arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 109 records · Page 6Linked to original sources

arXiv2009

Whether new data on $D_s\to f_0(980) e^+ ν_e$ can be understood if $f_0(980)$ consists of only the conventional $q\bar{q}$ structure

Only two isospin-singlet scalar mesons $f_0(600)$ ($σ$) and $f_0(980)$ exist below 1 GeV, so that it is natural to suppose that they are two energy eigenstates which are mixtures of ${1\over\sqrt 2}(u\bar u+d\bar d)$ and $s\bar s$. Is this picture right? Generally, it is considered that $f_0(600)$ mainly consists of ${1\over\sqrt 2}(u\bar u+d\bar d)$, if so, the dominant component of $f_0(980)$ should be $s\bar s$. The recent measurement of the CLEO collaboration on the branching ratio of $D_s\to f_0(980) e^+ ν_e$ provides an excellent opportunity to testify the structure of $f_0(980)$, namely whether the data can be understood as long as it consists of mainly the conventional $q\bar q$ structure. We calculate the form factors of $D_s\to f_0(980)$ in the light-front quark model (LFQM) and the corresponding branching ratio of the semileptonic decay. By fitting the data, we obtain the mixing angle $ϕ$. The obtained mixing angle shows that the $s\bar s$ component in $f_0(980)$ may not be dominant.

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arXiv2009

Probing unparticle theory via lepton flavor violating process $J/ψ\to ll'$ at BESIII

The lepton flavor violating process $J/ψ\to ll' (l\neq l')$ serves as an ideal place to probe the unparticle theory. Such process can only occur at loop level in the Standard model (SM), so that should be very suppressed, by contrast in unparticle scenario, it happens at tree level and its contribution may be sizable for practical measurement. Moreover, the BESIII will offer the largest database on $J/ψ$ which makes more accurate measurements possible. Furthermore, for such purely leptonic decays background is relatively low and signal would be cleaner. Our work carefully investigates the possibility of observing such processes from both theoretical and experimental aspects.

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arXiv2009

The puzzle of excessive non-$D\bar D$ component of the inclusive $ψ(3770)$ decay and the long-distant contribution

In this letter we suggest that the obvious discrepancy between theoretical prediction on the $\mathrm{non}-D\bar D$ decays of $ψ(3770)$ and data is to be alleviated by taking final state interaction (FSI) into account. By assuming that $ψ(3770)$ overwhelmingly dissociates into $D\bar D$, then the final state interaction induces a secondary process, we calculate the branching ratios of $ψ(3770)\to D\bar D\to J/ψη, ρπ, ωη, K^*K$. Our results show that the branching ratio of $ψ(3770)\to \mathrm{non}-D\bar{D}$ can reach up to $\mathcal{B}_{\mathrm{non}-D\bar{D}}^{FSI}=(0.2\sim1.1)%$ while typical parameters $I=0.4$ GeV$^{-2}$ and $α=0.8\sim 1.3$ are adopted. This indicates that the FSI is obviously non-negligible.

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arXiv2009

Testing the Bell Inequality at Experiments of High Energy Physics

Besides using the laser beam, it is very tempting to directly testify the Bell inequality at high energy experiments where the spin correlation is exactly what the original Bell inequality investigates. In this work, we follow the proposal raised in literature and use the successive decays $J/ψ\toγη_c\to Λ\barΛ\to pπ^-\bar pπ^+$ to testify the Bell inequality. Our goal is twofold, namely, we first make a Monte-Carlo simulation of the processes based on the quantum field theory (QFT). Since the underlying theory is QFT, it implies that we pre-admit the validity of quantum picture. Even though the QFT is true, we need to find how big the database should be, so that we can clearly show deviations of the correlation from the Bell inequality determined by the local hidden variable theory. There have been some critiques on the proposed method, so in the second part, we suggest some improvements which may help to remedy the ambiguities indicated by the critiques. It may be realized at an updated facility of high energy physics, such as BES III.

