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1. The $$^{12}$$C$$(\alpha ,\gamma )^{16}$$O reaction, in the laboratory and in the stars

   https://doi.org/10.1140/epja/s10050-025-01537-1  

   de_Boer, R J; Best, A; Brune, C R; Chieffi, A; Hebborn, C; Imbriani, G; Liu, W P; Shen, Y P; Timmes, F X; Wiescher, M (April 2025, The European Physical Journal A)

   Abstract The evolutionary path of massive stars begins at helium burning. Energy production for this phase of stellar evolution is dominated by the reaction path 3$$\alpha \rightarrow ^{12}$$ $\alpha {\to }^{12}$ C$$(\alpha ,\gamma )^{16}$$ ${(\alpha ,\gamma )}^{16}$ O and also determines the ratio of$$^{12}$$ $_{}^{12}$ C/$$^{16}$$ $_{}^{16}$ O in the stellar core. This ratio then sets the evolutionary trajectory as the star evolves towards a white dwarf, neutron star or black hole. Although the reaction rate of the 3$$\alpha $$ $\alpha$ process is relatively well known, since it proceeds mainly through a single narrow resonance in$$^{12}$$ $_{}^{12}$ C, that of the$$^{12}$$ $_{}^{12}$ C$$(\alpha ,\gamma )^{16}$$ ${(\alpha ,\gamma )}^{16}$ O reaction remains uncertain since it is the result of a more difficult to pin down, slowly-varying, portion of the cross section over a strong interference region between the high-energy tails of subthreshold resonances, the low-energy tails of higher-energy broad resonances and direct capture. Experimental measurements of this cross section require herculean efforts, since even at higher energies the cross section remains small and large background sources are often present that require the use of very sensitive experimental methods. Since the$$^{12}$$ $_{}^{12}$ C$$(\alpha ,\gamma )^{16}$$ ${(\alpha ,\gamma )}^{16}$ O reaction has such a strong influence on many different stellar objects, it is also interesting to try to back calculate the required rate needed to match astrophysical observations. This has become increasingly tempting, as the accuracy and precision of observational data has been steadily improving. Yet, the pitfall to this approach lies in the intermediary steps of modeling, where other uncertainties needed to model a star’s internal behavior remain highly uncertain. 

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2. Modular apparatus for nuclear reactions spectroscopy (MARS): characterization and first application to determine $$^{12}$$C($$^6$$Li,$$^4$$He)$$^{14}\hbox {N}^{g.s}$$ nuclear reaction cross sections

   https://doi.org/10.1140/epjp/s13360-025-06305-0  

   Garrido-Gómez, L; Vegas-Díaz, A; Alvarez, M_A G; Fernández-García, J P; Fernández, B; Ferrer, F J; Lopez-Aires, D (May 2025, The European Physical Journal Plus)

   Abstract This work presents MARS (Modular apparatus for nuclear reactions spectroscopy) and its characterization prior to its first application to measure$$^6$$ $_{}^6$ Li+$$^{12}$$ $_{}^{12}$ C nuclear reactions. Measurements were performed at the 3 MV tandem accelerator of the CNA (National Accelerator Center), in Seville, Spain. The$$^{6}$$ $_{}^6$ Li projectiles were accelerated at energies around the$$^6$$ $_{}^6$ Li+$$^{12}$$ $_{}^{12}$ C Coulomb barrier ($$V^{\text {cm}}_{B}\sim 3.0$$ $V_B^{\text{cm}}\sim 3.0$ MeV - center of mass and$$V^{\text {lab}}_{B}\sim 4.5$$ $V_B^{\text{lab}}\sim 4.5$ MeV - laboratory frame). Using a$$^{6}\hbox {Li}^{2+}$$ $_{}^6{\text{Li}}^{2+}$ beam, we measured at 13 laboratory energies from 4.00 to 7.75 MeV. Thus, we present the excitation function of$$^{12}$$ $_{}^{12}$ C($$^6$$ $_{}^6$ Li,$$^4$$ $_{}^4$ He)$$^{14}\hbox {N}^{g.s.}$$ $_{}^{14}{\text{N}}^{g.s.}$ reaction, at 2 backward angles ($$110.0^\circ $$ $110.0^{\circ }$ and$$140.0^\circ $$ $140.0^{\circ }$ ). The projectile dissociation, leading to this reaction, increases with the bombarding energies around the Coulomb barrier. This dissociation is favored at an optimum energy$$E_{b}^{\text {op}}$$ $E_b^{\text{op}}$ $$\ge $$ $\ge$ $$V_{B}$$ $V_B$ +$$|Q_{bu}|$$ $|Q_{\mathrm{bu}}|$ , where$$V_{B}$$ $V_B$ is the Coulomb barrier of the system, and$$|Q_{bu}|$$ $|Q_{\mathrm{bu}}|$ is the module ofQ-value for the$$^6$$ $_{}^6$ Li dissociation into$$^4$$ $_{}^4$ He+$$^2$$ $_{}^2$ H. This result corroborates a systematic analysis of weakly bound projectiles reacting on several targets [1]. 

