Editing Antihydrogen

{{Short description|Exotic particle made of an antiproton and positron}}

[[File:3D image of Antihydrogen.jpg|thumb|right|upright=1.2|Antihydrogen consists of an [[antiproton]] and a [[positron]]]]
[[File:Antihydrogen.gif|thumb|right|upright=1.2|Simplified model of an antihydrogen atom in ground state]]

{{Antimatter}}
'''Antihydrogen''' ('''{{physics particle|anti=yes|H}}''') 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]]. Scientists hope that studying antihydrogen may shed light on the question of why there is more [[matter]] than [[antimatter]] in the observable universe, known as the [[baryon asymmetry]] problem.<ref>{{Cite news |date=2011-06-06 |title=Antimatter atoms are corralled even longer |language=en-GB |work=BBC News |url=https://www.bbc.com/news/science-environment-13666892 |access-date=2023-09-28}}</ref> Antihydrogen is produced artificially in [[particle accelerator]]s. <!--In 1999, [[NASA]] gave a cost estimate of $62.5 trillion per gram of antihydrogen (equivalent to ${{Inflation|USD|62.5|1999}} trillion today), making it the most expensive material to produce.<ref name="NASA1999">{{cite web
 |date=12 April 1999
 |title=Reaching for the stars: Scientists examine using antimatter and fusion to propel future spacecraft
 |url=https://science.nasa.gov/science-news/science-at-nasa/1999/prop12apr99_1
 |publisher=[[NASA]]
 |quote=Antimatter is the most expensive substance on Earth
 |access-date=11 June 2010
}}</ref> This is due to the extremely low yield per experiment, and high [[opportunity cost]] of using a [[particle accelerator]]. [This sounds cool, but it is _highly_ misleading. Removing, for now. Please refer to the discussion at Talk:Antihydrogen#Cost_estimate-->

==Experimental history==
Accelerators first detected hot antihydrogen in the 1990s. [[ATHENA]] studied cold {{physics particle|anti=yes|H}} in 2002. It was first trapped by the Antihydrogen Laser Physics Apparatus ([[ALPHA Collaboration|ALPHA]]) team at [[CERN]]<ref name=natrev>{{cite journal|doi=10.1038/468355a |pmid=21085144 |author=Reich, Eugenie Samuel |title=Antimatter held for questioning |journal=Nature |volume=468 |issue=7322 |pages=355 |year=2010 |bibcode=2010Natur.468..355R |doi-access=free }}</ref><ref>[http://www.eiroforum.org/activities/scientific_highlights/201112_CERN/index.html eiroforum.org – CERN: Antimatter in the trap] {{webarchive |url=https://web.archive.org/web/20140203230601/http://www.eiroforum.org/activities/scientific_highlights/201112_CERN/index.html |date=February 3, 2014 }}, December 2011, accessed 2012-06-08</ref> in 2010, who then measured the structure and other important properties.<ref>{{cite web|url = http://physicsworld.com/cws/article/news/2012/mar/07/internal-structure-of-antihydrogen-probed-for-the-first-time|title = Internal Structure of Antihydrogen probed for the first time|date = March 7, 2012|website = Physics World}}</ref> ALPHA, [https://aegis.web.cern.ch/ AEgIS], and GBAR plan to further cool and study {{physics particle|anti=yes|H}} atoms.

===1s–2s transition measurement===
In 2016, the [[Antiproton Decelerator#ALPHA|ALPHA]] experiment measured the [[atomic electron transition]] between the two lowest [[energy level]]s of antihydrogen, 1s–2s. The results, which are identical to that of hydrogen within the experimental resolution, support the idea of matter–antimatter symmetry and [[CPT symmetry]].<ref>{{cite journal|url=http://www.nature.com/news/ephemeral-antimatter-atoms-pinned-down-in-milestone-laser-test-1.21193|title=Ephemeral antimatter atoms pinned down in milestone laser test|journal=Nature|date=19 December 2016|access-date=20 December 2016|author=Castelvecchi, Davide |doi=10.1038/nature.2016.21193|s2cid=125464517}}</ref>