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arXiv2009

Top-Higgs boson production via gluon fusion at LHC in the TC2 model

In our work: 0903.2612, we calculate the production rate of single top-Higgs boson in the TC2 model which is a modified version of the original top-technicolor model. The similar process was discussed in arXiv:hep-ph/9905347v2. The TC2 model, as we discussed in the introduction part remedies some shortcomings and loophole of the old version. The top-Higgs in the TC2 model is a mixture of the top-Higgs of the toptechnicolor model and that of the ETC model, thus a parameter $ε$ is introduced to denote the mixture. Moreover, we vary the mass range of the top-Higgs within 300 to 800 GeV while in arXiv:hep-ph/9905347v2, the mass range was taken as 200 to 400 GeV. In the work, our numerical results show that the production rate of single top-Higgs in the TC2 model is very close to that in the toptecnicolor model within the mass range of 200 to 400 GeV. This manifests that change from the original toptechnicolor model to the new TC2 version does not much affect the production rate of the top-Higgs even though the two top-Higgs in the two models are different. Beyond the 400 GeV, even the TC2 model predicts a negligible production rate at LHC. Since the phenomenological change is indeed not obvious, there is not much new to report. Even though the two models are somehow different, we believe that the result is not worth publishing. Therefore we decide to withdraw our manuscript.

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arXiv2008

What do we learn from the $ρ-π$ puzzle

The experimental observation indicates that the branching ratio of $ψ'\to ρπ$ is very small while the $ρ-π$ channel is a main one in $J/ψ$ decays. To understand the puzzle, various interpretations have been proposed. Meanwhile according to the hadronic helicity selection rule the decay mode $J/ψ\toρπ$ should be suppressed, but definitely, numerical computation is needed to determine how it is suppressed. We calculate the branching ratios corresponding to subprocesses $J/ψ\to ggg\toρπ$ and $\to ggg\toππ$ in the framework of QCD. The results show that the branching ratios are proportional to $({m_u+m_d\over M_{J/ψ}})^2$ for $ρπ$ mode and $({m_u-m_d\over M_{J/ψ}})^2$ for $ππ$ mode which is an isospin-violation channel. If only the OZI process is considered, the theoretical prediction on the ratio of $J/ψ\to ρπ$ is smaller than data, but not too drastically small. Meanwhile, a possible interpretation for the $ρπ$ puzzle is proposed that the suppression is due to interference between OZI and electromagnetic (EM) contributions. Thus based on this observation, we suggest that if the amplitudes of the strong OZI process via an s-channel three-gluon intermediate state and electromagnetic one via an s-channel virtual photon intermediate state have the same order of magnitude, and constructively interfere for $J/ψ\toρπ$, but destructively interfere for $ψ'\toρπ$, thus simultaneously the $ρπ$ puzzle disappears and the sizable width of $J/ψ\to\rhoı$ is understandable. However, so far, we cannot derive the phase difference from an underlying principle of QCD yet. Alternative interpretations are also discussed in the text.

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arXiv2008

Restudy on Dark Matter Time-Evolution in the Littlest Higgs model with T-parity

Following previous study, in the Littlest Higgs model (LHM), the heavy photon is supposed to be a possible dark matter candidate and its relic abundance of the heavy photon is estimated in terms of the Boltzman-Lee-Weinberg time-evolution equation. The effects of the T-parity violation is also considered. Our calculations show that when Higgs mass $M_H$ taken to be 300 GeV and don't consider T-parity violation, only two narrow ranges $133<M_{A_{H}}<135$ GeV and $167<M_{A_{H}}<169$ GeV are tolerable with the current astrophysical observation and if $135<M_{A_{H}}<167$ GeV, there must at least exist another species of heavy particle contributing to the cold dark matter. As long as the T-parity can be violated, the heavy photon can decay into regular standard model particles and would affect the dark matter abundance in the universe, we discuss the constraint on the T-parity violation parameter based on the present data. Direct detection prospects are also discussed in some detail.

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arXiv2008

FCNC-induced semileptonic decays of $J/ψ$ in the Standard Model

In this work, we calculate the form factors for $J/ψ\to \bar{D}^{(*)0}$ induced by the flavor changing neutral currents (FCNC) in terms of the QCD sum rules. Making use of these form factors, we further calculate the branching fractions of semileptonic decays $J/ψ\to \bar{D}^{(*)0} l ^+ l^-$ ($l=e, μ$). In particular, we formulate the matrix element $ $ with $T_{μν}$ being a tensor current, which was not fully discussed in previous literature. Our analysis indicates that if only the standard model (SM) applies, the production of single charmed mesons at the present electron-positron colliders is too small to be observed even the resonance effects are included, therefore if an anomalous production rates are observed, it would be a hint of new physics beyond SM. Even though the predicted branching ratios are beyond the reach of present facilities which can be seen from a rough order estimate, the more accurate formulation of the three point correlation function derived in this work has theoretical significance and the technique can also be applied to other places. In analog to some complicated theoretical derivations which do not have immediate phenomenological application yet, if the future experiments can provide sufficient luminosity and accuracy, the results would be helpful.

