Theorie Palaver

Programm für das Sommersemester 2019

Dienstags, 14:30 Uhr s.t.

Ort: Institut für Physik, MITP seminar room

16.04.19Zoltan Trocsanyi, Eötvös Lorand University
We consider an anomaly free extension of the standard model gauge group G by an abelian group to GxU(1)Z. The condition of anomaly cancellation is known to fix the Z-charges of the particles, but two. We fix one remaining charge by allowing for all possible Yukawa interactions of the known left handed neutrinos and new right-handed ones that obtain their masses through interaction with a new scalar field with spontaneously broken vacuum. We discuss some of the possible consequences of the model. Assuming that the new interaction is responsible for the observed differences between the standard model prediction for the anomalous magnetic moment of the muon or anti-muon and their measured values, we predict the size of the vacuum expectation value of the new scalar field.
23.04.19Brian Henning, University of Geneve
Common to every theoryfundamental, phenomenological, or even toyis a notion of degrees of freedom living on some space(time). While at first glance such a statement may sound so broad as to render it meaningless, it actually imposes quite a strong structure. The basic point is that the degrees of freedom allow for certain types of measurements (scattering, correlation functions, etc.), and that these measurements can only take values consistent with the relevant spacetime symmetry. Turning this around, we can use spacetime symmetry to parameterize the kinematics of experiments, thereby allowing a clearer way to study the dynamics. In this talk I will show that this is precisely what effective field theory (EFT) does. Viewing EFT through the lens of spacetime symmetry unveils previously unnoticed structures. For example, in an N particle phase space of massless particles we will find a certain U(N) actiongeneralizing the U(1)N little group scaling of the particleswhose harmonic decomposition also gives a harmonic decomposition of the spacetime symmetry group (in this case, the conformal group). This provides a basis not only for an EFT, but also a basis for the Hilbert space of free theories. As I will explain, the latter is an essential ingredient in the recently revived technique of Hamiltonian truncation to numerically study quantum field theories.
30.04.19Daniel G. Figueroa, EPFL, Switzerland
The inflationary sector might very well have no direct couplings to other species, apart from inevitable gravitational interactions. In the context of General Relativity, a thermal universe can still emerge after inflation if i) a radiation sector is excited towards the end of inflation, and ii) the equation of state after inflation becomes sufficiently stiff w >> 1/3. In such circumstances, the inflationary background of gravitational waves (GWs) is significantly enhanced, making this signal (potentially) observable by GW detectors. I will discuss first how LISA & LIGO could measure this signal, probing in this way the expansion rate of the early Universe. Secondly, I will show that the very same enhancement of the GW signal leads however to an inconsistency of the scenario, violating standard bounds on stochastic backgrounds of GWs. Finally, I will show that the very existence of the Standard Model Higgs can actually save the day, by simply demanding the Higgs to be non-minimally coupled to gravity.
03.05.19Fang Ye, Korean Advanced Institute of Science and Technology
Cosmological relaxation of the electroweak scale proposed by Graham, Kaplan and Rajendran (GKR) arises as a solution to the naturalness problem. However, certain downsides exist in the original GKR scenario which relies on the Hubble expansion to dissipate the relaxion energy, such as the extremely small parameters and large e-foldings. In order to avoid these issues, in this work, we investigate the plausibility of using fermion production as a dominant friction source to drain the relaxion energy, maintaining the slow-roll of the relaxion. Benchmark points will be given for successful relaxation, and some remaining issue and the phenomenology will also be discussed.


