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The kinetic mixing (KM) portal, by which the Standard Model (SM) photon mixes with a light dark photon arising from a new U(1)DU(1)D​ gauge group, allows for the possibility of viable scenarios of sub-GeV thermal dark matter (DM) with appropriately suppressed couplings to the SM. This KM can only occur if particles having both SM and dark quantum numbers, here termed portal matter (PM), also exist. The presence of such types of states and the strong suggestion of a need to embed U(1)DU(1)D​ into a non-abelian gauge structure not too far above the TeV scale based on the RGE running of the U(1)DU(1)D​ gauge coupling is potentially indicative of an enlarged group linking together the visible and dark sectors. The gauge group G=SU(3)c×SU(3)L×U(1)A×U(1)B=3c3L1A1BG=SU(3)c​×SU(3)L​×U(1)A​×U(1)B​=3c​3L​1A​1B​ is perhaps the simplest setup wherein the SM and dark interactions are partially unified in a non-abelian fashion that is not a simple product group of the form G=GSM×GDG=GSM​×GD​ encountered frequently in earlier work. The present paper describes the implications and phenomenology of this type of setup.

Portal matter (PM), having both Standard Model (SM) and dark sector charges, can induce kinetic mixing between the $U(1{)}_D$ dark photon and the SM gauge fields at the 1-loop level offering an attractive mechanism by which light ( $\lesssim 1\mathrm{GeV}$ ) thermal dark matter (DM) can interact with visible matter and obtain its observed relic density. In doing so, if the DM is fermionic, the CMB and other astrophysical observations inform us that it must be Majorana/pseudo-Dirac in nature to avoid velocity/temperature-independent $s$ -wave annihilation to SM final states. How does this idea fit into a more UV-complete picture also including the SM interactions? There are some reasons to believe that at least a first step along this path may not lie too far away in energy due to the renormalization group equations running of the dark gauge coupling, which for a significant range of parameters, becomes nonperturbative at/before the $\sim 10$ ’s of TeV energy range. This implies that $U(1{)}_D$ must become embedded in an asymptotically free, non-Abelian group, $G_D$ , before this can occur. The breaking of this larger group then produces the masses for the PM and the additional gauge fields associated with $G_D$ then can lead to new interactions between the SM and the dark sector. Following several bottom-up approaches, we have examined a set of distinctive and testable phenomenological features associated with this general setup, based upon a number of simplifying assumptions. Clearly, it behooves us to explore the impact of these specific assumptions on these predictions for the array of possible experimental tests of this class of models. In most past analyses it has been assumed that DM is a vectorlike, complex singlet under the group $G_D$ . If this assumption is relaxed, the dark sector must be augmented by additional fermion(s) and the associated scalar fields needed to break the gauge symmetries while generating the needed Majorana-like mass terms for the DM. In this paper, we analyze the simplest extension of this kind wherein the DM lies in a vectorlike doublet of $G_D$ , which we take to have the structure $SU(2{)}_I\times U(1{)}_{Y_I}$ as in earlier work, leading to new phenomenological implications. We find, for example, that given the current LHC search constraints on the masses of heavy gauge bosons, the production of these new dark states with large rates is unlikely to occur at colliders unless they are produced singly in $gg$ fusion or their pair production cross sections are resonantly enhanced. We also find that an additional mechanism arises to generate hierarchal neutrino masses in such a setup. Published by the American Physical Society 2024

Kinetic mixing of the dark photon, the gauge boson of a hidden U⁢(1)_D, with the Standard Model (SM) gauge fields to induce an interaction between ordinary matter and dark matter (DM) at 1-loop requires the existence of portal matter (PM) fields having both dark and SM charges. As discussed in earlier work, these same PM fields can also lead to other loop-level mechanisms besides kinetic mixing that can generate significant interactions between SM fermions and the dark photon in a manner analogous to those that can be generated between a Dirac neutrino and a SM photon, i.e., dark moments. In either case, there are reasons to believe, e.g., due to the renormalization group equation running of the U⁢(1)_D gauge coupling, that PM fields may have ~TeV-scale masses that lie at or above those directly accessible to the HL-LHC. If they lie above the reach of the HL-LHC, then the only way to possibly explore the physics at this high scale in the short term is via indirect measurements made at lower energies, e.g., at lepton colliders operating in the m_Z to 1 TeV range. In particular, processes such as e⁺⁢e^–→γ+DM or e⁺⁢e^–→ $$\bar{ƒ}ƒ$$, where ƒ is a SM fermion, may be most useful in this regard. Here we explore these possibilities within the framework of a simple toy PM model, introduced in earlier work, based on a non-Abelian dark gauge group completion operating at the PM scale. In the kinetic mixing setup, we show these efforts fail due to the inherently tiny cross sections in the face of substantial SM backgrounds. However, in the case of interactions via induced dark moments, since they necessarily take the form of higher dimensional operators whose influence grows with energy, we show that access to PM-scale information may become possible for certain ranges of the toy model parameters for both of these e⁺⁢e^– processes at a 1 TeV collider.

