Antihydrogen formation in collisions of positronium with antiprotons

Antihydrogen, consisting of a positron orbiting around an antiproton, is the simplest few body system consisting entirely of antimatter and as such is of considerable importance in providing additional tests of the validity of charge conjugation invariance. In addition, the nature of the gravitational interaction between matter and antimatter might more readily be investigated for an electrically neutral system than one which is charged. Before such studies can be undertaken the antihydrogen must, of course, be produced by attachment of a positron to an antipositron. Several production mechanisms have been proposed, the two most favored of which are radiative capture (spontaneous or stimulated) and charge exchange in positronium-antiproton collisions. The cross section for radiative capture is very much less than that for charge exchange, so that it might be thought that the latter process is greatly to be preferred. Various calculations of the cross section for the charge exchange process are briefly reviewed

ESTIMATING SURFICIAL SEAFLOOR SEDIMENT PROPERTIES USING AN EMPIRICAL ORTHOGONAL DECOMPOSITION ON ACOUSTIC BACKSCATTER WAVEFORM PROPERTIES

Seafloor classification and environmental assessment in shallow marine waters are crucial to habitat mapping, coastal management policies and maintaining navigational waterways. There are existing methods for remotely estimating some bottom properties, but the large variety of desired measured sediment properties frequently leads to insufficient quantifiable data to support marine policy decisions. This problem is exacerbated by the highly variable bottom composition of typical coastal and estuarine environments. In this work, field observations from an Odom Echotrac vertical-incidence echosounder with a 200 khz transducer were used to estimate seafloor sediment characteristics in regions with variable bottom types. Observations were obtained in water depths ranging 0.5-24 m of the Little Bay, New Hampshire, during February and March, 2013. Backscatter waveforms (the acoustic return representing the first interaction with the bottom) were analyzed and their properties compared to sediment grain size distributions. These comparisons showed varied degrees of predictive capability and require subjective a priori selection. In an effort to better capture the collective effects of seafloor sediment\u27s composition on acoustic returns, empirical orthogonal functions (EOF\u27s) were computed from an ensemble of seven waveform properties and compared with observed surficial sediment size fractions, bulk density, and porosity. A simple logarithmic model relating first mode EOF spatial variability to observed mud fractions explained 43% of the variability and well estimated the spatial pattern of mud across the bay (RMS errors in mud fraction of 10-15%) from deep channels (with no mud) to high concentrations of mud on the shallower flats near the sides of the estuary. This method produced greater coverage and higher resolution predictions of mud fraction than could be obtained using traditional sediment measuring techniques. Deviations from the model are shown to be correlated with lower sediment porosity most likely due to river inflow from the Bellamy River draining into the Bay. Application of the model coefficients to new data obtained in the Great Bay in 2014 with the same sonar and acoustic settings, showed similar predicted mud fractions with RMS errors of 11.9 and 13.2% along two surveyed lines. This empirical analysis provides a first order objective means to interpret acoustic backscatter, an important step towards a widespread quantitative assessment of shallow water seafloor sediments

Wind, Wave, and Engineering Effects on Tidal Inlet Morphodynamics

This body of work examines the hydro- and morpho-dynamics of tidal inlets using a diverse array of research tools. A custom X-band radar system (RIOS) is designed and used to track the evolution of shoal features over the ebb-tidal delta at Oregon Inlet, NC. Analysis of the data reveals that alongshore shoal migration rates are closely correlated to wave driven alongshore sediment transport, and also that the cross-shore bedform migration rates are tightly correlated with a sub-tidal water level gradient though the inlet. A follow up experiment with in-situ sensors suggests that the sub-tidal gradient is set up by regional winds and that the resulting pressure gradient significantly alters the tidal inlet hydrodynamics, at times entirely reversing tidal flows. Finally, the geomorphic impact of wind driven currents, as well as waves and the insertion of a terminal groin, is tested through an idealized numerical modeling study of a synthetic inlet system. The addition of a sub-tidal water level gradient has substantial and cascading effects on the inlet system morphology, potentially suggesting that the traditional inlet classification system, based only on wave and tidal properties, should be modified to include wind influence

The association between cognitive disengagement syndrome and symptoms of dissociation in college students

