Light stop searches at the LHC in events with one hard photon or jet and missing energy

Abstract

Low energy supersymmetric models provide a solution to the hierarchy problem and also have the necessary ingredients to solve two of the most outstanding issues in cosmology: the origin of the baryon asymmetry and the source of dark matter. In the MSSM, weak scale generation of the baryon asymmetry may be achieved in the presence of light stops, with masses lower than about 130 GeV. Moreover, the proper dark matter density may be obtained in the stop-neutralino co-annihilation region, where the stop-neutralino mass difference is smaller than a few tens of GeV. Searches for scalar top quarks (stops) in pair production processes at the Tevatron and at the Large Hadron Collider (LHC) become very challenging in this region of parameters. At the LHC, however, light stops proceeding from the decay of gluino pairs may be identified, provided the gluino mass is smaller than about 900 GeV. In this article we propose an alternative method for stop searches in the co-annihilation region, based on the search for these particles in events with missing energy plus one hard photon or jet. We show that this method is quite efficient and, when complemented with ongoing Tevatron searches, allows to probe stop masses up to about 160 GeV, fully probing the region of parameters consistent with electroweak baryogenesis in the MSSM. * Very recently, however, it was proposed that the reach of the Tevatron for small mass differences could be extended by not requiring full reconstruction of the two charm jets [10].

Compressed electroweakino spectra at the LHC

“…The possibility to search and constrain dark matter models using this signature has been suggested in [40][41][42] and has been successfully employed to constrain effective dark matter models by the ATLAS and CMS collaborations [43][44][45][46]. The importance of ISR radiation for SUSY searches has also been emphasised and exploited, for electroweakino production at the Tevatron [47], and in [48][49][50][51][52][53][54][55][56][57][58][59][60] in the context of strong SUSY production. A first application to direct electroweak production of Higgsinos has also appeared recently [61].…”

Light stop searches at the LHC in events with twobjets and missing energy

“…(4). The parameter space below the black curve is excluded by Tevatron searches [27], while in the region above the blue curve, the stop pair plus single jet ("monojet") signal has significance ≥ 5 [30]. The parameter region wheret1 decays into a charm and a neutralino are expected to dominate is given by the condition mχ0 1 + mc < mt 1 < mχ0…”

“…(4). The parameter space below the black curve is excluded by Tevatron searches at the 95% confidence level [27], whereas the parameter points above the blue curve allow detection of a monojet signal with at least 5σ significance [30]. In our analysis, we assume that the light stop decays dominantly into a charm quark and the lightest neutralino.…”

“…Ref. [30] therefore investigated QCD stop pair production in association with a very energetic photon or jet. The resulting signature is one hard photon or jet recoiling against large amounts of missing energy.…”

“…The latter are due to diagrams with an on-shell higgsino-like chargino and substantially increase the total cross section. Information about the magnitude of the EW contributions can in principle be obtained by measuring our process and stop pair production in association with a hard jet [30]. The latter process can be used to determine the stop mass, which in turn uniquely fixes the pure QCD contribution to our process.…”