Dark photon
The dark photon can also interact with the Standard Model if some of the fermions are charged under the new abelian group.
[3] The possible charging arrangements are restricted by a number of consistency requirements such as anomaly cancellation and constraints coming from Yukawa matrices.
Given the rich interaction structure of the well-known Standard Model particles, which make up only the subdominant component of the universe, it is natural to think about a similarly interactive behaviour of dark sector particles.
Dark photons could be part of these interactions among dark matter particles and provide a non-gravitational window (a so-called vector portal) into their existence by kinematically mixing with the Standard Model photon.
[5][6] Arguably the most interesting application of dark photons arises in the explanation of the discrepancy between the measured and the calculated anomalous magnetic moment of the muon,[7][8][9] although the simplest realisations of this idea are now in conflict with other experimental data.
[11] Adding a sector containing dark photons to the Lagrangian of the Standard Model can be done in a straightforward and minimal way by introducing a new U(1) gauge field.
[2] The specifics of the interaction between this new field, potential new particle content (e.g., a Dirac fermion for dark matter) and the Standard Model particles are virtually only limited by the creativity of the theorist and the constraints that have already been put on certain kinds of couplings.
The arguably most popular basic model involves a single new broken U(1) gauge symmetry and kinetic mixing between the corresponding dark photon field
is the mass of the dark photon (in this case it can be thought of as being generated by the Higgs or Stueckelberg mechanism),
The fundamental parameters of this model are thus the mass of the dark photon and the strength of the kinetic mixing.
Other models leave the new U(1) gauge symmetry unbroken, resulting in a massless dark photon carrying a long-range interaction.
[14] A massless dark photon, however, will be fully decoupled from the Standard Model and will not have any experimental consequence by itself.
A cavity, with resonant frequency tuned to match the mass of a dark photon candidate
With linear amplification, however, is difficult to overcome the effective noise imposed by the standard quantum limit and search for dark photon candidates that would produce a mean cavity population much less than 1 photon.
[18] The experiment has excluded dark photon candidates with mass centered around 24.86 μeV and
This has enabled a search speed up of over 1,000 as compared to the conventional linear amplification technique.
For a dark photon mass above the MeV, current limits are dominated by experiments based in particle accelerators.
Assuming that dark photons produced in the collisions would then decay mainly into positron-electron pairs, the experiments search for an excess of electron-positron pairs that would originate from the dark photon decay.
On average, experimental results now indicate that this hypothetical particle must interact with electrons at least a thousand times more feebly than the standard photon.
In more details, for a dark photon which would be more massive than a proton (thus with mass larger than a GeV), the best limits would arise from collider experiments.
