Searcharxiv⌕ Search

arXiv subjects

Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 307 records · Page 17Linked to original sources

Long Distance Contributions to Penguin Processes $b\rightarrow sγ$ and $b\rightarrow d γ$

We consider the long distance contributions to inclusive penguin processes through processes like $b\rightarrow s V$ and $b\rightarrow d V$ where $V$ are $^3S_1(c\bar c)$ states $ψ_i$ in the former case and include $ρ$, $ω$ for the latter case. We carefully examine vector dominance for $\bar c c$ states, and conclude that there is a large suppression of $ψ\sim γ$ transition when $ψ$ is at $q^2 = 0$. The long distance effects can be at most 10\% in both the amplitudes for $b\rightarrow sγ$ and $b\rightarrow d γ$. Although the long distance contributions are small, the ratio $BR(b\rightarrow dγ)/BR(b \rightarrow s γ) = |V_{td}/V_{ts}|^2$ does not hold due to significant $u$ and $c$ loop contributions to the short distance $b\rightarrow dγ$ amplitudes .

hep-ph↗

CP asymmetry Relations Between $\bar B^0\rightarrow ππ$ and $\bar B^0\rightarrow πK$ Rates

We prove that CP violating rate difference $Δ(\bar B^0 \rightarrow π^+π^-) = Γ(\bar B^0 \rightarrow π^+π^-) - Γ( B^0 \rightarrow π^-π^+)$ is related to $Δ(\bar B^0 \rightarrow π^+ K^-) = Γ(\bar B^0 \rightarrow π^+K^-) - Γ(B^0 \rightarrow π^-K^+)$ in the three generation Standard Model. Neglecting small annihilation diagrams, and in the SU(3) symmetry limit, we show $Δ(\bar B^0 \rightarrow π^+π^-) = - Δ(\bar B^0 \rightarrow π^+ K^-)$. The SU(3) breaking effects are estimated using factorization approximation, and yield $Δ(\bar B^0\rightarrow π^+π^-) \approx -(f_π/f_K)^2Δ(\bar B^0 \rightarrow π^+K^-)$. Usefulness of this remarkable relation for determining phases in the CKM unitarity triangle is discussed.

hep-ph↗

CP Violation In Hyperon Decays Due To Left-Right Mixing

We consider CP violation due to left-right mixing in a class of Left-Right symmetric models and show that it leads to observable effects in hyperon decays. For S-wave, the contribution to CP violating asymmetry A of the polarization in hyperon and anti-hyperon decays is proportional to $ε'/ε$. While the tree level L-R operators contribution is constrained to be less than $ 10^{-5}$, the gluon penguin operator contribution can be as large as $10^{-4}$. For P-wave, the contribution is not directly related to $ε'/ε$. Its contribution to A can be larger and may reach $6\times 10^{-4}$ in $Λ\rightarrow pπ^-$. This is much larger than the value expected for A in the Standard Model.

hep-ph↗

Penguin $B$ Decays $b\rightarrow s l^+l^-$ and $b\rightarrow s g$

The penguin mediated processes $b\rightarrow s g$ and $b\rightarrow s l^+l^-$ are studied. In the Standard Model, for the leptonic modes improvement in experimental limits will put strigient bounds on the top mass, where the present limit from $b\rightarrow s μ^+μ^-$ is 390 GeV. For hadronic penguin processes, although the gluonic penguin dominates, we find the electroweak contribution are around 30\% for the upper range allowed top mass. The branching ratio for $B\rightarrow X_s ϕ$ is predicted to be in the range $(0.6\sim 2)\times 10^{-4}$. Effects of the charged Higgs in two Higgs doublet models are discussed.

hep-ph↗

Isospin Structure of Penguins And Their Consequences in $B$ Physics

Isospin structure of gluon mediated or strong penguin is significantly altered when the full electroweak corrections are included. This has the consequence that some previous analyses which relied on a simple isospin structure in charmless $B$ decays become inapplicable. We present the general Hamiltonian in next-to-leading order QCD, and illustrate our conclusion quantitatively for $B\rightarrow ππ$ and $B\rightarrow Kπ$ decays in the factorization approximation. Some remarks on CP asymmetries in $B$ decays are also made.

hep-ph↗

A Light $Z'$ Boson

A model of a light $Z'$ boson based on gauged $L_μ- L_τ$ $U(1)$ symmetry is constructed. The $Z'$ boson mass is constrained to be in the range of 0.8 to 1 GeV from Z and $Z'$ mass relation, g-2 of muon, and tau decays. The two body decay $τ\rightarrow μZ'$ is possible. This will provide a striking signature to test the model. Talk presented at the Eighth Meeting of the American Physical Society, Division of Particles and Fields (DPF'94), Albuqurque, New Mexico, August 2-6, 1994.

hep-ph↗

CP Violation in a Multi-Higgs Doublet Model

We study CP violation in a multi-Higgs doublet model based on a $S_3 \times Z_3$ horizontal symmetry. We consider two mechanisms for CP violation in this model: a) CP violation due to complex Yukawa couplings; and b) CP violation due to scalar-pseudoscalar mixings. We find that the predictions for $ε'/ε$, CP violation in B decays and the electric dipole moments of neutron and electron are different between these two mechanisms. These predictions are also dramatically different from the minimal Standard Model predictions.

