Effective theory of the Delta(1232) in Compton scattering off the nucleon

We formulate a new power-counting scheme for a chiral effective field theory of nucleons, pions, and Deltas. This extends chiral perturbation theory into the Delta-resonance region. We calculate nucleon Compton scattering up to next-to-leading order in this theory. The resultant description of existing $\gamma$p cross section data is very good for photon energies up to about 300 MeV. We also find reasonable numbers for the spin-independent polarizabilities $\alpha_p$ and $\beta_p$.Comment: 29 pp, 9 figs. Minor revisions. To be published in PR

Dispersion Effects in Nucleon Polarisabilities

We present a formalism to extract the dynamical nucleon polarisabilities defined via a multipole expansion of the structure amplitudes in nucleon Compton scattering. In contradistinction to the static polarisabilities, dynamical polarisabilities gauge the response of the internal degrees of freedom of a composed object to an external, real photon field of arbitrary energy. Being energy dependent, they therefore contain additional information about dispersive effects induced by internal relaxation mechanisms, baryonic resonances and meson production thresholds of the nucleon. We give explicit formulae to extract the dynamical electric and magnetic dipole as well as quadrupole polarisabilities from low energy nucleon Compton scattering up to the one pion production threshold and discuss the connection to the definition of static nucleon polarisabilities. As a concrete example, we examine the results of leading order Heavy Baryon Chiral Perturbation Theory for the four leading spin independent iso-scalar polarisabilities of the nucleon. Finally, we consider the possible r{\^o}le of energy dependent effects in low energy extractions of the iso-scalar dipole polarisabilities from Compton scattering on the deuteron.Comment: 17 pages LaTeX2e with 2 figures, using includegraphicx (5 .eps files). Minor corrections, references updated. Contents identical to version to appear in Phys. Rev. C 65, spelling differen

Fixed-t subtracted dispersion relations for Compton scattering off the nucleon

We present fixed-$t$ subtracted dispersion relations for Compton scattering off the nucleon at energies $E_\gamma \leq$ 500 MeV, as a formalism to extract the nucleon polarizabilities with a minimum of model dependence. The subtracted dispersion integrals are mainly saturated by $\pi N$ intermediate states in the $s$-channel $\gamma N \to \pi N \to \gamma N$ and $\pi \pi$ intermediate states in the $t$-channel $\gamma \gamma \to \pi \pi \to N \bar N$. For the subprocess $\gamma \gamma \to \pi \pi$, we construct a unitarized amplitude and find a good description of the available data. We show results for Compton scattering using the subtracted dispersion relations and display the sensitivity on the scalar polarizability difference $\alpha - \beta$ and the backward spin polarizability $\gamma_\pi$, which enter directly as fit parameters in the present formalism

Field theory of nucleon to higher-spin baryon transitions

We discuss the nucleon to higher-spin $N$- and $\Delta$-resonance transitions by pions and photons. The higher-spin baryons are described by Rarita-Schwinger fields and, as we argue, this imposes a stringent consistency requirement on the form of the couplings. Popular $\pi N\Delta$ and $\gamma N\Delta$ couplings are inconsistent from this point of view. We construct examples of consistent interactions with the same nonrelativistic limit as the conventional ones.Comment: 5 pages, Revtex, 1 PostScript figure; published versio

Evidence for Anthropogenic Surface Loading as Trigger Mechanism of the 2008 Wenchuan Earthquake

Two and a half years prior to China's M7.9 Wenchuan earthquake of May 2008, at least 300 million metric tons of water accumulated with additional seasonal water level changes in the Minjiang River Valley at the eastern margin of the Longmen Shan. This article shows that static surface loading in the Zipingpu water reservoir induced Coulomb failure stresses on the nearby Beichuan thrust fault system at <17km depth. Triggering stresses exceeded levels of daily lunar and solar tides and perturbed a fault area measuring 416+/-96km^2. These stress perturbations, in turn, likely advanced the clock of the mainshock and directed the initial rupture propagation upward towards the reservoir on the "Coulomb-like" Beichuan fault with rate-and-state dependent frictional behavior. Static triggering perturbations produced up to 60 years (0.6%) of equivalent tectonic loading, and show strong correlations to the coseismic slip. Moreover, correlations between clock advancement and coseismic slip, observed during the mainshock beneath the reservoir, are strongest for a longer seismic cycle (10kyr) of M>7 earthquakes. Finally, the daily event rate of the micro-seismicity (M>0.5) correlates well with the static stress perturbations, indicating destabilization.Comment: 22 pages, 4 figures, 3 table

