arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 145 records · Page 8Linked to original sources

arXiv2006

PQCD Calculation for $Λ_{b}\to Λγ$ in the Standard Model

We calculate the branching ratio of $Λ_b \to Λγ$ in the standard model using the PQCD method. The predicted branching ratio $B(Λ_b \to Λγ)$ is about $(4.3\sim 8.6)\times 10^{-8}$, with reasonable parameter ranges in the heavy baryon distribution amplitude. This branching ratio is much smaller than those obtained in other hadronic model calculations. Future experimental data can provide important information on applicability of the PQCD method to heavy baryon radiative decay.

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arXiv2006

Probe Noncommutative Space-Time Scale Using $γγ\to Z$ At ILC

In the standard model production of on-shell Z boson at a photon collider (or Z decays into $γγ$) is strictly forbidden by angular momentum conservation and Bose statistics (the Yang's Theorem). In the standard model with noncommutative space-time this process can occur. Therefore this process provides an important probe for the noncommutative space-time. The $γγ$ collision at the ILC by laser backscattering of the electron and positron beams offers an ideal place to carry out such a study. Assuming an integrated luminosity of 500 fb$^{-1}$, we show that the constraint which can be achieved on $Γ(Z\to γγ)$ is three to four orders of magnitude better than the current bound of $5.2\times 10^{-5}$ GeV. The noncommutative scale can be probed up to a few TeVs.

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arXiv2006

The two-loop supersymmetric corrections to lepton anomalous magnetic and electric dipole moments

Using the effective Lagrangian method, we analyze the electroweak corrections to the anomalous dipole moments of lepton from some special two-loop topological diagrams which are composed of neutralino (chargino) - slepton (sneutrino) in the minimal supersymmetric extension of the standard model (MSSM). Considering the translational invariance of the inner loop momenta and the electromagnetic gauge invariance, we get all dimension 6 operators and derive their coefficients. After applying equations of motion to the external leptons, the anomalous dipole moments of lepton are obtained. The numerical results imply that there is a parameter space where the two-loop supersymmetric corrections to the muon anomalous dipole moments may be significant.

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arXiv2006

X(1812) in Quarkonia-Glueball-Hybrid Mixing Scheme

Recently a $J^{PC}=0^{++}$ (X(1812)) state with a mass near the threshold of $ω$ and $ϕ$ has been observed by the BES collaboration in $J/ψ\to γωϕ$ decay. It has been suggested that it is a $I^G = 0^+$ state. If it is true, this state fits in a mixing scheme based on quarkonia, glueball and hybrid (QGH) very nicely where five physical states are predicted. Together with the known $f_0(1370)$, $f_0(1500)$, $f_0(1710)$, and $f_0(1790)$ states, X(1812) completes the five members in this family. Using known experimental data on these particles we determine the ranges of the mixing parameters and predict decay properties for X(1812). We also discuss some features which may be able to distinguish between four-quark and hybrid mixing schemes.

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arXiv2006

Study on Strong Decays of $D_{sJ}(2632)$

The resonance $D_{sJ}(2632)$ observed by SELEX, has attracted great interests and meanwhile brought up serious dispute. Its spin-parity, so far has not finally determined and if it exists, its quark-structure might be exotic. Following the previous literature where $D_{sJ}(2632)$ is assumed to be a radial-excited state of $1^-$, we consider the possibilities that it might be a $q\bar q$ ground state of $2^+$ or the first radial-excited state of $0^+$ $D_{sJ}(2317)$ and re-calculate its strong decay widths in terms of the Bethe-Salpeter equation. Our results indicate that there still is a sharp discrepancy between the theoretical evaluation and data.

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arXiv2006

New Members in the $0^+$ ($0^{++})$ Family

Recent BES data on $J/ψ\to ϕππ$ indicate that there is a possible new spin-0 state ($f_0(1790)$) with a mass of $m=1790^{+40}_{-30} \mathrm{MeV}/\mathrm{c}^2$. Assuming it to be an iso-singlet $0^+(0^{++})$, we propose a new mixing scheme to describe this and the other three known iso-singlet $f_0(1370)$, $f_0(1500)$, $f_0(1710)$ states by adding iso-singlet hybrid states to the usual basis of two iso-singlet quarkonia and one glueball. Since there are two iso-singlet hybrid states, $(u\bar u+d\bar d)g/\sqrt 2$ and $s\bar sg$, this new basis implies existence of another iso-singlet state $X$. Using known data, we estimate the ranges of the mixing parameters. We find two sets of solutions with X mass predicted to be about 1820 MeV and 1760 MeV, respectively. We also study implications on the decay properties of these new states.

