663 research outputs found
Diving in Two Marine Lakes in Croatia
We describe the diving methods used for in-situ observations of the scyphozoan medusa, Aurelia sp., in two marine lakes on the island of Mljet, Croatia. Both lakes have a strong pycnocline at approximately 15 m. During this study (May, 2004) surface temperature was about 20o C; bottom temperature about 10o C. Visibility was 15 m to 30 m. Tide and currents were negligible. A dense resident population of Aurelia sp. and a predictable environment made this an ideal study site. Aurelia was most abundant in mid-water around the pycnocline. There were several dive objectives: specimen collection for laboratory analysis, population census, discrete plankton tows and direct observation of flow around swimming medusae. We used several methods for maintaining our orientation underwater including working from an anchor line, towing a tethered buoy, and use of a blue water rig. Because the environment was relatively benign we allowed the rig to drift free while the boat was standing by at a short distance. Often a tether was not required. This plan allowed the most freedom and provided an excellent reference throughout the dive
Is a Trineutron Resonance Lower in Energy than a Tetraneutron Resonance?
We present quantum Monte Carlo calculations of few-neutron systems confined
in external potentials based on local chiral interactions at
next-to-next-to-leading order in chiral effective field theory. The energy and
radial densities for these systems are calculated in different external
Woods-Saxon potentials. We assume that their extrapolation to zero
external-potential depth provides a quantitative estimate of three- and
four-neutron resonances. The validity of this assumption is demonstrated by
benchmarking with an exact diagonalization in the two-body case. We find that
the extrapolated trineutron resonance, as well as the energy for shallow well
depths, is lower than the tetraneutron resonance energy. This suggests that a
three-neutron resonance exists below a four-neutron resonance in nature and is
potentially measurable. To confirm that the relative ordering of three- and
four-neutron resonances is not an artifact of the external confinement, we test
that the odd-even staggering in the helium isotopic chain is reproduced within
this approach. Finally, we discuss similarities between our results and
ultracold Fermi gases.Comment: 6 pages, 5 figures, version compatible with published lette
Signatures of few-body resonances in finite volume
We study systems of bosons and fermions in finite periodic boxes and show how
the existence and properties of few-body resonances can be extracted from
studying the volume dependence of the calculated energy spectra. Using a
plane-wave-based discrete variable representation to conveniently implement
periodic boundary conditions, we establish that avoided level crossings occur
in the spectra of up to four particles and can be linked to the existence of
multi-body resonances. To benchmark our method we use two-body calculations,
where resonance properties can be determined with other methods, as well as a
three-boson model interaction known to generate a three-boson resonance state.
Finding good agreement for these cases, we then predict three-body and
four-body resonances for models using a shifted Gaussian potential. Our results
establish few-body finite-volume calculations as a new tool to study few-body
resonances. In particular, the approach can be used to study few-neutron
systems, where such states have been conjectured to exist.Comment: 13 pages, 10 figures, 2 tables, published versio
Enhanced molecular yield from a cryogenic buffer gas beam source via excited state chemistry
We use narrow-band laser excitation of Yb atoms to substantially enhance the brightness of a cold beam of YbOH, a polyatomic molecule with high sensitivity to physics beyond the standard model (BSM). By exciting atomic Yb to the metastable ³P₁ state in a cryogenic environment, we significantly increase the chemical reaction cross-section for collisions of Yb with reactants. We characterize the dependence of the enhancement on the properties of the laser light, and study the final state distribution of the YbOH products. The resulting bright, cold YbOH beam can be used to increase the statistical sensitivity in searches for new physics utilizing YbOH, such as electron electric dipole moment and nuclear magnetic quadrupole moment experiments. We also perform new quantum chemical calculations that confirm the enhanced reactivity observed in our experiment and compare reaction pathways of Yb(³P) with the reactants H₂O and H₂O₂. More generally, our work presents a broad approach for improving experiments that use cryogenic molecular beams for laser cooling and precision measurement searches of BSM physics
Collisional properties of cold spin-polarized nitrogen gas: theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules
We report a combined experimental and theoretical study of collision-induced
dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use
buffer gas cooling to create trapped samples of 14N and 15N atoms with
densities 5+/-2 x 10^{12} cm-3 and measure their magnetic relaxation rates at
milli-Kelvin temperatures. Rigorous quantum scattering calculations based on
accurate ab initio interaction potentials for the 7Sigma_u electronic state of
N2 demonstrate that dipolar relaxation in N + N collisions occurs at a slow
rate of ~10^{-13} cm3/s over a wide range of temperatures (1 mK to 1 K) and
magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are
insensitive to small variations of the interaction potential and to the
magnitude of the spin-exchange interaction, enabling the accurate calibration
of the measured N atom density. We find consistency between the calculated and
experimentally determined rates. Our results suggest that N atoms are promising
candidates for future experiments on sympathetic cooling of molecules.Comment: 48 pages, 17 figures, 3 table
An investigation of thermodynamics, microscopic structure, depolarized Rayleigh scattering, and collision dynamics in Xe-N-2 supercritical mixtures
We would like to dedicate this work to the late Professor W. A. Steele (W.A.S.), Penn State University, USA. NATO Research-Project SA 5-2-05(CRG 950087) JARC (97) 288 is acknowledged for project funding to J.S., H.V. and W.A.S. The Greek State Scholarships Foundation (IKY) is acknowledged for an award based on performance to S. M. This work was supported by computational time granted from the Greek Research & Technology Network (GRNET) in the National HPC facility ARIS. The CPU time of the Computing Centre of the University of Athens (Greece) is gratefully acknowledged. This research utilized Queen Mary’s Mid-Plus computational facilities, supported by QMUL Research-IT and funded by EPSRC grant EP/K000128/1. J.K. acknowledges financial support from the NSF Grant No. CHE-1565872 to Millard Alexander
Enhanced molecular yield from a cryogenic buffer gas beam source via excited state chemistry
We use narrow-band laser excitation of Yb atoms to substantially enhance the brightness of a cold beam of YbOH, a polyatomic molecule with high sensitivity to physics beyond the standard model (BSM). By exciting atomic Yb to the metastable ³P₁ state in a cryogenic environment, we significantly increase the chemical reaction cross-section for collisions of Yb with reactants. We characterize the dependence of the enhancement on the properties of the laser light, and study the final state distribution of the YbOH products. The resulting bright, cold YbOH beam can be used to increase the statistical sensitivity in searches for new physics utilizing YbOH, such as electron electric dipole moment and nuclear magnetic quadrupole moment experiments. We also perform new quantum chemical calculations that confirm the enhanced reactivity observed in our experiment and compare reaction pathways of Yb(³P) with the reactants H₂O and H₂O₂. More generally, our work presents a broad approach for improving experiments that use cryogenic molecular beams for laser cooling and precision measurement searches of BSM physics
- …