arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 91 records · Page 5Linked to original sources

arXiv2011

Dipion invariant mass distribution of the anomalous $Υ(1S) π^{+} π^{-}$ and $Υ(2S) π^{+} π^{-}$ production near the peak of $Υ(10860)$

Considering the defects of the previous work for estimating the anomalous production rates of $e^+e^-\to Υ(1S)π^+π^-,\,Υ(2S)π^+π^-$ near the peak of the $Υ(5S)$ resonance at $\sqrt s=10.87$ GeV [K.F. Chen {\it et al}. (Belle Collaboration), Phys. Rev. Lett. {\bf 100}, 112001 (2008)], we suggest a new scenario where the contributions from the direct dipion transition and the final state interactions interfere to result in not only the anomalously large production rates, but also the lineshapes of the differential widths consistent with the experimental measurement when assuming the reactions are due to the dipion emission of $Υ(5S)$. At the end, we raise a new puzzle that the predicted differential width $dΓ(Υ(5S)\toΥ(2S)π^+π^-)/d\cosθ$ has a discrepant trend from the data while other predictions are well in accord with the data. It should be further clarified by more accurate measurements carried by future experiments.

hep-ph↗

arXiv2011

Fraction of the gluonium component in $η'$ and $η$

It is interesting to determine the fraction of the gluonium component in $η$ and $η'$ which has been under serious discussion for many years. Measurements on different decay and/or production modes were employed in literatures, thus larger uncertainties were unavoidable. In this paper we suggest to determine the mixing angles of $η-η'-G$ using the data of semileptonic decays of $D$ and $D_s$. We extract the mixing angles $ϕ'$, $ϕ_G$ and the model parameters simultaneously. Thanks to the new measurements carried out by CLEO Collaboration and there are sufficient decay modes to determine both the model parameters and mixing angles. The mixing angles from data are $ϕ'=(41.5\pm2.0)^\circ$ and $\sinϕ_G=0.00\pm0.36$. Even though the central value of $\sinϕ_G$ is still zero, an upper bound is set. Moreover, as suggested in literature, $η(1405)$ is a glueball candidate whereas in our picture, $η(1405)$ may be identified as $G$ with glueonium being its main content. Using all the model-parameters obtained above, we estimate the branching ratios of $D_s^+(D^+)\rightarrow Ge^+ν_e$ where G is identified as $η(1405)$.

hep-ph↗

arXiv2011

Study on the mixing among the $0^{++}$ mesons around $1\sim 2$ $\mathrm{GeV}$ with the QCD sum rules

We calculate the correlation functions of $0^{++}$ $q\bar q$, $s\bar s$ and glueball in the QCD sum rules and obtain the mass matrix where non-diagonal terms are determined by the cross correlations among the three states. Diagonalizing the mass matrix and identifying the eigenstates as the physical $0^{++}$ scalar mesons, we can determine the mixing. Concretely, our calculations determine the fractions of $q\bar q$, $s\bar s$ and glueball in the physical states $f_0(1370)$, $f_0(1500)$ and $f_0(1710)$, the results are consistent with that gained by the phenomenological research.

hep-ph↗

arXiv2011

Earthquake Forecast via Neutrino Tomography

We discuss the possibility of forecasting earthquakes by means of (anti)neutrino tomography. Antineutrinos emitted from reactors are used as a probe. As the antineutrinos traverse through a region prone to earthquakes, observable variations in the matter effect on the antineutrino oscillation would provide a tomography of the vicinity of the region. In this preliminary work, we adopt a simplified model for the geometrical profile and matter density in a fault zone. We calculate the survival probability of electron antineutrinos for cases without and with an anomalous accumulation of electrons which can be considered as a clear signal of the coming earthquake, at the geological region with a fault zone, and find that the variation may reach as much as 3% for $\bar ν_e$ emitted from a reactor. The case for a $ν_e$ beam from a neutrino factory is also investigated, and it is noted that, because of the typically high energy associated with such neutrinos, the oscillation length is too large and the resultant variation is not practically observable. Our conclusion is that with the present reactor facilities and detection techniques, it is still a difficult task to make an earthquake forecast using such a scheme, though it seems to be possible from a theoretical point of view while ignoring some uncertainties. However, with the development of the geology, especially the knowledge about the fault zone, and with the improvement of the detection techniques, etc., there is hope that a medium-term earthquake forecast would be feasible.

hep-ph↗

arXiv2011

Determining the nuclear form factor for detection of dark matter in the relativistic mean field theory

In this work, we derive the nuclear form factor for the spin-independent collision between the WIMPs and nucleus in terms of the relativistic mean field (RMF) theory. Comparison with the traditional form factors which are commonly used in literature is given and it is found that our results are slightly above that of the 2PF model by 4% to 8%, but deviate from the Helm form factor by 15% to 25% for the whole recoil energy spectrum of 0$\sim $ 100 keV. Moreover, taking Xe and Ge as examples, we show the dependence of the form factor on the recoil energy.

