Shinsuke Kawai

Instability of 1-loop superstring cosmology

Abstract A stability analysis is made in the context of the previously discovered non-singular cosmological solution from 1-loop corrected superstring effective action. We found that this solution has an instability in graviton mode, which is shown to have a close relation to the avoidance of initial singularity via energy condition. We also estimate the condition for the breakdown of the background solution due to the overdominance of the graviton.

Evolution of Fluctuations during Graceful Exit in String Cosmology

We study the evolution of fluctuations in a universe dominated by a scalar field coupled to the Gauss-Bonnet term. During the graceful exit, we found non-negligible enhancements of both curvature perturbation and gravitational wave in the long wavelength limit, and we also found a short wavelength instability for steep background superinflation just after the completion of the graceful exit. This result for one possible graceful exit mechanism would provide a significant implication on the primordial spectrum from the string cosmology.

Higgs inflation in minimal supersymmetric SU(5) GUT

The Standard Model Higgs boson with large nonminimal coupling to the gravitational curvature can drive cosmological inflation. We study this type of inflationary scenario in the context of supersymmetric grand unification and point out that it is naturally implemented in the minimal supersymmetric SU(5) model, and hence virtually in any GUT models. It is shown that with an appropriate Kahler potential the inflaton trajectory settles down to the Standard Model vacuum at the end of the slow roll. The predicted cosmological parameters are also consistent with the 7-year WMAP data. Recently the idea that the Standard Model (SM) Higgs field may be identified with an inflaton field, has at- tracted much attention (1-9). The major role is played by the nonminimal coupling to gravity, which renders the Higgs mass to be within the range of 126−194 GeV (1-4), while keeping the amplitude of the primordial curvature perturbation at the scale of ∼ 10 5 . The idea of inflation by nonminimally coupled inflaton field itself is certainly not new (10). Nevertheless, the striking agreement with the present-day cosmological data, combined with the minimalistic nature of the model, makes this type of sce- nario very attractive. The predicted mass range of the Higgs particle is also interesting for the physics of the Large Hadron Collider. The Higgs potential in the SM is unstable against quantum corrections (the hierarchy problem) and it therefore is reasonable to reconsider Higgs inflation in supersymmetric theory (11, 12). It is shown in (11) that Higgs inflation cannot be implemented within the min- imal supersymmetric Standard Model (MSSM), as the field content of the latter is too restrictive. Instead, with an extra field (i.e. in the next-to-minimal super- symmetric Standard Model, NMSSM) a sensible scenario of Higgs inflation is found to be possible. The NMSSM model has tachyonic instability in the direction of the extra field, but this can be cured by considering a non- canonical Kahler potential (12). In this paper we discuss the possibility of Higgs in- flation in supersymmetric grand unified theory (GUT). There are several reasons to motivate this study. One obvious reason is that the energy scale of inflation is typically above the grand unification scale, and it is un- natural to suppose that the SM Lagrangian is valid all the way up to the scale of inflation; as the GUT scale destabilises the electroweak scale without supersymme- try, it seems that supersymmetric GUT is an appropri- ate theory to start with. Another reason is the puzzling necessity of the extra field besides the MSSM fields for successful Higgs inflation, as alluded to above; going be- yond the MSSM is somewhat against the minimalistic guiding principle of the original Higgs inflation, and as the NMSSM is structurally similar to the SU(5) GUT model, it seems natural to conjecture that the SU(5) GUT, rather than the NMSSM, may be a more appropri- ate minimal supersymmetric theory that accommodates Higgs inflation. Obvious questions are then whether it is possible to obtain enough inflation (e-folding) some- where between the Planck scale and the GUT scale, and if so whether the prediction of the cosmological parame- ters is consistent with the present observation. We shall address these issues below, and find that a viable Higgs inflationary scenario nicely fits into the minimal SU(5) model. We shall employ supergravity embedding of GUT (13), since the nonminimal coupling of the Higgs field to gravity naturally arises in that framework.

Rolling tachyon boundary conformal field theory on an orbifold

We consider the nontrivial boundary conformal field theory with exactly marginal boundary deformation. In recent years this deformation has been studied in the context of rolling tachyons and S-branes in string theory. Here we study the problem directly from an open string point of view, at one loop. We formulate the theory of the Z{sub 2} reflection orbifold. To do so, we extend fermionization techniques originally introduced by Polchinski and Thorlacius. We also explain how to perform the open string computations at arbitrary (rational) radius, by consistently constructing the corresponding shift orbifold, and show in what sense these are related to known boundary states. In a companion paper, we use these results in a cosmological context involving decaying branes.

