arXiv subjects

Xue-Qian Li

Publications and source records attributed to Xue-Qian Li.

At least 127 records · Page 7Linked to original sources

arXiv2007

The transition form factors for semi-leptonic weak decays of $J/ψ$ in QCD sum rules

Within the Standard Model, we investigate the semi-leptonic weak decays of $J/ψ$. The various form factors of $J/ψ$ transiting to a single charmed meson ($D^{(*)}_{(d,s)}$) are studied in the framework of the QCD sum rules. These form factors fully determine the rates of the weak semi-leptonic decays of $J/ψ$ and provide valuable information about the non-perturbative QCD effects. Our results indicate that the decay rate of the semi-leptonic weak decay mode $J/ψ\to D^{(*)-}_{s}+e^{+}+ν_{e}$ is at order of $10^{-10}$.

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arXiv2007

Constraints on Unparticle Interactions from Particle and Antiparticle Oscillations

We study unparticle effects on particle and antiparticle osillations in meson-antimeson, and muonium-antimuonium systems. Unlike usual tree level contributions to meson oscillations from heavy particle exchange with small $Γ_{12}$, the unparticle may have sizeable contributions to both $M_{12}$ and $Γ_{12}$ due to fractional dimension $d_\U$ of the unparticle. We find that very stringent constraints on the unparticle and particle interactions can be obtained. If unparticle effect dominates the contributions (which may happen in $D^0-\bar D^0$ mixing) to meson mixing parameters $x$ and $y$, we find that $x/y =\cot(πd_\U)$. Interesting constraints on unparticle and particle interactions can also be obtained using muonion and antimuonion oscillation data. We also comment on unparticle effects on CP violation in meson oscillations.

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arXiv2007

Analysis on Heavy Quarkonia Transitions with Pion Emission in Terms of the QCD Multipole Expansion and Determination of Mass Spectra of Hybrids

One of the most important tasks in high energy physics is search for the exotic states, such as glueball, hybrid and multi-quark states. The transitions $ψ(ns)\to ψ(ms)+ππ$ and $Υ(ns)\to Υ(ms)+ππ$ attract great attentions because they may reveal characteristics of hybrids. In this work, we analyze those transition modes in terms of the theoretical framework established by Yan and Kuang. It is interesting to notice that the intermediate states between the two gluon-emissions are hybrids, therefore by fitting the data, we are able to determine the mass spectra of hybrids. The ground hybrid states are predicted as 4.23 GeV (for charmonium) and 10.79 GeV (for bottonium) which do not correspond to any states measured in recent experiments, thus it may imply that very possibly, hybrids mix with regular quarkonia to constitute physical states. Comprehensive comparisons of the potentials for hybrids whose parameters are obtained in this scenario with the lattice results are presented.

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arXiv2007

New interpretation of matter-antimatter asymmetry based on branes and possible observational consequences

Motivated by the AMS project, we assume that after the Big Bang or inflation epoch, antimatter was repelled onto one brane which is separated from our brane where all the observational matter resides. It is suggested that CP may be spontaneously broken, the two branes would correspond to ground states for matter and antimatter respectively. Generally a complex scalar field which is responsible for the spontaneous CP violation, exists in the space between the branes and causes a repulsive force against the gravitation. A possible potential barrier prevents the mater(antimatter) particles to enter the space between two branes. However, by the quantum tunnelling, a sizable anti-matter flux may come to our brane. In this work by considering two possible models, i.e. the naive flat space-time and Randall-Sundrum models and using the observational data on the visible matter in our universe as inputs, we derive the antimatter flux which would be observed by the AMS detector.

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arXiv2007

Unparticle Physics Effects on D0-anti-D0 Mixing

The mixing of $K^0-\bar{K^0}$, $D^0-\bar{D^0}$ and $B_{(s)}^0-\bar{B^0_{(s)}}$ provides a sensitive probe to explore new physics beyond the Standard Model. The scale invariant unparticle physics recently proposed by Georgi can induce flavor-changing neutral current and contribute to the mixing at tree level. We investigate the unparticle effects on $B^0-\bar{B^0}$ and $D^0-\bar{D^0}$ mixing. Especially, the newly observed $D^0-\bar{D^0}$ mixing sets the most stringent constraints on the coupling of the unparticle to quarks.

