Highlighted Papers

Highlighted Papers are articles selected by the Editorial Board to increase the visibility of what are deemed to be especially important papers. The full PDF of the Highlighted Papers can be viewed and downloaded from this page free of charge. Every Highlighted Paper is introduced by a short paragraph explaining the novelty and particular importance of the published work.

December 2011
EPJ A - Vector Correlators in Lattice QCD: Methods and Applications

Vacuum polarisation, the modification of the photon propagator due to virtual electron-positron pairs, is one of the first quantum loop corrections encountered in field theory. In both QED and QCD it causes the running of the appropriate fine structure constant as the physical scale is varied, and also corrects the magnetic moments of electrons and muons from the value 2 predicted by the Dirac equation. For scales below a few GeV the QCD vacuum polarisation cannot be calculated perturbatively, but can be accessed via the optical theorem from the annihilation cross section of e+e- into hadrons, which is simply related to the spectral density ρ(s) in the vector isoscalar channel.. This paper opens a new direction by first assessing the current state-of-the-art in calculating the vacuum polarisation in lattice QCD, the most systematic non-perturbative approach, and then by setting out two different routes to improving on this, and identifying applications to strong interaction phenomenology.

Comparison with experimental data reveals that current results are badly finite-volume affected. The paper provides technical details enabling these distortions to be understood and ultimately extrapolated to the large volume limit. It also uses the same data to estimate the current-current correlator as a function of Euclidean time exposing the possibility that different ranges are amenable to different theoretical approaches; the dominant hadronic correction to (g-2) for the muon, about to be measured with unprecedented precision at Fermilab, comes from the range 0.5fm < ct < 1.5fm. This reasoning is also suggests a new QCD reference scale, to help callibrate the lattice spacing using high-precision numerical estimates of the vector correlator.

[David Bernecker and Harvey Meyer, Eur. Phys. J. A (2011) 47, 11: 148]

October 2011
EPJ A - The Similarity Renormalization Group for Three-Body Interactions in One Dimension

One important message that has emerged from developments of effective field theories and effective Hamiltonians for nuclear physics is that many-body forces are inevitable whenever degrees of freedom are eliminated. At the same time, first-principles calculations have shown that two-body forces alone are not able to give an accurate account of the energies of light nuclei and the saturation of nuclear matter. Three- (and possibly more-) body forces are thus essential in low-energy nuclear physics. The construction of effective interactions through elimination of degrees of freedom can be done either by imposing a cut-off on the Hilbert space or by applying a transformation to put the Hamiltonian into a simpler form, such as a diagonal matrix. The Similarity Renormalization Group follows the latter route by means of a continuous set of transformations. It has proved to be a powerful tool in low-energy nuclear physics, where it has been applied mainly in the context of expansions using harmonic-oscillator basis states.

The present paper provides the first application of this method to three-body interactions in a momentum-space basis. Although the models studied are simple ones, consisting of bosons in one dimension, the structure of the evolution equations has the full complexity of any set of three-body equations. The results show the expected decoupling of high- from low-momentum states for both two- and three-body interactions, which means that only low-momentum matrix elements of the evolved potentials are needed to describe low-energy states. This work paves the way for applications to few-nucleon scattering processes and nuclear matter, starting from realistic nuclear forces in three dimensions.

[O. Åkerlund et al ., Eur. Phys. J. A (2011) 47: 122]

September 2011
EPJ A - The Pionic Deuterium

A new precise measurement of the πD(3p-1s) X-ray transition in the pionic deuterium atom has been performed at the PSI accelerator in Switzerland [Ref.1]. The pionic deuterium is a short lifetime atom, where the negative pion (π ^- ) replaces the electron, resulting in an atomic size scaled down by the ratio of the pion mass over electron mass, a factor of about 270.
The experiment makes use of a high intensity decelerated beam of π ^- stopping in a cooled deuterium gas target where the π ^- is captured. Following the capture an atomic de-excitation quantum cascade of 0.1ns duration takes place and the atom ends up in the 1s ground state as shown in the Figure. A Bragg spectrometer equipped with a bent Silicon crystal and pixel semiconductor detectors provides the precise X-ray detection in the appropriate keV region.