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3. Simulation of the background from ¹³C(α,n)¹⁶O reaction in the JUNO scintillator

   https://doi.org/10.1140/epjc/s10052-025-14333-4  

   Adam, Thomas; Adamowicz, Kai; Ahmad, Shakeel; Ahmed, Rizwan; Aiello, Sebastiano; An, Fengpeng; Andreopoulos, Costas; Andronico, Giuseppe; Anfimov, Nikolay; Antonelli, Vito; et al (September 2025, The European Physical Journal C)

   Abstract Large-scale organic liquid scintillator detectors are highly efficient in the detection of MeV-scale electron antineutrinos. These signal events can be detected through inverse beta decay on protons, which produce a positron accompanied by a neutron. A noteworthy background for antineutrinos coming from nuclear power reactors and from the depths of the Earth (geoneutrinos) is generated by ($$\alpha ,\,n$$ $\alpha ,n$ ) reactions. In organic liquid scintillator detectors,$$\alpha $$ $\alpha$ particles emitted from intrinsic contaminants such as$$^{238}$$ $_{}^{238}$ U,$$^{232}$$ $_{}^{232}$ Th, and$$^{210}$$ $_{}^{210}$ Pb/$$^{210}$$ $_{}^{210}$ Po, can be captured on$$^{13}$$ $_{}^{13}$ C nuclei, followed by the emission of a MeV-scale neutron. Three distinct interaction mechanisms can produce prompt energy depositions preceding the delayed neutron capture, leading to a pair of events correlated in space and time within the detector. Thus, ($$\alpha ,\,n$$ $\alpha ,n$ ) reactions represent an indistinguishable background in liquid scintillator-based antineutrino detectors, where their expected rate and energy spectrum are typically evaluated via Monte Carlo simulations. This work presents results from the open-source SaG4n software, used to calculate the expected energy depositions from the neutron and any associated de-excitation products. Also simulated is a detailed detector response to these interactions, using a dedicated Geant4-based simulation software from the JUNO experiment. An expected measurable$$^{13}$$ $_{}^{13}$ C$$(\alpha ,\,n)^{16}$$ ${(\alpha ,n)}^{16}$ O event rate and reconstructed prompt energy spectrum with associated uncertainties, are presented in the context of JUNO, however, the methods and results are applicable and relevant to other organic liquid scintillator neutrino detectors. 

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4. The two-photon detachment of the positronium negative ion

   https://doi.org/10.1140/epjd/s10053-026-01156-5  

   Mitchell, William; Ward, Sandra J (May 2026, The European Physical Journal D)

   AbstractWe present our calculations of the two-photon detachment cross section of the positronium negative ion (Ps$$^-$$ $_{}^{-}$ ), in both the length and velocity forms for energies below the Ps($$n=2$$ $n=2$ ) threshold. We consider both the$$^1$$ $_{}^1$ S and$$^1$$ $_{}^1$ D final states. We employ the Rayleigh–Ritz method to determine the Ps$$^-$$ $_{}^{-}$ binding energy and the corresponding$$^1$$ $_{}^1$ S bound-state wave function. We use the complex Kohn variational method to obtain the$$^1$$ $_{}^1$ S and$$^1$$ $_{}^1$ D final-state wavefunctions, and we solve an inhomogeneous differential equation to obtain the$$^1$$ $_{}^1$ P intermediate-state wave function. The$$^1$$ $_{}^1$ S bound-state,$$^1$$ $_{}^1$ P intermediate-state, and final-state wave functions contain Hylleraas functions. We investigate the applicability of the Wigner’s and a modified Wigner’s threshold laws to the one- and two-photon detachment cross sections of Ps$$^-$$ $_{}^{-}$ near the ionization threshold. Graphic abstract 

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5. Lifetimes of excited states in $$^{16}$$C as a benchmark for ab initio developments

   https://doi.org/10.1140/epja/s10050-024-01396-2  

   Mathy, M; Petri, M; Roth, R; Wagner, L; Heil, S; Ayangeakaa, A D; Bottoni, S; Carpenter, M P; Crawford, H L; Fallon, P; et al (September 2024, The European Physical Journal A)

   Abstract Lifetimes of higher-lying states ($$2_2^+$$ $2_2^{+}$ and$$4_1^+$$ $4_1^{+}$ ) in$$^{16}$$ $_{}^{16}$ C have been measured, employing the Gammasphere and Microball detector arrays, as key observables to test and refine ab initio calculations based on interactions developed within chiral Effective Field Theory. The presented experimental constraints to these lifetimes of$$\tau ({2_2^+}) = [\,244, 446]\,~\textrm{fs}$$ $\tau \left(2_2^{+}\right)=[244,446]\text{fs}$ and$$\tau ({4_1^+}) = [\,1.8, 4]\,~\textrm{ps}$$ $\tau \left(4_1^{+}\right)=[1.8,4]\text{ps}$ , combined with previous results on the lifetime of the$$2_1^+$$ $2_1^{+}$ state of$$^{16}$$ $_{}^{16}$ C, provide a rather complete set of key observables to benchmark the theoretical developments. We present No-Core Shell-Model calculations using state-of-the-art chiral 2- (NN) and 3-nucleon (3N) interactions at next-to-next-to-next-to-leading order for both the NN and the 3N contributions and a generalized natural-orbital basis (instead of the conventional harmonic-oscillator single-particle basis) which reproduce, for the first time, the experimental findings remarkably well. The level of agreement of the new calculations as compared to the CD-Bonn meson-exchange NN interaction is notable and presents a critical benchmark for theory. 

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