0000000000202761
AUTHOR
S.s. Kamalov
showing 23 related works from this author
Generalized polarizabilities and electroexcitation of the nucleon
1999
Generalized nucleon polarizabilities for virtual photons can be defined in terms of electroproduction cross sections as function of the 4-momentum transfer $Q^2$. In particular, the sum of the generalized electric and magnetic polarizabilities $\Sigma=\alpha+\beta$ and the spin polarizability $\gamma$ can be expressed by virtual photon absorption cross sections integrated over the excitation energy. These quantities have been calculated within the framework of the recently developed unitary isobar model for pion photo- and electroproduction on the proton, which describes the available experimental data up to an excitation energy of about 1 GeV. Our results have been compared to the predicti…
<‘γ*N→Δtransition form factors: A new analysis of data onp(e,e′p)π0atQ2=2.8and4.0 (GeV/c)2
2001
Recent JLab data of the differential cross section for the reaction ${p(e,e}^{\ensuremath{'}}p){\ensuremath{\pi}}^{0}$ in the invariant mass region of $1.1lWl1.4 \mathrm{GeV}$ at four-momentum transfer squared ${Q}^{2}=2.8$ and $4.0 (\mathrm{GeV}{/c)}^{2}$ are analyzed with two models, both of which give an excellent description of most of the existing pion electroproduction data below $Wl1.5 \mathrm{GeV}.$ We find that at up to ${Q}^{2}=4.0 (\mathrm{GeV}{/c)}^{2},$ the extracted helicity amplitudes ${A}_{3/2}$ and ${A}_{/2}$ remain comparable with each other, implying that hadronic helicity is not conserved at this range of ${Q}^{2}.$ The ratios ${E}_{1+}{/M}_{1+}$ obtained show, starting …
Coherent π0-photoproduction from atomic nuclei
2003
Differential and total cross sections for coherent pi0-photoproduction from 12C, 40Ca, 93Nb and Pb targets have been measured throughout the region of the Delta(1232)-resonance. The experiments were performed with the TAPS-detector at the Mainz accelerator MAMI. The characteristic proportionality of the cross section to the square of the atomic mass number and to the nuclear mass form factor is clearly demonstrated. The data allow for the first time detailed tests of model predictions for this reaction. The comparison of the data to model predictions shows that the Delta-nucleus interaction saturates: it is described for heavy nuclei with the same potential parameters as for 4He.
The Generalized Gerasimov-Drell-Hearn Integral and the Spin Structure of the Nucleon
1998
The spin structure functions g1 and g2 have been calculated in the resonance region and for small and intermediate momentum transfer. The calculation is based on a gauge-invariant and unitary model for one-pion photo- and electroproduction. The predictions of the model agree with the asymmetries and the spin sturcture functions recently measured at SLAC, and the first moments of the calculated spin structure functions fullfil the Gerasimov-Drell-Hearn and Burkhardt-Cottingham sum rules within an error of typically 5-10 %.
Baryon resonance analysis from MAID
2009
The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the region below W=2 GeV and both single-Q^2 and Q^2 dependent transition form factors could be obtained for the Delta, Roper, D13(1520), S11(1535), S31(1620), S11(1650), D15(1675), F15(1680) and P13(1720). From the complete world data base, including also pi- data on the neutron, also Q^2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transver…
Pion interaction with the trinucleon up to the eta production threshold
1993
Pion elastic, charge exchange scattering and induced eta production on the trinucleon systems are investigated in a coupled-channels approach in momentum space with Fadeev wave functions. The channel $\pi N \rightarrow \eta N$ is included using an isobar model with S-, P-, and D-wave resonances. While the coherent reactions like $^3$He($\pi,\pi)^3$He can be reasonably well reproduced up to $T_{\pi}$=500 MeV, large discrepancies appear for the incoherent processes, $^3$He($\pi^-,\pi^0)^3$H and $^3$He($\pi^-,\eta)^3$H at backward angles and energies above $\Delta$-resonance. In the forward direction the $(\pi,\eta)$ calculations underestimate the experimental measurements very close to thresh…
Dispersion relation analysis of neutral pion photoproduction and electroproduction at threshold
2002
Neutral pion photoproduction and electroproduction at threshold is analyzed in the framework of dispersion relations. For this purpose, we evaluate the real threshold amplitudes in terms of Born contributions and dispersion integrals determined by the imaginary parts of the multipoles of the unitary isobar model (MAID) and the phenomenological partial-wave analysis (SAID). The results show considerable cancellations between Born terms and resonance contributions. Good agreement with the data is found for photoproduction. While our dispersion analysis suggests considerable discrepancies for electroproduction, the present state of the experimental multipole analysis at finite ${Q}^{2}$ does n…
Chiral dynamics in systems with strangeness
2002
In this talk a brief review of several problems involving systems with strangeness is made. In the first place one shows how the $\Lambda (1405)$, $\Lambda(1670)$ and $\Sigma(1620)$ states, for $S = -1$, and the $\Xi(1620)$ for $S= -2$ are generated dynamically in the context of unitarized chiral perturbation theory. The results for the $\bar{K}N$ interaction are then used to evaluate the $K^- d$ scattering length. Results obtained for the kaon selfenergy in a nuclear medium within this approach, with application to $K^-$ atoms, are also mentioned. Finally a few words are said about recent developments in the weak decay of $\Lambda$ hypernuclei and the puzzle of the $\Gamma_n/\Gamma_p$ rati…
Chiral unitary theory: Application to nuclear problems
2001
In this talk we briefly describe some basic elements of chiral perturbation theory, $\chi PT$, and how the implementation of unitarity and other novel elements lead to a better expansion of the $T$ matrix for meson meson and meson baryon interactions. Applications are then done to the $ \pi \pi $ interaction in nuclear matter in the scalar and vector channels, antikaons in nuclei and $K^-$ atoms, and how the $\phi$ meson properties are changed in a nuclear medium.
