We measured the triple coincidence A(e,e^{'}np) and A(e,e^{'}pp) reactions on carbon, aluminum, iron, and lead targets at Q^{2}>1.5 (GeV/c)^{2}, x_{B}>1.1 and missing momentum >400 MeV/c. This was ...the first direct measurement of both proton-proton (pp) and neutron-proton (np) short-range correlated (SRC) pair knockout from heavy asymmetric nuclei. For all measured nuclei, the average proton-proton (pp) to neutron-proton (np) reduced cross-section ratio is about 6%, in agreement with previous indirect measurements. Correcting for single-charge exchange effects decreased the SRC pairs ratio to ∼3%, which is lower than previous results. Comparisons to theoretical generalized contact formalism (GCF) cross-section calculations show good agreement using both phenomenological and chiral nucleon-nucleon potentials, favoring a lower pp to np pair ratio. The ability of the GCF calculation to describe the experimental data using either phenomenological or chiral potentials suggests possible reduction of scale and scheme dependence in cross-section ratios. Our results also support the high-resolution description of high-momentum states being predominantly due to nucleons in SRC pairs.
The EMC effect in deuterium and helium-3 is studied using a convolution formalism that allows isolating the impact of high-momentum nucleons in short-ranged correlated (SRC) pairs. We assume that the ...modification of the structure function of bound nucleons is given by a universal (i.e., nucleus independent) function of their virtuality, and find that the effect of such modifications is dominated by nucleons in SRC pairs. This SRC-dominance of nucleon modifications is observed despite the fact that the bulk of the nuclear inelastic scattering cross-section comes from interacting with low-momentum nucleons. These conclusions are found to be robust to model details including nucleon modification function parametrization, free nucleon structure function, and treatment of nucleon motion effects. While existing data cannot discriminate between such model details, we present predictions for measured, but not yet published, tritium EMC effect and tagged nucleon structure functions in deuterium that are sensitive to the neutron structure functions and bound nucleon modification functions.
The strong nuclear interaction is probed at short-distance and high-momenta using new measurements of the $^{12}$C$(e,e'p)$ and $^{12}$C$(e,e'pn)$ reactions, at high-$Q^2$ and $x_B>1$. The data span ...a missing-momentum range of 300-850 MeV/c and is predominantly sensitive to the dominant proton-neutron short-range correlated (SRC) pairs and complements previous $^{12}$C$(e,e'pp)$ measurements. The data are well reproduced by theoretical calculations using the Generalized Contact Formalism with both chiral and phenomenological nucleon-nucleon ($NN$) interaction models. This agreement, observed here for the first time, suggests that the measured high missing-momentum protons up to $850$ MeV/c belonged to SRC pairs. The measured $^{12}$C$(e,e'pn)$ / $^{12}$C$(e,e'p)$ ratio is consistent with a decrease in the fraction of proton-neutron SRC pairs with increasing missing-momentum. This confirms the transition from an isospin-dependent tensor $NN$ interaction at $\sim 400$ MeV/c to an isospin-independent scalar interaction at high-momentum around $\sim 800$ MeV/c.
Herein, short range correlated nucleon-nucleon (NN) pairs are an important part of the nuclear ground state. They are typically studied by scattering an electron from one nucleon in the pair and ...detecting its spectator correlated partner (“spectator-nucleon tagging”). The Electron Ion Collider (EIC) should be able to detect these nucleons, since they are boosted to high momentum in the laboratory frame by the momentum of the ion beam. To determine the feasibility of these studies with the planned EIC detector configuration, we have simulated quasielastic scattering for two electron and ion beam energy configurations: 5 GeV e- and 41 GeV/A ions, and 10 GeV e- and 110 GeV/A ions. We show that the knocked-out and recoiling nucleons can be detected over a wide range of initial nucleon momenta. We also show that these measurements can achieve much larger momentum transfers than current fixed target experiments. By detecting both low and high initial-momentum nucleons, the planned EIC has the potential to provide the data that should allow scientists to definitively show if the European Muon Collaboration effect and short-range correlation are connected, and to improve our understanding of color transparency.
