The Double-Dark Portal

In most models of the dark sector, dark matter is charged under some new symmetry to make it stable. We explore the possibility that not just dark matter, but also the force carrier connecting it to the visible sector is charged under this symmetry. This dark mediator then acts as a Double-Dark Portal. We realize this setup in the \emph{dark mediator Dark matter} model (dmDM), featuring a fermionic DM candidate $\chi$ with Yukawa couplings to light scalars $\phi_i$. The scalars couple to SM quarks via the operator $\bar q q \phi_i^* \phi_j/\Lambda_{ij}$. This can lead to large direct detection signals via the $2\rightarrow3$ process $\chi N \rightarrow \chi N \phi$ if one of the scalars has mass $\lesssim 10$ keV. For dark matter Yukawa couplings $y_\chi \sim 10^{-3} - 10^{-2}$, dmDM features a thermal relic dark matter candidate while also implementing the SIDM scenario for ameliorating inconsistencies between dwarf galaxy simulations and observations. We undertake the first systematic survey of constraints on light scalars coupled to the SM via the above operator. The strongest constraints are derived from a detailed examination of the light mediator's effects on stellar astrophysics. LHC experiments and cosmological considerations also yield important bounds. Observations of neutron star cooling exclude the minimal model with one dark mediator, but a scenario with two dark mediators remains viable and can give strong direct detection signals. We explore the direct detection consequences of this scenario and find that a heavy $\mathcal{O}(100)$ GeV dmDM candidate fakes different $\mathcal{O}(10)$ GeV WIMPs at different experiments. Large regions of dmDM parameter space are accessible above the irreducible neutrino background.Comment: 24 pages, 19 figures, + references and appendices, update the SIDM discussion and reference

Leptogenesis with Composite Neutrinos

Models with composite singlet neutrinos can give small Majorana or Dirac masses to the active neutrinos. The mechanism is based on the fact that conserved chiral symmetries give massless neutrinos at the renormalizable level. Thus, they acquire very small masses due to non-renormalizable terms. We investigate such models in two aspects. First, we find UV completions for them and then we investigate the possibility of giving leptogenesis. We find that these models offer new possibilities for leptogenesis. Models with Majorana masses can exhibit standard leptogenesis. Models with Dirac masses can provide a realization of Dirac type leptogenesis with mass scale that can be as low as 10 TeV.Comment: 20 pages, 4 figure

Magnetic Field Transfer From A Hidden Sector

Primordial magnetic fields in the dark sector can be transferred to magnetic fields in the visible sector due to a gauge kinetic mixing term. We show that the transfer occurs when the evolution of magnetic fields is dominated by dissipation due to finite electric conductivity, and does not occur at later times if the magnetic fields evolve according to magnetohydrodynamics scaling laws. The efficiency of the transfer is suppressed by not only the gauge kinetic mixing coupling but also the ratio between the large electric conductivity and the typical momentum of the magnetic fields. We find that the transfer gives nonzero visible magnetic fields today. However, without possible dynamo amplifications, the field transfer is not efficient enough to obtain the intergalactic magnetic fields suggested by the gamma-ray observations, although there are plenty of possibilities for efficient dark magnetogenesis, which are experimentally unconstrained.Comment: 26 pages, 2 figure

Direct Detection with Dark Mediators

We introduce dark mediator Dark matter (dmDM) where the dark and visible sectors are connected by at least one light mediator $\phi$ carrying the same dark charge that stabilizes DM. $\phi$ is coupled to the Standard Model via an operator $\bar q q \phi \phi^*/\Lambda$, and to dark matter via a Yukawa coupling $y_\chi \overline{\chi^c}\chi \phi$. Direct detection is realized as the $2\rightarrow3$ process $\chi N \rightarrow \bar \chi N \phi$ at tree-level for $m_\phi \lesssim 10 \ \mathrm{keV}$ and small Yukawa coupling, or alternatively as a loop-induced $2\rightarrow2$ process $\chi N \rightarrow \chi N$. We explore the direct-detection consequences of this scenario and find that a heavy $\mathcal{O}(100 \ \mathrm{GeV})$ dmDM candidate fakes different $\mathcal{O}(10 \ \mathrm{GeV})$ standard WIMPs in different experiments. Large portions of the dmDM parameter space are detectable above the irreducible neutrino background and not yet excluded by any bounds. Interestingly, for the $m_\phi$ range leading to novel direct detection phenomenology, dmDM is also a form of Self-Interacting Dark Matter (SIDM), which resolves inconsistencies between dwarf galaxy observations and numerical simulations.Comment: 9 pages, 8 figures + reference

The Dark Penguin Shines Light at Colliders

Collider experiments are one of the most promising ways to constrain Dark Matter (DM) interactions. For several types of DM-Standard Model couplings, a meaningful interpretation of the results requires to go beyond effective field theory, considering simplified models with light mediators. This is especially important in the case of loop-mediated interactions. In this paper we perform the first simplified model study of the magnetic dipole interacting DM, by including the one-loop momentum-dependent form factors that mediate the coupling -- given by the Dark Penguin -- in collider processes. We compute bounds from the monojet, monophoton, and diphoton searches at the $8$ and $14$ TeV LHC, and compare the results to those of direct and indirect detection experiments. Future searches at the $100$ TeV hadron collider and at the ILC are also addressed. We find that the optimal search strategy requires loose cuts on the missing transverse energy, to capture the enhancement of the form factors near the threshold for on-shell production of the mediators. We consider both minimal models and models where an additional state beyond the DM is accessible. In the latter case, under the assumption of anarchic flavor structure in the dark sector, the LHC monophoton and diphoton searches will be able to set much stronger bounds than in the minimal scenario. A determination of the mass of the heavier dark fermion might be feasible using the $M_{T2}$ variable. In addition, if the Dark Penguin flavor structure is almost aligned with that of the DM mass, a displaced signal from the decay of the heavier dark fermion into the DM and photon can be observed. This allows us to set constraints on the mixings and couplings of the model from an existing search for non-pointing photons.Comment: 32 pages + appendices and references, 22 figure