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arXiv2006

Spectra of Free Diquark in the Bethe-Salpeter Approach

In this work, we employ the Bethe-Salpeter (B-S) equation to investigate the spectra of free diquarks and their B-S wave functions. We find that the B-S approach can be consistently applied to study the diqaurks with two heavy quarks or one heavy and one light quarks, but for two light-quark systems, the results are not reliable. There are a few free parameters in the whole scenario which can only be fixed phenomenologically. Thus, to determine them, one has to study baryons which are composed of quarks and diquarks.

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arXiv2006

Mixing among the neutral Higgs bosons and rare B decays in the CP violating MSSM

Considering corrections from two-loop Feynman diagrams which involve gluino at large $\tanβ$, we analyze the effects of possible CP phases on the rare B decays: $\bar{B}_{_{s}} \to l^+l^-$ and $\bar{B}\to Kl^+l^-$ in the CP violating minimal supersymmetric extension of the standard model. It is shown that the results of exact two loop calculations obviously differ from that including one-loop contributions plus threshold radiative corrections. The numerical analysis indicates that the possibly large CP phases strongly affect the theoretical estimation of the branching ratios, and this results coincide with the conclusion of some other works appearing in recent literature.

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arXiv2006

Study on Decays of $D_{sJ}^{*}(2317)$ and $D_{sJ}(2460)$ in terms of the CQM Model

Based on the assumption that $D_{sJ}^{*}(2317)$ and $D_{sJ}(2460)$ are the $(0^+, 1^+)$ chiral partners of $D_{s}$ and $D^*_s$, we evaluate the strong pionic and radiative decays of $D_{sJ}^{*}(2317)$ and $D_{sJ}(2460)$ in the Constituent Quark Meson (CQM) model. Our numerical results of the relative ratios of the decay widths are reasonably consistent with data.

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arXiv2005

Study on production of exotic $0^+$ meson $D_{sJ}^{*}(2317)$ in decays of $ψ(4415)$

The newly observed $D_{sJ}^*$ family containing $D_{sJ}^{*}(2317)$, $D_{sJ}(2460)$ and $D_{sJ}(2632)$ attracts great interests. Determining their structure may be important tasks for both theorists and experimentalists. In this work we use the heavy quark effective theory (HQET) and a non-relativistic model to evaluate the production rate of $D_{sJ}^{*}(2317)$ from the decays of $ψ(4415)$, and we find that it is sizable and may be observed at BES III and CLEO, if it is a p-wave excited state of $D_s(1968)$. Unfortunately, the other two members of the family cannot be observed through decays of charmonia, because of the constraints from the final state phase space.

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arXiv2005

Density dependence of quark masses and stability of color-flavor locked phases

Considering the density dependence of quark masses, we investigate the color-flavor-locked matter and its stability relative to the ( unpaired ) strange quark matter. We find that, when the current mass of strange quark $m_s$ is small, the strange quark matter remains stable for the moderate baryon densities. When $m_s$ is large, the gapless phase of the color-flavor-locked matter is found to be difficult to be stable. A schematic phase diagram of three-flavor quark matter is presented, in which the color-flavor-locked phase region is suppressed in comparison with the previous results.

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arXiv2005

Revisiting the OZI-forbidden Radiative Decays of Orthoquarkonia

It is interesting to investigate the OZI-forbidden radiative orthoquarkonium decays $J/ψ\to γπ^0, γη$ and $γη'$ in perturbative QCD. In this work without the approximations adopted in literature we carry out a full one-loop calculation which involves integrations of 4-point and 5-point loop functions. Our numerical results are in agreement with the present data. We also briefly discuss the decays of $J/ψ\to γ+ρ^0$, as well as $Υ\to π^0γ, ηγ, η'γ$.

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arXiv2005

Possible Molecular Structure of the Newly Observed Y(4260)

We suggest that the newly observed resonance Y(4260) is a $χ_{c}-ρ^0$ molecule, which is an isovector. In this picture, we can easily interpret why $Y(4260)\to π^+π^-J/ψ$ has a larger rate than $Y(4260)\to D\bar D$ which has not been observed, and we also predict existence of the other two components of the isotriplet and another two possible partner states which may be observed in the future experiments. A direct consequence of this structure is that for this molecular structure $Y(4260)\to π^+π^-J/ψ$ mode is more favorable than $Y(4260)\to K\bar KJ/ψ$ which may have a larger fraction if other proposed structures prevail.

