SUSY searches in the electroweak sector

19 May 2017

The sensitivity of searches for supersymmetry (SUSY) has been boosted by the increased centre-of-mass energy of LHC Run 2. Analyses of the first Run 2 data recorded in 2015 and early 2016 focused on the production of strongly interacting SUSY particles – the partners of Standard Model (SM) gluons (“gluinos”) and quarks (“squarks”).

With the large data set accumulated during the rest of 2016, attention now turns to a more challenging but equally important part of the SUSY particle spectrum: the supersymmetric partners of SM electroweak gauge (“winos”, “binos”) and Higgs (“higgsinos”) bosons. The spectrum of the minimal supersymmetric extension of the SM contains six of these particles: two charged (“charginos”) and four neutral (“neutralinos”) ones. The cross-sections for the direct production of pairs of these particles are typically three to five orders of magnitude lower than that for gluino pair production, but such events might be the only indication of supersymmetry at the LHC if the partners of gluons, quarks and leptons are heavy.

CMS has recently reported searches for electroweak production of neutralinos and charginos in different final states. Decays of these particles to the lightest SUSY particle (LSP) – which are candidates for dark matter – are expected to produce Z, W and H bosons, or photons. If the SUSY partners of leptons (sleptons) are sufficiently light they can also be part of the decay chain. In all of these cases, since final states with two or more leptons constitute a large fraction of the signal events, CMS has searched for supersymmetry in final states with multiple leptons. These searches are complemented by analyses targeting hadronic decays of Higgs bosons in these events.

None of the searches performed by CMS show any significant deviation of the observed event counts from the estimated yields for SM processes. In benchmark models with reduced SUSY particle content, the strongest constraints on the electroweak production of pairs of the lightest chargino and the second-lightest neutralino are obtained by assuming their decay chains involve sleptons, with mass limits reaching up to 1.15 TeV, depending on the slepton’s mass and flavour. For direct decays of the chargino (neutralino) to a W (Z) boson and the lightest neutralino, the excluded regions reach up to 0.61 TeV.

A particularly interesting case, favoured by “natural” supersymmetry, are models with small mass differences between the lightest chargino and neutralino states. In these models, the transverse momenta of the leptons can be significantly lower than the typical thresholds of 10–20 GeV used in most analyses. CMS has designed a specific search to enhance the sensitivity to final states with two low-momentum leptons of opposite charge that includes a dedicated online selection for muons with transverse momenta as low as 3 GeV. The search reaches an unprecedented sensitivity: for a mass difference of 20 GeV, the exclusion reaches a mass of 230 GeV.

Based on data recorded in 2016, CMS has covered models of electroweak production of “wino”-like charginos and neutralinos with searches in different final states. More results are expected soon, and the sensitivity of the searches will largely profit from the extension of the data set in the remaining two years of LHC Run 2.

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