Theory Seminars SS 21

Thu 15.04.2021, 16.30 h

Maurizio Pierini (CERN)

Exploiting anomaly detection for new physics identification at the LHC

Host: M. Krämer

Anomaly detection techniques have been proposed as a way to mitigate the impact of model-specific assumptions when searching for new physics at the LHC, e.g., at event selection or at trigger. We will discuss how these techniques could be utilized in the data processing workflow and the impact they could have on how we search for new physics.


Thu 22.04.2021, 16.30 h

Ellen Zweibel (Madison)

Multiscale Challenges in Cosmic Ray Astrophysics

Cosmic rays have a major role in the dynamics and thermodynamics of interstellar gas. They can drive galactic outflows, and their influence may even be felt in the circumgalactic medium. The picture that has evolved over many decades, inspired originally by studies of cosmic ray confinement and propagation in the Milky Way, is that cosmic rays are coupled to the thermal gas through scattering off microscale fluctuations in the ambient magnetic field. Later it was appreciated that these fluctuations can be excited through kinetic instabilities of the cosmic rays themselves, and that the fluctuations mediate energy and momentum exchange with the thermal gas. Even more recently it has become recognized  that conditions on _intermediate_ scales have an important role in regulating these interactions as well. I will describe some of the challenges in understanding and modeling the coupling between cosmic rays and thermal gas accounting for conditions from plasma kinetic to global scales.

Host: P. Mertsch


Thu 29.04.2021, 16.30 h

Manfred Kraus (Florida State University)

Precision phenomenology for the ttW process

In the light of recently reported discrepancies between data and theory for the ttW normalization in either ttH measurements or tttt searches in the multi-lepton channels the accurate modelling of these complex signatures is revisited. I will discuss the current status of the modelling of the complex ttW signatures with special emphasis on our recent calculation of top-quark pair production in association with a W boson in the POWHEG-BOX framework. I will show a comparison with other Monte Carlo generators for the two same-sign lepton signature commonly used by the experiments. Special focus will be put on the assessment of the importance of subleading electroweak contributions in the modelling of the production process. In addition, I will discuss theoretical uncertainties of our predictions, which are estimated by variations of the renormalization and factorization scales, also matching uncertainties are estimated.

Host: M. Worek


Thu 06.05.2021, 16.30 h

Michael Kachelriess (Trondheim)

Antinuclei as probe for exotic physics

Antideuteron and antihelium nuclei have been proposed as promising detection channels for dark matter because of the low astrophysical backgrounds expected. After a brief review of the current experimental situation, I discuss some of the various flavors of the coalescence model used to describe the formation of light (anti-) nuclei. Then I present results for a coalescence model based on the Wigner function representations of the produced nuclei states, which includes both the process-dependent size of the formation region of antinuclei, and the momentum correlations of coalescing antinucleons in a semi-classical picture.  Therefore this model allows one to calculate in a consistent frame-work the antideuteron and antihelium fluxes both from secondary production and from dark matter annihilations, and I present results for resulting fluxes of  these antinuclei.

Host: P. Mertsch


Thu 20.05.2021, 16.30 h

Christopher Monahan (William&Mary)

Probing hadron structure on the lattice

Recent developments in theoretical approaches to the three-dimensional structure of hadrons have enabled the very first systematic calculations of parton distribution functions, which describe the internal longitudinal momentum distribution of fast-moving hadrons, directly from quantum chromodynamics. These computations are theoretically and numerically challenging, and renormalisation of the relevant non-local operators is tricky, in large part because of the multiple scales involved. I will introduce lattice QCD and review the recent developments in lattice calculations of three-dimensional hadron structure. In particular, I will highlight our proposal to use the gradient flow, which is a one-parameter mapping of the QCD fields that suppresses ultraviolet fluctuations, as a tool to explore hadron structure.

Host: R. Harlander


Thu 10.06.2021, 16.30 h

Simeon Bird (U of California, Riverside)

Simulations of Cosmological Structure and Machine Learning

The large scale distribution of matter in the Universe contains the answers to many mysteries, such as the nature of dark matter, the reionization of the Universe, and the growth of galaxies. Cosmological simulations are the only way to understand these questions. I will talk about how modern current simulation models, work, discuss some new models and improvements in our latest simulation runs, especially our implementations of reionization and cosmology. I will then talk about some new work to dramatically expand the region of applicability of these simulations using machine learning. This can both to expand their dynamic range and combine different simulations to infer the physical parameters of the Universe.

