Editing Hypernucleus (section)

== Types ==
=== Λ hypernuclei ===
The simplest, and most well understood, type of hypernucleus includes only the lightest hyperon, the Λ.<ref name="Feliciello">{{cite journal |last1=Feliciello |first1=A |last2=Nagae |first2=T |title=Experimental review of hypernuclear physics: recent achievements and future perspectives |journal=Reports on Progress in Physics |date=1 September 2015 |volume=78 |issue=9 |pages=096301 |doi=10.1088/0034-4885/78/9/096301|pmid=26317857 |bibcode=2015RPPh...78i6301F |s2cid=25818699 |url=https://www.openaccessrepository.it/record/75858 |archive-url=https://web.archive.org/web/20220530112234/https://www.openaccessrepository.it/record/75858 |url-status=dead |archive-date=May 30, 2022 }}</ref>

<!-- Wanted: Feynman diagrams (at the hadronic level) illustrating the dominant terms of the Λ–nucleon interaction -->
While two nucleons can interact through the [[nuclear force]] mediated by a [[virtual particle|virtual]] pion, the Λ becomes a Σ baryon upon emitting a pion,{{efn|name=isospin|[[Isospin]] ({{math|''I''}}), a number describing the up and down quark content of the system, is preserved in the strong interaction. Since the isospin of a pion is 1, the Λ baryon ({{math|1=''I'' = 0}}) must become a Σ ({{math|1=''I'' = 1}}) upon emitting a pion.{{sfn|Gal|Hungerford|Millener|2016|p=20}}}} so the Λ–nucleon interaction is mediated solely by more massive mesons such as the [[eta meson|η]] and [[omega meson|ω]] mesons, or through the simultaneous exchange of two or more mesons.{{sfn|Gal|Hungerford|Millener|2016|pp=2,20–21}} This means that the Λ–nucleon interaction is weaker and has a shorter range than the standard nuclear force, and the [[potential well]] of a Λ in the nucleus is shallower than that of a nucleon;{{sfn|Gal|Hungerford|Millener|2016|p=6}} in hypernuclei, the depth of the Λ potential is approximately 30&nbsp;[[MeV]].{{sfn|Tolos|Fabbietti|2020|p=50}} However, one-pion exchange in the Λ–nucleon interaction does cause quantum-mechanical mixing of the Λ and Σ baryons in hypernuclei (which does not happen in free space), especially in neutron-rich hypernuclei.{{sfn|Gal|Hungerford|Millener|2016|pp=20–21}}{{sfn|Tolos|Fabbietti|2020|p=52}}<ref>{{cite journal |last1=Umeya |first1=A. |last2=Harada |first2=T. |title=Λ–Σ coupling effect in the neutron-rich Λ hypernucleus <math>^{10}_{\Lambda}\mathrm{Li}</math> in a microscopic shell-model calculation |journal=Physical Review C |date=20 February 2009 |volume=79 |issue=2 |pages=024315 |doi=10.1103/PhysRevC.79.024315|arxiv=0810.4591|s2cid=117921775 }}</ref> Additionally, the [[three-body force]] between a Λ and two nucleons is expected to be more important than the three-body interaction in nuclei, since the Λ can exchange two pions with a virtual Σ intermediate, while the equivalent process in nucleons requires a relatively heavy [[delta baryon]] (Δ) intermediate.{{sfn|Gal|Hungerford|Millener|2016|pp=2,20–21}}

Like all hyperons, Λ hypernuclei can decay through the [[weak interaction]], which changes it to a lighter baryon and emits a meson or a [[lepton]]–antilepton pair. In free space, the Λ usually decays via the weak force to a proton and a π<sup>–</sup> meson, or a neutron and a π<sup>0</sup>, with a total half-life of {{val|263|2|ul=ps}}.<ref name="PDG">{{cite web|first1=C.|last1=Amsler|collaboration=Particle Data Group|display-authors=etal|year=2008|series=Particle listings|title={{Subatomic particle|Lambda}}|publisher=Lawrence Berkeley Laboratory|url=http://pdg.lbl.gov/2008/listings/s018.pdf}}</ref> A nucleon in the hypernucleus can cause the Λ to decay via the weak force without emitting a pion; this process becomes dominant in heavy hypernuclei, due to suppression of the pion-emitting decay mode.{{sfn|Tolos|Fabbietti|2020|p=50–51}} The half-life of the Λ in a hypernucleus is considerably shorter, plateauing to about {{val|215|14|u=ps}} near {{physics particle|[[Iron|Fe]]|TL=56|BL=Λ}},<ref>{{cite journal |last1=Sato |first1=Y. |last2=Ajimura |first2=S. |last3=Aoki |first3=K. |last4=Bhang |first4=H. |last5=Hasegawa |first5=T. |last6=Hashimoto |first6=O. |last7=Hotchi |first7=H. |last8=Kim |first8=Y. D. |last9=Kishimoto |first9=T. |last10=Maeda |first10=K. |last11=Noumi |first11=H. |last12=Ohta |first12=Y. |last13=Omata |first13=K. |last14=Outa |first14=H. |last15=Park |first15=H. |last16=Sekimoto |first16=M. |last17=Shibata |first17=T. |last18=Takahashi |first18=T. |last19=Youn |first19=M. |title=Mesonic and nonmesonic weak decay widths of medium-heavy Λ hypernuclei |journal=Physical Review C |date=9 February 2005 |volume=71 |issue=2 |pages=025203 |doi=10.1103/PhysRevC.71.025203|arxiv=nucl-ex/0409007v2|bibcode=2005PhRvC..71b5203S |s2cid=119428665 }}</ref> but some empirical measurements substantially disagree with each other or with theoretical predictions.{{sfn|Gal|Hungerford|Millener|2016|pp=17–18}}

