Theory Seminars WS 19/20

Thu 10.10.2019, 16.30 h

J. Thompson (Uni Heidelberg)

(Machine) Learning to be Unsure

Machine learning methods are becoming ubiquitous in particle physics. However, many of these techniques
suffer from overconfidence and a lack of associated uncertainties. In this talk, I will discuss how Bayesian neural networks can address these problems and help us to understand model uncertainties in a machine learning context. In order to do this, I will first explore how these uncertainties can be understood in a frequentist framework, before considering how the Bayesian neural network responds to statistical and systematic uncertainties.


Thu 17.10.2019, 16.30 h

S. Paßehr (RWTH Aachen)

Towards high-precision predictions for Higgs masses and decays in theories beyond the Standard Model

The particle with a mass of about 125GeV that has been discovered at the LHC has properties that, within the current theoretical and experimental uncertainties, leave plenty of room to interpret it as part of an extended Higgs sector in a theory beyond the Standard Model. Supersymmetric models are particularly interesting, since different problems of the Standard Model could be solved at the same time. In such theories, the Higgs mass is a predicted quantity, and in the most popular supersymmetric model, the MSSM, large radiative corrections occur. In order to be competitive with the experimental uncertainty, high-precision predictions are necessary that entail higher-order calculations. In view of feature collider experiments, the endeavor for high-precision predictions should also be extended to the decay widths of Higgs bosons. Apart from the MSSM, only a few other models have received attention. However, especially due to the absence of signs for new physics at the LHC, a model-agnostic approach can be pursued. This leads to generic calculations in general renormalizable theories.
I will discuss recent progress for the prediction of the Higgs-boson spectrum. At first, I will focus on the MSSM, and later I will show the possibilities of generic calculations. Finally, I want to address some properties of higher-order corrections to the decay widths of Higgs bosons.


Thu 24.10.2019, 16.30 h

K. Ng (GRAPPA, Amsterdam)

High-energy Gamma Rays and Neutrinos from the Sun

I will discuss recent results on gamma-ray observations of the Sun with Fermi, which revealed many interesting and surprising features. These gamma rays are expected to be produced by hadronic interactions between cosmic rays and the solar atmosphere. The high flux of gamma rays observed from the Sun requires a large boost of gamma-ray production by some mechanism, which is likely related to solar magnetic fields. Our new results include the first resolved image of the Sun and a mysterious dip in the spectrum between 30-50 GeV. In particular, we also find that the solar gamma-ray spectrum during solar minimum is hard (~E−2.1) and reaches at least 200 GeV. This suggests that ground-based experiments like HAWC and LHAASO will be important for probing the Sun at the TeV regime. Understanding solar gamma rays is crucial for predicting the solar atmospheric neutrino flux, which can potentially be detected by IceCube/KM3NeT, and is important for many aspects of solar dark matter searches.


Thu 31.10.2019, 16.30 h

M. Mühlleitner (KIT)

The Higgs Boson - A Dissection Tool for New Physics

Despite obvious flaws of the Standard Model of particle physics no direct sign of New Physics has been discovered so far. On the contrary, the discovered Higgs boson behaves very Standard-Model-like. With the increasing mass scale of the exclusion limits on New Physics, the Higgs boson itself becomes increasingly important as tool in the search for physics beyond the Standard Model. I will discuss several examples where New Physics might reveal itself indirectly in the Higgs sector.


Thu 14.11.2019, 16.30 h

L. Magnea (U. Torino)

Threshold logarithms beyond leading power
Universality and resummation

Resummations of threshold logarithms at leading power in the threshold variable have a long history and a vast range of phenomenological applications. In recent years, several groups have studied the possibility of extending the resummation technology to next-to-leading power (NLP) in the threshold expansion. I will review the theoretical motivations that suggest this generalisation, discuss some of the technical difficulties, and present recent results on the resummation of leading logarithms at NLP for processes with colour-singlet final states.


Thu 21.11.2019, 16.30 h

M. Bustamante (NBI, Copenhagen)

Pushing Neutrino Physics to the Cosmic Frontier

There is vast potential in high-energy cosmic neutrinos to test particle physics.  The cosmic neutrinos recently discovered by IceCube have the highest detected neutrino energies --- up to a few PeV --- and travel the longest distances --- up to a few Gpc, the size of the observable Universe.  These features make them attractive probes of particle-physics properties, possibly tiny in size, at energy scales unreachable by other means.  The decades before the IceCube discovery saw many proposals of particle-physics studies in this direction. Today, these proposals have become a reality, as we are finally turning them into data-driven tests, in spite of astrophysical unknowns. I will showcase examples of testing neutrino physics at these scales, including stringent tests of physics beyond the Standard Model.  For the coming decade, prospects are encouraging, thanks to upcoming experiments to detect neutrinos with energies a thousand times higher.


Thu 28.11.2019, 16.30 h

C. Garcia-Cely (DESY Hamburg)

Describing dark matter interactions by means of the effective-range theory

In the first part of the talk, I will show that the velocity dependence of the cross section in a non-relativistic scattering process is largely independent of the mediating force. In particular, light mediators, Breit-Wigner resonances and bound states give rise to approximately the same two-parameter cross section formula. This is the so-called effective-range theory, which was historically formulated in order to explain the scattering of low-energy nucleons. In the second part, I will discuss the implications for halos of self-interacting dark matter.


