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A. Barducci

Publications and source records attributed to A. Barducci.

At least 19 recordsLinked to original sources
arXiv2016

Relativistic two-body calculation of $b\bar{b}$-mesons radiative decays

This paper is a prosecution of a previous work where we presented a unified two-fermion covariant scheme which produced very precise results for the masses of light and heavy mesons. We extend the analysis to some radiative decays of mesons $Υ,\,χ_{b2}\,,h_b\,,χ_{b1}\,,χ_{b0}\,,η_b\,$ and we calculate their branching ratios and their widths. For most of them we can make a comparison with experimental data finding a good agreement.For the decays for which data are not available we compare ours with other recent theoretical previsions.

hep-ph↗
arXiv2014

Relativistic two fermion treatment of hyperfine transitions

A system of two fermions with different masses and interacting by the Coulomb potential is presented in a completely covariant framework. The spin-spin interaction, including the anomalous magnetic moments of the two fermions, is added by means of a Breit term. We solve the resulting fourth order differential system by evaluating the spectrum and the eigenfunctions. The interaction vertex with an external electromagnetic field is then determined. The relativistic eigenfunctions are used to study the photon emission from a hyperfine transition and are checked for the calculation of the Lamb shift due to the electron vacuum polarization in the muonic Hydrogen.

hep-ph↗
arXiv2012

Effective action for fermions with anomalous magnetic moment from Foldy-Wouthuysen transformation

In this paper we calculate the effective action for neutral particles with anomalous magnetic moment in an external magnetic and electric field. We show that we can take advantage from the Foldy Wouthuysen transformation for such systems, determined in our previous works: indeed, by this transformation we have explicitly evaluated the diagonalized Hamiltonian, allowing to present a closed form for the corresponding effective action and for the partition function at finite temperature from which the thermodynamical potentials can be calculated.

hep-th↗
arXiv2010

Particles with anomalous magnetic moment in external e.m. fields: the proper time formulation

In this paper we evaluate the expression for the Green function of a pseudo-classical spinning particle interacting with constant electromagnetic external fields by taking into account the anomalous magnetic and electric moments of the particle. The spin degrees of freedom are described in terms of Grassmann variables and the evolution operator is obtained through the Fock-Schwinger proper time method.

hep-th↗
arXiv2009

Foldy-Wouthuysen Transformation for a Spinning Particle with Anomalous Magnetic Moment

We study the Foldy-Wouthuysen transformation for a pseudoclassical particle with anomalous magnetic moment in an external, stationary electromagnetic field. We show that the transformation can be expressed in a closed form for neutral particles in purely electrostatic fields and for neutral and charged particles in external magnetostatic fields. The explicit expressions of the diagonalized Hamiltonians are calculated.

hep-th↗
arXiv2008

Spinning Particle with Anomalous Magnetic Moment in an External Plane Wave Field

In this paper we study the interaction of a Dirac-Pauli particle with an electromagnetic plane wave, by using a previously given generalization of the pseudo-classical Lagrangian for a spinning particle with an anomalous magnetic moment. We derive the explicit expressions for the eigenfunctions and the Green's functions of the theory. We discuss the validity of the semi-classical approach by comparing the wavefunctions with the (pseudo)-classical solutions of the Hamilton-Jacobi equation.

quant-ph↗
arXiv2005

A NJL-based study of the QCD critical line

We employ a 3 flavor NJL model to stress some general remarks about the QCD critical line. The dependence of the critical curve on $μ_q=(μ_u+μ_d)/2$ and $μ_I=(μ_u-μ_d)/2$ is discussed. The quark masses are varied to confirm that, in agreement with universality arguments, the order of transition depends on the number of active flavors $N_f$. The slope of the critical curve vs. chemical potential is studied as a function of $N_f$. We compare our results with those recently obtained in lattice simulations to establish a comparison among different models.

hep-ph↗
arXiv2004

Pion and kaon condensation in a 3-flavor NJL model

We analyze the phase diagram of a three-flavor Nambu-Jona-Lasinio model at finite temperature $T$ and chemical potentials $μ_u, μ_d, μ_s$. We study the competition of pion and kaon condensation and we propose a physical situation in which kaon condensation could be led only by light quark finite densities.

hep-ph↗
arXiv2004

Spinning particle in an external linearized gravitational wave field

We study the interaction of a scalar and a spinning particle with a coherent linearized gravitational wave field treated as a classical spin two external field. The spin degrees of freedom of the spinning particle are described by skew-commuting variables. We derive the explicit expressions for the eigenfunctions and the Green's functions of the theory. The discussion is exact within the approximation of neglecting radiative corrections and we prove that the result is completely determined by the semiclassical contribution.

