  We study a system of self-propelled particles (SPPs) in which individual
particles are allowed to switch between a fast aligning and a slow nonaligning
state depending upon the degree of the alignment in the neighborhood. The
switching is modeled using a threshold for the local order parameter. This
additional attribute gives rise to a mixed phase, in contrast to the ordered
phases found in clean SPP systems. As the threshold is increased from zero, we
find the sudden appearance of clusters of nonaligners. Clusters of nonaligners
coexist with moving clusters of aligners with continual coalescence and
fragmentation. The behavior of the system with respect to the clustering of
nonaligners appears to be very different for values of low and high global
densities. In the low density regime, for an optimal value of the threshold,
the largest cluster of nonaligners grows in size up to a maximum that varies
logarithmically with the total number of particles. However, on further
increasing the threshold the size decreases. In contrast, for the high density
regime, an initial abrupt rise is followed by the appearance of a giant cluster
of nonaligners. The latter growth can be characterized as a continuous
percolation transition. In addition, we find that the speed differences between
aligners and nonaligners is necessary for the segregation of aligners and
nonaligners.
