Search Results (27 total)

We investigate the model dependence of no-helicity flip generalized parton distribution of the pion upon different approaches for the quark-hadron and quark-photon vertices, in the spacelike region. In order to obtain information on contributions from both the valence and the nonvalence regions, we compare results for spacelike momentum transfers obtained from (i) an analytic covariant model with a bare quark-photon vertex, (ii) a light-front approach with a quark-photon vertex dressed through a microscopic vector-meson model, and (iii) a light-front approach based on the relativistic Hamiltonian dynamics. Our comparisons lead us to infer the same dynamical mechanism, the one-gluon-exchange dominance at short distances, as a source of both the electromagnetic form factor at large momentum transfer and the parton distribution close to the end points. The expected collinear behavior of the generalized parton distributions at high-momentum transfer, i.e. a maximum for x∼1, is also illustrated, independently of the different approaches. Finally, a comparison with recent lattice calculations of the gravitational form factors is presented.

We present a measurement of the spin-dependent cross sections for the ³He→(e→,e^′)X reaction in the quasielastic and resonance regions at a four-momentum transfer 0.1≤Q²≤0.9  GeV². The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

We propose a three-dimensional electromagnetic current operator within light-front dynamics that satisfies a light-front Ward-Takahashi identity for two-fermion systems. The light-front current operator is obtained by a quasipotential reduction of the four-dimensional current operator and acts on the light-front valence component of bound or scattering states. A relation between the light-front valence wave function and the four-dimensional Bethe-Salpeter amplitude both for bound or scattering states is also derived, such that the matrix elements of the four-dimensional current operator can be fully recovered from the corresponding light-front ones. The light-front current operator can be perturbatively calculated through a quasipotential expansion, and the divergence of the proposed current satisfies a Ward-Takahashi identity at any given order of the expansion. In the quasipotential expansion the instantaneous terms of the fermion propagator are accounted for by the effective interaction and two-body currents. We exemplify our theoretical construction in the Yukawa model in the ladder approximation, investigating in detail the current operator at the lowest nontrivial order of the quasipotential expansion of the Bethe-Salpeter equation. The explicit realization of the light-front form of the Ward-Takahashi identity is verified. We also show the relevance of instantaneous terms and of the pair contribution to the two-body current and the Ward-Takahashi identity.

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

The simultaneous investigation of the pion electromagnetic form factor in the space- and timelike regions within a light-front model allows one to address the issue of nonvalence components of the pion and photon wave functions. Our relativistic approach is based on a microscopic vector-meson-dominance model for the dressed vertex where a photon decays in a quark-antiquark pair, and on a simple parametrization for the emission or absorption of a pion by a quark. The results show an excellent agreement in the space like region up to -10  (GeV/c)², while in timelike region the model produces reasonable results up to 10  (GeV/c)².

We have searched for a deeply bound kaonic state by using the FINUDA spectrometer installed at the e⁺e^- collider DAΦNE. Almost monochromatic K^-’s produced through the decay of ϕ(1020) mesons are used to observe K^- absorption reactions stopped on very thin nuclear targets. Taking this unique advantage, we have succeeded to detect a kaon-bound state K^-pp through its two-body decay into a Λ hyperon and a proton. The binding energy and the decay width are determined from the invariant-mass distribution as 115_-5⁺⁶(stat)_-4⁺³(syst)   MeV and 67_-11⁺¹⁴(stat)_-3⁺²(syst)   MeV, respectively.

A high precision measurement of the transverse spin-dependent asymmetry A_T^′ in ³He→(e→,e^′) quasielastic scattering was performed in Hall A at Jefferson Lab at values of the squared four-momentum transfer, Q², between 0.1 and 0.6 (GeV/c)². A_T^′ is sensitive to the neutron magnetic form factor, G_Mⁿ. Values of G_Mⁿ at Q²=0.1 and 0.2 (GeV/c)², extracted using Faddeev calculations, were reported previously. Here, we report the extraction of G_Mⁿ for the remaining Q² values in the range from 0.3 to 0.6 (GeV/c)² using a plane-wave impulse approximation calculation. The results are in good agreement with recent precision data from experiments using a deuterium target.

We present the first precision measurement of the spin-dependent asymmetry in the threshold region of ³H→e(e→,e^′) at Q² values of 0.1 and 0.2 (GeV/c)². The agreement between the data and nonrelativistic Faddeev calculations which include both final-state interactions and meson-exchange current effects is very good at Q²  =  0.1 (GeV/c)², while a small discrepancy at Q²  =  0.2 (GeV/c)² is observed.

