HEP Theory Seminars WS 18/19


Thu 11.10.2018, 16.30 h

M. Millea (IAP, Paris)

Next-generation CMB (de)-lensing

In the next generation of CMB experiments are slated to measure the CMB polarization to noise levels and angular resolutions never before probed. One of the most exciting and revolutionary possibilities for this data would be a discovery of the non-zero tensor-to-scalar ratio r, i.e. the first detection of the background of gravitational waves produced by inflation. These gravitational waves are detectable via their impact on CMB B-mode polarization, however the B-modes are also significantly contaminated by the effects of gravitational lensing. Removing this lensing-induced B-mode foreground (called "delensing") will be necessary to obtain the tightest possible constraints on r, maybe turning a few-sigma hint of gravitational waves into a full blown discovery! However, to do so current methods need to be improved, but it is an open question how best to do so. In this talk, I will discuss an optimal Bayesian delensing method which we've developed to solve this problem, as well as a new formulation of weak gravitational lensing. Along the way, I will discuss how we plan to use the volunteer computing project I run, Cosmology@Home, to help us perform some of these computations. Ultimately, this method can yield not only improved constraints on $r$, but also yield better reconstructions of the lensing potential which can be used in cross-correlations with various other low-redshift probes of structures.


Thu 18.10.2018, 16.30 h

E. Vryonidou (CERN)

Precision in EFT studies for top quark and Higgs physics

In this talk I review recent progress in the computation of processes involving top quarks in the framework of Standard Model Effective Theory including NLO QCD corrections. In particular I will discuss the impact of QCD corrections on top pair production in association with a photon, a Z boson and a Higgs in the presence of higher-dimensional operators. Results for Higgs production in processes involving top quark loops will also be discussed. The complementarity of these two classes of processes in constraining top and Higgs quark operators will be demonstrated.


Thu 25.10.2018, 16.30 h

C. Pfrommer (AIP Potsdam)

How cosmic rays shape galaxies and galaxy clusters

Understanding the physics of galaxy formation is an outstanding problem in modern astrophysics. Recent cosmological simulations have demonstrated that feedback by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies and to slow down star formation to the small observed rates. However the particular physical processes underlying these feedback processes still remain elusive. In particular, these simulations neglected magnetic fields and relativistic particle populations (so-called cosmic rays). Those are known to provide a pressure support comparable to the thermal gas in our Galaxy and couple dynamically and thermally to the gas, which seriously questions their neglect. After introducing the underlying physical concepts, I will present our recent efforts to model cosmic ray physics in galaxy formation. I will demonstrate that cosmic rays play a decisive role on all scales relevant for the formation of galaxies, from individual supernova remnants up to scales relevant for entire galaxies and even galaxy clusters. Finally, I will discuss the non-thermal radio and gamma-ray emission of Milky-Way like galaxies and how the next-generation instruments can be used to infer properties relevant for galaxy formation.


Thu 08.11.2018, 16.30 h

S. Forte (Milano University)

Parton Distributions: past, present, future

I discuss the state of the art and future challenges in the determination of parton distributions (PDFs), in view of the physics needs at the LHC and beyond. I review recent developments and current progress both in theory and methodology. I then expose the main outstanding issues and  present ideas for future progress towards the goal of reaching sub-percent accuracy for discovery physics at hadron colliders.


Thu 15.11.2018, 16.30 h

T. Bringmann (Oslo)

Would we notice if dark matter just disappeared?

In the cosmological concordance model, dark matter is assumed to be cold, non-interacting and covariantly conserved, implying that its density decreases linearly with the volume of the expanding universe. The arguably least testable deviation from this simple picture would be that a small fraction of dark matter was, at any time, converted to an invisible form of radiation. I will discuss how cosmic microwave and large-scale structure observations can test such a scenario in a model-independent way, thus putting a conservative bound on how much dark matter could have disappeared at any point during the cosmological evolution. For late conversion times, but still before the onset of structure formation, such a 'disappearance' of a few percent of the dark matter would even mitigate a well-known discrepancy between these datasets. There is a variety of scenarios that can be mapped to this general idea, such as decaying dark matter or merging primordial black holes. In the second part of the talk, I will discuss yet another concrete particle physics realization, featuring a second era of dark matter annihilation after thermal freeze-out. As a bonus, this model naturally allows for velocity-dependent dark matter self-interactions strong enough to address the small-scale problems of structure formation.


