213 research outputs found
Three-body forces and shell structure in calcium isotopes
Understanding and predicting the formation of shell structure from nuclear
forces is a central challenge for nuclear physics. While the magic numbers
N=2,8,20 are generally well understood, N=28 is the first standard magic number
that is not reproduced in microscopic theories with two-nucleon forces. In this
Letter, we show that three-nucleon forces give rise to repulsive interactions
between two valence neutrons that are key to explain 48Ca as a magic nucleus,
with a high 2+ excitation energy and a concentrated magnetic dipole transition
strength. The repulsive three-nucleon mechanism improves the agreement with
experimental binding energies.Comment: 5 pages, 4 figures; improved version and added coupled-cluster
benchmark; published versio
The InDeVal insertion/deletion evaluation tool: a program for finding target regions in DNA sequences and for aiding in sequence comparison
BACKGROUND: The program InDeVal was originally developed to help researchers find known regions of insertion/deletion activity (with the exception of isolated single-base indels) in newly determined Poaceae trnL-F sequences and compare them with 533 previously determined sequences. It is supplied with input files designed for this purpose. More broadly, the program is applicable for finding specific target regions (referred to as "variable regions") in DNA sequence. A variable region is any specific sequence fragment of interest, such as an indel region, a codon or codons, or sequence coding for a particular RNA secondary structure. RESULTS: InDeVal input is DNA sequence and a template file (sequence flanking each variable region). Additional files contain the variable regions and user-defined messages about the sequence found within them (e.g., taxa sharing each of the different indel patterns). Variable regions are found by determining the position of flanking sequence (referred to as "conserved regions") using the LPAM (Length-Preserving Alignment Method) algorithm. This algorithm was designed for InDeVal and is described here for the first time. InDeVal output is an interactive display of the analyzed sequence, broken into user-defined units. Once the user is satisfied with the organization of the display, the information can be exported to an annotated text file. CONCLUSIONS: InDeVal can find multiple variable regions simultaneously (28 indel regions in the Poaceae trnL-F files) and display user-selected messages specific to the sequence variants found. InDeVal output is designed to facilitate comparison between the analyzed sequence and previously evaluated sequence. The program's sensitivity to different levels of nucleotide and/or length variation in conserved regions can be adjusted. InDeVal is currently available for Windows in Additional file 1 or from
Three-body forces and the limit of oxygen isotopes
The limit of neutron-rich nuclei, the neutron drip-line, evolves regularly
from light to medium-mass nuclei except for a striking anomaly in the oxygen
isotopes. This anomaly is not reproduced in shell-model calculations derived
from microscopic two-nucleon forces. Here, we present the first microscopic
explanation of the oxygen anomaly based on three-nucleon forces that have been
established in few-body systems. This leads to repulsive contributions to the
interactions among excess neutrons that change the location of the neutron
drip-line from O to the experimentally observed O. Since the
mechanism is robust and general, our findings impact the prediction of the most
neutron-rich nuclei and the synthesis of heavy elements in neutron-rich
environments.Comment: 4 pages, 4 figures, to be published in PR
Effective double-beta-decay operator for 76Ge and 82Se
We use diagrammatic many-body perturbation theory in combination with
low-momentum interactions derived from chiral effective field theory to
construct effective shell-model transition operators for the neutrinoless
double-beta decay of 76Ge and 82Se. We include all unfolded diagrams to first-
and second-order in the interaction and all singly folded diagrams that can be
constructed from them. The resulting effective operator, which accounts for
physics outside the shell-model space, increases the nuclear matrix element by
about 20% in 76Ge and 30% in 82Se.Comment: 8 pages, 5 figure
The effects of tides on the water mass mixing and sea ice in the Arctic Ocean
In this study, we use a novel pan-Arctic sea ice-ocean coupled model to examine the effects of tides on sea ice and the mixing of water masses. Two 30 year simulations were performed: one with explicitly resolved tides and the other without any tidal dynamics. We find that the tides are responsible for a ∼15% reduction in the volume of sea ice during the last decade and a redistribution of salinity, with surface salinity in the case with tides being on average ∼1.0–1.8 practical salinity units (PSU) higher than without tides. The ice volume trend in the two simulations also differs: −2.09 × 103 km3/decade without tides and −2.49 × 103 km3/decade with tides, the latter being closer to the trend of −2.58 × 103 km3/decade in the PIOMAS model, which assimilates SST and ice concentration. The three following mechanisms of tidal interaction appear to be significant: (a) strong shear stresses generated by the baroclinic clockwise rotating component of tidal currents in the interior waters; (b) thicker subsurface ice-ocean and bottom boundary layers; and (c) intensification of quasi-steady vertical motions of isopycnals (by ∼50%) through enhanced bottom Ekman pumping and stretching of relative vorticity over rough bottom topography. The combination of these effects leads to entrainment of warm Atlantic Waters into the colder and fresher surface waters, supporting the melting of the overlying ice
First Direct Double-Beta Decay Q-value Measurement of 82Se in Support of Understanding the Nature of the Neutrino
In anticipation of results from current and future double-beta decay studies,
we report a measurement resulting in a 82Se double-beta decay Q-value of
2997.9(3) keV, an order of magnitude more precise than the currently accepted
value. We also present preliminary results of a calculation of the 82Se
neutrinoless double-beta decay nuclear matrix element that corrects in part for
the small size of the shell model single-particle space. The results of this
work are important for designing next generation double-beta decay experiments
and for the theoretical interpretations of their observations.Comment: 5 pages, 2 figures, accepted for publication in Physical Review
Letter
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