arXiv subjects

Thomas G. Rizzo

Publications and source records attributed to Thomas G. Rizzo.

At least 19 recordsLinked to original sources

arXiv2026

Portal Matter and Scotogenic-like Dirac Neutrino Masses

Loops of portal matter (PM) fields, carrying both Standard Model (SM) and dark charges, can generate the necessary kinetic mixing (KM) between the ordinary and dark photons (DP) in vector portal scenarios thus allowing for interactions between visible and dark sector fields. Here we show that the field content of a previously considered model based on a partial $E_6$-like UV-completion of such setups can generate light Dirac neutrino masses in the interesting range, $\sim 0.05$ eV, at the one-loop level similar to what happens in scotogenic dark matter (DM) scenarios. While general $E_6-$like PM scenarios have been shown to be easily probed at colliders, such as the HL-LHC, uniquely testing this specific subclass of these setups in a direct fashion is found to be somewhat more challenging.

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arXiv2026

A Model for Dark Moments of the W Boson

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 vevs 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 $W^\pm+$MET and $W^+W^-+$MET channels.

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arXiv2026

Identifying Charged Lepton-like Portal Matter at Future Colliders

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, \ie, Portal Matter (PM). For the case of sub-GeV DM and DP, these PM states may lie in the $\sim 1-10$ 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, vector-like (VL) fermions having opposite dark charges, with an $O(1)$ mass splitting, yielding a finite value for the strength of the KM, \ie, $ε\sim a~ few \times10^{-4}$. 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.

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arXiv2025

Portal Matter Models of Kinetic Mixing with Two Light Dark Gauge Bosons

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 RGE 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 $G_D=SU(2)_I \times U(1)_{Y_I}$. 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 $\lsim 1$ GeV. However, this need not be the case and $G_D$ can instead break to $U(1)_{T_{3I}}\times U(1)_{Y_I}$, with $T_{3I}$ being the diagonal generator of $SU(2)_I$, now producing two light gauge bosons which obtain masses at the $\lsim 1$ GeV scale. In this paper we will explore the phenomenology of a very simple realization of this kind of alternative setup employing non-abelian KM and having a minimal, lepton-like 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, Higgs and PM states at colliders, as well as the corresponding signatures for the light dark gauge bosons are examined.

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arXiv2025

Towards UV-Models of Kinetic Mixing and Portal Matter VII: A Light Dark Photon in the $3_c3_L1_A1_B$ Model

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 (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 $U(1)_D$ into a non-abelian gauge structure not too far above the TeV scale based on the RGE 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 $G=SU(3)_c\times SU(3)_L\times U(1)_A\times U(1)_B=3_c3_L1_A1_B$ 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 $G=G_{SM}\times G_D$ encountered frequently in earlier work. The present paper describes the implications and phenomenology of this type of setup.

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arXiv2024

Towards UV Models of Kinetic Mixing and Portal Matter: A More Complex Dark Matter Sector?

Portal Matter, with both SM and dark charges, induces KM between the $U(1)_D$ dark photon and the SM gauge fields offering an attractive mechanism by which light thermal DM can obtain its observed relic density. If DM is fermionic, the CMB informs us that it must be Majorana/pseudo-Dirac in nature to avoid temperature-independent $s$-wave annihilation to SM states. How does this fit into a more complete picture with the SM? A first step along this path may not lie far away in energy due to the RGE running of the dark gauge coupling which can becomes non-perturbative before the $\sim 10$'s of TeV range. This implies that $U(1)_D$ must become embedded in a non-Abelian group, $G_D$, before this occurs. The breaking of this group then produces the PM masses and the heavy gauge fields associated with $G_D$ lead to new interactions between the SM and the dark sector. In the past we have examined a set of distinctive phenomenological features associated with this setup, based upon a number of simplifying assumptions. It behooves us to explore the impact of these assumptions on the predictions for possible experimental tests of these models. In past analyses we assumed that DM is a VL, complex singlet under $G_D$. If this assumption is relaxed, the dark sector must be augmented by additional fermion(s) and the scalar fields needed to break the symmetries while generating the needed Majorana-like mass terms for the DM. Here we analyze the simplest extension of this kind wherein the DM lies in a VL doublet of $G_D$, which we take to have the structure $SU(2)_I\times U(1)_{Y_I}$, leading to new phenomenology. We find 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 at colliders unless they are produced singly in $gg$-fusion or are resonantly enhanced.

