arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 163 records · Page 9Linked to original sources

arXiv2005

Does The $\triangle I ={1/2}$ Rule Hold In D And B $\to ππ$ Decays?

Although two pion decays of $K$, $D$ and $B$ have similar isospin structures, there are dramatic differences in the ratios $R_K$ and $R_{D,B}$ of amplitudes from $ΔI =3/2$ and $ΔI =1/2$ interactions. In $K\to ππ$ decays there is the famous $ΔI =1/2$ rule with $R_K \approx 1/22$, whereas in $B(D) \to ππ$ decays the ratios $R_{D,B}$ are of order one and therefore there is no such a rule. In this work we study decay amplitudes in $B(D)\to ππ$ using QCD factorization calculations paying particular attention to discrepancies between data and theoretical estimates. Since isospin does not play a special role in factorization calculations, no $ΔI =1/2$ rule is expected. We find that theoretical results on the size of the amplitudes are in qualitative agreement with data. However the phases for the amplitudes are very different. We show that the effects of re-scattering between the two pions in the final state can play a crucial rule in understanding the differences between $B(D)\to ππ$ and $K\to ππ$ decays. We also comment on the role of isospin analysis which applies to the study of CP violation in $B\to ππ$ decays.

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arXiv2005

The $K^0$ condensed phase of color-flavor locked quark matter : its formation as a stable state reexamined

Taking the vector kaon-quark interaction into account, we reexamine in detail the neutral kaon condensation in color-flavor locked quark matter. It is found that the pairing phenomena in the quark matter are somewhat influenced by $K^0$ condensation. Different from the previous predictions, we find that the $K^0$ condensed phase might no longer be more stable than the normal phase even if $K^0$ condensation can occur in the color-flavor locked matter.

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arXiv2004

The c-quark EDM and production of $h_c$ in $e^+e^-$ annihilation

We analyze the charm quark electric dipole moment (EDM) in the minimal supersymmetric extension of the standard model (MSSM), including important two-loop gluino contributions. Considering the experimental constraint on the neutron EDM, the theoretical prediction for the charm quark EDM can reach about $10^{-20}e\cdot cm$. If taking into account the mixing between the scharm and stop quarks in the effective supersymmetry scenario, the charm quark EDM can be enhanced to $\sim 10^{-19}e\cdot cm$. Direct production of the CP-odd $^1P_1$ state $h_c$ in $e^+e^-$ annihilation via the CP-violating process at the BES-III and CLEO-C is analyzed.

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arXiv2004

Parity, Charge Conjugation and SU(3) Constraints on Threshold Enhancement in $J/ψ$ decays into $γp\bar p$ and $K p \barΛ$

We study the threshold enhancement effects of baryon-anti-baryon systems in $J/ψ$ decays at BES using parity $P$, charge conjugation $C$, and flavor SU(3) symmetries. The $P$ and $C$ symmetries restrict the $p\bar p$ in $J/ψ\to γp\bar p$ to be in a state with $C=+1$, while the $p\bar p$ in $J/ψ\to π^0 p\bar p$ to be with $C=-1$. Combining the $C$ and $P$ symmetries with flavor SU(3) symmetry, i.e. the CPS symmetry, we find that the $Λ\bar p$ system cannot be in $0^+$ and $0^-$ states in $J/ψ\to K^+ Λ\bar p$. We provide a consistent explanation of observation and non-observation of near threshold enhancements in $J/ψ\to K^+ Λ\bar p$ and $J/ψ\to π^0 p \bar p$, respectively. We also find that near baryon pair threshold enhancement can happen in several channels for $J/ψ$ decays and can be several times larger than that observed in $J/ψ\to K^+ Λ\bar p$ in some channels.

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arXiv2004

Two-loop gluino contributions to neutron electric dipole moment in CP violating MSSM

We analyze two-loop gluino corrections to the neutron electric dipole moment (EDM) in the minimal supersymmetry extension of the standard model (MSSM). The dependence of two-loop corrections on the relevant CP violating phases differs from that of the one-loop contributions, and there is a region in the parameter space where the two-loop contributions are comparable with the one-loop contributions. Our numerical results show that the two-loop corrections can be as large as 30% of the one-loop results.

