Kinetic Mixing from Kaluza-Klein Modes: A Simple Construction for Portal Matter

Abstract

The vector portal/kinetic mixing simplified model of dark matter, in which thermal dark matter of a mass ranging from a few MeV to a few GeV can be realized in association with a dark-sector U (1), relies on the appearance of a small kinetic mixing term between this dark U (1) and the Standard Model (SM) hypercharge. It is well-known that kinetic mixing of the appropriate magnitude can be generated at one loop by the inclusion of "portal matter" fields which are charged under both the dark U (1) and the SM hypercharge, and it has been previously argued on phenomenological grounds that fermionic portal matter fields must exhibit a very specific set of characteristics: They must be vector-like and have the same Standard Model group representations as existing SM fermions. A natural explanation for the presence of dark U (1)-charged copies of SM fermions would be to enlarge the dark gauge group and embed the SM and the portal matter into a single dark multiplet, however, previous attempts at accomplishing this task require significant ad hoc additions to the model to ensure that the portal matter fields are vector-like while the SM fields are chiral, and rely on complicated dark Higgs sectors to realize the appropriate symmetry breaking. In this paper, we argue that embedding a portal matter model in extra dimensions can easily generate chiral SM fermions and vector-like portal matter from a single dark bulk multiplet, while allowing for a far simpler dark Higgs sector. We present a minimal construction of this "Kaluza-Klein portal matter" and explore its phenomenology at the LHC, noting that even in our simple realization of the paradigm such a construction exhibits phenomenology unique from that of more conventional theories of large extra dimensions and 4-dimensional models of portal matter.

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

“…In the most commonly discussed PM models, KM occurs at the 1-loop level in an abelian manner between the SM U (1) Y hypercharge gauge boson and the U (1) D DP via a set of states having both Y, Q D = 0. Such KM mixing is finite and, in principle, calculable in a class of models wherein the condition i Y i Q Di = 0 is satisfied, as was the case in our earlier works [11,12,[14][15][16][17][18]. In the present setup, the presence of the dark scalar triplet with a non-zero vev will also allow for non-abelian KM to occur between the DP and the neutral W 3 , SU (2) L field as the triplet itself carries both weak isospin as well as Q D = 0 and, in fact, alone satisfies the corresponding analogous condition i T 3i Q Di = 0 with T 3 being the third, diagonal SU (2) L generator.…”

“…We also see that, e.g., if z is less than 0.25, then Σ will lie (if g D = e) below 10 −4 and that other KM sources will then likely be needed for a DP in the mass range of interest here so that the range of this parameter is then somewhat restricted. Of course, as previously noted, in a more UV-complete framework beyond the current setup, other new fields, e.g., in the form of Q D = 0 heavy vector-like fermions [11][12][13][14][15][16][17][18], are also potentially present, carrying non-zero values the SM hypercharge so that additional contributions to KM from this more 'conventional' abelian source are obtainable. Here, for simplicity, we will ignore this possibility assuming the Σ is the only source of KM.…”

“…Of particular interest is the kinetic mixing (KM)/vector portal scenario [8,9] where a new gauge field, the dark photon (DP) [10], which is the gauge boson associated with a new U (1) D symmetry, couples to only DM and other dark sector fields. This new gauge field, V , can experience KM with the neutral SM gauge fields via a set of vacuum polarization-like diagrams wherein are exchanged new scalar and/or fermion fields, here termed Portal Matter (PM) [11][12][13][14][15][16][17][18], which carry both dark and SM quantum numbers. Being loop suppressed, the resulting strength of this interaction is rather weak and is usually described via a single parameter, ∼ 10 −4 − 10 −3 , which, for DM and DP in the mass range < ∼ 1 GeV can reproduce the observed DM relic abundance [19] via the conventional freeze-out mechanism while simultaneously avoiding numerous other existing experimental constraints.…”

“…Of course such new fields, e.g., heavy vector-like fermions with both SM and dark sector quantum numbers[11][12][13][14][15][16][17][18], might also be present in a more complete version of the present setup and they would also contribute to abelian KM as usual.…”

Lepton Flavor Portal Matter

“…As noted by Holdom [14,15], finite and calculable kinetic mixing of this magnitude can be generated at one loop if there exist additional fields in the theory which are charged under both U (1) D and U (1) Y , and the U (1) D and U (1) Y charges of the additional fields are such that the ultraviolet divergence from each field's loop contribution is cancelled by the others. Recently there has been interest in the theory and phenomenology of these "portal matter" fields [21][22][23][24][25][26][27][28][29][30][31], in scenarios in which they are light enough to be kinematically accessible at present or near-future experiments. It was noted in [21] that if the portal matter is fermionic, the most phenomenologically viable scenario would be for the portal matter to be charged under U (1) D , have identical SM quantum numbers to some SM field, and be vector-like under the SM gauge group and U (1) D .…”