Thomas G. Rizzo

Loops of portal matter (PM) fields carrying both dark and Standard Model (SM) quantum numbers can lead to the kinetic mixing (KM) of the SM photon and the analogous dark photon (DP) of a phenomenologically interesting magnitude. However, in specific frameworks, different loops of these same PM fields can also lead to other new types of interactions between some of the SM fields and the DP via the generation of “dark moment”-like couplings, even though the SM fields carry a zero dark charge at tree level. In recent work, this possibility has been explored for the case when these SM fields are fermionic. In this paper, we extend this idea to the case of the SM W boson, employing a previously examined model wherein PM consists of a complex scalar triplet plus a complex singlet, both of which obtain vacuum expectation values and thus also generate the DP mass. While this setup results in a somewhat stronger interaction between the DP and the W than in the familiar KM setup, the rate for W + DP production at the LHC is shown to still be rather small and is very difficult to observe due to large SM backgrounds. The direct production of the scalar PM itself, however, is shown to lead to similar new physics signatures but with much larger rates, and are considered here in their own right. The properties of such new PM states may already be constrained by LHC searches in both the W ± + missing transverse energy (MET) and W + W − + MET channels.

In the kinetic mixing (KM) portal scenario, the interaction of dark matter (DM) with the particles of the Standard Model (SM) is generated by diagrams connecting the familiar photon with its dark sector analog, the dark photon (DP), via loops of particles carrying both dark and SM quantum numbers, i.e., portal matter (PM). For the case of sub-GeV DM and DP, these PM states may lie in the ∼ 1 – 10 TeV range and be potentially accessible at the HL-LHC as well as at other future lepton and hadron colliders. In perhaps the simplest scenario of this kind, PM consists of just a pair of electrically charged, iso- and color-singlet, vectorlike (VL) fermions having opposite dark charges, with an O ( 1 ) mass splitting, yielding a finite value for the strength of the KM, i.e., ε ∼ a few × 10 − 4 . The dark Higgs induced mixing of PM states with their SM analogs allows for their decay but can also lead to significant distortions in the expected production properties for the PM at future lepton colliders due to t -channel dark Higgs exchange, potentially confusing PM identification. We show that the large set of clean observables available at lepton colliders is more that sufficient to resolve any of these ambiguities. The possibility of the production of like-sign PM fields via t / u -channel exchange of the same dark Higgs is also briefly explored.

The kinetic mixing (KM) portal mandates the existence of at least one new gauge boson, the dark photon (DP) based on the group U ( 1 ) D , which mixes with the Standard Model (SM) photon via loops of other new heavy particles carrying both SM and dark charges called portal matter (PM). Arguments exist based on the renormalization group equations running of the U ( 1 ) D gauge coupling suggesting that at higher scales U ( 1 ) D becomes part of a more complex non-Abelian group, a simple example being just the SM-like G D = S U ( 2 ) I × U ( 1 ) Y I . In our past analyses, it was always assumed that G D broke in a SM-like manner directly to the DP’s U ( 1 ) D which then subsequently broke at low energies ≲ 1 GeV . However, this need not be the case, and G D can instead break to U ( 1 ) T 3 I × U ( 1 ) Y I , with T 3 I being the diagonal generator of S U ( 2 ) I , now producing two light gauge bosons which obtain masses at the ≲ 1 GeV scale. In this paper, we explore the phenomenology of a very simple realization of this kind of alternative setup employing non-Abelian KM and having a minimal, leptonlike PM sector, demonstrating its distinctive nature in comparison to the previously examined symmetry breaking path. The effects of interference between these gauge bosons on thermal dark matter annihilation, the production of new heavy gauge, and Higgs and PM states at colliders, as well as the corresponding signatures for the light dark gauge bosons are examined. Collider signatures of this setup are found to be particularly challenging.

The kinetic mixing (KM) portal, by which the Standard Model (SM) photon mixes with a light dark photon arising from a new U ( 1 ) D gauge group, allows for the possibility of viable scenarios of sub-GeV thermal dark matter 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, also exist. The presence of such types of states and the strong suggestion of a need to embed U ( 1 ) D into a non-Abelian gauge structure not too far above the TeV scale based on the renormalization-group equation running of the U ( 1 ) D gauge coupling is potentially indicative of an enlarged group linking together the visible and dark sectors. The gauge group G = S U ( 3 ) c × S U ( 3 ) L × U ( 1 ) A × U ( 1 ) B = 3 c 3 L 1 A 1 B 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 = G SM × G D 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 ( ≲ 1 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 ∼ 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 S U ( 2 ) I × 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 g g 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