<p><span style="color: black;">I will describe a new portal to the dark sector that is a topological interaction, similar in structure to the WZW term in QCD. This portal connects pions and photons to a pair of dark matter (DM) pions, and can explain the observed DM relic abundance for GeV dark pion mass. Its topological nature implies the absence of signals in direct or indirect detection experiments despite the low-ish scale, consistent with observation. From the EFT perspective, this interaction is an unusual example of a quantised topological term that does not match an anomaly. Rather, it encodes a generalised symmetry structure known as 2-group symmetry. Matching this generalised symmetry guides our building a TeV-scale completion for the topological portal, which is here essential to make predictions for colliders like the LHC. I will conclude by putting this model in a broader context of possible topological couplings to dark sectors, some of which seem to arise naturally from string theory.</span></p>

<p><span style="color: rgb(0, 0, 0);">In this talk I will 1</span><sup style="color: rgb(0, 0, 0);">st</sup><span style="color: rgb(0, 0, 0);"> explore our efforts of trying to understand material properties of bacterial cell envelope, which is a unique active material whose mechanical properties are not yet understood despite decades of active research. In the case of the bacterium we are working with, the cell envelope consists of two lipid bilayers filled with membrane proteins, with an aqueous environment (termed periplasm) —containing a stiffer peptidoglycan cell wall —in-between. The cell wall exhibits stress stiffening with elasticity of the wall (E) behaving as E~P^1.2, where P is the pressure the cell is under. We have experimentally found that when subjected to an osmotic stress (swelling) and held under constant volumetric strain, cells undergo delayed lysis, and die abruptly. The timing of death is invariant to the volumetric strain impose by different stress magnitudes, but the probability of death rises with it. We observe time-dependent plasticity at sub-lethal volumetric strains, and the rate of active repair of the envelope sets the probability, but not the timing of death. Finally, the deformation is purely axial. I will go over the experimental results and our efforts to understand from them the material nature of the cell envelope.</span></p><p><br></p><p><span style="color: rgb(0, 0, 0);">In the 2</span><sup style="color: rgb(0, 0, 0);">nd</sup><span style="color: rgb(0, 0, 0);"> part of the talk, I will turn my attention to electrical properties of the same bacterial cells. Maintaining sufficient electrochemical gradient of protons, the so-called proton motive force (PMF), is the hallmark of life. Ordinarily, the main contribution to the electrochemical gradient of protons in bacterial cells comes from the electrical potential across the biological membrane. The generation of this gradient is assumed active in all living cells. We recently proposed that in the bacterium we study, proteins called proton-ion antiporters are used for this active generation. However, unlike in other living cells, bacteria have a large number of ribosomes, which are protein factories. They make all the proteins in the cell, including themselves, and significantly contribute to the total concentration of membrane-impermeable negative ions. This raises the question of whether active generation of membrane potential is even needed. We directly measured the contribution of the passive, Donnan potential, which is a result of the accumulation of these negative impermeable ions in the cells. We find that its value is significantly smaller than what one would expect from ribosomal content, confirming that membrane potential is actively generated. However, the result raises the question of why do all negative, impermeable ions in E. coli not contribute to the membrane potential. I will ponder over where did the charge ‘go’.</span></p>

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