07.05.19Claudius Krause, Fermilab
With no direct observation of physics beyond the Standard Model (SM) at the LHC, bottom-up Effective Field Theories (EFTs), especially in the newly-established Higgs sector, have become popular in the past years. Depending on the assumptions on the Higgs-like scalar, two different EFTs can be constructed: The Standard Model EFT (SMEFT) that assumes the Higgs is part of an SU(2) doublet; and the Electroweak Chiral Lagrangian (EWChL) that treats the Higgs scalar as independent singlet. In the first part of my talk, I will discuss the assumptions underlying these two EFTs and the different power counting schemes that arise within them. In the second part, I will derive a master formula for the complete one-loop renormalization of a generic Lagrangian employing background-field method and super-heat-kernel expansion. Then, I will apply the formula to the two Higgs EFTs and discuss the results.
14.05.19Georgios Papathanasiou, DESY
Scattering amplitudes form a bridge connecting theoretical particle physics with the real world of collider experiments, yet their computation by means of Feynman diagrams quickly becomes prohibitive. Focusing on the simplest case of N=4 super Yang–Mills theory, in this talk I present recent progress in bypassing these limitations and directly constructing amplitudes, by exploiting their expected analytic structure. First, I describe the discovery of new, possibly universal analytic properties known as the extended Steinmann relations, or equivalently cluster adjacency, as well as the coaction principle. Then, I demonstrate their power in computing the six-particle amplitude up to seven loops, as well as the seven-particle amplitude up to four loops, and discuss further applications.
21.05.19Aleksandrs Aleksejevs, Memorial University of Newfoundland
As the new generation of precision experiments aims to search for physics beyond the Standard Model, it becomes increasingly important to evaluate the relevant higher-order corrections. In collaboration with MOLLER and P2, we work to address the full set of two-loop electroweak radiative corrections to electron-electron and electron-proton scattering cross sections and asymmetries. In this presentation, we will describe the recent developments in dispersive sub-loop insertion approach in two-loop calculations, which would allow partial automatization.


11.06.19Vedran Brdar, MPI Heidelberg
In the first part of the talk I will discuss the low scale (10 - 100 TeV) left-right symmetric model with “naturally” small neutrino masses generated through the inverse seesaw mechanism. The inverse seesaw implies the existence of novel fermion singlets, S, with Majorana mass terms as well as the "left" and "right" Higgs doublets. These doublets provide the portal for S and break the left-right symmetry. The generic feature of the model is the appearance of heavy pseudo-Dirac fermions, formed by S and the right-handed neutrinos, which have the masses in the 1 GeV - 100 TeV range and can be searched at both current and future experiments such as LHC, SHiP, DUNE, and FCC-ee. In the second part of the talk I will introduce the "neutrino option", a recent proposal that the electroweak hierarchy problem is absent if the generation of the Higgs potential stems exclusively from quantum effects of heavy right-handed neutrinos which can also generate active neutrino masses via the type-I seesaw mechanism. In this framework, the tree-level scalar potential is assumed to vanish at high energies. Such a scenario therefore lends itself particularly well to be embedded in a classically scale-invariant theory. I will demonstrate that the minimal scale-invariant framework compatible with the "neutrino option" requires the Standard Model to be extended by two real scalar singlet fields in addition to right-handed neutrinos. In addition, the phase transition connected with scale symmetry breaking is of strong first order with a substantial amount of supercooling. This yields a sizable gravitational wave signal, so that the model can be fully tested by present and future gravitational wave observatories.
18.06.19Peter Lowdon, cole Polytechnique
Local formulations of quantum field theory imply that gauge theory correlators can potentially contain generalised infrared poles. In this talk I will outline the theoretical significance of these components, and report on recent lattice fit results for the gluon propagator.
25.06.19Johann Usovitsch, University of Dublin
We will give an introduction to Kira a Feynman integral reduction program. Further on we will report about the recent progress made in the development of this program. The development is focused on algorithmic improvements that are essential to extend the range of feasible high precision calculations for present and future colliders. Finally we will introduce the future feature of Kira: reconstruction of rational functions from samplings over the finite field.
09.07.19Michael Benzke, Hamburg University
If you want to look for new physics in decays that involve hadrons, you need to consider the related hadronic uncertainties. In this talk I will discuss hadronic uncertainties in the decay B to X_s l^+l^- in the framework of SCET at subleading power. The talk is based on 1705.10366 and work in progress.
Koordinator und Kontakt:
Dr. Zoltan Ször
Institut für Physik, THEP