Preface
New Particles Beyond the Standard Model have always been thought to be charged
under at least some of the same gauge interactions of ordinary particles. Although
this assumption has driven the theoretical speculations, as well as the experimental
searches of the last 50 years, it has also been increasingly challenged by the
negative results of all these searches—and the mounting frustration for the failure to
discover any of these hypothetical new particles.
As the hope of a breakthrough along these lines is waning, interest in a dark
sector—dark because not charged under the Standard Model gauge groups—is
growing: Maybe no new particles have been seen simply because they do not
interact through the Standard Model gauge interactions.
The dark sector can be simple or have a complex structure with many states—
some of which can be dark matter candidates.
If the dark and the visible sectors were to interact only gravitationally—which
they cannot avoid—there would be little hope of observing in the laboratory particles
belonging to the dark sector. A similar problem exists for dark matter: Although its
presence is motivated by gravitational physics, it is searched mostly through its
putative weak interactions—as in the direct- and indirect-detection searches of a
weakly interacting massive particle. For the same reason, we must pin our hopes on
assuming that dark and ordinary sectors also interact through a portal—as the
current terminology has it—that is, through a sallow glimmer, in a manner that,
though feeble, is (at least in principle) experimentally accessible.
The portal may take various forms that can be classified by the type and
dimension of its operators. The best motivated and most studied cases contain
relevant operators taking different forms depending on the spin of the mediator:
Vector (spin 1), Neutrino (spin 1/2), Higgs (scalar) and Axion (pseudo-scalar).
Among these possible portals, the vector portal is the one where the interaction
takes place because of the kinetic mixing between one dark and one visible Abelian
gauge boson (nonAbelian gauge bosons do not mix). The visible photon is taken to
be the boson of the Uð1Þ gauge group of electromagnetism—or, above the electroweak symmetry-breaking scale, of the hyper-charge—while the dark photon
comes to be identified as the boson of an extra Uð1Þ symmetry.
v
New Particles Beyond the Standard Model have always been thought to be charged
under at least some of the same gauge interactions of ordinary particles. Although
this assumption has driven the theoretical speculations, as well as the experimental
searches of the last 50 years, it has also been increasingly challenged by the
negative results of all these searches—and the mounting frustration for the failure to
discover any of these hypothetical new particles.
As the hope of a breakthrough along these lines is waning, interest in a dark
sector—dark because not charged under the Standard Model gauge groups—is
growing: Maybe no new particles have been seen simply because they do not
interact through the Standard Model gauge interactions.
The dark sector can be simple or have a complex structure with many states—
some of which can be dark matter candidates.
If the dark and the visible sectors were to interact only gravitationally—which
they cannot avoid—there would be little hope of observing in the laboratory particles
belonging to the dark sector. A similar problem exists for dark matter: Although its
presence is motivated by gravitational physics, it is searched mostly through its
putative weak interactions—as in the direct- and indirect-detection searches of a
weakly interacting massive particle. For the same reason, we must pin our hopes on
assuming that dark and ordinary sectors also interact through a portal—as the
current terminology has it—that is, through a sallow glimmer, in a manner that,
though feeble, is (at least in principle) experimentally accessible.
The portal may take various forms that can be classified by the type and
dimension of its operators. The best motivated and most studied cases contain
relevant operators taking different forms depending on the spin of the mediator:
Vector (spin 1), Neutrino (spin 1/2), Higgs (scalar) and Axion (pseudo-scalar).
Among these possible portals, the vector portal is the one where the interaction
takes place because of the kinetic mixing between one dark and one visible Abelian
gauge boson (nonAbelian gauge bosons do not mix). The visible photon is taken to
be the boson of the Uð1Þ gauge group of electromagnetism—or, above the electroweak symmetry-breaking scale, of the hyper-charge—while the dark photon
comes to be identified as the boson of an extra Uð1Þ symmetry.
v