arXiv subjects

Y. Yamada

Publications and source records attributed to Y. Yamada.

At least 55 records · Page 3Linked to original sources

arXiv2003

Scale- and gauge-independent mixing angles for scalar particles

The existing definitions of mixing angles (one-loop radiatively corrected and renormalization-scale-independent) for scalar particles turn out to be gauge-dependent when used in gauge theories. We show that a scale- and gauge-independent mixing angle can be obtained if the scalar self-energy is improved by the pinch technique, and give two relevant examples in the Minimal Supersymmetric Standard Model: the mixing of CP-even Higgs scalars and of top squarks. We also show that the recently proposed definition of mixing angle that uses the (unpinched) scalar two-point function evaluated at a particular value of the external momentum [p_*^2=(M_1^2+M_2^2)/2, where M_{1,2} are the masses of the mixed particles] computed in Feynman gauge coincides with the gauge-invariant pinched result. In alternative definitions (e.g. in the on-shell scheme), the improved Higgs mixing angle is different from that in Feynman gauge. An arbitrariness of the pinch technique for scalar particles is also discussed.

hep-ph↗

arXiv2002

Magneto-Optical Spectroscopy of Anatase TiO2 Doped with Co

Magneto-optical spectroscopy of a transparent ferromagnetic semiconductor, anatase TiO2 doped with Co, is carried out at room temperature. A large magneto-optical response with ferromagnetic field dependence is observed throughout from ultraviolet to visible range and increases with increasing Co content or carrier concentration. The magnitude of magnetic circular dichroism (MCD) per unit thickness has a peak around the absorption edge such a huge value of ~10400 degree/cm at 3.57 eV for a 10 mol% Co-doped specimen. Although the results are not sufficient to prove that the ferromagnetism is in the ordinary framework of diluted magnetic semiconductors, the coexistence of Co impurity and mobile carrier is shown to transform the band structure of host TiO2 to generate ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2002

Spin dynamics and antiferromagnetic order in PrBa2Cu4O8 studied by Cu nuclear respnance

Results of the nuclear resonance experiments for the planar Cu sites in PrBa2Cu4O8 are presented. The NMR spectrum at 1.5 K in zero magnetic field revealed an internal field of 6.1 T, providing evidence for an antiferromagnetic order of the planar Cu spins. This confirms that the CuO2 planes are insulating, therefore, the metallic conduction in this material is entirely due to the one-dimensional zigzag Cu2O2 chains. The results of the spin-lattice relaxation rates measured by zero field NQR above 245 K in the paramagnetic state are explained by the theory for a Heisenberg model on a square lattice.

cond-mat.str-el↗

arXiv2002

Static and dynamical dipolar / strain fluctuations in perovskite ferroelectric relaxors

We develop a theory to study the characteristics of dipolar / strain fluctuations in perovskite relaxors. In addition to the soft TO and TA phonons, we take into account another freedom of motion associated with the random hopping of $\mathrm{Pb^{2+}}$ ions between the off-center sites around the high symmetry corner site, thus constructing a coupled TO-TA-pseudospin model to describe the perovskite relaxors. It is shown that there is a possibility that prior to the on-set of instability of the uniform TO-mode (soft-mode), instability of TA mode with nanometer scale modulation $(\mathbf{q}\sim 0.1\mathbf{a}^{*})$ takes place, which produces static heterogeneous structure concerning polarization as well as shear strain. This seems to suggest the intrinsic origin of the heterogeneity in relaxors visualized as random distribution of PNR. The phonon spectral density distribution has also been investigated. It is shown that when the relaxation time of random hopping of $\mathrm{Pb^{2+}}$ ions is comparable to the TO phonon frequency at $\mathbf{q}\sim 0.1\mathbf{a}^{*}$, the calculated phonon spectral density reproduces the characteristic features called `waterfall'.

cond-mat.mtrl-sci↗

arXiv2001

One-loop corrections to neutral Higgs boson decays into neutralinos

We present the one-loop corrected decay widths for the decays of the neutral Higgs bosons h^0, H^0, and A^0 into a neutralino pair neu_m neu_n (m, n = 1, >..., 4) and to the decay neu_m -> (h^0, H^0, A^0) + neu_n. The corrections contain the one-loop contributions of all fermions and sfermions. All parameters are taken on-shell. This requires a proper treatment of the neutralino mass and mixing matrix. The dependence on the SUSY parameters is discussed. The corrections can be large in certain regions of the parameter space.