Nucleon resonances inπNscattering up to energiess⩽2.0GeV
2007
A meson-exchange model for pion-nucleon scattering was previously constructed using a three-dimensional reduction scheme of the Bethe-Salpeter equation for a model Lagrangian involving $\ensuremath{\pi},\ensuremath{\eta},N,\ensuremath{\Delta},\ensuremath{\rho}$, and \ensuremath{\sigma} fields. We thereby extend our previous work by including the $\ensuremath{\eta}N$ channel and all the $\ensuremath{\pi}N$ resonances with masses $~2$ GeV, up to the $F$ waves. The effects of the $\ensuremath{\pi}\ensuremath{\pi}N$ channels are taken into account by introducing an effective width in the resonance propagators. The extended model gives an excellent fit to both $\ensuremath{\pi}N$ phase shifts an…
Third and fourth S11 resonances in pion scattering and pion photoproduction
2003
Abstract A self-consistent analysis of pion scattering and pion photoproduction within a coupled channels dynamical model is presented. The results indicate the existence of a third and a fourth S11 resonance with the masses 1803 ± 7 and 2117 ± 64 MeV.
Gerasimov-Drell-Hearn sum rule and related integrals
2001
The spin structure of the nucleon resonance region is analyzed on the basis of our phenomenological model MAID. Predictions are given for the Gerasimov-Drell-Hearn sum rule as well as generalized integrals over spin structure functions. The dependence of these integrals on momentum transfer is studied and rigorous relationships between various definitions of generalized Gerasimov-Drell-Hearn integrals and spin polarizabilities are derived. These results are compared to the predictions of chiral perturbation theory and phenomenological models.
Backward electroproduction ofπ0mesons on protons in the region of nucleon resonances at four momentum transfer squaredQ2=1.0GeV2
2004
Exclusive electroproduction of pi{sup 0} mesons on protons in the backward hemisphere has been studied at Q2 = 1.0 GeV2 by detecting protons in the forward direction in coincidence with scattered electrons from the 4 GeV electron beam in Jefferson Lab's Hall A. The data span the range of the total (gamma*p) center-of-mass energy W from the pion production threshold to W = 2.0 GeV. The differential cross sections sigma{sub T} + epsilon sigma{sub L}, sigma{sub TL}, and sigma{sub TT} were separated from the azimuthal distribution and are presented together with the MAID and SAID parameterizations.
Pion electroproduction in the 3He(e,é π+)3H reaction at intermediate energies
1997
Abstract The differential coincident pion electroproduction cross section in the 3 He(e,e π + ) 3 H reaction has been measured with high resolution with the three-spectrometer set-up at the Mainz Microtron (MAMI) electron accelerator. Measurements were performed at the four incident energies E 0 = 855, 675, 600, and 555 MeV at fixed four-momentum transfer Q 2 = 0.045 GeV 2 , with the pions detected in parallel kinematics. This enables a separation of the measured cross section into the transverse and longitudinal structure functions by means of the Rosenbluth method. The experimental data are compared to model calculations, in which the elementary pion production amplitude includes the stan…
Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles
2009
We have made the first measurements of the virtual Compton scattering (VCS) process via the H$(e,e'p)\gamma$ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the $W$-dependence at fixed $Q^2=1$ GeV$^2$, and for the $Q^2$-dependence at fixed $W$ near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed $Q^2$-dependence is smooth. The measured ratio of H$(e,e'p)\gamma$ to H$(e,e'p)\pi^0$ cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to Real Compton Scattering (RCS) at high energy and large angles, our VCS data…
A unitary isobar model for pion photo- and electroproduction on the proton up to 1 GeV
1998
A new operator for pion photo- and electroproduction has been developed for nuclear applications at photon equivalent energies up to 1 GeV. The model contains Born terms, vector mesons and nucleon resonances ($P_{33}(1232)$, $P_{11}(1440)$, $D_{13}(1520)$, $S_{11}(1535)$, $F_{15}(1680)$, and $D_{33}(1700)$). The resonance contributions are included taking into account unitarity to provide the correct phases of the pion photoproduction multipoles. The $Q^2$ dependence of electromagnetic resonance vertices is described with appropriate form factors in the electromagnetic helicity amplitudes. Within this model we have obtained good agreement with the experimental data for pion photo- and elect…
Polarized structure functions of the nucleon in the resonance region
2000
Abstract We present predictions for the spin structure functions of the proton in the framework of a unitary isobar model for one-pion photo- and electroproduction. Our results are compared with recent experimental data from SLAC. The first moments of the calculated structure functions fullfil the Gerasimov-Drell-Hearn and Burkhardt-Cottingham sum rules within an error of typically 5–10%.
