arXiv subjects

S. S. Kamalov

Publications and source records attributed to S. S. Kamalov.

At least 37 records · Page 2Linked to original sources

arXiv2001

Chiral unitary theory: application to nuclear problems

In this talk we briefly describe some basic elements of chiral perturbation theory, $χPT$, 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 $ ππ$ interaction in nuclear matter in the scalar and vector channels, antikaons in nuclei and $K^-$ atoms, and how the $ϕ$ meson properties are changed in a nuclear medium.

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arXiv2001

Antikaons in nuclei and dense nuclear matter

We present recent progress on the properties of antikaons in nuclei and dense nuclear matter as obtained from two {\bar K}N interaction models: one based on the lowest-order meson-baryon chiral lagrangian and the other derived from a meson-exchange picture.

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arXiv2000

The E2/M1 and C2/M1 ratios and form factors in N -> Delta transitions

A partial wave analysis of pion photoproduction has been obtained in the framework of fixed-t dispersion relations valid from threshold up to 500 MeV. In the resonance region we have precisely determined the electromagnetic properties of the Δ(1232) resonance, in particular the E2/M1 ratio R_{EM}=(-2.5 +- 0.1) %. For pion electroproduction recent experimental data from Mainz, Bates and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed with two different models, an isobar model (MAID) and a dynamical model. The E2/M1 ratios extracted with these two models show, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and become positive with increasing Q^2. This is a possible indication of a very slow approach toward the pQCD region. The C2/M1 ratio near the photon point is found as R_{SM}(0)=(-6.5 +- 0.5) %. At high Q^2 the absolute value of the ratio is strongly increasing, a further indication that pQCD is not yet reached.

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arXiv2000

gamma* N -> Delta transition form factors: a new analysis of the JLab data on p(e,e'p)pi0 at Q^2=2.8 and 4.0 (GeV/c)^2

Recent JLab data of the differential cross section for the reaction p(e,e'p)pi0 in the invariant mass region of 1.1 < W < 1.4 GeV at four-momentum transfer squared Q^2= 2.8 and 4.0 (GeV/c)$^2$ are analyzed with two models, both of which give an excellent description of most of the existing pion electroproduction data below W < 1.5 GeV. We find that at up to Q^2 = 4.0(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 from a small and negative value at the real photon point, a clear tendency to cross zero, and to become positive with increasing Q^2. This is a possible indication of a very slow approach toward the pQCD region.Furthermore, we find that the helicity amplitude A_{1/2} and S_{1/2}, but not A_{3/2}, starts exhibiting the scaling behavior at about Q^2 \ge 2.5(GeV/c)^2.

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arXiv2000

Hypernuclear Physics with Photons

We review the opportunities and challenges in the field of hypernuclear physics with electromagnetic probes. An overview is presented regarding our current understanding of the elementary production process on the nucleon. This amplitude is then used in the nuclear environment to study the hyperon-nucleon (YN) interaction. We discuss two scenarios: hypernuclear excitation that allows the investigation of hypernuclear structure and the bound Lambda in the nucleus, and quasifree kaon production on the deuteron and on nuclei, which permits a more direct access to the YN force. Specific examples are given for few-body systems and shell-model nuclei.

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arXiv2000

Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the Δ(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.

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arXiv2000

The GDH Sum Rule and Related Integrals

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.

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arXiv2000

Pion electroproduction on the nucleon and the generalized GDH sum rule

Results of the recently developed unitary isobar model (MAID) are presented for the spin asymmetries, structure functions and relevant sum rules in the resonance region. The model describes the presently available data for single-pion photo- and electroproduction in the resonance region below Wcm = 2 GeV very well. It is based on Born terms and resonance contributions, and the respective multipoles are constructed in a gauge-invariant and unitary way for each partial wave. The eta production is included in a similar way, while the contribution of more-pion and higher channels is modeled by comparison with the total cross sections and simple phenomenological assumptions. Our evaluation of the energy-weighted integrals is in good agreement for the proton but shows big discrepancies for the neutron.

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arXiv2000

A Dynamical Model for the Resonant Multipoles and the $Δ$ Structure

We show that recent experiment data for the ratios $E_{1^+}/M_{1^+}$ and $S_{1^+}/M_{1^+}$ can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare $γ^* N Δ$ form factors. Within our model we find that the bare $Δ$ is almost spherical and the electric E2 and Coulomb C2 quadrupole excitations of the physical $Δ$ are nearly saturated by pion cloud contribution in $Q^2 \le 4.0 GeV^2$.

