70
4 Outlook
The constraints in the massless case seem to relegate the possible detection of the
dark photon to very large values of the effective scale in the dark dipole interaction,
as we discuss in Sect. 2.1. Exploring physics at such a large energy scale requires the
high sensitivity that can only be achieved either in future lepton colliders (where the
scaling with the energy of the dark dipole operator will also enhance its contribution)
or in searches for rare flavor-changing decays like those of the Kaon and B-meson
systems.
The constraints in the case of the massive dark photon have left open two important
regions in the parameter space. The first one is for the visible dark photon with
masses around 100 MeV or larger and mixing parameter between 10
−6 and 10
−4 .
Many future experiments aim at looking into this range, as we review in Sect. 3.3.1.
If also this window will be closed, it means that the already feeble interaction of the
vector portal is very weak indeed. Which leaves us with the second window still left
unexplored: an invisible dark photon with a very light mass and a mixing parameter
of order O(10
−8
) or even lighter and with smaller mixing parameter, as discussed in
Sects. 3.3.2 and 3.3.3. These two latter regions are of great interest for astrophysics
and cosmology and a very active area of speculations.
No single experiment or experimental approach is sufficient alone to cover the
large parameter space in terms of masses and couplings that dark photon models suggest: Synergy and complementarity among a great variety of experimental facilities
are paramount, calling for a broad collaboration across different communities.
4 Outlook
The constraints in the massless case seem to relegate the possible detection of the
dark photon to very large values of the effective scale in the dark dipole interaction,
as we discuss in Sect. 2.1. Exploring physics at such a large energy scale requires the
high sensitivity that can only be achieved either in future lepton colliders (where the
scaling with the energy of the dark dipole operator will also enhance its contribution)
or in searches for rare flavor-changing decays like those of the Kaon and B-meson
systems.
The constraints in the case of the massive dark photon have left open two important
regions in the parameter space. The first one is for the visible dark photon with
masses around 100 MeV or larger and mixing parameter between 10
−6 and 10
−4 .
Many future experiments aim at looking into this range, as we review in Sect. 3.3.1.
If also this window will be closed, it means that the already feeble interaction of the
vector portal is very weak indeed. Which leaves us with the second window still left
unexplored: an invisible dark photon with a very light mass and a mixing parameter
of order O(10
−8
) or even lighter and with smaller mixing parameter, as discussed in
Sects. 3.3.2 and 3.3.3. These two latter regions are of great interest for astrophysics
and cosmology and a very active area of speculations.
No single experiment or experimental approach is sufficient alone to cover the
large parameter space in terms of masses and couplings that dark photon models suggest: Synergy and complementarity among a great variety of experimental facilities
are paramount, calling for a broad collaboration across different communities.