Search for new phenomena with top quark pairs in final states with one lepton, jets, and missing transverse momentum in pp collisions at $$ \sqrt{s} $$ = 13 TeV with the ATLAS detector

A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of $$ \sqrt{s} $$ s = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb−1. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV.

Implementation of the ATLAS-SUSY-2018-31 analysis in the MadAnalysis 5 framework (sbottoms with multi-bottoms and missing transverse energy; 139 fb−1)

We present the implementation, in the MadAnalysis 5 framework, of the ATLAS-SUSY-2018-31 search for new physics, and document the validation of this implementation. This analysis targets, with 139 fb[Formula: see text] of proton–proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector between 2015 and 2018, the production of a pair of supersymmetric bottom squarks when they further decay through a cascade decay involving the second lightest neutralino and a Standard Model Higgs boson. The validation of our work is based on three benchmark scenarios targeting different kinematic configurations. The first of them considers a new physics spectrum leading to the presence of high-[Formula: see text] [Formula: see text]-jets originating from sbottom decays, whereas the last two, that differ by the neutralino mass spectrum, are dedicated to the compressed regime and thus yield the presence of soft [Formula: see text]-jets in the final state. We obtain an agreement between the MadAnalysis 5 predictions and the official ATLAS results at the level of 20–30%, the largest discrepancies being related to cases exhibiting a poor Monte Carlo numerical precision at the level of the official ATLAS results.

Direct dark matter research by observing electrons produced in neutralino-nucleus collisions

The most important process for directly detecting dark matter is the LSP-nucleus elastic scattering by measuring the energy of the recoiling nucleus. In the present work we explore a novel process that is the detection of the dark matter constituents by observing the low energy ionization electrons. We develop the formalism and apply it in calculating the ratio of the ionization rate to the nuclear recoil rate in a variety of atoms. The obtained ratios are essentially independent of all parameters of supersymmetry except the neutralino mass, but they crucially depend on the electron energy cut off. Based on our results it is both interesting and realistic to detect the LSP by measuring the ionization electrons following the-LSP nuclear collisions.

NEW CONSTRAINTS ON DARK MATTER FROM CMS AND ATLAS DATA

Constraints on dark matter from the first CMS and ATLAS SUSY searches are investigated. It is shown that within the minimal supergravity model, the early search for supersymmetry at the LHC has depleted a large portion of the signature space in dark matter direct detection experiments. In particular, the prospects for detecting signals of dark matter in the XENON and CDMS experiments are significantly affected in the low neutralino mass region. Here the relic density of dark matter typically arises from slepton coannihilations in the early universe. In contrast, it is found that the CMS and ATLAS analyses leave untouched the Higgs pole and the Hyperbolic Branch/Focus Point regions, which are now being probed by the most recent XENON results. Analysis is also done for supergravity models with non-universal soft breaking where one finds that a part of the dark matter signature space depleted by the CMS and ATLAS cuts in the minimal SUGRA case is repopulated. Thus, observation of dark matter in the LHC depleted region of minimal supergravity may indicate non-universalities in soft breaking.

Dalitz-esque Treatment of Neutralino Pair Production at the LHC

Processes of the form [Formula: see text] are studied via a technique that may be viewed as an adaptation of time-honored Dalitz plot analyses. Xi and Xj are new heavy states (with i,j = 1…n), which may be identical or distinct; and [Formula: see text] and [Formula: see text] are necessarily distinct Standard Model (SM) fermion pairs whose invariant masses can be measured. A Dalitz-like plot of said invariant masses, [Formula: see text]vs.[Formula: see text], exhibits a topology connected to the masses and specific decay chains of Xi and Xj. Aside from relatively minor details, observed patterns consist of a collection of box and wedge shapes. This collection is model-dependent: comparison of the observed pattern to the possibilities for a specific model yields information on which new particle pair combinations are actually being produced, information beyond that extractable from conventional one-dimensional invariant mass distributions. The technique is illustrated via application to the Minimal Supersymmetric Standard Model (MSSM) process [Formula: see text]. Here the heavy states are neutralinos [Formula: see text] (i = 2,3,4) — note [Formula: see text] is excluded — which are produced in gluino/squark [Formula: see text] cascade decay chains. Even with fairly modest expectations for the LHC performance during the first few years, this method still provides substantial insight into the neutralino mass spectrum and couplings if gluino/squark masses are relatively low (≃ 400 GeV).