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Chao-Hsi Chang

Publications and source records attributed to Chao-Hsi Chang.

At least 91 records · Page 5Linked to original sources
arXiv2002

The Spectrum of a Binding System for a Heavy Quark with an Anti-Sbottom or for a Sbottom and Anti-Sbottom Pair

Since long-lived light bottom squark (sbottom) and its anti-particle with a mass close to the bottom quark have not been excluded by experiments so far, we consider such a sbottom to combine with its anti-particle to form a color singlet meson-like bound state or to combine with a common anti-quark to form a fermion-like one, or accordingly their anti-particles to form an anti-particle bound system. Namely we calculate the low-lying spectrum of the systems based on QCD inspired potential model. To be as relativistic as possible, we start with the framework of Bethe-Salpeter (BS) equation even for non-relativistic binding systems. Finally, we obtain the requested spectrum by constructing general forms of the BS wave functions and solving the BS equations under instantaneous approximation.

hep-ph↗
arXiv2002

The meson $B_c$ annihilation to leptons and inclusive light hadrons

The annihilation of the $B_c$ meson to leptons and inclusive light hadrons is analyzed in the framework of nonrelativistic QCD (NRQCD) factorization. We find that the decay mode, which escapes from the helicity suppression, contributes a sizable fraction width. According to the analysis, the branching ratio due to the contribution from the color-singlet component of the meson $B_c$ can be of order (10^{-2}). We also estimate the contributions from the color-octet components. With the velocity scaling rule of NRQCD, we find that the color-octet contributions are sizable too, especially, in certain phase space of the annihilation they are greater than (or comparative to) the color-singlet component. A few observables relevant to the spectrum of charged lepton are suggested, that may be used as measurements on the color-octet and color-singlet components in the future $B_c$ experiments. A typical long distance contribution in the annihilation is estimated too.

hep-ph↗
arXiv2002

Transverse $τ$ polarization in $B_c \to τ\bar ν_τ γ$ decay

The possible transverse polarization of the $τ$-lepton, which is $T$-odd and can be used to measure CP-violation, is estimated precisely in the radiative decay mode $B_c \to τ\bar ν_τγ$ with possible parameters for the multi Higgs doublet model and the R-parity violating supersymmetric model. We find that the up-bound of the transverse polarization with possible parameters in the models is at very different levels i.e. it can be $\leq 15\sim 23%$ for the former but $\leq 0.1 %$ for the latter, and it is accessible experimentally soon.

hep-ph↗
arXiv2001

Decays of the Meson $B_c$ to a $P$-Wave Charmonium State $χ_c$ or $h_c$

The semileptonic decays, $B_{c}{\longrightarrow}{χ_c}(h_c)+{\ell}+{ν}_{\ell}$, and the two-body nonleptonic decays, $B_{c}{\longrightarrow}{χ_c}(h_c)+h$, (here $χ_c$ and $h_c$ denote $(c\bar c[^3P_J])$ and $(c\bar c[^1P_1])$ respectively, and $h$ indicates a meson) were computed. All of the form factors appearing in the relevant weak-current matrix elements with $B_c$ as its initial state and a $P$-wave charmonium state as its final state for the decays were precisely formulated in terms of two independent overlapping-integrations of the wave-functions of $B_c$ and the $P$-wave charmonium and with proper kinematics factors being `accompanied'. We found that the decays are quite sizable, so they may be accessible in Run-II at Tevatron and in the foreseen future at LHC, particularly, when BTeV and LHCB, the special detectors for B-physics, are borne in mind. In addition, we also pointed out that the decays $B_c\to h_c+...$ may potentially be used as a fresh window to look for the $h_c$ charmonium state, and the cascade decays, $B_c\to χ_c[^3P_{1,2}]+l+ν_l$ ($B_c\to χ_c[^3P_{1,2}]+h$) with one of the radiative decays $χ_c[^3P_{1,2}] \to J/ψ+γ$ being followed accordingly, may affect the observations of $B_c$ meson through the decays $B_{c}\to {J/ψ}+{l}+ν_{l}$ ($B_c\to J/ψ+h$) substantially.

hep-ph↗
arXiv2001

Study of the $B_c$-meson lifetime

We in terms of optical theorem estimate the lifetime of $B_c-$meson with the parameters which are deturmined by fitting the data for the lifetimes and inclusive semilepton-decays of various $B$ and $D$ mesons. In the estimate, we find that the bound-state effects are important, and take them into account carefully in the framework which attributes the effects to the effective masses of the decay heavy quarks in the inclusive processes. We also find that to $B_c$ lifetime the penguin contribution is enhanced due to possible interference between the penguin and the `tree part' $c_1O_1+c_2O_2$.

