A
bstract
A search for a Higgs boson produced via vector-boson fusion and decaying into invisible particles is presented, using 20.3 fb
−1
of proton-proton collision data at a centre-of-mass energy ...of 8 TeV recorded by the ATLAS detector at the LHC. For a Higgs boson with a mass of 125 GeV, assuming the Standard Model production cross section, an upper bound of 0.28 is set on the branching fraction of
H
→ invisible at 95% confidence level, where the expected upper limit is 0.31. The results are interpreted in models of Higgs-portal dark matter where the branching fraction limit is converted into upper bounds on the dark-matter-nucleon scattering cross section as a function of the dark-matter particle mass, and compared to results from the direct dark-matter detection experiments.
The production rates of prompt and non-prompt
J
/
ψ
and
ψ
(
2
S
)
mesons in their dimuon decay modes are measured using 2.1 and 11.4 fb
-
1
of data collected with the ATLAS experiment at the Large ...Hadron Collider, in proton–proton collisions at
s
=
7
and 8 respectively. Production cross-sections for prompt as well as non-prompt sources, ratios of
ψ
(
2
S
)
to
J
/
ψ
production, and the fractions of non-prompt production for
J
/
ψ
and
ψ
(
2
S
)
are measured as a function of meson transverse momentum and rapidity. The measurements are compared to theoretical predictions.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons (
e
or
μ
) with the same electric charge or at ...least three isolated leptons. The search also utilises
b
-tagged jets, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample of proton–proton collisions at
s
=
13
TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 corresponding to a total integrated luminosity of 3.2 fb
-
1
. No significant excess over the Standard Model expectation is observed. The results are interpreted in several simplified supersymmetric models and extend the exclusion limits from previous searches. In the context of exclusive production and simplified decay modes, gluino masses are excluded at
95
%
confidence level up to 1.1–1.3 TeV for light neutralinos (depending on the decay channel), and bottom squark masses are also excluded up to 540 GeV. In the former scenarios, neutralino masses are also excluded up to 550–850 GeV for gluino masses around 1 TeV.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
Inclusive four-jet events produced in proton-proton collisions at a centre-of-mass energy of
s
=
7
TeV are analysed for the presence of hard double-parton scattering using data ...corresponding to an integrated luminosity of 37
.
3 pb
−1
, collected with the ATLAS detector at the LHC. The contribution of hard double-parton scattering to the production of four-jet events is extracted using an artificial neural network, assuming that hard double-parton scattering can be approximated by an uncorrelated overlaying of dijet events. For events containing at least four jets with transverse momentum
p
T
≥ 20 GeV and pseudorapidity |
η
| ≤ 4
.
4, and at least one having
p
T
≥ 42
.
5 GeV, the contribution of hard double-parton scattering is estimated to be
f
DPS
= 0.092
− 0.011
+ 0.005
(stat.)
− 0.037
+ 0.033
(syst.). After combining this measurement with those of the inclusive dijet and four-jet cross-sections in the appropriate phase space regions, the effective cross-section,
σ
eff
, was determined to be
σ
eff
= 14. 9
− 1.0
+ 1.2
(stat.)
− 3.8
+ 5.1
(syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of
σ
eff
, performed at centre-of-mass energies between 63 GeV and 8 TeV using various final states, and it corresponds to 21
− 6
+ 7
% of the total inelastic cross-section measured at
s
=
7
TeV. The distributions of the observables sensitive to the contribution of hard double-parton scattering, corrected for detector effects, are also provided.
A search is presented for anomalous quartic gauge boson couplings in vector-boson scattering. The data for the analysis correspond to 20.2 fb−1 of s=8 TeV pp collisions and were collected in 2012 ...by the ATLAS experiment at the Large Hadron Collider. The search looks for the production of WW or WZ boson pairs accompanied by a high-mass dijet system, with one W decaying leptonically and a W or Z decaying hadronically. The hadronically decaying W/Z is reconstructed as either two small-radius jets or one large-radius jet using jet substructure techniques. Constraints on the anomalous quartic gauge boson coupling parameters α4 and α5 are set by fitting the transverse mass of the diboson system, and the resulting 95% confidence intervals are −0.024<α4<0.030 and −0.028<α5<0.033.
A search for neutral Higgs bosons of the minimal supersymmetric standard model (MSSM) and for a heavneutral
Z
′
boson is performed using a data sample corresponding to an integrated luminosity of ...3.2 fb
-
1
from proton–proton collisions at
s
=
13
TeV
recorded by the ATLAS detector at the LHC. The heavy resonance is assumed to decay to a
τ
+
τ
-
pair with at least one
τ
lepton decaying to final states with hadrons and a neutrino. The search is performed in the mass range of 0.2–1.2
TeV
for the MSSM neutral Higgs bosons and 0.5–2.5
TeV
for the heavy neutral
Z
′
boson. The data are in good agreement with the background predicted by the Standard Model. The results are interpreted in MSSM and
Z
′
benchmark scenarios. The most stringent constraints on the MSSM
m
A
–
tan
β
space exclude at 95 % confidence level (CL)
tan
β
>
7.6
for
m
A
=
200
GeV
in the
m
h
mod+
MSSM scenario. For the Sequential Standard Model, a
Z
SSM
′
mass up to 1.90
TeV
is excluded at 95 % CL and masses up to 1.82–2.17
TeV
are excluded for a
Z
SFM
′
of the strong flavour model.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb
−1
of integrated ...luminosity taken in 2012 at a centre-of-mass energy of 8 TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type
e
±
e
±
,
e
±
μ
±
, and
μ
±
μ
±
are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
The production of a
Z
boson and a photon in association with a high-mass dijet system is studied using 20.2 fb
−1
of proton-proton collision data at a centre-of-mass energy of
s
=
8
TeV ...recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Final states with a photon and a Z boson decaying into a pair of either electrons, muons, or neutrinos are analysed. Electroweak and total
pp
→
Zγjj
cross-sections are extracted in two fiducial regions with different sensitivities to electroweak production processes. Quartic couplings of vector bosons are studied in regions of phase space with an enhanced contribution from pure electroweak production, sensitive to vector-boson scattering processes
VV
→
Zγ
. No deviations from Standard Model predictions are observed and constraints are placed on anomalous couplings parameterized by higher-dimensional operators using effective field theory.
A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks to the lightest neutralino ( χ$~0\atop{1}$ ) is reported. It uses an LHC proton-proton data ...set at a center-of-mass energy √ s = 13 TeV with an integrated luminosity of 3.2 fb -1 collected with the ATLAS detector in 2015. The signal is searched for in events containing several energetic jets, of which at least three must be identified as b jets, large missing transverse momentum, and, potentially, isolated electrons or muons. Large-radius jets with a high mass are also used to identify highly boosted top quarks. No excess is found above the predicted background. For χ$~0\atop{1}$ masses below approximately 700 GeV, gluino masses of less than 1.78 TeV and 1.76 TeV are excluded at the 95% C.L. in simplified models of the pair production of gluinos decaying via sbottom and stop, respectively. These results significantly extend the exclusion limits obtained with the √ s = 8 TeV data set.
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