In the presence of a magnetic field the 1s–2s transition splits into two [[Hyperfine structure|hyperfine]] transitions with slightly different frequencies. The team calculated the transition frequencies for normal hydrogen under the magnetic field in the confinement volume as:

:f<sub>dd</sub> = {{val|2466061103064|(2)|u=kHz}} 
:f<sub>cc</sub> = {{val|2466061707104|(2)|u=kHz}}

A single-photon transition between s states is prohibited by quantum [[selection rule]]s, so to elevate ground state positrons to the 2s level, the confinement space was illuminated by a laser tuned to half the calculated transition frequencies, stimulating allowed [[two photon absorption]].

Antihydrogen atoms excited to the 2s state can then evolve in one of several ways:

*They can emit two photons and return directly to the ground state as they were
*They can absorb another photon, which ionizes the atom
*They can emit a single photon and return to the ground state via the 2p state—in this case the positron spin can flip or remain the same.

Both the ionization and spin-flip outcomes cause the atom to escape confinement. The team calculated that, assuming antihydrogen behaves like normal hydrogen, roughly half the antihydrogen atoms would be lost during the resonant frequency exposure, as compared to the no-laser case. With the laser source tuned 200&nbsp;kHz below half the transition frequencies, the calculated loss was essentially the same as for the no-laser case.

The ALPHA team made batches of antihydrogen, held them for 600 seconds and then tapered down the confinement field over 1.5 seconds while counting how many antihydrogen atoms were annihilated. They did this under three different experimental conditions:

*Resonance: exposing the confined antihydrogen atoms to a laser source tuned to exactly half the transition frequency for 300 seconds for each of the two transitions,
*Off-resonance: exposing the confined antihydrogen atoms to a laser source tuned 200 kilohertz below the two resonance frequencies for 300 seconds each,
*No-laser: confining the antihydrogen atoms without any laser illumination.

The two controls, off-resonance and no-laser, were needed to ensure that the laser illumination itself was not causing annihilations, perhaps by liberating normal atoms from the confinement vessel surface that could then combine with the antihydrogen.

The team conducted 11 runs of the three cases and found no [[statistically significant|significant]] difference between the off-resonance and no laser runs, but a 58% drop in the number of events detected after the resonance runs. They were also able to count annihilation events during the runs and found a higher level during the resonance runs, again with no significant difference between the off-resonance and no laser runs. The results were in good agreement with predictions based on normal hydrogen and can be "interpreted as a test of CPT symmetry at a precision of 200&nbsp;ppt."<ref>{{cite journal |last=Ahmadi|first=M|display-authors=et al|date=19 December 2016|title=Observation of the 1S–2S transition in trapped antihydrogen|journal=Nature|volume=541|issue=7638|pages=506–510|doi=10.1038/nature21040|pmid=28005057|bibcode = 2017Natur.541..506A |s2cid=3195564|url=http://discovery.ucl.ac.uk/1537231/1/Guiterrez_nature21040.pdf|doi-access=free}}</ref>

