Soft spectator scattering in the nucleon form factors at large Q2Q^2 within the SCET approach

The proton form factors at large momentum transfer are dominated by two contributions which are associated with the hard and soft rescattering respectively. Motivated by a very active experimental form factor program at intermediate values of momentum transfers, $Q^{2}\sim 5-15 \text{GeV}^{2}$, where an understanding in terms of only a hard rescattering mechanism cannot yet be expected, we investigate in this work the soft rescattering contribution using soft collinear effective theory (SCET). Within such description, the form factor is characterized, besides the hard scale $Q^2$, by a semi-hard scale $Q \Lambda$, which arises due to presence of soft spectators, with virtuality $\Lambda^2$ ($\Lambda \sim 0.5$ GeV), such that $Q^{2}\gg Q\Lambda\gg \Lambda^{2}$. We show that in this case a two-step factorization can be successfully carried out using the SCET approach. In a first step (SCET$_I$), we perform the leading order matching of the QCD electromagnetic current onto the relevant SCET$_I$ operators and perform a resummation of large logarithms using renormalization group equations. We then discuss the further matching onto a SCET$_{II}$ framework, and propose the complete factorization formula for the Dirac form factor, accounting for both hard and soft contributions. We also present a qualitative discussion of the phenomenological consequences of this new framework.Comment: 33 pages, 19 figures; typos corrected, text improved. Version to appear in Phys.Rev.

Scalar form-factor of the proton with light-cone QCD sum rules

In this article, we calculate the scalar form-factor of the proton in the framework of the light-cone QCD sum rules approach with the three valence quark light-cone distribution amplitudes up to twist-6, and observe the scalar form-factor $\sigma(t=-Q^2)$ at intermediate and large momentum transfers $Q^2> 2GeV^2$ has significant contributions from the end-point (or soft) terms. The numerical values for the $\sigma(t=-Q^2)$ are compatible with the calculations from the chiral quark model and lattice QCD at the region $Q^2>2GeV^2$.Comment: 18 pages, 7 figures, revised versio

The b quark low-scale running mass from Upsilon sum rules

The b quark low-scale running mass m_kin is determined from an analysis of the Upsilon sum rules in the next-to-next-to-leading order (NNLO). It is demonstrated that using this mass one can significantly improve the convergence of the perturbation series for the spectral density moments. We obtain m_kin(1 GeV) = 4.56 \pm 0.06 GeV. Using this result we derive the value of the MS-bar mass m: m(m) = 4.20 \pm 0.1 GeV. Contrary to the low-scale running mass, the pole mass of the b quark cannot be reliably determined from the sum rules. As a byproduct of our study we find the NNLO analytical expression for the cross section e+e- --> Q\bar Q of the quark antiquark pair production in the threshold region, as well as the energy levels and the wave functions at the origin for the ^1S_3 bound states of Q\bar Q.Comment: 22 pages, Late

Spectral density in resonance region and analytic confinement

We study the role of finite widths of resonances in a nonlocal version of the Wick-Cutkosky model. The spectrum of bound states is known analytically in this model and forms linear Regge tragectories. We compute the widths of resonances, calculate the spectral density in an extension of the Breit-Wigner {\it ansatz} and discuss a mechanism for the damping of unphysical exponential growth of observables at high energy due to finite widths of resonances.Comment: 13 pages, RevTeX, 6 figures. Revised version with typographical corrections and additional comments in conclusion

Radiative Correction to the Nuclear-Size Effect and Hydrogen-Deuterium Isotopic Shift

The radiative correction to the nuclear charge radius contribution to the Lamb shift of order $\alpha(Z\alpha)^5m_r^3$ is calculated. In view of the recent high precision experimental data, this theoretical correction produces a significant contribution to the hydrogen-deuterium isotopic shift.Comment: 5 pages, REVTEX, replaced with the final version, to be published in Phys.Rev. A, two references adde

Driving, conservation and absorbing states in sandpiles

We use a phenomenological field theory, reflecting the symmetries and conservation laws of sandpiles, to compare the driven dissipative sandpile, widely studied in the context of self-organized criticality, with the corresponding fixed-energy model. The latter displays an absorbing-state phase transition with upper critical dimension $d_c=4$. We show that the driven model exhibits a fundamentally different approach to the critical point, and compute a subset of critical exponents. We present numerical simulations in support of our theoretical predictions.Comment: 12 pages, 2 figures; revised version with substantial changes and improvement

Field theory of absorbing phase transitions with a non-diffusive conserved field

We investigate the critical behavior of a reaction-diffusion system exhibiting a continuous absorbing-state phase transition. The reaction-diffusion system strictly conserves the total density of particles, represented as a non-diffusive conserved field, and allows an infinite number of absorbing configurations. Numerical results show that it belongs to a wide universality class that also includes stochastic sandpile models. We derive microscopically the field theory representing this universality class.Comment: 13 pages, 1 eps figure, RevTex styl