arXiv subjects

Toru Goto

Publications and source records attributed to Toru Goto.

26 records · Page 2Linked to original sources

arXiv1999

Effect of an RRRR dimension 5 operator on the proton decay in the minimal SU(5) SUGRA GUT model

We reanalyze the proton decay in the minimal SU(5) SUGRA GUT model. Unlike previous analyses, we take into account a Higgsino dressing diagram of dimension 5 operator with right-handed matter fields ($RRRR$ operator). It is shown that this diagram gives a dominant contribution for $p\to K^+\barν_τ$ over that from $LLLL$ operator, and decay rate of this mode can be comparable with that of $p\to K^+\barν_μ$ which is dominated by the $LLLL$ contribution. It is found that we cannot reduce both the decay rate of $p\to K^+\barν_τ$ and that of $p\to K^+\barν_μ$ simultaneously by adjusting relative phases between Yukawa couplings at colored Higgs interactions. Constraints on the colored Higgs mass $M_C$ and a typical squark and slepton mass $m_{\tilde{f}}$ from Super-Kamiokande limit become considerably stronger due to the Higgsino dressing diagram of the $RRRR$ operator: $M_C > 6.5 \times 10^{16}\gev$ for $m_{\tilde{f}} < 1 \tev$, and $m_{\tilde{f}} > 2.5 \tev$ for $M_C < 2.5 \times 10^{16} \gev$.

hep-ph↗

arXiv1998

Effect of supersymmetric CP phases on the $B \to X_s γ$ and $B \to X_s l^+ l^-$ decays in the minimal supergravity model

We investigate the effect of supersymmetric CP violating phases on the $B\to X_s γ$ and $B\to X_s l^+l^-$ decays in the minimal supergravity model. We show that the phase of the trilinear scalar coupling constant for top squarks is strongly suppressed and aligned to that of the gaugino mass due to a renormalization effect from the Planck scale to the electroweak scale. As a result, the effect of supersymmetric CP violating phases on the $B \to X_s γ$ and $B \to X_s l^+ l^-$ decays are small taking into account the neutron and the electron electric dipole moment constraints. For the $B \to X_sγ$ decay, the amplitude has almost no new CP violating phase and the direct CP asymmetry is less than 2 %. For the $B \to X_s l^+ l^-$ decay, the branching ratio can be sizably different from that in the standard model only when the sign of the $B \to X_s γ$ amplitude is opposite to that in the standard model.

hep-ph↗

arXiv1998

Flavor changing neutral current processes in $B$ and $K$ decays in the supergravity model

Flavor changing neutral current processes such as $b \to s γ$, $b \to s l^+ l^-$, $b \to s ν\barν$, $ε_K$, $Δm_B$, $K^+ \to π^+ ν\barν$ and $\K_L \to π^0 ν\barν$ are calculated in the supersymmetric standard model based on supergravity. We consider two assumptions for the soft supersymmetry breaking terms. In the minimal case soft breaking terms for all scalar fields are taken to be universal at the GUT scale whereas those terms are different for the squark/slepton sector and the Higgs sector in the nonminimal case. In the calculation we have taken into account the next-to-leading order QCD correction to the $b \to s γ$ branching ratio, the results from the LEP II superparticles search, and the condition of the radiative electroweak symmetry breaking. We show that $Δm_B$ and $ε_K$ can be enhanced up to 40% compared to the Standard Model values in the nonminimal case. In the same parameter region the $b \to s ν\barν$, $K^+ \to π^+ ν\barν$ and $\K_L \to π^0 ν\barν$ branching ratios are reduced up to 10%. The corresponding deviation in the minimal case is 20% for $Δm_B$ and $ε_K$ and within 3% for the $b \to s ν\barν$, $K^+ \to π^+ ν\barν$ and $\K_L \to π^0 ν\barν$. For the $b \to s l^+ l^-$ process the significant deviation from the Standard Model is realized only when the $b \to s γ$ amplitude has an opposite sign to the Standard Model prediction. Significance on these results from possible future improvements of the $b \to s γ$ branching ratio measurement and top squark search is discussed.

