Species, chameleonic strings and the concept of particle
  We recently proposed a chameleonic solution to the cosmological constant
problem - Phys. Rev. D82 (2010) 044006. We further elaborate on our proposal
discussing also some new results. The basic ingredients of the model are A) a
chameleonic string dilaton parametrizing the amount of scale symmetry of the
problem in the Einstein frame; B) a dual nature of the concept of particle
where a splitting in local and global components is introduced (for every local
particle a corresponding global particle is present in the theory). In this
paper we proceed stepwise: 1) we point out that the value of the chameleonic
dilaton in the Einstein frame parametrizes also the string length and, in
particular, the string mass is an increasing function of the matter density; 2)
the concept of global particle is clarified (indeed global particles are simply
cosmic strings); 3) in the last quantization step, in the Feynman diagrams of
our model, the UV cut-off (which is chosen to be the string mass) is a function
of the chameleonic dilaton (naturally this stringy regularization is fully
compatible with the chameleonic scale invariance of the model); 4) we show that
a large number of particle species might be useful to keep locally under
control the dangerous variations of the fundamental constants, however, more
research efforts are necessary to make this point clear; 5) the chameleonic
behaviour of matter fields and of the de Broglie wavelength of local matter
particles is pointed out; 6) we briefly touch upon some ideas (which are still
waiting for a full confirmation) regarding a potential connection between the
shortest length scale of nature and the cosmological constant. A detailed
phenomenological analysis of the entire model is required to test these ideas.
