Nuclear clustering is one of the most puzzling phenomena in subatomic physics. Numerous examples of such structures include the ground state of the ¹¹Li nucleus with a halo of two neutrons or the famous Hoyle resonance at ¹²C, which plays a vital role in the synthesis of heavier elements in stars. The widespread presence of narrow resonances near the particle emission threshold suggests that this is a universal phenomenon in open quantum systems in which bound and unbound states strongly mix, resulting in the appearance of a collective state with the features of a nearby decay channel.  A new spectacular example of this phenomenon is the β^– delayed proton decay of the neutron halo ground state of ¹¹Be. Studies within a shell model embedded in the continuum (SMEC) suggest the existence of a J^𝜋 = 1/2⁺ collective resonance in ¹¹B, carrying many characteristics of a nearby proton-decay channel, which explains this puzzling decay. The proximity of proton and tritium emission thresholds suggests that this resonance may also contain an admixture of the ³H cluster configuration To clarify the nature of this hypothetical 1/2⁺ resonance, study of ¹⁰Be(p,p)¹⁰Be reaction will be needed.

The narrow 5/2⁺ resonance in ¹¹B at 11.600(20) MeV, which lies slightly above the neutron emission threshold and breaks down by the emission of the neutron or α particle, has a crucial effect   on the huge value of the ¹⁰B neutron capture cross-section. This suggests that the wave function of this resonance is strongly modified by the coupling to a nearby neutron emission channel. Indeed, in the SMEC calculations, there is a 5/2₆⁺ state near the neutron emission threshold, which strongly couples in L=2 partial wave to the channel [¹⁰B(3⁺)+n]^5/2+. The theoretically determined maximal collectivization for this state is found ~110 keV above the neutron emission threshold and close to the experimental energy of the 5/2⁺ state. In the future, to clarify the impact of the virtual neutron state on the ¹⁰B(n,𝛾)¹¹B reaction cross section, studies of the reaction ¹⁰Be(d,p)¹¹B will be needed.

J.Okołowicz, M. Płoszajczak, W. Nazarewicz

Convenient location of a near-threshold proton-emitting resonance in 11Be

Physical Review Letters 124, 042502 (2020)

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.042502

Few-nucleon systems are studied at energies below the pion production threshold with the purpose of  precise testing the state-of-the-art nuclear interaction potentials. In the case of three-nucleon system, theoretical calculations predict significant effects of dynamics beyond the pairwise interaction between nucleons: a so-called three-nucleon force (3 Nucleon Force, 3NF). Experiments conducted with the use of large acceptance (nearly 4π) detection systems provide data for the deuteron breakup reaction in collision with a proton in various kinematic configurations of the final state. A reach set of data for differential cross sections is a basis for determining the effect of 3NF, testing approaches to include Coulomb repulsion between protons into theoretical calculations and tracing relativistic effects. Measurements of this reaction using the WASA@COSY detector were carried out in the upper range of deuteron beam energies of interest, 150-200 MeV/nucleon. The first set of results shows that even at such high energies cross-section for configurations in which protons fly “close together” (with a small relative momentum) is dominated by Coulomb repulsion between protons. In other kinematic regions, we observe the effects of 3NF, but there are also cases where all the theoretical predictions, regardless of the model, underestimate the experimental data. Relativistic calculations, so far not including 3NF in the potential, do not improve the description either. The data measured with BINA@KVI detector at the beam energy of 80 MeV/nucleon, indicate discrepancies of similar nature and in similar phase space regions, while previous measurements at the beam energy of 65 MeV/nucleon were very well described by calculations taking into account both the 3NF and the Coulomb repulsion. The source of this discrepancy remains a puzzle: are the current 3NF models imperfect or does the difference stem from neglecting relativistic effects? The progress of theoretical calculations and the continuation of research in the intermediate energy range using the BINA detector at CCB can help in solving the problem.

B.Kłos (M. Berłowski, I.Ciepał, E.Czerwiński, L.Jarczyk, B. Kamys,  St.Kistryn, W.Krzemień, P.Kulessa, A.Kupść, A.Magiera, P.Moskal, W.Parol, D.Pszczel, K.Pysz, M.Skurzok, J.Smyrski, J.Stepaniak, E.Stephan, A.Szczurek, A.Trzciński, A.Wrońska, J.Zabierowski, M.J.Zieliński, P.Żuprański, J.Golak, A.Kozela R.Skibiński, I.Skwira-Chalot, A.Wilczek, H.Witała),  WASA@COSY collaboration at al.

Three-nucleon dynamics in dp breakup collisions using the WASA detector at COSY-Jülich

Physical Review C 101, 044001 (2020)

https://doi.org/10.1103/PhysRevC.101.044001

W.Parol, A.Kozela( K.Bodek, J.Golak, St.Kistryn, B.Kłos, J.Kuboś, P. Kulessa, A. Łobejko, A.Magiera, R.Skibiński, I.Skwira-Chalot, E.Stephan, D.Rozpędzik, A.Wilczek, H.Witała, B.Włoch, A.Wrońska, J.Zejma) et al.

Measurement of differential cross sections for deuteron-proton breakup reaction at 160 MeV

arXiv:2004.02651