Editing Positron (section)

=== Observation in cosmic rays ===
{{main|Cosmic ray}}
Satellite experiments have found evidence of positrons (as well as a few antiprotons) in primary cosmic rays, amounting to less than 1% of the particles in primary cosmic rays.<ref>{{cite journal |last1=Golden |title=Measurement of the Positron to Electron Ratio in Cosmic Rays above 5 GeV |journal=Astrophysical Journal Letters |date=February 1996 |volume=457 |issue=2 |doi=10.1086/309896 |bibcode=1996ApJ...457L.103G |hdl=11576/2514376 |s2cid=122660096 |url=https://ui.adsabs.harvard.edu/abs/1996ApJ...457L.103G/abstract |access-date=19 October 2021|hdl-access=free }}</ref> However, the fraction of positrons in cosmic rays has been measured more recently with improved accuracy, especially at much higher energy levels, and the fraction of positrons has been seen to be greater in these higher energy cosmic rays.<ref>{{cite journal |last1=Boudaud |title=A new look at the cosmic ray positron fraction |journal=Astronomy & Astrophysics |date=19 December 2014 |volume=575 |pages=A67 |url=https://www.aanda.org/articles/aa/full_html/2015/03/aa25197-14/aa25197-14.html |access-date=19 October 2021 |doi=10.1051/0004-6361/201425197 |doi-access=free|arxiv=1410.3799 }}</ref>

These do not appear to be the products of large amounts of antimatter from the Big Bang, or indeed complex antimatter in the universe (evidence for which is lacking, see below). Rather, the antimatter in cosmic rays appear to consist of only these two elementary particles. Recent theories suggest the source of such positrons may come from annihilation of dark matter particles, acceleration of positrons to high energies in astrophysical objects, and production of high energy positrons in the interactions of cosmic ray nuclei with interstellar gas.<ref>{{cite web |title=Towards Understanding the Origin of Cosmic-Ray Positrons |url=https://ams02.space/physics/towards-understanding-origin-cosmic-ray-positrons |website=The Alpha Magnetic Spectrometer on the International Space Station |access-date=19 October 2021}}</ref>

Preliminary results from the presently operating [[Alpha Magnetic Spectrometer]] (''AMS-02'') on board the [[International Space Station]] show that positrons in the cosmic rays arrive with no directionality, and with energies that range from 0.5 [[Wiktionary:gigaelectron volt|GeV]] to 500 GeV.<ref>
{{cite journal
 |last1=Accardo |first1=L.
 |collaboration=[[Alpha Magnetic Spectrometer#AMS-02|AMS Collaboration]]
 |date=2014
 |title=High Statistics Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–500 GeV with the Alpha Magnetic Spectrometer on the International Space Station
 |url=http://ams.nasa.gov/Documents/AMS_Publications/PhysRevLett.113.121101.pdf
 |journal=[[Physical Review Letters]]
 |volume=113 |issue=12
 |page=121101
 |bibcode=2014PhRvL.113l1101A
 |doi=10.1103/PhysRevLett.113.121101
 |pmid=25279616
 |doi-access=free
}}</ref><ref>
{{Cite journal|last1=Schirber |first1=M.
 |title=Synopsis: More Dark Matter Hints from Cosmic Rays?
 |journal=Physical Review Letters
 |volume=113
 |issue=12
 |pages=121102
 |doi=10.1103/PhysRevLett.113.121102
 |pmid=25279617
 |year=2014
 |arxiv=1701.07305
 |bibcode=2014PhRvL.113l1102A
 |url=https://cds.cern.ch/record/1756487
 |hdl=1721.1/90426
 |s2cid=2585508
 }}</ref> Positron fraction peaks at a maximum of about 16% of total electron+positron events, around an energy of 275 ± 32 GeV. At higher energies, up to 500 GeV, the ratio of positrons to electrons begins to fall again. The absolute flux of positrons also begins to fall before 500 GeV, but peaks at energies far higher than electron energies, which peak about 10 GeV.<ref>
{{cite web
 |title=New results from the Alpha Magnetic Spectrometer on the International Space Station
 |url=http://ams.nasa.gov/Documents/AMS_Publications/ams_new_results_-_18.09.2014.pdf
 |website=AMS-02 at NASA
 |access-date=21 September 2014
}}</ref><ref>{{Cite web|url=http://www1b.physik.rwth-aachen.de/~pebs/?PEBS_physics:Positron_fraction|title=Positron fraction|access-date=22 July 2018|archive-date=22 July 2018|archive-url=https://web.archive.org/web/20180722184917/http://www1b.physik.rwth-aachen.de/~pebs/?PEBS_physics:Positron_fraction|url-status=dead}}</ref> These results on interpretation have been suggested to be due to positron production in annihilation events of massive [[dark matter]] particles.<ref name="physrevltrs413">
{{Cite journal 
 |last1=Aguilar |first1=M.
 |display-authors=etal
 |year=2013
 |title=First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV
 |journal=[[Physical Review Letters]]
 |volume=110 |issue=14 |pages=141102
 |bibcode=2013PhRvL.110n1102A
 |doi=10.1103/PhysRevLett.110.141102
 |pmid=25166975
 |url=https://boa.unimib.it/bitstream/10281/44680/1/2013_PhysRevLett.110.141102_positron_fraction.pdf|doi-access=free
}}</ref>

Positrons, like anti-protons, do not appear to originate from any hypothetical "antimatter" regions of the universe. On the contrary, there is no evidence of complex antimatter atomic nuclei, such as [[antihelium]] nuclei (i.e., anti-alpha particles), in cosmic rays. These are actively being searched for. A prototype of the ''AMS-02'' designated ''AMS-01'', was flown into space aboard the {{OV|103}} on [[STS-91]] in June 1998. By not detecting any [[Antihelium#Antihelium|antihelium]] at all, the ''AMS-01'' established an upper limit of 1.1×10<sup>−6</sup> for the antihelium to helium [[flux]] ratio.<ref>
{{cite journal
 |last1=Aguilar |first1=M.
 |display-authors=etal
 |collaboration=[[Alpha Magnetic Spectrometer#AMS-02|AMS Collaboration]]
 |date=2002
 |title=The Alpha Magnetic Spectrometer (AMS) on the International Space Station: Part I – results from the test flight on the space shuttle
 |journal=[[Physics Reports]]
 |volume=366 |issue=6 |pages=331–405
 |bibcode=2002PhR...366..331A 
 |doi=10.1016/S0370-1573(02)00013-3
 |hdl=2078.1/72661
 |s2cid=122726107
}}</ref>