Antihydrogen
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{{Antimatter}}
'''Antihydrogen''' is the [[antimatter]] counterpart of [[hydrogen]]. Whereas the common hydrogen [[atom]] is composed of an [[electron]] and [[proton]], the antihydrogen atom is made up of a [[positron]] and [[antiproton]].
Its (proposed) [[chemical symbol]] is <font style="text-decoration: overline">H</font>, that is, H with an overbar ({{pronEng|ˌeɪtʃ ˈbɑr}} ''aitch-bar'').
==Antihydrogen characteristics==
According to the [[CPT theorem]] of particle physics, antihydrogen atoms should have many of the characteristics regular hydrogen atoms have, i.e. they should have the same [[mass]], [[magnetic moment]], and transition frequencies (see [[Atomic spectroscopy]]) between its atomic [[quantum states]]. Excited antihydrogen atoms are for example expected to glow with the same color as that of regular hydrogen. Antihydrogen atoms should be attracted to other matter or antimatter gravitationally with a force of the same magnitude as ordinary hydrogen atoms would experience. This would not be true if antimatter has negative [[gravitational mass]], which is considered highly unlikely, though not yet empirically disproven.
When antihydrogen atoms come into contact with ordinary matter, they quickly annihilate each other and produce energy in the form of [[gamma rays]] and high-energy particles called [[pion]]s. These pions in turn quickly decay into other particles called [[muons]], [[neutrinos]], [[positrons]], and [[electrons]], and these particles rapidly dissipate. If antihydrogen atoms were to be suspended in a [[Free space|perfect vacuum]], however, they should survive indefinitely.
==Production==
{{Cleanup-section|date=July 2008}}
In 1995, the [[CERN]] laboratory in [[Geneva]] first produced antihydrogen by shooting antiprotons, which were produced in a [[particle accelerator]], at [[xenon]] [[Cluster (physics)|clusters]]. When an antiproton gets close to a xenon nucleus, an electron-positron-pair can be produced, and with some probability the positron will be captured by the antiproton to form antihydrogen. The probability for producing antihydrogen from one antiproton was only about 10<sup>-19</sup>, so this method is not well suited for the production of substantial amounts of antihydrogen.
In recent experiments carried out by the [[ATRAP]] and [[ATHENA]] collaborations at CERN, positrons from a [[sodium]] [[radioactive]] source and antiprotons were brought together in a magnetic [[Penning trap]], where synthesis took place at a typical rate of 100 antihydrogen atoms per second. Antihydrogen was first produced by these two collaborations in [[2002]], and by 2004 perhaps a hundred thousand antihydrogen atoms were produced in this way.
The antihydrogen atoms synthesized so far have a very high temperature (a few thousand [[kelvin]]s); they will hit the walls of the experimental apparatus as a consequence and annihilate. A potential solution to this problem would be to produce antihydrogen atoms at such a low temperature (perhaps a fraction of a kelvin) that they can be captured in a [[magnetic trap]]. <!--The antihydrogen atoms can then be interrogated by laser beams, so that their atomic transition frequencies can be precisely measured. If any difference between hydrogen and antihydrogen were observed, however small, it would indicate that matter and antimatter do not behave in exactly the same way. This may help explain why the observable Universe appears to be made entirely of matter and not antimatter.-->
Simultaneous trapping of antiprotons and antielectrons was reported
<ref name="both">{{cite journal
|title=The ingredients of cold antihydrogen: Simultaneous confinement of antiprotons and positrons at 4 K
|author=G. Gabrielse
|coauthors=D. S. Hall, T. Roach, P. Yesley, A. Khabbaz, J. Estrada, C. Heimann and H. Kalinowsky
|journal=[[Physics Letters B]]
|volume=455
|issue= 1-4
|year=1999
|doi=10.1016/S0370-2693(99)00453-0
|url=http://www.freepatentsonline.com/6163587.html
|pages=311–315
}}</ref> and the cooling is achieved
<ref name="cool-H">
{{cite journal
|author=G. Andresen
|coauthors=at al.
|title=Antimatter Plasmas in a Multipole Trap for Antihydrogen
|journal=[[Physical Review Letters|PRL]]
|volume= 98
|pages= 023402
|year=2007
|url=http://link.aps.org/abstract/PRL/v98/e023402
|doi=10.1103/PhysRevLett.98.023402}}
</ref>; there are patents on the way of production of antihydrogen
<ref name="US6163587">{{cite journal
|title = Process for the production of antihydrogen
|journal=US patent
|volume= 6163587
|author = Hessels Eric Arthur
|coauthors=
|year = 2000
|month = December
|url = http://www.freepatentsonline.com/6163587.html
}}</ref>. In spite of this progress, the confinment time is not yet long, and the antimatter is not yet available at the market.
Antimatter atoms such as [[Deuterium#Anti-deuterium|antideuterium]] (<font style="text-decoration: overline">D</font>), [[antitritium]] (<font style="text-decoration: overline">T</font>), and [[antihelium]] (<font style="text-decoration: overline">He</font>) are much more difficult to produce than antihydrogen. Among these, only antideuterium nuclei have been produced so far, and these have such very high velocities that synthesis of antideuterium atoms may still be many decades ahead.
===Natural occurrence===
Today, no conclusive spectral signature for the presence of antihydrogen could be reported, since measuring the spectrum of antihydrogen, especially the 1S-2S interval, is exactly the goal of these CERN collaborations.
==See also==
*[[Gravitational interaction of antimatter]]
==References==
<references/>
[[Category:Hydrogen physics]]
[[Category:Antimatter]]
[[Category:Exotic atoms]]
[[de:Antiwasserstoff]]
[[es:Antihidrógeno]]
[[ja:反水素]]
[[no:Antihydrogen]]
[[pl:Antywodór]]
[[ru:Антиводород]]
[[zh:反氫]]