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Hadrons and Nuclei

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EPJ A Highlight - A new generation of chiral nuclear forces

Neutron-proton differential cross section at Elab=143 MeV up to fourth order in the chiral expansion.

Chiral effective field theory provides a systematically improvable perturbative approach to deriving nuclear forces in harmony with the symmetries of Quantum Chromodynamics. Combined with modern few- and many-body methods, this framework represents a commonly accepted procedure for ab initio studies of nuclear structure and reactions.

In this paper, the authors introduce a new generation of nucleon-nucleon forces up to fourth order in the chiral expansion. By employing an appropriate regularization in coordinate space, which maintains the analytic structure of the amplitude, the authors succeed in significantly reducing the amount of finite-cutoff artefacts. In addition, a simple approach to estimating the theoretical uncertainty in few- and many-nucleon calculations from the truncation of the chiral expansion is formulated. By calculating various two-nucleon scattering and bound-state observables, the authors verify that the results at different chiral orders and for different values of the regulator are indeed consistent with each other and with the experimental data.

The new generation of chiral nuclear forces is expected to provide an excellent starting point for applications in nuclear physics.

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EPJ A Highlight - Pionic-Hydrogen Atom and Quantum Chromodynamics

Spectrum of the simultaneous measurement of the H(3p-1s) and the 16O(6h-5g) transitions (top). Energy shift of the ground state at various H2 density (solid diamonds), the open diamond represents the previous experiment when using the latest value of the electromagnetic transition energy.

Analogous to the vast amount of knowledge acquired on the electronic hydrogen atom over the last century and the success of Quantum Electrodynamics (QED), hadronic physics is using a similar system, namely “pionic hydrogen” - a hydrogen atom where the electron is replaced by a negatively charged pion - as a laboratory for investigating Quantum Chromodynamics (QCD). Like in electronic hydrogen the finite size of the proton plays a role in the precise determination of the ground state of the atom. The smaller Bohr radius of the pion offers a larger sensitivity to the strong interaction between the pion and the proton, leading, e.g., to an energy shift compared to the ground state energy if only the electromagnetic interaction is considered. The precise determination of this shift provides a benchmark of our understanding of the pion-proton strong interaction from basic principles in QCD. To this end an exquisite experiment was devised and performed at the high intensity, low energy pion beam at the Paul Scherrer Institut using a cyclotron trap and an ultimate resolution Bragg spectrometer leading to an impressive four fold improvement compared to the previous best measurement as shown in Fig. 1.

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EPJ A Highlight - Subtracted dispersion relation estimate of two-photon exchange

Subtracted DR prediction for the elastic cross section. The subtraction constant, denoted by ε0, is fitted to data (blue band from the A1@MAMI Coll.).

Elastic electron-proton scatterings (with one-photon exchange) have always provided fundamental information on general properties of the proton. Recently, two experimental approaches, with and without polarized protons, gave strikingly different results for the electric over magnetic proton form factor ratio. Similarly, a mysterious discrepancy (“the proton radius puzzle”) has been observed in the measurement of the proton charge radius in different experiments, one of which is electron-proton scattering. Two-photon exchange (TPE) contributions have been proposed as a plausible solution to resolve the puzzles, but their estimates have strong model dependences. A quantitative understanding of TPE effects, based on general principles and avoiding model dependences, is necessary. A subtracted dispersion relation formalism for the TPE has been developed and tested. Its relative effect δ on the elastic cross section is in the 1-2 % range for a low value of the momentum transfer Q2 as function of the kinematic parameter ε, ranging between ε = 0 (backward scattering) and ε = 1 (forward scattering).

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EPJ A Highlight - Mechanisms of two-proton emission seen in three-body correlations

Sequential two-proton decay of the 16Ne Er=7.57 MeV state. The fractional energy distribution (left) gives resonance energy in 15F while the angular distribution (right) determines Iπ of the initial state.

Hitherto three-body correlations between decay products of nuclear resonances, unstable to the emission of two neutrons have been a very effective tool in the analysis of GSI-experiments on 5H, 10He, 13Li, and 14Be. Here the first report is given about the mechanisms for two-proton emission from states in 16Ne, representing the presently most complete study of this nucleus. One-neutron knockout from 17Ne populated the 16Ne(g.s.) (Er=1.39 MeV, Γ=0.08 MeV) above the 14O+p+p threshold, and resonances at Er=3.22 MeV and 7.57 MeV. The decay mechanisms were revealed analysing three-body energy correlations in the 14O+p+p system. It was found that the 16Ne(g.s.) undergoes a democratic three-body decay. In contrast to this, the 16Ne(21+) state emits protons through the 15F(g.s.) sequentially. The decay of 7.57 MeV state is well-described assuming emission of a proton from the d5/2 shell to 15F(5/2+), which decays by d5/2 proton emission to 14O(g.s.). By using R-matrix analysis and mirror symmetry this state was unambiguously identified as the third 2+ state in 16Ne.

EPJ A Highlight - Achieving high resolution in binary nuclear reactions with outgoing fast neutrons – at last!

