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Register a new userDispersive analysis of low energy $gamma^* Nrightarrowpi N$ process
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Xiong-Hui Cao
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We use a dispersion representation based on unitarity and analyticity to study the low energy $gamma^* Nrightarrow pi N$ process in the $S_{11}$ channel. Final state interactions among the $pi N$ system are critical to this analysis. The left-hand part of the partial wave amplitude is imported from $mathcal{O}(p^2)$ chiral perturbation theory result. On the right-hand part, the final state interaction is calculated through Omn`es formula in $S$ wave. It is found that a good numerical fit can be achieved with only one subtraction parameter, and the eletroproduction experimental data of multipole amplitudes $E_{0+}, S_{0+}$ in the energy region below $Delta(1232)$ are well described when the photon virtuality $Q^2 leq 0.1 mathrm{GeV}^2$.
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We present a dispersive representation of the $gamma Nrightarrow pi N$ partial-wave amplitude based on unitarity and analyticity. In this representation, the right-hand-cut contribution responsible for $pi N$ final-state-interaction effect are taken into account via an Omnes formalism with elastic $pi N$ phase shifts as inputs, while the left-hand-cut contribution is estimated by invoking chiral perturbation theory. Numerical fits are performed in order to pin down the involved subtraction constants. It is found that good fit quality can be achieved with only one free parameter and the experimental data of the multipole amplitude $E_{0}^+$ in the energy region below the $Delta(1232)$ are well described. Furthermore, we extend the $gamma Nrightarrow pi N$ partial-wave amplitude to the second Riemann sheet so as to extract the couplings of the $N^ast(890)$. The modulus of the residue of the multipole amplitude $E_{0}^+$ ($S_{11pE}$) is $2.41rm{mfmcdot GeV^2}$ and the partial width of $N^*(890)togamma N$ at the pole is about $0.369 {rm MeV}$, which is almost the same as the one of $N^*(1535)$, indicating that $N^ast(890)$ strongly couples to $pi N$ system.
The production of eta mesons in photon- and hadron-induced reactions has been revisited in view of the recent additions of high-precision data to the world data base. Based on an effective Lagrangian approach, we have performed a combined analysis of the free and quasi-free gamma N -> eta N, N N -> N N eta, and pi N -> eta N reactions. Considering spin-1/2 and -3/2 resonances, we found that a set of above-threshold resonances {S_{11}, P_{11}, P_{13}}, with fitted mass values of about M_R=1925, 2130, and 2050 MeV, respectively, and the four-star sub-threshold P_{13}(1720) resonance reproduce best all existing data for the eta production processes in the resonance-energy region considered in this work. All three above-threshold resonances found in the present analysis are essential and indispensable for the good quality of the present fits.
The $(n,gamma f)$ process is reviewed in light of modern nuclear reaction calculations in both slow and fast neutron-induced fission reactions on $^{235}$U and $^{239}$Pu. Observed fluctuations of the average prompt fission neutron multiplicity and average total $gamma$-ray energy below 100 eV incident neutron energy are interpreted in this framework. The surprisingly large contribution of the M1 transitions to the pre-fission $gamma$-ray spectrum of $^{239}$Pu is explained by the dominant fission probabilities of 0$^+$ and $2^+$ transition states, which can only be accessed from compound nucleus states formed by the interaction of $s$-wave neutrons with the target nucleus in its ground state, and decaying through M1 transitions. The impact of an additional low-lying M1 scissors mode in the photon strength function is analyzed. We review experimental evidence for fission fragment mass and kinetic energy fluctuations in the resonance region and their importance in the interpretation of experimental data on prompt neutron data in this region. Finally, calculations are extended to the fast energy range where $(n,gamma f)$ corrections can account for up to 3% of the total fission cross section and about 20% of the capture cross section.
Angular distributions in the final state of pi0-eta photoproduction on nucleons are considered. As a formal base the familiar isobar model is used in which the (pi0 eta N) state is a product of the resonance decay into eta-Delta(1232) and pi-S_{11}(1535) channels. One of the principal assumptions used is that in the actual energy region the reaction is dominated by a single resonance state. The developed formalism can serve as a tool for testing spin and parity of that resonance.
The neutron-neutron scattering length a_nn provides a sensitive probe of charge-symmetry breaking in the strong interaction. Here we summarize our recent efforts to use chiral perturbation theory in order to systematically relate a_nn to the shape of the neutron spectrum in the reaction pi- d --> n n gamma. In particular we show how the chiral symmetry of QCD relates this process to low-energy electroweak reactions such as p p --> d e+ nu_e. This allows us to reduce the uncertainty in the extracted a_nn (mainly due to short-distance physics in the two-nucleon system) by a factor of more than three, to <0.05 fm. We also report first results on the impact that two-nucleon mechanisms of chiral order P^4 have on the pi- d --> n n gamma neutron spectrum.
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