The measurement of the energy of the X-ray emitted in the πD(3p-1s) transition leads to a new value of 3075.583 ± 0.030 eV. A new and updated calculation of this transition energy assuming a pure electromagnetic system (pure QED - no strong interaction) leads to a value of 3077.939 ± 0.008 eV. The difference between these two quantities gives exactly the hadronic shift ε_1s = −2.336 ± 0.031 eV. The line-shape has been analysed, providing a new and precise value of the hadronic broadening Γ_1s = 1.171+0.023 -0.049 eV.
The accuracy of 1.3% achieved for the shift ε_1s leads to a more precise determination of the isoscalar scattering length a⁺ (πD being an isoscaler object). The new precise value obtained for the hadronic broadening Γ_1s leads to a new determination of the threshold parameter a, the transition strenght for a S-wave pion, with unprecedented accuracy.

Figure caption: De-excitation cascade in pionic deuterium.

[Th.Strauch et. al., Eur. Phys. J. A (2011) 47: 88]

February 2011
EPJ A – Validating Aspects of the Strong-Coupling Regime of QCD

A key to our understanding of Quantum Chromodynamics (QCD) in the strong regime is our ability to reproduce the hadronic excitation spectrum. Up to now, and due to their limited predictive power, quark models forecast of this spectrum at high excitation energies is unsatisfactory and is dubbed ``the missing resonances problem”. To explore the high excitation energies in the hadron spectrum production or scattering of heavier mesons from a nucleon target is essential.

In a recent tour-de-force experiment [Eur. Phys. J. A, Volume 47, n. 1, January 2011] I. Jaegle et al. report on an impressive first measurement of η’ photoproduction off a deuteron target at beam energies between 1.47 - 2.45 GeV at the tagged photon beam of the ELSA electron accelerator. Differential cross sections with a wide angular coverage were derived for quasi-free production both on protons and neutrons validating the quasi-free picture. And the first estimate of the coherent  γd → dη’ contribution is found consistent with an impulse approximation, pointing to a viable isospin composition model amplitudes and weak final state interactions.

Legendre polynomials coefficients from angular distributions fits of this experiment and world data are reported in Fig. 1 where proton and neutron cross sections for photon energies above 2 GeV, in a region where contributions from t-channel exchange are important, display a similar behavior. At lower photon energies from where the proton cross section peaks, the behavior is different and would require polarization observables for future investigation.

From left to right of panels in Fig. 1, we have the inclusive reaction data first, then the world free and, this experiment, quasi-free proton data and last quasi-free neutron data from coincidence and an extraction of proton from deuteron inclusive data. Solid lines: solution (I) NH model, dashed lines: η′-MAID for neutron and dotted lines: CLAS proton data.

July 2010
Collinear cluster tri-partition of ²⁵²Cf (sf) and in the ²³⁵U (n_th,f) reaction
Binary fission of a heavy nucleus into two fragments (e.g. Ba + Kr) is known since 1938. A fission process with three fragments ("ternary fission") of comparable mass was mostly considered to show up as a "star" with three fragments with comparable relative angles but was not observed on the level of 10^-8 per fission. The name "ternary fission" is nowadays usually applied to processes, where light nuclei (⁶He - ²⁸Mg) are observed perpendicular to the axis defined by the two heavy fragments.
In the present work "true ternary fission" is discovered as collinear cluster decay. The discovery relies on the fact that  two of the fragments emitted into the same direction become separated by an angle of ~1° on their way to the detector  while passing through material. One fragment may then be lost by hitting a grid at the detector entry. In the experiments masses and velocities are determined for two fragments observed at relative angles of ~180° in a two arm spectrometer. Evidence for the missing third fragment is obtained using the "missing-mass" approach.
In the figure ~10⁺⁷ fission events of ²⁵²Cf(sf) are plotted for M1 versus M2. The missing mass of the third fragment is visible as an additional peak (~35000 events) around A=200-212 (arrow) with a probability of 4.7x10^-3 relative to the binary fissions. Similar evidence was found also for neutron induced fission of ²³⁵U (n_th,f).
[Yu.V. Pyatkov et al., Eur. Phys. J. A Volume 45, 29-37 (2010)]

