- Superbradyon
**Superbradyons**are a new class ofsuperluminal particles. Unliketachyon s, they would have positive real values for bothmass andenergy . The term, as well as the existence of such particles, was suggested byLuis Gonzalez-Mestres as an antonym tobradyon (tardyon ).Superbradyons are similar to conventional particles (bradyons), but with a higher critical speed in vacuum, " c' ". This critical speed can be much larger than the speed of light "c", just as "c" is a million times larger than the speed of sound.

In a solid, it is possible to define, in the large wavelength limit for phonons, a form of Lorentz symmetry with the speed of sound playing the role of the critical speed. But the apparent space-time symmetry exhibited by the dispersion relation does not imply that phonons are elementary particles and that this is the fundamental symmetry of space and time.

According to Gonzalez-Mestres, superbradyons can be the ultimate building blocks of matter at the

Planck scale or beyond it. They would then be a new form ofpreons . Conventional special relativity would not be a fundamental property of space and time but would instead describe space-time as seen by matter at a given scale, just as the speed of sound reflects the properties of space and time as felt by phonons in a lattice. If so, the standard Lorentz symmetry of special relativity would be broken at Planck scale or at some other fundamental scale.Contrary to tachyons, superbradyons violate standard Lorentz invariance. But they can form a new sector of matter with a new invariance of the Lorentz type, where "c" would be replaced by a new, much larger, critical speed. The Lorentz symmetry of special relativity, with "c" as the critical speed in vacuum, is only a low-energy limit of the laws of Physics. Being composite, bare conventional "elementary" particles are not point-like field-theoretical objects at very small distance scales.

Lorentz symmetry violation is then expected to be governed by an energy-dependent parameter which tends to zero as momentum decreases. Such patterns require the existence of a privileged local inertial frame (the "vacuum rest frame"). They can be tested, at least partially, by ultra-high energy cosmic ray experiments like thePierre Auger Observatory Although this hypothesis may at first sight seem very unconventional, no inconsistency with experiment has yet been pointed out and the pattern may even look natural if quarks, leptons and gauge bosons are assumed to be composite. Why should then the actual constituents of matter have the same critical speed in vacuum as the composite objects ? Gonzalez-Mestres refers to this comment by Albert Einstein in "Geometry and Experiment" (1921) : "The interpretation of geometry advocated here cannot be directly applied to submolecular spaces... it might turn out that such an extrapolation is just as incorrect as an extension of the concept of temperature to particles of a solid of molecular dimensions", and argues that the space-time geometry beyond Planck scale has no reason to be the same as that felt by standard particles.

If superbradyons can exist in our Universe as free particles, they would spontaneously release radiation in the form of "conventional" particles and may be a source of ultra-high energy cosmic rays. They stop emitting such a radiation when their speed becomes equal to "c" or smaller. Therefore, the Universe may contain a sea of these superluminal particles with speeds close to the speed of light. Superbradyons can also provide a new approach to inflation, dark matter and dark energy.

To date, no experiment has been able to confirm or rule out the superbradyon hypothesis.

**References**# L. Gonzalez-Mestres, "Properties of a possible class of particles able to travel faster than light", Rencontre de Moriond on Dark Matter in Cosmology, Clocks and Tests of Fundamental Laws, Villars (Switzerland), Jan. 21-28,

1995 , arxiv|hep-ph/9505117

# L. Gonzalez-Mestres, "Observing Air Showers from Cosmic Superluminal Particles", Workshop on "Observing Giant Cosmic Ray Air Showers for > 10E20 eV Particles from Space", Univ. of Maryland, Nov 13-15,1997 , arxiv|physics/9712049

# L. Gonzalez-Mestres, "Superluminal Particles in Cosmic-Ray Physics", 26th International Cosmic Ray Conference, Salt Lake City, Utah,1999 , arxiv|hep-ph/9905454.

# L. Gonzalez-Mestres, "Superluminal Particles, Cosmology and Cosmic-Ray Physics", 28th International Cosmic Ray Conference, Tsukuba,2003 , arxiv|astro-ph/0407603 .

# L.Gonzalez-Mestres, "Lorentz symmetry violation and the results of the AUGER experiment", arxiv|0802.2536 .

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