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arXiv2024

Dark Moments for the Standard Model ?

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 $\sim eεQ$ 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, eg, 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 $e^+e^- \to γ+{\rm DP}$ process at Belle II are shown to provide some of the strongest future constraints on this scenario.

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arXiv2024

Towards UV-Models of Kinetic Mixing and Portal Matter V: Indirect Probes of the New Physics Scale

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, \ie, dark moments. In either case, there are reasons to believe, \eg, due to the RGE running of the $U(1)_D$ gauge coupling, that PM fields may have $\sim$ 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, \eg, at lepton colliders operating in the $m_Z$ to 1 TeV range. In particular, processes such as $e^+e^- \to γ+$DM or $e^+e^-\to \bar ff$, where $f$ 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 KM 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.

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arXiv2023

Towards UV-Models of Kinetic Mixing and Portal Matter IV: Quartification

As is well-known, Trinification, \ie, the extension of the Standard Model (SM) to $[SU(3)]^3=SU(3)_c\times SU(3)_L\times SU(3)_R$ as occurs in $E_6$ models, allows for a partial unification of the gauge forces even though quarks and leptons remain in separate multiplets so that no heavy gauge or scalar fields exist which can generate proton decay. The extension of this idea to Quartification, by including an additional $SU(3)'$ factor, has also been considered in the literature maintaining the basic attributes of Trinification but now allowing, \eg, for a more symmetric treatment of quarks and leptons at the price of new matter fields and gauge interactions. In this paper, we will consider this $SU(3)'$ to be the `dark' gauge group, now containing the familiar $U(1)_D$ subgroup, under which the SM fields are all neutral, which is associated with kinetic mixing (KM) and the existence of a light, $\lsim 1 $ GeV dark photon. This setup naturally predicts the existence of color-singlet portal matter (PM) fields, carrying both electromagnetic and $U(1)_D$ dark charges, that are necessary to generate this KM at the 1-loop level and whose masses are directly tied with those of the many new gauge bosons that originate from the extended gauge sector. In this paper, after a discussion of the detailed structure of this model, we present a broad survey of the collider phenomenology of the large set of new fields that must necessarily arise from this setup in a simplified version involving only a single generation of fermions. We demonstrate that several new signatures may be anticipated at the LHC as well as at future hadron and lepton colliders if such models are realized in nature.

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arXiv2023

Towards UV-Models of Kinetic Mixing and Portal Matter III: Relating Portal Matter and R-H Neutrino Masses

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 $G_{SM}=SU(3)_c\times SU(2)_L\times U(1)_Y$. However, we need to go beyond this simple model to gain a better understanding of the interplay between $G_{SM}$ 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, \eg, $SU(2)_I\times U(1)_{Y_I}$, 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 $G_{Dark}\to U(1)_D$ 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, \eg, the LHC and/or at the future FCC-hh.

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arXiv2022

Towards a UV-Model of Kinetic Mixing and Portal Matter II: Exploring Unification in an $SU(N)$ Group

If dark matter (DM) interacts with the Standard Model (SM) via the $U(1)_D$ kinetic mixing (KM) portal at low energies, it necessitates not only the existence of portal matter (PM) 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 GUT-like group, $G\to G_{SM}\times G_{Dark}$, 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 $SU(3)_c\times SU(2)_L\times U(1)_Y$ 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.

