View source for Physics:Matter creation

{{Short description|How to create matter from fundamentals particles to galaxies}}
Even restricting the discussion to [[Physics:Physics|physics]], scientists do not have a unique definition of what [[Physics:Matter|matter]] is. In the currently known [[Physics:Particle physics|particle physics]], summarised by the [[Physics:Standard Model|standard model]] of [[Physics:Elementary particle|elementary particles]] and interactions, it is possible to distinguish in an absolute sense particles of matter and particles of [[Physics:Antimatter|antimatter]]. This is particularly easy for those particles that carry [[Physics:Electric charge|electric charge]], such as [[Physics:Electron|electron]]s, [[Physics:Proton|proton]]s or [[Physics:Quark|quark]]s, while the distinction is more subtle in the case of [[Physics:Neutrino|neutrinos]], fundamental elementary particles that do not carry electric charge. In the standard model, it is not possible to create a net amount of matter particles—or more precisely, it is not possible to change the net number of [[Physics:Lepton|lepton]]s or of quarks in any [[Perturbation theory (quantum mechanics)|perturbative reaction]] among particles. This remark is consistent with all existing observations.

However, similar processes are not considered to be impossible and are expected in other models of the elementary particles, that extend the standard model.  They are necessary in speculative theories that aim to explain the cosmic excess of matter over antimatter, such as [[Physics:Leptogenesis|leptogenesis]] and [[Physics:Baryogenesis|baryogenesis]]. They could even manifest themselves in laboratory as [[Physics:Proton decay|proton decay]] or as creations of [[Physics:Electron|electrons]] in the so-called neutrinoless double beta decay. The latter case occurs if the neutrinos are [[Physics:Majorana fermion|Majorana particles]], being at the same time matter and antimatter, according to the definition given just above.<ref>
{{cite arXiv
 |last1=Dell'Oro |first1=S. 
 |last2=Marcocci |first2=S.
 |last3=Vissani |first3=F.
 |year=2018
 |title=Testing creation of matter with neutrinoless double beta decay
 |eprint=1710.06732 
 |class=hep-ph 
}}</ref> 

In a wider sense, one can use the word [[Physics:Matter|matter]] simply to refer to [[Physics:Fermion|fermions]]. In this sense, matter and antimatter particles (such as an electron and a [[Physics:Positron|positron]]) are identified beforehand. The process inverse to [[Physics:Annihilation|particle annihilation]] can be called '''matter creation'''; more precisely, we are considering here the process obtained under [[Physics:T-symmetry|time reversal]] of the annihilation process. This process is also known as [[Physics:Pair production|pair production]], and can be described as the conversion of light particles (i.e., photons) into one or more [[Physics:Massive particle|massive particle]]s {{citation needed|date=March 2017}}. The most common and well-studied case is the one where two photons convert into an [[Physics:Electron|electron]]–[[Physics:Positron|positron]] pair.

==Photon pair production==
{{main|Physics:Pair production}}
Because of [[Physics: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 these laws when in the presence of another particle (another boson, or even a fermion) which can share the primary photon's momentum. Thus, matter can be created out of two photons.

The [[Physics:Conservation of energy|law of conservation of energy]] sets a minimum [[Physics:Photon energy|photon energy]] required for the creation of a pair of fermions: this ''[[Physics:Threshold energy|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, in the rest frame, must be ''at least'' 2''m''<sub>e</sub>''c''<sup>2</sup> = 2&nbsp;×&nbsp;{{val|0.511|ul=MeV}} = {{val|1.022|u=MeV}} (''m''<sub>e</sub> is the mass of one electron and ''c'' is the [[Physics:Speed of light|speed of light]] in vacuum), an energy value that corresponds to soft [[Physics:Gamma ray|gamma ray]] photons. The creation of a much more massive pair, like a [[Physics:Proton|proton]] and [[Physics:Antiproton|antiproton]], requires photons with energy of more than {{val|1.88|u=GeV}} (hard gamma ray photons).

The first published calculations of the rate of e<sup>+</sup>–e<sup>−</sup> pair production in photon-photon collisions were done by [[Biography:Lev Landau|Lev Landau]] in 1934.<ref>
{{cite journal
 |last1=Landau |first1=L. D.
 |last2=Lifshits |first2=E. M.
 |year=1934
 |title=Production of electrons and positrons by a collision of two particles
 |url=http://usparc.ihep.su/spires/find/hep/www?irn=45942
 |journal=Physikalische Zeitschrift der Sowjetunion
 |volume=6 |pages=244–257
 |zbl=0010.23102
 |access-date=2011-09-11
 |archive-url=https://web.archive.org/web/20120324231409/http://usparc.ihep.su/spires/find/hep/www?irn=45942
 |archive-date=2012-03-24
 |url-status=dead
 }}</ref> It was predicted that the process of e<sup>+</sup>–e<sup>−</sup> pair creation (via collisions of photons) dominates in collision of [[Physics:Relativistic particle|ultra-relativistic]] charged particles—because those photons are radiated in narrow cones along the direction of motion of the original particle, greatly increasing photon flux.

In high-energy [[Physics:Particle collider|particle collider]]s, matter creation events have yielded a wide variety of exotic heavy particles precipitating out of colliding photon jets (see [[Physics:Two-photon physics|two-photon physics]]). Currently, two-photon physics studies creation of various fermion pairs both theoretically and experimentally (using [[Physics:Particle accelerator|particle accelerator]]s, [[Physics:Air shower|air shower]]s, radioactive isotopes, etc.).

It is possible to create all fundamental particles in the standard model, including quarks, leptons and bosons using photons of varying energies above some minimum threshold, whether directly (by pair production), or by decay of the intermediate particle (such as a W<sup>−</sup> boson decaying to form an electron and an electron-antineutrino).{{Citation needed|reason=Quarks carry color charge; photons do not.|date=April 2023}}

As shown above, to produce ordinary [[Physics:Baryonic matter|baryonic matter]] out of a [[Physics:Photon gas|photon gas]], this gas must not only have a very high photon density, but also be very hot – the energy ([[Physics:Temperature|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>&nbsp;[[Kelvin|K]], 10<sup>13</sup>&nbsp;K for [[Physics:Proton|proton]]s and [[Physics:Neutron|neutron]]s, etc. According to the [[Astronomy:Big Bang|Big Bang]] theory, in the early [[Astronomy:Universe|universe]], mass-less photons and massive fermions would inter-convert freely. As the photon gas expanded and cooled, some fermions would be left over (in extremely small amounts ~10<sup>−10</sup>) because low energy photons could no longer break them apart. Those left-over fermions would have become the matter we see today in the universe around us.

==See also==
*[[Physics:Annihilation|Annihilation]]
*[[Physics:Available energy (particle collision)|Available energy]]
*[[Physics:Pair production|Pair production]]
*[[Physics:Schwinger limit|Schwinger limit]]

==References==
<references />

{{DEFAULTSORT:Matter Creation}}
[[Category:Particle physics]]

{{Sourceattribution|Matter creation}}