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Francesco Giacosa

Publications and source records attributed to Francesco Giacosa.

At least 145 records · Page 8Linked to original sources
arXiv2013

Meson vacuum phenomenology in a three-flavor linear sigma model with (axial-)vector mesons

We study scalar, pseudoscalar, vector, and axial-vector mesons with non-strange and strange quantum numbers in the framework of a linear sigma model with global chiral $U(N_f)_L \times U(N_f)_R$ symmetry. We perform a global fit of meson masses, decay widths, as well as decay amplitudes. The quality of the fit is, for a hadronic model that does not consider isospin-breaking effects, surprisingly good. We also investigate the question whether the scalar $\bar{q}q$ states lie below or above 1 GeV and find the scalar states above 1 GeV to be preferred as $\bar{q}q$ states. Additionally, we also describe the axial-vector resonances as $\bar{q}q$ states.

hep-ph↗
arXiv2012

Pressure of the O(N) Model in 1+1 Dimensions

The O(N) model in 1+1 dimensions presents some features in common with Yang-Mills theories: asymptotic freedom, trace anomaly, non-petrurbative generation of a mass gap. An analytical approach to determine the termodynamical properties of the O(3) model is presented and compared to lattice results. Here the focus is on the pressure: it is shown how to derive the pressure in the CJT formalism at the one-loop level by making use of the auxiliary field method. Then, the pressure is compared to lattice results.

hep-ph↗
arXiv2012

Lattice versus 2PI: 2d O(N) model at nonzero T

The thermodynamics of the O(N) model in 1+1 dimensions is studied applying the CJT formalism and the auxiliary field method as well as fully nonperturbative finite temperature lattice simulations. The numerical results for the renormalized mass of the scalar particles, the pressure and the trace anomaly are presented and compared with the results from lattice simulation of the model. We find that when going to the two loop order we observe a good correspondence between the CJT formalism and the lattice study.

hep-ph↗
arXiv2012

Interaction of the pseudoscalar glueball with (pseudo)scalar mesons and nucleons

We study the interactions of the pseudoscalar glueball with scalar and pseudoscalar quark-antiquark meson fields and with the nucleon and its chiral partner. In both cases we introduce the corresponding chiral Lagrangian and discuss its properties. We calculate the mesonic and baryonic decays of a pseudoscalar glueball with mass of about 2.6 GeV as predicted by Lattice simulations.

hep-ph↗
arXiv2012

Study of chiral symmetry restoration in linear and nonlinear O(N) models using the auxiliary field method

We consider the O(N) linear σ model and introduce an auxiliary field to eliminate the scalar self-interaction. Using a suitable limiting process this model can be continuously transformed into the nonlinear version of the O(N) model. We demonstrate that, up to two-loop order in the CJT formalism, the effective potential of the model with auxiliary field is identical to the one of the standard O(N) linear σ model, if the auxiliary field is eliminated using the stationary values for the corresponding one- and two-point functions. We numerically compute the chiral condensate and the σ- and π-meson masses at nonzero temperature in the one-loop approximation of the CJT formalism. The order of the chiral phase transition depends sensitively on the choice of the renormalization scheme. In the linear version of the model and for explicitly broken chiral symmetry, it turns from crossover to first order as the mass of the σ particle increases. In the nonlinear case, the order of the phase transition turns out to be of first order. In the region where the parameter space of the model allows for physical solutions, Goldstone's theorem is always fulfilled.

hep-ph↗
arXiv2012

Oscillations in the decay law: A possible quantum mechanical explanation of the anomaly in the experiment at the GSI facility

We study the deviations from the usual exponential decay law for quantum mechanical systems. We show that simple and physically motivated deviations from the Breit-Wigner energy distribution of the unstable state are sufficient to generate peculiar deviations from the exponential decay law. Denoting with $p(t)$ the survival probability, its derivative $h(t)$ shows typically an oscillating behavior on top of the usual exponential function. We argue that this can be a viable explanation of the observed experimental results at GSI Darmstadt, where the function $h(t)$ has been experimentally measured for electron capture decays of Hydrogen-like ions. Moreover, if our interpretation is correct, we predict that by measuring $h(t)$ at times close to the initial one, the number of decays per second rapidly drops to zero.

nucl-th↗
arXiv2012

(Oscillating) non-exponential decays of unstable states

We discuss deviations from the exponential decay law which originate when going beyond the Breit-Wigner distribution for an unstable state. In particular, we concentrate on an oscillating behavior, reminiscent of the Rabi-oscillations, in the short-time region. We propose that these oscillations can explain the GSI experiment, which measured superimposed oscillations on top of the exponential law for hydrogen-like nuclides decaying via electron-capture (the so-called GSI anomaly). Moreover, we discuss the possibility, that the deviations from the Breit-Wigner distribution in the case of the GSI anomaly are (predominantly) caused by the interaction of the unstable state with the measurement apparatus. The consequences of this scenario, such as the non-existence of oscillations in an analogous experiment performed at the Berkeley Lab, are investigated.

nucl-th↗
arXiv2012

Non-exponential decay in quantum field theory and in quantum mechanics: the case of two (or more) decay channels

We study the deviations from the exponential decay law, both in quantum field theory (QFT) and quantum mechanics (QM), for an unstable particle which can decay in (at least) two decay channels. After a review of general properties of non-exponential decay in QFT and QM, we evaluate in both cases the decay probability that the unstable particle decays in a given channel in the time interval between $t$ and $t+dt.$ An important quantity is the ratio of the probability of decay into the first and the second channel: this ratio is constant in the Breit-Wigner limit (in which the decay law is exponential) and equals the quantity $Γ_{1}/Γ_{2}$, where $Γ_{1}$ and $Γ_{2}$ are the respective tree-level decay widths. However, in the full treatment (both for QFT and QM) it is an oscillating function around the mean value $Γ_{1}/Γ_{2}$ and the deviations from this mean value can be sizable. Technically, we study the decay properties in QFT in the context of a superrenormalizable Lagrangian with scalar particles and in QM in the context of Lee Hamiltonians, which deliver formally analogous expressions to the QFT case.

