arXiv subjects

T. Kobayashi

Publications and source records attributed to T. Kobayashi.

216 records · Page 12Linked to original sources

arXiv1997

Stops in R-parity Breaking Model for High-$Q^2$ Events at HERA

We investigate an event excess in the reaction $e^+p \to e^+ X$ with large $x$ and high $Q^2$ observed at HERA and show that the events could naturally be interpreted as a signature of the production of the scalar top quarks (stops) in a supersymmetric model with $R$-parity breaking interactions. The HERA events are characterized by the broad mass distribution and in fact it can be simulated by our specific scenario if we consider almost degenerate two mass eigenstates $\tilde{t_1}$ and $\tilde{t_2}$ of the stops.

hep-ph↗

arXiv1996

Quantum Deformation of igl(n) Algebra on Quantum Space

We study quantum deformed $gl(n)$ and $igl(n)$ algebras on a quantum space discussing multi-parametric extension. We realize elements of deformed $gl(n)$ and $igl(n)$ algebras by a quantum fermionic space. We investigate a map between deformed $igl(2)$ algebras of our basis and other basis.

q-alg↗

arXiv1996

Is a high $P_T$ muon of the $e^+p \to μ^+ X$ event observed at HERA a signature of the stop?

We investigate the $e^+p \to μ^+ X$ event with high transverse momenta observed at HERA (H1) and show that this event could be interpreted as a signature of the single production of the scalar top quark in a supersymmetric model with $R$-parity breaking interactions. The event topology of the H1 event is rather characteristic and in fact it can be simulated by our specific scenario if we reasonably choose our model parameters to be ({\romannumeral 1}) $m_{\widetilde{d},{\widetilde{b}}, {\widetildeν}}$ ${{\stackrel{>}{\sim}}}$ $1$TeV [$0.8$TeV] for ${λ'_{131}}$ $=$ $0.1$ [$0.05$] and ({\romannumeral 2}) $m_{\widetilde{W}_{1}}$ ${{\stackrel{<}{\sim}}}$ $150$GeV, $100$GeV ${{\stackrel{<}{\sim}}}$ $m_{\widetilde{t}_1}$ ${{\stackrel{<}{\sim}}}$ $200$GeV and ${λ'_{131}}$ ${{\stackrel{>}{\sim}}}$ $0.05$.

hep-ph↗

arXiv1995

Study of the levels in $^{17}$C above the $^{16}$C + neutron threshold

The $β$-delayed neutron decay of $^{17}$B was studied using a radioactive ion beam. The neutron energies, measured via time-of-flight, give information on states in $^{17}$C above the $^{16}$C + neutron threshold. States in $^{17}$C were found at excitation energies of 2.25(2) MeV, 2.64(2) MeV and 3.82(5) MeV, and possibly at 1.18(1) MeV. These low lying states are of possible interest for nuclear physics as well as for astrophysics.

nucl-ex↗

arXiv1995

Supersymmetric Particle Production at HERA

In the framework of the minimal supersymmetric standard model and the $R$-parity breaking model, we investigate various production processes of the supersymmetric partner at HERA energies. Our emphasis is paid upon the scalar top quark, the partner of top quark, characterized by its lighter mass than the top quark and other scalar quarks in a model. We propose experimentally feasible approaches to search for clean signals of the stop from either its production or decay processes.

hep-ph↗

arXiv1995

Quark Mass Matrices in Orbifold Models

We study left-right symmetric quark mass matrices whose up- and down-sectors have the same structure. This type of realistic mass matrices are derived from orbifold models. We cannot derive some of them by using an extra U(1) symmetry.

hep-ph↗

arXiv1995

Selection Rules for Nonrenormalizable Couplings in Superstring Theories

We study nonrenormalizable coupling terms in $Z_N$ orbifold models. Nontrivial selection rules of couplings are provided and cannot be understood in terms of a simple symmetry of effective field theories. We also discuss phenomenological implications of theses selection rules for the quark mass matrices.

hep-ph↗

arXiv1995

Implications of Non-universal Soft Masses on Gauge Coupling Unification

We study the gauge coupling unification of the minimal supersymmetric standard model with non-universal soft scalar and gaugino masses. The unification scale of the gauge couplings is estimated for non-universal cases. It is sensitive to the non-universality. It turns out that these cases can be combined with the assumption of string unification, which leads to a prediction of $\sin^2 θ_W(M_Z)$ and $k_1$, the normalisation of the $U(1)_Y$ generator. String unification predicts $k_1=1.3 - 1.4$. These values have non-trivial implications on string model building. Two-loop corrections are also calculated. Some of these cases exhibit a large discrepancy between experiment and string unification. We calculate string threshold corrections to explain the discrepancy.

hep-ph↗

arXiv1994

Minimal String Unification and Constraint on Hidden Sector

We examine whether a minimal string model possessing the same massless spectra as the MSSM can be obtained from $Z_4$, $Z_6$ and $Z_8$ orbifold constructions. Using an anomaly cancellation condition of the target space duality symmetry, we derive allowable values of a level $k_1$ of U(1)$_Y$ for the minimal string model on the orbifolds through computer analyses. We investigate threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)$_Y$ and examine consistencies of the model with the LEP experiments. Further we investigate what kinds of hidden sectors are consistent with the minimal string models. Also their gauge coupling constants of the hidden groups are estimated. We discuss Yukawa couplings of the models.