June 2010
Neutrino mean free paths in spin-polarized neutron Fermi liquids
The study of the behaviour of hadronic matter at high density and low temperature  allows to have a deeper understanding of fermionic systems known as Fermi liquids using the theory first developed by Landau in the 1950’s. The inclusion of an additional non-zero magnetic field, B, allows further testing properties of possibly magnetized matter.  Due to the tiny value of the neutron magnetic moment in principle huge magnetic fields, like those created on earth in heavy-ion collisions or thought to exist in magnetars, where log B (G)≈ 15, are needed to get sizable magnetization.
 In order to understand how neutrinos diffuse in a magnetized neutron medium like this the neutrino mean free paths (mfp) using the  Hartree-Fock approximation with effective nuclear Skyrme and Gogny forces is calculated, with the inclusion of magnetic fields.  It is shown that describing nuclear interaction with Skyrme forces and for magnetic field strengths larger than log B(G)=17, the neutrino mean free paths stay almost unchanged at intermediate densities but they largely increase at high densities when they are compared to the B=0 case.  The description with Gogny forces differs from the above mentioned using Skyrme forces and the mean free paths stay almost unchanged or decrease at densities  in the range (1-2) times that of nuclear saturation density. The resulting values of the neutrino mean free path can be explained by the combination of  two trends  i) common mild variation of the Landau parameters  ii)  very different values of the nucleon effective mass and induced magnetization of matter  as described with the two nuclear parametrizations.
[M. Ángeles Pérez-García, Eur. Phys. J. A Volume 44, 77-80 (2010)]

May 2010
Neutron-hole states in doubly-magic nucleus ²⁰⁸Pb
The doubly magic ²⁰⁸Pb nucleus continues to draw attention of nuclear spectroscopists due to its seemingly simple but rich structure features. This nucleus and its neighboring nuclei at the closed proton (Z=82) and the neutron (N=126) shells are ideal laboratories for studies of particle excitations and their interactions in a nuclear medium.
Precision measurements employing inelastic proton scattering and the high-resolution particle spectroscopy with the Q3D magnetic spectrograph at the Munich 14 MV tandem accelerator have discovered some earlier unobserved neutron particle-hole excitations with small cross sections.
The states of ²⁰⁸Pb lying between 5 and 6 MeV excitation with a neutron-hole configuration of (g _9/2, j ^-1 ) were observed in proton scattering at the resonance energy for the g _9/2 single-neutron isobaric-analog resonance (IAR) in ²⁰⁹Bi. The energy and on-resonance angular distributions for the scattered protons were measured and used to identify five missing members of the (g _9/2 f _7/2 ^-1) neutron multiplet. These results will aid nuclear structure theorists to improve their models and nuclear interactions in the domain of the heavy elements.
[A. Heusler et al., Eur. Phys. J. A Volume 44, 233-238 (2010)]

