Wed·Jan 22, 2020Wednesday·January 22, 2020

1. 01*

   A stringent upper limit on the direct 3$\alpha$ decay of the Hoyle State in ${}^{12}$C

   R. Smith🇬🇧 · M. Gai🇺🇸 · M. W. Ahmed🇺🇸 · M. Freer🇬🇧 · H. O. U. Fynbo🇩🇰 · D. Schweitzer🇩🇪 · S. R. Stern🇺🇸

   We investigate an implication of the most recent observation of a second $J^{\pi }=2^{+}$ state in ${}^{12}$C, which was measured using the ${}^{12}$C($\gamma$,$\alpha$)${}^8$Be${}_{\text{(g.s.)}}$ reaction. In addition to the dissociation of ${}^{12}$C to an $\alpha$-particle and ${}^8$Be in its ground state, a small fraction of events (2%) were identified as direct decays and decays to excited states in ${}^8$Be. This allowed a limit on the direct 3$\alpha$ partial decay width to be determined as ${\Gamma }_{3\alpha }<32(4)$ keV. Since this 2${}^{+}$ state is predicted by all theoretical models to be a collective excitation of the Hoyle state, the 3$\alpha$ partial width of the Hoyle state is calculable from the ratio of 3$\alpha$ decay penetrabilities of the Hoyle and 2${}^{+}$ states. This was calculated using the semi-classical WKB approach and we deduce a stringent upper limit for the direct decay branching ratio of the Hoyle state of $\frac{{\Gamma }_{3\alpha }}{\Gamma }<5.7\times 10^{-6}$, over an order of magnitude lower than previously reported. This result places the direct measurement of this rare decay mode beyond current experimental capabilities.

   nucl-exnucl-thPRC(2020)·28 citations
2. 02*

   Probing few-body nuclear dynamics via 3H and 3He (e,e'p)pn cross-section measurements

   R. Cruz-Torres · D. Nguyen · F. Hauenstein · A. Schmidt · S. Li · D. Abrams · H. Albataineh · S. Alsalmi · D. Androic · K. Aniol · W. Armstrong · J. Arrington and 116 other authors

   We report the first measurement of the \eep three-body breakup reaction cross sections in helium-3 (${}^3$He) and tritium (${}^3$H) at large momentum transfer ($⟨Q^2⟩\approx 1.9$ (GeV/c)${}^2$) and $x_B>1$ kinematics, where the cross section should be sensitive to quasielastic (QE) scattering from single nucleons. The data cover missing momenta $40\le p_{miss}\le 500$ MeV/c that, in the QE limit with no rescattering, equals the initial momentum of the probed nucleon. The measured cross sections are compared with state-of-the-art ab-initio calculations. Overall good agreement, within $\pm 20\%$, is observed between data and calculations for the full $p_{miss}$ range for ${}^3$H and for $100\le p_{miss}\le 350$ MeV/c for ${}^3$He. Including the effects of rescattering of the outgoing nucleon improves agreement with the data at $p_{miss}>250$ MeV/c and suggests contributions from charge-exchange (SCX) rescattering. The isoscalar sum of ${}^3$He plus ${}^3$H, which is largely insensitive to SCX, is described by calculations to within the accuracy of the data over the entire $p_{miss}$ range. This validates current models of the ground state of the three-nucleon system up to very high initial nucleon momenta of $500$ MeV/c.

   nucl-exnucl-thPRL(2020)·29 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.