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arXiv2004

$B\to Kπ$ Decays with $1/m_b$ Corrections in $QCD$ Factorization

It is commonly believed that a careful investigation of the subleading terms is crucial for a better understanding of the $QCD$ factorization in charmless B decays. In this work the penguin-dominated $B\to Kπ$ decays are discussed systematically, including the subleading corrections in $1/m_b$ due to soft and hard gluons, besides the annihilation contributions. Soft-gluon effects for all the relevant 4-quark effective operators are calculated within the framework of the light-cone QCD sum rules (LCSR). Our observation is that such soft and hard corrections are less important than the annihilation effects, enhancing only the branching ratios by a few percent; the resultant increase in the branching ratios due to the overall ${\cal O}(1/m_b)$ effects is between about $(22-27)%$ of the QCD factorization results with the ${\cal O}(α_s)$ corrections, as the weak phase $γ(=\textmd{Im}V_{ub}^*)$ ranges from $40^0$ to $80^0$. Impacts of the involved uncertainties are discussed in some details.

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arXiv2004

Photoproduction and Radiative Decay of Spin 1/2 and 3/2 Pentaquarks

We study photoproduction and radiative decays of pentauqarks paying particular attention to the differences between spin-1/2 and spin-3/2, positive and negative parities of pentaquarks. Detailed study of these processes can not only give crucial information about the spin, but also the parity of pentaquarks.

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arXiv2004

A Comprehensive Four-Quark Interpretation of D_s(2317), D_s(2457) and D_s(2632)

The recently observed new member of the charm-strange family D_s(2632) which has a surprisingly narrow width is challenging our theory. D_s(2317) and D_s(2457) which were observed earlier have similar behaviors and receive various theoretical explanations. Some authors use the heavy hadron chiral effective theory to evaluate heavy-light quark systems and obtain a reasonable evaluation on the masses of D_s(2317) and D_s(2457). An alternative picture is to interpret them as four-quark or molecular states. In this work, we are following the later and propose a unitive description for all the three new members D_s(2632), D_s(2317) and D_s(2457) and at least, so far our picture is consistent with the data.

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arXiv2004

Instantaneous Bethe-Salpeter Equation and Its Exact Solution

We present an approach to solve a Bethe-Salpeter (BS) equation exactly without any approximation if the kernel of the BS equation exactly is instantaneous, and take positronium as an example to illustrate the general features of the solutions. As a middle stage, a set of coupled and self-consistent integration equations for a few scalar functions can be equivalently derived from the BS equation always, which are solvable accurately. For positronium, precise corrections to those of the Schrödinger equation in order $v$ (relative velocity) in eigenfunctions, in order $v^2$ in eigenvalues, and the possible mixing, such as that between $S$ ($P$) and $D$ ($F$) components in $J^{PC}=1^{--}$ ($J^{PC}=2^{++}$) states as well, are determined quantitatively. Moreover, we also point out that there is a problematic step in the classical derivation which was proposed first by E.E. Salpeter. Finally, we emphasize that for the effective theories (such as NRQED and NRQCD etc) we should pay great attention on the corrections indicated by the exact solutions.

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arXiv2004

Can the observed enhancement in the mass spectrum of p \bar p in J/ψ\to γp \bar p be interpreted by a possible p \bar p bound state

Provided the enhancement in the $p \bar{p}$ spectrum in radiative decay $J/ψ\to γp \bar{p}$ observed by the BES collaboration is due to an existence of a $p \bar{p}$ molecular state, we calculate its binding energy and lifetime in the linear $σ$ model. We consider a possibility that the enhancement is due to a $p \bar p$ resonance which is in either S-wave or P-wave structure and compare our results with the data.

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arXiv2004

Evaluation of Spectra of Baryons Containing Two Heavy Quarks in Bag Model

In this work, we evaluate the energy spectra of baryons which consist of two heavy and one light quarks in the MIT bag model. The two heavy quarks constitute a heavy scalar or axial vector diquark. Concretely, we calculate the spectra of $|q(QQ')>_{1/2}$ and $|q(QQ')>_{3/2}$ where $Q$ and $Q'$ stand for $b$ and/or $c$ quarks. Especially, for $|q(bc)>_{1/2}$ there can be a mixing between $|q(bc)_0>_{1/2}$ and $|q(bc)_1>_{1/2}$ where the subscripts 0 and 1 refer to the spin state of the diquark (bc), the mixing is not calculable in the framework of quantum mechanics (QM) as the potential model is employed, but can be evaluated by the quantum field theory (QFT). Our numerical results indicate that the mixing is sizeable

