METing SUSY on the Z peak

Abstract

Recently the ATLAS experiment announced a 3 σ excess at the Z -peak consisting of 29 pairs of leptons together with two or more jets, E miss T > 225 GeV and H T > 600 GeV, to be compared with 10.6 ± 3.2 expected lepton pairs in the Standard Model. No excess outside the Z -peak was observed. By trying to explain this signal with SUSY we find that only relatively light gluinos, mg 1.2 TeV, together with a heavy neutralino NLSP of mχ 400 GeV decaying predominantly to Z -boson plus a light gravitino, such that nearly every gluino produces at least one Z -boson in its decay chain, could reproduce the excess. We construct an explicit general gauge mediation model able to reproduce the observed signal overcoming all the experimental limits. Needless to say, more sophisticated models could also reproduce the signal, however, any model would have to exhibit the following features: light gluinos, or heavy particles with a strong production cross section, producing at least one Zboson in its decay chain. The implications of our findings for the Run II at LHC with the scaling on the Z peak, as well as for the direct search of gluinos and other SUSY particles, are pointed out.

ATLAS on-Z excess via gluino-Higgsino-singlino decay chains in the NMSSM

“…After the report, many scenarios in the MSSM as well as in the NMSSM have been proposed to explain the ATLAS on-Z excess without conflicting with constraints from various SUSY searches including the CMS on-Z result [6,[8][9][10][11][12][13][14][15][16][17][18][19][20]. To have on-shell Z bosons in final states while escaping other SUSY search constraints, scenarios with a gravitino as the lightest SUSY particle (LSP) seems to be one of the simplest possibilities.…”

“…• The gluino mass is required to be lighter than about 1.2 TeV for a large enough production cross section to explain the number of the excess [6].…”

“…The cross section of pp → a 1 → tt is 1 fb at √ s = 14 TeV at the large λ benchmark point. 6 The current upper bound is 2 pb for m a 1 = 500 GeV at √ s = 8 TeV [63]. Therefore, the direct search for such the singlet-like Higgses would be challenging.…”

“…It is recently shown that the precision measurement of the double Higgs production can probe the singletlike Higgs bosons through the interference effect between a resonance production of the 5 In the small λ region, another promising channel for the heavy doublet-like Higgs searches is pp → h3/a2 → τ τ [60,61]. 6 In the small λ benchmark point, a1 → tt is forbidden by the kinematics, and mainly decays into bb.…”

“…In this paper, we call this excess "ATLAS on-Z excess". This excess seems to imply an existence of a gluino whose mass is lighter than 1.2 TeV or squarks lighter than 1.4 TeV [6]. The caveat is, though, that the CMS experiment has also analyzed the dileptons+jets+E miss T final state using √ s = 8 TeV data…”