Light-front Hamiltonian Dynamics Research Articles

The EMC effect for few-nucleon bound systems in light-front Hamiltonian dynamics

The light-front formalism for a covariant description of the European Muon Collaboration (EMC) effect, already applied to He3, is formally extended to any nucleus, and used for actually calculating the 3H and He4 cases. The realistic and accurate nuclear description of few-nucleon bound systems, obtained with both phenomenological and chiral potentials, has been properly combined with the Poincaré covariance and macroscopic locality, automatically satisfying both number of particles and momentum sum rule. While retaining the on-mass-shell nucleon structure functions, one is then able to predict a sizable EMC effect for He4, as already observed for He3. Moreover, the impact on the EMC effect of both i) the short-range correlations, such as those generated by modern nuclear interactions, and ii) the ratio between the neutron and proton structure functions has been studied. The short-range correlations generated by retaining only the standard nuclear degrees of freedom act on the depth of the minimum in the EMC ratio, while the uncertainties linked to the ratio of neutron to proton structure functions are found to be very small. These light-front results facilitate ascribing deviations from experimental data due to genuine QCD effects, not included in a standard nuclear description, and initiating unbiased investigations.

The European Muon Collaboration effect in light-front Hamiltonian dynamics

A new and rigorous light-front formalism for electron deep inelastic scattering on unpolarized nuclei, in Bjorken limit, is proposed. It preserves Poincaré covariance, macroscopic locality, both number of particles and momentum sum rules. The approach is applied to the A=3 iso-doublet, very relevant for the flavor decomposition of the parton distribution functions and in view of the planned operation with unpolarized and polarized beams at the Electron-Ion Collider. At variance with previous light-front estimates, our procedure, including a realistic nuclear description and free-nucleon structure functions, predicts a sizable European Muon Collaboration effect for 3He. This will allow to analyze deviations from the proposed baseline in terms of genuine QCD effects. The extension to heavier nuclei is straightforward, although numerically challenging.

Light-front spin-dependent spectral function and nucleon momentum distributions for a three-body system

Poincare' covariant definitions for the spin-dependent spectral function and for the momentum distributions within the light-front Hamiltonian dynamics are proposed for a three-fermion bound system, starting from the light-front wave function of the system. The adopted approach is based on the Bakamjian-Thomas construction of the Poincare' generators, that allows one to easily import the familiar and wide knowledge on the nuclear interaction into a light-front framework. The proposed formalism can find useful applications in refined nuclear calculations, like the ones needed for evaluating the EMC effect or the semi-inclusive deep inelastic cross sections with polarized nuclear targets, since remarkably the light-front unpolarized momentum distribution by definition fulfills both normalization and momentum sum rules. It is also shown a straightforward generalization of the definition of the light-front spectral function to an A-nucleon system.

Light-Front Dynamics and the $${{^{3}He}}$$ 3 H e Spectral Function

Two topics are presented. The first one is a novel approach for a Poincare' covariant description of nuclear dynamics based on light-front Hamiltonian dynamics. The key quantity is the light-front spectral function, where both normalization and momentum sum rule can be satisfied at the same time. Preliminary results are discussed for an initial analysis of the role of relativity in the EMC effect in 3He. A second issue, very challenging, is considered in a non-relativistic framework, namely a distorted spin-dependent spectral function for 3He in order to take care of the final state interaction between the observed pion and the remnant in semi-inclusive deep inelastic electron scattering off polarized 3He. The generalization of the analysis within the light-front dynamics is outlined.

Next-to-next-to-leading-order nucleon parton distributions from a light-cone quark model dressed with its virtual meson cloud

A detailed study of nucleon parton distribution functions is performed within a radiative Next-to-Next-to Leading Order (NNLO) parton model whose low-scale input is rigidly described by wave functions which include quarks and dressing meson-cloud. The light-front Hamiltonian dynamics fixes the three-quark wave functions and the meson cloud is introduced by means of high-order Fock component in time-ordered perturbation theory in the infinite momentum frame. Non-strange as well as strange mesons-baryon fluctuations are considered ($\pi$, $\rho$, $\omega$, $K$ and $K^*$, together with $N$, $\Delta$, $\Lambda$ and $\Sigma$) and the effects on strange and non-strange parton distributions investigated showing the large effects due to (non-strange) sea asymmetries and the delicate balance of the strange asymmetry. The non-strange and the strange components, the valence as well as the gluon distributions are compared with available experimental data confirming the need of both non-perturbative degrees of freedom and perturbative (NNLO) radiative effects.

