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S. Sirca

Publications and source records attributed to S. Sirca.

72 records · Page 4Linked to original sources
arXiv2006

Measurement of the proton electric to magnetic form factor ratio from \vec ^1H(\vec e, e'p)

We report the first precision measurement of the proton electric to magnetic form factor ratio from spin-dependent elastic scattering of longitudinally polarized electrons from a polarized hydrogen internal gas target. The measurement was performed at the MIT-Bates South Hall Ring over a range of four-momentum transfer squared $Q^2$ from 0.15 to 0.65 (GeV/c)$^2$. Significantly improved results on the proton electric and magnetic form factors are obtained in combination with previous cross-section data on elastic electron-proton scattering in the same $Q^2$ region.

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arXiv2006

Measurements of the Generalized Electric and Magnetic Polarizabilities of the Proton at Low Q2 Using the VCS Reaction

The mean square polarizability radii of the proton have been measured for the first time in a virtual Compton scattering experiment performed at the MIT-Bates out-of-plane scattering facility. Response functions and polarizabilities obtained from a dispersion analysis of the data at Q2=0.06 GeV2/c2 are in agreement with O(p3) heavy baryon chiral perturbation theory. The data support the dominance of mesonic effects in the polarizabilities, and the increase of beta with increasing Q2 is evidence for the cancellation of long-range diamagnetism by short-range paramagnetism from the pion cloud.

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arXiv2005

Direct calculation of the K-matrix for pion electro-production in the Delta channel

We present a method to calculate directly the K-matrices for the pion electro-production processes in the framework of chiral quark models which allows for a clean separation of the resonant amplitudes from the background. The method is applied to the calculation of the multipole amplitudes M_{1+}, E_{1+}, and S_{1+} in the Delta channel within the Cloudy Bag Model. A good overall description is found in a broad energy range.

hep-ph↗
arXiv2005

Measurement of the asymmetries in 3He(e,e'p)d and 3He(e,e'p)np

The electron-target-asymmetries A_parallel and A_perpendicular with target spin parallel and perpendicular to the momentum transfer q were measured for both the two-- and three-body breakup of 3He in the 3He(e,e'p)-reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knocked-out proton detected using the Three-Spectrometer-Facility at MAMI. The results are compared to Faddeev calculations which take into account Final State Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of Final State Interactions in the asymmetry of the three-body breakup and of an almost negligible one for the two-body breakup.

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arXiv2004

Measurement of Rlt and Atl in the 4He(e,e'p)3H Reaction at pmiss of 130-300 MeV/c

We have measured the 4He(e,e'p)3H reaction at missing momenta of 130-300 MeV/c using the three-spectrometer facility at the Mainz microtron MAMI. Data were taken in perpendicular kinematics to allow us to determine the response function Rlt and the asymmetry term Atl. The data are compared to both relativistic and non-relativistic calculations.

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arXiv2003

Final State Interaction Effects in pol 3He(pol e,e'p)

Asymmetries in quasi-elastic pol 3He(pol e,e'p) have been measured at a momentum transfer of 0.67 (GeV/c)^2 and are compared to a calculation which takes into account relativistic kinematics in the final state and a relativistic one-body current operator. With an exact solution of the Faddeev equation for the 3He-ground state and an approximate treatment of final state interactions in the continuum good agreement is found with the experimental data.

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arXiv2002

Axial Excitation of the Delta in Chiral Quark Models

We discuss electro-magnetic and weak axial N-Delta transition amplitudes in the linear sigma-model and the Cloudy-Bag Model as typical representatives of chiral quark models. We show that good qualitative understanding of the transition can be obtained in models which, in addition to the pion cloud, incorporate a fluctuating sigma-field inside the baryon.

