arXiv subjects

Zhenjun Xiao

Publications and source records attributed to Zhenjun Xiao.

At least 19 recordsLinked to original sources

arXiv2006

The single t-quark productions via the flavor-changing processes in the topcolor-assisted technicolor model at the hadron colliders

In the framework of topcolor-assisted technicolor(TC2) model, there exist tree-level flavor-changing (FC) couplings which can result in the loop-level FC coupling $tcg$. Such $tcg$ coupling can contribute significant clues at the forthcoming Large Hadron Collider (LHC) experiments. In this paper, based on the TC2 model, we study some single t-quark production processes involving $tcg$ coupling at the Tevatron and LHC: $pp(p\bar{p})\to t\bar{q}(q=u,d,s),tg$. We calculate the cross sections of these processes. The results show that the cross sections at the Tevatron are too small to observe the signal, but at the LHC it can reach a few pb. With the high luminosity, the LHC has considerable capability to find the single t-quark signal produced via some FC processes involving coupling $tcg$. On the other hand, these processes can also provide some valuable information of the coupling $tcg$ with detailed study of the processes and furthermore provide the reliable evidence to test the TC2 model.

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arXiv2006

B\to X_sγ, X_s l^+ l^- decays and constraints on the mass insertion parameters in the MSSM

In this paper, we study the upper bounds on the mass insertion parameters $(δ^{q}_{AB})_{ij}$ in the minimal supersymmetric standard model (MSSM). We found that the information from the measured branching ratio of $B \to X_s l^+ l^-$ decay can help us to improve the upper bounds on the mass insertions parameters $\left (δ^{u,d}_{AB})_{3j,i3}$. Some regions allowed by the data of $Br(B \to X_s γ) $ are excluded by the requirement of a SM-like $C_{7γ}(m_b)$ imposed by the data of $Br(B \to X_s l^+ l^-)$.

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arXiv2006

Branching Ratio and CP Asymmetry of $B \to ρη^{(\prime)}$ Decays in the Perturbative QCD Approach

In this paper,we calculate the branching ratios and CP-violating asymmetries for $B^0 \to ρ^0 η^{(\prime)}$ and $B^+\to ρ^+ \etap$ decays in the perturbative QCD factorization approach. In this approach, we not only calculate the usual factorizable contributions, but also evaluate the non-factorizable and annihilation type contributions. Besides the current-current operators, the contributions from the QCD and electroweak penguin operators are also taken into account. The theoretical predictions for the branching ratios are $Br(B^+ \to ρ^+ η^{(\prime)}) \approx 9 \times 10^{-6}$ and $Br(B^0 \to ρ^0 η^{(\prime)}) \approx 5 \times 10^{-8}$, which agree well with the measured values and currently available experimental upper limits. We also predict large CP-violating asymmetries in these decays: $A_{CP}^{dir}(ρ^\pm η)\approx -13 %$, $A_{CP}^{dir}(ρ^\pm η^\prime)\approx -18 %$, $A_{CP}^{dir}(ρ^0 η)\approx -41%$, $A_{CP}^{dir}(ρ^0 η^\prime)\approx -27%$, $A_{CP}^{mix}(ρ^0 η)\approx +25%$, and $A_{CP}^{mix}(ρ^0 η^\prime) \approx +11%$, which can be tested by the current or future B factory experiments.

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arXiv2006

CP violation of the two-body charmless hadronic B decays in the minimal supergravity Model

By choosing two typical input parameter points in the minimal supergravity (mSUGRA) model and using the QCD factorization (QCDF) approach, we studied the supersymmetric effects to the CP violation of the two-body charmless hadronic $B$ meson decays. We found that though the SUSY contributions can give large corrections to the CP asymmetries for some decay channels, they could not be distinguished experimentally from the SM values because of the large theoretical errors dominated by calculating the annihilation contributions in the QCDF approach.