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arXiv2005

Mixing of Pentaquark and Molecular States

There are experimental evidences for the existence of narrow states $Θ^+$ and $Θ_c$ with the same quantum numbers of $uudd\bar s$ and $uudd\bar c$ pentaquarks and also $NK^{(*)}$ and $ND^{(*)}$ molecular states. Their masses deviate from many theoretical estimates of the pure pentaquark and molecular states. In this work we study the possibility that the observed $Θ^+$ and $Θ_c$ are mixtures of pure pentaquark and molecular states. The mixing parameters are in general related to non-perturbative QCD which are not calculable at present. We determine them by fitting data from known states and then generalize the mechanism to $Θ_b$ to predict its mass and width. The mixing mechanism can also naturally explain the narrow width for $Θ^+$ and $Θ_c$ through destructive interferences, even if the pure pentaquark and molecular states have much larger decay widths. We also briefly discuss the properties of the partner eigenstates of $Θ^+$ and $Θ_c$ and the possibility of experimentally observe them. Moreover, probable consequences of multi-state mixing are also addressed.

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arXiv2005

Contributions from Goldstone-boson-exchange to baryon spectra in the MIT Bag Model

We discuss contributions of chiral bosons to baryon spectra in the MIT bag model. It is believed that within hadrons, chiral bosons are degrees of freedom which are independent of gluons to provide strong interactions between quarks. In the original MIT bag model, only interaction mediated by gluon exchanges was considered, by contrast, in this work we take into account the interaction mediated by the exchanges of chiral bosons $σ$ and $π^{(\pm,0)}$. Then following the standard approach, we minimize the effective hamiltonian which includes both the contributions from gluon and chiral-boson exchanges with respect to the bag radius to obtain the effective radius. By re-fitting the spectra of baryons, we find that the contributions from the boson-exchange may be 40% of that from gluon-exchanges and meanwhile the bag constant $B$, the zero-point energy $z_0$ almost do not change. It indicates that in the original version of the MIT bag model, the intermediate-distance interaction due to the chiral-boson exchanges is attributed into the effective coupling $α_c$ which stood for the short-distance interaction caused by the gluon exchanges and the long-distance effects reflected by $B$ and $z_0$ are not influenced.

hep-ph↗

arXiv2005

Effects of density-dependent quark mass on phase diagram of three-flavor quark matter

Considering the density dependence of quark mass, we investigate the phase transition between the (unpaired) strange quark matter and the color-flavor-locked matter, which are supposed to be two candidates for the ground state of strongly interacting matter. We find that if the current mass of strange quark $m_s$ is small, the strange quark matter remains stable unless the baryon density is very high. If $m_s$ is large, the phase transition from the strange quark matter to the color-flavor-locked matter in particular to its gapless phase is found to be different from the results predicted by previous works. A complicated phase diagram of three-flavor quark matter is presented, in which the color-flavor-locked phase region is suppressed for moderate densities.

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arXiv2005

Bi-Large Neutrino Mixing See-Saw Mass Matrix with Texture Zeros and Leptogenesis

We study constraints on neutrino properties from texture zeros in bi-large mixing See-Saw mass matrix and also from leptogenesis. Texture zeros may occur in the light (class a)) or in the heavy (class b)) neutrino mass matrices. Each of these two classes has 5 different forms which can produce non-trivial three generation mixing with at least one texture zero. We find that two types of texture zero mass matrices in both class a) and class b) can be consistent with present data on neutrino masses, mixing and produce the observed baryon asymmetry of the universe. None of the neutrinos can have zero masses with the lightest of the light neutrinos having a mass larger than about 0.039 eV for class a) and 0.002 eV for class b). In these models although CKM CP violating phase vanishes, non-zero Majorana phases, however, can exist and play an important role in producing the observed baryon asymmetry in our universe through leptogenesis mechanism. The requirement of producing the observed baryon asymmetry can further distinguish different models and also restrict the See-Saw scale to be in the range $10^{12}\sim 10^{15}$ GeV.

hep-ph↗

arXiv2005

Berry Phase in Neutrino Oscillations

We study the Berry phase in neutrino oscillations for both Dirac and Majorana neutrinos. In order to have a Berry phase, the neutrino oscillations must occur in a varying medium, the neutrino-background interactions must depend on at least two independent densities, and also there must be CP violation if the neutrino interactions with matter are mediated only by the standard model W and Z boson exchanges which implies that there must be at least three generations of neutrinos. The CP violating Majorana phases do not play a role in generating a Berry phase. We show that a natural way to satisfy the conditions for the generation of a Berry phase is to have sterile neutrinos with active-sterile neutrino mixing, in which case at least two active and one sterile neutrinos are required. If there are additional new CP violating flavor changing interactions, it is also possible to have a non-zero Berry phase with just two generations.

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