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arXiv1999

Pion Electroproduction on the Nucleon and the Generalized GDH Sum Rule

We present predictions for the spin structure functions of the proton and the neutron 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 fulfill the Gerasimov-Drell-Hearn and Burkhardt-Cottingham sum rules within an error of typically 5-10% for the proton. For the neutron target we find much bigger deviations, in particular the sum rule for I_1(0)+I_2(0) is heavily violated.

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arXiv1999

Coherent π^\circ photoproduction from ^4He

Differential cross sections and beam asymmetries for coherent π^\circ photoproduction from ^4He in the Δenergy-range have been measured with high statistical and systematic precisions using both decay photons for identifying the process.The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the Δ-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks.

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arXiv1999

Generalized polarizabilities and electroexcitation of the nucleon

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 $Σ=α+β$ and the spin polarizability $γ$ 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 predictions of chiral perturbation theory.

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arXiv1999

Medium Effects in Coherent Pion Photo- and Electroproduction on 4He and 12C

Coherent pi0 photo- and electroproduction on 4He and 12C nuclei is investigated in the framework of a distorted wave impulse approximation in momentum space. The elementary process is described by the recently developed unitary isobar model. Medium effects are considered by introducing a phenomenological Delta self-energy. The recent experimental data for 4He and 12C can be well described over a wide range of energies and emission angles by the assumption that the Delta-nuclear interaction saturates.

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arXiv1999

Pion cloud and the $Q^2-$dependence of $γ^* N \leftrightarrow Δ$ transition form factors

Recent experiment indicates that the behavior exhibited by the ratios $E_{1^+}/M_{1^+}$ and $S_{1^+}/M_{1^+}$ of the $γ^* N \leftrightarrow Δ$ transition remain small and negative for $Q^2 \le 4.0 GeV^2$. It implies that the perturbative QCD is still not applicable at these momentum transfers. We show that these data can be explained in a dynamical model for electromagnetic production of pions, together with a simple scaling assumption for the bare $γ^* N Δ$ form factors. Within our model we find that the bare $Δ$ is almost spherical and the electric E2 and Coulomb C2 quardrupole excitations of the physical $Δ$ are nearly saturated by pion cloud contribution in $Q^2 \le 4.0 GeV^2$.

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arXiv1999

The Generalized Gerasimov-Drell-Hearn Integral and the Spin Structure of the Nucleon

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 %.

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arXiv1998

A unitary isobar model for pion photo- and electroproduction on the proton up to 1 GeV

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 electroproduction on the nucleon for both differential cross sections and polarization observables. The model can be used as a starting point to predict and analyze forthcoming data.

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arXiv1998

Higher Nucleon Resonances in Exclusive $(γ, πN)$ Reactions on Nuclei

We report calculations for exclusive pion photoproduction on nuclei beyond the first resonance region in the distorted wave impulse approximation. The elementary operator contains contributions from the resonances $P_{33}(1232)$, $P_{11}(1440)$, $D_{13}(1520)$, $D_{33}(1740)$, $S_{11}(1535)$, $S_{11}(1650)$, and $F_{15}(1680)$, in addition to the usual Born plus vector meson contributions. It gives a good description of single pion photoproduction data up to about 1.1 GeV. Final state interactions are incorporated via optical potentials and are found to be substantial in the coincidence cross sections, but insensitive to the photon asymmetry. Clear signatures of possible medium modifications of the $D_{13}$ and $F_{15}$ resonances are predicted.

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arXiv1997

Coherent two pion photoproduction on 12C

We develop the formalism for coherent two pion photoproduction in nuclei and perform actual calculations of cross sections for $π^-π^+$ and $π^0π^0$ photoproduction on $^{12}C$. We find that due to the isospin symmetry the cross section for $π^0π^0$ 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 $π^0$ mesons are equal and their relative azimuthal angle $ϕ=180^0$ is forbidden. Conversely in the $π^-π^+$ 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 mechanism in both channels is related to the excitation of the $Δ$ isobar. Hence the reaction can serve as a source of information about $Δ$'s properties in nucleus. We have found that the reaction is sensitive to effects of the pion and $Δ$ renormalization in the nuclear medium, similar to those found in the coherent $(γ,π^0)$ reaction, but magnified because of the presence of the two pions.

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