Two-loop contribution to the pion transition form factor vs. experimental data
S. V. Mikhailov, N. G. Stefanis
备注：
5 pages, 2 figures, 3 tables. Presented by the first author at 3rd joint International HADRON STRUCTURE'09 Conference at Tatranska Strba, Slovakia, August 30th - September 3rd, 2009. v2 Eqs. (9), (10) corrected; wrong reference tag removed. v3 Fig. 2 and Ref. [13] corrected
We present predictions for the pion-photon transition form factor, derived with the help of light-cone sum rules and including the main part of the NNLO radiative corrections. We show that, when the Bakulev-Mikhailov-Stefanis (BMS) pion distribution amplitude is used, the obtained predictions agree well with the CELLO and the CLEO data. We found that no model distribution amplitude can reproduce the observed $Q^2$ growth of the new BaBar data, though the BMS model complies with several BaBar data points.
The Kroll-Lee-Zumino renormalizable Abelian quantum field theory of pionic strong interactions is used to compute the rho-meson propagator at the two-loop level.
We report on the fully analytic calculation of the leading-power contribution to the photon-pion transition form factor $γ\, γ^{\ast} \to π^0$ at two loops in QCD. The applied techniques are based on hard-collinear factorization, together with modern multi-loop methods. We focus both, on the technical details such as the treatment of evanescent operators, and the phenomenological implications. Our results indicate that the two-loop correction is numerically comparable to the one-loop effect in the same kinematic domain. We also demonstrate that our results will play a key role in disentangling…
Restrictions on the Hadronic Contribution to the Muon $(g-2)$-factor and to the Pion Electromagnetic Formfactor from Analytical Properties of the Pion Electromagnetic Formfactor
We calculate the width of the delta resonance at leading two-loop order in baryon chiral perturbation theory. This gives a correlation between the leading pion-nucleon-delta and pion-delta couplings, which is relevant for the analysis of pion-nucleon scattering and other processes.
It is shown that the basic shape of dipion mass distributions in the two-pion transitions of both charmonia and bottomonia states are explained by an unified mechanism based on the contribution of the $ππ$, $K\overline{K}$ and $ηη$ coupled channels including their interference.
We use the Polyakov-loop extended two-flavor quark-meson model as a low-energy effective model for QCD to study the phase diagram in the $μ_I$--$T$ plane where $μ_I$ is the isospin chemical potential. In particular, we focus on the Bose condensation of charged pions. At $T=0$, the onset of pion condensation is at $μ_I={1\over2}m_π$ in accordance with exact results. The phase transition to a Bose-condensed phase is of second order for all values of $μ_I$ and in the $O(2)$ universality class. The chiral critical line joins the critical line for pion condensation at a point whose position depends…
We accomplish the complete two-loop computation of the leading-twist contribution to the photon-pion transition form factor $γ\, γ^{\ast} \to π^0$ by applying the hard-collinear factorization theorem together with modern multi-loop techniques. The resulting predictions for the form factor indicate that the two-loop perturbative correction is numerically important. We also demonstrate that our results will play a key role in disentangling various models of the twist-two pion distribution amplitude thanks to the envisaged precision at Belle II.
I present higher loop order results for several calculations in Chiral perturbation Theory. 1) Two-loop results at finite volume for hadronic vacuum polarization. 2) A three-loop calculation of the pion mass and decay constant in two-flavour ChPT. For the pion mass all needed auxiliary parameters can be determined from lattice calculations of $ππ$-scattering. 3) Chiral corrections to neutron-anti-neutron oscillations.
A detailed discussion of the long-range one-pion exchange (Yukawa potential) contribution to the 2S hyperfine splitting in muonic hydrogen which had, until recently, been disregarded is presented. We evaluate the relevant vertex amplitudes, in particular $π^0μ^+μ^-$, combining low energy chiral expansions together with experimental data on $π^0$ and $η$ decays into two leptons. A value of $Δ{E}^π_{HFS}= -(0.09 \pm 0.06)\ μ\rm{eV}$ is obtained for this contribution.
