A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. E1-and E2-S of 12 alpha,gamma 0 16 O from gamma-ray angular distributions with a 4pi detector array

Single nucleon pickup reactions were performed with a 18:1 MeV=nucleon 14 O beam on a deuterium target. Within the coupled reaction channel framework, the measured cross sections were compared to theoretical predictions and analyzed using both phenomenological and microscopic overlap functions. The missing strength due to correlations does not show significant dependence on the nucleon separation energy asymmetry over a wide range of 37 MeV, in contrast with nucleon removal data analyzed within the sudden-eikonal formalism. DOI: 10.1103/PhysRevLett.110.122503 PACS numbers: 24.50.+g The existence of single-particle-like modes in nuclei, near the Fermi surface, is particularly important because these are at the basis of the nuclear shell model and thus govern the low energy nuclear dynamics. Yet, they result from nontrivial many-body correlations, which affect energy ordering and filling of active orbits. Spectroscopic factors (SFs) are a unique tool to address the question of correlations as they are strictly linked to the notion of shell occupancies and can be probed using direct reaction cross section measurements [1,2]. Information for stable nuclei was formerly provided by the electromagnetic probe (e, e 0 p) [3][4][5]. Even for closed shell nuclei like 16 O or 208 Pb, a cross section reduction by 30%-40% relative to an independent-particle-based model was observed. Different origins are now well established, like short range correlations [1] and couplings to collective modes at high excitation energy [6] or to the continuum [7]. Single nucleon pickup reactions were also used for stable nuclei yielding results consistent with (e, e 0 p) measurements [8,9].For nuclei away from the valley of stability, new approaches have been developed in inverse kinematics at various incident energies, knockout and transfer reactions. From knockout reactions at intermediate energy, a reduction factor R s was deduced as the ratio between the experimental cross section and a theoretical value obtained in a sudden-eikonal approach [10]. A strong dependence was claimed for R s versus the asymmetry (difference in separation energy) ÁS ¼ ðS p À S n Þ with ¼ þ1 (À1) for proton (neutron) removal reactions, with a reduction as high as 70% for large positive ÁS values. This reduction is still not understood and was first accounted for by possible missing correlations in shell-model calculations [10]. Different conclusions were drawn from (i) the possibility of dissipative processes beyond the sudden approximation [11,12], and (ii) transfer reactions at lower incident energies showing no ÁS dependence of R s [13]. From a theoretical point of view, ab initio calculations suggest only a mild dependence of SFs on ÁS [7,14], with equal SFs found for the nucleon removals from 56 Ni [6] despite significant ÁS values (AE 9:5 MeV). Coupled-cluster calculations [7] pointed out a further decrease of proton SFs for isotopes at the neutron dripline, due to coupling to the continuum. This has the substantial effect of enhancing the dependence on...