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Xiaoyuan Li

Publications and source records attributed to Xiaoyuan Li.

27 records · Page 2Linked to original sources
arXiv2001

Photon-3-Pion And Pion-2-Photon Form Factors From Dynamical Constituent Quarks

We study the form factors of the low energy anomalous pion-2-photon and photon-3-pion processes in the nonlocal chiral quark model which incorporates the momentum dependence of the dynamical quark mass and realizes correctly the chiral symmetries. The obtained slope parameter for pion-2-photon is in reasonable agreement with the direct experimental results but smaller than the ones invoking vector meson dominance. Our result for the photon-3-pion form factor interpolates between the two extremes of theoretical approaches, with the largest one provided by the vector meson dominance and the smallest one by the Schwinger-Dyson approach. But all of them are well below the single data point available so far. This situation will hopefully be clarified by the experiments at CEBAF and CERN.

hep-ph↗
arXiv2001

Non-unitarity of CKM matrix from the vector singlet quark mixing and neutron electric dipole moment

In the standard model (SM) the lowest order contribution to the quark electric dipole moment (EDM) occurs at the three loop level. We show that the non-unitarity of the CKM matrix in models with an extended quark sector typically gives rise to a quark EDM at the two loop level which has no GIM-like suppression factors except the external quark mass. The induced neutron EDM is of order 10^{-29} e cm and can be well within the reach of the next generation of experiments if it is further enhanced by long distance physics as happens in the SM.

hep-ph↗
arXiv1999

Enhanced contribution to quark and neutron electric dipole moments with small mixing of right-handed currents and CKM CP violation

We study the light quark and the neutron electric dipole moments (EDMs) under the assumptions that the CP source is still in the usual CKM matrix and that there is a small mixing of right-handed charged currents in the quark sector. We find that the EDMs arise already at two loop order that are much larger than the standard model (SM) result even for a small mixing.

hep-ph↗
arXiv1999

Vanishing Contribution to Quark Electric Dipole Moment in the 2HD Model with CKM CP Violation

In the standard model (SM) of electroweak interactions, CP noninvariance arises from the nonzero phase in the CKM matrix. Its contribution to the quark electric dipole moment (EDM) vanishes surprisingly at two loop order. This makes the quark EDM extremely small in the SM. In this paper, we consider the two Higgs doublet extension of the SM and assume that CP noninvariance is still encoded in the CKM matrix. We calculate the charged Higgs boson contribution to the quark EDM which naively should be of order $eG_F^2\tildeδ(4π)^{-4}m_{u(d)}m_t^2m_b^2m_H^{-2}$ for the up (down) quark with possible enhancement factors of $\tan^2β$. Here $\tildeδ$ is the rephasing invariant of CP violation. However, contrary to the above naive expectation, we find that the charged Higgs boson contribution vanishes strictly at two loop order. We show explicitly how this comes about and explains how it is related to the general form of Yukawa couplings in a spontaneously broken gauge theory.

hep-ph↗
arXiv1996

$O(α^2 G_F m^2_t)$ Contributions to $H\toγγ$

The rare decay $H\to γγ$ is a promising detection channel for an intermediate mass Higgs boson. We compute its two-loop $O(α^2 G_F m_t^2)$ correction in the standard model and find that the relative correction to the decay rate runs between $0.7%$ and $0.5%$ for $M_H=80-150$ GeV. The analogous correction to the amplitude for $gg\to H$ is recovered as a special case. The generalization of our result to other models is also briefly indicated.

hep-ph↗
arXiv1994

Renormalization of the Electroweak Theory in the Background Field Method

The applicability of the background field method in spontaneously broken gauge theories is examined with new features emphasized. An explicit one loop analysis in the electroweak theory shows that the method can be consistently implemented in the on-shell renormalization scheme, and that the choice of the background gauge cannot be arbitrary and must be fixed in the Landau gauge if one calculates scattering amplitudes involving unphysical Goldstone bosons. Some possible applications are also briefly indicated.

hep-ph↗
arXiv1994

Proof of the Equivalence Theorem in the Chiral Lagrangian Formalism

A general proof of the equivalence theorem in electroweak theories with the symmetry breaking sector described by the chiral Lagrangian is given in the $R_ξ$ gauge by means of the Ward-Takahashi identities. The precise form of the theorem contains a modification factor $C_{mod}$ associated with each external Goldstone boson similar to that in the standard model. $C_{mod}$ is exactly unity in our previously proposed renormalization scheme, {\it Scheme-II}.

hep-ph↗
arXiv1992

Gauge Model of Generation Nonuniversality Reexamined

A gauge model of generation nonuniversality proposed in 1981 is reexamined in the light of present precision electroweak measurements. Two necessary features of this model are that the tau lifetime should be longer than predicted by the standard model and that the rho parameter measured at the Z peak should have an additional negative contribution. Both trends are noticeable in present data. A future decisive test of this model would be the discovery of new W and Z bosons with nearly degenerate masses of a few TeV.

hep-ph↗