Where is all the new physics? In the decade since the Higgs boson's discovery, there have been no statistically significant hints of new particles in data from the Large Hadron Collider (LHC). Could they be sneaking past the standard searches? At the recent Rencontres de Moriond conference, the ATLAS collaboration at the LHC presented several results of novel types of searches for particles predicted by supersymmetry.
Supersymmetry, or SUSY for short, is a promising theory that gives each elementary particle a "superpartner", thus solving several problems in the current Standard Model of particle physics and even providing a possible candidate for dark matter. ATLAS's new searches targeted charginos and neutralinos - the heavy superpartners of force-carrying particles in the Standard Model - and sleptons - the superpartners of Standard Model matter particles called leptons. If produced at the LHC, these particles would each transform, or "decay", into Standard Model particles and the lightest neutralino, which does not further decay and is taken to be the dark-matter candidate.
ATLAS's newest search for charginos and sleptons studied a particle-mass region previously unexplored due to a challenging background of Standard Model processes that mimics the signals from the sought-after particles. The ATLAS researchers designed dedicated searches for each of these SUSY particle types, using all the data recorded from Run 2 of the LHC and looking at the particles' decays into two charged leptons (electrons or muons) and "missing energy" attributed to neutralinos. They used new methods to extract the putative signals from the background, including machine-learning techniques and "data-driven" approaches.
These searches revealed no significant excess above the Standard Model background. They allowed the ATLAS teams to exclude SUSY particle masses, including slepton masses up to 180 GeV. This slepton mass limit surpasses limits at low mass that were set by experiments at the LHC's predecessor - the Large Electron-Positron (LEP) collider - and that have stood for nearly twenty years. Moreover, it rules out some of the scenarios that could explain the long-standing anomaly associated with the magnetic moment of the muon, which has recently been corroborated by the Muon g-2 experiment at Fermilab in the US.
ATLAS physicists have also released the results of a new search for chargino-neutralino pairs, following up on some previous small excesses seen in early analyses of Run 2 data. They studied collision events where the chargino and neutralino decay via W and Z bosons respectively, with the W boson decaying to "jets" of particles and the Z boson to a pair of leptons. When the mass difference between the produced neutralino and the lightest possible neutralino lies below the Z boson mass, it is harder to select the signal events and the backgrounds are more challenging to model. This is the first ATLAS result in this decay channel to target this difficult mass region. The search found no significant deviation from the Standard Model prediction and led to new bounds on SUSY particle masses.
With the LHC set to begin its third data-taking run, ATLAS physicists are looking forward to building on these exciting results to continue their SUSY searches, in particular by targeting SUSY models that are well motivated theoretically and offer solutions to existing tensions between measurements and Standard Model predictions.
