Evidence from an international survey in the Atlantic biogeographic region of Europe indicates that chronic nitrogen deposition is reducing plant species richness in acid grasslands. Across the ...deposition gradient in this region (2–44 kg N ha
−1 yr
−1) species richness showed a curvilinear response, with greatest reductions in species richness when deposition increased from low levels. This has important implications for conservation policies, suggesting that to protect the most sensitive grasslands resources should be focussed where deposition is currently low. Soil pH is also an important driver of species richness indicating that the acidifying effect of nitrogen deposition may be contributing to species richness reductions. The results of this survey suggest that the impacts of nitrogen deposition can be observed over a large geographical range.
Atmospheric nitrogen deposition is reducing biodiversity in grasslands across Europe.
A
bstract
A determination of the top-quark mass is presented using 20.2 fb
−
1
of 8 TeV proton-proton collision data produced by the Large Hadron Collider and collected by the ATLAS experiment. The ...normalised differential cross section of top-quark pair production in association with an energetic jet is measured in the lepton+jets final state and unfolded to parton and particle levels. The unfolded distribution at parton level can be described using next-to-leading-order QCD predictions in terms of either the top-quark pole mass or the running mass as defined in the (modified) minimal subtraction scheme. A comparison between the experimental distribution and the theoretical prediction allows the top-quark mass to be extracted in the two schemes. The value obtained for the pole-mass scheme is:
m
t
pole
=
171.1
±
0.4
stat
±
0.9
syst
−
0.3
+
0.7
theo
GeV
The extracted value in the running-mass scheme is:
m
t
m
t
=
162.9
±
0.5
stat
±
1.0
syst
−
1.2
+
2.1
theo
GeV
.
The results for the top-quark mass using the two schemes are consistent, when translated from one scheme to the other.
A
bstract
A search for the electroweak production of pairs of charged sleptons or charginos decaying into two-lepton final states with missing transverse momentum is presented. Two simplified models ...of
R
-parity-conserving supersymmetry are considered: direct pair-production of sleptons
ℓ
~
ℓ
~
, with each decaying into a charged lepton and a
χ
~
1
0
neutralino, and direct pair-production of the lightest charginos
χ
~
1
±
χ
~
1
∓
, with each decaying into a
W
-boson and a
χ
~
1
0
. The lightest neutralino
χ
~
1
0
is assumed to be the lightest supersymmetric particle (LSP). The analyses target the experimentally challenging mass regions where
m
ℓ
~
−
m
χ
~
1
0
and
m
χ
~
1
±
−
m
χ
~
1
0
are close to the
W
-boson mass (‘moderately compressed’ regions). The search uses 139 fb
−
1
of
s
= 13 TeV proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider. No significant excesses over the expected background are observed. Exclusion limits on the simplified models under study are reported in the
ℓ
~
χ
~
1
0
and
χ
~
1
±
χ
~
1
0
mass planes at 95% confidence level (CL). Sleptons with masses up to 150 GeV are excluded at 95% CL for the case of a mass-splitting between sleptons and the LSP of 50 GeV. Chargino masses up to 140 GeV are excluded at 95% CL for the case of a mass-splitting between the chargino and the LSP down to about 100 GeV.
A
bstract
Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs (
t
t
¯
) are measured using the full LHC Run 2 data sample collected ...with the ATLAS detector. The data were collected at a
pp
collision energy of
s
= 13 TeV and correspond to an integrated luminosity of 140 fb
−
1
. The measurements use events containing an oppositely charged
eμ
pair and
b
-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton
p
T
distributions is obtained by reweighting the
t
t
¯
sample so as to reproduce the top-quark
p
T
distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to be
σ
t
t
¯
=
829
±
1
stat
±
13
syst
±
8
lumi
±
2
beam
pb
,
where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.
Our study investigates the negative impact of nitrogen (N) deposition on species richness in acidic grasslands, based on a temporal comparison of vegetation data spanning a period of almost 70 years. ...We compiled a large data base of plots assigned to the Violion caninae grassland type, composed of managed, but unfertilized semi-natural grasslands on nutrient-poor, acidic soils. In total 1114 plots, mainly from Great Britain, the Netherlands and Germany, were compiled, dating back to 1939. Environmental site information included geographical and soil (mean Ellenberg values) variables as well as estimates of cumulative N and sulphur (S) deposition since 1939. Statistical analyses were carried out separately for the data subsets from the three regions. In all regions, the vegetation differentiation was mainly related to soil acidity and nutrient availability, as well as to the year of sampling and the cumulative amounts of N and S deposition. Plot-species richness of vascular plants and bryophytes (analysed for Great Britain only) decreased with time and analyses suggest these are affected by various factors, notably soil pH, but also latitude and cumulative N deposition. The latter explained more of the variation in species number than the year of sampling and cumulative S deposition, which supports the interpretation that the decline in species richness is mainly caused by increasing N availability and less by altered management and soil acidification. For Great Britain and Germany, cumulative N deposition showed a strong negative relationship with several biodiversity measures, especially the proportion of dicots, whereas it was positively related to the proportion of grass species. In general, our results give temporal evidence for the negative effect of N deposition on species richness in semi-natural vegetation.