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arXiv2008

The NLO corrections of $H_{TC} Π^0$and $Π^ + Π^ - $ pair production at the ILC in the TC2 model

As well known, if the Higgs boson were not observed at LHC, the technicolor model would be the most favorable candidate responsible for the symmetry breaking. To overcome some defects in the previous model, some extended versions have been proposed. In the TC2 model typical signature is existence of heavy $H_{TC}$ and technipion $Π$. A direct proof of validity of the model is to produce them at accelerator. Thus we study the production rates of $ e^ + e^ - \to H_{TC} Π^0 $ and $e^ + e^ - \to Π^ + Π^ -$ at ILC in the topcolor-assisted technicolor (TC2) model. In fact, there is a flood of models belonging to new physics which can result in products with characteristics similar to $H_{TC}+Π$ of the TC2 model. Therefore to distinguish this model from others one may need to investigate some details by calculating the cross section to NLO. We indeed find that the NLO corrections are significant, namely the ratio $δ\equiv (σ_{NLO} - σ_{LO})/σ_{LO} $ in $ e^ + e^ - \to H_{TC} Π^0 $ exceeds 100% within a plausible parameter space.

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arXiv2008

Charm Physics - A Field Full with Challenges and Opportunities

In this review, we discuss some interesting issues in charm physics which is full with puzzles and challenges. So far in the field there exist many problems which have not obtained satisfactory answers yet and more unexpected phenomena have been observed at the present facilities of high energy physics. Charm physics may become an ideal place for searching new resonances and studying non-perturbative QCD effects, moreover probably is an area to explore new physics beyond the Standard Model. More data will be available at BESIII, B-factories, LHC and even future ILC which may open a wide window to a better understanding of the nature.

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arXiv2008

Contribution of final state interaction to the branching ratio of $B\to J/ψD$

To testify the validity of the perturbative QCD (pQCD) and investigate its application range, one should look for a suitable process to do the job. $B\to J/ψD$ is a promising candidate. The linear momentum of the products is relatively small, so that there may exist a region where exchanged gluons are soft and the perturbative treatment may fail, so that the non-perturbative effect would be significant. We attribute such non-perturbative QCD effects into the long-distance final state interaction (FSI) which is estimated in this work. We find that the contribution from the FSI to the branching ratio is indeed sizable and may span a rather wide range of $10^{-6}\sim 10^{-5}$, and cover a region where the pQCD prediction has the same order. A more accurate measurement on its branching ratio may provide important information about the application region of pQCD and help to clarify the picture of the inelastic rescattering (i.e. FSI) which is generally believed to play an important role in B decays.

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arXiv2008

Neutrino decay as a possible interpretation to the MiniBooNE observation with unparticle scenario

In a new measurement on neutrino oscillation $ν_μ\toν_e$, the MiniBooNE Collaboration observes an excess of electron-like events at low energy and the phenomenon may demand an explanation which obviously is beyond the oscillation picuture. We propose that heavier neutrino $ν_2$ decaying into a lighter one $ν_1$ via the transition process $ν_μ\to ν_e+X$ where $X$ denotes any light products, could be a natural mechanism. The theoretical model we employ here is the unparticle scenario established by Georgi. We have studied two particular modes $ν_μ\to ν_e+\Un$ and $ν_μ\to ν_e+\barν_e+ν_e$. Unfortunately, the number coming out from the computation is too small to explain the observation. Moreover, our results are consistent with the cosmology constraint on the neutrino lifetime and the theoretical estimation made by other groups, therefore we can conclude that even though neutrino decay seems plausible in this case, it indeed cannot be the source of the peak at lower energy observed by the MiniBooNE collaboration and there should be other mechanisms responsible for the phenomenon.