To date, the two-nucleon pick-up and stripping counterparts of the (p,t) and (t,p) reactions, the (3He,n) and (n, 3He) reactions, have been poorly investigated due to the difficulty in performing high-resolution measurements of fast-neutron energies. The best time-of-flight (3He,n) measurements report resolutions not better than 250 keV. This lack of experimental resolution has hindered a full understanding of the role of proton pairing in nuclei.

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EPJ A Highlight - Modern three-body forces make neutron stars collapse

Density profile of the collapsed state of 10000 neutrons in the X-Y-plane along the symmetry axis Z=0 (schematic illustration). Polarized neutrons, which interact through incorrect three-body forces, concentrate in small spheres separated by 0.9 fermi. © Dmitry K. Gridnev et al.

Nuclear systems ranging from light nuclei to massive neutron stars can be well described by nucleons interacting through two-body and three-body forces. From electrostatics we know that two identical uniformly charged spheres repel at any distance but the repulsion disappears when the spheres completely overlap. Similarly, in some modern expressions of nuclear three-body force it is assumed that the nuclear repulsion between the three nucleons is zero when they occupy the same position in space.

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EPJ A Highlight - Photocouplings at the Pole from Pion Photoproduction

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Тelected fit results for the beam asymmetry Σ (left) and the double polarization E (right)

While the strong force is well understood at high energies in terms of perturbative QCD, the precise mechanism responsible for the confinement of quarks and gluons in color-neutral hadrons at low energies remains a mystery to date. The intermediate energy region is characterized by rich and complex spectra of excited baryons and mesons. Its phenomenology provides a key to our understanding of the fundamental properties of matter.

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EPJ A Highlight – COMPASS: Radiative widths of the a2(1320) and π2(1670) mesons

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Intensity of the 2-+ partial-wave in π^- γ→π^- π^- π^+, interpreted as radiative width of the π2(1670).

Radiative transitions are among the most important and insightful processes for the investigation of atomic, nuclear and hadronic systems. They reveal the electromagnetic substructure of the involved particles. The a2(1320) meson is known since the 1980s to decay radiatively with a branching of about 0.3% into a pion and a photon. Theoretically this can be linked, for example through the vector meson dominance model, to the main hadronic decay channels.

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EPJ A Highlight - MINOS: A vertex tracker coupled to a thick liquid-hydrogen target for in-beam spectroscopy of exotic nuclei

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View of the MINOS device inside the DALI2 gamma array at the RIKEN Radioactive Isotope Beam Factory.

MINOS is a new apparatus dedicated to in-beam nuclear structure experiments with low-intensity exotic beams at energies above 150 MeV/nucleon.

It is intended to provide increased luminosity compared to standard solid-target experiments in hydrogen-induced studies, while simultaneously improving experimental resolution. This article exposes the concept of the device developed at the CEA in France and reviews in detail the associated recent technical advances. MINOS is composed of a thick finger-shaped liquid hydrogen target, from 50 to 200 mm thick, combined with a compact time projection chamber serving as a vertex tracker, the first of its kind in low-energy nuclear physics. This innovative setup offers access to the first spectroscopy of a new range of very exotic nuclei beyond our current reach. An exciting program on the search for new 21+ states in neutron-rich even-even nuclei, spectroscopy of unbound oxygen nuclei and di-neutron correlations in borromean nuclei will be performed with MINOS at the RIKEN Radioactive Isotope Beam Factory in Japan over the next few years. MINOS is funded by the European Research Council.

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EPJ A Highlight - Prompt x-rays emitted in neutron-induced fission help unveil the evolution of fission fragment charge yields as a function of incident neutron energy

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Charge distribution determined from the x-ray yield measurements (symbols) for different incident neutron intervals: threshold to 6MeV ~3MeV, from 6 to 11MeV ~8MeV, from 11 to 20MeV ~14MeV, from 20 to 50MeV ~32MeV and from 50to 400MeV ~180MeV. For more detail see text.

Nuclear fission is accompanied by the prompt emission of neutrons, gamma rays and x-rays. It has been known since the sixties that fission prompt x-rays originate essentially as a consequence of the internal conversions occurring in the prompt gamma deexcitation cascades of fission fragments.

This work presents for the first time a measurement of the prompt fission x-ray yields in 238U(n,f) for average incident neutron energies ranging from 3 to 200 MeV. These results provide new information on fission fragment deexcitation and allow testing the current knowledge of fission fragment nuclear structure. These results provide also a means to investigate the evolution, as a function of incident neutron energy, of fission fragment charge yields and elemental prompt x-ray emission probabilities.

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Editors-in-Chief
David Blaschke, Thomas Duguet and Maria Jose Garcia Borge
Thank you, yet again, for an excellent set of proofs and the meticulous work you do! ... We are glad to see that EPJA maintains its high standards.

Harald W. Grießhammer, The George Washington University, Washington DC, USA

ISSN (Print Edition): 1434-6001
ISSN (Electronic Edition): 1434-601X

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