Particle Theory Seminars

The particle theory seminars are organized by the Theory Group of the Laboratory for Particle Physics (LTP) at PSI.

Thursday, May 7, 2026, at 10:00 AM in WBGB/019.

Speaker: Heleen Mulder (Nikhef, Amsterdam)

Title: Probing hadronic CP violation with Electric Dipole Moments of paramagnetic molecules

Abstract

Electric Dipole Moments (EDMs) are CP-violating observables, which can be measured in a wide range of low-energy experimental systems. While no non-zero EDM has been measured yet, the strong upper bounds on EDMs allow us to bound possible underlying sources of (BSM) CP violation. In this talk, I will focus on how to interpret the bounds on the EDMs of paramagnetic molecules. The precision of EDM experiments on these specific systems has increased rapidly over the last decade or so. While these increasingly precise bounds are usually translated to upper bounds on the electron EDM, paramagnetic EDMs also receive contributions through CP-violating electron-nuclear interactions. These effective interactions dominantly arise from hadronic sources of CP violation, like the QCD theta term and nucleon EDMs. Calculating electron-nuclear interactions involving meson exchanges or nucleon EDMs in an EFT framework, we have been able to translate paramagnetic EDM measurements to bounds on various hadronic sources of CP violation. While currently less stringent than equivalent bounds from diamagnetic atom and neutron EDM searches, we stress that the paramagnetic bounds are completely independent, and will be crucial in disentangling possible underlying sources of CP violation when non-zero EDM measurements appear. 

Thursday, March 26, 2026, at 10:30 AM in the auditorium WHGA/001.

Speaker: Maxim Mai (University of Bern)

Title: Three-hadron systems: A Tridge between Lattice QCD, Phenomenology and EFTs

Abstract

What is the pattern and mechanisms behind the labyrinthine spectrum of excited hadrons? This question challenges our understanding of strong interaction and is of relevance to many applications where the exact parameters of these states influence interaction patterns. Many open questions in this regard hinge on a precise understanding of the three-body dynamics. 
In the last decade a large progress has been made in this regard triggered and triggering specifically (by) strong progress of Lattice QCD. In my talk, I will show the main ideas and recent outcomes of this journey. Specifically, I will introduce the Finite Volume Unitarity approach, allowing to cross connect Lattice QCD finite volume spectra with input from Effective Field Theory or phenomenology, including results on a1(1260), pi(1300), omega(782) resonances.

Thursday, March 20, 2025, at 11:00 AM in WBGB/020.

Speaker: Eleftheria Solomonidi (IFIC, Universitat de València)

Title: Rare charm decays with cascade topologies & implications for CP violation

Abstract

The CP violation observed in the hadronic decays of charmed mesons remains a puzzling open question for theorists. Calculations relying on the assumption of inelastic final-state interactions occurring between the pairs of pions and kaons fall short of the experimental value. It has been pointed out that a third channel of four pions can leave imprints on the CP asymmetries of the two-body decays. At the same time, plenty of data are available for rare decays such as D⁰ → π⁺π^-l⁺l^-, which provide a promising environment for the search for new physics. With this motivation, we study the cascade topology D⁰ → a₁(1260)⁺( → ρ(770)⁰ π⁺) π^-, which has been measured to contribute significantly to the 4π decays of the same meson, and estimate its effect on the branching ratio of the rare decays. I will also comment on the possibility of this topology contributing to the decay amplitude of D⁰ → π⁺π^- and by extension to the related CP asymmetry.

Thursday, March 20, 2025, at 11:00 AM in WBGB/020.

Speaker: Eirini Lymperiadou (HISKP, University of Bonn)

Title: A model for light tensor meson transition form factors

Abstract

This work deals with the construction of a mostly dynamical approach for the derivation of tensor meson transition form factors (TFFs). The specific case of the a2(1320) tensor meson is examined in the context of the hadronic light-by-light (HLbL) term of the anomalous magnetic moment of the muon a_μ. The goal is to provide a TFF description that will minimize the uncertainties introduced due to model dependencies, approximation and the scarcity of data for this case. The constructed framework and the calculations performed will be presented along with the utilized theoretical concepts, such as dispersion relations, left-hand cuts and the Passarinio-Veltman decomposition. 
 

Friday, February 21, 2025, at 11:30 AM in WHGA/121

Speaker: Lisa Biermann (PSI)

Title: Aspects of the Electroweak Phase Transition

Abstract

The hot early universe might have been in a different ground state than our universe today. This opens the possiblity that our universe transitioned from a high-temperature vacuum (which for example restores the electroweak symmetry) to the low-temperature vacuum of today in which the electroweak symmetry is spontaneously broken via an electroweak phase transition (EWPT). Models beyond the Standard Model (BSM) can (unlike the Standard Model) encorporate a (strong) first-order EWPT while also reproducing all experimental collider data. This additional possibility of a strong first-order EWPT can then serve as the required departure from thermal equilibrium, allowing to explain the observed matter-antimatter asymmetry of the universe at the electroweak scale. Furthermore, a first-order EWPT sources a stochastic gravitational wave background to which e.g. the future space-bound gravitational wave observatory LISA might become sensitive.

In this talk, I will dive into finite-temperature field theory and present several BSM extensions of the scalar sector that allow for first-order electroweak phase transitions, as well as observable gravitational waves. I will highlight how EWPTs position these models at the intersection of collider phenomenology and cosmology, i.e. how they are tested from these two perspectives.