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arXiv1996

$b \to s l \bar l $ in the minimal supergravity model

The $b \rightarrow s l^+ l^-$ process is studied in the minimal supergravity model in detail. Taking account of the long distance contributions from the $c\overline{c}$ resonances, we calculate the branching ratio and the lepton forward-backward asymmetry in this model. We find that there is a strong correlation between the branching ratios of $b \rightarrow s γ$ and $b \rightarrow s l^+ l^-$ processes and that the interference effect can change the $b \rightarrow s l^+ l^-$ branching ratio in the off-resonance regions by up to $\pm 15%$ depending on the relative phase between the long and short distance contributions. Using various phenomenological constraints including the branching ratio of $b \rightarrow s γ$, we show that there are regions in the parameter space where the branching ratio of $b \rightarrow s l^+ l^-$ is enhanced by about 50% compared to the SM. We also show that the branching ratio of $b \rightarrow s ν{\overline ν}$ is reduced at most by 10% from the SM prediction.

hep-ph↗

arXiv1996

Charged Higgs mass bound from the $b \to s γ$ process in the minimal supergravity

The constraint on the mass of the charged Higgs boson from the recent measurement of the inclusive $b \rightarrow s γ$ decay is studied in a framework of the minimal supergravity. It is shown that the lower bound for the charged Higgs mass crucially depends on the sign of the higgsino mass parameter ($μ$). For $μ<0$, the bound exceeds 400 GeV when the ratio of two Higgs vacuum expectation values ($\tanβ$) is larger than 10. No strong bound is obtained for $μ>0$ due to cancellations between charged Higgs and supersymmetric particle contributions.

hep-ph↗

arXiv1995

$B^0$--$\overline{B}^0$ mixing and $ε_K$ parameter in the minimal supergravity model

\bb mixing and a CP violation parameter in \kk mixing $ε_K$ are studied in the minimal supergravity model. We solve the one-loop renormalization group equations for the minimal supersymmetric standard model (MSSM) parameters numerically in order to determine the masses and mixings of the supersymmetric particles, while all off-diagonal (generation mixing) elements and phases of Yukawa coupling matrices and those of squark mass matrices are taken into account. Applying the radiative electroweak symmetry breaking condition and phenomenological constraints including the recent measurement of the \bsg inclusive branching ratio, we obtain the allowed parameter region. We have found that the present constraints still allow a parameter region where both \bb mass splitting $ΔM_B$ and $ε_K$ are $\sim 20$\% larger than the standard model values. By explicit numerical calculations, we have also found that the complex phase of \bb mixing matrix element in this model is almost the same as the standard model value in a good accuracy in the whole allowed parameter region. It is shown that $ΔM_B$ and $ε_K$ can put useful constraints on the supersymmetry's parameter space when the Cabibbo-Kobayashi-Maskawa matrix elements are determined through the measurements of CP violations in $B$ decay with future $B$-factories.

hep-ph↗

arXiv1994

Charged Higgs mass bound from the $b\rightarrow sγ$ process in the minimal supergravity model

We study the constraint on the mass of the charged Higgs boson in the minimal supergravity model based on the recent measurement of the inclusive $b\rightarrow sγ$ decay. It is shown that the lower bound for the charged Higgs mass crucially depends on the sign of the higgsino mass parameter ($μ$). For $μ<0$, the bound exceeds 400 GeV when the ratio of two Higgs vacuum expectation values ($\tanβ$) is larger than 10. No strong bound is obtained for $μ>0$ due to cancellations between charged Higgs and supersymmetric particle contributions. For $3\lsim\tanβ\lsim5$, a charged Higgs lighter than 180 GeV is excluded by this process irrespective of the sign of $μ$.

hep-ph↗

arXiv1994

Flavor mixing in the gluino coupling and the nucleon decay

Flavor mixing in the quark-squark-gluino coupling is studied for the minimal SU(5) SUGRA-GUT model and applied to evaluation of the nucleon lifetime. All off-diagonal (generation mixing) elements of Yukawa coupling matrices and of squark/slepton mass matrices are included in solving numerically one-loop renormalization group equations for MSSM parameters, and the parameter region consistent with the radiative electroweak symmetry breaking condition is searched. It is shown that the flavor mixing in the gluino coupling for a large $\tanβ$ is of the same order of magnitude as the corresponding Kobayashi-Maskawa matrix element in both up-type and down-type sector. There exist parameter regions where the nucleon decay amplitudes for charged lepton modes are dominated by the gluino dressing process, while for all the examined regions the neutrino mode amplitudes are dominated by the wino dressing over the gluino dressing.

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