Thu 05.12.2019, 16.30 h

D.J.E. Marsh (Uni Göttingen)

Axions: the Lightest DM Candidate

Axions are a hypothetical class of particle which may constitute all, or a part of, the dark matter in the Universe. Due to their unique symmetry properties, they are the lightest viable dark matter candidate, with the mass possibly being as low as 10-22 eV. I will discuss how this lower limit is derived from observations of the cosmic microwave background anisotropies, and the abundance of galaxies at high redshift, and the observed spin distribution of supermassive black holes. At the lower mass limit, axions manifest novel wavelike behaviour in the inner regions of galaxies that can be used to search for evidence of them in stellar motions. Direct detection of such ultralight axions is extremely challenging, and I will discuss some ideas.


Thu 12.12.2019, 16.30 h

M. Wiesemann (CERN und MPI München)

MiNNLOPS: A new method to match NNLO QCD to parton showers

Measurements of electroweak interactions at the LHC require precision that can be reached only by highest-order perturbative calculations. In this talk, I will discuss the advancement of the current state-of-the-art of theory predictions, focusing on vector-boson pair production processes. This involves the combination of higher-order corrections in both QCD and EW perturbation theory on the one hand, but also the inclusion of all-order logarithmic contributions in regions affected by soft-gluon effects on the other hand. In particular, I will introduce a novel approach to obtain parton-shower predictions with NNLO QCD accuracy.


Thu 19.12.2019, 16.30 h

R. Teyssier (Uni Zürich)

Galaxy formation and cosmic magnetism

Our universe is threaded by billions of galaxies organised in walls, filaments and clusters. The Cold Dark Matter theory is very successful at predicting the dynamics of cosmic structure on large scale. Within the same model, we also want to predict the rich variety of galaxy shapes and colours, a much more challenging endeavour. I will present the current status of the theory of galaxy formation, with an emphasis on small scale physics, such as star formation and the associated stellar feedback. I will show that gas turbulence in the interstellar medium is likely to play a key role. I will finally describe one current frontier in computational galaxy formation, namely the modelling of magnetic fields and their possible impact on the structure of galaxies.


Thu 09.01.2020, 16.30 h

A. Fialkov (Kavli Institute for Cosmology, Cambridge)

Unveiling cosmic dawn

Cosmic dawn - the era of primordial star and black hole formation - is one of the least-explored epochs in the history of the Universe. One of the potentially powerful probes of cosmic dawn is the predicted 21 cm signal of neutral intergalactic hydrogen. The 21 cm signal is tied to the intensity of radiation generated by the first sources of light, and, thus, can be used to constrain the process of primordial star and black hole formation as well as reionization. The first claimed detection of the signal from redshift z ~ 17 by the EDGES Low Band instrument (announced in February 2018) is very controversial and disagrees with standard astrophysics and cosmology. In my talk I will review the status of 21 cm cosmology. I will discuss astrophysical implications of the existing upper limits from z ~ 6-14, the status of the EDGES Low detection, and methods to verify the 21 cm observations in the future.


Thu 23.01.2020, 16.30 h

A. Font-Ribera (UC London)

Studying the expansion of the Universe with quasar spectra

From 2009 to 2014, the Baryon Oscillation Spectroscopic Survey (BOSS) used the SDSS telescope to obtain spectra of 1.5 million galaxies to get very accurate measurements of the Baryon Acoustic Oscillations (BAO) scale at redshift z ~0.5. At the same time, BOSS observed over 184 000 high redshift quasars (z>2.15) with the goal of detecting the BAO feature in the clustering of the intergalactic medium, using a technique known as the Lyman alpha forest (LyaF).

In this talk, I will overview the final results from the LyaF working group in BOSS, and I will present updated results obtained with the extended BOSS survey (eBOSS, 2014-2019). This includes the measurement of BAO at z=2.4 both from the auto-correlation of the LyaF (Sainte Agathe et al. 2019), and from its cross-correlation with quasars (Blomqvist et al. 2019). From the combination of these studies, we are able to measure the expansion rate of the Universe 11 billion years ago with a 2% uncertainty.

Starting in 2020, the Dark Energy Spectroscopic Instrument (DESI) will increase this data set by an order of magnitude. DESI will provide an exquisite measurement of the expansion over cosmic time, while at the same time addressing other interesting questions: the sum of the mass of the neutrino species, properties of dark matter particles, tests of general relativity and the shape of the primordial power spectrum of density fluctuations.


Thu 30.01.2020, 16.30 h

C. Sturm (U. Würzburg)

Static post-Newtonian corrections to the gravitational two-body potential

Deviations from Newton's description of gravitation due to effects of general relativity(GR) can be determined systematically through the calculation of post-Newtonian(PN) corrections. In the talk I will discuss the computation of the static post-Newtonian corrections to the gravitational two-body potential up to fifth PN order. These corrections can be calculated in an effective field theory approach to GR with methods commonly used in high energy particle physics.