The kinetic mixing (KM) of a dark photon (DP) with the familiar one of the Standard Model (SM) requires the existence of a new set of fields, called portal matter (PM), which carry both SM and dark sector quantum numbers, some whose masses may lie at the TeV scale. In the vanilla KM model, the dark gauge group is just the simple $$G$$_$Dark$ = $U(1)$_$$D$$ needed to describe the DP while the SM gauge interactions are described by the usual $$G$$_$SM$=$SU(3)$_c × $SU(2)$_$$L$$ × $U(1)$_$$Y$$. However, we need to go beyond this simple model to gain a better understanding of the interplay between GSM and $$G$$_$Dark$ and, in particular, determine how they both might fit into a more unified construction. Following our previous analyses, this generally requires $$G$$_$Dark$ to be extended to a non-abelian group, e.g., $SU(2)$_$$I$$ × $U(1)$_{$$Y_I$$}, under which both the PM and SM fields may transform non-trivially. In this paper, also inspired by our earlier work on top-down models, we consider extending the SM gauge group to that of the Left-Right Symmetric Model (LRM) and, in doing so, through common vacuum expectation values, link the mass scales associated with the breaking of $$G$$_$Dark$ → $U(1)$_$$D$$ and the PM fields to that of the RH-neutrino as well as the heavy gauge bosons of the LRM. This leads to an interesting interplay between the now coupled phenomenologies of both visible and dark sectors at least some of which may be probed at, e.g., the LHC and/or at the future FCC-hh.

If dark matter interacts with the Standard Model (SM) via the $U(1)$_$$D$$ kinetic mixing portal at low energies, it necessitates not only the existence of portal matter particles which carry both dark and SM quantum numbers, but also a possible UV completion into which this $U(1)$_$$D$$ and the SM are both embedded. In earlier work, following a bottom-up approach, we attempted to construct a more unified framework of these SM and dark sector interactions. In this paper, we will instead begin to explore, from the top down, the possibility of the unification of these forces via the decomposition of a grand-unified-theory-like group, $$G$$ → $$G$$_$SM$ × $$G$$_$Dark$, where $U(1)$_$$D$$ is now a low-energy diagonal subgroup of $$G$$_$Dark$ and where the familiar $$G$$_$SM$ = $SU(5)$ will play the role of a proxy for the conventional $SU(3)$_$$c$$ × $SU(2)$_$$L$$ × $U(1)$_$$Y$$ SM gauge group. In particular, for this study it will be assumed that $$G$$ = $SU(N)$ with $$N$$ = $6–10$. Although not our main goal, models that also unify the three SM generational structure within this same general framework will also be examined. The possibilities are found to be quite highly constrained by our chosen set of model-building requirements, which are likely too strong when they are employed simultaneously to obtain a successful model framework.

As experimental searches for WIMP dark matter continue to yield null results, models beyond the WIMP paradigm have proliferated in order to elude ever improving observational constraints, among them that of sub-GeV dark matter mediated by a massive vector portal (a dark photon) associated with a new dark U(1) gauge symmetry. It has been previously noted that for a significant range of the parameter space of this class of models, the annihilation of dark matter particles into a pair of dark photons can dominate the freeze-out process even when this process is kinematically forbidden for dark matter at rest — this is known as the “forbidden dark matter” (FDM) regime. Prior studies of this regime, however, assume that any “dark Higgs” associated with breaking the dark U(1) and imparting mass to the dark photon is decoupled from the dark matter and as such plays no role in the freeze-out process. In this paper, we explore the effects of a dark Higgs on sub-GeV dark matter phenomenology in this FDM regime by considering the simplest possible construction in which there exist non-trivial dark matter-dark Higgs couplings: a model with a single complex scalar DM candidate coupled directly to the dark Higgs field. We find that for a wide range of parameter space, the dark Higgs can alter the resulting relic abundance by many orders of magnitude, and that this effect can remain significant even for a small dark matter-dark Higgs coupling constant. Considering measurements from direct detection and measurements of the CMB, we further find that points in this model’s parameter space which recreate the appropriate dark matter relic abundance suffer only mild constraints from other sources at present, but may become accessible in near-future direct detection experiments.