hep-ph↗

Hadronic Penguin B Decays In The Standard And The Two-Higgs-Doublet Models

We study in next-to-leading order QCD hadronic penguin $B$ decays in the Standard and two-Higgs-doublet models. Although the gluonic penguin dominates, we find the electroweak contribution non-negligible. In the Standard Model, the branching ratio for $B \rightarrow X_s ϕ$ is predicted to be in the range $(0.6\sim 2)\times 10^{-4}$. The ranges of branching ratios for $B\rightarrow K ϕ$, $B\rightarrow K^*ϕ$, and $B_s\rightarrow ϕϕ$ are $(0.4\sim 2)\times 10^{-5}$, $(0.2\sim 1)\times 10^{-5}$, and $(0.15\sim 0.5)\times 10^{-5}$, respectively. The contribution from the charged Higgs boson in two Higgs doublet models depend on $cotβ$, and can be as large as 40\%.

hep-ph↗

$ε'/ε$ And Anomalous Gauge Boson Couplings

We study $ε'/ε$ in the Standard Model and $ε'/ε$ due to anomalous $WWγ$ and $WWZ$ interactions %using recent result on the top quark mass from CDF. as a function of the top quark mass. In the Standard Model, $ε'/ε$ is in the range $10^{-3} \sim 10^{-4}$ for the central value of top quark mass reported by CDF. The anomalous gauge couplings can have large contributions to the $CP$ violating $I=2$ amplitude in $K \rightarrow ππ$. Within the allowed regions for the anomalous gauge couplings, $ε'/ε$ can be dramatically different from the standard model prediction.

hep-ph↗

Proton Life-Time Problem In Finite Grand Unified Theories

We study proton decay in finite supersymmetric SU(5) grand unified theories. We find that the dimension-five operators due to color triplet higgsino induce too rapid a proton decay. This behaviour can be traced to the large Yukawa couplings to the first generation that are necessary for finiteness.

hep-ph↗

Gluon dipole penguin contributions to $ε'/ε$ and CP violation in Hyperon decays in the Standard Model

We consider the gluon dipole penguin operator contributions to $ε'/ε$ and CP violation in hyperon decays. It has been proposed by Bertolini et al. that the contribution to $ε'/ε$ may be significant. We show that there is a cancellation in the leading order contribution and this contribution is actually suppressed by a factor of order O($m_π^2, m_K^2)/Λ^2$. We find that the same operator also contributes to CP violation in hyperon decays where it is not suppressed. The gluon dipole penguin operator can enhance CP violation in hyperon decays by as much as 25\%.

hep-ph↗

CP violation in a multi-Higgs doublet model with flavor changing neutral current

We study CP violation in a multi-Higgs doublet model based on a $S_3 \times Z_3$ horizontal symmetry where CKM phase is not the principal source of CP violation. We consider two mechanisms for CP violation in this model: a) CP violation due to complex Yukawa couplings; and b) CP violation due to scalar-pseudoscalar Higgs boson mixings. Both mechanisms can explain the observed CP violation in the neutral Kaon system. $ε'/ε$ due to neutral Higgs boson exchange is small in both mechanisms, but charged Higgs boson con- tributions can be as large as $10^{-3}$ for a), and $10^{-4}$ for b). CP violation in the neutral B system is, however, quite different from the Minimal Standard Model. The neutron Electric Dipole Moment can be as large as the present ex- perimental bound, and can be used to constrain charged Higgs boson masses. The electron EDM is one order of magnitude below the experimental bound in case b) and smaller in case a).

hep-ph↗

CP Violation in Fermion Pair Decays of Neutral Boson Particles

We study CP violation in fermion pair decays of neutral boson particles with spin 0 or 1. We study a new asymmetry to measure CP violation in $η, K_L \rightarrow μ^+μ^-$ decays and discuss the possibility of measuring it experimentally. For the spin-1 particles case, we study CP violation in the decays of $J/ψ$ to $SU(3)$ octet baryon pairs. We show that these decays can be used to put stringent constraints on the electric dipole moments of $Λ$, $Σ$ and $Ξ$.

hep-ph↗

Constraints on the anomalous $WWγ$ couplings from $b\rightarrow sγ$

We study contributions to $b \rightarrow s γ$ from anomalous $WWγ$ interactions. Although these anomalous interactions are not renormalizable, the contributions are cut-off independent. Using recent results from the CLEO collaboration on inclusive radiative B decays, we obtain bounds for the anomalous CP conserving and CP violating couplings. The constraints on the CP conserving couplings are comparable with or better than constraints from other experiments.

hep-ph↗

Anomalous $WWZ$ couplings and $K_L\rightarrow μ^+ μ^-$

We study contributions to $K_L \rightarrow μ^+μ^-$ from anomalous $WWZ$ interactions. There are, in general, seven anomalous couplings. Among the seven anomalous couplings, only two of them contribute significantly. The others are suppressed by factors like $m_s^2/M_W^2$, $m_d^2$, or $m_K^2/M_W^2$. Using the experimental data on $K_L\rightarrow μ^+μ^-$, we obtain strong bounds on the two anomalous couplings.

hep-ph↗

A Possible Way of Connecting the Grassmann Variables and the Number of Generation

We construct a Left-Right symmetric model in which the number of generation is related to Grassmann variables. We introduce two sets of complex Grassmann variables ($θ^1_q$,$θ^2_q$), ($θ^1_l$, $θ^2_l$) and associate each variable with left- and right-handed quark and lepton fields, respectively. Expanding quark and lepton fields in powers of the Grassmann variables, we find that there are exactly three generations of quarks and leptons. Integrating out the Grassmann variables, we obtain phenomenologically acceptable fermion mass matrices.

hep-ph↗