To appear in the proceedings of the Joint Meeting Heidelberg-Liege-Paris-Wroclaw (HLPW08), Spa, Belgium, 6-8 March 2008; uses aip-6s.clo, aipproc.cls and aipxfm.sty (included)
The newly introduced Transition Distribution Amplitudes (TDAs) are discussed for the $π$-$γ$ transitions. Relations between $π$-$γ$ and $γ$-$π$ TDAs for different cases are given. Numerical values for the $π$-$γ$ TDAs in different models are compared. GPD's features are extended to TDAs and the role of PCAC highlighted. We give hints for the evaluation of cross sections for meson pair production in our approach.
We discuss the pion and kaon distribution amplitudes up to twist-3 in the context of the random instanton vacuum (RIV). We construct explicitly the pertinent quasi-pion and quasi-kaon distributions in the RIV, and analyze them in leading order in the diluteness factor, at a resolution fixed by the inverse instanton size. The distribution amplitudes (DA) follow from the large momentum limit. The results at higher resolution are discussed using QCD evolution, and compared to their asymptotic limits and some lattice results.
Roy's equations and the pion-pion experimental data
用Roy方程检验与实验数据拟合的标量-等差π介子-π介子散射振幅是否满足交叉对称条件。结果表明，振幅描述的“向下平坦”的相移解决方案满足交叉对称性低于1 GeV，而振幅拟合的“向上平坦”的数据不。这样，长期存在的“上下”的模糊性的唯象确定的标量等腰π介子-π介子振幅已得到解决，确认独立的结果，最近的联合分析的pi+pi-和pi 0 pi 0数据。
We compute the decay rate of dimuonium into a neutral pion and a photon. We find that approximately one in 10^5 ortho-dimuonia decays into this channel. We also determine the contribution of the virtual photon-pion loop to the hyperfine splitting in dimuonium and reproduce its leading effect in the anomalous magnetic moment of the muon.
We propose a model-independent parametrization for the one-pion-to-vacuum matrix elements of the vector and axial vector hadronic currents in the presence of an external uniform magnetic field. It is shown that, in general, these hadronic matrix elements can be written in terms of several gauge covariant Lorentz structures and form factors. Within this framework we obtain a general expression for the weak decay $π^- \to l\,\barν_l$ and discuss the corresponding limits of strong and weak external magnetic fields.
We analyse confining gauge theories where the 750 GeV di-photon resonance is a composite techni-pion that undergoes anomalous decays into SM vectors. These scenarios naturally contain accidentally stable techni-pions Dark Matter candidates. The di-photon resonance can acquire a larger width by decaying into Dark Matter through the CP-violating $θ$-term of the new gauge theory reproducing the cosmological Dark Matter density as thermal relic.
We present some results on the analysis of three pion resonances. The analyses are motivated by the recent release of the largest data set on diffractively produced three pions by the COMPASS collaboration. We construct reaction amplitudes that satisfy fundamental $S$-matrix principles, which allows the use of models that have physical constraints to be used in fitting data. The models are motivated by the isobar model that satisfy unitarity constraints. The model consist of a Deck production amplitude with which final state interactions are constrained by unitarity. We employ the isobar model…
Generalized Distribution Amplitudes at the $Z$ pole
我们研究了通过过程$e^+e^-\to Z \to ππγ$在$Z$ -极上独家产生的二π介子。在两个π介子的不变质量远小于$Z$玻色子质量的运动学区域，这个过程可以分解为一个硬系数和一个软矩阵元的卷积，即两个π介子的广义分布振幅。我们计算了带电π介子对产生和中性π介子对产生的微分截面，发现前者比后者大得多。我们表明，结合测量的带电π介子对生产和中性π介子对生产的$Z$极提供了一个方便的方法来访问的$C$奇数部分的$2π$分布振幅。
The pion-photon transition form factor is studied by employing two types of Sum Rules: Light Cone Sum Rules (LCSR) and Anomaly Sum Rules (ASR). By comparing the predictions for the pion-photon transition form factor, obtained from these two approaches, the applicability limit of the LCSRs at low momenta is determined. Reciprocally, the ASR threshold dependence on the momentum was extracted using our LCSR-based method in combination with two different types of pion distribution amplitudes and found that at higher Q2 it approaches a constant.
We analyzed two twist-3 distribution amplitudes of pion, i.e. pseudoscalar $ϕ^P_{3;π}(x)$ and pseudotensor $ϕ^σ_{3;π}(x)$, within the LFQM. Our LFQM descriptions both for twist-3 $ϕ^P_{3;π}$ and $ϕ^σ_{3;π}$ not only satisfy the fundamental constraint required from the isospin symmetry, but also reproduce exactly the asymptotic forms anticipated from QCD's conformal limit.
We use a simple, instanton motivated, nonlocal chiral quark model to calculate pion Generalized Distribution Amplitudes (GDAs). The nonlocality appears due to the momentum dependence of the constituent quark mass, which we take in a form of a generalized dipole formula. With this choice all calculations can be performed directly in the Minkowski space and the sensitivity to the shape of the cutoff function can be studied. We demonstrate that the model fulfills soft pion theorems both for chirally even and chirally odd GDAs. The latter one cannot be derived by the methods of current algebra. Wh…
We suggest to probe the pion light-cone distribution amplitude, applying a dispersion relation for the pion electromagnetic form factor. Instead of the standard dispersion relation, we use the equation between the spacelike form factor $F_π(Q^2)$ and the integrated modulus of the timelike form factor. For $F_π(Q^2)$, the QCD light-cone sum rule with a dominant twist-2 term is used. Adopting for the pion twist-2 distribution amplitude a certain combination of the first few Gegenbauer polynomials, it is possible to fit their coefficients $a_{2,4,6,...}$ (Gegenbauer moments) from this equation, e…
We compare the low-energy partial wave analyses $πN$ scattering with a high-energy data via finite energy sum rules. We construct a new set of amplitudes by matching the imaginary part from the low-energy analysis with the high-energy, Regge parametrization and reconstruct the real parts using dispersion relations.
