An electron beam, characterized by a high-angular discrimination (≃0.7°), has been used to measure the total (elastic plus inelastic) cross section of H 2 O in the energy range 3-100 eV. Broad coincidence is found with recent experiments, including a pronounced shoulder in the 6-12 eV region. However, at energies ≲6 eV, the present cross sections are ≃30% higher. Furthermore, forward scattering has been probed in the angular range 0°-3.5°and measures of the average (rotationally and vibrationally summed) differential elastic cross sections for incident energies ≤12 eV are obtained at a scattering angle ≃1 ∘ . The measurements, which provide the first test of theoretical predictions in an angular region experimentally unexplored until now, are found to be within 1 standard deviation of corresponding ab initio R-matrix calculations.

Using a purely electrostatic positron beam, the total cross section of positrons scattering from H 2 O has been measured for the first time with a high angular discrimination (≃1°) against forward scattered projectiles. Results are presented in the energy range (10-300) eV. Significant deviations from previous measurements are found which are, if ascribed entirely to the angular acceptances of various experimental systems, in quantitative accord with ab initio theoretical predictions of the differential elastic scattering cross section. DOI: 10.1103/PhysRevLett.117.253401 While the apparent imbalance between matter and antimatter in the Universe remains a major puzzle in science [1,2], much progress in the understanding of the interactions between the two has been achieved through studies of controlled collisions of positrons ðe þ Þ and positronium (Ps, the short-lived atom made of an electron and a positron) with atoms and molecules [3][4][5][6][7].At low energies, the static and polarization interactions tend to cancel for positrons reducing their scattering probability in comparison with electrons. However, polarization often enhances direct ionization by positrons, so that they can be more penetrating and more ionizing than electrons, a result of potential import in analyses of astrophysical (e.g., [8]) and atmospheric events (e.g., [9]) as well as in positron-track simulations for dosimetry in positron emission tomography (e.g., [10]). In turn, these studies contribute to the motivation for investigating the interaction of e þ with water which accounts for about 60% of the human body and which is the most abundant greenhouse gas in the atmosphere.Measurements of positron-water total cross section (σ T ) were first carried out 30 years ago [11,12]. Since then, only a few new results have been added, experimentally [13][14][15] and theoretically [16,17], without a satisfactory agreement emerging among them. The integral (σ el ) and differential (dσ el =dΩ) elastic (el) scattering cross sections for e þ þ H 2 O have also been measured recently [17], complementing theoretical determinations [18,19].Because of the long-range forces involved in the scattering of charged projectiles from a polar molecule such as H 2 O, one of the major difficulties in measuring σ T (even in the case of electrons, e.g., [20][21][22][23]) lies in discriminating against the considerable flux of small forward-angle scattered particles (FSPs) (e.g., [16,19]). The largest error associated with FSPs arises from elastic scattering and rovibrational inelastic processes which cannot be easily distinguished from the incident flux via energy loss discrimination since this is smaller than (or comparable to) typical beam energy resolutions (e.g., the first vibrational excitation from the ground state J ¼ 0 is ≃1595 cm −1 [24]). Detection of FSPs leads to a systematic underestimate of the beam attenuation and, thus, the measured total cross sections. In this respect, beams that employ magnetic fields are more likely to transport FSPs from the...

Experimental determinations of the absolute differential positronium-formation cross sections near 0 • for Ne, Ar, Kr, and Xe are presented and compared with theory. The degree of forward collimation, expressed by the ratios of the differential-to-integral positronium-formation cross sections, is also computed and compared with theories and other targets. Trends among targets and structures at low energies emerge when considered as a function of the reduced total energy.

Quantum physics is undoubtedly the most successful theory of the microscopic world, yet the complexities which arise in applying it even to simple atomic and molecular systems render the description of basic collision probabilities a formidable task. For this reason, approximations are often employed, the validity of which may be restricted to given energy regimes and/or targets and/or projectiles. Now we have found that the lognormal function, widely used for the probability distribution of macroscopic stochastic events (as diverse as periods of incubation of and recovery from diseases, size of grains, abundance of species, fluctuations in economic quantities, etc.) may also be employed to describe the energy dependence of inelastic collisions at the quantum level (including ionization, electron capture and excitation by electrons, positrons, protons, antiprotons, etc.), by allowing for the relevant threshold energy. A physical interpretation is discussed in this article by analogy with the heat capacity of few-level systems in solid state physics. We find the generality of the analysis to extend also to nuclear reactions. As well as aiding the description of collision probabilities for quantum systems, this finding is expected to impact also on the fundamental understanding of the interface between the classical and quantum domains.

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