17 research outputs found
Measurement of the relative B-c(+/-)/B-+/- production cross section with the ATLAS detector at root s=8 TeV
No full textThe total cross section and differential cross sections for the production of
B
±
c
mesons, times their branching fraction to
J
/
ψ
π
±
, are measured relative to those for the production of
B
±
mesons, times their branching fraction to
J
/
ψ
K
±
. The data used for this study correspond to an integrated luminosity of
20.3
fb
−
1
of
p
p
collisions recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a center-of-mass energy of
√
s
=
8
TeV
. The measurement is performed differentially in bins of transverse momentum
p
T
for
13
GeV
<
p
T
(
B
±
c
)
<
22
GeV
and
p
T
(
B
±
c
)
>
22
GeV
and in bins of rapidity
y
for
|
y
|
<
0.75
and
0.75
<
|
y
|
<
2.3
. The relative cross section times branching fraction for the full range
p
T
>
13
GeV
and
|
y
|
<
2.3
is
(
0.34
±
0.0
4
stat
+
0.06
−
0.02
sys
±
0.0
1
lifetime
)
%
. The differential measurements suggest that the production cross section of the
B
±
c
decreases faster with
p
T
than the production cross section of the
B
±
, while no significant dependence on rapidity is observed
Medium-Induced Modification of Z-Tagged Charged Particle Yields in Pb+Pb Collisions at 5.02 TeV with the ATLAS Detector.
No full textThe yield of charged particles opposite to a Z boson with large transverse momentum (p_{T}) is measured in 260 pb^{-1} of pp and 1.7 nb^{-1} of Pb+Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The Z boson tag is used to select hard-scattered partons with specific kinematics, and to observe how their showers are modified as they propagate through the quark-gluon plasma created in Pb+Pb collisions. Compared with pp collisions, charged-particle yields in Pb+Pb collisions show significant modifications as a function of charged-particle p_{T} in a way that depends on event centrality and Z boson p_{T}. The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a p_{T} regime difficult to measure through other channels
Measurements of jet observables sensitive to b -quark fragmentation in tt¯ events at the LHC with the ATLAS detector
No full textSeveral observables sensitive to the fragmentation of
b
quarks into
b
hadrons are measured using
36
fb
−
1
of
√
s
=
13
TeV
proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing
b
hadrons are obtained from a sample of dileptonic
t
¯
t
events, and the associated set of charged-particle tracks is separated into those from the primary
p
p
interaction vertex and those from the displaced
b
-decay secondary vertex. This division is used to construct observables that characterize the longitudinal and transverse momentum distributions of the
b
hadron within the jet. The measurements have been corrected for detector effects and provide a test of heavy-quark-fragmentation modeling at the LHC in a system where the top-quark decay products are color connected to the proton beam remnants. The unfolded distributions are compared with the predictions of several modern Monte Carlo parton-shower generators and generator tunes, and a wide range of agreement with the data is observed, with
p
values varying from
5
×
10
−
4
to 0.98. These measurements complement similar measurements from
e
+
e
−
collider experiments in which the
b
quarks originate from a color singlet
Z
/
γ
∗
Measurement of the inclusive tt¯ production cross section in the lepton+jets channel in pp collisions at √s = 7 TeV with the ATLAS detector using support vector machines
No full textA measurement of the top quark pair-production cross section in the
lepton
+
jets
decay channel is presented. It is based on
4.6
fb
−
1
of
√
s
=
7
TeV
p
p
collision data collected during 2011 by the ATLAS experiment at the CERN Large Hadron Collider. A three-class, multidimensional event classifier based on support vector machines is used to differentiate
t
¯
t
events from backgrounds. The
t
¯
t
production cross section is found to be
σ
t
¯
t
=
168.5
±
0.7
(
stat
)
+
6.2
−
5.9
(
syst
)
+
3.4
−
3.2
(
lumi
)
pb
. The result is consistent with the Standard Model prediction based on QCD calculations at next-to-next-to-leading order
Longitudinal flow decorrelations in Xe+Xe collisions at √sNN=5.44 TeV with the ATLAS detector
No full textThe first measurement of longitudinal decorrelations of harmonic flow amplitudes
v
n
for
n
=
2
–
4
in
Xe
+
Xe
collisions at
√
s
N
N
=
5.44
TeV
is obtained using
3
μ
b
−
1
of data with the ATLAS detector at the LHC. The decorrelation signal for
v
3
and
v
4
is found to be nearly independent of collision centrality and transverse momentum (
p
T
) requirements on final-state particles, but for
v
2
a strong centrality and
p
T
dependence is seen. When compared with the results from
Pb
+
Pb
collisions at
√
s
N
N
=
5.02
TeV
, the longitudinal decorrelation signal in midcentral
Xe
+
Xe
collisions is found to be larger for
v
2
, but smaller for
v
3
. Current hydrodynamic models reproduce the ratios of the
v
n
measured in
Xe
+
Xe
collisions to those in
Pb
+
Pb
collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between
Xe
+
Xe
and
Pb
+
Pb
. The results on the system-size dependence provide new insights and an important lever arm to separate effects of the longitudinal structure of the initial state from other early and late time effects in heavy-ion collisions
Search for new phenomena in events with an energetic jet and missing transverse momentum in collisions at TeV with the ATLAS detector
