孤子局域共振激发的一种物理模型

作为非线性领域的重要内容,孤子自从被发现以来就得到了广泛的关注。虽然目前关于孤子的传播及其相互作用性质等方面的研究已经相当成熟,但是对于孤子的产生问题仍然没有系统的认识。孤子产生问题的深入研究,不仅对于理解自然界及物理系统中的非线性现象十分重要,对孤子现象的工程应用也极为重要。基于这些原因,一些孤子激发方法的研究不断被提出。在非线性阵列带隙中的能量超透射现象激发孤子理论的基础上,局域共振孤子激发的方法被提出。该方法通过在半无限长β-FPU链中加入缺陷来引入局域共振机制,不仅有效降低了孤子激发的临界驱动振幅,还实现了孤子激发的可控操作。然而,虽然理论分析与数值模拟都证明了这一方法的可行性,但关于该方法的相关实验验证还是空白。为了进一步将该方法推向实际实验,提出了一种简易可行的实验方案,并做了深入的理论分析与大量的数值模拟。该方案给出一种物理模型,该模型以线性弹簧为基本元件,利用结构的几何非线性来构建β-FPU链,并以耦合摆阵列来实现。通过大量数值模拟证明了该方法的可行,同时研究了孤子释放周期与驱动频率、振幅的关系,结果与前述理论一致。此外,还通过数值模拟研究了孤子的激发周期与缺陷处阻尼大小的关系,以及单元质量不均匀性对孤子激发与传播的影响,为进一步实验提供依据。

Role of Electromagnetic Dipole Operator in the Electroweak Penguin Dominated Vector Meson Decays of B Meson

The pure annihilation type decays Dd^0→Фγand Bs→ργ receive only colour suppressed penguin contributions with a very small branching ratio in the standard model. When we include the previously neglected electromagnetic dipole operator, the branching ratios can be enhanced to R（^-Bd^0→Фγ）≈ 1×10^-11 and R（^-Bs→ργ）～（6 - 16） ×10^-10, which are one order magnitude larger than previous study using the QCD factorization approach. The new effect can also give a large contribution, of order 10^-9, to transverse polarization of B→Фρ and B →ωФ, which is comparable to the longitudinal part. These effects can be detected in the LHCb experiment and the super-B factories.

Effects from hadronic structure of photon on B→φγ and B_(s)→(ρ^(0),ω)γ decays

Using the perturbative QCD approach,we studied the effects of the hadronic structure of photons on the pure annihilation rediative decays B→φγ and B_(s)→(ρ^(0),ω)γ.These decays have small branching fractions due to the power suppression by Λ/mB,which makes them very sensitive to next-leading power corrections.The quark components and the related two-particle distribution amplitudes of a final state photon are introduced.The branching fractions can be enhanced remarkably by factorizable and nonfactorizable emission diagrams.The branching fraction of B→φγ increases by approximately 40 times,and those of B_(s)→ρ^(0)γ and B_(s)→ωγ are on the order of O(10-10).We also note that the ratio of branching fractions of B_(s)→ρ^(0)γ and B_(s)→ωγ is very sensitive to the effects of the hadronic structure of photons.All these results can be tested in future.

Revisiting radiative leptonic B decay

In this paper,we summarize the existing methods of solving the evolution equation of the leading-twist B-meson LCDA.Then,in the Mellin space,we derive a factorization formula with next-to-leading-logarithmic(NLL)resummation for the form factors FA,V in the B→γℓνdecay at leading power inΛ/mb.Furthermore,we investigate the power suppressed local contributions,factorizable non-local contributions(which are suppressed by 1/Eγand 1/mb),and soft contributions to the form factors.In the numerical analysis,which employs the two-loop-level hard function and the jet function,we find that both the resummation effect and the power corrections can sizably decrease the form factors.Finally,the integrated branching ratios are also calculated for comparison with future experimental data.

Exclusive production of B_c mesons in e^+e^- colliders

Within the framework of the perturbative QCD approach,we calculate the time-like B（c ）Bc（Bc＊）form factors F（Q^2）and A2（Q^2）.We include relativistic corrections and QCD corrections,either of which can give about20%correction to the leading-order contribution,but there are cancellation effects between them.We calculate the cross sections of the e^＋e^-→Bc-Bc＋（Bc＊＋）processes.The cross sections are enhanced at the Z pole to beσ^（P P）（Q=m Z）～1.3×10^-5pb andσ^（P V）（Q=m Z）～2.5×10^-5pb,which are still too small to be detected by proposed e~＋e^-colliders such as the Circular Electron Positron Collider.

Λ_(b)→Λ_(c) form factors from QCD light-cone sum rules

In this study,we calculate the transition form factors of Λ_(b) decaying into Λ_(c) within the framework of light-cone sum rules with the distribution amplitudes(DAs)of the Λ_(b)-baryon.In the hadronic representation of the correlation function,we isolate both the Λ_(c) and Λ_(c)^(∗) states so that the Λ_(b)→Λ_(c) form factors can be obtained without ambiguity.We investigate the P-type and A-type currents to interpolate light baryons for comparison because the interpolation current for the baryon state is not unique.We also employ three parametrization models for the DAs of Λ_(b) in the numerical calculation.We present the numerical predictions for the Λ_(b)→Λ_(c) form factors and branching fractions,averaged forward-backward asymmetry,averaged final hadron polarization,and averaged lepton polarization of the Λ_(b)→Λ_(c)ℓμ decays,as well as the ratio of the branching ratios RΛ_(c).The predicted RΛ_(c) is consistent with LHCb data.

Weak decays of doubly heavy baryons:"decay constants"

Inspired by the recent observation of the ∑c^＋c^＋ by the LHCb Collaboration, we explore the ＂decay constants＂ of doubly heavy baryons in the framework of QCD sum rules. With the ∑cc,∑bb, and Ωcc,Ωbc,Ωbb baryons interpolated by three-quark operators, we calculate the correlation functions using the operator product expansion and include the contributions from operators up to dimension six. On the hadron side, we consider both contributions from the lowest-lying states with J^P 1/2^＋ and from negative parity baryons with JP=1/2^-. We find that the results are stable and the contaminations from negative parity baryons are not severe. These results are ingredients for the QCD study of weak decays and other properties of doubly-heavy baryons.