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arXiv2002

N-Delta axial transition form factors

We review some basic properties of the N-Delta transition axial amplitudes and relate them to the strong pi-N-Delta form-factor. In models with the pion cloud we derive a set of constraints on the pion wave function which guaranty the correct behaviour of the amplitudes in the vicinity of the pion pole. Corrections due to the spurious center-of-mass motion are calculated to the leading order in the inverse baryon mass. We give explicit expressions for the amplitudes in the Cloudy Bag Model and show that they rather strongly underestimate the experimental values.

hep-ph↗
arXiv2002

Axial amplitudes for Delta excitation in chiral quark models

We study the axial amplitudes for the N-Delta transition in models with quarks and chiral mesons. A set of constraints on the pion field is imposed which enforces PCAC and the off-diagonal Goldberger-Treiman relation. The quark contribution to the amplitudes in general strongly underestimates the C^A_5 amplitude as well as the piNDelta strong coupling constant. We show that the results are considerably improved in models that, in addition to the pion cloud, incorporate a fluctuating sigma field inside the baryon.

hep-ph↗
arXiv2002

Radial excited states of the nucleon in quark models with dynamical confinement

The Roper state is described in the framework of the chiral chromodielectric model as a single quark excitation in self-consistent meson potentials. The interplay between quark and meson excitations is discussed and we emphasise the role of non-quark degrees of freedom in the electroproduction of nucleon excitations.

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arXiv2001

Precise Neutron Magnetic Form Factors

Precise data on the neutron magnetic form factor G_{mn} have been obtained with measurements of the ratio of cross sections of D(e,e'n) and D(e,e'p) up to momentum transfers of Q^2 = 0.9 (GeV/c)^2. Data with typical uncertainties of 1.5% are presented. These data allow for the first time to extract a precise value of the magnetic radius of the neutron.

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arXiv2001

Recent measurements with the out-of-plane spectrometer system at MIT-Bates

The recent experimental program with the out-of-plane spectrometer system (OOPS) at MIT-Bates encompassed an extensive set of $\mathrm{d}(\vec\mathrm{e},\mathrm{e}'\mathrm{p})$ measurements, investigations of the $\mathrm{N}\toΔ$ transition using $\mathrm{p}(\vec\mathrm{e},\mathrm{e}'\mathrm{p})π^0$ and $\mathrm{p}(\vec\mathrm{e},\mathrm{e}'π^+)\mathrm{n}$ reaction channels, and studies of virtual Compton scattering (VCS) $\mathrm{p}(\mathrm{e},\mathrm{e}'\mathrm{p})γ$ below the pion threshold. Preliminary results are presented.

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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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arXiv1996

The role of the pion cloud in electroproduction of the $Δ$(1232)

We calculate the ratios $E2/M1$ and $C2/M1$ of the multipole amplitudes for electroproduction of the $Δ$(1232) in the range of photon virtuality $0<-K^2<1$~GeV$^2$ in a chiral chromodielectric model and a linear $σ$-model. We find that relatively large experimental values can be explained in terms of the pion contribution alone; the contribution arising from d-state quark admixture remains below 10\%. We describe the pion cloud as a coherent state and use spin and isospin projection to obtain the physical nucleon and the $Δ$. The $A_{1/2}$ and $A_{3/2}$ amplitudes are reasonably well reproduced in the $σ$-model; in the chromodielectric model, however, they are a factor of two too small.

hep-ph↗
arXiv1995

E2 and C2 amplitudes for electroproduction of $Δ$

We compute the amplitudes for the electromagnetic $γ_v N \to Δ$ transition in the framework of soliton models with quarks and mesons. The ratios E2/M1 and C2/M1 as functions of the photon four-momentum squared are dominated by the contribution of the pion cloud and are in reasonable agreement with the available experimental data.

hep-ph↗
arXiv1995

Counting pions in the nucleon

The number of pions in the nucleon is a theoretical concept and its value is model dependent. It is, however, a useful tool to understand the pionic contribution to several nucleon observables such as electric charge radii, magnetic moments, axial charge, pion-nucleon coupling constant, amplitudes for electroexcitation of baryon resonances and electroproduction of pions above such resonances.

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