We investigate a new class of dark matter: superweakly-interacting massive particles (superWIMPs). As with conventional WIMPs, superWIMPs appear in well-motivated particle theories with naturally the correct relic density. In contrast to WIMPs, however, superWIMPs are impossible to detect in all conventional dark matter searches. We consider the concrete examples of gravitino and graviton cold dark matter in models with supersymmetry and universal extra dimensions, respectively, and show that superWIMP dark matter satisfies stringent constraints from Big Bang nucleosynthesis and the cosmic microwave background.PACS numbers: 95.35.+d, 4.65.+e, 11.10.Kk, 12.60.Jv There is ample evidence that luminous matter makes up only a small fraction of all matter in the universe. Results from the Wilkinson Microwave Anisotropy Probe, combined with other data, constrain the non-baryonic dark matter density to Ω DM = 0.23 ± 0.04 [1], far in excess of the luminous matter density. We therefore live in interesting times: while the amount of dark matter is becoming precisely known, its identity remains a mystery.WIMPs, weakly-interacting massive particles with weak-scale masses, are particularly attractive dark matter candidates. WIMPs have several virtues. First, their appearance in particle physics theories is independently motivated by the problem of electroweak symmetry breaking. Second, given standard cosmological assumptions, their thermal relic abundance is naturally that required for dark matter. Third, the requirement that WIMPs annihilate efficiently enough to give the desired relic density generically implies that WIMP-matter interactions are strong enough for dark matter to be discovered in current or near future experiments.Here we consider a new class of non-baryonic cold dark matter: superweakly-interacting massive particles (superWIMPs or SWIMPs). As with WIMPs, superWIMPs appear in well-motivated theoretical frameworks, such as supersymmetry and extra dimensions, and their (nonthermal) relic density is also naturally in the desired range. In contrast to conventional WIMPs, however, they interact superweakly and so evade all direct and indirect dark matter detection experiments proposed to date.For concreteness, we consider two specific superWIMPs: gravitinos in supersymmetric theories, and Kaluza-Klein (KK) gravitons in theories with extra dimensions. Gravitino and graviton superWIMPs share many features, and we investigate them in parallel.For gravitino superWIMPs, we consider supergravity, where the gravitinoG and all standard model (SM) superpartners have weak-scale masses. Assuming R-parity conservation, the lightest supersymmetric particle (LSP) is stable. In supergravity, the LSP is usually assumed to be a SM superpartner. Neutralino LSPs are excellent WIMP candidates, giving the desired thermal relic density for masses of 50 GeV to 2 TeV, depending on Higgsino content. In contrast, here we assume aG LSP. The gravitinos considered here couple gravitationally and form cold dark matter, in contrast to the ca...

We investigate supergravity models in which the lightest supersymmetric particle (LSP) is a stable gravitino. We assume that the next-lightest supersymmetric particle (NLSP) freezes out with its thermal relic density before decaying to the gravitino at time t ∼ 10 4 − 10 8 s. In contrast to studies that assume a fixed gravitino relic density, the thermal relic density assumption implies upper, not lower, bounds on superpartner masses, with important implications for particle colliders. We consider slepton, sneutrino, and neutralino NLSPs, and determine what superpartner masses are viable in all of these cases, applying CMB and electromagnetic and hadronic BBN constraints to the leading two-and three-body NLSP decays. Hadronic constraints have been neglected previously, but we find that they provide the most stringent constraints in much of the natural parameter space. We then discuss the collider phenomenology of supergravity with a gravitino LSP. We find that colliders may provide important insights to clarify BBN and the thermal history of the Universe below temperatures around 10 GeV and may even provide precise measurements of the gravitino's mass and couplings.

