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Seminare im Jahr 2019

Seminare in früheren Jahren

Dark Matter Direct Detection Theory - From Extensions of the Standard Model to Observables
Dozent/Speaker: Dr. Christopher Körber (RUB, TPII)
Termin/Date: Do. 14.05.2020, 16:15 - 17:45
Ort/Location: virtual
Übersicht/Abstract: What is the nature of so called Dark Matter and does it interact with regular matter except through gravity? Direct detection experiments aim to answer this question. Yet, propagating measurements (or constraints) to the fundamental theory requires bridging several scales - from target nucleus to individual nucleons to the level of quarks & gluons and beyond. In this talk I describe the methods used, assumptions made and challenges faced by the bridging of scales to address implications of measurements.
Spinning black holes with Skyrme hairs and soliton stars
Dozent/Speaker: Dr. Yakov Shnir (JINR, Dubna, Russian Federation)
Termin/Date: Do. 12.03.2020, 15:00 - 16:30
Ort/Location: NB 6/173
Übersicht/Abstract: We study a new families of stationary rotating axially symmetric hairy black holes which represent solutions of the Einstein-Skyrme model in the Kerr spacetime. We found that the spinning axially symmetric cloudy solutions of the model also exist in the regular asymptotically flat space-time without the event horizon. The latter congurations are similar to the usual rotating boson stars, which, in the at flat space limit are linked to the axially symmetric Q-balls, in both cases the solutions exist for some restricted range of values of the angular frequency and possess a quantized angular momentum. Considering similar solution in the O(3) sigma model, we show that, depending on the values of the parameters of the model and the Hawking temperature, the branch structure of the corresponding cloudy solutions varies from the usual inspirraling pattern, which is typical for the boson stars, to the two branch structure, similar to that of the black holes with Skyrme hairs. As another examples of spinning black holes with synchronized hairs we consider families of parity-odd solutions of the Einstein-Klein-Gordon model. Finally, we discuss Dirac stars and compare their properties with the usual boson stars.
Calculation of The Three-Pion Exchange Nucleon-Nucleon Potential in Chiral Effective Field Theory
Dozent/Speaker: Victor Springer (RUB)
Termin/Date: Do. 27.02.2020, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Chiral effective field theory is a powerful tool in order to describe low energy nucleon-nucleon (NN) reactions. In this framework, a long range part of an NN potential is described by the exchange of pions. Whereas one and two pion exchanges were studied well in the past, only a few calculations to the three pion exchange exit in literature. Since in one of them singularities in the spectral function representation were found, it is of interest to study this potential in another way. In this talk, I will discuss the three pion exchange NN potential in a chiral EFT with the method of unitary transformation. Using dimensional regularization, the occurring two-loop tensor integrals are reduced to two so-called sunset master integrals. For those, a one-dimensional integral representation in terms of Bessel functions was found.
Novel chiral Hamiltonian and observables in light and medium-mass nuclei
Dozent/Speaker: Dr. Vittorio Somã, CEA Saclay, France
Termin/Date: Do. 30.01.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Recent advances in nuclear structure theory have led to the availability of several complementary ab initio many-body techniques applicable to light and medium-mass nuclei as well as nuclear matter. After successful benchmarks between different approaches, the focus is moving to the development of improved models of nuclear Hamiltonians, currently representing the largest source of uncertainty in ab initio calculations of nuclear systems. In this context, a novel parameterisation of a Hamiltonian based on chiral effective field theory has been recently introduced. Specifically, three-nucleon operators at next-to-next-to-leading order are combined with an existing (and successful) two-body interaction containing terms up to next-to-next-to-next-to-leading order. The resulting potential is labelled NN+3N(lnl). Using results from no-core shell model and self-consistent Green's function approaches, in this seminar I will explore the performance of this new interaction across light and medium-mass nuclei. Both systematics along complete isotopic chains and specific medium-mass isotopes of high experimental relevance will be discussed. Finally, perspectives on the description of doubly open-shell nuclei will be addressed.
Eigenvector Continuation as a Tool for Ab Initio Simulations
Dozent/Speaker: Dr. Dillon Frame (Forschungszentrum Jülich)
Termin/Date: Do. 28.11.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Eigenvector Continuation is a new mathematical tool that has been developed for use in determining the eigenvectors of Hamiltonian matrices in situations where obtaining them directly is otherwise impractical. This method relies on projecting the Hamiltonian into a low-dimensional subspace formed out of known eigenvectors, and using analytic continuation to determine the eigenvector of interest. In this talk, I will show the applications of this method to three systems: the Bose-Hubbard model, neutron matter on the lattice, and the non-perturbative inclusion of the Coulomb interaction in C-12 and O-16.