Gauge mediation scenario with hidden sector renormalization in MSSM

We study the hidden sector effects on the mass renormalization of a simplest gauge-mediated supersymmetry breaking scenario. We point out that possible hidden sector contributions render the soft scalar masses smaller, resulting in drastically different sparticle mass spectrum at low energy. In particular, in the 5+5 minimal gauge-mediated supersymmetry breaking with high messenger scale (that is favored by the gravitino cold dark matter scenario), we show that a stau can be the next lightest superparticle for moderate values of hidden sector self-coupling. This provides a very simple theoretical model of long-lived charged next lightest superparticles, which imply distinctive signals in ongoing and upcoming collider experiments.

Fractional S-branes on a spacetime orbifold

Unstable D-branes are central objects in string theory, and exist also in time-dependent backgrounds. In this paper we take first steps to studying brane decay in spacetime orbifolds. As a concrete model we focus on the R 1,d /Z2 orbifold. We point out that on a spacetime orbifold there exist two kinds of S-branes, fractional S-branes in addition to the usual ones. We investigate their construction in the open string and closed string boundary state approach. As an application of these constructions, we consider a scenario where an unstable brane nucleates at the origin of time of a spacetime, its initial energy then converting into energy flux in the form of closed strings. The dual open string description allows for a well-defined description of this process even if it originates at a singular origin of the spacetime.

Higgs inflation in minimal supersymmetricSU(5)grand unified theory

The Standard Model Higgs boson with large nonminimal coupling to the gravitational curvature can drive cosmological inflation. We study this type of inflationary scenario in the context of supersymmetric grand unification and point out that it is naturally implemented in the minimal supersymmetric SU(5) model, and hence virtually in any GUT models. It is shown that with an appropriate Kahler potential the inflaton trajectory settles down to the Standard Model vacuum at the end of the slow roll. The predicted cosmological parameters are also consistent with the 7-year WMAP data. Recently the idea that the Standard Model (SM) Higgs field may be identified with an inflaton field, has at- tracted much attention (1-9). The major role is played by the nonminimal coupling to gravity, which renders the Higgs mass to be within the range of 126−194 GeV (1-4), while keeping the amplitude of the primordial curvature perturbation at the scale of ∼ 10 5 . The idea of inflation by nonminimally coupled inflaton field itself is certainly not new (10). Nevertheless, the striking agreement with the present-day cosmological data, combined with the minimalistic nature of the model, makes this type of sce- nario very attractive. The predicted mass range of the Higgs particle is also interesting for the physics of the Large Hadron Collider. The Higgs potential in the SM is unstable against quantum corrections (the hierarchy problem) and it therefore is reasonable to reconsider Higgs inflation in supersymmetric theory (11, 12). It is shown in (11) that Higgs inflation cannot be implemented within the min- imal supersymmetric Standard Model (MSSM), as the field content of the latter is too restrictive. Instead, with an extra field (i.e. in the next-to-minimal super- symmetric Standard Model, NMSSM) a sensible scenario of Higgs inflation is found to be possible. The NMSSM model has tachyonic instability in the direction of the extra field, but this can be cured by considering a non- canonical Kahler potential (12). In this paper we discuss the possibility of Higgs in- flation in supersymmetric grand unified theory (GUT). There are several reasons to motivate this study. One obvious reason is that the energy scale of inflation is typically above the grand unification scale, and it is un- natural to suppose that the SM Lagrangian is valid all the way up to the scale of inflation; as the GUT scale destabilises the electroweak scale without supersymme- try, it seems that supersymmetric GUT is an appropri- ate theory to start with. Another reason is the puzzling necessity of the extra field besides the MSSM fields for successful Higgs inflation, as alluded to above; going be- yond the MSSM is somewhat against the minimalistic guiding principle of the original Higgs inflation, and as the NMSSM is structurally similar to the SU(5) GUT model, it seems natural to conjecture that the SU(5) GUT, rather than the NMSSM, may be a more appropri- ate minimal supersymmetric theory that accommodates Higgs inflation. Obvious questions are then whether it is possible to obtain enough inflation (e-folding) some- where between the Planck scale and the GUT scale, and if so whether the prediction of the cosmological parame- ters is consistent with the present observation. We shall address these issues below, and find that a viable Higgs inflationary scenario nicely fits into the minimal SU(5) model. We shall employ supergravity embedding of GUT (13), since the nonminimal coupling of the Higgs field to gravity naturally arises in that framework.