==Characteristics==
The [[CPT theorem]] of particle physics predicts antihydrogen atoms have many of the characteristics regular hydrogen has; i.e. the same [[mass]], [[magnetic moment]], and atomic state transition frequencies (see ''[[atomic spectroscopy]]'').<ref>{{cite magazine|url=http://focus.aps.org/story/v26/st1 |title=The Coolest Antiprotons |magazine=Physical Review Focus |author=Grossman, Lisa |date=July 2, 2010|volume=26|issue= 1}}</ref> For example, excited antihydrogen atoms are expected to glow the same color as regular hydrogen. Antihydrogen atoms should be [[gravitational interaction of antimatter|attracted to other matter or antimatter gravitationally]] with a force of the same magnitude that ordinary hydrogen atoms experience.<ref name=natrev/> This would not be true if antimatter has negative [[gravitational mass]], which is considered highly unlikely, though not yet empirically disproven (see ''[[gravitational interaction of antimatter]]'').<ref>{{cite magazine|url=http://www.technologyreview.com/view/423901/antihydrogen-trapped-for-1000-seconds|date=May 2, 2011|title=Antihydrogen trapped for a thousand seconds|magazine=Technology Review|access-date=March 18, 2014|archive-date=April 14, 2015|archive-url=https://web.archive.org/web/20150414171504/http://www.technologyreview.com/view/423901/antihydrogen-trapped-for-1000-seconds/|url-status=dead}}</ref> Recent theoretical framework for negative mass and repulsive gravity (antigravity) between matter and antimatter has been developed, and the theory is compatible with CPT theorem.<ref>{{Cite web|last=Du|first=Hong|title=Application of New Relativistic Quantum Wave Equation on Hydrogen Atom and its Implications on Antimatter Gravitational Experiments|url=https://www.researchgate.net/publication/344381683|url-status=live|archive-url=https://web.archive.org/web/20210426065717/https://www.researchgate.net/publication/344381683_Application_of_New_Relativistic_Quantum_Wave_Equation_on_Hydrogen_Atom_and_its_Implications_on_Antimatter_Gravitational_Experiments |archive-date=2021-04-26 }}</ref>

When antihydrogen comes into contact with ordinary matter, its constituents quickly [[Annihilation|annihilate]]. The positron annihilates with an electron to produce [[gamma ray]]s. The antiproton, on the other hand, is made up of antiquarks that combine with quarks in either neutrons or protons, resulting in high-energy [[pion]]s, that quickly decay into [[muon]]s, [[neutrino]]s, [[positron]]s, and [[electron]]s. If antihydrogen atoms were suspended in a [[Free space|perfect vacuum]], they should survive indefinitely.

As an anti-element, it is expected to have exactly the same properties as hydrogen.<ref>{{cite news|url=https://www.bbc.co.uk/news/science-environment-17284822|title=Antihydrogen undergoes its first-ever measurement|first=Jason|last=Palmer|work=BBC News |date=14 March 2012}}</ref> For example, antihydrogen would be a gas under standard conditions and combine with antioxygen to form antiwater, {{physics particle|anti=yes|H}}<sub>2</sub>{{physics particle|anti=yes|O}}.

==Production==
The first antihydrogen was produced in 1995 by a team led by [[Walter Oelert]] at CERN<ref>{{cite news |last=Freedman |first=David H. |date=January 1997 |title=Antiatoms: Here Today ... |newspaper=Discover Magazine |url=http://discovermagazine.com/1997/jan/antiatomsheretod1029}}</ref> using a method first proposed by [[Charles Munger Jr]], [[Stanley Brodsky]] and [[Ivan Schmidt Andrade]].<ref>{{cite journal|title=Production of relativistic antihydrogen atoms by pair production with positron capture|last=Munger|first=Charles T.|s2cid=12149672|date=1994|journal=[[Physical Review D]]|volume=49|number=7|pages=3228–3235|doi=10.1103/physrevd.49.3228|pmid = 10017318|bibcode = 1994PhRvD..49.3228M |osti=1449799 }}</ref>

