143 research outputs found
Generalized Contact Formalism Analysis of the He Reaction
Measurements of short-range correlations in exclusive He
reactions are analyzed using the Generalized Contact Formalism (GCF). We
consider both instant-form and light-cone formulations with both the AV18 and
local N2LO(1.0) nucleon-nucleon () potentials. We find that kinematic
distributions, such as the reconstructed pair opening angle, recoil neutron
momentum distribution, and pair center of mass motion, as well as the measured
missing energy, missing mass distributions, are all well reproduced by GCF
calculations. The missing momentum dependence of the measured He
/ He cross-section ratios, sensitive to nature of the
interaction at short-distacnes, are also well reproduced by GCF calculations
using either interaction and formulation. This gives credence to the GCF
scale-separated factorized description of the short-distance many-body nuclear
wave-function.Comment: Accepted for publication in Physics Letters B. 8 pages, 4 figures and
online supplementary material
The CLAS12 Backward Angle Neutron Detector (BAND)
The Backward Angle Neutron Detector (BAND) of CLAS12 detects neutrons emitted
at backward angles of to , with momenta between
and MeV/c. It is positioned 3 meters upstream of the target, consists of
rows and layers of cm by 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 ps (150 ps) for energy depositions above 2 MeVee (5 MeVee).
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, \% for neutron momentum 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.Comment: 17 pages, 25 figures, 3 tables. Accepted for publication in NIM-
Observation of a correlated free four-neutron system
A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades(1), with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far(2-4), leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy He-8 beam was key
Measurement of nuclear transparency ratios for protons and neutrons
This paper presents, for the first time, measurements of neutron transparency ratios for nuclei relative to C measured using the (e,e′n) reaction, spanning measured neutron momenta of 1.4 to 2.4 GeV/c. The transparency ratios were extracted in two kinematical regions, corresponding to knockout of mean-field nucleons and to the breakup of Short-Range Correlated nucleon pairs. The extracted neutron transparency ratios are consistent with each other for the two measured kinematical regions and agree with the proton transparencies extracted from new and previous (e,e′p) measurements, including those from neutron-rich nuclei such as lead. The data also agree with and confirm the Glauber approximation that is commonly used to interpret experimental data. The nuclear-mass-dependence of the extracted transparencies scales as Aα with α=−0.289±0.007, which is consistent with nuclear-surface dominance of the reactions
2016 update of the EULAR recommendations for the management of early arthritis
Objectives: Since the 2007 recommendations for the management of early arthritis have been presented, considerable research has been published in the field of early arthritis, mandating an update of the 2007 EULAR recommendations for management of early arthritis. Methods: In accordance with the 2014 EULAR Standardised Operating Procedures, the expert committee pursued an approach that was based on evidence in the literature and on expert opinion. The committee involved 20 rheumatologists, 2 patients and 1 health professional representing 12 European countries. The group defined the focus of the expert committee and target population, formulated a definition of “management” and selected the research questions. A systematic literature research (SLR) was performed by 2 fellows with the help of a skilled librarian. A set of draft recommendations was proposed on the basis of the research questions and the results of the SLR. For each recommendation the categories of evidence were identified, the strength of recommendations was derived and the level of agreement was determined through a voting process. Results: The updated recommendations comprise 3 overarching principles and 12 recommendations for managing early arthritis. The selected statements involve the recognition of arthritis, referral, diagnosis, prognostication, treatment (information, education, pharmacological and non-pharmacological interventions), monitoring and strategy. Eighteen items were identified as relevant for future research. Conclusion: These recommendations provide rheumatologists, general practitioners, health professionals, patients and other stakeholders with an updated EULAR consensus on the entire management of early arthritis
Comparing proton momentum distributions in and 3 nuclei via H H and He measurements
We report the first measurement of the reaction cross-section
ratios for Helium-3 (He), Tritium (H), and Deuterium (). The
measurement covered a missing momentum range of
MeV, at large momentum transfer (
(GeV)) and , which minimized contributions from non
quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave
impulse approximation (PWIA) calculations using realistic spectral functions
and momentum distributions. The measured and PWIA-calculated cross-section
ratios for He and H extend to just above the typical nucleon
Fermi-momentum ( MeV) and differ from each other by , while for He/H they agree within the measurement accuracy of
about 3\%. At momenta above , the measured He/H ratios differ from
the calculation by . Final state interaction (FSI) calculations
using the generalized Eikonal Approximation indicate that FSI should change the
He/H cross-section ratio for this measurement by less than 5\%. If
these calculations are correct, then the differences at large missing momenta
between the He/H experimental and calculated ratios could be due to the
underlying interaction, and thus could provide new constraints on the
previously loosely-constrained short-distance parts of the interaction.Comment: 8 pages, 3 figures (4 panels
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