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arXiv2008

Double heavy-quarkonium production from electron-positron annihilation in the Bethe-Salpeter formalism

In this work we evaluate the cross section of the process $e^+e^-\to J/ψη_c$ at energy $\sqrt{s}\approx 10.6$ GeV in the Bethe-Salpeter formalism. To simplify our calculation, the heavy quark limit is employed. Without taking the beyond-leading-order contribution(s) into account, the cross section calculated in this scenario is comparable with the experimental data. We also present our prediction for the cross section of double bottomonium production $e^+e^-\to Υ(1S)η_b$ for the energy range of $\sqrt{s}\approx (25 \hbox{-} 30)$ GeV which may be experimentally tested, even though there is no facility of this range available at present yet.

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arXiv2008

Effects of hadronic loops on the direct CP violation of $B_{c}$

It is well known that the final state interaction plays an important role in the decays of $B$-meson. The contribution of the final state interaction which is supposed to be long-distance effects, to the concerned processes can interfere with that of the short-distance effects produced via the tree and/or loop diagrams at quark-gluon level. The interference may provide a source for the direct CP violation $\mathcal{A}_{CP}$ in the process $B_{c}^{+}\to D^{0}π^{+}$. We find that a typical value of $\mathcal{A}_{CP}$ when the final state interaction effect is taken into account can be about -22% which is different from that without the final state interaction effect. Therefore, when we extract information on CP violation from the data which will be available at LHCb and the new experiments in $B$-factories, the contribution from the final state interaction must be included. This study may be crucial for searching new physics in the future.

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arXiv2008

Weak decays of $J/ψ$: the non-leptonic case

In our previous study, we calculated the transition from factors of $J/ψ\to D^{(*)}_{(s)}$ using the QCD sum rules. Based on the factorization approximation, the obtained form factors can be applied to evaluate the weak non-leptonic decay rates of $J/ψ\to D^{(*)}_{(s)}+M$, where $M$ stands for a light pseudoscalar or vector meson. We predict that the branching ratio for inclusive non-leptonic two-body weak decays of $J/ψ$ which are realized via the spectator mechanism, can be as large as $1.3 \times 10^{-8}$, in particular, the branching ratio of $J/ψ\to D^{*\pm}_s+ρ^\mp$ can reach $5.3 \times 10^{-9}$. Such values will be marginally accessed by the ability of BESIII which will begin running very soon.

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arXiv2008

A Possibility of Search for New Physics at LHCb

It is interesting to search for new physics beyond the standard model at LHCb. We suggest that weak decays of doubly charmed baryon such as $Ξ_{cc}(3520)^+, Ξ_{cc}^{++}$ to charmless final states would be a possible signal for new physics. In this work, we consider two models, i.e. the unparticle and $Z'$ as examples to study such possibilities. We also discuss the cases for $Ξ^0_{bb}, Ξ_{bb}^-$ which have not been observed yet, but one can expect to find them when LHCb begins running. Our numerical results show that these two models cannot result in sufficiently large decay widths, therefore if such modes are observed at LHCb, there must be a new physics other than the unparticle or $Z'$ models.

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arXiv2008

Diquarks and Λ_{b}\toΛ_c weak decays

In this work we investigate the weak $Λ_{b}\toΛ_c$ semi-leptonic and non-leptonic decays. The light-front quark model and diquark picture for heavy baryons are adopted to evaluate the $Λ_{b}\toΛ_c$ transition form factors. In the heavy quark limit we study the Isgur-Wise function. By fitting the data of the semi-leptonic process we obtain the mass of the light scalar diquark as 817 MeV. The numerical predictions on the branching ratios of non-leptonic decay modes $Λ_{b}\toΛ_c M$ and various polarization asymmetries are made. A comparison with other approaches is discussed.

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arXiv2007

Equation of State for Neutralino Star as a Form of Cold Dark Matter

In order to study the structure of neutralino star and dark galaxy, we consider dynamical interactions due to boson-exchange in the neutralino matter. Taking into account interactions of neutralinos with bosons, we derive the equation of state (EOS) of neutralino stars in terms of the relativistic mean field approach. Then we apply the resulting EOS to investigate properties of the neutralino star such as its density profile and mass limit. For example, if the neutralino mass is around 1 TeV, the Oppenheimer mass limit of the neutralino star is obtained as $6.06\times 10^{-7}M_\odot$, and the corresponding radius is about 7.8 mm. Actually, due to an increasing annihilation rate as indicated by our calculation, this dense state can never be realized in practice. Our results also show that the low density neutralino star may be a possible aggregation of the cold dark matter.

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