A search for chargino–neutralino pair production in three-lepton final states with missing transverse momentum is presented. The study is based on a dataset of
s
=
13
TeV
pp
collisions recorded with ...the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139
fb
-
1
. No significant excess relative to the Standard Model predictions is found in data. The results are interpreted in simplified models of supersymmetry, and statistically combined with results from a previous ATLAS search for compressed spectra in two-lepton final states. Various scenarios for the production and decay of charginos (
χ
~
1
±
) and neutralinos (
χ
~
2
0
) are considered. For pure higgsino
χ
~
1
±
χ
~
2
0
pair-production scenarios, exclusion limits at 95% confidence level are set on
χ
~
2
0
masses up to 210 GeV. Limits are also set for pure wino
χ
~
1
±
χ
~
2
0
pair production, on
χ
~
2
0
masses up to 640 GeV for decays via on-shell
W
and
Z
bosons, up to 300 GeV for decays via off-shell
W
and
Z
bosons, and up to 190 GeV for decays via
W
and Standard Model Higgs bosons.
Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H→invisible decays where H is produced ...according to the standard model via vector boson fusion, Z(ℓℓ)H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb^{-1} of pp collisions at a center-of-mass energy of sqrts=13 TeV at the LHC. In combination with the results at sqrts=7 and 8 TeV, an exclusion limit on the H→invisible branching ratio of 0.26(0.17_{-0.05}^{+0.07}) at 95% confidence level is observed (expected).
A
bstract
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to ...an integrated luminosity of 36.1 fb
−1
at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (
e
or
μ
). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.
A search for charginos and neutralinos at the Large Hadron Collider using fully hadronic final states and missing transverse momentum is reported. Pair-produced charginos or neutralinos are explored, ...each decaying into a high-pT Standard Model weak boson. Fully hadronic final states are studied to exploit the advantage of the large branching ratio, and the efficient rejection of backgrounds by identifying the high-pT bosons using large-radius jets and jet substructure information. An integrated luminosity of 139 fb−1 of proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 13 TeV is used. No significant excess is found beyond the Standard Model expectation. Exclusion limits at the 95% confidence level are set on wino or higgsino production with various assumptions about the decay branching ratios and the type of lightest supersymmetric particle. A wino (higgsino) mass up to 1060 (900) GeV is excluded when the lightest supersymmetry particle mass is below 400 (240) GeV and the mass splitting is larger than 400 (450) GeV. The sensitivity to high-mass winos and higgsinos is significantly extended relative to previous LHC searches using other final states.
A search for the associated production of the Higgs boson with a top quark pair (tt¯H) is reported. The search is performed in multilepton final states using a data set corresponding to an integrated ...luminosity of 36.1 fb−1 of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy s=13 TeV at the Large Hadron Collider. Higgs boson decays to WW*, ττ, and ZZ* are targeted. Seven final states, categorized by the number and flavor of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass of 125 GeV and a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. The best fit for the tt¯H production cross section is σ(tt¯H)=790−210+230 fb, in agreement with the Standard Model prediction of 507−50+35 fb. The combination of this result with other tt¯H searches from the ATLAS experiment using the Higgs boson decay modes to bb¯, γγ and ZZ*→4ℓ, has an observed significance of 4.2 standard deviations, compared to an expectation of 3.8 standard deviations. This provides evidence for the tt¯H production mode.
A search for long-lived, massive particles predicted by many theories beyond the Standard Model is presented. The search targets final states with large missing transverse momentum and at least one ...high-mass displaced vertex with five or more tracks, and uses 32.8 fb−1 of s=13 TeV pp collision data collected by the ATLAS detector at the LHC. The observed yield is consistent with the expected background. The results are used to extract 95% C.L. exclusion limits on the production of long-lived gluinos with masses up to 2.37 TeV and lifetimes of O(10−2)−O(10) ns in a simplified model inspired by split supersymmetry.
A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. ...This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015–2016, corresponding to 36.1 fb−1 of integrated luminosity at s=13 TeV. Events with same-flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R-parity-conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with nonuniversal Higgs boson masses.
Algorithms used for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC) are presented in this paper; these algorithms are ...used in ATLAS physics analyses that involve electrons in the final state and which are based on the 2015 and 2016 proton–proton collision data produced by the LHC at
s
= 13
TeV
. The performance of the electron reconstruction, identification, isolation, and charge identification algorithms is evaluated in data and in simulated samples using electrons from
Z
→
e
e
and
J
/
ψ
→
e
e
decays. Typical examples of combinations of electron reconstruction, identification, and isolation operating points used in ATLAS physics analyses are shown.
The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of
s
=
13
TeV
pp
collisions from Run 2 of the Large Hadron Collider are presented. These new ...tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77%
b
-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model
t
t
¯
events; similarly, at a
c
-jet identification efficiency of 30%, a light-jet (
b
-jet) rejection factor of 70 (9) is obtained.
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