The possibility of a reliable extraction of the neutron deep inelastic structure function, F₂ⁿ(x), for x<0.85 from joint measurements of deep inelastic structure functions of deuteron, ³He, and ³H is investigated. The model dependence in this extraction, linked to the possible different interactions between nucleons in nuclei, is shown to be weak, if the nuclear structure effects are properly taken into account. A combined analysis of the deep inelastic structure functions of these nuclei is proposed to study effects beyond the impulse approximation.

The deuteron electromagnetic form factors, A(Q²) and B(Q²), and the tensor polarization T₂₀(Q²), are unambiguously calculated within the front-form relativistic Hamiltonian dynamics, by using a novel current, built up from one-body terms, which fulfills Poincaré, parity, and time reversal covariance, together with Hermiticity and the continuity equation. A simultaneous description of the experimental data for the three deuteron form factors is achieved up to Q²<0.4 (GeV/c)². At higher momentum transfer, different nucleon-nucleon interactions strongly affect A(Q²), B(Q²), and T₂₀(Q²), and the effects of the interactions can be related to S-state kinetic energy in the deuteron. Different nucleon form factor models have huge effects on A(Q²), smaller effects on B(Q²), and essentially none on T₂₀(Q²).

We have measured the transverse asymmetry A_T^′ in ³He→(e→,e^′) quasielastic scattering in Hall A at Jefferson Laboratory with high precision for Q² values from 0.1 to 0.6 (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted based on Faddeev calculations for Q²  =  0.1 and 0.2 (GeV/c)² with an experimental uncertainty of less than 2%.

The deuteron magnetic and quadrupole moments are unambiguosly determined within the front-form Hamiltonian dynamics, by using a new current operator which fulfills Poincaré, parity, and time reversal covariance, together with Hermiticity and the continuity equation. For both quantities the usual disagreement between theoretical and experimental results is largely removed.

Experimental deep inelastic neutron-scattering data from fluid deuterium at a temperature of 20.7 K, collected over a wide range of momentum transfer q (28 Å^-1<q<70 Å^-1), are presented. These experimental results, together with data previously collected from fluid hydrogen, are compared with a quantum-mechanical calculation, based on a Morse anharmonic intramolecular interaction. For both fluids it is shown that the present calculation is able to reproduce the experimental response function F(y,q) over a wide momentum-transfer range, which spans from the molecular rotovibrational structure up to the molecular dissociation regime. Thus this calculation represents a clear improvement on previous models, based on harmonic or impulse approximations, which provide a good description of these fluids in the two opposite limits only, i.e., at the lowest and highest q values, respectively.

In the theory of deep inelastic scattering the final state interaction between the struck quark and the remnants of the target is usually assumed to be negligible in the Bjorken limit. This assumption, still awaiting a full validation within nonperturbative QCD, is investigated in a model composed of two relativistic particles, interacting via a relativistic harmonic oscillator potential, within light-cone Hamiltonian dynamics. An electromagnetic current operator whose matrix elements behave properly under Poincaré transformations is adopted. It is shown that (i) the parton model is recovered, once the standard parton model assumptions are adopted, and (ii) when relativistic, interacting eigenfunctions are exactly taken into account for both the initial and final states, the values of the structure functions, averaged over small, but finite intervals of the Bjorken variable x, coincide with the results of the parton model in the Bjorken limit.

The electromagnetic inclusive responses of polarized ³He and ³H are thoroughly investigated at the quasielastic peak for squared momentum transfers up to 2 (GeV/c)², within the plane wave impulse approximation. Great emphasis is put on the effects in the bound state due to different two- and three-body nuclear forces, and to the Coulomb interaction as well. A careful analysis of the polarized responses allows one to select possible experiments for minimizing the model dependence in the extraction of the neutron electromagnetic form factors. In particular, the relevant role played by the proton in the transverse-longitudinal response of polarized ³He, at low momentum transfer, can be utilized for obtaining valuable information on the proton contribution to the total polarized response and eventually on the neutron charge form factor.

A model-independent procedure is proposed which allows us to obtain the asymptotic scaling function at infinite momentum transfer and a comprehensive assessment of deviations of deep inelastic-neutron-scattering data from a perfect scaling behavior, which are due to the finite values of energy and momentum transfers to the target atom. The procedure is applied to simulated and experimental data on polycrystalline ZrH₂ at T=20 and 290 K. We observe that the impulse approximation, valid for infinite energy and momentum transfers, is not yet completely reached at the finite momentum transfer values q of neutron experiments even at q≃60 Å^-1. It is argued that, beyond the specific case of ZrH₂, this procedure can be used for the description of deep inelastic-neutron scattering from other molecular and atomic systems.

