Chang Sub Shin

Dark radiation and small-scale structure problems with decaying particles

Although the standardCDM model describes the cosmic microwave background radiation and the large scale structure of the Universe with great success, it has some tensions with observations in the effective number of neutrino species (dark radiation) and the number of small scale structures (overabundance problem). Here we propose a scenario which can relax these tensions by producing both dark matter and dark radiation by late decays of heavy particle. Thanks to the generation mechanism, dark matters are rather warm so that the small-scale structure problem is resolved. This scenario can be naturally realized in supersymmetric axion model, in which axions produced by saxion decays provide dark radiation, while axinos from saxion decays form warm dark matter. We identify a parameter region of supersymmetric axion model satisfying all known cosmological constraints.

Thermal inflation and baryogenesis in heavy gravitino scenario

We present a thermal inflation model that incorporates the Affleck-Dine leptogenesis in heavy gravitino/moduli scenario, which solves the moduli-induced gravitino problem while producing a correct amount of baryon asymmetry and relic dark matter density. The model involves two singlet flat directions stabilized by radiative corrections associated with supersymmetry breaking, one direction that generates the Higgs μ and B parameters, and the other direction that generates the scale of spontaneous lepton number violation. The dark matter is provided by the lightest flatino which might be identified as the axino if the model is assumed to have a U(1)PQ symmetry to solve the strong CP problem. We derive the conditions for the model to satisfy various cosmological constraints coming from the Big-Bang nucleosynthesis and the dark matter abundance.

Sparticle spectrum of large volume compactification

We examine the large volume compactification of Type IIB string theory or its F theory limit and the associated supersymmetry breakdown and soft terms. It is crucial to incorporate the loop-induced moduli mixing, originating from radiative corrections to the Kähler potential. We show that in the presence of moduli mixing, soft scalar masses generically receive a D-term contribution of the order of the gravitino mass m3/2 when the visible sector cycle is stabilized by the D-term potential of an anomalous U(1) gauge symmetry, while the moduli-mediated gaugino masses and A-parameters tend to be of the order of m3/2/8π2. It is noticed also that a too large moduli mixing can destabilize the large volume solution by making it a saddle point.

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We revisit the idea of generating the Higgs {mu} parameter through a spontaneously broken Peccei-Quinn (PQ) symmetry in a gauge-mediated supersymmetry breaking scenario. For the messenger scale of gauge mediation higher than the PQ scale, the setup naturally generates {mu}{approx}m{sub soft} and the Higgs soft parameter B < or approx. O(m{sub soft}) with the CP phase of B aligned to the phase of gaugino masses, while giving the PQ scale v{sub PQ{approx}{radical}}(m{sub soft{Lambda}}), where m{sub soft} denotes the gauge-mediated gaugino or sfermion masses and {Lambda} is the cutoff scale which can be identified as the Planck scale or the grand unified theory scale. The PQ sector of the model results in distinctive cosmology including a late thermal inflation. We discuss the issue of dark matter and baryogenesis in the resulting thermal inflation scenario, and find that a right amount of gravitino dark matter can be produced together with a successful Affleck-Dine leptogenesis, when the gravitino mass m{sub 3/2}=O(100) keV.

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We examine dark matter production rates in supersymmetric axion models typified by the mass hierarchy m_gravitino << m_neutralino << m_axino. In such models, one expects the dark matter to be composed of an axion/gravitino admixture. After presenting motivation for how such a mass hierarchy might arise, we examine dark matter production in the SUSY Kim-Shifman-Vainshtein-Zakharov (KSVZ) model, the SUSY Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model and a hybrid model containing contributions from both KSVZ and DFSZ. Gravitinos can be produced thermally and also non-thermally from axino, saxion or neutralino decay. We obtain upper bounds on T_R due to overproduction of gravitinos including both the thermal and non-thermal processes. For T_R near the upper bound, then dark matter tends to be gravitino dominated, but for T_R well below the upper bounds, then axion domination is more typical although in many cases we find a comparable mixture of both axions and gravitinos. In this class of models, we ultimately expect detection of relic axions but no WIMP signal, although SUSY should ultimately be discovered at colliders.

-problem and axion in gauge mediation

We show that in a large volume scenario of type IIB string or F-theory compactifications, single thermal inflation provides only a partial solution to the cosmological problem of the light volume modulus. We then clarify the conditions for double thermal inflation, being a simple extension of the usual single thermal inflation scenario, to solve the cosmological moduli problem in the case of relatively light moduli masses. Using a specific example, we demonstrate that double thermal inflation can be realized in large volume scenario in a natural manner, and the problem of the light volume modulus can be solved for the whole relevant mass range. We also find that right amount of baryon asymmetry and dark matter can be obtained via a late-time Affleck-Dine mechanism and the decays of the visible sector NLSP to flatino LSP.

Mixed axion/gravitino dark matter from SUSY models with heavy axinos

A bstractWe study the general effects of anomalous U(1)A gauge symmetry on soft supersymmetry (SUSY) breaking terms in large volume scenario, where the MSSM sector is localized on a small cycle whose volume is stabilized by the D-term potential of the U(1)A . Since it obtains SUSY breaking mass regardless of the detailed form of Kähler potential, the U(1)A vector superfield acts as a messenger mediating the SUSY breaking in the moduli sector to the MSSM sector. Then, through the loops of U(1)A vector superfield, there arise soft masses of the order of m3/2/8π2 for scalar mass squares, m3/2/(8π2)2 for gaugino masses, and m3/2/8π2 for A-paramteres. In addition, the massive U(1)A vector superfield can have non-zero F and D-components through the moduli mixing in the Kähler potential, and this can result in larger soft masses depending upon the details of the moduli mixing. For instance, in the presence of one-loop induced moduli mixing between the visible sector modulus and the large volume modulus, the U(1)AD-term provides soft scalar mass squares of the order of m3/2. However, if the visible sector modulus is mixed only with small cycle moduli, its effect on soft terms depends on how to stabilize the small cycle moduli.

Cosmological moduli problem in large volume scenario and thermal inflation

In this paper, we discuss the Kaluza–Klein threshold correction to low-energy gauge couplings in theories with warped extra dimension, which might be crucial for the gauge coupling unification when the warping is sizable. Explicit expressions of one-loop thresholds are derived for generic five-dimensional (5D) gauge theory on a slice of 5D anti-de Sitter space (AdS5), where some of the bulk gauge symmetries are broken by orbifold boundary conditions and/or by bulk Higgs vacuum values. The effects of the mass mixing between the bulk fields with different orbifold parities are included, as such a mixing is required in some classes of realistic warped unification models.

Anomalous U(1) mediation in large volume compactification

It is known that the smallest size of the structures of the Universe with the weakly interacting massive dark matter is determined by the scale that enters the Hubble horizon at the time of kinetic decoupling of WIMP. This comes from the fact that the perturbation at smaller scales is erased due to the collisional damping during the kinetic decoupling. However the isocurvature mode is not affected and continue to be constant. We discuss about the generation of the isocurvature mode of WIMP dark matter at small scales recently found by Choi, Gong, and Shin1 and its implications for the indirect detection of dark matter through the formation of the small size of halos.