hep-ph↗
arXiv2001

$V_{cs}$ from Pure Leptonic Decays of $D_s$ with Radiative Corrections

The radiative corrections to the pure leptonic decay $D_s{\longrightarrow} {\ell}{ν}_{\ell}$ up-to one-loop order is presented. We find the virtual photon loop corrections to $D_s{\longrightarrow} τ{ν}_τ$ is negative and the corresponding branching ratio is larger than $3.51\times 10^{-3}$. Considering the possible experimental resolutions, our prediction of the radiative decay $D_s{\longrightarrow} τ{ν}_τγ$ is not so large as others, and the best radiative channel to determine the $V_{cs}$ or $f_{D_s}$ is $D_s{\longrightarrow} μ{ν}_μγ$.

hep-ph↗
arXiv2001

The Pure Leptonic Decays of $D_s$ Meson and Their Radiative Corrections

The radiative corrections to the pure leptonic decay $D_s{\longrightarrow} {\ell}{ν}_{\ell}$ up-to one-loop order is presented. We find the virtual photon loop corrections to $D_s{\longrightarrow} τ{ν}_τ$ is negative and the corresponding branching ratio is larger than $3.51\times 10^{-3}$. Considering the possible experimental resolutions, our prediction of the radiative decay $D_s{\longrightarrow} τ{ν}_τγ$ is not so large as others, and the best channel to determine the $V_{cs}$ or $f_{D_s}$ is $D_s{\longrightarrow} μ{ν}_μγ$. How to cancel the infrared divergences appearing in the loop calculations, and the radiative decay $D_s{\longrightarrow} {\ell}{ν}_{\ell}γ$ is shown precisely. It is emphasized that the radiative decay may be separated properly and may compare with measurements directly as long as the theoretical `softness' of the photon corresponds to the experimental resolutions.

hep-ph↗
arXiv2001

Semileptonic Decays of $B_c$ Meson to a P-Wave Charmonium State $χ_c$ or $h_c$

The semileptonic decays of meson $B_c$ to a P-wave charmonium state $χ_c(^3P_J)$ or $h_c(^1P_1)$ are computed. The results show that the decays are sizable so they are accessible in Tevatron and in LHC, especially, with the detectors LHCB and BTeV in the foreseeable future, and of them, the one to the $^1P_1$ charmonium state potentially offers us a novel window to see the unconfirmed $h_c$ particle. In addition, it is pointed out that since the two charmonium radiative decays $χ_c(^3P_{1,2}) \to J/ψ+γ$ have sizable branching ratios, the cascade decays of the concerned decays and the charmonium radiative decays may affect the result of the observing the $B_c$ meson through the semileptonic decays $B_{c}\to {J/ψ}+{l}+ν_{l}$ substantially.

hep-ph↗
arXiv2000

The lifetime of B_c-meson and some relevant problems

The lifetime of the B_c-meson is estimated with consistent considerations on all of the heavy mesons ($B^0, B^\pm, B_s, D^0, D^\pm D_s$) and the double heavy meson B_c. In the estimate, the framework, where the non-spectator effects for nonleptonic decays are taken into account properly, is adopted, and the parameters needed to be fixed are treated carefully and determined by fitting the available data. The bound-state effects in it are also considered. We find that in decays of the meson B_c, the QCD correction terms of the penguin diagrams and the main component terms c_1O_1, c_2O_2 of the effective interaction Lagrangian have direct interference that causes an enhancement about 3 ~ 4% in the total width of the B_c meson.

hep-ph↗
arXiv2000

Probing flavor changing interactions in hadron collisions

The subprocess $gg \to t\bar{c}+\bar{t}c$ in the two-Higgs-doublet model with flavor-changing scalar couplings is examined at the one loop level. With perturbative QCD factorization theorem, the corresponding cross sections for hadron-hadron collisions are computed numerically. The results are applicable to the whole mass range of the weakly coupled Higgs bosons. In case we could efficiently exclude the severe backgrounds of the $t\bar{c}(\bar{t}c)$ production signal, probing the flavor-changing top-charm-scalar vertex at hadron colliders would be very promising and accessible experimentally.

