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G. Salme`

Publications and source records attributed to G. Salme`.

At least 19 recordsLinked to original sources
arXiv2008

Trinucleon Electromagnetic Form Factors and the Light-Front Hamiltonian Dynamics

This contribution briefly illustrates preliminary calculations of the electromagnetic form factors of $^3$He and $^3$H, obtained within the Light-front Relativistic Hamiltonian Dynamics, adopting i) a Poincaré covariant current operator, without dynamical two-body currents, and ii) realistic nuclear bound states with $S$, $P$ and $D$ waves. The kinematical region of few $(GeV/c)^2$, relevant for forthcoming TJLAB experiments, has been investigated, obtaining possible signatures of relativistic effects for $Q^2>2.5 ~(GeV/c)^2$.

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arXiv2008

Light-front Ward-Takahashi Identity for Two-Fermion Systems

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 quasi-potential 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 quasi-potential expansion, and the divergence of the proposed current satisfies a Ward-Takahashi identity at any given order of the expansion. In the quasi-potential 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 quasi-potential 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.

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arXiv2007

Relativistic Hamiltonian Dynamics and Few-Nucleon Systems

We present a preliminary calculation of the electromagnetic form factors of $^3$He and $^3$H, performed within the Light-Front Hamiltonian Dynamics. Relativistic effects show their relevance even at the static limit, increasing at higher values of momentum transfer, as expected.

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arXiv2006

Electromagnetic form factors of the nucleon in spacelike and timelike regions

An approach for a unified description of the nucleon electromagnetic form factors in spacelike and timelike regions is presented. The main ingredients of our model are: $i)$ a Mandelstam formula for the matrix elements of the nucleon electromagnetic current; $ii)$ a 3-dimensional reduction of the problem on the Light-Front performed within the so-called {\tt Propagator Pole Approximation} ({\bf PPA}), which consists in disregarding the analytical structure of the Bethe-Salpeter amplitudes and of the quark-photon vertex function in the integration over the minus components of the quark momenta; $iii)$ a dressed photon vertex in the $q\bar{q}$ channel, where the photon is described by its spin-1, hadronic component.

hep-ph↗
arXiv2006

Timelike and spacelike hadron form factors, Fock state components and light-front dynamics

A unified description of spacelike and timelike hadron form factors within a light-front model was successfully applied to the pion. The model is extended to the nucleon to study the role of $q \bar q$ pair production and of nonvalence components in the nucleon form factors. Preliminary results in the spacelike range $0 \le Q^2 \le 10 ~ (GeV/c)^2$ are presented.

hep-ph↗
arXiv2005

A light-front Quark Model for the electromagnetic form factor of the pion

In this contribution, an approach for a unified description of the pion electromagnetic form factor, in the space- and time-like regions, within a constituent quark model on the light front, will be reviewed. Our approach is based on i) the on-shell quark-hadron vertex functions in the valence sector, ii) the dressed photon vertex where a photon decays in a quark-antiquark pair, and iii) the emission and absorption amplitudes of a pion by a quark. Results favorably compare with the existing experimental data.

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arXiv2005

A Study of Final State Effects in the electrodisintegration of a polarized Helium-3 target

An approach for the description of the final state interaction in the evaluation of inclusive electromagnetic responses of a polarized Helium-3 target, is briefly illustrated. Preliminary results of calculations, where the final state interaction is fully taken into account for the two-body break-up channel, are compared with experimental data, showing a very encouraging improvement with respect to the plane wave impulse approximation results.

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arXiv2003

Electromagnetic Form Factors of Hadrons in the Space- and Time-like Regions within a Constituent Quark Model on the Light Front

An approach for describing the electromagnetic form factors of hadrons, in the space- and time-like regions, within a constituent quark model on the light front is shortly illustrated and calculations for the pion case are reported. Three main ingredients enter our approach: i) the dressed photon vertex where a photon decays in a quark-antiquark pair, ii) the on-shell quark-hadron vertex functions in the valence sector, and iii) the non-valence component of the hadron state.

hep-ph↗
arXiv2003

Electromagnetic form factor of the pion in the space- and time-like regions within the front-form dynamics

The pion electromagnetic form factor is calculated in the space- and time-like regions from -10 $(GeV/c)^2$ up to 10 $(GeV/c)^2$, within a front-form model. The dressed photon vertex where a photon decays in a quark-antiquark pair is depicted generalizing the vector meson dominance ansatz, by means of the vector meson vertex functions. An important feature of our model is the description of the on-mass-shell vertex functions in the valence sector, for the pion and the vector mesons, through the front-form wave functions obtained within a realistic quark model. The theoretical results show an excellent agreement with the data in the space-like region, while in the time-like region the description is quite encouraging.

