Abstract
The results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using 36.1~hbox {fb}^{-1} of integrated luminosity from proton–proton collisions at sqrt{s}=13 TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark tilde{t} and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay tilde{t} rightarrow b tilde{chi }_{1}^{pm } into a b-quark and the lightest chargino with tilde{chi }_{1}^{pm } rightarrow W tilde{chi }_{1}^{0}, the decay tilde{t} rightarrow t tilde{chi }_{1}^{0} into an on-shell top quark and the lightest neutralino, the three-body decay tilde{t} rightarrow b W tilde{chi }_{1}^{0} and the four-body decay tilde{t} rightarrow b ell nu tilde{chi }_{1}^{0}. No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the tilde{t} and tilde{chi }_{1}^{0} masses. The results exclude at 95% confidence level tilde{t} masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.
Highlights
The standard model (SM) of particle physics is extremely successful in describing the phenomena of elementary particles and their interactions
The three-body selection uses a number of “super-razor” variables that are defined in Ref. [55]. They are designed to identify events with two massive parent particles each decaying into a set of visible and invisible particles
A 3.2% uncertainty in the luminosity measurement is taken into consideration for all signal and background estimates that are directly derived from Monte Carlo (MC) simulations
Summary
The standard model (SM) of particle physics is extremely successful in describing the phenomena of elementary particles and their interactions. In its current form, it fails to explain several observations, such as the nature of dark matter, the baryon asymmetry of the universe and the stabilisation of the Higgs boson mass against radiative corrections from the Planck scale These shortcomings could be remedied by the existence of new particles at the TeV scale, which motivates extensive searches at the Large Hadron Collider (LHC). The ATLAS detector [33] at the LHC is a multi-purpose particle detector with a cylindrical forward–backward symmetric geometry and an approximate 4π coverage in solid angle It consists of an inner tracking detector (ID) surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field, electromagnetic and hadron calorimeters, and a muon spectrometer. When considering simplified models including the t → tχ decay, top squark masses up to about 700 GeV have been excluded for a nearly massless lightest neutralino. For the same assumptions about the lightest neutralino mass, if the t → bχ1± decay is dominant, top squark masses up to about 500 GeV have been excluded
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