Editing Hypernucleus (section)

=== 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}}