The CLAS12 Backward Angle Neutron Detector (BAND) Segarra, E.P.; Hauenstein, F.; Schmidt, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
10/2020, Letnik:
978
Journal Article
Recenzirano
Odprti dostop
The Backward Angle Neutron Detector (BAND) of CLAS12 detects neutrons emitted at backward angles of 155° to 175°, with momenta between 200 and 600 MeV/c0. It is positioned 3-m upstream of the target, ...consists of 18 rows and 5 layers of 7.2-cm by 7.2-cm scintillator bars, and read out on both ends by PMTs to measure time and energy deposition in the scintillator layers. Between the target and BAND there is a 2-cm thick lead wall followed by a 2-cm veto layer to suppress gammas and reject charged particles. This paper discusses the component-selection tests and the detector assembly. Timing calibrations (including offsets and time-walk) were performed using a novel pulsed-laser calibration system, resulting in time resolutions better than 250 ps (150 ps) for energy depositions above 2 MeV (5 MeV). Cosmic rays and a variety of radioactive sources were used to calibration the energy response of the detector. Scintillator bar attenuation lengths were measured. The time resolution results in a neutron momentum reconstruction resolution, δp/p<1.5% for neutron momentum 200≤p≤600 MeV/c. Final performance of the BAND with CLAS12 is shown, including electron–neutral particle timing spectra and a discussion of the off-time neutral contamination as a function of energy deposition threshold.
Short range correlated nucleon-nucleon (NN) pairs are an important part of the nuclear ground state. They are typically studied by scattering an electron from one nucleon in the pair and detecting ...its spectator correlated partner (spectator-nucleon tagging). The Electron Ion Collider (EIC)should be able to detect these nucleons, since they are boosted to high momentum in the lab frame by the momentum of the ion beam. To determine the feasibility of these studies with the planned EIC detector configuration, we have simulated quasi-elastic scattering for two electron and ion beam energy configurations: 5 GeV electrons and 41 GeV/A ions, and 10 GeV electrons and 110 GeV/A ions. We show that the knocked-out and recoiling nucleons can be detected over a wide range of initial nucleon momenta. We also show that these measurements can achieve much larger momentum transfers than current fixed target experiments. By detecting both low and high initial-momentum nucleons,the EIC will provide the data that should allow scientists to definitively show if the EMC effect and short-range correlation are connected, and to improve our understanding of color transparency
Understanding the causes and consequences of the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern is crucial to pandemic control yet difficult to achieve ...because they arise in the context of variable human behavior and immunity. We investigated the spatial invasion dynamics of lineage B.1.1.7 by jointly analyzing UK human mobility, virus genomes, and community-based polymerase chain reaction data. We identified a multistage spatial invasion process in which early B.1.1.7 growth rates were associated with mobility and asymmetric lineage export from a dominant source location, enhancing the effects of B.1.1.7's increased intrinsic transmissibility. We further explored how B.1.1.7 spread was shaped by nonpharmaceutical interventions and spatial variation in previous attack rates. Our findings show that careful accounting of the behavioral and epidemiological context within which variants of concern emerge is necessary to interpret correctly their observed relative growth rates.
The first UK epizootic of highly pathogenic (HP) H5N1 influenza in wild birds occurred in 2008, in a population of mute swans that had been the subject of ornithological study for decades. Here we ...use an innovative combination of ornithological, phylogenetic and immunological approaches to investigate the ecology and age structure of HP H5N1 in nature. We screened samples from swans and waterbirds using PCR and sequenced HP H5N1-positive samples. The outbreak's origin was investigated by linking bird count data with a molecular clock analysis of sampled virus sequences. We used ringing records to reconstruct the age-structure of outbreak mortality, and we estimated the age distribution of prior exposure to avian influenza. Outbreak mortality was low and all HP H5N1-positive mute swans in the affected population were <3 years old. Only the youngest age classes contained an appreciable number of individuals with no detectable antibody responses to viral nucleoprotein. Phylogenetic analysis indicated that the outbreak strain circulated locally for ∼1 month before detection and arrived when the immigration rate of migrant waterbirds was highest. Our data are consistent with the hypothesis that HP H5N1 epizootics in wild swans exhibit limited mortality due to immune protection arising from previous exposure. Our study population may represent a valuable resource for investigating the natural ecology and epidemiology of avian influenza.
Thirty years after the discovery of HIV-1, the early transmission, dissemination, and establishment of the virus in human populations remain unclear. Using statistical approaches applied to HIV-1 ...sequence data from central Africa, we show that from the 1920s Kinshasa (in what is now the Democratic Republic of Congo) was the focus of early transmission and the source of pre-1960 pandemic viruses elsewhere. Location and dating estimates were validated using the earliest HIV-1 archival sample, also from Kinshasa. The epidemic histories of HIV-1 group M and nonpandemic group O were similar until ~1960, after which group M underwent an epidemiological transition and outpaced regional population growth. Our results reconstruct the early dynamics of HIV-1 and emphasize the role of social changes and transport networks in the establishment of this virus in human populations.