Coherent two pion photoproduction on C-12
1997
We develop the formalism for coherent two pion photoproduction in nuclei and perform actual calculations of cross sections for $\pi^-\pi^+$ and $\pi^0\pi^0$ photoproduction on $^{12}C$. We find that due to the isospin symmetry the cross section for $\pi^0\pi^0$ production is very small and has a maximum when the pions propagate together. However, the kinematical region where the energies and polar angles of the two $\pi^0$ mesons are equal and their relative azimuthal angle $\phi=180^0$ is forbidden. Conversely in the $\pi^-\pi^+$ production the pions prefer to have a relative azimuthal angle 180$^0$ and the production of the pions propagating together is suppressed. The dominant one-body m…
Threshold π0 photo- and electro-production in a meson-exchange model
2003
We show that, within a meson-exchange dynamical model describing well most of the existing pion electromagnetic production data up to the second resonance region, one is also able to obtain a good agreement with the pi^0 photo- and electroproduction data near threshold. In the case of pi^0 production, the effects of final state interaction in the threshold region are nearly saturated by single charge exchange rescattering. This indicates that in ChPT, it might be sufficient to carry out the calculation just up to one-loop diagrams for threshold neutral pion production.
Chiral unitary approach to the K^- deuteron scattering length
2000
Starting from a recent model where the Kbar N amplitudes are evaluated from the chiral Lagrangians using a coupled channel unitary method, we evaluate here the scattering length for K^- deuteron scattering. We find that the double scattering contribution is very large compared to the impulse approximation and that the charge exchange contribution of this rescattering is as large as the sequential K^- scattering on the two nucleons. Higher order rescattering corrections are evaluated using coupled channels with K^- and Kbar^0 within the integral form of the fixed centre approximation to the Faddeev equations. The higher order corrections involving intermediate pions and hyperons are found ne…
Measurement of the Beam-Helicity Asymmetry in thep(e→,e′p)π0Reaction at the Energy of theΔ(1232)Resonance
2002
In a p((e) over right arrow, e' p)pi(0) out-of-plane coincidence experiment at the three-spectrometer setup of the Mainz Microtron MAMI, the beam-helicity asymmetry has been precisely measured around the energy of the Delta(1232) resonance and Q(2) = 0.2(GeV/c)(2). The results are in disagreement with three up-to-date model calculations. This is interpreted as a lack of understanding of the nonresonant background, which in dynamical models is related to the pion cloud.
The asymmetry in π−3He scattering
1992
Abstract We show that the π + − 3 He asymmetry data obtained in a recent TRIUMF experiment at T π = 100 MeV can be understood in terms of the elementary π N amplitudes. Through a comparison to a full optical-potential calculation that employs realistic Faddeev wave functions for the trinucleon we show that all nuclear-structure dependence cancels for A y at lower energies. At higher pion energies around the Δ-resonance region the sign flip of A y offers the possibility of extracting information about the smaller partial waves.
S11 RESONANCES IN A COUPLED-CHANNEL DYNAMICAL MODEL
2005
A self-consistent analysis of pion scattering and pion photoproduction within a coupled channels dynamical model is presented. The results indicate the existence of a third and a fourth S11 resonance with the masses 1846±47 and 2113±70 MeV . In the case of pion photoproduction, we found that background contributions to the imaginary part of the S-wave multipole become large and negative in the region of the S11(1535) resonance. Due to this fact, much larger resonance contributions are required in order to explain the results of the recent multipole analysis. For the first S11(1535) resonance we obtain a value of 72±2×10-3 GeV -1/2 for the electromagnetic helicity amplitude A1/2.