58 research outputs found

    A search for an unexpected asymmetry in the production of e+μ− and e−μ+ pairs in proton-proton collisions recorded by the ATLAS detector at root s = 13 TeV

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    This search, a type not previously performed at ATLAS, uses a comparison of the production cross sections for e(+)mu(-) and e(-)mu(+) pairs to constrain physics processes beyond the Standard Model. It uses 139 fb(-1) of proton-proton collision data recorded at root s = 13 TeV at the LHC. Targeting sources of new physics which prefer final states containing e(+)mu(-) and e(-)mu(+), the search contains two broad signal regions which are used to provide model-independent constraints on the ratio of cross sections at the 2% level. The search also has two special selections targeting supersymmetric models and leptoquark signatures. Observations using one of these selections are able to exclude, at 95% confidence level, singly produced smuons with masses up to 640 GeV in a model in which the only other light sparticle is a neutralino when the R-parity-violating coupling lambda(23)(1)' is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880 GeV when g(1R)(eu) = g(1R)(mu c) = 1, at 95% confidence level. The limit on the coupling reduces to g(1R)(eu) = g(1R)(mu c) = 0.46 for a mass of 1420 GeV

    Measurement of the nuclear modification factor for muons from charm and bottom hadrons in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

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    Heavy-flavour hadron production provides information about the transport properties and microscopic structure of the quark-gluon plasma created in ultra-relativistic heavy-ion collisions. A measurement of the muons from semileptonic decays of charm and bottom hadrons produced in Pb+Pb and pp collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV with the ATLAS detector at the Large Hadron Collider is presented. The Pb+Pb data were collected in 2015 and 2018 with sampled integrated luminosities of 208 mu b(-1) and 38 mu b(-1), respectively, and pp data with a sampled integrated luminosity of 1.17 pb(-1) were collected in 2017. Muons from heavy-flavour semileptonic decays are separated from the light-flavour hadronic background using the momentum imbalance between the inner detector and muon spectrometer measurements, and muons originating from charm and bottom decays are further separated via the muon track's transverse impact parameter. Differential yields in Pb+Pb collisions and differential cross sections in pp collisions for such muons are measured as a function of muon transverse momentum from 4 GeV to 30 GeV in the absolute pseudorapidity interval vertical bar eta vertical bar < 2. Nuclear modification factors for charm and bottom muons are presented as a function of muon transverse momentum in intervals of Pb+Pb collision centrality. The bottom muon results are the most precise measurement of b quark nuclear modification at low transverse momentum where reconstruction of B hadrons is challenging. The measured nuclear modification factors quantify a significant suppression of the yields of muons from decays of charm and bottom hadrons, with stronger effects for muons from charm hadron decays

    Productive Development Policies in Latin American Countries: The Case of Peru, 1990-2007

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    Differential cross-section measurements of the production of four charged leptons in association with two jets using the ATLAS detector

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    Differential cross-sections are measured for the production of four charged leptons in association with two jets. These measurements are sensitive to final states in which the jets are produced via the strong interaction as well as to the purely-electroweak vector boson scattering process. The analysis is performed using proton-proton collision data collected by ATLAS at √s = 13 TeV and with an integrated luminosity of 140 fb−1. The data are corrected for the effects of detector inefficiency and resolution and are compared to state-of-the-art Monte Carlo event generator predictions. The differential cross-sections are used to search for anomalous weak-boson self-interactions that are induced by dimension-six and dimension-eight operators in Standard Model effective field theory

    The Macaronesian Sparrowhawk diet in native and exotic forests

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    The Macaronesian Sparrowhawk (Accipiter nisus granti) is an endemic subspecies restricted to the forest habitats of Madeira and the Canary Islands. We studied its inter-habitat diet variation on the largest of the Canaries, Tenerife, during the 2014-2015 breeding seasons. We also compared the current food spectrum (2014-2015) with that reported in a study conducted 30 years ago. Prey remains analyses were conducted at the three main forested habitats, two native (laurel forest and Canarian pinewood) and one exotic (exotic tree plantations). Birds formed the main dietary component of the Sparrowhawk (26 species identified), but mammals and reptiles were also consumed in small numbers. Avian prey of around 200-300 g were preferred by Sparrowhawks. Three species accounted for 63.4% of the total number of prey counted (Atlantic Canary Serinus canarius, Rock Pigeon Columba livia and Blackbird Turdus merula), but their importance varied among habitats. In laurel forests and exotic tree plantations diet niche breadth and prey diversity was greater than in the Canarian pinewoods as reflected by prey densities. A comparison of our data with those from 1984-85 showed that large-sized avian prey might have become more important in the diet spectrum and thus indicating changes in the population status of prey species. The majority of Macaronesian Sparrowhawk hunting habitats are exposed to agricultural and forestry practices that could result in negative population trends of Sparrowhawk prey. Further studies are required to fully understand predator-prey relationships and to develop effective management actions to guarantee the long-term conservation of this island raptor

    Magnetic structure of the spin-1/2 layer compound NaNiO2

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    We have carried out high resolution neutron powder diffraction experiments aiming at a determination of the magnetic structure of the S=1/2 layer compound NaNiO2. The magnetic moments are ferromagnetically aligned in the NiO2 layers and antiparallel between layers. The direction of the magnetic moment has a small component along the a-direction. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2005
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