The results of a new search for positronium decays into invisible final states are reported. Convincing detection of this decay mode would be a strong evidence for new physics beyond the standard model (SM): for example, the existence of extra--dimensions, of milli-charged particles, of new light gauge bosons or of mirror particles. Mirror matter could be a relevant dark matter candidate. In this paper the setup and the results of a new experiment are presented. In a collected sample of about $(6.31\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ orthopositronium decays, no evidence for invisible decays in an energy [0,80] keV was found and an upper limit on the branching ratio of orthopositronium $\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}}$ could be set: $\mathrm{Br}(\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}})l4.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ Our results provide a limit on the photon mirror-photon mixing strength $ϵ\ensuremath{\le}1.55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ and rule out particles lighter than the electron mass with a fraction ${Q}_{x}\ensuremath{\le}3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium $\mathrm{Br}(\mathrm{p}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}})\ensuremath{\le}4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ and the direct annihilation $\mathrm{Br}({e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{\text{invisible}})\ensuremath{\le}2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}(90%\mathrm{C}.\mathrm{L}.)$ could be set.

Improved limit on invisible decays of positronium

The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositronium in the gas. The momentum distribution of the positronium is observed by using the one-dimensional angular correlation of the annihilation radiation method. Silica aerogel is used to form a sufficient amount of positronium in a restricted region necessary for the high-resolution measurements. The average energies of orthopositronium for mean lifetimes ranging from 3.2 to 86 ns are determined by applying static magnetic fields. A thermalization model which includes the momentum-transfer cross section as an adjustable parameter is fitted to the average energy. The cross section thus obtained is for positronium in the energy range below 0.3 eV.

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Momentum-transfer cross section for slow positronium-He scattering

Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

http://iopscience.iop.org/article/10.1088/0953-4075/27/5/017/pdf

Angular correlation studies of positron annihilation in the noble gases

Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)], has been detected experimentally. We have found that the lifetime of the magnetically unperturbed ortho-Ps in Xe gas of 1 atm decreases significantly when a magnetic field of 1.0 T is applied. This decrease is attributed to the Ps spin conversion caused by spin-orbit interaction during Ps-Xe collision. The annihilation cross section of ortho-Ps due to this interaction has been determined to be 3 times as large as that expected by Mitroy and Novikov.

Experimental Evidence for Spin-Orbit Interactions in Positronium-Xe Collisions

We have recently introduced the idea of making use of three-photon positron annihilations in positron emission tomography. In this paper, the basic characteristics of the three-gamma imaging in PET are studied by means of Monte Carlo simulations and analytical computations. Two typical configurations of human and small animal scanners are considered. Three-photon imaging requires high-energy resolution detectors. Parameters currently attainable by CdZnTe semiconductor detectors, the technology of choice for the future development of radiation imaging, are assumed. Spatial resolution is calculated as a function of detector energy resolution and size, position in the field of view, scanner size and the energies of the three-gamma annihilation photons. Possible ways to improve the spatial resolution obtained for nominal parameters, 1.5 cm and 3.2 mm FWHM for human and small animal scanners, respectively, are indicated. Counting rates of true and random three-photon events for typical human and small animal scanning configurations are assessed. A simple formula for minimum size of lesions detectable in the three-gamma based images is derived. Depending on the contrast and total number of registered counts, lesions of a few mm size for human and sub mm for small animal scanners can be detected.

https://iopscience.iop.org/article/10.1088/0031-9155/50/23/019/pdf

Performance of three-photon PET imaging: Monte Carlo simulations.

Conservation of ortho-positronium in oxygen has been investigated by the high-resolution 2 gamma angular correlation method. The oxygen pressure was low enough to suppress possible conversion of para-positronium to ortho-positronium. Silica aerogel was used to form a sufficient number of positronium atoms in a restricted region necessary for the high-resolution angular correlation measurements. The existence of an inelastic conversion process has been reconfirmed. The cross sections for the elastic and inelastic conversion processes have been estimated to be (1.0+or-0.3)*10-19 cm2 and some 10-17 cm2, respectively.

https://iopscience.iop.org/article/10.1088/0953-4075/23/3/025/pdf

T Hyodo

Papers

Conversion of ortho-positronium in low-density oxygen gas