hep-ph↗

arXiv2001

Difference L operators related to q-characters

We introduce a factorized difference operator L(u) annihilated by the Frenkel-Reshetikhin screening operator for the quantum affine algebra U_q(C^{(1)}_n). We identify the coefficients of L(u) with the fundamental q-characters, and establish a number of formulas for their higher analogues. They include Jacobi-Trudi and Weyl type formulas, canceling tableau sums, Casorati determinant solution to the T-system, and so forth. Analogous operators for the orthogonal series U_q(B^{(1)}_n) and U_q(D^{(1)}_n) are also presented.

math.QA↗

arXiv2001

One-loop corrections to the chargino and neutralino mass matrices in the on-shell scheme

We present a consistent procedure for the calculation of the one-loop corrections to the charginos and neutralinos by using their on-shell mass matrices. The on-shell gaugino mass parameters M and M', and the Higgsino mass parameter μare defined by the elements of these on-shell mass matrices. The on-shell mass matrices are different by finite one-loop corrections from the tree-level ones given in terms of the on-shell parameters. When the on-shell M and μare determined by the chargino sector, the neutralino masses receive corrections up to 4%. This must be taken into account in precision measurements at future e^+ e^- linear colliders.

hep-ph↗

arXiv2001

Universal Phase Diagram for High-Piezoelectric Perovskite Systems

Strong piezoelectricity in the perovskite-type PbZr(1-x)TixO3 (PZT) and Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) systems is generally associated with the existence of a morphotropic phase boundary (MPB) separating regions with rhombohedral and tetragonal symmetry. An x-ray study of PZN-9%PT has revealed the presence of a new orthorhombic phase at the MPB, and a near-vertical boundary between the rhombohedral and orthorhombic phases, similar to that found for PZT between the rhombohedral and monoclinic phases. We discuss the results in the light of a recent theoretical paper by Vanderbilt and Cohen, which attributes these low-symmetry phases to the high anharmonicity in these oxide systems.

cond-mat.mtrl-sci↗

arXiv2001

Scattering rules in soliton cellular automata associated with crystal bases

Solvable vertex models in a ferromagnetic regime give rise to soliton cellular automata at q=0. By means of the crystal base theory, we study a class of such automata associated with the quantum affine algebra U_q(g_n) for non exceptional series g_n = A^{(2)}_{2n-1}, A^{(2)}_{2n}, B^{(1)}_n, C^{(1)}_n, D^{(1)}_n and D^{(2)}_{n+1}. They possess a commuting family of time evolutions and solitons labeled by crystals of the smaller algebra U_q(g_{n-1}). Two-soliton scattering rule is identified with the combinatorial R of U_q(g_{n-1})-crystals, and the multi-soliton scattering is shown to factorize into the two-body ones.

math.QA↗

arXiv2000

Improved SUSY QCD corrections to Higgs boson decays into quarks and squarks

The O(alpha_s) SUSY QCD corrections to the decays of the MSSM Higgs bosons into bottom quarks and squarks can be very large for large tan beta in the on--shell renormalization scheme. We improve the calculation by a careful choice of the tree-level Higgs boson couplings in terms of running parameters of quarks and squarks.

hep-ph↗

arXiv2000

Improved SUSY QCD corrections to Higgs boson decays into quarks and squarks

We improve the calculation of the supersymmetric $O(α_s)$ QCD corrections to the decays of Higgs bosons into quarks and squarks in the Minimal Supersymmetric Standard Model. In the on-shell renormalization scheme these corrections can be very large, which makes the perturbative expansion unreliable. This is especially serious for decays into bottom quarks and squarks for large $\tanβ$. Their corrected widths can even become negative. We show that this problem can be solved by a careful choice of the tree-level Higgs boson couplings to quarks and squarks, in terms of the QCD and SUSY QCD running quark masses, running trilinear couplings $A_q$, and on-shell left-right mixing angles of squarks. We also present numerical results for the corrected partial decay widths for the large $\tanβ$ case.

hep-ph↗

arXiv1999

Muon's Behaviors under Bremsstrahlung with both the LPM effect and the Ter-Mikaelian effect and Direct Pair Production with the LPM effect

