An analysis is given of the Stueckelberg extension of the standard model with a hidden sector gauge group U1 X where the mass growth for the extra gauge boson occurs via the Stueckelberg mechanism, and where the kinetic mixing in the U1 X U1 Y sector is included. Such a kinetic mixing is generic in a broad class of supergravity and string models. We carry out a detailed global fit of the model with the precision CERN LEP data on and off the Z pole, with 2 within 1% of the 2 of the standard model fit. Further, it is shown that, in the absence of matter in the hidden sector, there is a single effective parameter that controls the deviations from the standard model predictions, and the dependence on the kinetic mixing emerges only when matter in the hidden sector is included. An analysis is also given of millicharged dark matter arising from the hidden sector, where it is shown that such dark matter from the Stueckelberg extension can satisfy WMAP-3 data while allowing for a sharp Z 0 resonance which can be detected at the Tevatron and at the CERN LHC via a dilepton signal generated by the Drell-Yan process.

We present a simplified version of the atomic dark matter scenario, in which charged dark constituents are bound into atoms analogous to hydrogen by a massless hidden sector U(1) gauge interaction. Previous studies have assumed that interactions between the dark sector and the standard model are mediated by a second, massive Z ′ gauge boson, but here we consider the case where only a massless γ ′ kinetically mixes with the standard model hypercharge and thereby mediates direct detection. This is therefore the simplest atomic dark matter model that has direct interactions with the standard model, arising from the small electric charge for the dark constituents induced by the kinetic mixing. We map out the parameter space that is consistent with cosmological constraints and direct searches, assuming that some unspecified mechanism creates the asymmetry that gives the right abundance, since the dark matter cannot be a thermal relic in this scenario. In the special case where the dark "electron" and "proton" are degenerate in mass, inelastic hyperfine transitions can explain the CoGeNT excess events. In the more general case, elastic transitions dominate, and can be close to current direct detection limits over a wide range of masses.

The recent positron excess observed in the PAMELA satellite experiment strengthens previous experimental findings. We give here an analysis of this excess in the framework of the Stueckelberg extension of the standard model which includes an extra U (1) X gauge field and matter in the hidden sector. Such matter can produce the right amount of dark matter consistent with the WMAP constraints. Assuming the hidden sector matter to be Dirac fermions it is shown that their annihilation can produce the positron excess with the right positron energy dependence seen in the HEAT, AMS and the PAMELA experiments. The predictions of thep/p flux ratio also fit the data.

There has been renewed interest in the possibility that dark matter exists in the form of atoms, analogous to those of the visible world. An important input for understanding the cosmological consequences of dark atoms is their self-scattering. Making use of results from atomic physics for the potentials between hydrogen atoms, we compute the low-energy elastic scattering cross sections for dark atoms. We find an intricate dependence upon the ratio of the dark proton to electron mass, allowing for the possibility to "design" low-energy features in the cross section. Dependences upon other parameters, namely the gauge coupling and reduced mass, scale out of the problem by using atomic units. We derive constraints on the parameter space of dark atoms by demanding that their scattering cross section does not exceed bounds from dark matter halo shapes. We discuss the formation of molecular dark hydrogen in the universe, and determine the analogous constraints on the model when the dark matter is predominantly in molecular form.

The CDF and D0 data of nearly 475 pb ÿ1 in the dilepton channel is used to probe a recent class of models, Stueckelberg extensions of the standard model (StSM), which predict a Z 0 boson whose mass is of topological origin with a very narrow decay width. A Drell-Yan analysis for dilepton production via this Z 0 shows that the current data put constraints on the parameter space of the StSM. With a total integrated luminosity of 8 fb ÿ1 , the very narrow Z 0 can be discovered up to a mass of about 600 GeV. The StSM Z 0 will be very distinct since it can occur in the region where a Randall-Sundrum graviton is excluded.

It has been suggested that cold dark matter (CDM) has difficulties in explaining tentative evidence for noncuspy halo profiles in small galaxies, and the low velocity dispersions observed in the largest Milky Way satellites ("too big to fail" problem). Strongly self-interacting dark matter has been noted as a robust solution to these problems. The elastic cross sections required are much larger than predicted by generic CDM models, but could naturally be of the right size if dark matter is composite. We explore in a general way the constraints on models where strongly interacting CDM is in the form of dark "atoms" or "molecules," or bound states of a confining gauge interaction ("hadrons"). These constraints include considerations of relic density, direct detection, big bang nucleosynthesis, the cosmic microwave background, and LHC data.

One of the simplest hidden sectors with signatures in the visible sector is fermionic dark matter χ coupled to a Z gauge boson that has purely kinetic mixing with the standard model hypercharge. We consider the combined constraints from relic density, direct detection and collider experiments on such models in which the dark matter is either a Dirac or a Majorana fermion. We point out sensitivity to details of the UV completion for the Majorana model. For kinetic mixing parameter ≤ 0.01, only relic density and direct detection are relevant, while for larger , electroweak precision, LHC dilepton, and missing energy constraints become important. We identify regions of the parameter space of m χ , m Z , dark gauge coupling and that are most promising for discovery through these experimental probes. We study the compatibility of the models with the galactic center gamma ray excess, finding agreement at the 2-3σ level for the Dirac model.

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