The concept of a portal—which at first blush might seem rather harmless—
actually represents a radical departure from what is the main conceptual outcome of
our study of particle physics, namely, the gauge principle and idea that all interactions must be described by a gauge theory. The portal, and the new interactions
that it brings into the picture, adds a significant exception to this principle. Among
the possible portals, the vector case deviates the least from the gauge principle as it
only introduces a mixing for the gauge bosons while the interaction to matter
remains of the gauge type (albeit with an un-quantized charge). Instead, the other
kinds of portal imply a manifest new violation to the gauge principle, the other’s
being the notable case of the Yukawa and self-interactions of the Higgs boson—
which are themselves, exactly because of their not being gauge interactions, the
least understood part of the Standard Model.
There is an additional and important reason to study the dark sector in general,
and the dark photon in particular: The main motivation in introducing new-physics
scenarios is to use them as a foil for the Standard Model in mapping possible
experimental discrepancies. In the absence of clearly identified new states, the many
parameters, for instance, of the supersymmetric extensions to the Standard Model or
even of the effective field theory approach to physics beyond the Standard Model,
are working against their usefulness. Instead, each dark sector can be reduced to
few parameters—to wit, just two in the case of the dark photon—in terms of which
the possible discrepancies with respect to the Standard Model are more effectively
mapped in the experimental searches and the potential discovery more discernible.
Trieste, Italy
Marco Fabbrichesi
Genève, Switzerland
Emidio Gabrielli
October 2020
Gaia Lanfranchi
vi
Preface
actually represents a radical departure from what is the main conceptual outcome of
our study of particle physics, namely, the gauge principle and idea that all interactions must be described by a gauge theory. The portal, and the new interactions
that it brings into the picture, adds a significant exception to this principle. Among
the possible portals, the vector case deviates the least from the gauge principle as it
only introduces a mixing for the gauge bosons while the interaction to matter
remains of the gauge type (albeit with an un-quantized charge). Instead, the other
kinds of portal imply a manifest new violation to the gauge principle, the other’s
being the notable case of the Yukawa and self-interactions of the Higgs boson—
which are themselves, exactly because of their not being gauge interactions, the
least understood part of the Standard Model.
There is an additional and important reason to study the dark sector in general,
and the dark photon in particular: The main motivation in introducing new-physics
scenarios is to use them as a foil for the Standard Model in mapping possible
experimental discrepancies. In the absence of clearly identified new states, the many
parameters, for instance, of the supersymmetric extensions to the Standard Model or
even of the effective field theory approach to physics beyond the Standard Model,
are working against their usefulness. Instead, each dark sector can be reduced to
few parameters—to wit, just two in the case of the dark photon—in terms of which
the possible discrepancies with respect to the Standard Model are more effectively
mapped in the experimental searches and the potential discovery more discernible.
Trieste, Italy
Marco Fabbrichesi
Genève, Switzerland
Emidio Gabrielli
October 2020
Gaia Lanfranchi
vi
Preface