In the Standard Model, translation, rotation and parity correspond to three fundamental symmetries to which elementary particles are subject. Among the four elementary interactions, only the weak interaction violates parity symmetry (P symmetry). However, the P symmetry can be restored as fundamental symmetry if there are "mirror" particles that can be associated with the known elementary particles.
It was in 1956 that Chinese physicists Tsung Dao Lee and Chen Ning Yang suggested that the weak interaction violates parity symmetry to explain the meson decay anomaly (also known as the τ–θ puzzle). They then propose a series of experiments to test their hypothesis. A few months later, the results of the experiments will confirm that the weak interactions of elementary particles indeed violate the P symmetry.
Between the 1960s and 1990s, theoretical physicists were already beginning to look for solutions that would naturally restore parity symmetry. One of the solutions previously put forward is the existence of mirror particles, elementary particles already known. But it was not until 1991 that the idea was formally described.
In the Standard Model, the W bosons responsible for the weak interaction are left-handed, and only interact with left-handed quarks. In the case of mirror particles, these would be of right chirality and the weak mirror interaction would therefore only have W bosons of right chirality. The sector of mirror particles thus contains right neutrinos, right gluons, etc. Each mirror particle has the same mass as its ordinary counterpart.
Apart from this difference in chirality, mirror particles interact with each other in the same way as ordinary particles. However, the interaction between mirror particles and ordinary particles is impossible, due to the physical incompatibility of the bosons involved; mirror bosons cannot interact with classical particles, and vice versa. With the exception of gravity, which is the only interaction acting on all particles, since it is inherent in the very geometry of space-time.
In 1985, University of Toronto physicist Bob Holdom suggested that the two types of particles could interact through the exchange of a Holdom boson. However, this interaction would be extremely weak. Thus, one particle can transform into another via the exchange of such a boson. Nevertheless, this mechanism would be strongly constrained by the principle of conservation of charge.
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It is, for example, forbidden for an ordinary electron to transform into a mirror electron, because the charge would be modified. Only electrically neutral particles can therefore exchange Holdom bosons. This is particularly the case for photons. Thus, ordinary photons can interact with mirror photons and transform into each other. Many experiments involving neutrino oscillation and ortho-positronium decay seem to confirm Holdom's hypothesis.
Mirror matter would have been created at the end of the era of Planck, when gravity separated from the other three elementary interactions. Although the concrete mechanism of formation is still very vague, nothing implies that ordinary matter and mirror matter were created in identical quantities. This is why mirror matter is a candidate for dark matter. Indeed, she is currently one of the few candidates who can explain the positive signal of the DAMA/NaI experiment while being compatible with the negative results of others.
Beyond the restoration of P symmetry, the existence of mirror matter would make it possible to solve other cosmological pitfalls. It would thus offer a solution to the GZK limit (upper limit of the energy of cosmic rays) by involving the oscillation of mirror neutrinos into ordinary neutrinos. In addition, nothing prevents mirror matter from forming stars, planets, or even galaxies. The gravitational influence of such objects could therefore be detectable.