hep-ph↗

arXiv2010

Re-study on the contribution of scalar potential and spectra of $c\bar c$, $b\bar b$ and $b\bar c(\bar b c)$ families

We indicated in our previous work that for QED the role of the scalar potential which appears at the loop level is much smaller than that of the vector potential and in fact negligible. But the situation is different for QCD, one reason is that the loop effects are more significant because $α_s$ is much larger than $α$, and secondly the non-perturbative QCD effects may induce a sizable scalar potential. In this work, we phenomenologically study the contribution of the scalar potential to the spectra of charmonia, bottomonia and $b\bar c(\bar b c)$ family. Taking into account both vector and scalar potentials, by fitting the well measured charmonia and bottomonia spectra, we re-fix the relevant parameters and test them by calculating other states of not only the charmonia, bottomonia, but also further the $b\bar c$ family. We also consider the Lamb shift of the spectra.

hep-ph↗

arXiv2010

Phenomenological study on the significance of the scalar potential and Lamb shift

We indicated in our previous work that for QED the contributions of the scalar potential which appears at the loop level is much smaller than that of the vector potential and in fact negligible. But the situation may be different for QCD, one reason is that the loop effects are more significant because $α_s$ is much larger than $α$, and secondly the non-perturbative QCD effects may induce the scalar potential. In this work, we phenomenologically study the contribution of the scalar potential to the spectra of charmonia. Taking into account both vector and scalar potentials, by fitting the well measured charmonia spectra, we re-fix the relevant parameters and test them by calculating other states of the charmonia family. We also consider the role of the Lamb shift and present the numerical results with and without involving the Lamb shift.

hep-ph↗

arXiv2010

$B^{()}\bar B^{()}$ intermediate state contribution to $Υ(4S,5S)\to η_b+γ$ radiative decay

In this work, we investigate the re-scattering effects in the radiative decay $Υ(5S)\toη_b+γ$, which were suggested to be crucially important for understanding the anomalous largeness of the branching ratios $B(Υ(5S)\to Υ(1S)+ππ)$ and $B(Υ(5S)\to Υ(1S)+η)$. Our calculations show that the re-scattering effects may enhance $Γ(Υ(10860)\to η_b+γ)$ by four orders, but the tetraquark structure does not. Recently the BaBar and CLEO collaborations have measured the mass of ${η_b}$ and the branching ratios $\mathcal{B}(Υ(2S)\rightarrowη_b+γ)$, $\mathcal{B}(Υ(3S)\rightarrowη_b+γ)$. We hope that very soon, $Υ(10860)\to η_b+γ$ will be measured and it would be an ideal opportunity for testing whether the re-scattering or the tetraquark structure is responsible for the anomaly of $\mathcal{B}\big(Υ(5S)\rightarrowΥ(nS)π^+π^- (n=1,2,3)\big)$, $i.e.$, the future measurements on the radiative decays of $Υ(5S)$ might be a touchstone of the two mechanisms.

hep-ph↗

arXiv2010

Re-Study on the wave functions of $Υ(nS)$ states in LFQM and the radiative decays of $Υ(nS)\to η_b+γ$

The Light-front quark model (LFQM) has been applied to calculate the transition matrix elements of heavy hadron decays. However, it is noted that using the traditional wave functions of the LFQM given in literature, the theoretically determined decay constants of the $Υ(nS)$ obviously contradict to the data. It implies that the wave functions must be modified. Keeping the orthogonality among the $nS$ states and fitting their decay constants we obtain a series of the wave functions for $Υ(nS)$. Based on these wave functions and by analogy to the hydrogen atom, we suggest a modified analytical form for the $Υ(nS)$ wave functions. By use of the modified wave functions, the obtained decay constants are close to the experimental data. Then we calculate the rates of radiative decays of $Υ(nS)\to η_b+γ$. Our predictions are consistent with the experimental data on decays $Υ(3S)\to η_b+γ$ within the theoretical and experimental errors.

hep-ph↗

arXiv2010

Determining the $η-η'$ mixing by the newly measured $BR(D(D_s)\toη(η')+\bar l+ν_l$