Thu 22.11.2018, 16.30 h

G. Buchalla (LMU München)

New Physics in the Higgs Sector - An Effective Theory Approach

The properties of the Higgs boson will be investigated with increasing precision during the coming years in order to probe the dynamics of electroweak symmetry breaking. While the Higgs couplings are compatible with the Standard Model at present, deviations of order 10% or more are currently still allowed. With the precision goal for Higgs couplings of a few percent at LHC Run II and III, it will be possible to test a scenario, in which anomalous Higgs couplings are the dominant effects of new physics in the electroweak sector. Such a scenario leads to an effective field theory (EFT) that has the form of an electroweak chiral Lagrangian, including a light Higgs. We discuss the systematics and the power counting of this approach, its relation to an EFT organized in terms of the canonical dimension of operators, and phenomenological applications. We discuss in particular how the electroweak chiral EFT provides us with a quantum field theory justification for the usual kappa parametrization of Higgs couplings.


Thu 29.11.2018, 16.30 h

J. Adamek (Queen Mary, London)

Relativistic effects in N-body simulations of cosmic large-scale structure

As our advanced telescopes produce ever larger and deeper maps of our Universe we need to consider that observations are taken on our past light cone and on a spacetime geometry that is pervaded by small distortions. A precise understanding of the weak-field regime of General Relativity allows one to model these aspects consistently within N-body simulations of cosmic structure formation. The subtle relativistic effects in cosmic structure can tell us how gravity operates on the largest scales that we observe and may hold the key to unraveling the mystery of dark energy.


Thu 06.12.2018, 16.30 h

A. Penin (University of Alberta)

High-Energy Limit of Mass-Suppressed Amplitudes in Gauge Theories

We study the high-energy limit of the scattering amplitudes suppressed by the leading power of the quark mass in perturbative QCD. We prove the factorization and perform all-order resummation of the double-logarithmic radiative corrections which determine the asymptotic behavior of the amplitudes. We present explicit results for the Higgs boson production in gluon fusion mediated by a light-quark loop and for the leading power suppressed contributions to the quark form factors, which reveal “magical” universality. Nontrivial relations between the asymptotic behavior of different amplitudes and the amplitudes in different gauge theories are found.


Thu 13.12.2018, 16.30 h

F. Day (Cambridge)

Axions and X-ray polarimetry

X-Ray telescope observations have already placed world leading bounds on the axion-photon coupling by searching for axion-photon interconversion in the magnetic fields of galaxy clusters. However, current X-ray telescopes are unable to exploit one of the most striking features of this effect: only photons polarised parallel to the background magnetic field mix with axions. This leads to distinctive polarisation signatures from astrophysical sources. The next generation of polarising X-ray telescopes could detect these signatures. I will discuss the opportunities and difficulties of detecting axions with X-ray polarimetry.


Thu 10.01.2019, 16.30 h

J. Quevillon (LPSC, Grenoble)

The Universal One-Loop Effective Action

In this talk, we present the universal one-loop effective action obtained by integrating out massive, non-degenerate multiplets, focusing on the coefficients of operators of dimension up to six composed of Standard Model fields. Our general results may be applied to loops of non-degenerate heavy fermions or bosons. We discuss the relationship of our results to the SM Effective Field Theory and their ongoing improvements. As an example, we apply our method to the general MSSM with non-degenerate stop squarks.


Thu 17.01.2019, 16.30 h

P. Blasi (Gran Sasso Science Institute)

Cosmic ray self regulation

The transport of cosmic rays inside accelerators as well as around the sources and during their journey throughout the Galaxy is strongly affected by the presence of cosmic rays themselves, in that the electric current they carry is responsible for excitation of electro-magnetic instabilities that may strongly modify cosmic ray propagation. In this sense cosmic ray transport, at least in some energy range, may be self-regulated by a chain of non-linear phenomena. I will discuss the physics of some of these phenomena and the implications that may arise from them. In particular I will discuss the role of these non-linear phenomena for particle acceleration and the possible features in the cosmic spectrum that may arise from them. Special emphasis will be given to a discussion of the physical meaning of cosmic ray escape from the Galaxy and the related neutrino flux.


Thu 24.01.2019, 16.30 h

A. Lenz (Durham)

Pinning down the flavour anomalies

We review current hints for deviations of flavour observables from the corresponding standard model (SM) predictions and emphasize a precise knowledge of the involved hadronic quantities. We also discuss several of the proposed extensions of the  SM that might be able to explain these anomalies.


Thu 31.01.2019, 16.30 h

E. Ishida (U. Clermont-Ferrand)

Adaptive Learning for Astronomy

The full exploitation of the next generation of large scale photometric surveys depends heavily on our ability to provide reliable classification and redshifts based solely on photometric data. In preparation for this scenario, there has been many attempts to approach the supernova photometric classification and the photometric redshift estimation issues. Although different methods present different degree of success, text-book machine learning methods fail to address the crucial issue of lack of representativeness between spectroscopic (training) and photometric (target) samples. In this talk I will show how Adaptive Learning can be used to construct a recommendation systems specially design for the astronomical case and ensure optimal exploration of spectroscopic resources. I will also illustrate how this crucial issue lies in the boundary between machine learning and statistics, helping us to appreciate the connection between these two fields.