Distinguished EPJ Referees

EPJ A Highlight - Celebrating the life and work of Mahir Hussein through cutting-edge research

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A new collection of papers focusing on the dynamics of nuclei pays fitting tribute to Iraqi/Brazilian physicist Mahir Saleh Hussein.

On 16th May 2019 Iraqi/ Brazilian physicist Mahir Saleh Hussein passed away leaving behind a distinguished career of achievements in a broad range of physics disciplines. There is, perhaps, no better way of paying tribute to this influential figure in physics than presenting a collection of cutting-edge, peer-reviewed papers. A special edition of EPJ A, edited by Valdir Guimarães, Universidade de São Paulo Instituto de Física, Brazil, Carlos Bertulani, Department of Physics of the Texas A&M University-Commerce, USA, and Nicolas Alamanos, Deputy Director of the Institute of Research into the Fundamental Laws of the Universe (IRFU), France, brings together a collection of papers focusing on nuclei clustering and dynamics as a fitting tribute to Hussein.

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EPJ A Highlight - THOR: Driving collaboration in heavy-ion collision research

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Analysing collisions between heavy ions. Image by Pcharito - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/ index.php?curid=31433194

As an expansive platform for collaboration between different research groups, the THOR COST Action has enabled hundreds of physicists studying the aftermath of high-energy collisions between heavy ions to improve their predictions.

In the universe’s earliest moments, particles existed in an unimaginably hot plasma, whose behaviour was governed by deeply complex webs of interaction between individual particles. Today, researchers can recreate these exotic conditions through high-energy collisions between heavy ions, whose products can tell us much about how hot, strongly-interacting matter behaves. Yet without extensive, highly coordinated collaborations between researchers across many different backgrounds, studies like this simply wouldn’t be possible. This Topical Issue of EPJ A draws together a large collection of papers inspired by the theory of hot matter and relativistic heavy-ion collisions (THOR) European Cooperation in Science and Technology (COST) Action. Running between November 2016 and April 2021, THOR has provided a way for over 300 researchers involved in heavy-ion collision analysis to freely exchange their ideas, leading to exciting new advances in the wider field of particle physics.

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EPJ A Highlight - Paving the way for effective field theories

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Cover picture: image courtesy of Germain Caminade (http://germaincaminade.com/)

A detailed analysis of theories which approximate the underlying properties of physical systems could lead to new advances in studies of low-energy nuclear processes

Over the past century, a wide variety of models have emerged to explain the complex behaviours which unfold within atomic nuclei at low energies. However, these theories bring up deep philosophical questions regarding their scientific value. Indeed, traditional epistemological tools have been rather elaborated to account for a unified and stabilised theory rather than to apprehend a plurality of models. Ideally, a theory is meant to be reductionist, unifying and fundamentalist. In view of the intrinsic limited precision of their prediction and of the difficulty in assessing a priori their range of applicability, as well as of their specific and disconnected character, traditional nuclear models are necessarily deficient when analysed by means of standard epistemological interpretative frameworks.

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EPJ A Highlight - Automated symmetry adaption in nuclear many-body theory

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Symmetry reduction process of a prototypical many-body expression leading to an equivalent symmetry-reduced form. Recoupling coefficients arising from the AMC program are shown in red.

The extreme cost of solving the A-nucleon Schrödinger equation can be minimized by leveraging rotational symmetry and, thus, enable the computation of observables in heavy nuclei and/or with high precision.

The associated reduction process, which amounts to re-expressing the working equations in terms of rotationally-invariant objects, requires lengthy symbolic manipulations of elaborate algebraic identities.

For the first time, this involved process is automated by a powerful graph-theory-based tool, the AMC code, which condenses months of error-prone derivations into a simple computational task performed within seconds.

The AMC program tightens the gap for a full automation of the many-body workflow, thereby lowering the time required to build and test novel quantum many-body formalisms.

EPJ A Highlight - Emergence of nuclear rotation from elementary interactions between the nucleons

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Rotational bands in an ab initio calculations for the nuclear excitation spectrum of 11Be.

Nuclei are quantum many-body systems which exhibit emergent degrees of freedom, from shell structure and clustering to collective rotations and vibrations. Such emergent phenomena are traditionally the domain of phenomenological models, yet their description can now be placed on a more fundamental footing in terms of microscopic theory. The nature and emergence of rotational bands are presently investigated in light nuclei through ab initio nuclear many-body calculations. Beyond simply analyzing spectroscopic signatures, the structural insight are investigated in terms of angular momentum coupling schemes and group theoretical correlations as underpinnings for the rotational structure.