If dark matter (DM) interacts with the Standard Model (SM) via the kinetic mixing (KM) portal, it necessitates the existence of portal matter (PM) particles which carry both dark and SM quantum numbers that will appear in vacuum polarization-like loop graphs. In addition to the familiar ~ eϵQ strength, QED-like interaction for the dark photon (DP), in some setups different loop graphs of these PM states can also induce other coupling structures for the SM fermions that may come to dominate in at least some regions of parameter space regions and which can take the form of ‘dark’ moments, e.g., magnetic dipole-type interactions in the IR, associated with a large mass scale, Λ. In this paper, motivated by a simple toy model, we perform a phenomenological investigation of a possible loop-induced dark magnetic dipole moment for SM fermions, in particular, for the electron. We show that at the phenomenological level such a scenario can not only be made compatible with existing experimental constraints for a significant range of correlated values for Λ and the dark U(1)_D gauge coupling, g_D, but can also lead to quantitatively different signatures once the DP is discovered. In this setup, assuming complex scalar DM to satisfy CMB constraints, parameter space regions where the DP decays invisibly are found to be somewhat preferred if PM mass limits from direct searches at the LHC and our toy model setup are all taken seriously. High precision searches for, or measurements of, the e⁺e^- → γ + DP process at Belle II are shown to provide some of the strongest future constraints on this scenario.

In this paper, we present a model which attempts to unify a new dark sector force with a local SU(3) flavor symmetry. Dark matter (DM) and its potential interactions with the Standard Model (SM) continue to present a rich framework for model building. In the case of thermal DM of a mass between a few MeV and a few GeV, a compelling and much-explored framework is that of a dark photon/vector portal, which posits a new U(1) “dark photon” which only couples to the SM via small kinetic mixing (KM) with the SM hypercharge. This mixing can be mediated at the one-loop level by portal matter (PM) fields which are charged under both the dark U(1) and the SM gauge group. In earlier work, one of the authors has noted that models with appropriate portal matter content to produce finite and calculable kinetic mixing can arise from nonminimal dark sectors, in which the dark U(1) is a subgroup of a larger gauge symmetry under which SM particles might have nontrivial representations. We expand on this idea here by constructing a model in which in which the dark U(1) is unified with another popular extension to the SM gauge group, a local SU(3) flavor symmetry. The full dark/flavor symmetry group is SU(4)_F × U(1)_F, incorporating the local SU(3) flavor symmetry with PM appearing as a vectorlike “fourth generation” to supplement the three generations of the SM. To ensure finite contributions to KM, the SM gauge group is arranged into Pati-Salam multiplets. The new extended dark gauge group presents a variety of interesting experimental signatures, including nontrivial consequences of the flavor symmetry being unified with the dark sector.

The possibility of light dark matter (DM) annihilating through a dark photon (DP) which kinetically mixes (KM) with the Standard Model (SM) hypercharge field is a very attractive scenario. For DM in the interesting mass range below ~1 GeV, it is well known that bounds from the CMB provide a very strong model building constraint forcing the DM annihilation cross section to be roughly 3 orders of magnitude below that needed to reproduce the observed relic density. Under most circumstances this removes the possibility of an s-wave annihilation process for DM in this mass range as would be the case, e.g., if the DM were a Dirac fermion. In an extra-dimensional setup explored previously, it was found that the s-channel exchange of multiple gauge bosons could simultaneously encompass a suppressed annihilation cross section during the CMB era while also producing a sufficiently large annihilation rate during freeze-out to recover the DM relic density. In this paper, we analyze more globally the necessary requirements for this mechanism to work successfully and then realize them within the context of a simple model with two ‘dark’ gauge bosons having masses of a similar magnitude and whose contributions to the annihilation amplitude destructively interfere. We show that if the DM mass threshold lies appropriately in the saddle region of this destructive interference between the two resonance humps it then becomes possible to satisfy these requirements simultaneously provided several ancillary conditions are met. The multiple constraints on the parameter space of this setup are then explored in detail to identify the phenomenologically successful regions.

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe standard model (SM) processes and search for physics beyond the standard model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF’s physics potential.