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Macroscopic Fluctuation Theory and Current Fluctuations in Active Lattice Gases
by Tal Agranov, Sunghan Ro, Yariv Kafri and Vivien Lecomte
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Authors (as registered SciPost users):  Tal Agranov 
Submission information  

Preprint Link:  scipost_202208_00013v1 (pdf) 
Date submitted:  20220805 18:24 
Submitted by:  Agranov, Tal 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study the current large deviations for a lattice model of interacting active particles displaying a motilityinduced phase separation (MIPS). To do this, we first derive the exact fluctuating hydrodynamics of the model in the large system limit. On top of the usual Gaussian noise terms the theory also presents Poissonian noise terms, that we fully account for. We find a dynamical phase transition between flat density profiles and sharply phaseseparated traveling waves, and we derive the associated phase diagram together with the large deviation function for all phases, including the one displaying MIPS. We show how the results can be obtained using methods similar to those of equilibrium phase separation, in spite of the nonequilibrium nature of the problem.
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Reports on this Submission
Anonymous Report 2 on 20221031 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202208_00013v1, delivered 20221031, doi: 10.21468/SciPost.Report.6013
Report
This is an important and timely paper that builds on the authors substantial track record on large deviations and dynamical phase transitions. Here they study the full counting statistics of current fluctuations in a onedimensional model of active matter (that of KourbaneHoussene et al, ref.39). The most salient feature of (intrinsically nonequilibrium) active matter is the socalled motility induced phase separation (the nonhomogeneous clustered phase akin to that of sticky spheres but in the absence of interactions, which occurs at high drive even in 1D). The model is a generalisation of a SEP with tumbling particles carrying charge which determines their motional asymmetry, with rates scaled with size appropriately to give a good diffusive limit of times and length.
There are two main results. The first one is the formulation of the MFT for this model (and by extension for similar active lattice systems). The presence of stochastic jumps due to tumbling events makes the MFT for the particle and "charge" currents nonGaussian. The second result is the computation of the relevant LD functions, and in particular the observation of a "dynamical phase transition" (i.e. a transition in the optimal trajectories of rare events). The fact that these can be shown analytically is quite remarkable, making this paper an important starting point for the application of LD/MFT methods to this class of systems. An interesting result is that the LD transition coincides with the steady state MIPS one at the critical point of the model (as it occurs for LD phase transitions in systems with equilibrium phase transitions).
The paper is well written and mostly clear, and the results correct as far as I can check. One small issue is the use of "bias" which can be confusing: on the one hand it can refer to the asymmetry in the particle hopping (as after eq.22), while in LDs bias often refers to what is also called "tilting" (the exponential rewighting of the path probability to probe atypical events, controlled in this case by \Lambda). After fixing this and several typos the paper can be published.
Anonymous Report 1 on 20221023 (Invited Report)
 Cite as: Anonymous, Report on arXiv:scipost_202208_00013v1, delivered 20221023, doi: 10.21468/SciPost.Report.5965
Report
In this paper, the authors study a discretespace and continuous time model
of active particles, which was introduced in Ref. [39]. At variance with standard
active particle systems, the tumbling rate in this model is scaled with the system size
in such a way that all the processes occur on diffusive time scales. Despite of this, the
model shares many similarities with more standard active matter models, in particular
motility induced phase separation (MIPS). One of the main results of the paper is to derive
exact fluctuating hydrodynamic equations (in the limit of a large system).
In a previous work [43], the same authors, using Macroscopic Fluctuation Theory (MFT),
studied the typical fluctuations, e.g. of the integrated current through the system, which
are Gaussian (this was done using a mapping to the ABC model). The main result of this paper
is to go beyond that work [43] and derive a nonGaussian MFT, which in particular retains
the Poissonian nature of the noise arising from the random tumbling events.
Within this framework, they derive the large deviation function (LDF) associated to the integrated current.
Using an analogy with equilibrium phase transitions, they study in detail the dynamical
phase transition exhibited by this LDF. They also connect this phase transition to the rich
phase diagram of the model, which exhibits MIPS.
The present paper presents original, interesting and timely results for active particle systems.
In particular, I think that their study of the Poissonian noise should be relevant to study
other active systems. My only criticism is that the presentation of the results (and somehow
of the whole paper in general) could be made a bit more accessible to a wider audience (see also below
for other comments), not necessarily familiar with MIPS (which at the end is not really the main
topic of the paper). Once the authors take these remarks into account,
the paper can be published in SciPost.
Here is a list of comments that the authors might wish to consider (some of them are just typos or minor comments):
1) At the end of Section II, it would be useful if the authors could explain the phase diagram with
more details. As it is now, it is a bit cryptic to follow, in particular for readers who are not familiar with MIPS.
2) In Fig. 1, there are arrows to illustrate the mechanism 'D_0'. The second arrow (the rightmost one) is a bit misleading: what does it mean?
3) After Eq. (2), it would be useful to give a clear definition of $J_\rho$, $J_m$ and $K$.
4) In Eq. (5), I am not sure that ${\mathbb C}$ is the best notation since this usually denotes the ensembles of complex numbers...
5) Above (5): "account" > "accounts"
6) Below (22): "with a conserved order parameters" > "with conserved order parameters"
7) At the beginning of Section V, the authors should recall what $\ell_s$ is.
8) At the bottom of p. 6 "reminiscence" > "reminiscent"
9) Below Eq. (30), "in low density phase" > "in THE low density phase"
10) Below Eq. (30) it is not clear what is meant by "This dynamics are illustrated schematically in Fig. 5(B)"
11) Above A2): "transmutation reaction" > "transmutation reactions"