We investigate electroproduction of pion pairs on the nucleon in the framework of QCD factorization for hard exclusive processes. We extend previous analyses by taking the hard-scattering coefficients at next-to-leading order in α s. The dynamics of the produced pion pair is described by two-pion distribution amplitudes, for which we perform a detailed theoretical and phenomenological analysis....

The pion structure function is investigated in a simple model, where pion and constituent quark fields are coupled through the simplest pseudoscalar coupling. The imaginary part of the forward γπ → γπ scattering amplitude is evaluated and related to the structure functions. It is shown that the introduction of nonperturbative effects, linked to the size of the pion and preserving gauge invarian...

We study the pion production in heavy-ion collisions at SIS energies within the coupled resonance BUU model. The standard calculation overpredicts the pion multiplicity in the central Au+Au collision at 1 A GeV by about a factor of 2, which is interpreted as a signature for the quenching of the resonance production/absorption in nucleon-nucleon collisions at high nuclear density. Results of the...

We study in this paper the possible signals of a pion string associated with the QCD chiral phase transition in LHC Pb Pb collision at energy √ s = 5.5 A TeV. In terms of the Kibble-Zurek mechanism we discuss the production and evolution of the pion string. The pion string is not topologically stable, its decay gives rise to the neutral pions and the sigma mesons which decay into pions. We esti...

We study the possible signals of a pion string associated with the QCD chiral phase transition in LHC Pb Pb collision at energy √ s = 5.5 TeV. In terms of the Kibble-Zurek mechanism we discuss the production and evolution of the pion string. The pion string is not topologically stable, it decays into neutral pions and sigma mesons which in turn decay into pions. We estimate the numbers of the p...

We study the flavour breaking effects appearing in the Wilson twisted mass formulation of lattice QCD. In this quenched study, we focus on the mass splitting between the neutral and the charged pion, determining the neutral pion mass with a stochastic noise method to evaluate the disconnected contributions. We find that these disconnected contributions are significant. Using the Osterwalder-Sei...

The role of pion rescattering in ππ decay of radially excited heavy quarkonia modeled in terms of a Qππ coupling, is investigated within the framework of the covariant Blankenbecler-Sugar equation. The effects of pion rescattering (or pion exchange) are shown to be large, unless the coupling of the two-pion system to the heavy quarks is mediated by a fairly light scalar σ meson, which couples t...

We present a detailed quark-model study of pion photoproduction within the effective Lagrangian approach. Cross sections and single-polarization observables are investigated for the four charge channels, γp → π+n, γn → π−p, γp → π0p, and γn → π0n. Leaving the πN∆ coupling strength to be a free parameter, we obtain a reasonably consistent description of these four channels from threshold to the ...