![]() In the framework of non-relativistic or cold DM, which shows the best agreement with observations, an important class of candidate particles are the so-called weakly interacting massive particles (WIMPs) (for a recent review of the WIMP paradigm, see e.g. Several particles have been proposed as DM candidates. While there could be some contributions to DM from the Standard Model (SM) of particle physics in the form of neutrinos as hot dark matter or from large astrophysical objects consisting of protons and neutrons, it is generally assumed that these are only subleading effects and the dominant contribution to DM is a non-baryonic type of matter which is composed of a new type of particles as predicted within many models of physics beyond the SM. Nevertheless, the composition of DM is basically unknown, as all attempts to detect it – directly or indirectly – have so far been unsuccessful. These observations allowed to understand the abundance and distribution of DM in the universe and in many galaxies. The LLNL scientists created the positrons by shooting the lab's high-powered Titan laser onto a one-millimeter-thick piece of gold.The idea that dark matter (DM) is the main component of the matter content of the universe has been around for several decades and is well supported by astrophysical observations at large scales. ^ "Laser technique produces bevy of antimatter".Using four vector notation, the conservation of energy-momentum before and after the interaction gives: p γ = p e − + p e + + p ʀ : CS1 maint: multiple names: authors list ( link) These properties can be derived through the kinematics of the interaction. The reverse of this process is electron–positron annihilation. Because of this, when pair production occurs, the atomic nucleus receives some recoil. The photon must be near a nucleus in order to satisfy conservation of momentum, as an electron–positron pair produced in free space cannot satisfy conservation of both energy and momentum. (Thus, pair production does not occur in medical X-ray imaging because these X-rays only contain ~150 keV.) The photon must have higher energy than the sum of the rest mass energies of an electron and positron (2 ⋅ 511 keV = 1.022 MeV, resulting in a photon-wavelength of 1.2132 picometer) for the production to occur. The photon's energy is converted to particle mass in accordance with Einstein's equation, E = m ⋅ c 2 where E is energy, m is mass and c is the speed of light. If the photon is near an atomic nucleus, the energy of a photon can be converted into an electron–positron pair: These interactions were first observed in Patrick Blackett's counter-controlled cloud chamber, leading to the 1948 Nobel Prize in Physics. The black dot labelled 'Z' represents an adjacent atom, with atomic number Z.įor photons with high photon energy ( MeV scale and higher), pair production is the dominant mode of photon interaction with matter. In reality the produced pair are nearly collinear. Photon to electron and positron Diagram showing the process of electron–positron pair production. The probability of pair production in photon–matter interactions increases with photon energy and also increases approximately as the square of atomic number of (hence, number of protons in) the nearby atom. For instance, if one particle has electric charge of +1 the other must have electric charge of −1, or if one particle has strangeness of +1 then another one must have strangeness of −1. Īll other conserved quantum numbers ( angular momentum, electric charge, lepton number) of the produced particles must sum to zero – thus the created particles shall have opposite values of each other. (As the electron is the lightest, hence, lowest mass/energy, elementary particle, it requires the least energetic photons of all possible pair-production processes.) Conservation of energy and momentum are the principal constraints on the process. As energy must be conserved, for pair production to occur, the incoming energy of the photon must be above a threshold of at least the total rest mass energy of the two particles created. Pair production often refers specifically to a photon creating an electron–positron pair near a nucleus. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson.
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