arXiv subjects

Koichi Hamaguchi

Publications and source records attributed to Koichi Hamaguchi.

100 records · Page 6Linked to original sources

arXiv2006

Local Grand Unification

In the standard model matter fields form complete representations of a grand unified group whereas Higgs fields belong to incomplete `split' multiplets. This remarkable fact is naturally explained by `local grand unification' in higher-dimensional extensions of the standard model. Here, the generations of matter fields are localized in regions of compact space which are endowed with a GUT gauge symmetry whereas the Higgs doublets are bulk fields. We realize local grand unification in the framework of orbifold compactifications of the heterotic string, and we present an example with SO(10) as a local GUT group, which leads to the supersymmetric standard model as an effective four-dimensional theory. We also discuss different orbifold GUT limits and the unification of gauge and Yukawa couplings.

hep-ph↗

arXiv2005

The Gravitino in Gaugino Mediation

We show that for gaugino mediated supersymmetry breaking the gravitino mass is bounded from below. For a size of the compact dimensions of order the unification scale and a cutoff given by the higher-dimensional Planck mass, we find $m_{3/2} \gtrsim 10 GeV$. In a large domain of parameter space, the gravitino is the lightest superparticle with a scalar $\tildeτ$-lepton as the next-to-lightest superparticle.

hep-ph↗

arXiv2004

Probing lepton flavour violation in slepton NLSP scenarios

In supersymmetric models where the gravitino is the lightest superparticle, the next-to-lightest superparticle (NLSP) is long-lived, and hence could be collected and studied in detail. We study the prospects of direct detection of lepton flavour violation in charged slepton NLSP decays. Mixing angles in the slepton sector as small as ~ 3\times 10^{-2} (9\times 10^{-3}) could be probed at the 90% confidence level if 3\times 10^3 (3\times 10^4) sleptons could be collected.

hep-ph↗

arXiv2004

Dual Models of Gauge Unification in Various Dimensions

We construct a compactification of the heterotic string on an orbifold T^6/Z_6 leading to the standard model spectrum plus vector--like matter. The standard model gauge group is obtained as an intersection of three SO(10) subgroups of E_8. Three families of SO(10) 16-plets are localized at three equivalent fixed points. Gauge coupling unification favours existence of an intermediate GUT which can have any dimension between five and ten. Various GUT gauge groups occur. For example, in six dimensions one can have E_6 \times SU(3), SU(4) \times SU(4) \times U(1)^2 or SO(8) \times SO(8), depending on which of the compact dimensions are large. The different higher--dimensional GUTs are `dual' to each other. They represent different points in moduli space, with the same massless spectrum and ultraviolet completion.

hep-ph↗

arXiv2004

Maximal Temperature in Flux Compactifications

Thermal corrections have an important effect on moduli stabilization leading to the existence of a maximal temperature, beyond which the compact dimensions decompactify. In this note, we discuss generality of our earlier analysis and apply it to the case of flux compactifications. The maximal temperature is again found to be controlled by the supersymmetry breaking scale, T_{crit} \sim \sqrt{m_{3/2} M_P}.

hep-th↗

arXiv2004

A study of late decaying charged particles at future colliders

In models where the gravitino is the lightest supersymmetric particle (LSP), the next-to-lightest supersymmetric particle (NLSP) is long-lived. We consider an important charged NLSP candidate, the scalar tau $\tildeτ$. Slow charged NLSPs may be produced at future colliders and they may be stopped in a massive stopper which simultaneously serves as a detector for the NLSP and its decay products. We found the number of events at a 1 kton to O(10) kton detector could be significant enough to study the NLSP decays with lifetime shorter than $10^{10}$ sec at the LHC. The performance of existing 1 kton detectors may be good enough to do such studies at the LHC, if they can be placed close to the ATLAS/CMS detectors. At a future $e^- e^-$ collider, scalar electrons $\tilde{e}^-$s are copiously produced. Slow NLSPs may be produced from the $\tilde{e}^-$ decay. The number of stopped NLSPs at a future linear collider could be large enough to study rare decay modes of the NLSP.

hep-ph↗

arXiv2004

Dilaton Destabilization at High Temperature

Many compactifications of higher-dimensional supersymmetric theories have approximate vacuum degeneracy. The associated moduli fields are stabilized by non-perturbative effects which break supersymmetry. We show that at finite temperature the effective potential of the dilaton acquires a negative linear term. This destabilizes all moduli fields at sufficiently high temperature. We compute the corresponding critical temperature which is determined by the scale of supersymmetry breaking, the beta-function associated with gaugino condensation and the curvature of the K"ahler potential, T_crit ~ (m_3/2 M_P)^(1/2) (3/β)^(3/4) (K'')^(-1/4). For realistic models we find T_crit ~ 10^11-10^12 GeV, which provides an upper bound on the temperature of the early universe. In contrast to other cosmological constraints, this upper bound cannot be circumvented by late-time entropy production.

hep-th↗

arXiv2004

Gravitino and Goldstino at Colliders

We consider theories with spontaneously broken global or local supersymmetry where the pseudo-goldstino or the gravitino is the lightest superparticle (LSP). Assuming that the long-lived next-to-lightest superparticle (NSP) is a charged slepton, we study several supergravity predictions: the NSP lifetime, angular and energy distributions in 3-body NSP decays. The characteristic couplings of the gravitino, or goldstino, can be tested even for very small masses.

hep-ph↗

arXiv2004

Supergravity at Colliders

We consider supersymmetric theories where the gravitino is the lightest superparticle (LSP). Assuming that the long-lived next-to-lightest superparticle (NSP) is a charged slepton, we investigate two complementary ways to prove the existence of supergravity in nature. The first is based on the NSP lifetime which in supergravity depends only on the Planck scale and the NSP and gravitino masses. With the gravitino mass inferred from kinematics, the measurement of the NSP lifetime will test an unequivocal prediction of supergravity. The second way makes use of the 3-body NSP decay. The angular and energy distributions and the polarizations of the final state photon and lepton carry the information on the spin of the gravitino and on its couplings to matter and radiation.

hep-ph↗

arXiv2003

Curvatons in Supersymmetric Models

We study the curvaton scenario in supersymmetric framework paying particular attention to the fact that scalar fields are inevitably complex in supersymmetric theories. If there are more than one scalar fields associated with the curvaton mechanism, isocurvature (entropy) fluctuations between those fields in general arise, which may significantly affect the properties of the cosmic density fluctuations. We examine several candidates for the curvaton in the supersymmetric framework, such as moduli fields, Affleck-Dine field, $F$- and $D$-flat directions, and right-handed sneutrino. We estimate how the isocurvature fluctuations generated in each case affect the cosmic microwave background angular power spectrum. With the use of the recent observational result of the WMAP, stringent constraints on the models are derived and, in particular, it is seen that large fraction of the parameter space is excluded if the Affleck-Dine field plays the role of the curvaton field. Natural and well-motivated candidates of the curvaton are also listed.

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