December 2009
Molecular and cluster structures in ¹⁸O
The cluster model has been used to describe certain states of selected nuclei as clusters of lighter nuclei which, because they are particularly stable, retain some of their identity in a molecular state. For instance, when excited in a particular way a carbon nucleus can take up a structure like three alpha-particles while a magnesium nucleus can exist as two orbiting carbon nuclei - in effect, a "nuclear molecule". The present authors have recently discovered that heavier N=Z alpha-conjugate nuclei, such as 36Ar, ⁵⁶Ni and ⁶⁰Zn, may have ternary decays from extremely deformed collinear shapes. Alpha-clustering was fairly well illustrated 4 decades ago by the Ikeda diagram for N=Z nuclei.
Based upon complete spectroscopy measurements of beryllium and carbon isotopes performed by the present collaboration, von Oertzen has proposed an extension of the Ikeda diagram to picture neutron-rich nuclei with alpha-particles and ¹⁶O nuclei as basic clusters. The present article on molecular structures in ¹⁸O and forthcoming investigations of more exotic neutron-rich oxygen isotopes, such as the schematic illustrations of the figure, will necessitate revision of this extended Ikeda diagram to include also the strongly bound ¹⁴C nucleus as a basic cluster by analogy to ¹⁶O.
Nuclear clustering has an important bearing on energy production in stars and the abundance of the elements, since the rate at which nuclei are produced in stars and supernovae is profoundly influenced by their structure. Therefore, further spectroscopic exploration at Radioactive Ion Beams facilities, such as GANIL-SPIRAL2, RIKEN-RIBF, FAIR, FRIB and EURISOL, will have to be undertaken to discover new surprising clusters (e.g. nuclear polymers) in exotic nuclei far from the stability line.
[W. von Oertzen et al., Eur. Phys. J. A Volume 43, 17-33 (2010)]

December 2009
Antiproton-nucleus annihilation and timelike form factors of the proton
Electromagnetic form factors tell us about the electric charge distribution and magnetic moment inside the nucleon and are functions of t, the square of the momentum transfer, which mathematically can be of either sign. They have been measured in the spacelike region (t < 0) for fifty years by elastic electron-nucleon scattering, while in the timelike region they are measured in proton- antiproton annihilation into electron-positron pairs or its inverse reaction providing information on nucleon-antinucleon interaction. An interesting exploration would be in the under-threshold region, requiring the proton or the antiproton to be off-mass-shell, where among other things quasi-nuclear bound states are recurringly predicted to exist. The present work relies on the fact that a nucleus provides nucleons with various degrees of off-shellness. This idea explored the 1980's in the case of deuteron, and almost stopped antiprotons, is now revived in view of future experiments with relatively high-energy beams. An exploratory study of this reaction has been undertaken in the context of the PANDA detector at FAIR, in Darmstadt, Germany. While noting that with energetic antiprotons the relevant under-threshold region is very hard to reach, and that one has to fight against a large background, the feasibility relies on the expected detector performances.
[H. Fonvieille and V. Karmanov, Eur. Phys. J. A Volume 42, 287-298 (2009)]

August 2009
Study of the 133Ba Nucleus with the (d,p) Reaction
In a recent article Suliman et al. present a study of the single-neutron levels of the 133Ba nucleus using the 132Ba(d,p)133Ba reaction with a polarised deuteron beam. Many unambiguous spin-parity assignments were made, up to an excitation energy of 2.2 MeV, due to the sensitivity of the angular distributions of the vector analysing powers to the total spin, J, of the final state (it will be recalled that angular distributions of the differential cross section are only sensitive to the transferred angular momentum, L). These assignments enabled a detailed comparison with the results of different structure models.
Along the chain of odd-A Ba isotopes it is conspicuous that the lowest 1/2+ and 3/2+ states cross each other, the 133Ba nucleus forming the point at which these states are almost degenerate. It thus forms a severe constraint on structure models in this mass region. It may also be considered as a single fermion (a neutron hole) coupled to a nearly E(5) 134Ba core, making it a suitable nucleus for comparison with IBFM (Interacting Boson-Fermion Model) calculations.
Suliman et al. show that in fact none of the IBFM schemes that they investigated was able to provide a satisfactory description of the low-lying levels of 133Ba, suggesting that the structure is rather more complicated than this simple picture suggests. This conclusion seems to be borne out by spherical shell model calculations which provided the best overall description of the low-lying levels.
[G. Suliman et al., Eur. Phys. J. A 41, 299-313 (2009)]

