Superbradyon

A superbradyon is a hypothetical elementary particle that can travel much faster than the speed of light. But unlike tachyons, superbradyons would have positive real values for both mass and energy. A superbradyon would be a new kind of existent particle able to transmit faster than light information and to violate standard causality while possibly preserving a new definition of this concept with a higher critical speed. Superbradyons can be, for instance, the "bradyonic" component of a new Lorentz-like symmetry. They are not related to any form of instability, as it is the case for tachyons from which they are fundamentally different.
The term, as well as the existence, of such particles, were suggested by Luis González-Mestres as an extension of the concept of bradyon (tardyon) to matter with a higher critical speed in vacuum. The relevance of the work by Gonzalez-Mestres on Lorentz symmetry violation has been recognized in 2002 by the CERN Courier and The New York Times. Already in 1997, his work had been quoted by Sidney Coleman and Sheldon Glashow. In 1997, he had also been an invited speaker at the AUGER pre-conference Workshop of ICRC 1997 and at the Maryland Worksop "Observing giant cosmic ray air showers from >10E20 eV particles from space".
Contrary to tachyons, which are a particular representation of special relativity, superbradyons explicitly and radically break standard Lorentz invariance. They are similar to conventional particles (bradyons), but with a higher critical speed in vacuum, c' . The superbradyon 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. This would most likely imply that standard Lorentz symmetry is not a fundamental symmetry but a low-energy composite effect. Superbradyons can then form a new sector of matter with a new invariance of the Lorentz type, which would be the actual fundamental symmetry and 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, would be only a low-energy limit of the laws of Physics for conventional matter.
==The ultimate (and "pre-Big Bang") constituents of matter ?==
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 González-Mestres, superbradyons can be the ultimate building blocks of matter at the Planck scale or beyond it. They would then be a new form of preons. Conventional special relativity would not be a fundamental property of space and time but would instead describe space-time as seen by standard 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.
In his August 2009 paper, to derive dual amplitudes from very high-order Feynmann diagrams was not sensitive to the actual space-time structure felt by the constituents of hadrons. He concludes: "Therefore, fishnet diagrams involving superbradyons as internal lines can in principle lead in the same kind of limit to a string structure for conventional particles compatible with standard special relativity."
Thus, superbradyons can also be at the origin of non-cyclic pre-Big Bang cosmologies and provide alternatives to standard inflation. Our present vacuum may be a piece of superbradyonic condensed matter (or some other phase) inside a possibly much larger superbradyonic world, and our standard particles, excitations of this matter. The apparent expansion of our Universe would then correspond to the nucleation of a specific phase of superbradyonic matter where our standard particles can exist as excitations. Such a scenario would also provide an alternative to Conformal Cyclic Cosmology in the field of pre-Big Bang theories.
On the validity of standard relativity
Although the superbradyon 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 his 1921 lecture Geometry and Experiment : "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.
Another English translation of Einstein's text, published in 1922, can be found at the MacTutor History of Mathematics archive. The relevant paragraph is :
"It is true that this proposed physical interpretation of geometry breaks down when applied immediately to spaces of sub-molecular order of magnitude. But nevertheless, even in questions as to the constitution of elementary particles, it retains part of its importance. For even when it is a question of describing the electrical elementary particles constituting matter, the attempt may still be made to ascribe physical importance to those ideas of fields which have been physically defined for the purpose of describing the geometrical behaviour of bodies which are large as compared with the molecule. Success alone can decide as to the justification of such an attempt, which postulates physical reality for the fundamental principles of Riemann's geometry outside of the domain of their physical definitions. It might possibly turn out that this extrapolation has no better warrant than the extrapolation of the idea of temperature to parts of a body of molecular order of magnitude."
Gonzalez-Mestres emphasizes that it is in itself very remarkable that special relativity holds for particle physics with such a good accuracy, but that it seems normal to consider applying Einstein's remark to preonic patterns. Not only concerning the validity of standard relativity, but also for quantum mechanics and other fundamental principles
Possible phenomenological implications
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,.
Referring to the 1995-96 papers by González-Mestres on superluminal particles, Sidney Coleman and Sheldon Glashow used a similar "Cherenkov" radiation in vacuum to set bounds on Lorentz symmetry violation in a standard THεμ scenario where conventional particles would not have exactly the same critical speed. by ultra-high energy cosmic ray experiments like the Pierre Auger Observatory. Such experiments can also be directly sensitive to the existence of cosmic superluminal particles,.
As the phenomenological LSV parameters are expected to be energy-dependent, existing low-energy bounds cannot be applied to high-energy phenomena.
In a May 2009 paper,<ref name="Gonzalez-Mestres2009d"/> Gonzalez-Mestres has argued that if the free superbradyons in our Universe are just quasiparticles generated by a new kind of superbradyonic condensed matter, it may happen that the original fundamental superbradyons do not obey conventional quantum mechanics. He also suggests that superbradyon spontaneous decays and similar interactions can be candidates to explain data on electron and positron abundances (PAMELA, ATIC, Fermi LAT, HESS, PPB-BETS) considered as possible dark matter signatures.

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