In the [[Low Energy Antiproton Ring|LEAR]], antiprotons from an [[particle accelerator|accelerator]] were shot at [[xenon]] [[Cluster (physics)|clusters]],<ref name="first-AH">{{cite journal|title=Production of Antihydrogen |first1=G. |last1=Baur |first2=G. |last2=Boero |first3=S. |last3=Brauksiepe |first4=A. |last4=Buzzo |first5=W. |last5=Eyrich |first6=R. |last6=Geyer |first7=D. |last7=Grzonka |first8=J. |last8=Hauffe |first9=K. |last9=Kilian |first10=M. |last10=LoVetere |first11=M. |last11=Macri |first12=M. |last12=Moosburger |first13=R. |last13=Nellen |first14=W. |last14=Oelert |first15=S. |last15=Passaggio |first16=A. |last16=Pozzo |first17=K. |last17=Röhrich |first18=K. |last18=Sachs |first19=G. |last19=Schepers |first20=T. |last20=Sefzick |first21=R.S. |last21=Simon |first22=R. |last22=Stratmann |first23=F. |last23=Stinzing |first24=M. |last24=Wolke |journal=[[Physics Letters B]] |volume=368 |year=1996 |pages=251ff |bibcode = 1996PhLB..368..251B |doi=10.1016/0370-2693(96)00005-6 |issue=3|url=http://ikpe1101.ikp.kfa-juelich.de/ps210/PL_paper_CERN_preprint.ps }}</ref> producing electron-positron pairs. Antiprotons can capture positrons with probability about {{val|e=-19}}, so this method is not suited for substantial production, as calculated.<ref>{{cite journal |last1=Bertulani |first1=Carlos A. |last2=Baur |first2=Gerhard |year=1988 |title=Pair production with atomic shell capture in relativistic heavy ion collisions |url=http://faculty.tamuc.edu/cbertulani/cab/papers/BJP3.pdf |journal=Brazilian Journal of Physics |volume=18 |pages=559}}</ref><ref>{{cite journal|doi=10.1016/0370-1573(88)90142-1|title=Electromagnetic processes in relativistic heavy ion collisions|journal=Physics Reports|volume=163|issue=5–6|pages=299|year=1988|last1=Bertulani|first1=Carlos A.|last2=Baur|first2=Gerhard|bibcode=1988PhR...163..299B|url=http://juser.fz-juelich.de/record/845594/files/J%C3%BCl_2163_Bertulani.pdf}}</ref><ref name="first-calc">{{cite journal |title=Electromagnetic Pair Production with Capture |last1=Aste |first1=Andreas |last2=Hencken |first2=Kai |last3=Trautmann |first3=Dirk |last4=Baur |first4=G.|journal=Physical Review A |volume=50 |year=1993 |pages=3980–3983 |bibcode=1994PhRvA..50.3980A |doi=10.1103/PhysRevA.50.3980 |issue=5 |pmid=9911369|url=http://edoc.unibas.ch/9325/1/20100604145900_4c08f8941070d.pdf }}</ref> [[Fermilab]] measured a somewhat different cross section,<ref>{{cite journal|last1=Blanford|first1=G.|first2=D.C. |last2=Christian |first3=K. |last3=Gollwitzer |first4=M. |last4=Mandelkern |first5=C.T. |last5=Munger |first6=J. |last6=Schultz |first7=G. |last7=Zioulas|s2cid=58942287|date=December 1997|title=Observation of Atomic Antihydrogen|journal=Physical Review Letters|publisher=Fermi National Accelerator Laboratory|quote=FERMILAB-Pub-97/398-E E862 ... p and H experiments| doi=10.1103/PhysRevLett.80.3037 | bibcode=1997APS..APR.C1009C|volume=80|issue=14|pages=3037}}</ref> in agreement with predictions of [[quantum electrodynamics]].<ref>{{cite journal |last1=Bertulani |first1=C. A. |last2=Baur |first2=G. |year=1998 |title=Antihydrogen production and accuracy of the equivalent photon approximation |journal=Physical Review D |volume=58 |issue=3 |pages=034005 |arxiv=hep-ph/9711273 |bibcode=1998PhRvD..58c4005B |doi=10.1103/PhysRevD.58.034005 |s2cid=11764867}}</ref> Both resulted in highly energetic, or hot, anti-atoms, unsuitable for detailed study.

