Revised rates for the stellar triple-α process from measurement of 12C nuclear resonances

α-Clustering in atomic nuclei from first principles with statistical learning and the Hoyle state character

A long-standing crucial question with atomic nuclei is whether or not α clustering occurs there. An α particle (helium-4 nucleus) comprises two protons and two neutrons, and may be the building block of some nuclei. This is a very beautiful and fascinating idea, and is indeed plausible because the α particle is particularly stable with a large binding energy. However, direct experimental evidence has never been provided. Here, we show whether and how α(-like) objects emerge in atomic nuclei, by means of state-of-the-art quantum many-body simulations formulated from first principles, utilizing supercomputers including K/Fugaku. The obtained physical quantities exhibit agreement with experimental data. The appearance and variation of the α clustering are shown by utilizing density profiles for the nuclei beryllium-8, -10 and carbon-12. With additional insight by statistical learning, an unexpected crossover picture is presented for the Hoyle state, a critical gateway to the birth of life.

Alpha particles are considered the building blocks for some nuclei in alpha-clustering. Here the authors discuss quantum many-body simulations with nucleon-nucleon interaction to characterize the Hoyle state, the first excited 0+ state of the 12C nucleus, and find complexity in its alpha-clustering.

Formation of α clusters in dilute neutron-rich matter

The surface of neutron-rich heavy nuclei, with a neutron skin created by excess neutrons, provides an important terrestrial model system to study dilute neutron-rich matter. By using quasi-free α cluster-knockout reactions, we obtained direct experimental evidence for the formation of α clusters at the surface of neutron-rich tin isotopes. The observed monotonous decrease of the reaction cross sections with increasing mass number, in excellent agreement with the theoretical prediction, implies a tight interplay between α-cluster formation and the neutron skin. This result, in turn, calls for a revision of the correlation between the neutron-skin thickness and the density dependence of the symmetry energy, which is essential for understanding neutron stars. Our result also provides a natural explanation for the origin of α particles in α decay.

Enhanced triple-α reaction reduces proton-rich nucleosynthesis in supernovae

The rate of the triple-α reaction that forms ¹²C affects^1,2 the synthesis of heavy elements in the Ga-Cd range in proton-rich neutrino-driven outflows of core-collapse supernovae^3-5. Initially, these outflows contain only protons and neutrons; these later combine to form α particles, then ¹²C nuclei via the triple-α reaction, and eventually heavier nuclei as the material expands and cools. Previous experimental work^6,7 demonstrated that despite the high temperatures encountered in these environments, the reaction is dominated by the well characterized Hoyle state resonance in ¹²C nuclei. At sufficiently high nucleon densities, however, proton- and neutron-scattering processes may alter the effective width of the Hoyle state^8,9. This raises the questions of what the reaction rate in supernova outflows is, and how changes affect nucleosynthesis predictions. Here we report that in proton-rich core-collapse supernova outflows, these hitherto neglected processes enhance the triple-α reaction rate by up to an order of magnitude. The larger reaction rate suppresses the production of heavy proton-rich isotopes that are formed by the νp process^3-5 (where ν is the neutrino and p is the proton) in the innermost ejected material of supernovae^10-13. Previous work on the rate enhancement mechanism⁹ did not anticipate the importance of this enhancement for proton-rich nucleosynthesis. Because the in-medium contribution to the triple-α reaction rate must be present at high densities, this effect needs to be included in supernova nucleosynthesis models. This enhancement also differs from earlier sensitivity studies that explored variations of the unenhanced rate by a constant factor^1,2, because the enhancement depends on the evolving thermodynamic conditions. The resulting suppression of heavy-element nucleosynthesis for realistic conditions casts doubt on the νp process being the explanation for the anomalously high abundances of ^92,94Mo and ^96,98Ru isotopes in the Solar System^1,3,14 and for the signatures of early Universe element synthesis in the Ga-Cd range found in the spectra of ancient metal-poor stars^15-20.

Revisiting the bound on axion-photon coupling from globular clusters

We derive a strong bound on the axion-photon coupling g(aγ) from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the R parameter, i.e., the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard, we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for g(aγ). We show that the estimated value of g(aγ) substantially depends on the adopted He mass fraction Y, an effect often neglected in previous investigations. Taking as a benchmark for our study the most recent determinations of the He abundance in H ii regions with O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound g(aγ)<0.66×10(-10)  GeV(-1) at 95% confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.

Signatures of the Z = 82 shell closure in α-decay process

In recent experiments at the velocity filter Separator for Heavy Ion reaction Products (SHIP) (GSI, Darmstadt), an extended and improved set of α-decay data for more than 20 of the most neutron-deficient isotopes in the region from lead to thorium was obtained. The combined analysis of this newly available α-decay data, of which the (186)Po decay is reported here, allowed us for the first time to clearly show that crossing the Z = 82 shell to higher proton numbers strongly accelerates the α decay. From the experimental data, the α-particle formation probabilities are deduced following the Universal Decay Law approach. The formation probabilities are discussed in the framework of the pairing force acting among the protons and the neutrons forming the α particle. A striking resemblance between the phenomenological pairing gap deduced from experimental binding energies and the formation probabilities is noted. These findings support the conjecture that both the N = 126 and Z = 82 shell closures strongly influence the α-formation probability.

Low-temperature triple-alpha rate in a full three-body nuclear model

A new three-body method is used to compute the rate of the triple-alpha capture reaction, which is the primary source of 12C in stars. In this Letter, we combine the Faddeev hyperspherical harmonics and the R-matrix method to obtain a full solution to the three-body α+α+α continuum. Particular attention is paid to the long-range effects caused by the pairwise Coulomb interactions. The new rate agrees with the Nuclear Astrophysics Compilation of Reaction rates for temperatures greater than 0.07 GK, but a large enhancement at lower temperature is found (≈10(12) at 0.02 GK). Our results are compared to previous calculations where additional approximations were made. We show that the new rate does not significantly change the evolution of stars around one solar mass. In particular, such stars still undergo a red-giant phase consistent with observations, and no significant differences are found in the final white dwarfs.

Improved limit on direct α decay of the Hoyle state

The current evaluation of the triple-α reaction rate assumes that the α decay of the 7.65 MeV, 0+ state in 12C, commonly known as the Hoyle state, proceeds sequentially via the ground state of 8Be. This assumption is challenged by the recent identification of two direct α-decay branches with a combined branching ratio of 17(5)%. If correct, this would imply a corresponding reduction in the triple-α reaction rate with important astrophysical consequences. We have used the 11B(3He,d) reaction to populate the Hoyle state and measured the decay to three α particles in complete kinematics. We find no evidence for direct α-decay branches, and hence our data do not support a revision of the triple-α reaction rate. We obtain an upper limit of 5×10(-3) on the direct α decay of the Hoyle state at 95% C.L., which is 1 order of magnitude better than a previous upper limit.

Pair decay width of the Hoyle state and its role for stellar carbon production

The pair decay width of the first excited 0+ state in 12C (the Hoyle state) is deduced from a novel analysis of the world data on inelastic electron scattering covering a wide momentum transfer range, thereby resolving previous discrepancies. The extracted value Γπ=(62.3±2.0) μeV is independently confirmed by new data at low momentum transfers measured at the S-DALINAC and reduces the uncertainty of the literature values by more than a factor of 3. A precise knowledge of Γπ is mandatory for quantitative studies of some key issues in the modeling of supernovae and of asymptotic giant branch stars, the most likely site of the slow-neutron nucleosynthesis process.