those measuring rare B-meson decays and the magnetic moment of the muon.
Today, the higher-energy of the 13 TeV LHC is bringing increased sensitivity to rare processes and to higher-mass particles. The ATLAS physics teams are excited to be using their fresh knowledge about where SUSY might be hiding to start the hunt afresh.