ATLAS hunting second Higgs boson
After the discovery of the Standard Model (SM) Higgs boson in 2012 a key question is whether there is an extended Higgs sector in which the observed particle would be only a part of the story. One of these extensions is a supersymmetric version of the Standard Model. Tests of supersymmetric models of particle physics are one of the major goals of the physics program at the Large Hadron Collider (LHC).
The minimal supersymmetric standard model (MSSM) incorporates all Standard Model particles and adds their supersymmetric partners. There are five physical Higgs bosons in this model: three neutral and two oppositely charged. The observed Higgs boson would be identified with one, probably the lightest one, of the neutral MSSM Higgs bosons. The others could exist in a wide mass range above that of the discovered boson. Supersymmetric theories have many parameters which values are not known. The most important one for a new Higgs boson, apart from its mass, is called tan β. For high values of tan β parameter the coupling of the heavy Higgs to third generation bottom-type fermions (τ and b-quark) is enhanced. This makes the search channel H → ττ sensitive in high tan β parameter space because the branching ratio to τ leptons can reach 10%. Moreover, the production rate in association with b-quark is also enhanced leading to a higher cross section. The analysis is designed to look at events in which one of the τs decays into a lepton and another into hadrons or both τs decay into hadrons (τ leptons themselves have too small a lifetime to reach the active part of the detector). The presence of b-tagged jets helps to classify events as belonging to gluon-fusion or b-associated production modes, giving extra sensitivity. The LHC data collected with the ATLAS detector in the years 2015 and 2016 indicate no significant deviations from predictions of the Standard Model. Therefore, upper limits on cross section times branching ratio of a generic scalar particle is set for masses of up to 2.25TeV. The most stringent limits yet on MSSM parameters are set, see figure, but there still remains a sizeable area where it could be hiding.