# Mirror Matter

In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to ordinary matter. Modern physics deals with three basic types of spatial symmetry: reflection, rotation and translation. The known elementary particles respect rotation and translation symmetry but do not respect mirror reflection symmetry (also called P-symmetry or parity). Of the four fundamental interactions—electromagnetism, the strong interaction, the weak interaction, and gravity—only the weak interaction breaks parity.

Parity violation in weak interactions was first postulated by Tsung Dao Lee and Chen Ning Yang in 1956 as a solution to the τ-θ puzzle. They suggested a number of experiments to test if the weak interaction is invariant under parity. These experiments were performed half a year later and they confirmed that the weak interactions of the known particles violate parity.

However parity symmetry can be restored as a fundamental symmetry of nature if the particle content is enlarged so that every particle has a mirror partner. The theory in its modern form was written down in 1991, although the basic idea dates back earlier. Mirror particles interact amongst themselves in the same way as ordinary particles, except where ordinary particles have left-handed interactions, mirror particles have right-handed interactions. In this way, it turns out that mirror reflection symmetry can exist as an exact symmetry of nature, provided that a "mirror" particle exists for every ordinary particle. Parity can also be spontaneously broken depending on the Higgs potential. While in the case of unbroken parity symmetry the masses of particles are the same as their mirror partners, in case of broken parity symmetry the mirror partners are lighter or heavier.

Mirror matter, if it exists, would have to be very weakly interacting with ordinary matter. This is because the forces between mirror particles are mediated by mirror bosons. With the exception of the graviton, none of the known bosons can be identical to their mirror partners. The only way mirror matter can interact with ordinary matter via forces other than gravity is via so-called kinetic mixing of mirror bosons with ordinary bosons or via the exchange of Holdom particles. These interactions can only be very weak. Mirror particles have therefore been suggested as candidates for the inferred dark matter in the universe.

In another context, mirror matter has been proposed to give rise to an effective Higgs mechanism responsible for the electroweak symmetry breaking. In such a scenario, mirror fermions have masses on the order of 1 TeV since they interact with an additional interaction, while some of the mirror bosons are identical to the ordinary gauge bosons. In order to emphasize the distinction of this model from the ones above, these mirror particles are usually called katoptrons.

Read more about Mirror Matter:  Observational Effects of Mirror Matter, Alternate Terminology

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