Theorie Palaver

Programm für das Wintersemester 2019/2020

Tuesdays, 14:30 Uhr s.t.

Ort: Institut für Physik, MITP seminar room

15.10.19Fatemeh Elahi, IPM, Tehran
A few hundred cold gas clouds were recently discovered, each situated a few hundred parsecs from the center of the Milky Way Galaxy. These gas clouds can provide unprecedented sensitivity to dark matter-standard model interactions. The main physical basis is simple: dark matter tends to have a higher temperature than the coldest interstellar and intergalactic gas. Therefore, dark matter can heat the gas to higher-than-observed temperatures, if dark matter interacts enough with baryons or electrons in the gas cloud. In this talk, I will discuss the bounds cold gas clouds give on ultra-light dark photon, vector portal, and millicharged dark matter.

22.10.19Andreas Helset, Bohr Institute
We formulate an effective field theory describing large mass scalars and fermions minimally coupled to gravity. The operators of this effective field theory are organized in powers of the transfer momentum divided by the mass of the matter field, an expansion which lends itself to the efficient extraction of classical contributions from loop amplitudes in both the post-Newtonian and post-Minkowskian regimes. We use this effective field theory to calculate the classical and leading quantum gravitational scattering amplitude of two heavy spin-1/2 particles at the second post-Minkowskian order.

29.10.19Alexey Vladimirov, Regensburg University
The evolution of transverse momentum dependent distributions or TMD-evolution differs from ordinary renormalization group evolution in several aspects. Two main ones are its double-scale nature and the presence of non-perturbative component. In the talk, I review the recent progress in studies of TMD-evolution, including the proof of rapidity-divergence renormalization, soft-rapidity correspondence, zeta-prescription, and comparison with the experimental measurements.

05.11.19Enrico Morgante, Johannes Gutenberg-Universität
The relaxion mechanism is a proposed solution to the hierarchy problem, in which the EW scale is set by the classical evolution of a scalar field in the early universe. In this talk, I will review this construction and discuss some recent developments.

19.11.19Leonardo Vernazza, Nikhef
Scattering processes near threshold develop large logarithms, that need to be resummed. In my talk I will focus in particular on electroweak annihilation processes, such as Drell-Yan and Higgs production in gluon fusion, and discuss the underlying factorisation theorems which allow the resummation of such logarithms at next-to-leading power, comparing diagrammatic and effective field theory methods.

03.12.19Matthias Heller, Johannes Gutenberg-Universität
The Drell-Yan production of charged lepton pairs is one of the key processes measured at hadron colliders. The QCD corrections to the cross-section are known to order \(\alpha_s^2\) and electroweak corrections are known to order \(\alpha\). The next important step for a better theoretical understanding is the complete calculation of the mixed QCD-EW corrections of order \(\alpha \alpha_s\). In this talk, I report on the first calculation of the virtual two-loop corrections of order \(\alpha \alpha_s\) to the cross-section. The calculation is carried out analytically using tensor reduction, IBP relations and the method of differential equations. We validate a previous calculation of the subset of mixed QCD-QED corrections and show how the jet and soft functions of that reference can be used to subtract the infrared divergencies of the complete mixed QCD-electroweak virtual corrections. In the talk, I will focus on the calculation of the master integrals, which involve algebraic letters in the differential equation.

10.12.19Guoxing Wang, Peking University
The invariant mass distribution of top quark pair production is an important observable whose threshold behavior is sensitive to top-quark mass. In this talk, I will focus on the invariant mass distribution of top-quark pair production in threshold region and soft region. I will first talk about the resummation of Coulomb corrections in threshold limit β → 0. We combine the resummation with fixed-order results and present phenomenologically relevant numeric results. We find that the resummation effect significantly enhances the differential cross section in the threshold region, and makes the theoretical prediction more compatible with experimental data. As for soft limit z → 1, I’ll talk about the calculation of NNLO soft function which is a major bottleneck in pushing up the soft resummation accuracy of top-quark pair production. We show the non-trivial structure of three-parton correlations and the consistent extraction of NNLO soft fragmentation function. At the end, I’ll show the preliminary results of hard function and its application to transverse momentum resummation for top-quark pair production and decay in the future.

17.12.19Fazlollah Hajkarim, Frankfurt University
Using the quantum chromodynamics (QCD) equation of state (EoS) from lattice calculations we investigated QCD effects on the first order primordial gravitational waves (PGWs) produced during the inflationary era. We also considered the cases for vanishing and nonvanishing lepton asymmetry where the latter one is constrained by cosmic microwave background experiments. Also, we investigated scenarios that inflation is succeeded by a phase where the energy density of the Universe was dominated by a scalar component with a general equation of state. Then we evaluated the spectrum of primordial gravitational waves induced in the post-inflationary Universe. We showed that if the energy density of the Universe was dominated by some specific fluid 𝜙 before Big Bang Nucleosynthesis (BBN), its equation of state could be constrained by gravitational wave experiments. Moreover, we studied the effect of QCD and electroweak transitions on the induced (or second order) PGW from scalar perturbations which is different from the first order PGW spectrum. Finally, I briefly discuss the production of dark matter (DM) in an early matter era dominated by a heavy long lived scalar field.

28.01.20Fabian Lange, RWTH Aachen
The Gradient-Flow formalism was primarily introduced for simulations of Quantum Chromodynamics (QCD) on the lattice and has now found widespread use in this field. In addition, it offers the potential for cross-fertilization between perturbative and lattice calculations. In my talk I will introduce the Gradient-Flow formalism and outline the perturbative approach. Afterwards, I will present two examples of its applications. First, I will show how it could be used to extract the strong coupling constant from lattice simulations. As second application, I will illustrate how it helps to define the energy-momentum tensor of QCD on the lattice.

04.02.20Avirup Ghosh, Harish Chandra Research Institute
The existence of Dark Matter (DM) as one of the major component of our universe is inevitable today. Though there are several evidences of DM, all of them are gravitational in nature. The particle nature of DM is yet unknown. The most popular theory of particle DM is Weakly Interacting Massive Particles(WIMP) which though very interesting from the point of view of naturalness, is not yet tested in experiments. On the other hand, Feebly Interacting Massive Particles (FIMP) as an alternative scenario is gaining attention in recent times which due to their feeble interaction can not be observed in current generation of experiments. I shall discuss in my talk how in some FIMP scenarios the tiny coupling of FIMP DM give rise to interesting signals.


Dr. Zoltan Ször
Institut für Physik, THEP