nucl-th↗
arXiv2011

Properties of hadrons in a chiral model with (axial-)vector mesons

Recent advances in the development of a chiral linear $σ$-model with (axial-)vector mesons are presented. The model is based on the basic requirements of global chiral symmetry and dilatation invariance. The role of (axial-)vector states turns out to be crucial both in the meson and the baryon sectors. First results at nonzero temperature and density are discussed.

hep-ph↗
arXiv2011

Deviation from the exponential decay law in relativistic quantum field theory: the example of strongly decaying particles

We show that a short-time regime, in which a deviation from the exponential decay law occurs, exists also in the framework of a superrenormalizable relativistic quantum field theory. This, in turn, implies the possibility of a quantum Zeno effect also for elementary decays. The attention is then focused on the typical order of magnitude of strong decay rates of mesons: for these particles, strong deviations from the exponential decay law are present during a period of time comparable with their mean life time. As a concrete example, the case of the $ρ$ meson is studied.

hep-ph↗
arXiv2011

Restoration of chiral symmetry in the large-N_c limit

We study the large-N_c behavior of the critical temperature T_c for chiral symmetry restoration in the framework of the Nambu-Jona-Lasinio (NJL) model and the linear σ-model. While in the NJL case T_c scales as N_c^0 and is, as expected, of the same order as Λ_QCD (just as the deconfinement phase transition), in the σ-model the scaling behavior reads T_c \propto N_c^1/2. We investigate the origin of the different scaling behavior and present two improvements of the σ-model: (i) a simple, phenomenologically motivated temperature dependence of the parameters and (ii) the coupling to the Polyakov loop. Both approaches lead to the scaling T_c \propto N_c^0.

hep-ph↗
arXiv2011

Does nuclear matter bind at large $N_c$?

The existence of nuclear matter at large $N_{c}$ is investigated in the framework of effective hadronic models of the Walecka type. This issue is strongly related to the nucleon-nucleon attraction in the scalar channel, and thus to the nature of the light scalar mesons. Different scenarios for the light scalar sector correspond to different large $N_{c}$ scaling properties of the parameters of the hadronic models. In all realistic phenomenological scenarios for the light scalar field(s) responsible for the attraction in the scalar channel it is found that nuclear matter does not bind in the large $N_{c}$ world. We thus conclude that $N_{c}=3$ is in this respect special: 3 is fortunately not large at all and allows for nuclear matter, while large $N_{c}$ would not.

hep-ph↗
arXiv2011

Non exponential decays of hadrons

We analyze the survival probability of unstable particles in the context of quantum field theory. After introducing the spectral function of resonances, we show that deviations from the exponential decay law occur at short times after the creation of the unstable particle. For hadronic decays, these deviations are sizable and could lead to observable effects.

hep-ph↗
arXiv2011

Nuclear matter and chiral phase transition at large-$N_{c}$

Two aspects of the QCD phase diagrams are studied in the limit of a large number of colors: at zero temperature and nonzero density the (non)existence of nuclear matter, and at zero density and nonzero temperature the chiral phase transition.

hep-ph↗
arXiv2011

Nuclear matter within a dilatation-invariant parity doublet model: the role of the tetraquark at nonzero density

We investigate the role of a scalar tetraquark state for the description of nuclear matter within the parity doublet model in the mirror assignment. In the dilatation-invariant version of the model a nucleon-nucleon interaction term mediated by the lightest scalar tetraquark field naturally emerges. At nonzero density one has, beyond the usual chiral condensate, also a tetraquark condensate. The behavior of both condensates and the restoration of chiral symmetry at high density are studied. It is shown that this additional scalar degree of freedom affects non negligibly the properties of the medium.

hep-ph↗
arXiv2011

Analytical study of a gas of gluonic quasiparticles at high temperature: effective mass, pressure and trace anomaly

The thermodynamical properties of a pure Yang-Mills theory SU(N) is described by a gas of gluonic quasiparticles with temperature dependent mass $m(T)$ and a bag function $B(T).$ The analytic behavior of $m(T)$ and the pressure $p$ at high $T$ are derived and constraints on the parameters defining $B(T)$ are discussed. The trace anomaly $θ=ρ-3p$ is evaluated in the high $T$ domain: it is dominated by a quadratic behavior $θ=nKT^{2},$ where $n=2(N^{2}-1)$ is the number of degrees of freedom and $K$ is an integration constant which does not depend on the bag function $B(T)$. This is a general result which is in very good agreement with recent lattice simulations.

hep-ph↗
arXiv2011

The Glueball in a Chiral Linear Sigma Model with Vector Mesons

We present a two-flavour linear sigma model with global chiral symmetry and (axial-)vector mesons as well as an additional glueball degree of freedom. We study the structure of the well-established scalar resonances f0(1370) and f0(1500): by a fit to experimentally known decay widths we find that f0(1370) is predominantly a \bar{q}q state and f0(1500) is predominantly a glueball state. The overall phenomenology of these two resonances can be well described. Other assignments for our mixed quarkonium-glueball states are also tested, but turn out to be in worse agreement with the phenomenology. As a by-product of our analysis, the gluon condensate is determined.

hep-ph↗