hep-ph↗
arXiv1999

Further Study On U(1) Gauge Invariance Restoration

To further investigate the applicability of the projection scheme for eliminating the unphysical divergence $s/m_e^2$ due to U(1) gauge invariance violation, we study the process $e^-+W^+\to e^-+\bar t+b$ which possesses advantages of simplicity and clearness. Our study indicates that the projection scheme can indeed eliminate the unphysical divergence $s/m_e^2$ caused by the U(1) gauge invariance violation and the scheme can apply to very high energy region.

hep-ph↗
arXiv1998

Radiative Leptonic Decays of $B_c $ Meson

To the leading order, the radiative leptonic decays $B_c \to γ\ell \bar ν$ ($ \ell=e, μ$) are studied carefully. In the study, a non-relativistic constituent quark model and the effective Lagrangian for the heavy flavour decays are used. As a result, the branching ratios turn out to be of the orders of $10^{-5}$ for $B_c \to γμ\bar ν$ or for $B_c \to γe \bar ν$. Based on this study, we point out the decays being accessible experimentally at the future LHC, and the possibility to determine the decay constant $f_{B_c}$ through the radiative decays.

hep-ph↗
arXiv1997

J/ψElectromagnetic Production at e^+e^- colliders

Within the framework of QED we investigate the inclusive J/ψproduction at $e^+e^-$ colliders. It is expected to be as a further test to the charmonium production mechanisms and QED. We find that at the energies of CESR, BEPC, and TRISTAN (e.g. 4.0 GeV \leq \sqrt s \leq 60 GeV), the contributions of the concerned electromagnetic processes to the $J/ψ$ inclusive production are great, even dominant, thus they greatly affect the observation of the color-octet $J/ψ$ production signature at $e^+e^-$ colliders. The production of ψ', being similar to that of J/ψ's, is roughly estimated, and its influence on the observation of the color-octet J/ψproduction signature in e^+e^- collision is also discussed.

hep-ph↗
arXiv1997

New Types of Accessible Environmental Influence on Neutrino Oscillation

To describe possible effects of medium, besides the hamiltonian introduced by Mikheev, Smirnov and Wolfenstein (MSW), in principle, there may exist new terms in the evolution equation for the neutrino oscillation due to the environment. Considering a two-energy-level quantum system: $ν_1-ν_2$, two-species neutrino system which is embedded in a matter environment, we solve the neutrino evolution equation precisely with the possible extra terms, especially, those cause alternations between pure and mixed states of the neutrino quantum system. We obtain some remarkable features for the neutrino oscillations, which are different from that of MSW and accessible to be observed in solar neutrino observation and/or in the planned long-baseline neutrino experiments.

hep-ph↗
arXiv1997

The J/ψProduction Associated with a Hard Photon and the Color-Octet Singnature in e^+e^- Annihilation

J/ψproduction associated with a hard photon in e^+e^- annihilation, being of QED nature, is invastigated thoroughly in the paper. To show its influence on the observation of the color-octet singnature in the e^+e^- annihilation via J/ψinclusive production, the cross sections of the J/ψproductions through different mechanisms at various energies are compared quantatively by presenting them in figures together. The contribution from the production associated with a hard photon to the inclusive production of J/ψis pointed out to be significant at the concerned energies, thus the influence from it on the observation of the color-octet signature should be dealt with carefully.

hep-ph↗
arXiv1997

The QCD corrections to scalar top pair production in photon-photon collisions

The complete QCD corrections to the scalar top quark pair production via $γγ$ fusion in parent $e^{+}e^{-}$ collider are calculated in the Supersymmetric Model. The various origins of corrections are discussed. We find that the correction due to gluino exchange is smaller than that due to conventional gluonic contribution, and the contribution due to squark quartic interactions is much smaller. Moreover, the dependences of the corrections on the masses and the mixing angle of the supersymmetric particles are also investigated.

hep-ph↗
arXiv1996

On the hadronic production of $B_c$ and $B_c^*$ mesons

With respect to the two theoretical approaches for estimating the hadronic productions of the mesons $B_c$ and $B_c^*$: the full $α_s^4$ calculation and the fragmentation approximation, with a thorough comparative study a quite remarkable conclusion may be drawn. In hadronic collisions the condition for the applicability of the fragmentation approximation is the transverse momentum $P_T$ of the produced $B_c$ and $B_c^*$ being much greater than the meson masses, and the higher the energy of the relevant subprocess the higher the $P_T$ is requested. The mechanisms for the productions are also clarified. In order to show the resultant differences of the two approaches, newly introduced distribution of the fraction of the $B_c$ (or $B_c^*$) meson energy (a measurable observable) in the center of mass of the subprocess is emphasized.

hep-ph↗