Kinetic mixing and portal matter phenomenology

Dark photons are interesting as potential mediators between the dark matter sector and the fields of the Standard Model (SM). The interaction of the dark photon, described by a broken $U(1{)}_D$ gauge symmetry, with the SM is usually generated at the one-loop level via kinetic mixing through the existence of portal matter, here assumed to be fermionic, which carries both a dark charge as well as a SM $U(1{)}_Y$ hypercharge. For theoretical consistency, as well as for many phenomenological reasons, this portal matter must be vectorlike with respect to the SM and dark gauge groups and, in particular, is shown to be allowed only to transform as vectorlike copies of the usual SM fields. The dark Higgs that is responsible for the breaking of $U(1{)}_D$ can then generate a mixing between the portal matter and SM fields with the same electric charge thus altering the dark photon/portal matter interactions with (at least some of) the SM fields and also providing a path for the portal matter fields to decay. In this paper we briefly explore the phenomenology of some specific simple models of this portal matter including, for the case where the portal matter is leptonic in nature, their potential impact on experiments probing low energy parity violation and the g-2 of the muon. In the case of color triplet, bottom quarklike portal matter, their direct pair and single production at the LHC is shown to be observable in final states that include missing $E_T$ and/or very highly boosted lepton jets together with pairs of $\text{high}\text{−}{p}_T$ $b$-jets that can be used to trigger on such events. These signatures are quite distinct from those usually employed in the search for vectorlike quarks at the LHC and, furthermore, we demonstrate that the conventional signal channels for vectorlike quarks involving the SM Higgs and gauge fields are essentially closed in the case of portal matter. Many other more complex, and more realistic, portal matter scenarios of the type discussed here are possible which can lead to wide-ranging signatures in various classes of experiments.

Figure 1

(Top) Maximum value of $\epsilon$ as a function of $m_V$ allowed by the present APV data for positive (negative) values of $y$ as solid (dashed) lines. From top to bottom, $|y|=0.1$, 0.3 or 1 has been assumed, respectively. (Bottom) Parameter space to be probed by the MOLLER experiment assuming the SM result is recovered. Here, again we see the maximum value of $\epsilon$ as a function of $m_V$ for, from top to bottom $|y|=0.1$, 0.3, 1.0, respectively.

Figure 2

The parameter $R$, as defined in the text, as a function of the $V$ mass $m_V$ assuming $y=-0.5(-0.25,0,0.25,0.5)$ corresponding to the red (blue, green, magenta, cyan) curves, respectively.

Figure 3

(Top) $B\overline{B}$ (magenta) and $b$-squark (green) pair-production cross section at $\sqrt{s}=13\mathrm{TeV}$ as discussed in the text. (Bottom) Values of the 3-body phase space function $F$ for the decay $S\to V{e}^{+}{e}^{-}$ as a function of $r=m_V/m_S$ as described in the text.

Figure 4

(Top) Boosted decay length ($d$) distribution of the dark photon from the decay $B\to bV$ assuming that $m_B=1\mathrm{TeV}$ as described in the text. (Bottom) Boosted leptonic opening angle distributions, averaged over the model parameter space, for the boosted $V\to e^{+}{e}^{-}$ (red) and $V\to {\mu }^{+}{\mu }^{-}$ (blue) decay modes.

Figure 5

(Top) Event rate estimate for the number of single lepton jets for the idealized ATLAS detector as discussed in the text assuming an integrated luminosity of $100{\mathrm{fb}}^{-1}$. From top to bottom the histograms assume that $m_1=1$, 1.5, 2 TeV, respectively. (Bottom) Boosted decay length ($d$) distribution of the $S$ from the decay $B\to bS$ assuming that $m_B=1\mathrm{TeV}$ as described in the text assuming $m_V<m_S<2m_V$. Note that values of $d>{10}^{12}\mathrm{m}$ or so are likely excluded by consideration of cosmological and nucleosynthesis constraints.

Figure 6

(Top) Boosted decay length ($d$) distribution of the $V$ arising from the decay chain $B\to bS$, $S\to VV$ assuming that $m_B=1\mathrm{TeV}$ as described in the text. (Bottom) Ratio of the boosts of the two $V$’s produced from a single initial $B$ (or $\overline{B}$) and the $B\to bS$, $S\to VV$ decay chain. $V_1$ is always defined to be the state with the larger boost.

Figure 7

Distribution of the opening angle between the two $V$’s in $S$ decay, $\mathrm{\Delta }\varphi$, as described in the text.

Figure 8

(Top) lowest order cross section for single $B$ production in association with $S/V$ as a function of the $B$ mass. From top to bottom a cut of $p_T(S,V)>50$, 100, 200 GeV, respectively, has been applied. (Bottom) Same as in the top panel but now as a function of $p_T^{\min }(S,V)$ for, from top to bottom, a $B$ mass of 1, 1.5, or 2 TeV, respectively, has been assumed. In both panels a cut of $|{\eta }_{V,S}|<2.5$ has also been applied.

Figure 9

Associated production cross section for $B+V/S$ as a function of the $|{\eta }_{V,S}{|}_{\min }$; from top to bottom, we assume a $B$ mass of 1, 1.5, or 2 TeV, respectively.