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arXiv2005

Exclusive $B \to V γ$ decays in the T2HDM

By employing the QCD factorization approach for the exclusive $B \to V γ$ decays, we calculated the new physics contributions to the branching ratios, CP asymmetries, isospin and U-spin symmetry breaking of $B \to K^*γ$ and $B \to ργ$ decays, induced by the charged Higgs penguin diagrams appeared in the top-quark two-Higgs-doublet model(T2HDM). Within the considered parameter space, we found that (a) a charged-Higgs boson with a mass larger than 300 GeV are always allowed by the date of $B \to V γ$ decay, and such lower limit on $\mhp$ are comparable with those obtained from the inclusive $B \to X_s γ$ decay; (b) the CP asymmetry of $B \to ργ$ in the T2HDM can be as large as 10% in magnitude and has a strong dependence on the angle $θ$ and the CKM angle $γ$; (c) the isospin symmetry breakings of $B \to V γ$ decays in the T2HDM are generally small in size: around 6% for $B \to K^* γ$ decay and less than 20% for $B \to ργ$ decay; and (d) the U-spin symmetry breaking $ΔU(K^*,ρ)$ in the T2HDM is also small in size, only about 8% of the branching ratio $\calb (B \to ρ^0 γ)$.

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arXiv2005

Branching Ratio and CP Asymmetry of $B \to πη^{(\prime)}$ Decays in the Perturbative QCD Approach

We calculate the branching ratios and CP-violating asymmetries for $B^0 \to π^0 η^{(\prime)}$ and $B^+\to π^+ η^{(\prime)}$ decays in the perturbative QCD (pQCD) factorization approach here. We not only calculate the usual factorizable contributions, but also evaluate the non-factorizable and annihilation type contributions. Besides the current-current operators, the contributions from the QCD and electroweak penguin operators are also taken into account. The pQCD results for the CP-averaged branching ratios are $Br(B^+ \to π^+ η) \approx 4.1 \times 10^{-6}$, $Br(B^+ \to π^+ η^\prime) \approx 2.4 \times 10^{-6}$, and $Br(B^0 \to π^0 η^{(\prime)}) \approx 0.2 \times 10^{-6}$, which agree very well with the measured values or currently available experimental upper limits. We also predict large CP-violating asymmetries in these decays: $A_{CP}^{dir}(π^\pm \etap)\sim A_{CP}^{dir}(π^0 \etap)\sim -0.35 $, and $A_{CP}^{mix}(π^0 \etap)\sim 0.67$, but with large errors. The pQCD prediction for $A_{CP}^{dir}(π^\pm η)$ ($A_{CP}^{dir}(π^\pm η^\prime))$ has the same (opposite) sign with the primary measured values. Further improvements in both theory and experiments are needed to clarify this discrepancy.

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arXiv2005

Charmless $B \to PV, VV $ decays and new physics effects in the mSUGRA model

By employing the QCD factorization approach, we calculate the new physics contributions to the branching radios of the two-body charmless $ B \to PV$ and $B \to VV$ decays in the framework of the minimal supergravity (mSUGRA) model. we choose three typical sets of the mSUGRA input parameters in which the Wilson coefficient $C_{7γ}(m_b)$ can be either SM-like (the case A and C) or has a flipped-sign (the case B). We found numerically that (a) the SUSY contributions are always very small for both case A and C; (b) for those tree-dominated decays, the SUSY contributions in case B are also very small; (c) for those QCD penguin-dominated decay modes, the SUSY contributions in case B can be significant, and can provide an enhancement about $30% \sim 260%$ to the branching ratios of $B \to K^*(π,ϕ,ρ)$ and $K ϕ$ decays, but a reduction about $30% \sim 80%$ to $ B\to K(ρ, ω)$ decays; and (d) the large SUSY contributions in the case B may be masked by the large theoretical errors dominated by the uncertainty from our ignorance of calculating the annihilation contributions in the QCD factorization approach.

