Search for top-squark pair production in final states with one lepton, jets, and missing transverse momentum using 36 fb−1 of $$ \sqrt{s}=13 $$ TeV pp collision data with the ATLAS detector

Abstract

The results of a search for the direct pair production of top squarks, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, several energetic jets, and missing transverse momentum are reported. The analysis also targets spin-0 mediator models, where the mediator decays into a pair of dark-matter particles and is produced in association with a pair of top quarks. The search uses data from proton-proton collisions delivered by the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of √ s = 13 TeV and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36 fb −1 . A wide range of signal scenarios with different mass-splittings between the top squark, the lightest neutralino and possible intermediate supersymmetric particles are considered, including cases where the W bosons or the top quarks produced in the decay chain are offshell. No significant excess over the Standard Model prediction is observed. The null results are used to set exclusion limits at 95% confidence level in several supersymmetry benchmark models. For pair-produced top-squarks decaying into top quarks, top-squark masses up to 940 GeV are excluded. Stringent exclusion limits are also derived for all other considered top-squark decay scenarios. For the spin-0 mediator models, upper limits are set on the visible cross-section.The search presented is based on 16 dedicated analyses that target the various scenarios mentioned above. Each of these analyses corresponds to a set of event selection criteria, referred to as a signal region (SR), and is optimised to target one or more signal scenarios. Two different analysis techniques are employed in the definition of the SRs, which are referred to as 'cut-and-count' and 'shape-fit'. The former is based on counting events in a single region of phase space, and is employed in the 16 analyses. The latter is used in some SRs in addition to the 'cut-and-count' technique and employs SRs split into multiple bins in a specific discriminating kinematic variable, that can cover a range that is larger than the 'cut-and-count' SR. By utilising different signal-to-background ratios in the various bins, the search sensitivity is enhanced in challenging scenarios where it is particularly difficult to separate signal from background.The main background processes after the signal selections include tt, single-top Wt, tt + Z(→ νν), and W+jets. Each of those SM processes are estimated by building dedicated control regions (CRs) enhanced in each of the processes, making the analysis more robust against potential mis-modelling effects in simulated events and reducing the uncertainties in the background estimates. The backgrounds are then simultaneously normalised in data using a likelihood fit for each SR with its associated CRs. The background modelling as predicted by the fits is tested in a series of validation regions (VRs). ATLAS detector and data collectionThe ATLAS detector [82] is a multipurpose particle physics detector with nearly 4π co...

Scalar dark matter coannihilating with a coloured fermion

“…On the other hand, in the region of parameter space relevant for coannihiltion, the large mass of the top precludes two-body decays and we need to consider more complicate decays such a F → SW b or even four-body decays F → Sbf f where f and f denote light SM fermions. Phenomenologically, this process is similar to the stop-coannihilation region in simplified models of Supersymmetry (SUSY) [61,62] and there exist limits on the pair production cross section of stop pairstt † . We use the limits on stops-production reported by [61] to estimate the LHC limits on the model under consideration here.…”

“…Phenomenologically, this process is similar to the stop-coannihilation region in simplified models of Supersymmetry (SUSY) [61,62] and there exist limits on the pair production cross section of stop pairstt † . We use the limits on stops-production reported by [61] to estimate the LHC limits on the model under consideration here. We would like to caution that simplified SUSY is similar but not identical to the fermionic t-channel mediator model.…”

Simplified dark matter models with loop effects in direct detection and the constraints from indirect detection and collider search *

“…The Large Hadron Collider (LHC) performed the search for DM in association with energetic jet [45,46] or the third generation quarks [47][48][49] for simplified DM models with spin-0 mediator at √ s = 13 TeV collisions. The most severe limits are from final states with tt and missing transverse momentum [50].…”

“…The most severe limits are from final states with tt and missing transverse momentum [50]. For model D4 with pseudo-scalar mediator, assuming unitary couplings g χ = g q = 1, the range of mediator mass between 15 and 25 GeV is excluded [48]. This limit is valid for all DM masses as long as m a > 2m χ and closely related models D2 and D3 should have very similar collider constraint.…”

SUSY signals from QCD production at the upgraded LHC

“…Because of the ≈ 20% leptonic branching fraction of the W boson produced in each top decay, the final states with more leptons are more rare despite being cleaner to search for experimentally due to lower backgrounds. Guided by existing limits [31,32,33], we choose to estimate the reach of an all-hadronic stop search with similar methodology to [34]. Our final state of interest is thus six jets and missing energy, including two b-tagged jets, and with two triplets of jets each forming tops.…”