Lorentz Invariance and Superluminal Particles

If textbook Lorentz invariance is actually a property of the equations describing a sector of matter above some critical distance scale, several sectors of matter with different critical speeds in vacuum can coexist and an absolute rest frame (the vacuum rest frame, possibly related to the local rest frame of the expanding Universe) may exist without contradicting the apparent Lorentz invariance felt by "ordinary" particles (particles with critical speed in vacuum equal to $c$ , the speed of light). Sectorial Lorentz invariance, reflected by the fact that all particles of a given dynamical sector have the same critical speed in vacuum, will then be an expression of a fundamental sectorial symmetry (e.g. preonic grand unification or extended supersymmetry) protecting a parameter of the equations of motion. We study the breaking of Lorentz invariance in such a scenario, with emphasis on mixing between the "ordinary" sector and a superluminal sector, and discuss with examples the consequences of existing data. The sectorial universality of the value of the high-energy speed in vacuum, even exact, does not necessarily imply that Lorentz invariance is not violated and does not by itself exclude the possibility to produce superluminal particles at accelerators or to find them in experiments devoted to high-energy cosmic rays. Similarly, the stringent experimental bounds on Lorentz symmetry violation at low energy cannot be extrapolated to high-energy phenomena. Several basic questions related to possible effects of Lorentz symmetry violation are discussed, and potential signatures are examined.