Cosmological issues are examined when gravitino is the lightest superparticle (LSP) and R-parity is broken. Decays of the next lightest superparticles occur rapidly via R-parity violating interaction, and thus they do not upset the bigbang nucleosynthesis, unlike the R-parity conserving case. The gravitino LSP becomes unstable, but its lifetime is typically much longer than the age of the Universe. It turns out that observations of diffuse photon background coming from radiative decays of the gravitino do not severely constrain the gravitino abundance, and thus the gravitino weighing less than around 1 GeV can be dark matter of the Universe when bilinear R-parity violation generates a neutrino mass which accounts for the atmospheric neutrino anomaly. *

In models of universal extra dimensions, gravity and all standard model fields propagate in the extra dimensions. Previous studies of such models have concentrated on the Kaluza-Klein (KK) partners of standard model particles. Here we determine the properties of the KK gravitons and explore their cosmological implications. We find the lifetimes of decays to KK gravitons, of relevance for the viability of KK gravitons as dark matter. We then discuss the primordial production of KK gravitons after reheating. The existence of a tower of KK graviton states makes such production extremely efficient: for reheat temperature T RH and d extra dimensions, the energy density stored in gravitons scales as T 2+3d/2 RH . Overclosure and Big Bang nucleosynthesis therefore stringently constrain T RH in all universal extra dimension scenarios. At the same time, there is a window of reheat temperatures low enough to avoid these constraints and high enough to generate the desired thermal relic density for KK WIMP and superWIMP dark matter.

We consider big bang nucleosynthesis (BBN) with long-lived charged massive particles. Before decaying, the long-lived charged particle recombines with a light element to form a bound state like a hydrogen atom. This effect modifies the nuclear-reaction rates during the BBN epoch through the modifications of the Coulomb field and the kinematics of the captured light elements, which can change the light element abundances. It is possible for heavier nuclei abundances such as 7 Li and 7 Be to decrease sizably, while the ratios Y p , D/H, and 3 He=H remain unchanged. This may solve the current discrepancy between the BBN prediction and the observed abundance of 7 Li. If future collider experiments find signals of a long-lived charged particle inside the detector, the information of its lifetime and decay properties could provide insights into not only the particle physics models but also the phenomena in the early Universe, in turn.

Cold dark matter may be made of superweakly interacting massive particles, super-WIMP's, that naturally inherit the desired relic density from late decays of metastable WIMP's. Well-motivated examples are weakscale gravitinos in supergravity and Kaluza-Klein gravitons from extra dimensions. These particles are impossible to detect in all dark matter experiments. We find, however, that super-WIMP dark matter may be discovered through cosmological signatures from the early Universe. In particular, super-WIMP dark matter has observable consequences for big bang nucleosynthesis and the cosmic microwave background ͑CMB͒, and may explain the observed underabundance of 7 Li without upsetting the concordance between deuterium and CMB baryometers. We discuss the implications for future probes of CMB blackbody distortions and collider searches for new particles. In the course of this study, we also present a model-independent analysis of entropy production from late-decaying particles in light of Wilkinson microwave anisotropy probe data.

Collisionless, cold dark matter in the form of weakly interacting massive particles (WIMPs) is well motivated in particle physics, naturally yields the observed relic density, and successfully explains structure formation on large scales. On small scales, however, it predicts too much power, leading to cuspy halos, dense cores, and large numbers of subhalos, in apparent conflict with observations. We consider super-WIMP dark matter, produced with large velocity in late decays at times 10(5) - 10(8) s. As analyzed by Kaplinghat in a more general setting, we find that super-WIMPs have sufficiently large free-streaming lengths and low phase space densities to help resolve small scale structure problems while preserving all of the above-mentioned WIMP virtues.

Dark matter may be composed of superWIMPs, superweakly-interacting massive particles produced in the late decays of other particles. We focus on the case of gravitinos produced in the late decays of sleptons or sneutrinos and assume they are produced in sufficient numbers to constitute all of non-baryonic dark matter. At leading order, these late decays are two-body and the accompanying energy is electromagnetic. For natural weak-scale parameters, these decays have been shown to satisfy bounds from Big Bang nucleosynthesis and the cosmic microwave background. However, sleptons and sneutrinos may also decay to three-body final states, producing hadronic energy, which is subject to even more stringent nucleosynthesis bounds. We determine the three-body branching fractions and the resulting hadronic energy release. We find that superWIMP gravitino dark matter is viable and determine the gravitino and slepton/sneutrino masses preferred by this solution to the dark matter problem. In passing, we note that hadronic constraints disfavor the possibility of superWIMPs produced by neutralino decays unless the neutralino is photino-like. PACS numbers: 95.35.+d, 98.80.Cq, 26.35.+c, 98.80.Es