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)



Thu 29.11.2018, 16.30 h

J. Adamek (Queen Mary, London)



Thu 06.12.2018, 16.30 h

A. Penin (University of Alberta)



Thu 13.12.2018, 16.30 h

F. Day (Cambridge)



Thu 10.01.2019, 16.30 h

J. Quevillon (LPSC, Grenoble)



Thu 17.01.2019, 16.30 h

P. Blasi (Gran Sasso)



Thu 24.01.2019, 16.30 h

A. Lenz (Durham)



Thu 31.01.2019, 16.30 h

E. Ishisa (Clermont-Ferrand)