Although cliffs form approximately 75% of the world's coastline, the understanding of the processes through which they evolve remains limited because of a lack of quantitative data on the ...morphological changes they undergo. In this paper the combination of terrestrial time‐of‐flight laser scanning with high‐resolution digital photogrammetry is examined to generate high‐quality data‐sets pertaining to the geomorphic processes governing cliff development. The study was undertaken on a section of hard rock cliffs in North Yorkshire, UK, which has been monitored over a 12‐month period. High‐density, laser‐scanned point clouds have been used to produce an accurate representation of these complex surfaces, free from the optical variations that degrade photographic data. These data‐sets have been combined with high‐resolution photographic monitoring, resampled with the fixed accuracies of the terrestrial laser survey, to generate a new approach to recording the volumetric changes in complex coastal cliffs. This has led to significant improvements in the understanding of the activity patterns of coastal cliffs.
Résumé
Bien que les falaises forment environ 75% du littoral mondial, notre compréhension des processus qui marquent leur évolution reste limitée par suite du manque de données quantitatives sur les modifications morphologiques qu'elles subissent. On étudie dans cet article une combinaison de levés comprenant un balayage terrestre au laser (basé sur les temps de propagation) et une saisie par photogrammétrie numérique à haute résolution permettant de générer des jeux de données de grande qualité se rapportant aux processus géographiques qui régissent le développement des falaises. On a mené cette étude sur une partie des falaises de roche dure du Yorkshire du Nord (Royaume Uni) en s'assurant d'un suivi sur une période de 12 mois. A partir des nuages de points, à haute densité, issus du balayage laser, on a réalisé une représentation précise de ces surfaces complexes, exempte des variations optiques qui perturbent les données photographiques. En combinant le suivi photographique à haute résolution avec le relevé terrestre au laser qui en assoyait le ré‐échantillonnage précis, on a obtenu une nouvelle façon de traiter les enregistrements des changements volumétriques des falaises littorales complexes. Cela a amenéà des améliorations significatives dans notre compréhension des schémas de la dynamique des falaises littorales.
Zusammenfassung
Obwohl ungefähr 75% der Küstenlinien in der Welt durch Klippen geformt sind, ist unser Verständnis ihres Entwicklungsprozesses begrenzt, da quantitative Daten zu den ablaufenden morphologischen Veränderungen fehlen. In diesem Beitrag wird die Kombination von terrestrischem Laserscanning mit hochauflösender Digitaler Photogrammetrie beschrieben, um qualitativ hochwertige Datensätze zu den geomorphologischen Prozessen zu erfassen, die die Entwicklung der Küstenklippen bestimmen. Ein Abschnitt der Steilküste in North Yorkshire in UK wurde über einen Zeitraum von 12 Monaten intensiv überwacht. Die Geometrie der komplexen Oberflächen wird dabei durch dichte Punktwolken aus dem terrestrischen Laserscanning repräsentiert. Hochauflösende photographische Aufnahmen werden auf die Geometrie der Laserdaten umgebildet und mit diesen kombiniert. Mit diesem kombinierten Datenmodell lassen sich volumetrische Änderungen von komplexen Küstenklippen aufnehmen und analysieren. Dies trug wesentlich dazu bei, die komplexen geomorphologischen Bewegungsmuster von Küstenklippen besser zu verstehen.
Resumen
Aunque los acantilados están presentes en el 75% de las costas continentales del mundo, nuestro conocimiento de los procesos evolutivos continua siendo limitado por la falta de datos cuantitativos sobre los cambios morfológicos que sufren. En este artículo examinamos la combinación del escaneado láser terrestre y la fotogrametría digital de alta resolución para obtener datos de alta calidad sobre los procesos geomorfológicos implicados en el desarrollo de los acantilados. El estudio se ha llevado a cabo en una sección de acantilados de roca dura en North Yorkshire, Reino Unido, que ha sido monitorizado durante 12 meses. Las nubes de puntos de alta densidad obtenidas por escaneado láser han sido utilizadas para obtener una representación exacta de esas complejas estructuras libre de las variaciones ópticas que degradan las fotografías. Esta información se combinó con una secuencia fotográfica de alta resolución, remuestreada con los datos exactos del levantamiento láser terrestre, lo que constituye una nueva forma de registrar los cambios volumétricos en acantilados costeros complejos. Ello ha permitido mejorar significativamente nuestro conocimiento de los patrones de actividad de los mismos.
PDF
