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Register a new userPion couplings of the Delta(1232)
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We calculate the strong couplings of pions to the Delta(1232) resonance using a QCD parameterization method that includes in addition to the usual one-quark also two-quark and previously uncalculated three-quark operators. We find that three-quark operators are necessary to obtain results consistent with the data and other QCD based baryon structure models. Our results are also in quantitative agreement with a model employing large D state admixtures to the nucleon and Delta wave functions indicating that the pion-nucleon and pion-Delta couplings are sensitive to the spatial shape of these baryons.
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We use lattice QCD and the Luscher method to study elastic pion-nucleon scattering in the isospin $I = 3/2$ channel, which couples to the $Delta(1232)$ resonance. Our $N_f=2+1$ flavor lattice setup features a pion mass of $m_pi approx 250$ MeV, such that the strong decay channel $Delta rightarrow pi N$ is close to the threshold. We present our method for constructing the required lattice correlation functions from single- and two-hadron interpolating fields and their projection to irreducible representations of the relevant symmetry group of the lattice. We show preliminary results for the energy spectra in selected moving frames and irreducible representations, and extract the scattering phase shifts. Using a Breit-Wigner fit, we also determine the resonance mass $m_Delta$ and the $g_{Delta-pi N}$ coupling.
We calculate the single spin asymmetry for the $e p to e Delta(1232)$ process, for an electron beam polarized normal to the scattering plane. Such single spin asymmetries vanish in the one-photon exchange approximation, and are directly proportional to the absorptive part of a two-photon exchange amplitude. As the intermediate state in such two-photon exchange process is on its mass shell, the asymmetry allows one to access for the first time the on-shell $Delta to Delta$ as well as $N^ast to Delta$ electromagnetic transitions. We present the general formalism to describe the $e p to e Delta$ beam normal spin asymmetry, and provide a numerical estimate of its value using the nucleon, $Delta(1232)$, $S_{11}(1535)$, and $D_{13}(1520)$ intermediate states. We compare our results with the first data from the Qweak@JLab experiment and give predictions for the A4@MAMI experiment.
We determine the $Delta(1232)$ resonance parameters using lattice QCD and the Luscher method. The resonance occurs in elastic pion-nucleon scattering with $J^P=3/2^+$ in the isospin $I = 3/2$, $P$-wave channel. Our calculation is performed with $N_f=2+1$ flavors of clover fermions on a lattice with $Lapprox 2.8$ fm. The pion and nucleon masses are $m_pi =255.4(1.6)$ MeV and $m_N=1073(5)$ MeV, and the strong decay channel $Delta rightarrow pi N$ is found to be above the threshold. To thoroughly map out the energy-dependence of the nucleon-pion scattering amplitude, we compute the spectra in all relevant irreducible representations of the lattice symmetry groups for total momenta up to $vec{P}=frac{2pi}{L}(1,1,1)$, including irreps that mix $S$ and $P$ waves. We perform global fits of the amplitude parameters to up to 21 energy levels, using a Breit-Wigner model for the $P$-wave phase shift and the effective-range expansion for the $S$-wave phase shift. From the location of the pole in the $P$-wave scattering amplitude, we obtain the resonance mass $m_Delta=1378(7)(9)$ MeV and the coupling $g_{Deltatext{-}pi N}=23.8(2.7)(0.9)$.
Hard bremsstrahlung production in proton-proton collisions has been studied with the ANKE spectrometer at COSY-Juelich in the energy range of 353-800 MeV by detecting the final proton pair {pp}_s from the pp -> {pp}_s reaction with very low excitation energy. Differential cross sections were measured at small diproton c.m. angles from 0 to 20 degrees and the average over this angular interval reveals a broad peak at a beam energy around 650 MeV with a FWHM of about 220 MeV, suggesting the influence of Delta(1232)N intermediate states. Comparison with deuteron photodisintegration shows that the cross section for diproton production is up to two orders of magnitude smaller, due largely to differences in the selection rules.
The pion structure in Minkowski space is described in terms of an analytic model of the Bethe-Salpeter amplitude combined with Euclidean Lattice QCD results for the running quark mass. In the present work, a pion model previously proposed, which allows for a Nakanishi integral representation, is studied in order to verify the sensitivity of the pion electromagnetic form factor to small variations of the quark self-energy. In addition, we extend the previous work, providing the Nakanishi integral representation for the invariants associated with a decomposition of the pion Bethe-Salpeter amplitude.
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