Subsequently, CERN built the [[Antiproton Decelerator]] (AD) to support efforts towards low-energy antihydrogen, for tests of fundamental symmetries. The AD supplies several CERN groups. CERN expects their facilities will be capable of producing 10 million antiprotons per minute.<ref name="madsen">{{cite journal |author=Madsen |first=N. |year=2010 |title=Cold antihydrogen: a new frontier in fundamental physics |journal=Philosophical Transactions of the Royal Society A |volume=368 |issue=1924 |pages=3671–3682 |bibcode=2010RSPTA.368.3671M |doi=10.1098/rsta.2010.0026 |pmid=20603376 |doi-access=free}}</ref>

===Low-energy antihydrogen===
Experiments by the [[ATRAP]] and ATHENA collaborations at CERN, brought together positrons and antiprotons in [[Penning trap]]s, resulting in synthesis at a typical rate of 100 antihydrogen atoms per second. Antihydrogen was first produced by ATHENA in 2002,<ref>{{cite journal |author=Amoretti |first=M. |display-authors=etal |year=2002 |title=Production and detection of cold antihydrogen atoms |url=https://cds.cern.ch/record/581488/files/cer-2340034.pdf |journal=[[Nature (journal)|Nature]] |volume=419 |issue=6906 |pages=456–459 |bibcode=2002Natur.419..456A |doi=10.1038/nature01096 |pmid=12368849 |s2cid=4315273}}</ref> and then by ATRAP<ref>{{cite journal |author=Gabrielse |first=G. |display-authors=etal |year=2002 |title=Driven Production of Cold Antihydrogen and the First Measured Distribution of Antihydrogen States |url=http://juser.fz-juelich.de/record/25767/files/17128.pdf |journal=Physical Review Letters |volume=89 |issue=23 |page=233401 |bibcode=2002PhRvL..89w3401G |doi=10.1103/PhysRevLett.89.233401 |pmid=12485006}}</ref> and by 2004, millions of antihydrogen atoms were made. The atoms synthesized had a relatively high temperature (a few thousand [[kelvins]]), and would hit the walls of the experimental apparatus as a consequence and annihilate. Most precision tests require long observation times.

ALPHA, a successor of the ATHENA collaboration, was formed to stably trap antihydrogen.<ref name=madsen/> While electrically neutral, its spin [[magnetic moment]]s interact with an inhomogeneous magnetic field; some atoms will be attracted to a magnetic minimum, created by a combination of mirror and multipole fields.<ref>{{cite journal
 |author=Pritchard, D. E. 
 |year=1983
 |title=Cooling neutral atoms in a magnetic trap for precision spectroscopy
 |journal=Physical Review Letters
 |volume=51  |page=1983
 |doi= 10.1103/PhysRevLett.51.1983
|bibcode = 1983PhRvL..51.1983T
 |last2=Heinz
 |first2=T.
 |last3=Shen
 |first3=Y.
 |issue=21 }}</ref>

In November 2010, the ALPHA collaboration announced that they had trapped 38 antihydrogen atoms for a sixth of a second,<ref>
{{cite journal
 |author=Andresen, G. B. ([[ALPHA Collaboration]])
 |year=2010
 |title=Trapped antihydrogen
 |journal=[[Nature (journal)|Nature]]
 |doi=10.1038/nature09610
|bibcode = 2010Natur.468..673A
 |volume=468
 |issue=7324
 |pages=673–676
 |pmid=21085118|s2cid=2209534
 |display-authors=etal}}</ref> the first confinement of neutral antimatter. In June 2011, they trapped 309 antihydrogen atoms, up to 3 simultaneously, for up to 1,000 seconds.<ref>{{cite journal
 |author=Andresen, G. B. ([[ALPHA Collaboration]])
 |title=Confinement of antihydrogen for 1,000 seconds
 |journal=[[Nature Physics]]
 |doi=10.1038/nphys2025
 |volume=7
 |issue=7
 |bibcode = 2011NatPh...7..558A
 |year=2011
 |pages=558–564 |arxiv = 1104.4982 |s2cid=17151882
 |display-authors=etal}}</ref> They then studied its hyperfine structure, gravity effects, and charge. ALPHA will continue measurements along with experiments ATRAP,  [https://aegis.web.cern.ch/ AEgIS] and GBAR.

