Nuclear Physics ++

Prof. Dr. Robert Roth
Institut für Kernphysik - Theoriezentrum
Schlossgartenstr. 2 - Gebäude S211
Welcome to the TNP++ web site. You will find information about the group, our research program, our teaching activities, and much more on these pages.
  • Physical Review Letter on the In-Medium No-Core Shell Model
    April 2017
    R. Wirth
    Ab initio many-body methods have different strengths and weaknesses. The No-Core Shell Model (NCSM), for example, can handle odd-mass systems and excited states but is limited in particle number. Medium-mass methods like the In-Medium Similarity Renormalization Group (IM-SRG) can handle higher particle numbers but are restricted to ground-states of even nuclei. With this Physical Review Letter we present a hybrid of the two methods that has the merits of both: the In-Medium No-Core Shell Model (IM-NCSM). The IM-NCSM uses the IM-SRG to decouple a small NCSM model space from higher excitations, dramatically improving convergence of NCSM calculations. This way, the IM-NCSM enables access to excited states and electromagnetic observables of medium-mass nuclei. Check out the publication for more information.
  • hoch3 FORSCHEN - Pieces of Cosmic Puzzles
    April 2017
    R. Roth
    Based on our recent back-to-back publications in Physics Review Letters on the tetraneutron and the hyperon puzzle in neutrons stars, the TU Darmstadt has produced a little news piece for our hoch3 FORSCHEN magazine entitled 'Pieces of Cosmic Puzzles'. You can download the english version of the complete issue here and the german version here.
  • Physical Review Letter on the Tetraneutron
    October 2016
    R. Roth
    Simultaneously with our letter on hypernuclei a second letter on an equally exotic nuclear system appeared: the tetraneutron, i.e., a systems composed of 4 neutrons. In collaboration with researchers from Russia and the US we have explored the existence of a low-lying resonance in the tetraneutron and we have predicted its energy and lifetime using novel ab initio methods including continuum degrees of freedom. Our results for the resonance energy are remarkably close to a recent experimental claim by a group from RIKEN. We will certainly continue to study this elusive system, which provides a sensitive probe for the neutron-neutron interaction. Several groups within the SFB 1245, both theory and experiment, will hunt for further insights into these systems beyond the neutron dripline.
  • Physical Review Letter on Induced Hyperon-Nucleon-Nucleon Interactions
    October 2016
    R. Roth
    Unitary transformations are a key component for accelerating convergence of many-body calculations in order to reliably compute observables. These transformations induce many-body terms that have to be included in the calculation. We worked out the Similarity Renormalization Group (SRG) transformation of hyperon-nucleon interactions at the three-body level and can now precisely compute binding and excitation energies of hypernuclei. The induced three-body terms are stronger than expected and their appearance links into a long-standing puzzle in the physics of neutron stars. Check out the publications section...
  • New Paper with LENPIC Collaboration
    April 2016
    R. Roth
    With the LENPIC Collaboration we have published a new paper in Physical Review C that marks another step towards a systematic uncertainty quantification in ab initio calculations. Using a new generation of semi-local chiral two-nucleon interactions from leading order (LO) to next-to-next-to-next-to-next-to leading order (N4LO) we study the systematics of various few-body observables and quantify the theoretical uncertainties. In the next step we will extend these studies to nuclear structure observables in heavier systems and include consistent 3N interactions up to N3LO. Stay tuned...
  • Physics Letters B on Hartree-Fock Many-Body Perturbation Theory
    March 2016
    R. Roth
    We have studied many-body perturbation theory (MBPT) as a tool for nuclear structure calculations for quite some time now. Its simplicity makes it very a very attractive alternative to more complicated and computationally demanding methods like coupled-cluster theory. However, the problem with MBPT is the order-by-order convergence of the perturbation series. In this Letter we explore the impact of the partitioning, i.e. the definition of the unperturbed basis, on the convergence. Using high-order MBPT we explicitly show that a Hartree-Fock basis leads to convergent ground-state energies so that low-order approximation are meaningful. Moreover, we show that third-order MBPT is in excellent agreement with the most advances coupled-cluster approximations up to Sn isotopes at the fraction of the computational cost.
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