Editing Protonium

{{Short description|Bound state of a proton and antiprotron}}
[[File:Structure of Protonium.svg|thumb|An illustration of the protonium atom.]]
{{distinguish|Positronium}}
{{hatnote|For the isotope <sup>1</sup>H and the corresponding hydrated ion, see [[Hydrogen-1|Protium]] and [[Hydronium]].}}

'''Protonium''', also known as '''antiprotonic hydrogen''', is a type of [[exotic atom]] in which a [[proton]] (symbol: p) and an [[antiproton]] (symbol: {{overline|p}}) are bound to each other.<ref>{{cite journal |last=Zurlo |first=N. |display-authors=etal.  |title=Production Of Slow Protonium In Vacuum |journal=[[Hyperfine Interactions]] |volume=172 |issue=1–3 |pages=97–105 |arxiv=0801.3193  |bibcode= 2006HyInt.172...97Z |doi=10.1007/s10751-007-9529-0 |year=2006 |s2cid=119182686 }}</ref>

Since protonium is a [[bound state|bound system]] of a [[particle]] and its corresponding [[antiparticle]], it is an example of a type of [[exotic atom]] called an ''[[onium]]''.

Protonium has a [[Exponential decay#Mean lifetime|mean lifetime]] of approximately 1.0&nbsp;[[Microsecond|μs]] and a binding energy of −0.75&nbsp;[[Electronvolt|keV]].<ref>{{cite journal |last=Abdel-Raouf |first=Mohamed Assad |title=Binding energy of protonium ions |journal=Journal of Physics: Conference Series |volume=194 |issue=7 |doi=10.1088/1742-6596/194/7/072003 |year=2009 |page=072003 |bibcode=2009JPhCS.194g2003A |doi-access=free }}</ref>

Like all onia, protonium is a [[boson]] with all quantum numbers ([[baryon number]], [[flavour quantum number]]s, etc.) and [[electrical charge]] equal to 0.

==Production==
There are two known methods to generate protonium. One method involves violent particle collisions. The other method involves putting antiprotons and [[proton]]s into the same magnetic cage. The latter method was first used during the experiment [[ATHENA]] (ApparaTus for High precision Experiment on Neutral Antimatter)  at the [[CERN]] laboratory in [[Geneva]] in 2002, but it was not until 2006 that scientists realized protonium was also generated during the experiment.<ref>{{cite journal |url=https://www.openaccessrepository.it/record/136861/files/fulltext.pdf |archive-url=https://web.archive.org/web/20240110154115/https://www.openaccessrepository.it/record/136861/files/fulltext.pdf |url-status=dead |archive-date=January 10, 2024 |title=Protonium production in ATHENA |author=L. Venturelli |display-authors=etal |collaboration=Athena collaboration |journal=[[Nuclear Instruments and Methods in Physics Research Section B]] |volume=261 |issue=1–2 |date=August 2007 |pages=40–43 |doi=10.1016/j.nimb.2007.04.135 |bibcode=2007NIMPB.261...40V}}</ref>

Reactions involving a proton and an antiproton at high energies give rise to many-particle final states. In fact, such reactions are the basis of [[particle collider]]s such as the [[Tevatron]] at [[Fermilab]]. Indirect searches for protonium at [[LEAR]] (Low Energy Antiproton Ring at [[CERN]]) have used antiprotons impinging on nuclei such as [[helium]], with unclear results. Very low energy collisions in the range of 10&nbsp;[[Electronvolt|eV]] to 1&nbsp;[[keV]] may lead to the formation of protonium.

==Studies==
Planned experiments will use traps as the source of low energy antiprotons. Such a beam would be allowed to impinge on atomic [[hydrogen]] targets, in the field of a laser, which is meant to excite the bound proton–antiproton pairs into an excited state of protonium with some efficiency (whose computation is an open theoretical problem). Unbound particles are rejected by bending them in a magnetic field. Since the protonium is uncharged, it will not be deflected by such a field. This undeflected protonium, if formed, would be allowed to traverse a meter of high vacuum, within which it is expected to decay via annihilation of the proton and antiproton. The decay products would give unmistakable signatures of the formation of protonium.{{Fact|date=January 2025}}

Theoretical studies of protonium have mainly used non-relativistic [[quantum mechanics]]. These give predictions for the [[binding energy]] and [[mean lifetime|lifetime]] of the states. Computed lifetimes are in the range of 0.1 to 10 [[microsecond]]s. Unlike the [[Bohr atom|hydrogen atom]], in which the dominant interactions are due to the [[Coulomb's law|Coulomb attraction]] of the electron and the proton, the constituents of protonium interact predominantly through the [[strong interaction]]. Thus multiparticle interactions involving [[meson]]s in intermediate states may be important. Hence the production and study of protonium would be of interest also for the understanding of [[internucleon force]]s.{{Fact|date=January 2025}}

==See also==
* [[Positronium]]
* [[Antiprotonic helium]]
* [[Antiproton Collector]]
* [[Antiproton Accumulator]]

== References ==
{{reflist}}

==Further reading==
* {{cite web
 |last1=Battersby |first1=S.
 |date=13 October 2006
 |title=Antimatter and matter combine in chemical reaction
 |url=https://www.newscientist.com/channel/fundamentals/dn10302-antimatter-and-matter-combine-in-chemical-reaction.html
 |work=[[New Scientist]]
 |access-date=2015-06-26
}}
* {{cite journal
 |last1=Klempt |first1=E.
 |last2=Bradamante |first2=F.
 |last3=Martin |first3=A.
 |last4=Richard |first4=J.-M.
 |year=2002
 |title=Antinucleon-nucleon interaction at low energy: scattering and protonium
 |journal=[[Physics Reports]]
 |volume=368 |issue=2–3 |pages=119–316
 |bibcode=2002PhR...368..119K
 |doi=10.1016/S0370-1573(02)00144-8
|url=http://hal.in2p3.fr/in2p3-00011702/file/Klempt2002.pdf
 }}
*{{cite journal
 |last=Zurlo |first=N.
 |display-authors=etal.
 |title=Evidence For The Production Of Slow Antiprotonic Hydrogen In Vacuum
 |journal=[[Physical Review Letters]]
 |volume=97 |issue=15 |pages=153401
 |arxiv=0708.3717
 |bibcode=2006PhRvL..97o3401Z
 |doi=10.1103/PhysRevLett.97.153401
 |pmid=17155325
|year=2006
 |s2cid=36091971
 }}

{{Particles}}

[[Category:Onia]]
[[Category:Proton]]