Researchers Reveal Universal Multicritical Behaviour in Disordered Contact Processes with Activated Scaling

Infection spreading represents a fundamental process across many scientific disciplines, from epidemiology to ecology, and understanding its behaviour in complex environments remains a significant challenge. Leone V. Luzzatto, Juan Felipe Barrera López, and István A. Kovács, researchers at Northwestern University and associated institutes, investigate a particularly intricate scenario where infection spreads amidst both varying environmental conditions and differing infection rates. Their work focuses on the multicritical contact process, a model exhibiting a sensitive balance between active and inactive states, and reveals that this process displays universal behaviour consistent with theoretical predictions. This finding not only clarifies the dynamics of infection in disordered environments, but also establishes a surprising connection to the well-studied multicritical Ising model, potentially allowing researchers to explore entanglement-like properties through classical computer simulations.

Simple Contact Process and Epidemic Transitions

The contact process serves as a foundational model for understanding how infections spread, demonstrating a critical transition between an outbreak dying out and persisting indefinitely. While simple, real-world epidemics often exhibit more complex behaviours, prompting researchers to explore extensions to the basic model, incorporating factors like spatial variations and multiple infection stages to better reflect real-world nuances. One such extension introduces a “recovery rate” that changes depending on the stage of infection, allowing for more realistic modelling of diseases with prolonged infectious periods. This approach can lead to “multicritical” behaviour, where the system undergoes multiple phase transitions, reflecting the interplay between different infection stages and recovery mechanisms. This work investigates a modified contact process incorporating stage-dependent recovery rates, aiming to characterise the conditions under which multicriticality emerges and to map out the phase diagram of the modified process.

Disorder Alters Contact Process Critical Behaviour

This research investigates the critical behaviour of absorbing-state phase transitions, specifically focusing on the contact process in the presence of quenched disorder. A central question is how disorder affects the universality class of the transition. The study explores the role of infinite-randomness fixed points and their connection to strong disorder renormalization group (RG) approaches, and investigates quantum multicritical points in disordered systems. The research employs Monte Carlo simulations, renormalization group techniques, finite-size scaling analysis, and theoretical analysis based on the strong disorder RG framework.

The main findings demonstrate that strong quenched disorder can significantly alter the critical behaviour of the contact process, sometimes leading to a breakdown of standard theories and the emergence of new universality classes. The study confirms the existence of infinite-randomness fixed points in the strong disorder RG flow, associated with unusual critical exponents and logarithmic corrections to scaling. This research makes several important contributions to the fields of statistical physics and condensed matter physics, providing a deeper understanding of the effects of strong disorder on critical phenomena and shedding light on quantum criticality in disordered systems. It has implications for understanding a wide range of real-world systems, such as disordered magnets and biological systems.

Universal Dynamics Link Infection and Magnetism

This research investigates the multicritical contact process, a model of infection spread exhibiting complex behaviour when both dilution and infection rates vary. Through large-scale Monte Carlo simulations, the study demonstrates that this process displays universal, ultra-slow dynamical scaling. The observed exponents align with predictions from the strong disorder renormalization group (SDRG) method, indicating that the multicritical contact process belongs to the same universality class as the multicritical Ising model. This finding establishes a connection between these seemingly different systems and opens possibilities for exploring entanglement properties through classical simulations.