Nonlinear optical response in gapped graphene

We present a formulation for the nonlinear optical response in gapped graphene, where the low-energy single-particle spectrum is modeled by massive Dirac theory. As a representative example of the formulation presented here, we obtain closed form formula for the third harmonic generation (THG) in gapped graphene. It turns out that the covariant form of the low-energy theory gives rise to a peculiar logarithmic singularities in the nonlinear optical spectra. The universal functional dependence of the response function on dimension-less quantities indicates that the optical nonlinearity can be largely enhanced by tuning the gap to smaller values.Comment: http://iopscience.iop.org/0953-8984/labtalk-article/4938

Endpoint behavior of the pion distribution amplitude in QCD sum rules with nonlocal condensates

Starting from the QCD sum rules with nonlocal condensates for the pion distribution amplitude, we derive another sum rule for its derivative and its "integral" derivatives---defined in this work. We use this new sum rule to analyze the fine details of the pion distribution amplitude in the endpoint region $x\sim 0$. The results for endpoint-suppressed and flat-top (or flat-like) pion distribution amplitudes are compared with those we obtained with differential sum rules by employing two different models for the distribution of vacuum-quark virtualities. We determine the range of values of the derivatives of the pion distribution amplitude and show that endpoint-suppressed distribution amplitudes lie within this range, while those with endpoint enhancement---flat-type or CZ-like---yield values outside this range.Comment: 20 pages, 10 figures, 1 table, conclusions update

Plane wave limit of local conserved charges

We study the plane wave limit of the Backlund transformations for the classical string in AdS space times a sphere and obtain an explicit expression for the local conserved charges. We show that the Pohlmeyer charges become in the plane wave limit the local integrals of motion of the free massive field. This fixes the coefficients in the expansion of the anomalous dimension as the sum of the Pohlmeyer charges.Comment: v2: added explanation

Improved estimates of the pion-photon transition form factor in the (1≤Q2≤5)(\mathbf{1\leq Q^2\leq5})~GeV2^\mathbf{2} range and their theoretical uncertainties

We consider the pion-photon transition form factor at low to intermediate spacelike momenta within the theoretical framework of light-cone sum rules. We derive predictions which take into account all currently known contributions stemming from QCD perturbation theory up to the next-to-next-to-leading order (NNLO) and by including all twist terms up to order six. In order to enable a more detailed comparison with forthcoming high-precision data, we also estimate the main systematic theoretical uncertainties, stemming from various sources, and discuss their influence on the calculations --- in particular the dominant one related to the still uncalculated part of the NNLO contribution. The analysis addresses, in broad terms, also the role of the twist-two pion distribution amplitude derived with different approaches.Comment: 10 pages, 3 figures, Based on the talk given at the XIIth International Conference "Quark Confinement and the Hadronic Spectrum" (Thessaloniki, Greece, 28 August - 4 September 2016