The mixing of $η-η'$ or $η-η'-G$ is of a great theoretical interest, because it concerns many aspects of the underlying dynamics and hadronic structure of pseudoscalar mesons and glueball. Determining the mixing parameters by fitting data is by no means trivial. In order to extract the mixing parameters from the available processes where hadrons are involved, theoretical evaluation of hadronic matrix elements is necessary. Therefore model-dependence is somehow unavoidable. In fact, it is impossible to extract the mixing angle from a unique experiment because the model parameters must be obtained by fitting other experiments. Recently $BR(D\toη+\bar l+ν_l)$ and $BR(D_s\toη(η')+\bar l+ν_l)$ have been measured, thus we are able to determine the $η-η'$ mixing solely from the semileptonic decays of D-mesons where contamination from the final state interactions is absent. Thus we hope that the model-dependence of the extraction can be somehow alleviated. Once $BR(D\toη'+\bar l+ν_l)$ is measured, we can further determine all the mixing parameters for $η-η'-G$. As more data are accumulated, the determination will be more accurate. In this work, we obtain the transition matrix elements of $D_{(s)}\to η^{(\prime)}$ using the light-front quark model whose feasibility and reasonability for such processes have been tested.

hep-ph↗

arXiv2010

The Simplest Dark-Matter Model, CDMS II Results, and Higgs Detection at LHC

The direct-search experiment for dark matter performed by the CDMS II Collaboration has observed two candidate events. Although these events cannot be interpreted as significant evidence for the presence of weakly interacting massive particle (WIMP) dark matter (DM), the total CDMS II data have led to an improved upper-limit on the WIMP-nucleon spin-independent cross-section. We study some implications of these results for the simplest WIMP DM model, the SM+D, which extends the standard model (SM) by the addition of a real SM-singlet scalar field dubbed darkon to play the role of the DM. We find that, although the CDMS II data rule out a sizable portion of parameter space of the model, a large part of the parameter space is still allowed. We obtain strong correlations among the darkon mass, darkon-nucleon cross-section, mass of the Higgs boson, and branching ratio of its invisible decay. We point out that measurements of the Higgs invisible branching-ratio at the LHC can lift some possible ambiguities in determining the darkon mass from direct DM searches.

hep-ph↗

arXiv2010

Constraints on Scalar Dark Matter from Direct Experimental Searches

The standard model (SM) plus a real gauge-singlet scalar field dubbed darkon (SM+D) is the simplest model possessing a weakly interacting massive particle (WIMP) dark-matter candidate. The upper limits for the WIMP-nucleon elastic cross-section as a function of WIMP mass from the recent XENON10 and CDMS-II experiments rule out darkon mass ranges from 10 to (50,70,75) GeV for Higgs-boson masses of (120,200,350) GeV, respectively. This may exclude the possibility of the darkon providing an explanation for the gamma-ray excess observed in the EGRET data. We show that by extending the SM+D to a two-Higgs-doublet model plus a darkon the experimental constraints on the WIMP-nucleon interactions can be circumvented due to suppression occurring at some values of the product tan(alpha)tan(beta), with alpha being the neutral-Higgs mixing angle and tan(beta) the ratio of vacuum expectation values of the Higgs doublets. We also comment on the implication of the darkon model for Higgs searches at the LHC.

hep-ph↗

arXiv2010

Understanding the branching ratios of χ_{c1}\toϕϕ, ωω, ωϕobserved at BES-III

In this work, we discuss the contribution of the mesonic loops to the decay rates of $χ_{c1}\to ϕϕ,\,ωω$ which are suppressed by the helicity selection rules and $χ_{c1}\to ϕω$ which is a double-OZI forbidden process. We find that the mesonic loop effects naturally explain the clear signals of $χ_{c1}\to ϕϕ,\,ωω$ decay modes observed by the BES collaboration. Moreover, we investigate the effects of the $ω-ϕ$ mixing which may result in the order of magnitude of the branching ratio $BR(χ_{c1} \to ωϕ)$ being $10^{-7}$. Thus, we are waiting for the accurate measurements of the $BR(χ_{c1} \to ωω)$, $BR(χ_{c1} \to ϕϕ)$ and $BR(χ_{c1} \to ωϕ)$, which may be very helpful for testing the long-distant contribution and the $ω-ϕ$ mixing in $χ_{c1}\toϕϕ,\,ωω,\,ωϕ$ decays.

hep-ph↗

arXiv2010

Study on the radiative decays of $Υ(nS)\to η_b+γ$

In this work, we investigate the characteristics of the spin-singlet state $η_b$ of the bottomonia family via the radiative decays of $Υ(nS)\to η_b+γ$. The theoretical estimation of the decay widths is carried out in terms of the light-front quark model (LFQM). Recently CLEO and BaBar collaborations have measured $\mathcal{B}(Υ(3S)\toγη_b)$ and the mass of ${η_b}$. In terms of the data we fix the concerned input parameters in our calculations of $Υ(nS)\to η_b+γ$. A special attention is paid on the transition of $Υ(5S)\to η_b+γ$. The BELLE data showed that the width of $Υ(5S)\to Υ(2S,1S)+ππ$ is two orders larger than that of $Υ(4S)\to Υ(2S,1S)+ππ$, thus some theoretical explanations have been proposed. Among them, it is suggested the inelastic final state interaction (IFSI) $Υ(5S)\to B\bar B\to Υ(1S)+ππ$ may be a natural one. If so, a similar mechanism also applies to $Υ(5S)\to B^{(*)}\bar B^{(*)}\to η_b+γ$, the precise measurement would serve as a good test whether $Υ(5S)$ possess exotic components. Our calculation in the LFQM indicates that the rate of the direct process $Υ(5S)\toη_b+γ$ is not anomalous compared to $Υ(mS)\toη_b+γ(m=1,2,3,4)$, thus if the IFSI does apply, the rate of $Υ(5S)\toη_b+γ$ should be larger than the others by orders.

