arXiv subjects

Makiko Nagashima

Publications and source records attributed to Makiko Nagashima.

29 records · Page 2Linked to original sources

arXiv2004

B->J/psi K* in a Supersymmetric Right-handed Flavor Mixing Scenario

A supersymmetric extension of the Standard Model, with maximal s_R-b_R mixing and a new source of CP violation, contains all the necessary ingredients to account for a possible anomaly in the measured CP asymmetry in B->phi K_s decay. In the same framework we study the decay B->J/psi K*, paying particular attention to observables that can be extracted by performing a time dependent angular analysis, and become nonzero because of new physics effects.

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arXiv2004

Resolving the B-> phi K^* Polarization Anomaly

The experimental observation of sizable transverse components for B-> phi K^* decay is in strong contrast to all other VV modes, and poses a challenge to our understanding of B decay dynamics. Observing that the gluon emitted from b-> sg^{(*)} chromodipole transition is transverse, we give a heuristic model where the transverse phi descends from the emitted gluon, hence similar phenomena should occur for B-> omega K^* but not for B-> rho^0 K^*. New physics in bsg chromodipole coupling, perhaps needed for the \bar B -> phi K_S CP violation anomaly, may lead to different patterns of CP and T violation in transverse components of \bar B -> phi \bar K^*, omega \bar K^* decays.

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arXiv2004

Two-parton twist-3 factorization in perturbative QCD

We prove collinear factorization theorem for the process $πγ^*\toπ$ at the twist-3 level in the covariant gauge by means of the Ward identity, concentrating on the two-parton case. It is shown that soft divergences cancel and collinear divergences are grouped into the pseudo-scalar and pseudo-tensor two-parton twist-3 pion distribution amplitudes. The delicate summation of a complete set of diagrams for achieving factorization in momentum, spin, and color spaces is emphasized. The proof is then extended to the exclusive semileptonic decay $B\toπl\barν$, assuming the hard scale to be of $O(\sqrt{\barΛM_B})$, where $\barΛ$ is a hadronic scale and $M_B$ the $B$ meson mass. We explain the distinction between the factorization of collinear divergences for a pion distribution amplitude and of soft divergences for a $B$ meson distribution amplitude. The gauge invariance and universality of the two-parton twist-3 pion distribution amplitudes are confirmed. The proof presented here can accommodate the leading twist-2 case. We then compare our proof with that performed in the framework of soft-collinear effective theory.

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arXiv2004

Effect of Supersymmetric Right-handed Flavor Mixing on $B_s$ decays

Motivated by the possibility of ${\cal S}_{ϕK_S}<0$, we study the implications for $B_s$ meson system. In a specific model that realizes ${\cal S}_{ϕK_S}<0$ with large $s$-$b$ mixing, right-handed dynamics and a new CP phase, we present predictions for CP asymmetries in $B_s\to J/ψϕ$, $K^+K^-$ and $ϕγ$ decays. Even if the measurement of time-dependent CP asymmetry becomes hampered by very fast $B_s$ oscillation, a finite difference between the decay rates of $B_s$ mass eigenstates may enable the studies of CP violations with untagged data samples. Thus, studies of CP violation in the $B_s$ system would remain useful for the extraction of new physics information.

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arXiv2003

$\bar B^0\to ϕK_S$ and $\bar K^{*0}γ$ CP Asymmetries from Supersymmetric Right-handed Flavor Mixing: Implications for Heavy Quark Phenomenology

Two recent experimental developments, when combined, may have far reaching implications. $S_{ϕK_S} < 0$, if confirmed, would imply large $s$-$b$ mixing, a new CP phase, and right-handed dynamics. Large $Δm_{B_s}$ would be likely, making the $B_s$ program at hadron machines difficult. Reconstruction of $B$ vertex from $K_S$ at B factories, as shown by BaBar's first measurement of $S_{K_Sπ^0}$, makes $S_{K_Sπ^0γ}$ in $\bar B\to \bar K^{*0}γ$ accessible. This would be a boon for B factory upgrades. Supersymmetric Abelian flavor symmetry, independently motivated, can realize all of this with a light $\widetilde{sb}_1$ squark. B factory and collider studies of flavor, CP and SUSY may not be what we had expected.