hep-ph↗
arXiv2002

Pair term in the Electromagnetic Current within the Front-Form Dynamics: Spin-0 Case

The frame and scale dependence of the pair-term contribution to the electromagnetic form factor of a spin-zero composite system of two-fermions is studied within the Light Front. The form factor is evaluated from the plus-component of the current in the Breit frame, using for the first time a nonconstant, symmetric ansatz for the Bethe-Salpeter amplitude. The frame dependence is analyzed by allowing a nonvanishing plus component of the momentum transfer, while the dynamical scale is set by the masses of the constituents and by mass and size of the composite system. A transverse momentum distribution, associated with the Bethe-Salpeter amplitude, is introduced which allows to define strongly and weakly relativistic systems. In particular, for strongly relativistic systems, the pair term vanishes for the Drell-Yan condition, while is dominant for momentum transfer along the light-front direction. For a weakly relativistic system, fitted to the deuteron scale, the pair term is negligible up to momentum transfers of 1(GeV/c)$^2$. A comparison with results obtained within the Front-Form Hamiltonian dynamics with a fixed number of constituents is also presented.

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arXiv2001

Neutron structure function $F_2^n(x)$ from deep inelastic electron scattering off few-nucleon systems

The possibility of a reliable extraction of the neutron deep inelastic structure function, $F_2^n(x)$, for $ x < 0.85$ from joint measurements of deep inelastic structure functions of deuteron, $^{3}He$ and $^{3}H$ 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.

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arXiv2001

Deuteron Electromagnetic Properties with a Poincaré-Covariant Current Operator within Front-Form Hamiltonian Dynamics

Deuteron elastic and deep inelastic electromagnetic properties have been studied within the front-form Hamiltonian dynamics, using a Poincaré-covariant current operator. The deuteron elastic form factors are strongly sensitive to different realistic $N-N$ interactions, while the relevance of different nucleon form factor models is huge for $A(Q^2)$, weak for $B(Q^2)$ and negligible for the tensor polarization. The possibility to gain information on the neutron charge form factor from an analysis of $A(Q^2)$ has been investigated. The extraction of the neutron structure functions from the deuteron deep inelastic structure functions at high $x$ is largely affected by the use of our Poincaré-covariant relativistic approach instead of the usual impulse approximation within an instant-form approach.

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arXiv2000

Poincare' Covariant Current Operator and Elastic Electron-Deuteron Scattering in the Front-form Hamiltonian Dynamics

The deuteron electromagnetic form factors, $A(Q^2)$ and $B(Q^2)$, and the tensor polarization $T_{20}(Q^2)$, 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 $Q^2 < 0.4 (GeV/c)^2$. At higher momentum transfer, different nucleon-nucleon interactions strongly affect $A(Q^2)$, $B(Q^2)$, and $T_{20}(Q^2)$ 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 $A(Q^2)$, smaller effects on $B(Q^2)$ and essentially none on $T_{20}(Q^2)$.

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arXiv1999

Deuteron Magnetic and Quadrupole Moments with a Poincaré Covariant Current Operator in the Front-Form Dynamics

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.

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arXiv1999

A Poincaré-covariant current operator for interacting systems and deuteron electromagnetic form factors

In front-form dynamics a current operator for systems of interacting particles, which fulfills Poincaré, parity and time reversal covariance, together with hermiticity, can be defined. The electromagnetic form factors can be extracted without any ambiguity and in the elastic case the continuity equation is automatically satisfied. Applications to the calculation of deuteron form factors are presented, and the effects of different nucleon-nucleon interactions, as well as of different nucleon form factors are investigated.

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arXiv1998

A Poincaré Covariant Current Operator for Deep Inelastic Structure Functions and Deuteron Electromagnetic Properties

In front-form dynamics the current operator can be constructed from auxiliary operators, defined in a Breit frame where initial and final three-momenta of the system are directed along the $z$ axis. Poincaré covariance constraints reduce for auxiliary operators to the ones imposed only by kinematical rotations around the $z$ axis. Elastic and transition form factors can be extracted without any ambiguity and in the elastic case the continuity equation is automatically satisfied, once Poincaré, ${\cal P}$ and ${\cal T}$ covariance, together with hermiticity, are imposed. Applications to deep inelastic structure functions in an exactly solvable model and to the calculation of the deuteron electromagnetic form factors are presented.

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