Matter creation 2362494 183134798 2008-01-09T07:33:02Z Frongle 5915317 /* Photon pair production */ fixing disambig link '''Matter creation''' is the process inverse to particle [[annihilation]]. It is the conversion of [[mass|massless]] particles into one or more massive particles. This process is the [[T-symmetry|time reversal]] of annihilation. Since all known massless particles are [[bosons]] and the most familiar massive particles are [[fermions]], usually what is considered is the process which converts two bosons (e.g. [[photons]]) into two fermions (e.g., an [[electron]]-[[positron]] pair). == Photon pair production == Because of [[Conservation of momentum#Conservation of momentum|momentum conservation laws]], the creation of a pair of fermions (matter particles) out of a single photon cannot occur. However, matter creation is allowed by momentum conservation law when in the presence of another particle (it may be another photon or other boson, or even a fermion) which can share photon's momentum. Thus, matter can be created out of two photons, for example (this is the process inverse to annihilation). The [[energy conservation|law of energy conservation]] sets a minimum photon energy required for creation of a pair of fermions: this ''[[threshold energy]]'' must be greater than the total [[rest energy]] of the fermions created. To create an electron-positron pair the total energy of the photons must be ''at least'' 2''m''<sub>e</sub>''c''<sup>2</sup> = 2×0.511&nbsp;MeV = 1.022&nbsp;[[MeV]] (''m''<sub>e</sub> is the mass of one electron and ''c'' is the [[speed of light]] in vacuum), an energy value that corresponds to soft [[gamma ray]] photons. The creation of a much more massive pair, like a [[proton]] and [[antiproton]], requires photons with energy of more than 1.88 GeV (hard gamma ray photons). First calculations of rate of e<sup>+</sup>/e<sup>-</sup> pair production in photon-photon collision was done by Lev Landau in 1934.<ref> L.D. Landau, E.M. Lifshits, Sov. Phys., 6 (1934), 244</ref> It was predicted that the process of e<sup>+</sup>/e<sup>-</sup> pair creation (via collisions of photons) dominates in collision of ultrarelativistic charged particles — because those photons are radiated in narrow cones along the direction of motion of original particle greatly increasing photon flux. In high-energy [[particle collider]]s, matter creation events have yielded a wide variety of exotic heavy particles precipitating out of colliding photon jets (see [[two-photon physics]]). Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using [[particle accelerators]], [[Air shower (physics)|air showers]], [[radioactive isotopes]], etc). As shown above, to produce ordinary [[baryonic matter]] out of a photon gas, this gas must not only have a very high [[photon density]], but also be very hot – the energy ([[temperature]]) of photons must obviously exceed the rest mass energy of the given matter particle pair. The threshold temperature for production of electrons is about 10<sup>10</sup> [[kelvin]]s, 10<sup>13</sup> K for [[proton]]s and [[neutron]]s, etc. In the early [[universe]] (''see [[Big Bang]]'') photons and fermions (massive particles of matter) freely interconverted. As photon gas expanded and cooled, some fermions were left over (in extremely small amount ~10<sup>-10</sup>) because low energy photons could no longer break them apart - and that is the matter we see today in the universe around us. ==See also== * [[Annihilation]] * [[Available energy (particle collision)|Available energy]] ==References== <references /> [[Category:Particle physics]] [[es:Creación de masa]]