Nutrient pollution presents a serious threat to biodiversity conservation. In terrestrial ecosystems, the deleterious effects of nitrogen pollution are increasingly understood and several mitigating ...environmental policies have been developed. Compared to nitrogen, the effects of increased phosphorus have received far less attention, although some studies have indicated that phosphorus pollution may be detrimental for biodiversity as well. On the basis of a dataset covering 501 grassland plots throughout Europe, we demonstrate that, independent of the level of atmospheric nitrogen deposition and soil acidity, plant species richness was consistently negatively related to soil phosphorus. We also identified thresholds in soil phosphorus above which biodiversity appears to remain at a constant low level. Our results indicate that nutrient management policies biased toward reducing nitrogen pollution will fail to preserve biodiversity. As soil phosphorus is known to be extremely persistent and we found no evidence for a critical threshold below which no environmental harm is expected, we suggest that agro‐environmental schemes should include grasslands that are permanently free from phosphorus fertilization.
A
bstract
A measurement of inclusive and differential fiducial cross-sections for the production of the Higgs boson decaying into two photons is performed using 139 fb
−
1
of proton-proton collision ...data recorded at
s
= 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio, in a fiducial region closely matching the experimental selection, is measured to be 67
±
6 fb, which is in agreement with the state-of-the-art Standard Model prediction of 64
±
4 fb. Extrapolating this result to the full phase space and correcting for the branching ratio, the total cross-section for Higgs boson production is estimated to be 58
±
6 pb. In addition, the cross-sections in four fiducial regions sensitive to various Higgs boson production modes and differential cross-sections as a function of either one or two of several observables are measured. All the measurements are found to be in agreement with the Standard Model predictions. The measured transverse momentum distribution of the Higgs boson is used as an indirect probe of the Yukawa coupling of the Higgs boson to the bottom and charm quarks. In addition, five differential cross-section measurements are used to constrain anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework.
In a special run of the LHC with
β
⋆
=
2.5
km, proton–proton elastic-scattering events were recorded at
s
=
13
TeV with an integrated luminosity of
340
μ
b
-
1
using the ALFA subdetector of ATLAS ...in 2016. The elastic cross section was measured differentially in the Mandelstam
t
variable in the range from
-
t
=
2.5
·
10
-
4
GeV
2
to
-
t
=
0.46
GeV
2
using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section
σ
tot
, parameters of the nuclear slope, and the
ρ
-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit
t
→
0
. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the
t
-dependence. The results for
σ
tot
and
ρ
are
σ
tot
(
p
p
→
X
)
=
104.7
±
1.1
mb
,
ρ
=
0.098
±
0.011
.
The uncertainty in
σ
tot
is dominated by the luminosity measurement, and in
ρ
by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb
−
1
of proton-proton collision data collected by the ATLAS detector at the LHC in ...2015–2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for values of
c
times LLP mean proper lifetime in the range between 20 mm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for scalars with a mass of 60 GeV and for masses between 200 GeV and 1 TeV.
Field observations and experimental data of effects of nitrogen (N) deposition on plant species diversity have been used to derive empirical critical N loads for various ecosystems. The great ...advantage of such an approach is the inclusion of field evidence, but there are also restrictions, such as the absence of explicit criteria regarding significant effects on the vegetation, and the impossibility to predict future impacts when N deposition changes. Model approaches can account for this. In this paper, we review the possibilities of static and dynamic multispecies models in combination with dynamic soil–vegetation models to (1) predict plant species composition as a function of atmospheric N deposition and (2) calculate critical N loads in relation to a prescribed protection level of the species composition. The similarities between the models are presented, but also several important differences, including the use of different indicators for N and acidity and the prediction of individual plant species vs. plant communities. A summary of the strengths and weaknesses of the various models, including their validation status, is given. Furthermore, examples are given of critical load calculations with the model chains and their comparison with empirical critical N loads. We show that linked biogeochemistry–biodiversity models for N have potential for applications to support European policy to reduce N input, but the definition of damage thresholds for terrestrial biodiversity represents a major challenge. There is also a clear need for further testing and validation of the models against long-term monitoring or long-term experimental data sets and against large-scale survey data. This requires a focused data collection in Europe, combing vegetation descriptions with variables affecting the species diversity, such as soil acidity, nutrient status and water availability. Finally, there is a need for adaptation and upscaling of the models beyond the regions for which dose–response relationships have been parameterized, to make them generally applicable.
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