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arXiv2007

Comprehensive Calculations on the OZI-forbidden Nonleptonic Decays of Orthoquarkonia $J/ψ(Υ)\to ππ,ρπ$

In this work, we calculate the decay rates of the OZI-forbidden processes $J/ψ(Υ)\to ππ, ρπ$ at the order of the leading-twist distribution amplitude. The process of $J/ψ(Υ)\to π^+ π^-$ violates isospin conservation and the amplitude is explicitly proportional to the isospin violation factor $m_u-m_d$, our numerical results on their decay rates are consistent with the data. The process $J/ψ(Υ)\to ρπ$ violates the hadronic helicity conservation and should be suppressed, as indicated in literature, its decay rate can only be proportional to $m_q^2$ at the order of leading twist. Our theoretical evaluation confirms this statement that the theoretical evaluation on $Γ(J/ψ(Υ)\to ρπ)$ is almost one order smaller than the data unless the model parameters take certain extreme values. It may imply that the sizable branching ratio of $J/ψ(Υ)\to ρπ$ should be explained by either higher twist contributions or other mechanisms.

hep-ph↗

arXiv2007

$Λ_{c}^{+}(2940)$: A Possible Molecular State?

A new baryonic state $Λ_{c}(2940)^{+}$ has recently been discovered by the Babar collaboration in the $D^{0}p$ channel. Later Belle collaboration also observed this state in the $Σ_c(2455)^{0,++} π^{\pm}\to Λ_{c}^{+}π^{+}π^{-}$ channel. The mass of $Λ_{c}(2940)^{+}$ is just a few MeV below the sum of $D^{*0}$ and $p$ masses suggesting a possibility that this state may be a $D^{*0} p$ molecular state. In this paper we study whether such a molecular state can be consistent with data. We find that the molecular structure can explain data and that if $Λ_c(2940)^+$ is a $D^{*0} p$ molecular state it is likely a $1/2^-$ state. Several other decays modes are also suggested to further test the molecular structure of $Λ_c^+(2940)$.

hep-ph↗

arXiv2007

Lifetime of doubly charmed baryons

In this work, we evaluate the lifetimes of the doubly charmed baryons $Ξ_{cc}^{+}$, $Ξ_{cc}^{++}$ and $Ω_{cc}^{+}$. We carefully calculate the non-spectator contributions at the quark level where the Cabibbo-suppressed diagrams are also included. The hadronic matrix elements are evaluated in the simple non-relativistic harmonic oscillator model. Our numerical results are generally consistent with that obtained by other authors who used the diquark model. However, all the theoretical predictions on the lifetimes are one order larger than the upper limit set by the recent SELEX measurement. This discrepancy would be clarified by the future experiment, if more accurate experiment still confirms the value of the SELEX collaboration, there must be some unknown mechanism to be explored.

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arXiv2007

Baryon spectra and non-strange baryon strong decays in the chiral SU(3) quark model

In the framework chiral SU(3) quark model, the baryon spectra within the band of $N\leq 2$ are studied, and the effect of the confining potential in different configurations, namely the $Δ$-mode and Y-mode are compared. In the same way, the baryon spectra in the extended chiral SU(3) quark model, in which additional vector meson exchanges are introduced, are also calculated. It is shown that a reasonable baryon spectrum in the chiral SU(3) quark model can be achieved no matter whether the $Δ$-mode or the Y-mode confining potential is employed. In the extended chiral SU(3) quark model, several energy levels are further improved. The resultant binding energies of excited baryon states in different confining modes deviate just by a few to several tens MeV, and it is hard to justify which confining mode is the dominant one. The non-strange baryon strong decay widths are further discussed in the point-like meson emission model by using the wave-function obtained in the spectrum calculation. The resultant widths can generally explain the experimental data but still cannot distinguish which confining mode is more important in this simple decay mode.

