The inert doublet model (IDM), a two Higgs extension of the standard model with an unbroken Z 2 symmetry, is a simple and yet rich model of dark matter. We present a systematic analysis of the dark matter abundance and investigate the potentialities for direct and gamma indirect detection. We show that the model should be within the range of future experiments, like GLAST and ZEPLIN. The lightest stable scalar in the IDM is a perfect example, or archetype of a weakly interacting massive particle.

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We argue that for QCD in the limit of a large number of colors, the axial U (1) symmetry of massless quarks is effectively restored at the deconfining phase transition. If this transition is of second order, metastable states in which parity is spontaneously broken can appear in the hadronic phase. These metastable states have dramatic signatures, including enhanced production of η and η mesons, which can decay through parity violating decay processes such as η → π 0 π 0 , and global parity odd asymmetries for charged pions.It may be possible to observe the phase transition(s) from hadronic to quark and gluon degrees of freedom through the collisions of heavy nuclei at ultrarelativistic energies. In the region of central rapidity, the relevant phase transitions are those at nonzero temperature; these phase transitions can be studied by numerical simulations of lattice gauge theory. At present, simulations indicate that for three colors coupled to light quarks, there is at most one phase transition, controlled by the chiral dynamics of the light quarks [1]. The order of the phase transition in QCD, in which two flavors are very light, and one flavor not too heavy (up, down, and strange), is still unsettled.The nature of the chiral phase transition depends crucially upon the dynamics of the axial U (1) symmetry of the light quarks [2,3]. Notably, for two massless flavors, if the axial U (1) symmetry is not restored about the chiral phase transition, then the transition can be of second order; if it is restored, the transition may be driven first order by fluctuations.There are two approaches to understanding the dynamical breaking of the axial U (1) symmetry. The first assumes that the dominant fluctuations are semiclassical instantons [4]- [7]. The second is based upon the large N limit of an SU (N ) gauge theory [8]- [15], and assumes that the dominant fluctuations are not semiclassical, but quantum.At zero temperature, both approaches give a reasonably successful phenomenology for the η mass and related processes. In this Letter we show that these two approaches give radically different predictions at nonzero temperature. In instanton models of the hadronic vacuum [4], the topological susceptibility is essentially constant below the phase transition, and only drops off above the phase transition. We argue that at large N , the topological susceptibility essentially vanishes at the phase transition. If the deconfining phase transition is of second order, then the axial U (1) symmetry is dynamically restored as the phase transition is approached from below. Under this assumption, using a nonlinear sigma model [11]- [15] we show that metastable states with spontaneous parity violation arise in the hadronic phase, and would produce striking experimental signatures.The large N limit of SU (N ) gauge theories is believed to be a reasonable approximation even for N = 3 [8]. We assume that confinement holds for all N , with the masses of mesons and glueballs of order one as N → ∞; interactions between mesons and/or glu...

We consider the possibility that along the thermal history of the Universe, dark matter (DM) would have been created from Standard Model particles, either through a kinetic mixing portal to an extra U (1) gauge field, or through the Higgs portal. Depending solely on the DM particle mass, on the portal and on the DM hidden sector interaction, we show how the observed DM relic density can be obtained. There are four possible freeze-in/reannihilation/freeze-out regimes, which together result in a simple characteristic relic density phase diagram, with the shape of a "Mesa". In the case of the kinetic mixing portal, we show that, unlike other freeze-in scenarios discussed in the literature, the freeze-in regime can be probed by forthcoming DM direct detection experiments. These results are well representative of any scenario where a DM hidden sector would be created out of the Standard Model sector.