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arXiv2004

Charmless $B \to PP $ decays and the new physics effects in the minimal supergravity model

By employing the QCD factorization approach, we calculate the new physics contributions to the branching radios of the two-body charmless $ B \to PP$ decays in the framework of the minimal supergravity (mSUGRA) model. Within the considered parameter space, we find that (a) the supersymmetric (SUSY) corrections to the Wilson coefficients $C_k$ ($k=3-6$) are very small and can be neglected safely, but the leading order SUSY contributions to $C_{7γ}(M_W)$ and $C_{8g}(M_W)$ can be rather large and even change the sign of the corresponding coefficients in the standard model; (b) the possible SUSY contributions to those penguin-dominated decays in mSUGRA model can be as large as $30-50%$; (c) for the well measured $B \to K π$ decays, the significant SUSY contributions play an important rule to improve the consistency of the theoretical predictions with the data; (d) for $B \to K η'$ decays, the theoretical predictions of the corresponding branching ratios become consistent with the data within one standard deviation after the inclusion of the large SUSY contributions in the mSUGRA model.

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arXiv2003

Exclusive $B \to (K^*, ρ) γ$ decays in the general two-Higgs-doublet models

By employing the QCD factorization approach, we calculated the next-to-leading order new physics contributions to the branching ratios, CP asymmetries, isospin and U-spin symmetry breaking of the exclusive decays $B \to V γ$ ($V=K^*, ρ$), induced by the charged Higgs penguins in the general two-Higgs-doublet models. Within the considered parameter space, we found that (a) the new physics corrections to the observables are generally small in the model I and model III-A, moderate in model II, but large in model III-B; (b) from the well measured branching ratios and upper limits, a lower bound of $\mhp > 200$ GeV in model II was obtained, while the allowed range of $\mhp$ in model III-B is $ 226 \leq \mhp \leq 293 $ GeV; these bounds are comparable with those from the inclusive $B \to X_s γ$ decay; (c)the NLO Wilson coefficient $C_7(m_b)$ in model III-B is positive and disfavered by the measured value of isospin symmetry breaking $Δ_{0-}^{exp} (K^*γ) = (3.9 \pm 4.8)%$, but still can not be excluded if we take the large errors into account; (d) the CP asymmetry $\acp(B \to ργ)$ in model III-B has an opposite sign with the one in the standard model (SM), which may be used as a good observable to distinguish the SM from model III-B; (e) the isospin symmetry breaking $Δ(ργ)$ is less than 10% in the region of $γ= [ 40 \sim 70]^\circ$ preferred by the global fit result, but can be as large as 20 to 40% in the regions of $γ\leq 10^\circ$ and $γ\geq 120^\circ$. The SM and model III-B predictions for $Δ(ργ)$ are opposite in sign for small or large values of the CKM angles; (f) the U-spin symmetry breaking $ΔU(K^*,ρ)$ in the SM and the general two-Higgs-doublet models is generally small in size: $\sim 10^{-7}$.

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arXiv2003

B^0-\bar{B}^0 mixing and B \to X_s γdecay in the third type 2HDM: effects of NLO QCD contributions

In this paper, we calculated the next-to-leading order (NLO) new physics contributions to the mass splitting $\dmd$ and the branching ratio $\brbxsga$ induced by the charged Higgs loop diagrams in the third type of two-Higgs-doublet models (model III) and draw the constraints on the free parameters of model III. For the model III under consideration, we found that (a) an upper limit $|\ltt|\leq 1.7$ is obtained from the precision data of $\dmd=0.502 \pm 0.007 ps^{-1}$, while $|\ltt| \approx 0.5$ is favored phenomenologicaly; (b) for $B \to X_s γ$ decay, the NLO QCD contributions tend to cancel the LO new physics contributions; (c) a light charged Higgs boson with a mass around or even less than 200 GeV is still allowed at NLO level by the measured branching ratio $\brbxsga$: numerically, $188 \leq \mh \leq 215$ GeV for $(|\ltt|,|\lbb|)=(0.5,18)$; (d) the NLO QCD contributions tend to cancel the LO contributions effectively, the lower limit on $\mh$ is consequently decreased by about 200 GeV; (e) the allowed region of $\mh$ will be shifted toward heavy mass end for a non-zero relative phase $θ$ between the Yukawa couplings $\ltt$ and $\lbb$. The numerical results for the conventional model II are also presented for the sake of a comparison.