EPJ A Highlight - Advancing AGATA – Future Science with The Advanced Gamma Tracking Array

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Artist's view of the 4p AGATA spectrometer showing the mechanical holding frame (yellow) and cryostat dewars (blue) of the Ge detectors.

AGATA – the Advanced Gamma Tracking Array is a multi-national European project for the ultimate high-resolution gamma-ray spectrometer for nuclear physics capable of measuring γ rays from a few tens of keV to beyond 10 MeV, with unprecedented efficiency, excellent position resolution for individual γ-ray interactions and correspondingly unparalleled angular resolution, and very high count-rate capability. AGATA will be a flag ship spectrometer and have an enormous impact on nuclear structure studies at the extremes of isospin, mass, angular momentum, excitation energy and temperature. It will enable us to uncover and understand hitherto hidden secrets of the atomic nucleus.

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EPJ A Highlight - Towards the solution of the “hyperon puzzle”

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Neutron star’s mass-radius relation with and without hyperons. Masses of the pulsars PSR J0348+0432 and PSR J0740+6620 are shown with their observation uncertainties.

The possible presence of strange matter in the core of neutron stars has given rise to the so-called hyperon puzzle: hyperonic degrees of freedom are energetically allowed in the extreme density conditions believed to exist in the core of Neutron Stars, but hyperons reduce the internal pressure of the star, which then cannot compensate the gravitational field to sustain the most massive compact stars observed.

This work reports on the effect of three-body interactions when including a Lambda hyperon on the properties of hyper-nuclei and Neutron Stars. State-of-the-art three-body chiral effective interactions are introduced in a microscopic Brueckner-Hartree-Fock calculation.

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EPJ A Highlight - Confirming the validity of the Silver-Blaze property for QCD at finite chemical potential

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Sketch of the QCD phase diagram in the temperature and baryon chemical potential plane.

The properties of the theory of strong interactions, QCD, at finite chemical potential are of great interest for at least two reasons: (i) model studies suggest a potentially rich landscape of different phases with highly interesting analogies to those found in solid state physics; (ii) the resulting thermodynamic properties have far reaching consequences for the physics of neutron stars and neutron star mergers.

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EPJ A Highlight - A Liquid-Lithium Target for Nuclear Physics

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The free-surface LiLiT flow, photographed while bombarded by a ~ 3 kW continuous-wave proton beam from the SARAF linac. The liquid lithium jet, ~1.5 mm thick, forced-flown at a velocity of 2.5 m/s at ~ 195 °C and supported by a 0.5 mm thick stainless steel backing wall, serves both as a neutron producing target and the power beam dump. The target chamber pressure connected to the accelerator beam line is 1×10-6 mbar.

A liquid-lithium target (LiLiT) bombarded by a 1.5 mA, 1.92 MeV proton beam from the SARAF superconducting linac acts as a ~30 keV quasi-Maxwellian neutron source via the 7Li(p,n) reaction with the highest intensity (5×1010 neutrons/s) available todate. We activate samples relevant to stellar nucleosynthesis by slow neutron capture (s-process). Activation products are detected by α, β or γ spectrometry or by direct atom counting (accelerator mass spectrometry, atom-trap trace analysis). The neutron capture cross sections, corrected for systematic effects using detailed simulations of neutron production and transport, lead to experimental astrophysical Maxwellian averaged cross sections (MACS). A parallel effort to develop a LiLiT-based neutron source for cancer therapy is ongoing, taking advantage of the neutron spectrum suitability for Boron Neutron Capture Therapy (BNCT) and the high neutron yield available.

EPJ A Highlight - Shape stability of pasta phases: Lasagna case

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Exotic non-spherical shapes of nuclear matter, so called pasta phases, are possible because of the competition between the short-ranged nuclear attraction and the long-ranged Coulomb repulsion, leading to the phenomenon of Coulomb frustration, well known in statistical mechanics. Such complex phases are expected in the inner crust of neutron stars, as well as in core-collapse supernova cores.

The authors of the EPJ A (2018) 54:215 paper examine for the first time the stability of the «lasagna» phase, consisting of periodically placed slabs, by means of exact geometrical methods. Calculations are done in the framework of the compressible liquid drop model but obtained results are universal and do not depend on model parameters like surface tension and charge density. The stability analysis is done with respect to the different types of deformations corresponding to the eigenvalues of the deformation matrix.

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