Centre de Physique Théorique

Relativistic microcausality is the statement that local field operators commute outside the light cone. This condition is known to break down in low-energy effective theories, such as <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>P</a:mi><a:mo stretchy="false">(</a:mo><a:mi>X</a:mi><a:mo stretchy="false">)</a:mo></a:mrow></a:math> models with a derivative interaction term of the “wrong sign.” Despite their Lorentz-invariant form, these theories can exhibit superluminal propagation on Lorentz-breaking backgrounds. We approach this phenomenon by computing the full operator-valued commutator in position space, perturbatively in interaction picture. After testing this formalism on a <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>λ</e:mi><e:msup><e:mi>ϕ</e:mi><e:mn>4</e:mn></e:msup></e:math> theory, we apply it to a <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>P</g:mi><g:mo stretchy="false">(</g:mo><g:mi>X</g:mi><g:mo stretchy="false">)</g:mo></g:math> model. There, we show that the perturbative corrections to the free-theory commutator contain derivatives of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>δ</k:mi></k:math> functions with support on the standard Minkowski light cone. While these corrections vanish on Lorentz-invariant states, they become “activated” on states where Lorentz symmetry is spontaneously broken. In this case, they approximate the new “sound cone” by means of a Taylor expansion. By applying linear response theory to an extended source, we show that deviations from standard causality are already present at first order in this expansion. Finally, we try to understand what goes wrong with the standard argument according to which Lorentz invariance implies microcausality.

Understanding the complex landscape of protein interactions, especially those involving intrinsically disordered proteins (IDPs), is fundamental yet challenging due to their structural heterogeneity and flexibility. Traditional sequence-based homology methods frequently fall short in characterizing IDP functions and interactions. Here, we present a novel approach leveraging supervised and unsupervised machine learning techniques, focusing exclusively on the compositional features of proteins. An Edmond-Ogston-inspired mixing model can reliably predict the degree of survivin (BIRC5) binding as a function of peptide composition alone, revealing a first interesting connection with the composition diagrams of chemical thermodynamics. By representing protein sequences through their functional group compositions, we demonstrate that specific compositions robustly predict binding interactions with survivin, an important human protein in cellular regulation pathways. Experimental validation via peptide microarray confirms the predictive power of our simplified compositional model, independent of exact amino acid sequences. Extending this method across the human proteome, we identified distinct compositional signatures correlating with survivin interactions and revealed fine grained biologically meaningful functional clusters based on compositional similarity. Our findings suggest a compositional blueprint underpinning protein interactions, offering a powerful, simplified framework to decode complex biological networks.

Recent observational evidence of axially symmetric anisotropies in the local cosmic expansion rate motivates an investigation of whether they can be accounted for within the Lemaître-Tolman-Bondi (LTB) framework with an off-center observer. Within this setting, we compute the exact relativistic luminosity distance via the Sachs equation and compare it with the approximate expression obtained from the covariant cosmographic approach (including Hubble, deceleration, jerk and curvature parameters). This comparison allows us to identify the regimes in which the covariant cosmographic method remains reliable. In addition, we compare the LTB relativistic distance for small inhomogeneities with the corresponding result derived from linear perturbation theory (LPT) in the standard cosmological model. This analysis establishes a precise correspondence between the LTB and LPT approaches, offering a consistent dictionary for the interpretation of the observed anisotropies of the large-scale gravitational field. This analysis will be instrumental in interpreting expansion-rate anisotropies, facilitating investigations of the local Universe beyond the FLRW framework with a fully non-perturbative metric approach.

We theoretically investigate the low-temperature magnetic and quantum properties of hexanuclear Fe^{3+}_6 complexes under an external magnetic field. We primarily study the impact of competing exchange interactions and their asymmetries induced by the Jahn-Teller distortion on the quantum properties of the complexes. The inequality in exchange interactions lifts the ground-state degeneracy that gives rise to complex quantum behavior. By constructing the ground-state phase diagram and analyzing magnetization, we identify key magnetic phases and critical phenomena. We further quantify quantum correlations using tripartite entanglement negativity and conditional von Neumann entropy to unveil how the Jahn-Teller effect enhances intra-triangle entanglement while modulating inter-triangle correlations. Our findings highlight the Fe^{3+}_6 complex as a promising molecular platform for tunable quantum correlations, with potential applications in quantum information processing and molecular qubits.

Cells are active mechanical objects: they are subject to forces, exert force, and interpret changes in force as biological information. We now understand much about how this occurs at the molecular and single-cell level. We also appreciate that mechanobiology gains even greater complexity when it operates at the multicellular level of tissues and organisms. Here, cells exert forces on other cells within tissues to support morphogenesis and homeostasis; but these forces must also be accommodated to ensure that tissue integrity is preserved. Cell-cell adhesion junctions play important roles in transmitting, resisting, as well as detecting mechanical forces in coherent tissues. In this brief article we consider how epithelia adapt to mechanical stresses, focusing on recent developments in understanding the sources of force and new mechanisms for adherens junctions and desmosomes in mechanotransduction.

Consider a conformally covariant four-point function of identical scalar operators with a discrete spectrum, a twist gap, and compatible with the unitarity conditions. We give a mathematical proof confirming that the spectrum and OPE coefficients at large spin and fixed twist always become that of a generalized free field theory.

SUMMARY During hippocampal Sharp Wave Ripples, sequences of awake coding activities are replayed with a rhythmic timing and high level of synchrony favorable for synaptic plasticity and transfer of information to downstream structures. Previous work reported the emergence of ripples at P10 in the CA1 region, together with the development of inhibition. On the other hand, neither the relationships between perisomatic inhibition and ripples nor their developmental emergence have been investigated in CA3, in which ripples have a different frequency profile (90-110Hz instead of 140-200Hz in CA1), functional perisomatic inhibitory circuits have different properties, and some developmental aspects such as neurogenesis or interneuron maturation are early compared to CA1. We have here investigated the hypothesis of a conjoint and earlier appearance and maturation of ripples and perisomatic inhibition in the CA3 hippocampal region compared to CA1. We report a parallel sequence of events in CA3 and CA1, starting with the early expression of perisomatic GABAergic synaptic activity combined with the emergence of ripple activity. Interestingly, perisomatic inhibition and ripple activity follow parallel maturation trajectories, beginning in CA3 at P7 with immature (i.e. not functional yet) inhibition and immature ripples (proto-ripples) with labile oscillatory frequency. Mature functional perisomatic inhibition and clear high-frequency ripple activity progressively emerge between P10 and P12, reaching adult-like properties by P13. A similarly progressive maturation of ripples occurs in CA1, from P11 to P15. The progressive emergence of functional inhibition and specific patterns of neuronal activity likely support the progressive emergence of cognitive function to which they are necessary prerequisite.