A
bstract
This paper presents measurements of
t
t
¯
production in association with additional
b
-jets in
pp
collisions at the LHC at a centre-of-mass energy of 13 TeV. The data were recorded with the ...ATLAS detector and correspond to an integrated luminosity of 36.1 fb
−1
. Fiducial cross-section measurements are performed in the dilepton and lepton-plus-jets
t
t
¯
decay channels. Results are presented at particle level in the form of inclusive cross-sections of
t
t
¯
final states with three and four
b
-jets as well as differential cross-sections as a function of global event properties and properties of
b
-jet pairs. The measured inclusive fiducial cross-sections generally exceed the
t
t
¯
b
b
¯
predictions from various next-to-leading-order matrix element calculations matched to a parton shower but are compatible within the total uncertainties. The experimental uncertainties are smaller than the uncertainties in the predictions. Comparisons of state-of-the-art theoretical predictions with the differential measurements are shown and good agreement with data is found for most of them.
In order to study further the long-range correlations (“ridge”) observed recently in p+Pb collisions at sNN=5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v2, has ...been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 μb−1, the parameter v2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |η|<2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1<η<4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.
Searches for scalar leptoquarks pair-produced in proton–proton collisions at
s
=
13
TeV at the Large Hadron Collider are performed by the ATLAS experiment. A data set corresponding to an integrated ...luminosity of 36.1 fb
-
1
is used. Final states containing two electrons or two muons and two or more jets are studied, as are states with one electron or muon, missing transverse momentum and two or more jets. No statistically significant excess above the Standard Model expectation is observed. The observed and expected lower limits on the leptoquark mass at 95% confidence level extend up to 1.29 TeV and 1.23 TeV for first- and second-generation leptoquarks, respectively, as postulated in the minimal Buchmüller–Rückl–Wyler model, assuming a branching ratio into a charged lepton and a quark of 50%. In addition, measurements of particle-level fiducial and differential cross sections are presented for the
Z
→
e
e
,
Z
→
μ
μ
and
t
t
¯
processes in several regions related to the search control regions. Predictions from a range of generators are compared with the measurements, and good agreement is seen for many of the observables. However, the predictions for the
Z
→
ℓ
ℓ
measurements in observables sensitive to jet energies disagree with the data.
Measurements of the production cross section of a
Z
boson in association with jets in proton–proton collisions at
s
=
13
TeV are presented, using data corresponding to an integrated luminosity of ...3.16 fb
-
1
collected by the ATLAS experiment at the CERN Large Hadron Collider in 2015. Inclusive and differential cross sections are measured for events containing a
Z
boson decaying to electrons or muons and produced in association with up to seven jets with
p
T
>
30
GeV and
|
y
|
<
2.5
. Predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements for up to two additional partons interfaced with parton shower and fixed-order predictions at next-to-leading order and next-to-next-to-leading order are compared with the measured cross sections. Good agreement within the uncertainties is observed for most of the modelled quantities, in particular with the generators which use next-to-leading-order matrix elements and the more recent next-to-next-to-leading-order fixed-order predictions.
A
bstract
An angular analysis of the decay
B
d
0
→
K
∗
μ
+
μ
−
is presented, based on proton-proton collision data recorded by the ATLAS experiment at the LHC. The study is using 20.3 fb
−1
of ...integrated luminosity collected during 2012 at centre-of-mass energy of
s
=
8
TeV. Measurements of the
K
*
longitudinal polarisation fraction and a set of angular parameters obtained for this decay are presented. The results are compatible with the Standard Model predictions.
This Letter presents a search for exotic decays of the Higgs boson to a pair of new (pseudo)scalar particles, H → aa, where the a particle has a mass in the range 20–60 GeV, and where one of the a ...bosons decays into a pair of photons and the other to a pair of gluons. The search is performed in event samples enhanced in vector-boson fusion Higgs boson production by requiring two jets with large invariant mass in addition to the Higgs boson candidate decay products. The analysis is based on the full dataset of pp collisions at √s = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36.7 fb−1. The data are in agreement with the Standard Model predictions and an upper limit at the 95% confidence level is placed on the production cross section times the branching ratio for the decay H → aa → γγ gg. This limit ranges from 3.1 pb to 9.0 pb depending on the mass of the a boson.
A search for squarks and gluinos in final states containing hadronic jets, missing transverse momentum but no electrons or muons is presented. The data were recorded in 2015 by the ATLAS experiment ...in s√= 13 TeV proton--proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 3.2 fb−1 of analyzed data. Results are interpreted within simplified models that assume R-parity is conserved and the neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1.51 TeV for a simplified model incorporating only a gluino octet and the lightest neutralino, assuming the lightest neutralino is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.03 TeV are excluded for a massless lightest neutralino. These limits substantially extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.
A
bstract
Charged Higgs bosons heavier than the top quark and decaying via
H
±
→
tb
are searched for in proton-proton collisions measured with the ATLAS experiment at
s
=
8
TeV corresponding to an ...integrated luminosity of 20
.
3fb
−1
. The production of a charged Higgs boson in association with a top quark,
gb
→
tH
±
, is explored in the mass range 200 to 600 GeV using multi-jet final states with one electron or muon. In order to separate the signal from the Standard Model background, analysis techniques combining several kinematic variables are employed. An excess of events above the background-only hypothesis is observed across a wide mass range, amounting to up to 2.4 standard deviations. Upper limits are set on the
gb
→
tH
±
production cross section times the branching fraction BR(
H
±
→
tb
). Additionally, the complementary
s
-channel production,
qq
′
→
H
±
, is investigated through a reinterpretation of
W
′
→
tb
searches in ATLAS. Final states with one electron or muon are relevant for
H
±
masses from 0.4 to 2.0 TeV, whereas the all-hadronic final state covers the range 1.5 to 3.0 TeV. In these search channels, no significant excesses from the predictions of the Standard Model are observed, and upper limits are placed on the
qq
′
→
H
±
production cross section times the branching fraction BR(
H
±
→
tb
).
A
bstract
Searches for new heavy resonances decaying to
WW
,
WZ
, and
ZZ
bosons are presented, using a data sample corresponding to 3.2 fb
−1
of
pp
collisions at
s
=
13
TeV collected with the ATLAS ...detector at the CERN Large Hadron Collider. Analyses selecting
ννqq
,
ℓνqq
,
ℓℓqq
and
qqqq
final states are combined, searching for an arrow-width resonance with mass between 500 and 3000 GeV. The discriminating variable is either an invariant mass or a transverse mass. No significant deviations from the Standard Model predictions are observed. Three benchmark models are tested: a model predicting the existence of a new heavy scalar singlet, a simplified model predicting a heavy vector-boson triplet, and a bulk Randall-Sundrum model with a heavy spin-2 graviton. Cross-section limits are set at the 95% confidence level and are compared to theoretical cross-section predictions for a variety of models. The data exclude a scalar singlet with mass below 2650 GeV, a heavy vector-boson triplet with mass below 2600 GeV, and a graviton with mass below 1100 GeV. These results significantly extend the previous limits set using
pp
collisions at
s
=
8
TeV.
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