# Universality class of non-reciprocal interacting systems

When |
Oct 15, 2024
from 11:00 to 12:05 |
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Where | Salle des thèses |

Attendees |
Giulia Pisegna |

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Self-organization in living and active systems arises from simple microscopic interactions, often leading to collective behavior via spontaneous symmetries breaking. To study these phenomena, we use a field-theoretical approach, which enables the computation of properties independent of the underlying microscopic details. In this seminar, I will focus on a specific example of such an active matter system, for which the composition of distinct units produce a physical behavior that is different from what it is expected. Precisely, I will present a generic class of active matter models with two scalar fields describing the concentration of a molecular species interacting non-reciprocally. When non- reciprocity crosses a threshold, the system transitions from a phase-separated equilibrium configuration to an out-of-equilibrium stationary state, where parity and time-reversal symmetries are broken. The two species system enters a traveling wave state, with one particle chasing the other in a spontaneously chosen direction. This is a striking example that polar order can arise from non- polar particles, contrarily to many active matter models that assume polarity at a microscopic level. We study analytically and numerically the stability of the ordered state and demonstrate the existence of true long-range orientational order in two dimensions and higher. We go beyond a linear approximation and perform Renormalization Group analysis to study the effect of non-linearities. We show that the dynamics of concentration fluctuations around the ordered state map onto the Kardar-Parisi-Zhang universality class. This result enables us to predict the large-scale behavior of systems with non-reciprocal interactions at any dimension.