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arXiv2003

Charmless hadronic decays $B \to VV$ in the Topcolor-assisted Technicolor model

Based on the effective Hamiltonian with the generalized factorization approach, we calculate the branching ratios and CP asymmetries of $B \to VV$ decays in the Topcolor-assisted Technicolor (TC2) model. Within the considered parameter space we find that: (a) for the penguin-dominated $B \to K^{*+}ϕ$ and $K^{*0}ϕ$ decays, the new physics enhancements to the branching ratios are around 40%; (b) the measured branching ratios of $B \to K^{*+} ϕ$ and $ K^{*0} ϕ$ decays prefer the range of $3 \lesssim \nceff \lesssim 5$; (c) the SM and TC2 model predictions for the branching ratio ${\cal B}(B^+ \to ρ^+ ρ^0)$ are only about half of the Belle's measurement; and (d) for most $B \to VV$ decays, the new physics corrections on their CP asymmetries are generally small or moderate in magnitude and insensitive to the variation of $\mpcc$ and $\nceff$.

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arXiv2003

Constraints on the CKM angle $α$ from the measured CP asymmetries and branching ratios of $B \to π^+ π^-, π^+ π^0$ and $K^0π^+$ decays

In this paper, we draw constraints on the Cabibbo-Kobayashi-Maskawa (CKM) angle $α$ and the strong phase $δ$ from the experimental measurements of $\ssb$, the CP-violating asymmetries and branching ratios of $B \to π^+ π^-, π^+ π^0$ and $K^0 π^+$ decays. In the SM, the measured $\ssb$ leads to an upper limit on $α$: $α\leq 180^\circ-β$. Taking the weighted-average of the newest BaBar and Belle measurements of $\spp$ and $\app$ in $B^0 \to π^+π^-$ decay as the experimental input, we find the allowed range of angle $α$, $76^\circ \leq α\leq 135^\circ$ for $r=|P/T| =0.3 \pm 0.1 $. From the measured CP-averaged branching ratios of $B \to π^+ π^-, π^+ π^0$ and $K^0 π^+$ decays, we found an inequality, $\cosα \cosδ \geq 0.45$. The region of $70^\circ \leq α\leq 110^\circ$ is thus excluded by applying this inequality. The combined constraints on $α$ and $δ$ are $117^\circ \leq α\leq 135^\circ$ and $-160^\circ \leq δ\leq -132^\circ$ if we take $\spp=0.49 \pm 0.27$ and $\app=0.51 \pm 0.19$ as the experimental input. The new lower limit on the angle $α$ is dominated by the measured branching ratios considered in this paper, and is much stronger than those obtained before.

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arXiv2003

Technicolor corrections on $B_{s,d} \to γγ$ decays in QCD factorization

Within the framework of the Top-color-assisted Technicolor (TC2) model, we calculate the new physics contributions to the branching ratios $\calb(B_{s,d} \to γγ)$ and CP violating asymmetries $\rcpm(B_{s,d} \to γγ)$ in the QCD factorization based on the heavy-quark limit $m_b \gg Λ_{QCD}$. Using the considered parameter space, we find that (a) for both $B_s\to γγ$ and $B_d \to γγ$ decays, the new physics contribution can provide a factor of two to six enhancement to their branching ratios, (b) for the $B_s \to γγ$ decay, its direct CP violation is very small in both the SM and TC2 model, and (c) the CP violating asymmetry $\rcpm(B_d \to γγ)$ is around the ten percent level in both the SM and TC2 model, but the sign of CP asymmetry in the TC2 model is different from that in the SM.

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arXiv2002

Rare decay $B \to X_{s} ν\bar ν$ in the two-Higgs-doublet model of type-III

In this paper, we calculated the new physics contribution to theoretically very clean rare decay $B\to X_{s} ν\barν$ in the general two-Higgs-doublet model (model III). Within the considered parameter space, we found that (a) the new physics contribution can provide one to two orders of enhancement to the branching ratio $\calb(B\to X_s ν\barν)$ and can saturate the experimental bound on $\calb(B \to X_s ν\barν)$ in some regions of the parameter space; (b) besides the CLEO data of $B \to X_s γ$, the ALEPH upper limit on $\calb (B \to X_s ν\barν)$ also lead to further constraint on the size of the Yukawa coupling $λ_{tt}$: $λ_{tt}< 6.4$ for $λ_{bb}=2.7$ and $\mhp=200$ GeV.