Study of Axial Pion Production in Chiral Perturbation Theory
Dozent/Speaker: Tim Höhne (University of Bochum)
Termin/Date: Do. 26.09.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: In the present work we study axial pion production within heavy-baryon chiral perturbation theory. We set up the necessary theoretical framework and give the chiral Lagrangians describing the interactions among pions, nucleons and external sources. Afterwards we employ these Lagrangians to calculate the full axial pion production amplitude up to NNLO including tree- and loop contributions and renormalize it. For the comparison to experiments we then focus on weak charged current neutrino induced pion production containing axial pion production as a subprocess. This process does not only involve axial but also vector pion production. After also calculating and renormalizing the vector pion production amplitude up to NNLO, both amplitudes are combined and used to calculate the neutrino induced pion production total cross section. Plenty of consistency checks are deployed to assure the calculation was done correctly. Finally the obtained cross sections are compared to a covariant calculation as well as existing experimental data.
Lattice simulations with chiral effective field theory for light and medium-mass nuclei
Dozent/Speaker: Prof. Dr. Serdar Elhatisari (Karamanoglu Mehmetbey University, Karaman, Turkey)
Termin/Date: Do. 11.07.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Chiral effective field theory ($\chi$EFT) is a model-independent and systematic approach for describing the interactions between nucleons having energies below the chiral symmetry breaking scale. Nuclear lattice effective field theory (NLEFT) is a powerful numerical method formulated in the framework of $\chi$EFT to study the few- and many-nucleon problems. There has been significant progress in \emph{ab initio} nuclear structure and nuclear scattering calculations using NLEFT. In this talk I discuss recent developments in NLEFT calculations including degrees of locality of the nuclear forces, a new lattice formulation of $\chi$EFT interactions with a simpler decomposition into spin channels, Galilean invarience restoration interaction on the lattice etc. I also present recent results on the ground state energies of light and medium-mass nuclei from lattice simulations with $\chi$EFT at next-to-next-to-next-to-leading order.
Studies of 3-D Structure of the Nucleon from CLAS and CLAS12 at Jefferson Lab
Dozent/Speaker: Prof. Dr. Kyungseon Joo (University of Connecticut)
Termin/Date: Mi. 10.07.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Recent developments of generalized parton distributions (GPDs) and transversely dependent momentum distributions (TMDs) have opened a new window on a 3D imaging of the nucleon, going far beyond the one-dimensional, longitudinal structure probed in inclusive deep-inelastic scattering, and the transverse structure encoded in the different form factors. The 3D imaging of the valence quarks is a cornerstone of the new 12 GeV experimental program at Jefferson Lab, and a future proposed US-based electron ion collider (EIC) will extend this imaging to sea quarks and gluons. In this talk, I will describe recent measurements in exclusive and semi-inclusive deep processes with the CLAS detector, the CEBAF Large Acceptance Spectrometer at Jefferson Lab to study 3D structure of the nucleon, and I will also explore future directions using the new CLAS12 detector.
Symmetries, Ergodicity, and the Sign problem
Dozent/Speaker: Prof. Dr. Tom Luu (FZ Jülich)
Termin/Date: Do. 23.05.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: The hybrid monte carlo algorithm (HMC) is a numerical workhorse in simulations of strongly interacting phenomena, such as lattice QCD, nuclear lattice EFT, and strongly correlated electrons in low dimensions. An implicit assumption in these HMC simulations is that the algorithm is ergodic, i.e. any state space can be reached with positive probability. However, this is not always the case and depends on the underlying symmetries of the system. HMC also requires a positive definite probability measure, a lack of which is known as the sign problem. These two aspects are not independent of each, and I show how attempts to solve one aspect may worsen the other. In particular, I describe how attempts to alleviate the sign problem via holomorphic flow leads to an ergodicity problem. I present solutions to such ergodicity issues via the use of `ergodicity jumps'. The interplay between ergodicity and the sign problem has direct bearing on lattice QCD simulations at finite baryon chemical potential and nuclear lattice EFT calculations of neutron-rich nuclei.
Few-body bound states and resonances in finite volume
Dozent/Speaker: Dr. Sebastian König (TU Darmstadt)
Termin/Date: Do. 16.05.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Since the pioneering work of Lüscher in the 1980s it is well known that considering quantum systems in finite volume, specifically, finite periodic boxes, can be used as a powerful computational tool to extract physical observables. While this formalism has been worked out in great detail in the two-body sector, much effort is currently being invested into deriving analogous relations for systems with more constituents. Such work is relevant not only for nuclear physics, where lattice methods are now able to calculate few- and many-nucleon states, but also for other fields such as simulations of cold atomic systems. In this talk, I will present recent progress that has been achieved for the extraction of few-body bound-state and resonances properties from finite-volume simulations, focusing on results that are valid for systems with an arbitrary number of constituents.
