We study the dynamics of a tracer particle (TP) on a comb lattice populatedby randomly moving hard-core particles in the dense limit. We first considerthe case where the TP is constrained to move on the backbone of the comb only,and, in the limit of high density of particles, we present exact analyticalresults for the cumulants of the TP position, showing a subdiffusive behavior$\sim t^{3/4}$. At longer times, a second regime is observed, where standarddiffusion is recovered, with a surprising non analytical dependence of thediffusion coefficient on the particle density. When the TP is allowed to visitthe teeth of the comb, based on a mean-field-like Continuous Time Random Walkdescription, we unveil a rich and complex scenario, with several successivesubdiffusive regimes, resulting from the coupling between the inhomogeneouscomb geometry and particle interactions. Remarkably, the presence of hard-coreinteractions speeds up the TP motion along the backbone of the structure in allregimes.
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