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The edge of SUSY

2 June 2018 | By

Figure 1: Lepton invariant mass distribution in a signal region designed to look for either an “on-Z” mass peak, or an edge structure in the distribution indicative of a kinematic endpoint resulting from SUSY particle decays. Two examples of how a SUSY signal model would appear are shown with a dashed (“on-Z”) and solid (“edge”) red line. The data points show ATLAS data, while the predicted Standard Model background contributions are displayed in block colours. (Image: ATLAS Collaboration/CERN)

Physicists dream about finding bumps, or excess signals in the data. Bumps tend to point to something new that cannot be explained by the Standard Model, which gives the currently best understanding of the universe in spite of its deficiency to explain several important observations.

The ATLAS experiment has just completed a new search for evidence of supersymmetry (SUSY), a theory that predicts the existence of new “super-partner” particles, with different properties from their Standard Model counterparts. This search looks for SUSY particles decaying to produce two leptons and scrutinises the invariant mass distribution of these leptons, hoping to find a bump.

The search is exciting, as it has a history of small bumps that could point to new physics. The ATLAS and CMS collaborations both reported moderate excesses in searches with two leptons, jets and missing transverse momentum using the 8 TeV LHC dataset. For ATLAS, there was a bump in the data around the Z boson mass, while CMS reported an excess in data at a slightly lower mass. As neither experiment was able to confirm each other’s findings the first time around, a repeat was scheduled for Run 2. After waiting through the first long shutdown of the LHC, both ATLAS and CMS checked each other’s analyses with new 13 TeV LHC data and ruled out any excess. It was time to move on, to update the searches to target new possible bumps at 13 TeV.

The new ATLAS paper reports on a search for new physics processes beyond the Standard Model, taking advantage of the larger Run 2 dataset.

Figure 2: Exclusion limits at 95% confidence level on a SUSY model where gluinos are pair-produced and subsequently decay to an on- or off-shell Z boson and the lightest SUSY particle. Limits are presented in a plane defined by the gluino mass on the x-axis and the neutralino (lightest SUSY particle) mass on the y-axis. The blue dashed and solid red lines give the expected sensitivity of this search and the sensitivity based on the observed yields in data, respectively. The yellow band shows the one standard deviation variation of the blue dashed expected band based on the uncertainties in the background prediction. The region excluded is encompassed by the solid red line. (Image: ATLAS Collaboration/CERN)

The new ATLAS paper reports on a search for new physics processes beyond the Standard Model, taking advantage of the larger Run 2 dataset by requiring stricter, more signal-like criteria that reject more Standard Model background events. The expected shape of the lepton invariant mass distribution is exploited to identify either an “on-Z” mass peak, or an edge structure in the distribution indicative of a kinematic endpoint resulting from SUSY particle decays, as shown in Figure 1. By lowering the transverse momentum threshold for the leptons, the search additionally probes more “compressed” SUSY signal models, where the difference in mass between new particles is small. This results in lower momentum, harder-to-detect decays in the detector. Could SUSY be hiding within these more difficult-to-find models?

In this case, it appears not. No excess has been observed in the Run 2 data so far, and so these results further extend our limits on excluded SUSY models, as presented in Figure 2. With the excesses reported by ATLAS and CMS at 8 TeV now a distant memory, all eyes have turned to the full Run 2 dataset currently being collected, as we continue to explore the corners where new physics might be hiding.