hep-ph↗

arXiv2010

The magnetic dipole transitions in the $(c\bar{b})$ binding system

The magnetic dipole transitions between the vector mesons $B_c^*$ and their relevant pseudoscalar mesons $B_c$ ($B_c$, $B_c^*$, $B_c(2S)$, $B_c^*(2S)$, $B_c(3S)$ and $B_c^*(3S)$ etc, the binding states of $(c\bar{b})$ system) of the $B_c$ family are interesting. To see the `hyperfine' splitting due to spin-spin interaction is an important topic for understanding the spin-spin interaction and the spectrum of the the $(c\bar{b})$ binding system. The knowledge about the magnetic dipole transitions is also very useful for identifying the vector boson $B_c^*$ mesons experimentally, whose masses are just slightly above the masses of their relevant pseudoscalar mesons $B_c$ accordingly. Considering the possibility to observe the vector mesons via the transitions at $Z^0$ factory and the potentially usages of the theoretical estimate on the transitions, we fucus our efforts on calculating the magnetic dipole transitions, i.e. precisely to calculate the rates for the transitions such as decays $B_c^*\to B_cγ$ and $B_c^*\to B_c e^+e^-$, and particularly work in the Behte-Salpeter framework. In the estimate, as a typical example, we carefully investigate the dependance of the rate $Γ(B_c^*\to B_cγ)$ on the mass difference $ΔM=M_{B_c^*}-M_{B_c}$ as well.

hep-ph↗

arXiv2010

Search for $B_c(ns)$ via the $B_c(ns)\to B_c(ms)π^+π^-$ transition at LHCb and $Z_0$ factory

It is interesting to study the characteristics of the whole family of $B_c$ which contains two different heavy flavors. LHC and the proposed $Z^0$ factory provide an opportunity because a large database on the $B_c$ family will be achieved. $B_c$ and its excited states can be identified via their decay modes. As suggested by experimentalists, $B_c^*(ns)\to B_c+γ$ is not easy to be clearly measured, instead, the trajectories of $π^+$ and $π^-$ occurring in the decay of $B_c(ns)\to B_c(ms)+π^+π^-$ ($n>m$) can be unambiguously identified, thus the measurement seems easier and more reliable, therefore this mode is more favorable at early running stage of LHCb and the proposed $Z^0$ factory. In this work, we calculate the rate of $B_c(ns)\to B_c(ms)+π^+π^-$ in terms of the QCD multipole-expansion and the numerical results indicate that the experimental measurements with the luminosity of LHC and $Z^0$ factory are feasible.

hep-ph↗

arXiv2009

Evaluating decay Rates and Asymmetries of $Λ_b$ into Light Baryons in LFQM

In this work we calculate the branching ratios of semi-leptonic and non-leptonic decays of $Λ_b$ into light baryons ($p$ and $Λ$), as well as the measurable asymmetries which appear in the processes, in the light front quark model (LFQM). In the calculation, we adopt the diquark picture and discuss the justifiability of applying the picture in our case. Our result on the branching ratio of $Λ_b\toΛ+J/ψ$ is in good agreement with data. More predictions are made in the same model and the results will be tested in the future experiments which will be conducted at LHCb and even ILC.

hep-ph↗

arXiv2009

Symmetry of Dirac Equation and Corresponding Phenomenology

It has been suggested that the high symmetries in the Schrödinger equation with the Coulomb or harmonic oscillator potentials may remain in the corresponding relativistic Dirac equation. If the principle is correct, in the Dirac equation the potential should have a form as ${(1+β)\over 2}V(r)$ where $V(r)$ is ${-e^2\over r}$ for hydrogen atom and $κr^2$ for harmonic oscillator. However, in the case of hydrogen atom, by this combination the spin-orbit coupling term would not exist and it is inconsistent with the observational spectra of hydrogen atom, so that the symmetry of SO(4) must reduce into SU(2). The governing mechanisms QED and QCD which induce potential are vector-like theories, so at the leading order only vector potential exists. However, the higher order effects may cause a scalar fraction. In this work, we show that for QED, the symmetry restoration is very small and some discussions on the symmetry breaking are made. At the end, we briefly discuss the QCD case and indicate that the situation for QCD is much more complicated and interesting.

hep-ph↗