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arXiv2007

Scalar Dark Matter Effects in Higgs and Top Quark Decays

We study possible observational effects of scalar dark matter, the darkon D, in Higgs h and top quark t decay processes, h->DD and t->cDD in the minimal Standard Model (SM) and its two Higgs doublet model (THDM) extension supplemented with a SM singlet darkon scalar field D. We find that the darkon D can have a mass in the range of sub-GeV to several tens of GeV, interesting for LHC and ILC colliders, to produce the required dark matter relic density. In the SM with a darkon, t->cDD only occurs at loop level giving a very small rate, while the rate for Higgs decay h->DD can be large. In THDM III with a darkon, where tree level flavor changing neutral current (FCNC) interaction exists, a sizable rate for t->cDD is also possible.

hep-ph↗

arXiv2007

Diquarks and the Semi-Leptonic Decay of $Λ_{b}$ in the Hybrid Scheme

In this work we use the heavy-quark-light-diquark picture to study the semileptonic decay $Λ_b \to Λ_c+l+\barν_l$ in the so-called hybrid scheme. Namely, we apply the heavy quark effective theory (HQET) for larger $q^2$ (corresponding to small recoil), which is the invariant mass square of $l+\barν$, whereas the perturbative QCD approach for smaller $q^2$ to calculate the form factors. The turning point where we require the form factors derived in the two approaches to be connected, is chosen near $ρ_{cut}=1.1$. It is noted that the kinematic parameter $ρ$ which is usually adopted in the perturbative QCD approach, is in fact exactly the same as the recoil factor $ω=v\cdot v'$ used in HQET where $v$, $v'$ are the four velocities of $Λ_b$ and $Λ_c$ respectively. We find that the final result is not much sensitive to the choice, so that it is relatively reliable. Moreover, we apply a proper numerical program within a small range around $ρ_{cut}$ to make the connection sufficiently smooth and we parameterize the form factor by fitting the curve gained in the hybrid scheme. The expression and involved parameters can be compared with the ones gained by fitting the experimental data. In this scheme the end-point singularities do not appear at all. The calculated value is satisfactorily consistent with the data which is recently measured by the DELPHI collaboration within two standard deviations.

hep-ph↗

arXiv2007

Annihilation Contributions and CP Asymmetries in $B^+ \to π^+ K^0, K^+ K^0$ and $B^0\to K^0 \bar K^0$

Recently the branching ratios for $B^+\to K^+\bar K^0$ and $B^0 \to K^0 \bar K^0$ have been measured. Data indicate that the annihilation amplitudes in these decays are not zero. A non-zero annihilation amplitude plays an important role in CP violation for $B^+\to π^+ K^0, K^+ \bar K^0$. Using the measured branching ratios for these decays, we show that there is an absolute bound of 5% for the size of CP asymmetry in $B^+\to π^+ K^0$ from a relation between the amplitudes of these decays. The size of CP asymmetry in $B^+ \to K^+\bar K^0$ can, however, be as large as 90%. Future experimental data will test these predictions.

hep-ph↗

arXiv2006

Study on the Form Factors at Effective Vertices of Diqarks Interacting with Gauge Bosons

The diquark structure in baryons is commonly accepted as a reasonable approximation which can much simplify the picture and reduce the length of calculations. However, a diquark by no means is a point-like particle, even though it is treated as a whole object. Therefore, to apply the diquark picture to the phenomenological calculations, at the effective vertices for the diquark-gauge boson interactions, suitable form factors must be introduced to compensate the effects caused by the inner structure of the diquark. It is crucial to derive the appropriate form factors for various interactions. In this work, we use the Bethe-Salpeter equation to derive such form factors and numerically evaluate their magnitudes.