In 2018, [https://aegis.web.cern.ch/ AEgIS] has produced a novel pulsed source of antihydrogen atoms with a production time spread of merely 250 nanoseconds. <ref>{{cite journal
 |author=Amsler, C. et al. ([[AEgIS experiment]])
 |title=Pulsed production of antihydrogen
 |journal=[[Communications Physics]]
 |doi=10.1038/s42005-020-00494-z
 |volume=4
 |year=2021
 |issue=1
 |pages=19
 |bibcode=2021CmPhy...4...19A
 |display-authors=etal|hdl=2434/813338
 |hdl-access=free
 }}</ref>
The pulsed source is generated by the [[charge exchange]] reaction between Rydberg [[positronium]] atoms -- produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses -- and antiprotons, trapped, cooled and manipulated in electromagnetic traps. The pulsed production enables the control of the antihydrogen temperature, the formation of an antihydrogen beam, and in the next phase a precision measurement on the gravitational behaviour using an atomic interferometer, the so-called [[Moiré deflectometry|Moiré deflectormeter]].

===Larger antimatter atoms===
Larger antimatter atoms such as [[Deuterium#Antideuterium|antideuterium]] ({{physics particle|anti=yes|D}}), [[antitritium]] ({{physics particle|anti=yes|T}}), and [[antihelium]] ({{physics particle|anti=yes|He}}) are much more difficult to produce. Antideuterium,<ref>{{Cite journal|author = Massam, T|year = 1965 |title = Experimental observation of antideuteron production |journal = Il Nuovo Cimento |volume = 39|issue = 1 |pages = 10–14 |doi = 10.1007/BF02814251|last2 = Muller|first2 = Th.|last3 = Righini|first3 = B.|last4 = Schneegans|first4 = M.|last5 = Zichichi|first5 = A.|bibcode = 1965NCimS..39...10M |s2cid = 122952224 }}</ref><ref>{{Cite journal|author = Dorfan, D. E|date=June 1965|title = Observation of Antideuterons|journal = Phys. Rev. Lett.|volume = 14|issue = 24 |pages = 1003–1006| doi = 10.1103/PhysRevLett.14.1003|last2 = Eades|first2 = J.|last3 = Lederman|first3 = L. M.|last4 = Lee|first4 = W.|last5 = Ting|first5 = C. C.|bibcode=1965PhRvL..14.1003D}}</ref> antihelium-3 ({{SimpleNuclide|anti=yes|helium|3}})<ref>{{cite journal |author=Antipov |first=Y. M. |display-authors=etal |year=1974 |title=Observation of antihelium3 (in Russian) |journal=Yadernaya Fizika |volume=12 |page=311}}</ref><ref>{{cite journal|author=Arsenescu, R. |year=2003|title=Antihelium-3 production in lead-lead collisions at 158 ''A'' GeV/''c''|journal=[[New Journal of Physics]]|volume=5 |issue=1|page=1|doi=10.1088/1367-2630/5/1/301|bibcode = 2003NJPh....5....1A|display-authors=etal|doi-access=free}}</ref> and antihelium-4 ({{SimpleNuclide|anti=yes|helium|4}}) nuclei<ref>{{cite journal |author=Agakishiev |first=H. |display-authors=etal |year=2011 |title=Observation of the antimatter helium-4 nucleus |journal=Nature |volume=473 |issue=7347 |pages=353–6 |arxiv=1103.3312 |bibcode=2011Natur.473..353S |doi=10.1038/nature10079 |pmid=21516103 |s2cid=118484566}}</ref> have been produced with such high velocities that synthesis of their corresponding atoms poses several technical hurdles.

==See also==
* [[Gravitational interaction of antimatter]]

==References==
{{Reflist}}

==External links==
* {{cite web |last1=Merrifield |first1=Michael |title=Antihydrogen |url=http://www.sixtysymbols.com/videos/antihydrogen.htm |work=Sixty Symbols |publisher=[[Brady Haran]] for the [[University of Nottingham]] |last2=Copeland |first2=Ed}}

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[[Category:Antimatter]]
[[Category:Hydrogen]]
[[Category:Hydrogen physics]]
[[Category:Gases]]