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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.

The electric dipole strength distribution in 120 Sn between 5 and 22 MeV has been determined at RCNP Osaka from polarization transfer observables measured in proton inelastic scattering at E0 = 295 MeV and forward angles including 0 • . Combined with photoabsorption data a highly precise electric dipole polarizability αD( 120 Sn) = 8.93(36) fm 3 is extracted. The dipole polarizability as isovector observable par excellence carries direct information on the nuclear symmetry energy and its density dependence. The correlation of the new value with the well established αD( 208 Pb) serves as a test of its prediction by nuclear energy density functionals (EDFs). Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.PACS numbers: 21.10. Ky, 25.40.Ep, 21.60.Jz, 27.60.+j The nuclear equation of state (EOS) describing the energy of nuclear matter as function of its density has wide impact on nuclear physics and astrophysics [1] as well as physics beyond the standard model [2,3]. The EOS of symmetric nuclear matter with equal proton and neutron densities is well constrained from the ground state properties of finite nuclei, especially in the region of saturation density ρ 0 ≃ 0.16 fm −3 [4]. However, the description of astrophysical systems as, e.g., neutron stars requires knowledge of the EoS for asymmetric matter [5][6][7][8] which is related to the leading isovector parameters of nuclear matter, viz. the symmetry energy (J) and its derivative with respect to density (L) [9]. For a recent overview of experimental and theoretical studies of the symmetry energy see Ref. [10]. In spite of steady extension of knowledge on exotic nuclei, just these isovector properties are poorly determined by fits to experimental ground state data because the valley of nuclear stability is still extremely narrow along isotopic chains [11][12][13]. Thus one needs observables in finite nuclei specifically sensitive to isovector properties to better confine J and L. There are two such observables, the neutron skin r skin in nuclei with large neutron excess and the (static) dipole polarizability α D .The neutron skin thickness r skin = r n − r p defined as the difference of the neutron and proton root-meansquare radii r n,p is determined by the interplay between the surface tension and the pressure of excess neutrons on the core described by L [14,15]. Studies within nuclear density-funtional theory [16] show for all EDFs a strong correlation between r skin and the isovector symmetry energy parameters [17][18][19]. The most studied case so far is 208 Pb, where r skin has been derived from coherent photoproduction of π 0 mesons [20], antiproton annihilation [21,22], proton elastic scattering at 650 MeV [23] and 295 MeV [24], and from the dipole polarizability [25]. A nearly model-independent determination of the neutron skin is possible by measuring the weak form factor of nuclei with parity-violating elastic electron scattering [26]. Such an experiment has b...

The gamma strength function and level density of 1^{-} states in ^{96}Mo have been extracted from a high-resolution study of the (p[over →], p[over →]^{'}) reaction at 295 MeV and extreme forward angles. By comparison with compound nucleus γ decay experiments, this allows a test of the generalized Brink-Axel hypothesis in the energy region of the pygmy dipole resonance. The Brink-Axel hypothesis is commonly assumed in astrophysical reaction network calculations and states that the gamma strength function in nuclei is independent of the structure of the initial and final state. The present results validate the Brink-Axel hypothesis for ^{96}Mo and provide independent confirmation of the methods used to separate gamma strength function and level density in γ decay experiments.

Precision data are presented for the break-up reaction, 2 H( p, pp)n, within the framework of nuclear-force studies. The experiment was carried out at KVI using a polarized-proton beam of 190 MeV impinging on a liquid-deuterium target and by exploiting the detector, BINA. Some of the vector-analyzing powers are presented and compared with state-of-the-art Faddeev calculations including three-nucleon forces effect. Significant discrepancies between the data and theoretical predictions were observed for kinematical configurations which correspond to the 2 H( p, 2 He)n channel. These results are compared to the 2 H( p, d)p reaction to test the isospin sensitivity of the present three-nucleon force models. The current modeling of two and three-nucleon forces is not sufficient to describe consistently polarization data for both isospin states. Understanding the exact nature of the nuclear force is one of the long-standing questions in nuclear physics. In 1935, Yukawa successfully described the pair-wise nucleon-nucleon (NN) interaction as an exchange of a boson [1]. Current NN models are mainly based on Yukawa's idea and provide an excellent description of the high-quality database of proton-proton and neutron-proton scattering [2] and of the properties of the deuteron. However, for the simplest three-nucleon system, triton, three-body calculations employing NN forces clearly underestimate the experimental binding energies [3], demonstrating that NN forces are not sufficient to describe the three-nucleon system accurately. Some of the discrepancies between experimental data and calculations solely based on the NN interaction can be resolved by introducing an additional three-nucleon force (3NF). Most of the current models for the 3NF are based on a refined version of Fujita-Miyazawa's 3NF model [4], in which a 2π-exchange mechanism is incorporated by an intermediate ∆ excitation of one of the nucleons [5,6].The structure of the 3NF can be studied via a measurement of observables in three-nucleon scattering processes. More detailed information on the spin dependence of the 3NF can be obtained by measuring polarization observables such as the analyzing powers. For this, a series of * Electronic address: messchendorp@kvi.nl extensive studies of 3NF effects in elastic-scattering reactions have been performed at KVI and other laboratories. Precision measurements of the vector analyzing power of the proton in elastic proton-deuteron scattering have been performed at various beam energies ranging from 90 to 250 MeV [7,8,9,10,11]. Also, vector and tensor analyzing powers in elastic deuteron-proton scattering have been obtained at various beam energies ranging from 75 to 270 MeV [12,13,14,15,16,17]. In these measurements, systematic discrepancies between data and theoretical predictions which rigorously solve the Faddeev equations and using only NN potentials were observed. A large part of the discrepancies were removed by adding a 3NF to the NN potentials. Nevertheless, there are still unresolved problems specially at higher...