hep-ph↗

arXiv2006

Calculation of the Branching Ratio of $B^{-}\to h_{c}+K^{-}$ in PQCD

The branching ratio of $B^-\to h_c+K^-$ is re-evaluated in the PQCD approach. In this theoretical framework all the phenomenological parameters in the wavefunctions and Sudakov factor are priori fixed by fitting other experimental data, and in the whole numerical computations we do not introduce any new parameter. Our results are consistent with the upper bounds set by the Babar and Belle measurements.

hep-ph↗

arXiv2006

Detecting the ambient neutralino dark matter particles at accelerator

In this work, we present a new strategy to investigate the possibility of direct detection of the ambient neutralino matter at accelerator. We calculate the cross sections for both elastic and inelastic scattering processes of the dark matter particles with the beam particles at $e^+e^-$ and hadron colliders.

hep-ph↗

arXiv2006

Calculation of BR($\bar{B}^0\to Λ_{c}^++\bar p)$ in the PQCD Approach

We calculate the branching ratio of $\bar{B}^0 \to Λ_c^+ \bar{p}$ in the PQCD approach. Most previous model calculations obtained branching ratios significantly larger than experimental data. We find that the predicted branching ratio for BR$(\bar{B}^0 \to Λ_c^+ \bar{p})$ in the PQCD approach can vary over a range of $(2.3\sim 5.1)\times 10^{-5}$ with the largest uncertainty coming from the parameters in the wave function of $Λ_c$. With the favored values for the parameters in the $Λ_c^+$ wave function, $β= 1$ GeV and $m_q = 0.3$ GeV, the branching ratio is about $2.3\times 10^{-5}$ which is satisfactorily consistent with the value measured by experiments.

hep-ph↗

arXiv2006

Some properties of the newly observed X(1835) state at BES

Recently the BES collaboration has announced observation of a resonant state in the $π^+π^- η'$ spectrum in $J/ψ\to γπ^+π^-η'$ decay. Fitting the data with a $0^{-+}$ state, the mass is determined to be 1833.7 MeV with $7.7σ$ statistic significance. This state is consistent with the one extracted from previously reported $p \bar p$ threshold enhancement data in $J/ψ\to γp \bar p$. We study the properties of this state using QCD anomaly and QCD sum rules assuming X(1835) to be a pseudoscalar and show that it is consistent with data. We find that this state has a sizeable matrix element $<0|G\tilde G|G_p>$ leading to branching ratios of $(2.61\sim 7.37)\times 10^{-3}$ and $(2.21\sim 10.61)\times 10^{-2}$ for $J/ψ\to γG_p$ and for $G_p \to π^+π^- η'$, respectively. Combining the calculated branching ratio of $J/ψ\to γG_p$ and data on threshold enhancement in $J/ψ\to γp \bar p$, we determine the coupling for $G_p- p-\bar p$ interaction. We finally study branching ratios of other $J/ψ\to γ+ {three mesons}$ decay modes. We find that $J/ψ\to γG_p \to γ(π^+π^- η, K K π^0)$ can provide useful tests for the mechanism proposed.

hep-ph↗

arXiv2006

Study on contributions of hadronic loops to decays of $J/ψ\to$ vector $+$ pseudoscalar mesons

In this work, we evaluate the contributions of the hadronic loops to the amplitudes of $J/ψ\to PV$ where $P$ and $V$ denote light pseudoscalar and vector mesons respectively. By fitting data of two well measured channels of $J/ψ\to PV$, we obtain the contribution from the pure OZI process to the amplitude which is expressed by a phenomenological quantity $|\mathcal{G}^{PV}_{S}|$, and a parameter $α$ existing in the calculations of the contribution of hadronic loops. In terms of $α$ and $|\mathcal{G}^{PV}_{S}|$, we calculate the branching ratios of other channels and get results which are reasonably consistent with data. Our results show that the contributions from the hadronic loops are of the same order of magnitude as that from the OZI processes and the interference between the two contributions are destructive. The picture can be applied to study other channels such as PP or VV of decays of $J/ψ$.

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