Chapter 1
Introduction
The fundamental interactions among the elementary particles of the Standard Model
(SM) are provided by the photon, the weak gauge bosons W
± and Z
0 and the gluons.
These are the gauge bosons, spin 1 vector particles that make the symmetries on
which the SM is based into local symmetries and the SM into a gauge theory.
The dark photon is a new gauge boson whose existence has been conjectured. The
names para- [1], hidden-sector, secluded photon and U-boson [2] have also being
used to indicate the same particle. The dark photon is dark because it arises from
a symmetry of a hypothetical dark sector comprising particles completely neutral
under the SM interactions. The neutrality of ordinary matter makes it blind to this
new gauge boson which is accordingly invisible and therefore characterized as dark.
The idea of adding to the SM a new gauge boson similar to the photon was first
considered in the context of supersymmetric theories in [3, 4]. It was discussed more
in general shortly after in [5, 6].
The dark sector is assumed to exist as a world parallel to our own. It may contain
few or many states, and these can be fermions or scalars or both, depending on the
model. Dark matter proper—the existence of which is deemed necessary to explain
astrophysical data—is found among these states. Its relic density can be computed
and constrained by observational data. In addition, the dark states can interact; their
interactions can be Yukawa-like or mediated by dark gauge bosons or both depending
on the model.
Dark though it is, the dark photon can be detected because of its kinetic mixing
with the ordinary, visible photon. This kinetic mixing is always possible because
the field strengths of two Abelian gauge fields can be multiplied together to give a
dimension four operator. The existence of such an operator means that the two gauge
bosons can go into each other as they propagate. This kinetic mixing provides the
portal linking the dark and visible sectors. It is this portal that makes possible to
detect the dark photon in the experiments.
In this primer, we review the physics of this new gauge boson from the theoretical
and the experimental point of view. We explain how the dark photon enters laboratory,
astrophysical and cosmological observations as well as dark matter physics.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
M. Fabbrichesi et al., The Physics of the Dark Photon,
SpringerBriefs in Physics, https://doi.org/10.1007/978-3-030-62519-1_1
1
Introduction
The fundamental interactions among the elementary particles of the Standard Model
(SM) are provided by the photon, the weak gauge bosons W
± and Z
0 and the gluons.
These are the gauge bosons, spin 1 vector particles that make the symmetries on
which the SM is based into local symmetries and the SM into a gauge theory.
The dark photon is a new gauge boson whose existence has been conjectured. The
names para- [1], hidden-sector, secluded photon and U-boson [2] have also being
used to indicate the same particle. The dark photon is dark because it arises from
a symmetry of a hypothetical dark sector comprising particles completely neutral
under the SM interactions. The neutrality of ordinary matter makes it blind to this
new gauge boson which is accordingly invisible and therefore characterized as dark.
The idea of adding to the SM a new gauge boson similar to the photon was first
considered in the context of supersymmetric theories in [3, 4]. It was discussed more
in general shortly after in [5, 6].
The dark sector is assumed to exist as a world parallel to our own. It may contain
few or many states, and these can be fermions or scalars or both, depending on the
model. Dark matter proper—the existence of which is deemed necessary to explain
astrophysical data—is found among these states. Its relic density can be computed
and constrained by observational data. In addition, the dark states can interact; their
interactions can be Yukawa-like or mediated by dark gauge bosons or both depending
on the model.
Dark though it is, the dark photon can be detected because of its kinetic mixing
with the ordinary, visible photon. This kinetic mixing is always possible because
the field strengths of two Abelian gauge fields can be multiplied together to give a
dimension four operator. The existence of such an operator means that the two gauge
bosons can go into each other as they propagate. This kinetic mixing provides the
portal linking the dark and visible sectors. It is this portal that makes possible to
detect the dark photon in the experiments.
In this primer, we review the physics of this new gauge boson from the theoretical
and the experimental point of view. We explain how the dark photon enters laboratory,
astrophysical and cosmological observations as well as dark matter physics.
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
M. Fabbrichesi et al., The Physics of the Dark Photon,
SpringerBriefs in Physics, https://doi.org/10.1007/978-3-030-62519-1_1
1