May 2009
Empirical Transverse Charge Densities in the Deuteron
The study of the space distribution of quarks inside hadrons is a fundamental issue in hadron physics. This paper tackles the problem of extracting information on the spatial distribution of the quark charges inside longitudinally and transversely polarized deuterons, in a model independent way, exploiting available experimental information on the electromagnetic form factors. The authors develop a general formalism for spin-1 particles and apply it to the deuteron; a formalism for the nucleons had been presented previously. The results are very interesting; in the figure it is shown the quark transverse charge density for longitudinally polarized deuterons, with helicity 1 (upper panel) and 0 (middle panel). The lowest panel compares the two cases (for the density along the y-axis), showing how the charge density for zero helicity states exhibits a dip at the center of the deuteron.
[C.E. Carlson, M. Vanderhaeghen, Eur. Phys. J. A 41, 1-5 (2009)]

March 2009
Universal Properties and Structure of Halo Nuclei
There has been renewed interest in quantum many-body systems with large scattering length a recently, largely because in ultracold atomic gases a can be tuned to arbitrary values via Feshbach resonances. In the so-called "unitary limit" of infinite scattering length such systems exhibit universal properties, independent of the microphysics. Since the neutron-neutron scattering length is large compared to the range r0 of the nucleon-nucleon interaction, the question arises whether such universal properties can also be observed in nuclei. Of particular interest is the question of "Efimov states" in the three-body system, which arise if at least two of the three pairs of particles have a large scattering length compared to interaction range. The number of the ensuing three-body bound states, which are geometrically spaced between h/mr02 and h/ma2 grows to infinity as a diverges.
Treating neutron-rich light nuclei ("halo nuclei") as an effective three-body problem with a core and two valence neutrons, the possible existence of Efimov states is adressed by using an effective field theory approach. To leading order in a (r0 /a) - expansion, the Faddeev equations for the core and the two halo neutrons are solved and next-to-leading order corrections are estimated as systematic errors. As a function of the two-body bound (virtual) state energies of the neutron-core (Enc) and the neutron-neutron (Enn) systems the boundary of the region with at least one excited Efimov state is calculated and an error band provided. It is found that none of the known halo nuclei is likely to have an excited Efimov state, with the possible exception of 20C.
[D.L. Canham, H.-W. Hammer, Eur. Phys. J. A 37, 367-380 (2008)]

September 2008
A Fitter Code for Deep Virtual Compton Scattering and Generalized Parton Distributions
The Generalized Parton Distributions (GPDs) have recently emerged as a powerful tool to study the space and momentum distributions of quarks inside the nucleons; they could offer the first ever access to the orbital motion of quarks. The GPDs depend on several variables and are, in various ways, related to experimental information. Dedicated experiments are gathering and will gather data, which, for the first time, will allow an extraction of the GPDs. Such an effort requires best-fit procedures, co-ordination among various groups, and, above all, a clearly defined fitting procedure to be followed. This is exactly what the paper offers.
[M. Guidal, Eur. Phys. J. A 37, 319-332 (2008)]