Two-body Current Operators and Deuteron Form Factors Within the Light-Front Hamiltonian Dynamics

The deuteron electromagnetic properties are investigated within the Light-Front Hamiltonian dynamics using one-body and two-body currents. The dynamical nature of the latter is generated within a Yukawa model from an analysis of the Light-Front current that acts on the three-dimensional valence component and fulfills the Ward–Takahashi identity. Preliminary results for the magnetic moment and the probability of the nonvalence component are shown.

A covariant investigation of Neutral Vector Mesons: dynamical properties and electromagnetic decay widths

A simple, but fully-covariant model for describing neutral Vector Mesons, in both light and heavy sectors, is briefly illustrated. The main ingredients of our relativistic constituent model are i) an Ansatz for the Bethe-Salpeter vertex for Vector Mesons, and ii) a Mandelstam-like formula for the electromagnetic decay widths. The free parameters of our approach are fixed through a comparison with the valence transverse-momentum distribution, $n(k_\perp)$, obtained within phenomenological Light-Front Hamiltonian Dynamics models reproducing the mass spectra. Preliminary results for the transverse-momentum distributions, the parton distribution and the electromagnetic decay constants are shown.

Relativistic Hamiltonian dynamics and few-nucleon systems

We present a preliminary calculation of the electromagnetic form factors of 3He and 3H, 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.

Spin force dependence of the parton distributions: The ratioF2n(x,Q2)/F2p(x,Q2)

Light-front Hamiltonian dynamics is used to relate low-energy constituent quark models to deep inelastic unpolarized structure functions of the nucleon. The approach incorporates the correct Pauli principle prescription consistently and it allows a transparent investigation of the effects due to the spin-dependent SU(6)-breaking terms in the quark model Hamiltonian. Both Goldstone-boson-exchange interaction and hyperfine-potential models are discussed in a unified scheme and a detailed comparison, between the two(apparently) different potential prescriptions, is presented.

Renormalization group procedure for effective particles in light-front Hamiltonian dynamics

We describe the renormalization group procedure for effective particles, and we present results of its application to the f 3 vertex in scalar field theory in 6 dimensions and to the triple-gluon vertex counterterm of QCD in 4 dimensions.

Entanglement of Fock-space expansion and covariance in light-front Hamiltonian dynamics

We investigate in a model with scalar ``nucleons'' and mesons the contributions of higher Fock states that are neglected in the ladder approximation of the Lippmann-Schwinger equation. This leads to a breaking of covariance, both in light-front and in instant-form Hamiltonian dynamics. The lowest Fock sector neglected has two mesons in the intermediate state and corresponds to the stretched box. First we show in a simplified example that the contributions of higher Fock states are much smaller on the light-front than in instant-form dynamics. Then we show for a scattering amplitude above threshold that the stretched boxes are small, however necessary to retain covariance. For an off-energy-shell amplitude covariance is not necessarily maintained and this is confirmed by our calculations. Again, the stretched boxes are found to be small.

Relativistic scattering and bound-state properties in a special Hamiltonian model.

We present numerical results for fermion-boson scattering cross sections and bound-state form factors in an elementary model of relativistic renormalized light-front Hamiltonian dynamics. The model Hamiltonian describes a fermion emitting and absorbing one scalar boson. Renormalization of the coupling constant leads to triviality and the cutoff cannot be arbitrarily large. Nevertheless, the resulting total fermion-boson scattering cross section is found to be practically independent of the cutoff within the triviality bounds. We also study cutoff dependence of the fermion-boson bound-state form factors. The cutoff dependence is negligible for boson masses considerably smaller than the fermion mass even for momentum transfers exceeding many times the fermion mass, as long as the momentum transfer is small compared to the cutoff. For heavy bosons the cutoff dependence of the fermion-boson sector contribution is stronger but the bound-state structure is dominated by the cutoff-independent bare fermion component. Thus, the model Hamiltonian leads to almost cutoff-independent results in a whole range of mass and coupling parameters within the triviality bounds.