Three-particle dynamics and resonances in a finite volume
Dozent/Speaker: Prof. Dr. Hans-Werner Hammer (TU Darmstadt)
Termin/Date: Do. 9.05.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: The three-particle quantization condition relates finite-volume energies obtained on the lattice to infinite-volume scattering amplitudes. It can be partially diagonalized in the center-of-mass frame by using the cubic symmetry on the lattice. Instead of spherical harmonics, the amplitudes are expanded in the basis functions of different irreducible representations of the octahedral group. We discuss the numerical solution and properties of such a projected quantization condition in a simple model and discuss their interpretation in terms of bound and scattering states.
Moreover, we discuss the extraction of multi-body resonance properties from avoided level crossings in finite volume spectra.
Exact summation of chiral logs in 2D: quasi-renormalizable QFTs, Dixon's elliptic functions, and all that
Dozent/Speaker: Dr. Kirill Semenov-Tian-Shansky (Petersburg Nuclear Physics Institute)
Termin/Date: Do. 18.04.2019, 16:00 - 17:30
Ort/Location: NB 6/173
Übersicht/Abstract: We discuss the non-linear recurrence relations for the coefficients of Leading Logarithms (LLs) in massless Effective Field Theories (EFTs). These relations can be worked out relying on the fundamental properties of quantum field theory amplitudes: analyticity, unitarity and crossing symmetry and generalize the renormalization group technique for the case of non-renormalizable EFTs. We put particular emphasize on the 2D case and address the mathematical properties of the relevant class of non-linear recurrence relations. We present a number of highly non-trivial solutions in terms of elliptic functions. We introduce the class of quasi-renormalizable quantum field theories for which the summation of leading logarithmic corrections for $2 \rightarrow 2$ scattering amplitudes results in manifestation of an infinite number of the Landau poles. Finally, we present explicit examples of quasi-renormalizable QFTs in 2D.
Exclusive decays $\chi_{cJ}\rightarrow K^*(892)K $ within the effective field theory framework
Dozent/Speaker: Dr. Nikolay Kivel (Petersburg Nuclear Physics Institute)
Termin/Date: Do. 17.04.2019, 16:00 - 17:30
Ort/Location: NB 6/173
Übersicht/Abstract: Charmonia hadronic decays $\chi_{cJ}\rightarrow K^*(892)\bar{K} $ are considered within the effective field theory framework. The colour-singlet and colour-octet contributions are studied using potential NRQCD effective theory. It is shown that the heavy quark spin symmetry allows one to establish the relation between the colour-octet matrix elements and to compute the hard spin symmetry breaking corrections. The obtained results are used for the phenomenological description of the measured branching fractions.
Weak reactions of the deuteron in $\chi$EFT
Dozent/Speaker: Dr. Bijaya Acharya (Uni Mainz)
Termin/Date: Do. 24.01.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Chiral effective field theory ($\chi$EFT) provides a unified framework for constructing nuclear potentials as well as the couplings of nuclei to external electroweak sources with controlled uncertainties. However, quantification of these uncertainties even for the simplest of nuclear systems present several challenges. I will present the results of the latest $\chi$EFT calculations, along with uncertainty estimates, for several electroweak observables involving the deuteron: (i) the proton-proton fusion reaction which can not be measured reliably in the lab, but is an important ingredient in stellar astrophysics models, (ii) the muon-deuteron capture rate whose upcoming measurement will determine the low energy constant $c_D$ that parametrizes the pion-range part of the three-nucleon force, and (iii) the response functions that enter the neutrino-deuteron scattering cross section, which upon extension to the medium-mass region of the nuclear chart, will provide important input for current and future neutrino experiments.
Relativistic ab initio calculation in finite nuclear systems and its promotion to nuclear density functional
Dozent/Speaker: Dr. Shihang Shen (University of Milan)
Termin/Date: Do. 17.01.2019, 16:15 - 17:45
Ort/Location: NB 6/173
Übersicht/Abstract: Nuclear ab initio calculation is one of the most fundamental tasks of nuclear physics, and there have been many exciting progresses in the past. Yet it is still difficult to describe the whole nuclear chart using ab initio methods with a low-enough cost to achieve high-enough precisions, comparing with nuclear density functional theory. On the other hand, nuclear density functional has its difficulty and certain terms in the functional are difficult to be determined. In this case, ab initio calculation is important and can help to provide valuable information. I will discuss recent progress of one of the ab initio methods, the relativistic Brueckner-Hartree-Fock theory, in finite nuclear system, with emphasis on the importance of relativistic effect. Then I will show how such ab initio calculation can help to build better nuclear density functionals.