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arXiv2004

Hadronic Decays Involving Heavy Pentaquarks

Recently several experiments have reported evidences for pentaquark $Θ^+$. H1 experiment at HERA-B has also reported evidence for $Θ_c$. $Θ^+$ is interpreted as a bound state of an $\bar s$ with other four light quarks $udud$ which is a member of the anti-decuplet under flavor $SU(3)_f$. While $Θ_c$ is a state by replacing the $\bar s$ in $Θ^+$ by a $\bar c$. One can also form $Θ_b$ by replacing the $\bar s$ by a $\bar b$. The charmed and bottomed heavy pentaquarks form triplets and anti-sixtets under $SU(3)_f$. We study decay processes involving at least one heavy pentaquark using $SU(3)_f$ and estimate the decay widths for some decay modes. We find several relations for heavy pentaquarks decay into another heavy pentaquark and a $B (B^*)$ or a $D(D^*)$ which can be tested in the future. $B$ can decay through weak interaction to charmed heavy pentaquarks. We also study some $B$ decay modes with a heavy pebtaquark in the final states. Experiments at the current $B$ factories can provide important information about the heavy pentaquark properties.

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arXiv2004

The anomalous Higgs-top couplings in the MSSM

The anomalous couplings of the top quark and the Higgs boson has been studied in an effective theory resulting in the framework of the minimal supersymmetric extension of the standard model (MSSM) when the heavy fields are integrated out. Constraints on the parameters of the model from the experimental data on the ratio $R_b={Γ(Z\to b\bar{b})/Γ(Z\to hadrons)}$ are derived.

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arXiv2004

Estimating Mass of Sigma-Meson and Study on Application of the Linear Sigma-Model

Whether the $σ-meson$ ($f_0(600)$) exists as a real particle is a long-standing problem in both particle physics and nuclear physics. In this work, we analyze the deuteron binding energy in the linear $σ$ model and by fitting the data, we are able to determine the range of $m_σ$ and also investigate applicability of the linear $σ$ model for the interaction between hadrons in the energy region of MeV's. Our result shows that the best fit to the data of the deuteron binding energy and other experimental data about deuteron advocates a narrow range for the $σ-$meson mass as $520\leq m_σ\leq 580$ MeV and the concrete values depend on the input parameters such as the couplings. Inversely fitting the experimental data, our results set constraints on the couplings. The other relevant phenomenological parameters in the model are simultaneously obtained.

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arXiv2003

Lepton Dipole Moments and Rare Decays in the CP-violating MSSM with Nonuniversal Soft-Supersymmetry Breaking

We investigate the muon anomalous magnetic dipole moment (MDM), the muon electric dipole moment (EDM) and the lepton-flavour-violating decays of the $τ-$lepton, $τ\to μγ$ and $τ\to 3μ$, in the CP-violating Minimal Supersymmetric Standard Model (MSSM) with nonuniversal soft-supersymmetry breaking. We evaluate numerically the muon EDM and the branching ratios $B(τ\to μγ)$ and $B(τ\to 3μ)$, after taking into account the experimental constraints from the electron EDM and muon MDM. Upon imposition of the experimental limits on our theoretical predictions for the aforementioned branching ratios and the muon MDM, we obtain an upper bound of about $10^{-23} e\cdot cm$ on the muon EDM which lies well within the explorable reach of the proposed experiment at BNL.

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arXiv2003

Non-spectator Contributions To The Lifetime of $Λ_{b}$

In this work, we evaluate the contributions of non-spectator effects to the lifetimes of $Λ_b$ and B-mesons. Based on the well-established models and within a reasonable range of the concerned parameters, the contributions can reduce the lifetime of $Λ_b$ by $7 \sim 8%$ compared to that of B-mesons which are not significantly affected. This might partly explain the measured ratio $τ(Λ_{b})/τ(B^{0})=0.79 $ \cite{Data}, which has been a long-standing discrepancy between theory and experimental data.

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arXiv2002

Neutrinoless Double Beta Decay in Supersymmetric Seesaw model

Inspired by the recent HEIDELBERG-MOSCOW double beta decay experiment, we discuss the neutrinoless double beta decay in the supersymmetric seesaw model. Our numerical analysis indicates that we can naturally explain the data of the observed neutrinoless double beta decay, as well as that of the solar and atmospheric neutrino experiments with at least one Majorana-like sneutrino of middle energy scale in the model.

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arXiv2002

Cabibbo-suppressed non-leptonic decays of Lambda_c and final state interaction

With the diquark structure of Lambda_c, we investigate the branching ratios of Lambda_c -> n pi^+ and p pi^0. The results show that without considering the final state interaction (FSI), the branching ration of Lambda_c -> p pi^+ is only of order 10^{-6} whereas taking into account the FSI effects, this ratio could reach 10^{-4} and is at the same order as Lambda_c -> n pi^0. Concrete values depend on phenomenological parameters adopted in the calculations. These branching ratios can be measured in the experiments to come.

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