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.
A search is presented for four-top-quark production using an integrated luminosity of 139 fb
-
1
of proton–proton collision data at a centre-of-mass energy of
13
TeV
collected by the ATLAS detector ...at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be
24
-
6
+
7
fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.
A search for the electroweak production of charginos, neutralinos and sleptons decaying into final states involving two or three electrons or muons is presented. The analysis is based on 36.1 fb
of
... TeV proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider. Several scenarios based on simplified models are considered. These include the associated production of the next-to-lightest neutralino and the lightest chargino, followed by their decays into final states with leptons and the lightest neutralino via either sleptons or Standard Model gauge bosons; direct production of chargino pairs, which in turn decay into leptons and the lightest neutralino via intermediate sleptons; and slepton pair production, where each slepton decays directly into the lightest neutralino and a lepton. No significant deviations from the Standard Model expectation are observed and stringent limits at 95% confidence level are placed on the masses of relevant supersymmetric particles in each of these scenarios. For a massless lightest neutralino, masses up to 580 GeV are excluded for the associated production of the next-to-lightest neutralino and the lightest chargino, assuming gauge-boson mediated decays, whereas for slepton-pair production masses up to 500 GeV are excluded assuming three generations of mass-degenerate sleptons.
A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic
t
t
¯
plus missing transverse momentum final ...state is presented. The analysis of 139
fb
-
1
of
s
=
13
TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via
t
~
→
t
(
∗
)
χ
~
1
0
, with
t
(
∗
)
denoting an on-shell (off-shell) top quark and
χ
~
1
0
the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where
m
t
~
>
m
t
+
m
χ
~
1
0
, top squark masses are excluded in the range 400–1250 GeV for
χ
~
1
0
masses below 200 GeV at 95% confidence level. In the situation where
m
t
~
∼
m
t
+
m
χ
~
1
0
, top squark masses in the range 300–630 GeV are excluded, while in the case where
m
t
~
<
m
W
+
m
b
+
m
χ
~
1
0
(with
m
t
~
-
m
χ
~
1
0
≥
5
GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.
A
bstract
This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated ...luminosity of 36.1 fb
−1
of
pp
collisions at
s
=
13
TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns.
The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with
pp
collisions at a centre-of-mass energy
s
=
13
TeV. The absolute luminosity scale is determined using ...van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosity for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2
pp
data sample corresponds to an integrated luminosity of
140.1
±
1.2
fb
-
1
, i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 2017–2018 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of
338.1
±
3.1
pb
-
1
.
Single- and double-differential cross-section measurements are presented for the production of top-quark pairs, in the lepton + jets channel at particle and parton level. Two topologies, resolved and ...boosted, are considered and the results are presented as a function of several kinematic variables characterising the top and
t
t
¯
system and jet multiplicities. The study was performed using data from
pp
collisions at centre-of-mass energy of 13 TeV collected in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of
36
fb
-
1
. Due to the large
t
t
¯
cross-section at the LHC, such measurements allow a detailed study of the properties of top-quark production and decay, enabling precision tests of several Monte Carlo generators and fixed-order Standard Model predictions. Overall, there is good agreement between the theoretical predictions and the data.
A
bstract
A search for new phenomena in events with two same-charge leptons or three leptons and jets identified as originating from
b
-quarks in a data sample of 36.1 fb
−1
of
pp
collisions at
s
=
...13
TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No significant excess is found and limits are set on vector-like quark, four-top-quark, and same-sign top-quark pair production. The observed (expected) 95% CL mass limits for a vector-like
T
- and
B
-quark singlet are
m
T
> 0.98 (0.99) TeV and
m
B
> 1.00 (1.01) TeV respectively. Limits on the production of the vector-like
T
5/3
-quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the
T
5/3
-quark is (expected to be) 1.19 (1.21) TeV. The Standard Model four-top-quark production cross-section upper limit is (expected to be) 69 (29) fb. Constraints are also set on exotic four-top-quark production models. Finally, limits are set on same-sign top-quark pair production. The upper limit on
uu
→
tt
production is (expected to be) 89 (59) fb for a mediator mass of 1 TeV, and a dark-matter interpretation is also derived, excluding a mediator of 3 TeV with a dark-sector coupling of 1.0 and a coupling to ordinary matter above 0.31.
A
bstract
A search for charged Higgs bosons heavier than the top quark and decaying via
H
±
→
tb
is presented. The data analysed corresponds to 36.1 fb
−1
of
pp
collisions at
s
=
13
TeV and was ...recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark,
pp
→
tbH
±
, is explored in the mass range from
m
H
± = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at
m
H
± = 200 GeV to 0.070 pb at
m
H
± = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model.
The inclusive top quark pair (
t
t
¯
) production cross-section
σ
t
t
¯
has been measured in proton–proton collisions at
s
=
13
TeV
, using 36.1 fb
-
1
of data collected in 2015–2016 by the ATLAS ...experiment at the LHC. Using events with an opposite-charge
e
μ
pair and
b
-tagged jets, the cross-section is measured to be:
σ
t
t
¯
=
826.4
±
3.6
(
stat
)
±
11.5
(
syst
)
±
15.7
(
lumi
)
±
1.9
(
beam
)
pb
,
where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the LHC beam energy, giving a total uncertainty of 2.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. It is used to determine the top quark pole mass via the dependence of the predicted cross-section on
m
t
pole
, giving
m
t
pole
=
173
.
1
-
2.1
+
2.0
GeV
. It is also combined with measurements at
s
=
7
TeV
and
s
=
8
TeV
to derive ratios and double ratios of
t
t
¯
and
Z
cross-sections at different energies. The same event sample is used to measure absolute and normalised differential cross-sections as functions of single-lepton and dilepton kinematic variables, and the results are compared with predictions from various Monte Carlo event generators.
PDF