Differential and integral cross sections of the muon are calculated in the materials: water, standard rock, iron and lead with and without the LPM effect. The corresponding cross sections are also calculated with dielectric supression effect (Ter-Mikaelian effect), in addition to the LPM effect. To demonstrate the importance of the LPM effect in the bremsstrahlung process, depth intensity relation of muon, energy spectrum, range distribution and survival probability are calculated taking into account bremsstrahlung spectrum.

hep-ph↗

arXiv1999

Impact of bosonic decays on the search for stau_2 and tau-sneutrino

We perform a detailed study of the decays of the heavier tau slepton (stau_2) and tau-sneutrino (snu_tau) in the Minimal Supersymmetric Standard Model (MSSM). We show that the decays into Higgs or gauge bosons, i.e. stau_2 -> stau_1 + (h^0, H^0, A^0 or Z^0), stau_2 -> snu_tau + (H^- or W^-), and snu_tau -> stau_1 + (H^+ or W^+), can be very important due to the sizable tau Yukawa coupling and large mixing parameters of stau. Compared to the decays into fermions, such as stau_2 -> tau + neutralino_i and stau_2 -> nu_tau + chargino_j^-, these bosonic decay modes can have significantly different decay distributions. This could have an important influence on the search for stau_2 and snu_tau and the determination of the MSSM parameters at future colliders.

hep-ph↗

arXiv1998

SUSY-QCD corrections to stop and sbottom decays into Higgs bosons

We calculate the order(α_s) SUSY-QCD corrections to the widths of stop and sbottom decays into Higgs bosons within the Minimal Supersymmetric Standard Model. We give the complete analytical formulae paying particular attention to the on-shell renormalization of the soft SUSY-breaking parameters. We also perform a detailed numerical analysis of both stop and sbottom decays into all Higgs bosons h^0, H^0, A^0, and H^\pm. We find that the SUSY-QCD corrections are significant, mostly negative and of the order of a few ten percent.

hep-ph↗

arXiv1998

Bosonic decays of stop_2 and sbottom_2

We perform a detailed study of the decays of the heavier top and bottom squarks (stop_2 and sbottom_2) in the Minimal Supersymmetric Standard Model (MSSM). We show that the decays into Higgs or gauge bosons can be dominant in a wide range of the model parameters due to the large Yukawa couplings and mixings of stops and sbottoms. Compared to the conventional decays into fermions these bosonic decay modes can have significantly different decay structures and distributions. This could have an important impact on the search for stop_2 and sbottom_2 and the determination of the MSSM parameters at future colliders.

hep-ph↗

arXiv1997

SUSY--QCD Corrections to Higgs Particle Decays into Quarks and Squarks

We study the decays of the Higgs bosons $H^{\pm}$, $H^0$, and $A^0$ within the Minimal Supersymmetric Standard Model. For decays into quarks and squarks we include the supersymmetric QCD radiative corrections. We find that the corrections are significant and can go up to 50%. The supersymmetric decay modes $H^+ \to \tilde t \bar{\tilde b}$ and $H^0 (A^0) \to \tilde t \bar{\tilde t}$ can be dominant in a wide range of the model parameters due to the large Yukawa couplings and mixings of $\tilde t$ and $\tilde b$.

hep-ph↗

arXiv1997

SUSY-QCD corrections to stop and sbottom decays into W and Z bosons

We calculate the supersymmetric ${\cal O}(α_s)$ QCD corrections to stop and sbottom decays into vector bosons within the Minimal Supersymmetric Standard Model. We give analytic formulae and perform a numerical analysis of these decays. We find that SUSY-QCD corrections to the decay widths are typically -5% to -10% depending on the squark masses, squark mixing angles, and the gluino mass.

hep-ph↗

arXiv1997

Higgs Particle Decays in Supersymmetry

We present a detailed study of the decays of the Higgs bosons $H^+$, $H^0$, and $A^0$ within the Minimal Supersymmetric Standard Model including SUSY--QCD corrections. We find that the supersymmetric modes $\tilde t \bar{\tilde b}$ ($\tilde t \bar{\tilde t}$, and for large $\tanβ$ $\tilde b \bar{\tilde b}$) can dominate the $H^+$ ($H^0$, $A^0$) decays in a wide range of the model parameters due to the large Yukawa couplings and mixings of $\tilde t$ and $\tilde b$. Compared to the conventional modes $H^+ \to τ^+ ν_τ, t \bar b$, and $H^0, A^0 \to t \bar t, b \bar b$, the supersymmetric modes can have an important impact on the Higgs boson searches at future colliders.

hep-ph↗