Search for Supersymmetry in Final States Containing two Hadronically Decaying Taus with the ATLAS Detector at √s = 8 and 13 TeV

Two analyses will be presented searching for the production of supersymmetric particles decaying into final states containing at least two hadronically decaying taus and miss- ing transverse energy in proton-proton collisions. The collisions were recorded by the ATLAS experiment at the LHC. Two different datasets are analyzed, one recorded at a center-of-mass energy of 8 TeV in 2012 and one recorded at a center-of-mass energy of 13 TeV in 2015. The first analysis is performed using data collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb−1. The electroweak production of supersymmetric particles which decay into final states containing at least two hadron- ically decaying taus, missing transverse energy, as well as few to no jets, is studied. Particular attention is paid to the validation of the diboson background, the combina- tion with the channel where one of the two taus decays hadronically and the other one leptonically, as well as the estimate of the sensitivity of the analysis at √s = 13 TeV. No excess over the Standard Model expectation is found, thus limits are set using the CLs method. Model independent limits on the cross section are computed, the observed upper limit on the visible cross section is 0.37 fb. Model dependent limits are computed for three different pMSSM scenarios as well as two simplified models. One of the sim- plified models is characterized by chargino pair production, decaying via intermediate staus or tau sneutrinos into neutrinos, taus and the lightest neutralino. The other sim- plified model is characterized by the associated production of a lightest chargino and a next-to-lightest neutralino, also decaying via intermediate staus into taus, neutrinos and the lightest neutralino. The parameters of one of the pMSSM models are chosen such that the dominant process is the direct pair production of staus decaying into taus. The parameters of the other two pMSSM models are chosen such that the three pro- cesses compete. For one of the pMSSM models the stau mass is chosen to be fixed, while for the other it is chosen to be halfway between the χ˜1± and χ˜10 mass. In the case of simultaneous χ˜1±χ˜1∓ and χ˜1±χ˜20 production, χ˜1±/χ˜10 masses of up to 410/140 GeV and in the case of χ˜1± pair production alone, χ˜1±/χ˜10 masses of up to 345/90 GeV can be excluded. The presented analysis is not yet sensitive to large parts of the phase-space of the pMSSM model targeting direct-stau pair production, because the cross section is too small. In the other two pMSSM parameter planes, similar lightest chargino mass ranges can be excluded, namely approximately 100 GeV − 350 GeV. The second analysis makes uses of data collected throughout 2015 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 3.2 fb−1. The strong production of supersymmetric particles is studied which decay into final states con- taining at least two hadronically decaying taus, missing transverse energy, as well as jets. The focus of the analysis is on the development of the signal region targeting low mass-splittings and the estimate of the multi-jet contribution to the signal, as well as control regions. Again, no excess over the Standard Model expectation is found and thus limits are set using the CLs method. The obtained model independent observed upper limit on the visible cross section, hσvisi95 obs, is 1.07 fb. Model dependent limits are interpreted in two models: a simplified model of gluino pair production, subsequently decaying asymmetrically via a lightest chargino and a next-to-lightest neutralino which decay via staus or tau sneutrinos into final states containing lightest neutralinos, jets, taus as well as tau neutrinos; and a gauge-mediated-symmetry-breaking, GMSB, model where, in addition to the previous process, squarks are produced in pairs and decay via intermediate neutralinos and staus/sleptons into a pair of quarks and up to four taus. Other processes are possible as well in the GMSB modell, but they are sub-dominant for most of the studied phase-space. In the first model gluino masses up to 1550 GeV can be excluded while masses of the lightest neutralino up to 750 GeV can be excluded. In case of the GMSB model gluino masses up to 2.2 TeV can be excluded for large values of tan β, while for lower values only gluino masses up to 2.0 TeV can be excluded.