hep-ph↗

arXiv1994

Neutron Electric Dipole Moment under Non-Universal Soft SUSY Breaking Terms

The electric dipole moment of the neutron (EDMN) is re-examined in a general framework of the soft supersymmetry breaking. We point out some features of the relation between the EDMN and non-universal soft supersymmetry breaking terms. We give the constraints on the soft scalar masses and the soft CP phases, which have the rather large dependence on the non-universality of the soft breaking terms. We also show that the soft CP phase $ϕ_B$ which has no natural suppression mechanism may not have large contribution to the EDMN in a certain parameter region where the radiative symmetry breaking occurs successfully. $ϕ_B$ may not need to be so small.

hep-ph↗

arXiv1994

CP in Orbifold Models

We study CP in orbifold models. It is found that the orbifolds always have some automorphisms as CP symmetry. The symmetries are restricted non-trivially due to geometrical structure of the orbifolds. Explicit analysis on Yukawa couplings also shows that CP is not violated in orbifold models.

hep-th↗

arXiv1994

Non-universal Soft Scalar Masses in Superstring Theories

We study soft scalar masses comparing with gaugino masses in 4-dimensional string models. In general non-universal soft masses are derived in orbifold models. We give conditions on modular weights to lead to the large non-universality in the soft scalar masses. This non-universality is applied to the unification of the gauge coupling constants in the minimal string model.

hep-ph↗

arXiv1994

Minimal String Unification and Yukawa Couplings in Orbifold Models

We study the minimal supersymmetric standard model derived from $Z_N \times Z_M$ orbifold models. Moduli dependent threshold corrections of the gauge couplings are investigated to explain the measured values of the coupling constants. Also we study Yukawa couplings of the models. We find that the $Z_2 \times Z_6'$, $Z_2\times Z_6$, $Z_3 \times Z_6$ and $Z_6 \times Z_6$ orbifold models have the possibility to derive Yukawa couplings for the second and third generations as well as the measured gauge coupling constants. Allowed models are shown explicitly by combinations of modular weights for the matter fields.

hep-ph↗

arXiv1994

Gauge Coupling Unification due to Non-universal Soft Supersymmetry Breaking

Gauge coupling unification is studied in the MSSM with non-universal soft supersymmetry breaking terms. If gaugino masses are sufficiently smaller than scalar soft masses and the scalar soft masses have also certain types of non-universality, gauge coupling unification scale can be larger than $3\times 10^{16}$~GeV even within the MSSM contents. String unification may not need a large threshold correction or a large modulus value. We also discuss the relation to the string model building.

hep-ph↗

arXiv1994

Minimal String Model in $Z_4$, $Z_6$ and $Z_8$ Orbifold Constructions

We examine whether a minimal string model possessing the same massless spectra as the MSSM can be obtained from $Z_4$, $Z_6$ and $Z_8$ orbifold constructions. Using an anomaly cancellation condition of the target space duality symmetry, we derive allowable values of a level $k_1$ of U(1)$_Y$ for the minimal string model on the orbifolds through computer analyses. We investigate threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)$_Y$ and examine consistencies of the model with the LEP experiments. It is found that $Z_4$ and $Z_8$-II can not derive the minimal string model but $Z_6$-I, $Z_6$-II and $Z_8$-I are possible to derive it with $13/12 \leq k_1\leq 41/30$, $16/15\leq k_1\leq 17/12$ and $1\leq k_1\leq 41/21$ respectively. The minimum values of the moduli on unrotated planes are estimated within the ranges of the levels.

hep-th↗

arXiv1993

Levels of U(1)y in Minimal String Model on Z(N)*Z(M) Orbifolds

We study a minimal string model possessing the same massless spectra as the MSSM on $Z_N\times Z_M$ orbifolds. Threshold corrections of the gauge coupling constants of SU(3), SU(2) and U(1)$_Y$ are investigated in a case of an overall modulus. Using computer analyses, we search ranges of levels of U(1)$_Y$ allowed by the LEP experiments. It is found that $Z_3\times Z_3$ can not derive the minimal string model for a $M_Z$ SUSY breaking scale. The minimum values of the overall moduli are estimated within the ranges of the levels.

hep-th↗

arXiv1993

Minimal String Unification and Hidden Sector in Z_8 Orbifold Models

We study the minimal supersymmetric standard model derived from the $Z_8$ orbifold models and its hidden sectors. We use a target-space duality anomaly cancellation so as to investigate hidden sectors consistent with the MSSM unification. For the allowed hidden sectors, we estimate the running gauge coupling constants making use of threshold corrections due to the higher massive modes. The calculation is important from the viewpoint of gaugino condensations, which is one of the most promissing mechanism to break the supersymmetry.

hep-th↗

arXiv1993

Study of Minimal String Unification in $Z_8$ Orbifold Models

We study the construction of the minimal supersymmetric standard model from the $Z_8$ orbifold models. We use a target-space duality anomaly cancellation and a unification of gauge couplings as constraints. It is shown that some models obtained through a systematical search realize the unification of SU(3) and SU(2) coupling constants.

hep-th↗