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arXiv2022

Kinetic Mixing, Dark Higgs Triplets, $M_W$ and All That

The kinetic mixing (KM) portal is a popular mechanism which allows light dark matter (DM) in the mass range below $\sim 1$ GeV to achieve the observed relic density by thermal means and can be effectively described by only a few parameters, e.g., $ε$, the strength of this KM. In the simplest setup, the Standard Model (SM) $U(1)_Y$ hypercharge gauge boson and a $U(1)_D$ dark photon (DP), which only couples to fields in the dark sector, experience KM via loops of portal matter (PM) fields which have both SM and dark charges thus generating a small coupling between us and dark matter (DM). However, if one wishes to understand the underlying physics behind this idea in a deeper fashion we need to take a step upward to a more UV-complete picture. Meanwhile, CDFII has measured a value for the $W$-boson mass which lies significantly above SM expectations. In this paper we speculate that this shift in the $W$'s mass may be related to the $\sim 1$ GeV mass of the DP within a framework of scalar PM that leads to phenomenologically interesting values of $ε$ via non-abelian KM due to the existence of an $SU(2)_L$, $Y=0$, {\it complex} Higgs triplet which carries a non-zero value of the $U(1)_D$ dark charge. Possible gauge boson plus missing energy signatures of this scenario that can appear at the LHC and elsewhere are examined. Indeed, with modest assumptions, all of the new scalar PM states are predicted to have masses below roughly $\simeq 630$ GeV and so should be at least kinematically accessible. The HL-LHC will very likely to be able to explore all of this model's allowed parameter space.

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arXiv2022

Forbidden Scalar Dark Matter and Dark Higgses

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.

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arXiv2022

Portal Matter and Dark Sector Phenomenology at Colliders

If dark matter (DM) interacts with the Standard Model (SM) via the kinetic mixing (KM) portal, it necessitates the existence of massive, likely $\gsim 1$ TeV, enabler portal matter (PM) particles that carry both dark and SM quantum numbers which will appear in vacuum polarization-like loop graphs. Such heavy states are only directly accessible at high energy colliders and apparently lie at mass scales beyond the direct kinematic reach of the ILC, CEPC and FCC-ee. A likely possibility is that these new particles are part of the 'next step' toward a UV-complete scenario describing both the SM and dark sector physics. A simple and straightforward example of such a scenario involving a non-abelian dark sector gauge group is employed in this work to demonstrate some of the range expected from this new physics. Here we present a broad survey of existing analyses designed to explore the nature of such PM states in a array of collider contexts, particularly at the LHC, and point out some of the future directions where additional work is obviously required in the hunt for new signatures as well as in model building directions.

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arXiv2022

$SU(4)$ Flavorful Portal Matter

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 non-minimal dark sectors, in which the dark $U(1)$ is a subgroup of a larger gauge symmetry under which SM particles might have non-trivial 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 $SU(4)_F \times U(1)_F$, incorporating the local $SU(3)$ flavor symmetry with PM appearing as a vector-like "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 non-trivial consequences of the flavor symmetry being unified with the dark sector.

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arXiv2021

C$^3$: A "Cool" Route to the Higgs Boson and Beyond

We present a proposal for a cold copper distributed coupling accelerator that can provide a rapid route to precision Higgs physics with a compact 8 km footprint. This proposal is based on recent advances that increase the efficiency and operating gradient of a normal conducting accelerator. This technology also provides an $e^{+}e^{-}$ collider path to physics at multi-TeV energies. In this article, we describe our vision for this technology and the near-term R&D program needed to pursue it.

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arXiv2021

The Bactrian Effect: Multiple Resonances and Light Dirac Dark Matter

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 $\sim 1$ 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.

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arXiv2020

Kinetic Mixing, Dark Photons and Extra Dimensions III: Brane Localized Dark Matter

Extra dimensions have proven to be a very useful tool in constructing new physics models. In earlier work, we began investigating toy models for the 5-D analog of the kinetic mixing/vector portal scenario where the interactions of dark matter, taken to be, e.g., a complex scalar, with the brane-localized fields of the Standard Model (SM) are mediated by a massive $U(1)_D$ dark photon living in the bulk. These models were shown to have many novel features differentiating them from their 4-D analogs and which, in several cases, avoided some well-known 4-D model building constraints. However, these gains were obtained at the cost of the introduction of a fair amount of model complexity, e.g., dark matter Kaluza-Klein excitations. In the present paper, we consider an alternative setup wherein the dark matter and the dark Higgs, responsible for $U(1)_D$ breaking, are both localized to the 'dark' brane at the opposite end of the 5-D interval from where the SM fields are located with only the dark photon now being a 5-D field. The phenomenology of such a setup is explored for both flat and warped extra dimensions and compared to the previous more complex models.

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