The present study examines the relationship between cognitive disengagement syndrome and symptoms of disassociation in college students. Using a Hierarchical Linear Regression we found that CDS does indeed have a significant positive relationship with dissociation, over and above co-morbid disorders (PTSD, ACES, GAD, Depression). This finding suggests that dissociation should be added to the symptomatology of CDS

Enhancement of antihydrogen formation in antiproton collisions with excited-state positronium

© Published under licence by IOP Publishing Ltd. Antihydrogen formation in positronium scattering on antiprotons is investigated using the two-centre convergent close-coupling method. A several orders of magnitude enhancement in the formation of antihydrogen is found when positronium is in an excited state. The effect is greatest at the lowest energies considered which encompass those achievable in experiment. This suggests a practical approach to creating neutral antimatter for testing its interaction with gravity and for spectroscopic measurements

Positronium formation from valence and inner shells in noble gas atoms

When recent experimental positronium (Ps) formation cross sections in noble gases have been compared with the most up-to date theoretical studies, the agreement is qualitative, but not quantitative. In this paper we re-examine this process and show that at low energies Ps formation must be treated nonperturbatively. We also look at Ps formation with inner shell electrons.Comment: 12 pages, 5 figures, to be published in Nuclear Instruments and Methods

Convergent variational calculation of positronium-hydrogen-atom scattering lengths

We present a convergent variational basis-set calculational scheme for elastic scattering of positronium atom by hydrogen atom in S wave. Highly correlated trial functions with appropriate symmetry are needed for achieving convergence. We report convergent results for scattering lengths in atomic units for both singlet ($=3.49\pm 0.20$) and triplet ($=2.46\pm 0.10$) states.Comment: 11 pages, 1 postscript figure, Accepted in J. Phys. B (Letter

Many-body theory of positron-atom interactions

A many-body theory approach is developed for the problem of positron-atom scattering and annihilation. Strong electron-positron correlations are included non-perturbatively through the calculation of the electron-positron vertex function. It corresponds to the sum of an infinite series of ladder diagrams, and describes the physical effect of virtual positronium formation. The vertex function is used to calculate the positron-atom correlation potential and nonlocal corrections to the electron-positron annihilation vertex. Numerically, we make use of B-spline basis sets, which ensures rapid convergence of the sums over intermediate states. We have also devised an extrapolation procedure that allows one to achieve convergence with respect to the number of intermediate-state orbital angular momenta included in the calculations. As a test, the present formalism is applied to positron scattering and annihilation on hydrogen, where it is exact. Our results agree with those of accurate variational calculations. We also examine in detail the properties of the large correlation corrections to the annihilation vertex.Comment: 25 pages, 16 figure

Induced long range dipole field enhanced antihydrogen formation in the pˉ+Ps(n=2)→e−+Hˉ(n≤2)\bar{p}+ Ps(n=2)\to e^- + \bar{H}(n\le 2) reaction

We assume all interaction to be Coulombic and solve the modified Faddeev equation for energies between the $Ps(n=2)$ and $\bar{H}(n=3)$, which involve six and eight open channels. We find that 99% of the antihydrogen are formed in $\bar{H}(n=2)$. Just above the $Ps(n=2)$ threshold the S, P, and D partial waves contribute more than 4000 square Bohr radii near the maximum. Evidences indicate that the induced long range dipole potential from the degenerate $Ps(n=2)$ targets is responsible for such a large antihydrogen formation cross section.Comment: 2 ps figure

Cold neutral atoms via charge exchange from excited state positronium: a proposal

We present a method for generating cold neutral atoms via charge exchange reactions between trapped ions and Rydberg positronium. The high charge exchange reaction cross section leads to efficient neutralisation of the ions and since the positronium-ion mass ratio is small, the neutrals do not gain appreciable kinetic energy in the process. When the original ions are cold the reaction produces neutrals that can be trapped or further manipulated with electromagnetic fields. Because a wide range of species can be targeted we envisage that our scheme may enable experiments at low temperature that have been hitherto intractable due to a lack of cooling methods. We present an estimate for achievable temperatures, neutral number and density in an experiment where the neutrals are formed at a milli-Kelvin temperature from either directly or sympathetically cooled ions confined on an ion chip. The neutrals may then be confined by their magnetic moment in a co-located magnetic minimum well also formed on the chip. We discuss general experimental requirements