hep-th↗
arXiv2003

Scalar and spinning particles in a plane wave field

We study the quantization problem of relativistic scalar and spinning particles interacting with a radiation electromagnetic field by using the path integral and the external source method. The spin degrees of freedom are described in terms of Grassmann variables and the Feynman kernel is obtained through functional integration on both Bose and Fermi variables. We provide rigourous proof that the Feynman amplitudes are only determined by the classical contribution and we explicitly evaluate the propagators.

hep-th↗
arXiv2003

Ladder-QCD at finite isospin chemical potential

We use an effective QCD model (ladder-QCD) to explore the phase diagram for chiral symmetry breaking and restoration at finite temperature with different $u,d$ quark chemical potentials. In agreement with a recent investigation based on the Nambu-Jona-Lasinio model, we find that a finite pion condensate shows up for high enough isospin chemical potential $μ_{I}=(μ_{u}-μ_{d})/2$. For small $μ_{I}$ the phase diagram in the $(μ_B,T)$ plane shows two first order transition lines and two critical ending points.

hep-ph↗
arXiv2000

Chiral Meson Masses at Finite Temperature and Density

The ratio of the sigma mass to the pion mass at finite temperatures and densities provides for a quantitative signal of chiral symmetry breaking. We calculate this ratio by using an extension to finite chemical potential of the field theoretic composite operator formalism as applied to QCD. The calculation is limited to regions of the phase diagram where only quark-antiquark condensates dominate (no quark-quark condensates) and it confirms the expected behaviours. In particular the sigma becomes an essentially stable particle in a narrow region bordering the transition line from broken to restored chirality . This pattern is qualitatively the same both for the region where the transition is of second order as for the region where it is of first order (apart from the discontinuities expected in the latter case).

hep-ph↗
arXiv1998

Pseudoscalar and scalar meson masses at finite temperature

The composite operator formalism is applied to QCD at finite temperature to calculate the masses of scalar and pseudoscalar mesons. In particular the ratio of the sigma mass to the pion mass is an interesting measure of the degree of chiral symmetry breaking at different temperatures. We calculate the temperature T* at which M_sigma(T) < 2M_pi(T), above which the sigma partial width into two pions vanishes. We find T*=0.95T_c (where T_c is the critical temperature for the chiral phase transition), within the full effective potential given by the formalism. We find that an expansion a-la Landau of the effective potential around the critical point in the limit of small quark mass provides for a very good determination of T*.

hep-ph↗
arXiv1997

Corrections to the Pagels-Stokar Formula for f_π

Within the composite operator formalism we derive a formula for the pion decay constant $f_π$, as defined directly from the residue at the pion pole of the meson propagator, rather than from the matrix element of the axial current. The calculation is performed under some simplifying assumptions, and we verify the complete consistency with soft-pion results, in particular with the Adler-Dashen relation. The formula one obtains for (the pole-defined) $f_π^2$ differs from the previous Pagels-Stokar expression by an additive term, and it still provides $f_π^2$ in terms of the quark self-energy. We make some numerical estimates leading to $(30 ÷40)%$ deviation for $f_π^2$ with respect to the Pagels-Stokar formula.

hep-ph↗
arXiv1996

Low Temperature Dominance of Pion-like Excitations in the Massive Gross-Neveu Model at Order 1/N

We perform a 1/N-expansion of the partition function of the massive Gross-Neveu model in 1+1 dimensions. The procedure allows for the inclusion of the contribution of scalar and pseudoscalar composites (of order 1/N) to the equation of state. The naive expectation that the bosonic fluctuations correct significantly the mean field approximation at low temperatures is confirmed by our calculations. Actually the relevant degrees of freedom of hadronic matter at low temperatures are found to be pion-like excitations, rather than the fundamental constituents.

hep-ph↗
arXiv1996

1/N_c expansion for the partition function in four fermion models

We present a derivation of the bosonic contribution to the thermodynamical potential of four fermion models by means of a $1/N_c$-expansion of the functional integral defining the partition function. This expansion turns out to be particularly useful to correct the mean field approximation expecially at low temperatures, where the relevant degrees of freedom are low-mass bosonic excitations (pseudogoldstones).

hep-ph↗
arXiv1995

Disoriented Chiral Condensates: A Dynamical Simulation in the (2+1)-Dimensional Gross-Neveu Model

We simulate the formation and growth of disoriented chiral condensate (DCC) regions which follow the expansion of a high energy density region into the ``cold'' vacuum. The numerical study is based on the one-loop effective potential for the massive 2+1-dimensional Gross-Neveu model. We pay attention to the setting of the initial conditions and to determining which parameters are relevant for a strong amplification of the pion field. We find that the size of the ``hot'' source plays a significant role. For large enough source radii, we observe strong correlation phenomena, corresponding to the growth of large regions where the pion field oscillates along a given direction. We give our results in terms of the $θ$ angle which defines the DCC disorientation, of the other $O(4)$ angles distributions, of the local ratios $R_{a}=π_{a}^{2}/{\vecπ}^2$ $(a=1,2,3)$, and of the energies associated with the fields at representative times.

hep-ph↗