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arXiv2002

$k_T$ factorization of exclusive processes

We prove $k_T$ factorization theorem in perturbative QCD (PQCD) for exclusive processes by considering $πγ^*\to γ(π)$ and $B\toγ(π) l\barν$. The relevant form factors are expressed as the convolution of hard amplitudes with two-parton meson wave functions in the impact parameter $b$ space, $b$ being conjugate to the parton transverse momenta $k_T$. The point is that on-shell valence partons carry longitudinal momenta initially, and acquire $k_T$ through collinear gluon exchanges. The $b$-dependent two-parton wave functions with an appropriate path for the Wilson links are gauge-invariant. The hard amplitudes, defined as the difference between the parton-level diagrams of on-shell external particles and their collinear approximation, are also gauge-invariant. We compare the predictions for two-body nonleptonic $B$ meson decays derived from $k_T$ factorization (the PQCD approach) and from collinear factorization (the QCD factorization approach).

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arXiv2001

Mixing Matrix of Quarks Having Natural Twisting in E_6 Grand Unified Model

Mass matrix of quarks is studied in the Supersymmetric $E_6$ Grand Unified Theory (GUT). The fundamental representation $\textbf{27}$ in $E_6$ which corresponds to one generation contains two sets of $\textbf{5}^{*}$'s in SU(5), so that there are six flavors of lepton doublets and right-handed down-quark triplets. It is known that the twisting (interchange) among the $\textbf{5}^{*}$ representations may reproduce the observed quark and lepton mixing matrices. If $E_6$ is directly broken down to $SU(3)_C \times SU(2)_L \times U(1)_Y$ and two extra U(1)'s, the extra 3 sets of down-type quarks do not necessarily decouple from the quark mass matrix, under the appropriate choice of the U(1) flavor charges on the quark and Higgs fields. Then, by diagonalizing the 6 $\times$ 6 down-quark mass matrix, we find a certain set of parameters, in which the twisting between a pair of the right-handed down-quarks occurs naturally, reproducing the reasonable values for the $d$-, $s$- and $b$-quark masses, and for $V_{ud}$, $V_{us}$, $V_{cd}$, $V_{cs}$ and $V_{tb}$ of the CKM matrix elements. As a by-product, one vector-like down-quark appears at the experimentally accessible TeV scale.

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arXiv2001

Large effects on $\BsBs$ mixing by vector-like quarks

We calculate the contributions of the vector-like quark model to $\BsBs$ mixing, taking into account the constraints from the decay $B\to X_sγ$. In this model the neutral bosons mediate flavor-changing interactions at the tree level. However, $\BsBs$ mixing is dominated by contributions from the box diagrams with the top quark and the extra up-type quark. In sizable ranges of the model parameters, the mixing parameter $x_s$ is much different from the standard model prediction.

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arXiv2001

Inclusive dileptonic rare B decays with an extra generation of vector-like quarks

We investigate the leading effects of extending the Standard Model of electroweak interactions by an extra iso-singlet up- and down- type quark pair on various distributions and total branching ratio of the inclusive B-> X_s l^+ l^- (l =e,μ) rare B decays. The presence of the extra vector-like down quark $D$ results in the non-unitarity of the extended quark mixing matrix V, which in turn leads to b-> s FCNC at the tree level proportional to (V^\dagger V)_{sb}. On the other hand, the effective penguin and box vertex functions are sensitive to the mass of the extra iso-singlet up quark m_U. The experimental upper bound on BR(B-> X_s μ^+ μ^-) is used to constrain the parameters of the model. It is shown that the shapes of the differential branching ratio and forward-backward asymmetry distribution are very sensitive to the value of the model parameters. We also calculate the CP aymmetry distribution of the dileptonic decay in the vector-like quark model. It is shown that, for a typical choice of the model parameters, asymmetries up to around 10% can be achieved for certain values of the dilepton invariant mass.

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arXiv2001

Contributions of Vector-Like Quarks to $B \to X_s γ$ and $B_s^0 \bar B_s^0$ Mixing

The radiative decay $\BSgamma$ and the mixing $\BsBs$ are discussed in the vector-like quark model which contains extra SU(2)-singlet quarks with the same electric charges as the up-type and down-type quarks. Constraints on the extended Cabibbo-Kobayashi-Maskawa matrix are obtained from the experimental results for the branching ratio of $\BSgamma$ . Within these experimental bounds, any value of the branching ratio can be accounted for by the contributions from the vector-like quarks. In sizable ranges of the model parameters, the mixing parameter for $\BsBs$ is much different from the prediction of the standard model.

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arXiv2000

Contributions of vector-like quarks to radiative B meson decay

We study the decay B\to X_sγin a minimal extension of the standard model with extra up- and down-type quarks whose left- and right-handed components are both SU(2) singlets. Constraints on the extended Cabibbo-Kobayashi-Maskawa matrix are obtained from the experimental results for the branching ratio. Even if the extra quarks are too heavy to be detected in near-future colliders, the branching ratio could have a value which is non-trivially different from the prediction of the standard model.

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