Low-Energy Compton Scattering of Polarized Photons on Polarized Nucleons

The general structure of the cross section of $\gamma N$ scattering with polarized photon and/or nucleon in initial and/or final state is systematically described and exposed through invariant amplitudes. A low-energy expansion of the cross section up to and including terms of order $\omega^4$ is given which involves ten structure parameters of the nucleon (dipole, quadrupole, dispersion, and spin polarizabilities). Their physical meaning is discussed in detail. Using fixed-t dispersion relations, predictions for these parameters are obtained and compared with results of chiral perturbation theory. It is emphasized that Compton scattering experiments at large angles can fix the most uncertain of these structure parameters. Predictions for the cross section and double-polarization asymmetries are given and the convergence of the expansion is investigated. The feasibility of the experimental determination of some of the struture parameters is discussed.Comment: 41 pages of text, 9 figures; minor revisions prior to publication in Phys. Rev.

Nucleon to Delta Weak Excitation Amplitudes in the Non-relativistic Quark Model

We investigate the nucleon to Delta(1232) vector and axial vector amplitudes in the non-relativistic quark model of the Isgur-Karl variety. A particular interest is to investigate the SU(6) symmetry breaking, due to color hyperfine interaction. We compare the theoretical estimates to recent experimental investigation of the Adler amplitudes by neutrino scattering.Comment: \documentstyle[aps]{revtex}, 21pages; 11 postscript figures. Accepted for publication by Phys. Rev.

Radiative decays of decuplet hyperons

We calculate the radiative decay widths of decuplet hyperons in a chiral constituent quark model including electromagnetic exchange currents between quarks. Exchange currents contribute significantly to the E2 transition amplitude, while they largely cancel for the M1 transition amplitude. Strangeness suppression of the radiative hyperon decays is found to be weakened by exchange currents. Differences and similarities between our results and other recent model predictions are discussed.Comment: 11 pages, 1 eps figure, revtex, accepted for publication in Phys. Rev.

Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the \Delta(1232) resonance

The recoil proton polarization has been measured in the p (\vec e,e'\vec p) pi^0 reaction in parallel kinematics around W = 1232 MeV, Q^2 = 0.121 (GeV/c)^2 and epsilon = 0.718 using the polarized c.w. electron beam of the Mainz Microtron. Due to the spin precession in a magnetic spectrometer, all three proton polarization components P_x/P_e = (-11.4 \pm 1.3 \pm 1.4) %, P_y = (-43.1 \pm 1.3 \pm 2.2) %, and P_z/P_e = (56.2 \pm 1.5 \pm 2.6) % could be measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio CMR = (-6.4\pm 0.7_{stat}\pm 0.8_{syst}) % was determined from P_x in the framework of the Mainz Unitary Isobar Model. The consistency among the reduced polarizations and the extraction of the ratio of longitudinal to transverse response is discussed.Comment: 5 pages LaTeX, 1 table, 2 eps figure

Measurement of the Transverse-Longitudinal Cross Sections in the p (e,e'p)pi0 Reaction in the Delta Region

Accurate measurements of the p(e,e?p)pi0 reaction were performed at Q^2=0.127(GeV/c)^2 in the Delta resonance energy region. The experiments at the MIT-Bates Linear Accelerator used an 820 MeV polarized electron beam with the out of plane magnetic spectrometer system (OOPS). In this paper we report the first simultaneous determination of both the TL and TL? (``fifth" or polarized) cross sections at low Q^{2} where the pion cloud contribution dominates the quadrupole amplitudes (E2 and C2). The real and imaginary parts of the transverse-longitudinal cross section provide both a sensitive determination of the Coulomb quadrupole amplitude and a test of reaction calculations. Comparisons with model calculations are presented. The empirical MAID calculation gives the best overall agreement with this accurate data. The parameters of this model for the values of the resonant multipoles are |M_{1+}(I=3/2)|= (40.9 \pm 0.3)10^{-3}/m_pi, CMR= C2/M1= -6.5 \pm 0.3%, EMR=E2/M1=-2.2 \pm 0.9%, where the errors are due to the experimental uncertainties.Comment: 10 pages, 3 figures, minor corrections and addition