The charge form factor of π and K mesons is evaluated adopting a relativistic constituent quark model based on the light-front formalism. The relevance of the high-momentum components of the meson wave function, for values of the momentum transfer accessible to CEBAF energies, is illustrated. The predictions for the elastic form factor of π and K mesons are compared with the results of different relativistic approaches, showing that the measurements of the pion and kaon form factors planned at CEBAF could provide information for discriminating among various model of the meson structure.

A quantum-mechanical calculation for the description of neutron scattering at very high momentum transfer off both homonuclear and heteronuclear diatomic molecules at T=0 K in terms of the nuclear momentum distribution, n(k), of the nuclei in these molecules is proposed. The results of this calculation compare well with neutron-scattering measurements on liquid D₂ at T=20 K in the momentum transfer range 55<q<80 Å^-1 and with previous measurements on solid H₂ at T=4 K and liquid H₂ at T=20 K at high momentum transfer (q∼100 Å^-1), showing that high-energy neutrons can be a sensitive probe to investigate the momentum distribution of the nuclei in these molecules.

The theory of quasielastic inclusive scattering of polarized leptons off polarized ³He is critically reviewed and the origin of different expressions for the polarized nuclear response function appearing in the literature is explained. The sensitivity of the longitudinal asymmetry upon the neutron form factors is thoroughly investigated and the role played by the polarization angle for minimizing the proton contribution is illustrated.

It is shown that the nuclear effects playing a relevant role in deep inelastic scattering of polarized electrons by polarized ³He are mainly those arising from the effective proton and neutron polarizations generated by the S’ and D waves in ³He. A simple and reliable equation relating the neutron g₁ⁿ, and ³He, g₁³, spin structure functions is proposed. It is shown that the measurment of the first moment of the ³He structure function can provide a significant check of the Bjorken sum rule.

The asymmetry for quasielastic inclusive scattering of polarized electrons by polarized ³He has been computed for the first time in terms of a spin dependent spectral function obtained from a realistic three-body wave function without using the closure approximation. It is shown that a proper choice of kinematics minimizes the proton contribution to the asymmetry, which therefore becomes very sensitive to different models of the electromagnetic form factors of the neutron. The calculated asymmetry qualitatively agrees with the available experimental data.

The approach to y scaling previously adopted to obtain the nucleon momentum distribution in the two- and three-nucleon systems is extended to the case of complex nuclei and nuclear matter. The basic elements of this approach, which takes properly into account nucleon binding and momentum, are reviewed. A new method of analysis, which allows one to obtain the experimental asymptotic scaling function from inclusive cross sections even if these data are affected by final-state interactions, is proposed and illustrated. By such a method, the asymptotic scaling functions of ³He, ⁴He, ¹²C, ⁵⁶Fe, and nuclear matter are obtained from recent experimental data and it is demonstrated that, particularly at high negative values of the scaling variable, the available data points at the highest value of the momentum transfer are affected by final-state interaction and cannot therefore be considered to represent the asymptotic scaling function. It is shown that, unlike what is commonly stated, the nucleon momentum distribution is not simply defined in terms of the derivative of the asymptotic scaling function, but as a sum of such a derivative plus the derivative of a quantity, the binding correction, generated by the removal energy distribution of nucleons embedded in the nuclear medium.

The binding correction and its derivative are evaluated with various types of spectral functions, and the nucleon momentum distributions in ³He, ⁴He, ¹²C, ⁵⁶Fe, and nuclear matter are obtained up to nucleon momenta k≊500 MeV/c. For few-body systems the obtained momentum distributions satisfactorily agree with the ones extracted from (e,e’p) reactions and with theoretical calculations performed within Faddeev or variational approaches, whereas for complex nuclei they qualitatively agree with predictions of theoretical many-body approaches which take nucleon-nucleon correlations into account and, at the same time, at k≥350 MeV/c they are larger by orders of magnitude than the ones predicted by mean field approaches. Such a result does represent unambiguous evidence of correlation effects in nuclei.

The method, previously used to obtain the nucleon momentum distribution in ²H from the y-scaling analysis of inclusive electron scattering data, is extended to ³He. It is shown that the binding correction which has to be handled in this case does not hinder the extraction of the momentum distribution from the scaling function. The obtained momentum distribution satisfactorily agrees with the one extracted from exclusive (e,e’p) reactions.