TTK Theory Seminars WS 18/19

Thu 04.04.2019, 16.30 h

S. Recchia (Paris)

Cosmic rays in the turbulent interstellar medium

The cosmic ray(CR) spectrum detected at Earth is a non-trivial combination of the spectrum released by the sources and of the CR propagation and interaction with the interstellar medium (ISM). CRs in the ISM are scattered by the turbulent magnetic field, and, depending on their energy and species, they can incur in severe energy losses. In addition, CRs  can  themselves  excite magnetic  turbulence  in  the background plasma and generate large scale flows like galactic winds, thus affecting their own transport. In this talk I will discuss some  implications of the CR-ISM interaction, in particular  the formation of galactic winds,  the CR escape and propagation in the source proximity,  the identification of the sources of TeV CR electrons and  the ionization of molecular clouds.


Thu 18.04.2019, 16.30 h

N. Bozorgnia (IPPP Durham)

The dark matter distribution in the Solar neighbourhood

The dark matter density and velocity distribution in the Solar neighbourhood are important inputs in the analysis of data from direct detection experiments. Uncertainties in these astrophysical inputs complicate the interpretation of direct detection results, and prevent a precise determination of the particle physics properties of dark matter. High resolution hydrodynamic simulations of galaxy formation have recently become possible and provide important information on the properties of the dark matter halo. I will discuss the local dark matter distribution of Milky Way-like galaxies extracted from state-of-the-art hydrodynamic simulations, present an analysis of direct detection data using this distribution, and discuss the possibility of dark disks. I will also explore possible correlations between the stellar and dark matter velocity distributions, and discuss if there is a subset of stars which trace the dark matter distribution in various simulated Milky Way-like galaxies.


Thu 25.04.2019, 16.30 h

C. Byrnes (U. Sussex)

Cosmology with primordial black holes

Primordial black holes (PBHs) could be part or all of the dark matter, but even if they don’t exist, they constrain the initial conditions of the Universe and inflation. In this talk I discuss how the non-detection of PBHs constrains the primordial power spectrum over a wider range of scales than any other probe. I will discuss some of the subtleties associated with making these constraints, including their dependence on the shape of the primordial power spectrum (which cannot be steeper than k^4), the background equation-of-state parameter and the non-linear relation between the curvature and density perturbations.
More optimistically, if LIGO and Virgo has detected even one PBH, I will show that the reduction in pressure during the QCD transition naturally leads to an exponentially enhanced number of solar mass PBHs. Being below the Chandrasekhar mass, the detection of such light black holes would be a smoking gun for a primordial black hole. Finally, I will show that a mixed dark matter model with WIMPs and PBHs is already observationally excluded, unless the PBHs are at least one million times lighter than the sun.


Thu 09.05.2019, 16.30 h

J. Pradler (HEPHY Wien)

The photon as new physics messenger

In this talk we will discuss cosmological and laboratory probes of dark states directly coupled to the photon. In a first scenario, the phenomenology of sub-GeV dark particles, directly coupled to electromagnetism through higher dimensional operators, is considered. The prospects of detecting such states at the intensity frontier are confronted with low-energy precision tests as well as with cosmological and astrophysical observables. In a second scenario, the perfect possibility that our Universe is filled with some form of dark radiation is explored. If the latter is composed of ultra-light dark photons, they may convert into ordinary photons and induce a variety of signatures. Among them is a modification of the cosmological 21cm signal. An explanation of tentative EDGES result is offered.


Thu 16.05.2019, 16.30 h

J.-N. Lang (U. Zürich)

Automation of Beyond Standard Model electroweak corrections

In search of new fundamental forces and constituents of matter, particle physics will continue looking for smallest deviations from the Standard Model in the coming years. As the windows for new physics shrink and no candidate for new physics emerged, automated precision tools become indispensable for studying Beyond Standard Models (BSM) at particle colliders and beyond. In my talk I review the technology used by the community to automate the computation of next-to-leading order amplitudes in UV complete models and effective field theory. I will discuss obstacles in the automation of UV complete models using the example of renormalization of mixing angles. Finally, I conclude that although BSM EW renormalization is not trivial, automation of EW corrections can be achieved for a large class of BSM models.