===Hypertriton===
The simplest hypernucleus is the [[hypertriton]] ({{PhysicsParticle|[[Hydrogen|H]]|TL=3|BL=Λ}}), which consists of one proton, one neutron, and one Λ hyperon. The Λ in this system is very loosely bound, having a [[separation energy]] of 130&nbsp;keV and a large radius of 10.6&nbsp;[[Femtometer|fm]],{{sfn|Tolos|Fabbietti|2020|p=53}} compared to about {{val|2.13|u=fm}} for the [[deuteron]].<ref>{{cite journal |last1=Tiesinga |first1=Eite |last2=Mohr |first2=Peter J. |last3=Newell |first3=David B. |last4=Taylor |first4=Barry N. |title=CODATA Recommended Values of the Fundamental Physical Constants: 2018 |journal=Journal of Physical and Chemical Reference Data |date=1 September 2021 |volume=50 |issue=3 |pages=033105 |doi=10.1063/5.0064853 |pmid=36733295 |pmc=9890581 |bibcode=2021JPCRD..50c3105T |language=en |issn=0047-2689}}</ref>

This loose binding would imply a lifetime similar to a free Λ. However, the measured hypertriton lifetime averaged across all experiments (about {{val|206|15|13|u=ps}}) is substantially shorter than predicted by theory, as the non-mesonic decay mode is expected to be relatively minor; some experimental results are substantially shorter or longer than this average.{{sfn|Tolos|Fabbietti|2020|pp=52–53}}<ref>{{cite journal |author=ALICE Collaboration |title=<math>^{3}_{\Lambda}\mathrm{H}</math> and <math>\overline{^{3}_{\Lambda}\mathrm{H}}</math> lifetime measurement in Pb–Pb collisions at s NN = 5.02 TeV via two-body decay |journal=Physics Letters B |date=October 2019 |volume=797 |pages=134905 |doi=10.1016/j.physletb.2019.134905| s2cid=204776807 |doi-access=free |arxiv=1907.06906 }}</ref>

=== Σ hypernuclei ===
The existence of hypernuclei containing a Σ baryon is less clear. Several experiments in the early 1980s reported bound hypernuclear states above the Λ [[separation energy]] and presumed to contain one of the slightly heavier Σ baryons, but experiments later in the decade ruled out the existence of such states.<ref name="Feliciello"/> Results from [[exotic atoms]] containing a Σ<sup>−</sup> bound to a nucleus by the [[electromagnetic force]] have found a net repulsive Σ–nucleon interaction in medium-sized and large hypernuclei, which means that no Σ hypernuclei exist in such mass range.<ref name="Feliciello"/> However, an experiment in 1998 definitively observed the light Σ hypernucleus {{PhysicsParticle|[[Helium|He]]|TL=4|BL=Σ}}.<ref name="Feliciello"/>

=== ΛΛ and Ξ hypernuclei ===
Hypernuclei containing two Λ baryons have been made. However, such hypernuclei are much harder to produce due to containing two strange quarks and, as of 2016, only seven candidate ΛΛ hypernuclei have been observed.{{sfn|Gal|Hungerford|Millener|2016|p=41}} Like the Λ–nucleon interaction, empirical and theoretical models predict that the Λ–Λ interaction is mildly attractive.{{sfn|Tolos|Fabbietti|2020|pp=43–45,59}}<ref>{{cite journal |author=ALICE Collaboration|title=Study of the Λ–Λ interaction with femtoscopy correlations in pp and p–Pb collisions at the LHC |journal=Physics Letters B |date=10 October 2019 |volume=797 |pages=134822 |doi=10.1016/j.physletb.2019.134822 |arxiv=1905.07209 |bibcode=2019PhLB..79734822A |s2cid=161048820 |url=https://www.sciencedirect.com/science/article/pii/S0370269319305362 |language=en |issn=0370-2693}}</ref>

