Collisions involving antiprotons and antihydrogen: an overview Jonsell, S.
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
03/2018, Volume:
376, Issue:
2116
Journal Article
Peer reviewed
Open access
I give an overview of experimental and theoretical results for antiproton and antihydrogen scattering with atoms and molecules (in particular H, He). At low energies (1 keV) there are practically no ...experimental data available. Instead I compare the results from different theoretical calculations, of various degrees of sophistication. At energies up to a few tens of eV, I focus on simple approximations that give reasonably accurate results, as these allow quick estimates of collision rates without embarking on a research project.
This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.
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The formation of a beam of antihydrogen atoms when antiprotons pass through cold, dense positron plasmas is simulated for various plasma properties and antiproton injection energies. There are marked ...dependences of the fraction of injected antiprotons which are emitted as antihydrogen in a beam-like configuration upon the temperature of the positrons, and upon the antiproton kinetic energy. Yields as high as 13% are found at the lowest positron temperatures simulated here (5 K) and at antiproton kinetic energies below about 0.1 eV. By 1 eV the best yields are as low as 10−3, falling by about two orders of magnitude with an increase of the positron temperature to 50 K. Example distributions for the antihydrogen angular emission, binding energy and kinetic energy are presented and discussed. Comparison is made with experimental information, where possible.
The formation of antihydrogen atoms from antiprotons injected into a positron plasma is simulated, focussing on the fraction that fulfil the conditions necessary for confinement of anti-atoms in a ...magnetic minimum trap. Trapping fractions of around 10−4 are found under conditions similar to those used in recent experiments, and in reasonable accord with their results. We have studied the behaviour of the trapped fraction at various positron plasma densities and temperatures and found that collisional effects play a beneficial role via a redistribution of the antihydrogen magnetic moment, allowing enhancements of the yield of low-field seeking states that are amenable to trapping.
We present an investigation of resonances with natural and unnatural parities in positron scattering with atomic hydrogen. The complex scaling method has been used. Resonance states for natural ...parity with total angular momenta and unnatural parity with are calculated. Resonance energies and widths are reported and compared with other theoretical calculations.
Antihydrogen, the bound state of an antiproton and a positron, is of interest for use in precision tests of nature's fundamental symmetries. Antihydrogen formed by carefully merging cold plasmas of ...positrons and antiprotons has recently been trapped in magnetic traps. The efficiency of trapping is strongly dependent on the temperature of the nascent antihydrogen, which, to be trapped, must have a kinetic energy less than the trap depth of . In the conditions in the ALPHA experiment, the antihydrogen temperature seems dominated by the temperature of the positron plasma used for the synthesis. Cold positrons are therefore of paramount interest in that experiment. In this paper, we propose an alternative route to make ultra-cold positrons for enhanced antihydrogen trapping. We investigate theoretically how to extend previously successful sympathetic cooling of positrons by laser-cooled positive ions to be used for antihydrogen trapping. Using simulations, we investigate the effectiveness of such cooling in conditions similar to those in ALPHA, and discuss how the formation process and the nascent antihydrogen may be influenced by the presence of positive ions. We argue that this technique is a viable alternative to methods such as evaporative and adiabatic cooling, and may overcome limitations faced by these. Ultra-cold positrons, once available, may also be of interest for a range of other applications.
Physicists have long wondered whether the gravitational interactions between matter and antimatter might be different from those between matter and itself. Although there are many indirect ...indications that no such differences exist and that the weak equivalence principle holds, there have been no direct, free-fall style, experimental tests of gravity on antimatter. Here we describe a novel direct test methodology; we search for a propensity for antihydrogen atoms to fall downward when released from the ALPHA antihydrogen trap. In the absence of systematic errors, we can reject ratios of the gravitational to inertial mass of antihydrogen >75 at a statistical significance level of 5%; worst-case systematic errors increase the minimum rejection ratio to 110. A similar search places somewhat tighter bounds on a negative gravitational mass, that is, on antigravity. This methodology, coupled with ongoing experimental improvements, should allow us to bound the ratio within the more interesting near equivalence regime.
Atoms made of a particle and an antiparticle are unstable, usually surviving less than a microsecond. Antihydrogen, made entirely of antiparticles, is believed to be stable, and it is this longevity ...that holds the promise of precision studies of matter-antimatter symmetry. We have recently demonstrated trapping of antihydrogen atoms by releasing them after a confinement time of 172 ms. A critical question for future studies is: how long can anti-atoms be trapped? Here, we report the observation of anti-atom confinement for 1,000 s, extending our earlier results by nearly four orders of magnitude. Our calculations indicate that most of the trapped anti-atoms reach the ground state. Further, we report the first measurement of the energy distribution of trapped antihydrogen, which, coupled with detailed comparisons with simulations, provides a key tool for the systematic investigation of trapping dynamics. These advances open up a range of experimental possibilities, including precision studies of charge-parity-time reversal symmetry and cooling to temperatures where gravitational effects could become apparent. PUBLICATION ABSTRACT
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We derive general universal scaling relations governing resonances induced by the dipole moment of excited positronium interacting with atomic ions. A single non-universal parameter, which contains ...all the system-dependent information, is defined. Our results are compared to numerical calculations, using complex scaling, for S, P, and D-wave resonances below the positronium n = 2 threshold in the e + -(H, Li, Na, K) systems. The energy and width ratios of the successive resonances are found to agree well with the analytically derived scaling law.
At the historic Shelter Island Conference on the Foundations of Quantum Mechanics in 1947, Willis Lamb reported an unexpected feature in the fine structure of atomic hydrogen: a separation of the 2S
...and 2P
states
. The observation of this separation, now known as the Lamb shift, marked an important event in the evolution of modern physics, inspiring others to develop the theory of quantum electrodynamics
. Quantum electrodynamics also describes antimatter, but it has only recently become possible to synthesize and trap atomic antimatter to probe its structure. Mirroring the historical development of quantum atomic physics in the twentieth century, modern measurements on anti-atoms represent a unique approach for testing quantum electrodynamics and the foundational symmetries of the standard model. Here we report measurements of the fine structure in the n = 2 states of antihydrogen, the antimatter counterpart of the hydrogen atom. Using optical excitation of the 1S-2P Lyman-α transitions in antihydrogen
, we determine their frequencies in a magnetic field of 1 tesla to a precision of 16 parts per billion. Assuming the standard Zeeman and hyperfine interactions, we infer the zero-field fine-structure splitting (2P
-2P
) in antihydrogen. The resulting value is consistent with the predictions of quantum electrodynamics to a precision of 2 per cent. Using our previously measured value of the 1S-2S transition frequency
, we find that the classic Lamb shift in antihydrogen (2S
-2P
splitting at zero field) is consistent with theory at a level of 11 per cent. Our observations represent an important step towards precision measurements of the fine structure and the Lamb shift in the antihydrogen spectrum as tests of the charge-parity-time symmetry
and towards the determination of other fundamental quantities, such as the antiproton charge radius
, in this antimatter system.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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