No full textResults of a search for new physics 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 139 fb at a center-of-mass energy of 13 TeV collected in the period 2015-2018 with the ATLAS detector at the Large Hadron Collider. Compared to previous publications, in addition to an increase of almost a factor of four in the data size, the analysis implements a number of improvements in the signal selection and the background determination leading to enhanced sensitivity. Events are required to have at least one jet with transverse momentum above 150 GeV and no reconstructed leptons (, or ) or photons. Several signal regions are considered with increasing requirements on the missing transverse momentum starting at 200 GeV. Overall agreement is observed between the number of events in data and the Standard Model predictions. Model-independent confidence-level limits on visible cross sections for new processes are obtained in the range between 736 fb and 0.3 fb. Results are also translated into improved exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, supersymmetric particles in several compressed scenarios, axion-like particles, and new scalar particles in dark-energy-inspired models. In addition, the data are translated into bounds on the invisible branching ratio of the Higgs boson.57 pages in total, author list starting page 41, 11 figures, 10 tables. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2018-0
Observation of electroweak production of two jets and a Z-boson pair
No full textElectroweak symmetry breaking explains the origin of the masses of elementary particles through their interactions with the Higgs field. Besides the measurements of the Higgs boson properties, the study of the scattering of massive vector bosons with spin 1 allows the nature of electroweak symmetry breaking to be probed. Among all processes related to vector-boson scattering, the electroweak production of two jets and a Z-boson pair is a rare and important one. Here we report the observation of this process from proton–proton collision data corresponding to an integrated luminosity of 139 fb−1 recorded at a centre-of-mass energy of 13 TeV with the ATLAS detector at the Large Hadron Collider. We consider two different final states originating from the decays of the Z-boson pair: one containing four charged leptons and another containing two charged leptons and two neutrinos. The hypothesis of no electroweak production is rejected with a statistical significance of 5.7σ, and the measured cross-section for electroweak production is consistent with the Standard Model prediction. In addition, we report cross-sections for inclusive production of a Z-boson pair and two jets for the two final states
Search for displaced leptons in √s = 13 TeV pp collisions with the ATLAS detector
No full textA search for charged leptons with large impact parameters using
139
fb
−
1
of
√
s
=
13
TeV
p
p
collision data from the ATLAS detector at the LHC is presented, addressing a long-standing gap in coverage of possible new physics signatures. Results are consistent with the background prediction. This search provides unique sensitivity to long-lived scalar supersymmetric lepton partners (sleptons). For lifetimes of 0.1 ns, selectron, smuon, and stau masses up to 720, 680, and
340
GeV
, respectively, are excluded at 95% confidence level, drastically improving on the previous best limits from LEP
Search for new phenomena in events with an energetic jet and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector
No full textResults of a search for new physics 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
139
fb
−
1
at a center-of-mass energy of 13 TeV collected in the period 2015–2018 with the ATLAS detector at the Large Hadron Collider. Compared to previous publications, in addition to an increase of almost a factor of four in the data size, the analysis implements a number of improvements in the signal selection and the background determination leading to enhanced sensitivity. Events are required to have at least one jet with transverse momentum above 150 GeV and no reconstructed leptons (
e
,
μ
or
τ
) or photons. Several signal regions are considered with increasing requirements on the missing transverse momentum starting at 200 GeV. Overall agreement is observed between the number of events in data and the Standard Model predictions. Model-independent 95% confidence-level limits on visible cross sections for new processes are obtained in the range between 736 fb and 0.3 fb. Results are also translated into improved exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, supersymmetric particles in several compressed scenarios, axionlike particles, and new scalar particles in dark-energy-inspired models. In addition, the data are translated into bounds on the invisible branching ratio of the Higgs boson
Search for squarks and gluinos in final states with jets and missing transverse momentum using 139 fb − 1 of s = 13 TeV pp collision data with the ATLAS detector
No full textAbstract: A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing jets and missing transverse momentum, but no electrons or muons, is presented. The data used in this search were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of s = 13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1. The results are interpreted in the context of various R-parity-conserving models where squarks and gluinos are produced in pairs or in association and a neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 2.30 TeV for a simplified model containing only a gluino and the lightest neutralino, assuming the latter is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.85 TeV are excluded if the lightest neutralino is massless. These limits extend substantially beyond the region of supersymmetric parameter space excluded previously by similar searches with the ATLAS detector