Hypernuclei containing a Ξ baryon are known.{{citation needed|date=January 2024}} Empirical studies and theoretical models indicate that the Ξ<sup>–</sup>–proton interaction is attractive, but weaker than the Λ–nucleon interaction.{{sfn|Tolos|Fabbietti|2020|pp=43–45,59}} Like the Σ<sup>–</sup> and other negatively charged particles, the Ξ<sup>–</sup> can also form an exotic atom. When a Ξ<sup>–</sup> is bound in an exotic atom or a hypernucleus, it quickly decays to a ΛΛ hypernucleus or to two Λ hypernuclei by exchanging a strange quark with a proton, which releases about 29&nbsp;MeV of energy in free space:{{efn|name=qvalue|The initial proton and Ξ<sup>–</sup> have respective masses of approximately 938.3 and 1321.7&nbsp;MeV, while the outgoing Λ's are each about 1115.7&nbsp;MeV;<ref>{{cite journal |last1=Workman |first1=R L |last2=Burkert |first2=V D |last3=Crede |first3=V |last4=Klempt |first4=E |last5=Thoma |first5=U |last6=Tiator |first6=L |display-authors=1|collaboration=Particle Data Group|title=Review of Particle Physics |journal=Progress of Theoretical and Experimental Physics |date=8 August 2022 |volume=2022 |issue=8 |page=083C01 |doi=10.1093/ptep/ptac097|doi-access=free |hdl=11585/900713 |hdl-access=free }}</ref> the energy that is released is equal to the amount of mass that is lost (times ''c''<sup>2</sup>).}}
:Ξ<sup>−</sup> + p → Λ + Λ<ref name="JPARC E07"/>{{sfn|Gal|Hungerford|Millener|2016|pp=16,43}}{{sfn|Tolos|Fabbietti|2020|p=53}}

=== Ω hypernuclei ===
Hypernuclei containing the [[omega baryon]] (Ω) were predicted using [[lattice QCD]] in 2018; in particular, the proton–Ω and Ω–Ω [[dibaryon]]s (bound systems containing two baryons) are expected to be stable.<ref>{{cite journal |last1=Iritani |first1=Takumi |collaboration=HALQCD Collaboration |title=NΩ dibaryon from lattice QCD near the physical point |journal=Physics Letters B |date=May 2019 |volume=792 |pages=284–289 |doi=10.1016/j.physletb.2019.03.050|arxiv=1810.03416
|bibcode=2019PhLB..792..284I |s2cid=102481007 }}</ref><ref>{{cite journal |last1=Gongyo |first1=Shinya |collaboration=HALQCD Collaboration |title=Most Strange Dibaryon from Lattice QCD |journal=Physical Review Letters |date=23 May 2018 |volume=120 |issue=21 |pages=212001 |doi=10.1103/PhysRevLett.120.212001|pmid=29883161 |arxiv=1709.00654 |bibcode=2018PhRvL.120u2001G |s2cid=43958833 }}</ref> {{As of|2022}}, no such hypernuclei have been observed under any conditions, but the lightest such species could be produced in heavy-ion collisions,<ref>{{cite journal |last1=Zhang |first1=Liang |last2=Zhang |first2=Song |last3=Ma |first3=Yu-Gang |title=Production of ΩNN and ΩΩN in ultra-relativistic heavy-ion collisions |journal=The European Physical Journal C |date=May 2022 |volume=82 |issue=5 |pages=416 |doi=10.1140/epjc/s10052-022-10336-7|arxiv=2112.02766 |bibcode=2022EPJC...82..416Z |s2cid=244908731 |doi-access=free }}</ref> and measurements by the STAR experiment are consistent with the existence of the proton–Ω dibaryon.<ref>{{cite journal |author=STAR Collaboration |title=The proton–Ω correlation function in Au + Au collisions at s NN = 200 GeV |journal=Physics Letters B |date=March 2019 |volume=790 |pages=490–497 |doi=10.1016/j.physletb.2019.01.055| s2cid=127339678 |doi-access=free |hdl=11368/2940231 |hdl-access=free }}</ref>

=== Hypernuclei with higher strangeness ===
Since the Λ is electrically neutral and its nuclear force interactions are attractive, there are predicted to be arbitrarily large hypernuclei with high strangeness and small net charge, including species with no nucleons. [[nuclear binding energy|Binding energy]] per baryon in multi-strange hypernuclei can reach up to 21&nbsp;MeV/''A'' under certain conditions,<ref name=jpg08/> compared to 8.80&nbsp;MeV/''A'' for the ordinary nucleus [[Nickel-62|<sup>62</sup>Ni]].<ref>{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html |website=hyperphysics.phy-astr.gsu.edu |title=The Most Tightly Bound Nuclei |accessdate=October 23, 2019 }}</ref> Additionally, formation of Ξ baryons should quickly become energetically favorable, unlike when there are no Λ's, because the exchange of strangeness with a nucleon would be impossible due to the Pauli exclusion principle.{{sfn|Gal|Hungerford|Millener|2016|p=43}}