arXiv subjects

L. Tiator

Publications and source records attributed to L. Tiator.

At least 91 records · Page 5Linked to original sources

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.

hep-ph↗

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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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

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

Analysis of resonance multipoles from polarization observables in eta photoproduction

A combined analysis of new eta photoproduction data for total and differential cross sections, target asymmetry and photon asymmetry is presented. Using a few reasonable assumptions we perform the first model-independent analysis of the E0+, E2- and M2- eta photoproduction multipoles. Making use of the well-known A3/2 helicity amplitude of the D13(1520) state we extract its branching ratio to the eta-N channel, Gamma(eta,N)/Gamma = (0.08 +- 0.01)%. At higher energies, we show that the photon asymmetry is extremely sensitive to small multipoles that are excited by photons in the helicity 3/2 state. The new GRAAL photon asymmetry data at higher energy show a clear signal of the F15(1680) excitation which permits extracting an F15(1680)->eta,N branching ratio of (0.15 +0.35 -0.10)%.

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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 %.

hep-ph↗

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

Electromagnetic Production of the Hypertriton

Kaon photoproduction on He3, gamma + He3 --> K+ + Hypertriton, is studied in the framework of the impulse approximation. Realistic He3 wave functions obtained as solutions of Faddeev equations with the Reid soft-core potential are used along with different Hypertriton wave functions. Results are compared for several elementary operator models, which can successfully describe the elementary kaon production off the proton up to a photon lab energy of k = 2.2 GeV. It is found that the corresponding cross sections are small, of the order of several nanobarns. It is also shown that the influence of Fermi motion is important, while the effect of different off-shell assumptions on the cross section is not too significant.

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arXiv1998

Eta photoproduction

We present a combined analysis of the new eta photoproduction data for total and differential cross sections, target asymmetry and photon asymmetry. Using basic assumptions, this allows a model-independent extraction of the $E_{2-}$ and $M_{2-}$ multipoles as well as resonance parameters of the $D_{13}(1520)$ state. At higher energy, we show that the photon asymmetry is extremely sensitive to small multipoles that are excited by photons in the helicity 3/2 state. These could be, e.g., the $F_{15}(1680)$, the $F_{17}(1990)$, or the $G_{17}(2190)$ resonances.

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arXiv1998

Analysis of eta production using a generalized Lee model

We have investigated the processes N($π$, $π$)N and N($π$, $η$)N close to eta threshold using a simple, nonrelativistic Lee model which has the advantage of being analytically solvable. It is then possible to study the Riemann sheets of the S-matrix and the behavior of its resonance poles especially close to threshold. A theoretical simulation of the experimental cusp effect at eta threshold leads to a characteristic distribution of poles on the Riemann sheets. We find a pole located in the $4^{th}$ Riemann sheet that up to now has not been discussed. It belongs to the cusp peak at eta threshold. In addition we obtain the surprising result using the Lee model that the resonance $S_{11}(1535)$ does not play a large role. The main features of the experimental data can be reproduced without explicitly introducing this resonance. Furthermore, we have also studied the reactions N($γ$, $π$)N and N($γ$, $η$)N and find reasonable agreement between the data and both models with and without the $S_{11}(1535)$ resonance.

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arXiv1997

Dispersion Theoretical Analysis of Pion Photoproduction at Threshold and in the Delta Region

A new partial wave analysis of pion photoproduction has been obtained in the framework of fixed-t dispersion relations valid form threshold up to 500 MeV. It is based on new Mainz data for π^{0} and π^{+} production off the proton and both older and more recent data from Bonn, Frascati and TRIUMF for π^{+} and π^{-}. At threshold we obtain a good agreement with the existing data for both charged and neutral pion production. 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 \pm 0.1) %. We show that a model independent resonance background subtraction is possible with the speed-plot technique and obtain the Δpole at W =(1211 - 50i) MeV and the E2/M1 ratio of the residues as R_Δ = - 0.035 - 0.046 i.

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arXiv1997

Multipole analysis of pion photoproduction based on fixed t dispersion relations and unitarity

We have analysed pion photoproduction imposing constraints from fixed t dispersion relations and unitarity. Coupled integral equations for the S and P wave multipoles were derived from the dispersion relations and solved by the method of Omnes and Muskhelishvili. The free parameters were determined by a fit to the most recent data for π^{+} and π^{0} production on the proton as well as π^{-} production on the neutron, in the energy We have analysed pion photoproduction imposing constraints from fixed t dispersion relations and unitarity. Coupled integral equations for the S and P wave multipoles were derived from the dispersion relations and solved by the method of Omnes and Muskhelishvili. The free parameters were determined by a fit to the most recent data for π^{+} and π^{0} production on the proton as well as π^{-} production on the neutron, in the energy range 160 MeV \leq E_γ \leq 420 MeV. The lack of high precision data on the neutron and of polarization observables leads to some limitations of our results. Especially the multipole M_{1-} connected with the Roper resonance P_{11}(1440) cannot be determined to the required precision. Our predictions for the threshold amplitudes are in good agreement with both the data and chiral perturbation theory. In the region of the Δ(1232) we have determined the ratio of electric quadrupole and magnetic dipole excitation. The position of the resonance pole is obtained in excellent agreement with pion-nucleon scattering, and the complex residues of the multipoles are determined with the speed-plot technique.

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arXiv1997

Photoproduction of the Hypertriton

In the framework of the impulse approximation we study the photoproduction of the hypertriton $^3_Λ$H by using realistic $^3$He wave functions obtained as solutions of Faddeev equations with the Reid soft-core potential for different $^3_Λ$H wave functions. We obtain relatively small cross sections of the order of 1 nb. We also find that the influence of Fermi motion is important, while the effect of different off-shell assumptions on the cross section is not too significant.

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arXiv1996

A dispersion theoretical approach to the threshold amplitudes of pion photoproduction

We give predictions for the partial wave amplitudes of pion photoproduction near threshold by means of dispersion relations at fixed t. The free parameters of this approach are determined by a fit to experimental data in the energy range 160 MeV $\le E_γ \le$ 420 MeV. The observables near threshold are found to be rather sensitive to the amplitudes in the resonance region, in particular to the $Δ$ (1232) and $N^*$ (1440). We obtain a good agreement with the existing threshold data for both charged and neutral pion production. Our predictions also agree well with the results of chiral perturbation theory, except for neutral pion production off the neutron.

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