Parity violation in weak interactions was first postulated by
Tsung Dao Lee and
Chen Ning Yang[2] in 1956 as a solution to the
τ-θ puzzle. In consultation with the experimental physicist
Chien-Shiung Wu a number of possibilities were proposed to test whether the weak interaction was in fact invariant under parity. One of the group's suggestions involved monitoring the decay of
Cobalt-60,
to determine whether the electrons it emitted were radiated isotropically, like the two gamma rays. Wu performed this experiment at the
National Bureau of Standards in
Washington, D.C. after nine months of work. Contrary to most expectations, in December 1956 she and her team observed anisotropic electron radiation, proving that the weak interactions of the known particles violate parity.[3][4][5][6][7][8]
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 described in 1991,[9] although the basic idea dates back further.[2][10][11] 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.[12][13] 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 interact weakly in strength 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
kinetic mixing of mirror bosons with ordinary bosons. These interactions can only be very weak. Mirror particles have therefore been suggested as candidates for the inferred
dark matter in the universe.[14][15][16][17][18]
In another context,[19] 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 acquire masses on the order of 1 TeV since they interact with an additional gauge interaction not only becoming strong around the characteristic energy scale of the electroweak interactions but also being theoretically unified with Standard Model interactions under a larger gauge symmetry near the Planck energy scale. In order to emphasize the distinction of this model from the ones above,[14][15][16][17][18] these mirror particles are usually called
katoptrons[19][20][21] within the context of the Katoptron Model and they are expected to decay to Standard Model particles shortly after their creation.
Observational effects
Abundance
Mirror matter could have been diluted to unobservably low densities during the
inflation epoch.
Sheldon Glashow has shown that if at some high energy scale particles exist which interact strongly with both ordinary and mirror particles,
radiative corrections will lead to a mixing between
photons and mirror photons.[22] This mixing has the effect of giving mirror electric charges a very small ordinary electric charge. Another effect of photon–mirror photon mixing is that it induces oscillations between
positronium and mirror positronium. Positronium could then turn into mirror positronium and then decay into mirror photons.
The mixing between photons and mirror photons could be present in tree-level
Feynman diagrams or arise as a consequence of quantum corrections due to the presence of particles that carry both ordinary and mirror charges. In the latter case, the quantum corrections have to vanish at the one and two loop-level Feynman diagrams, otherwise the predicted value of the kinetic mixing parameter would be larger than experimentally allowed.[22]
An experiment to measure this effect was being planned in November 2003.[23]
Dark matter
If mirror matter does exist in large abundances in the universe and if it interacts with ordinary matter via photon—mirror photon mixing, then this could be detected in dark matter direct detection experiments such as
DAMA/NaI and its successor
DAMA/LIBRA. In fact, it is one of the few dark matter candidates which can explain the positive DAMA/NaI dark matter signal whilst still being consistent with the null results of other dark matter experiments.[24][25]
Electromagnetic effects
Mirror matter may also be detected in electromagnetic field penetration experiments[26] and there would also be consequences for planetary science[27][28] and astrophysics.[29]
GZK puzzle
Mirror matter could also be responsible for the
GZK puzzle.
Topological defects in the mirror sector could produce mirror neutrinos which can oscillate to ordinary neutrinos.[30] Another possible way to evade the GZK bound is via neutron–mirror neutron oscillations.[31][32][33][34]
Gravitational effects
If mirror matter is present in the universe with sufficient abundance then its gravitational effects can be detected. Because mirror matter is analogous to ordinary matter, it is then to be expected that a fraction of the mirror matter exists in the form of mirror galaxies, mirror stars, mirror planets etc. These objects can be detected using gravitational
microlensing.[35] One would also expect that some fraction of stars have mirror objects as their companion. In such cases one should be able to detect periodic
Doppler shifts in the spectrum of the star.[17] There are some hints that such effects may already have been observed.[36]
Neutron to mirror-neutron oscillations
Neutrons which are electrically neutral particles of ordinary matter could oscillate into its mirror partner, the mirror neutron.[37] Recently experiments looked for neutrons vanishing into the mirror world. Most experiments found no signal and hence gave limits on transition rates to the mirror state,[38][39][40][41] one paper claimed signals.[42] Current research looks for signals where an applied magnetic field adjust the energy level of the neutron to the mirror world.[43][44] This energy difference can be interpreted due to
a mirror magnetic field present in the mirror world or a mass difference of the neutron and its mirror partner. Such a transition to the mirror world could also solve the neutron lifetime puzzle.[45] Experiments searching for mirror neutron oscillation are ongoing at the
Paul Scherrer Institute's
UCN source, Switzerland,
Institut Laue-Langevin, France and
Spallation Neutron Source, USA.
See also
Antimatter – Material composed of antiparticles of the corresponding particles of ordinary matter
Dark energy – Energy driving the accelerated expansion of the universe
^
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