Host: J. Lesgourgues


Thu 17.06.2021, 16.30 h

Andrzej Czarnecki (Alberta University)

Muon's anomalous magnetic moment g-2

The recent Fermilab result on the muon g-2 is put in perspective, including the history of g-2 measurements and theoretical evaluations. The behaviour of a relativistic muon in a magnetic field will be described in an accessible manner. A variety of Standard Model contributions and some possible New Physics effects will be discussed. Electron g-2 experiments will be mentioned as well.

Host: R. Harlander


Thu 24.06.2021, 16.30 h

Ben Safdi (University of Michigan)

Compact stars as axion laboratories

The quantum chromodynamics axion and axion-like particles are some of the most sought-after beyond the Standard Model particles at present because of their possible connections with the strong-CP problem, dark matter, and ultraviolet physics such as Grand Unification and String Theory.  Laboratory searches are underway around the world to search for these hypothetical particles, but certain regions of axion parameter space -- such as ultralight axions with weak couplings to matter -- are notoriously difficult to probe with terrestrial experiments, despite their theoretical motivations.  However, axions in this part of the parameter space may be produced in abundance within compact stars such as white dwarfs and neutron stars.  It has long been recognized that axion production in compact stars opens up a new pathway for them to cool.  I will point out, however, that axions may also lead to novel X-ray signatures around these stars, whereby the axions are produced within the stellar cores and then convert to photons in the strong magnetic fields surrounding the stars.  I will discuss recent data taken by the XMM-Newton and Chandra telescopes from nearby neutron stars and white dwarfs that provide some of the strongest probes to-date of axions by searching for these processes.

Host: S. Manconi


Thu 01.07.2021, 16.30 h

Yu-Dai Tsai (Fermilab and University of Chicago)

Resonant Self-Interacting Dark Mesons and New Millicharge Searches

I will first talk about a new dark matter model, resonant self-interacting dark meson, and the observational and experimental signatures.
I will then give a general overview of experimental facilities with high energies and high intensities, focusing on proton fixed-target (at Fermilab) and hadron collider experiments (at the LHC). I will classify the searches as "decay" searches and "scattering" searches, and detail the new physics models of interest. These experiments can help close the gap between the low-mass/high-mass gap for the dark sector searches, and the low-energy/high-energy gap for the neutrino study.
I will present two new experimental proposals, LongQuest and FORMOSA. LongQuest is a multi-purpose proton fixed-target experiment, studying the decay particles, including dark photons and axion-like particles. FORMOSA is a specialized LHC forward experiment that is the world-most sensitive proposal to study millicharged particles, and also has the potential ability to study heavy neutrino and tau neutrino dipole moments.
Finally, I will discuss the possibility of constructing TeV forward neutrino detectors at the LHC forward physics region to conduct high-energy neutrino cross-section measurements.
This talk is based on arXiv:2008.08608, arXiv:1908.07525, arXiv:2010.07941, arXiv:1812.03998, and ongoing works. Similar concepts can be applied to electron colliders (e.g., ILC & Belle) and other fixed-target (e.g., J-PARC) experiments.
A pedagogical version of this talk can be found here.

Host: F. Kahlhoefer


Thu 08.07.2021, 16.30 h

Suvodip Mukherjee (Amsterdam)

Panorama of the Universe with multi-messenger observations

The discovery of astrophysical gravitational waves has opened a new avenue to explore the cosmos using transients. I will discuss a few new frontiers in the field of physical cosmology and fundamental physics that can be explored using gravitational wave signal detectable from the currently ongoing network of gravitational wave detectors such as LIGO/Virgo, and in the future from gravitational wave detectors such as KAGRA, LIGO-India, LISA, Einstein Telescope, and Cosmic Explorer. I will elucidate the existence of synergies between electromagnetic probes and gravitational wave probes and their importance in understanding the standard model of cosmology and the fundamental laws of physics that govern it.

Host: J. Lesgourgues


Thu 15.07.2021, 16.30 h

Claude Duhr (CERN)

Inclusive colour-singlet production at N3LO

I will review recent advances in our ability to compute QCD corrections for colour-singlet production at N3LO, like Higgs, Drell-Yan, W or WH production, and I will discuss some phenomenological implications. These results give us for the first time an insight into the stability and the convergence of perturbative QCD predictions at this order and at LHC energy scales.

Host: M. Worek