Hypernuclei and strange-baryon interactions

4 problems

• Charge-symmetry breaking in the mass-4 mirror hypernuclei

  Open

  The hypernuclei $4_{\Lambda}\,\mathrm{H}$ and $4_{\Lambda}\,\mathrm{He}$ differ only by swapping one proton for a neutron, so apart from small electric effects the $\Lambda$ should be bound equally in both. The measured difference is much larger than in ordinary mirror nuclei and changes sign between the ground and excited states, and theory does not yet reproduce it reliably.

• $\Lambda$ separation energy of the hypertriton

  Contested

  The hypertriton, a $\Lambda$ hyperon bound to a deuteron, is the lightest hypernucleus and the anchor of fits to hyperon-nucleon forces. Measurements of how strongly the $\Lambda$ is held differ by a factor of about four.

• Density dependence of the $\Lambda$ potential in nuclear matter

  Open

  Binding energies of $\Lambda$ hyperons in heavy hypernuclei show that a $\Lambda$ feels an attraction of about $30\,\mathrm{MeV}$ inside ordinary nuclear matter. Whether this attraction turns into repulsion at two to three times nuclear density decides whether Lambdas appear in neutron stars.

• Two-pole structure of the $\Lambda(1405)$

  Partially resolved

  The $\Lambda(1405)$ is a strange baryon that lies just below the energy needed to make an antikaon and a nucleon, and is often described as a bound state of the two. Theory based on chiral symmetry (an approximate symmetry of QCD with nearly massless light quarks) predicts that it is really two nearby states, and whether the second, broader one exists is debated.