July 2008
Neutron Velocity Distribution from a Superthermal Solid 2H2 Ultracold Neutron Source
Neutrons with velocities below ~ 7 m/s are termed ultracold (UCN). They are produced in non-equilibrium superthermal converters, e.g., superfluid helium or solid deuterium, and can be stored in vessels with a storage time constant close to the neutron lifetime. UCN are used for the measurement of fundamental properties of the neutron, such as the electric dipole moment, the lifetime of the free neutron, and the gravitational interaction. For an increased precision of these experiments, very intense new UCN sources are in construction. The velocity distribution of the neutrons from a solid deuterium converter at 5 K is very important for the design of such sources. This velocity distribution has now been determined for the first time at the UCN source at the TRIGA reactor in Mainz using the time-of-flight (TOF) method: it is shown that the neutron spectrum rises sharply above 4.5 m/s. After transport in an 8 m stainless-steel neutron guide, the distribution has a maximum around 7 m/s, and decreases approximately exponentially above this velocity. Though expected from, e.g., deceleration in neutron optics, this experiment confirms the recent first experimental verification of neutron acceleration by the material optical potential of a solid 2H2 UCN converter. As a consequence, the number of storable neutrons in an experiment can be increased by a factor of ~ 2 by placing the corresponding experimental setup outside the biological shielding about 1 m above the UCN converter.
[I. Altarev et al., Eur. Phys. J. A 37, 9-14 (2008)]

July 2008
Study of the Internal Structure of the Proton Using Virtual Compton Scattering
A property of paramount importance in the investigation of the nucleon structure is its "polarizability", a measure of how stiffly the nucleon responds to external electric and magnetic fields by rearranging its internal constituents. The most direct method of determining such polarizabilities is real Compton scattering, the direct scattering of a photon from the nucleon. As with the nucleon electromagnetic distributions, the formalism to describe the polarizabilities can be extended to probe differing distance scales, using the technique of virtual Compton scattering. This provides stringent tests of calculations that link the effective low-energy description of nucleons to Quantum chromodynamics (QCD) the theory of strong interactions. Furthermore, spin polarizabilities can also be explored using spin degrees of freedom of a polarized virtual photon beam and polarized nucleon target. In a recent experiment [Eur. Phys. J. A 37, 1-8 (2008)] a new measurement of the virtual Compton scattering cross section was performed at the MAMI accelerator using the standard setup of the A1 collaboration at a photon four-momentum transfer Q2 = 0.33 (GeV/c)2. The results of this experiment set a new stage of precision and confirm the non-trivial behavior of the response of the nucleon as the probe distance scale is varied from a fraction of the nucleon size to its full size. Collectively, the results from different laboratories indicate that the nucleon's paramagnetic (or intrinsic) and diamagnetic (or induced) polarizabilities are of the same order of magnitude (in absolute value). As a result, the total magnetic polarizability of the proton is smaller than its electric polarizability. Clearly, this experiment calls for future precision measurements at low Q2 setting a benchmark test for further theoretical advances, and enables the unraveling, for example, of the nucleon pion cloud structure.
[The A1 Collaboration, Eur. Phys. J. A 37, 1-8 (2008)]

June 2008
A Possible Signature of Hyper-Deformation From Ternary Fission
Search for hyper-deformed shapes in atomic nuclei has been challenging nuclear physicists since the discovery of super-deformation by Paul Nolan and Peter Twin in the mid-eighties. For such exotic configurations nuclei are approximated by ellipsoidal shapes with major-to-minor axis ratios of 2:1 and 3:1 for super- and hyper-deformations, respectively. In their recent paper Wolfram von Oertzen and coworkers report on observations of fission and ternary cluster decay events of a high-spin state in 56Ni. This state with angular momentum of about 45 units of ħ was produced in heavy-ion fusion reactions between 32S and 24Mg nuclei at the VIVITRON facility in Strasbourg. Its decay modes were observed by using a highly sophisticated Binary Reaction Spectrometer of the Hahn Meitner Institute Berlin coupled to the EUROBALL Ge-array. The observed ternary coplanar fission was interpreted to be a signature of the decay of an extremely deformed nucleus at high angular momentum. It is an interesting observation since 56Ni can be considered as spherical doubly magic nucleus in its ground state. For deformed configurations, shell corrections for quadrupole deformation could provide explanations for the observed elongated shape of 56Ni. As suggested by the findings in this paper, measurements of the ternary fission process offer new possibility for a detailed spectroscopy of extremely deformed shapes.
[W. von Oertzen et al., Eur. Phys. J. A 36, 3 (2008)]