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arXiv2002

Constraint on the CKM angle alpha from the experimental measurements of CP violation in B_d^0 --> pi^+ pi^- decay

In this paper, we study and try to find the constraint on the CKM angle alpha from the experimental measurements of CP violation in B_d^0 --> pi^+ pi^- decay, as reported very recently by BaBar and Belle Collaborations. After considering uncertainties of the data and the ratio r of penguin over tree amplitude, we found that strong constraint on both the CKM angle alpha and the strong phase delta can be obtained from the measured CP asymmetries S_{pi pi} and A_{pi pi}: (a) the ranges of 87 degrees <= alpha <= 131 degrees and 36 degrees <= delta <= 144 degrees are allowed by 1 sigma of the averaged data for r = 0.31; (b) for Belle's result alone, the limits on alpha and delta are 104 degrees <= alpha <= 139 degrees and 42 degrees <= delta <= 138 degrees for 0.32 <= r <= 0.41; and (c) the angle alpha larger than 90 degrees is preferred.

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arXiv2002

Extraction of the CKM angle $γ$ from the new "mixed " system of $B^+ \to π^+ K^0$ and $B_d^0 \to π^0 K^0$ decays

In this paper we try to extract the CKM angle $γ$ from the new "mixed" system of $B^+ \to π^+ K^0$ and $B_d^0 \to π^0 K^0$ decays. We also made an update for the constraints on the angle $γ$ from the observables $R$ and $A_0$. In the parametrization, the SU(2) isospin symmetry of strong interactions has been applied. We found the following results: (a) the measured value of $R$ is now very close to unit, the bound on the angle $γ$ from the measurement of $R$ is therefore not as promising as before, but some bounds on $γ$ can still be read off from $r-γ$ plane if $r$ could be fixed by using an additional input; (b) the measured $R_1$ implies a limit on the strong phase $Δ_1$; (c) due to the contribution from the color allowed electroweak penguin, the minimal value of $R_1$ can be larger than unit. For $ε_1=0.2$ and $R_1=1.2$, the range of $65^\circ \leq γ\leq 115^\circ$ will be excluded, such bounds on $γ$ are interesting and complimentary to the limits from global fit; (d) the dependences of extraction of $γ$ on the variation of parameters $ε_1$, $ρ$, $r_1$ and strong phases are also studied.

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arXiv2002

$B \to π^+ π^-, K π, K \etap$ decays and new physics effects in the general two-Higgs-doublet model: an update

In this paper, we reexamine the new physics contributions to seven well measured $B \to PP$ decays in the standard model (SM) and the general two-Higgs-doublet model (model III) and compare the theoretical predictions with the new data. Within the considered parameter space we found that: (a) the measurements of the branching ratios for $B\to π^+ π^-, K^- π^+$ and $K^0 π^+$ lead to a strong constraint on the form factor $F_0^{Bπ}(0)$: $F_0^{Bπ}(0)=0.24 \pm 0.03$; (b) the new physics enhancements to the penguin-dominated $B \to K π$ and $K \etap$ decays are significant in size, $\sim (40-65)%$ $w.r.t$ the SM predictions, and play an important role in restoring the consistency between the data and the theory.

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arXiv2002

$\etap K$ puzzle of B meson decays and new physics effects in the general two-Higgs-doublet model

we calculate the new physics contributions to seven measured decays $B \to π^+ π^-, K π$ and $ K \etap$ in the general two-Higgs-doublet model (Model III). Within the considered parameter space, we find that: (a) the CLEO/BaBar measurement of $B\to π^+ π^-$ decay prefers a small $F_0^{Bπ}(0)$: $F_0^{Bπ}(0)=0.25 \pm 0.03$; (b) the new physics enhancements to the penguin-dominated $B \to Kπ$ and $B\to K \etap$ decays are significant in size, $\sim (40-70)%$ $w.r.t$ the standard model predictions; and (c) the new physics enhancements can boost the branching ratios ${\cal B}(B \to K^+ \etap)$ and ${\cal B}(B \to K^0 \etap)$ to be consistent with the data within one standard deviation, and hence lead to a simple and plausible new physics interpretation for the $\etap K$ puzzle.

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