Unknown dark matter and familiar positron annihilation

Xiang Zheng Tu 

  

I must admit that I knew nothing about dark matter, but I am familiar with positron annihilation. In 1986 I utilized positron annihilation measurement to study of vacancy defects in GaAs liquid phase epitaxial layers. In the following year the research result was published in “Journal of Applied Physics” which was titled as “Positron-annihilation study of vacancy defects in GaAs liquid-phase epitaxial layers”. The paper concluded that the defects observed to trap positrons in undoped GaAs liquid phase epitaxial layers are neutral arsenic vacancies. Systematic trends of the epitaxial growth temperature on positron lifetime are observed. The setup for the measurement is shown in the above figure. Positrons are emitted from a radioactive source. The positron is the antiparticle of the electron, and when a positron enters a GaAs liquid phase epitaxial layer, it will find abundant supply of electrons with which to annihilate. The energy release by the annihilation forms two highly energetic gamma rays, and if one assumes that the momenta of the positron and electron before the annihilation, the two gamma rays photos must in opposite directions in order to conserve momentum. These coincident gamma rays at 180 degrees provide a powerful tool for eliminating all gamma events which are not coincident at 180 degrees.

It is interesting to know that positron annihilation measurement has been used to find dark matter. Despite striking evidence for the existence of dark matter from astrophysical observations, dark matter has still escaped any direct or indirect detection until today. Therefore a proof for its existence and the revelation of its nature belongs to one of the most intriguing challenges of nowadays cosmology and particle physics. A lot of work has been done to investigate the nature of dark matter through indirect signatures from dark matter annihilation into electron-positron pairs. It is thought that the dark matter particles are thermal relics, then dark matter particles and antiparticles exist in equal amounts, and they could also annihilate or decay to standard model particles that can be detected. As a two-body process, the rate of annihilation is proportional to the square of the dark matter density, whereas single-body decay process is proportional to the dark matter density. The primary products of the annihilations or decays, i.e. cosmic ray protons, antiprotons, electrons, positrons, gamma-rays and neutrinos, could in principle be observed on or around the Earth, while secondary radiation like gamma-rays, and radio or microwaves from synchrotron could be detected.

Alpha Magnetic Spectrometer (AMS-02) is a powerful state-of-the-art particle physics detector. This detector was installed on the International Space Station and operated by an international team composed of 56 institutes from 16 countries and organized under United States Department of Energy (DOE) sponsorship. It has collected the antiproton-to-proton ratio stays constant which cannot be explained by the secondary antiprotons from collisions of ordinary cosmic rays with interstellar medium.  A new source such as astrophysical accelerators and annihilating or decaying dark matter was subjected.

Samuel C. C. Ting who awarded the Nobel Prize in Physics said that more high-energy positrons than expected are buzzing around the galaxy—has not impressed the doubters. That positron excess, which a European satellite found in the mid-2000s and the AMS confirmed, has sparked hundreds of theory papers connecting it to hypothetical dark matter particles. The mutual annihilation of those particles might create a half-and-half blend of electrons and positrons in a narrow energy range. The electrons would fade into a sea of electrons from other sources, but the rarer positrons might stand out. To Ting, the best explanation for the extra positrons is a dark matter particle with a mass of 1 million megavolts —about as much energy as a flying mosquito.