Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hydrodynamic Equations for Flocking Models without Velocity Alignment   The spontaneous emergence of collective motion patterns is usually associated
with the presence of a velocity alignment mechanism that mediates the
interactions among the moving individuals. Despite of this widespread view, it
has been shown recently that several flocking behaviors can emerge in the
absence of velocity alignment and as a result of short-range, position-based,
attractive forces that act inside a vision cone. Here, we derive the
corresponding hydrodynamic equations of a microscopic position-based flocking
model, reviewing and extending previously reported results. In particular, we
show that three distinct macroscopic collective behaviors can be observed: i)
the coarsening of aggregates with no orientational order, ii) the emergence of
static, elongated nematic bands, and iii) the formation of moving, locally
polar structures, which we call worms. The derived hydrodynamic equations
indicate that active particles interacting via position-based interactions
belong to a distinct class of active systems fundamentally different from other
active systems, including velocity-alignment-based flocking systems.

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The categories of this paper are: plasm-ph
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
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The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
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The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
13
The categories of this paper are: quant-ph