Thu 23.05.2019, 16.30 h

E. Sellentin (U. Genf)

A particle physicist's view of Cosmology

Evidently cosmology has the potential to probe particle physics at energetic scales and densities which are inaccessible to human-built detectors. In reality however, cosmology is often convincing to itself, but leaves doubts in particle physicists, especially when it comes to differences of frequentist and Bayesian inference: how can the physics inferred from cosmological data convince particle physicists when the machinery of the inference differs so strongly between our fields? These differences are often belittled, but are a hot and evolving topic in mathematics and statistics itself. This talk will thus focus on the most frequently asked questions that particle physicists pose to cosmology, and answer them with the aid of modern research in


Thu 06.06.2019, 16.30 h

M. Grazzini (U. Zürich)

Heavy-quark production in NNLO QCD

The study of top-quark production and decay is central in the LHC physics programme, allowing precise tests of the Standard Model and offering a window on possible new physics. Accurate theoretical predictions are crucial for these analyses. In this talk, we report on a new calculation of the next-to-next-leading order QCD radiative corrections to the production of top-quark pairs at hadron colliders. The calculation is performed by using the qT-subtraction method to handle and cancel infrared singular contributions at intermediate stages of the computation, and represents its first complete application to the hadroproduction of a colourful high-mass system at next-to-next-leading order. We discuss the calculation of the additional soft contributions needed to implement qTsubtraction for this process, and show first numerical results.



Thu 27.06.2019, 16.30 h

A. Refregier (ETH Zürich)

Cosmological Weak Lensing

Weak gravitational lensing is a unique technique to map the distribution of dark matter in the universe. It is also a sensitive probe of dark energy, large scale structures in the universe, and cosmological parameters. We will first briefly describe the principles of weak lensing. We will then review the current observational status of this field, highlighting several new measurements especially from the ongoing Dark Energy Survey (DES). We will then discuss the status of tensions between cosmological probes, and a new approach based on forward modelling and machine learning.


Thu 04.07.2019, 16.30 h

C. White (Queen Mary London)

The classical double copy

Non-abelian gauge theories underly particle physics, including collision processes at particle accelerators. Recently, quantum scattering probabilities in gauge theories have been shown to be closely related to their counterparts in gravity theories, by the so-called double copy. This suggests a deep relationship between two very different areas of physics, and may lead to new insights into quantum gravity, as well as novel computational methods. This talk will review the double copy for amplitudes, before discussing how it may be extended to describe exact classical solutions such as black holes. Finally, I will discuss hints that the double copy may extend beyond perturbation theory.


Thu 11.07.2019, 16.30 h

D. Malyshev (Erlangen)

The hunt for primordial black holes

The existence of primordial black holes (PBHs) was proposed soon after the discovery of the cosmic microwave background and the realization that the Universe was likely created in a Big Bang. PBHs can form in large energy density fluctuations in the early Universe. Their masses, depending on the time of creation, may range from the Planck mass up to masses of supermassive black holes (SMBHs). Thus, PBHs can provide natural solutions to the problems of dark matter, existence of SMBHs at high redshifts, existence of stellar-mass black holes observed by LIGO/Virgo. In the talk, I will review observational constraints on the PBHs. For small initial masses of PBHs, the most important constraints come from the non-observation of the associated Hawking radiation from PBHs, while at high masses, where the Hawking radiation can be neglected, constraints are derived from gravitational effects, such as gravitational lensing and survival of neutron stars. I will also talk about the recent constraints from the Fermi Large Area Telescope data on Hawking radiation of individual PBHs in the vicinity of the Earth.