arXiv subjects

Francesco Giacosa

Publications and source records attributed to Francesco Giacosa.

At least 127 records · Page 7Linked to original sources

arXiv2014

Decays of open charmed mesons in the extended Linear Sigma Model

We enlarge the so-called extended linear Sigma model (eLSM) by including the charm quark according to the global $U(4)_r\times U(4)_l$ chiral symmetry. In the eLSM, besides scalar and pseudoscalar mesons, also vector and axial-vector mesons are present. Almost all the parameters of the model were fixed in a previous study of mesons below 2 GeV. In the extension to the four-flavor case, only three additional parameters (all of them related to the bare mass of the charm quark) appear. We compute the (OZI-dominant) strong decays of open charmed mesons. The results are compatible with the experimental data, although the theoretical uncertainties are still large.

hep-ph↗

arXiv2014

The role of the next-to-leading order triangle-shaped diagram in two-body hadronic decays

The next-to-leading-order contribution to the amplitude of a two-body decay process is a triangle-shaped diagram in which the unstable state is exchanged by the emitted particles. In this work we calculate this diagram in the framework of a scalar quantum field theory and we estimate its role in hadronic physics, i.e., we apply our results to the well-known scalar-isoscalar resonances $f_{0}(500)$, $f_{0}(980)$, $f_{0}(1370)$, $f_{0}(1500)$, $f_{0}(1710)$ and the scalar-isovector resonance $a_{0}(1450)$. It turns out that, with the exception of the broad resonance $f_{0}(500)$, the next-to-leading-order contribution is small and can be neglected.

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arXiv2014

Is f0(1710) a glueball?

We study the three-flavor chirally and dilatation invariant extended Linear Sigma Model with (pseudo)scalar and (axial-)vector mesons as well as a scalar dilaton field whose excitations are interpreted as a glueball. The model successfully describes masses and decay widths of quark-antiquark mesons in the low-energy region up to 1.6 GeV. Here we study in detail the vacuum properties of the scalar-isoscalar channel and find that (i) a narrow glueball is only possible if the vacuum expectation value of the dilaton field is (at tree-level) quite large (i.e., larger than what lattice QCD and QCD sum rules suggest) and (ii) that only solutions in which f0(1710) is predominantly a glueball are found. Moreover, the resonance f0(1370) turns out to be mainly nonstrange quark-antiquark state and thus corresponds to the chiral partner of the pion, while the resonance f0(1500) is mainly strange quark-antiquark state.

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arXiv2014

The scalar-isovector sector in the extended Linear Sigma Model

We study basic properties of scalar hadronic resonances within the so-called extended Linear Sigma Model (eLSM), which is an effective model of QCD based on chiral symmetry and dilatation invariance. In particular, we focus on the mass and decay width of the scalar isovector state $a_{0}(1450)$ and perform a numerical study of the propagator pole(s) on the unphysical Riemann sheets. In this work, this meson is understood as a seed state explicitly included in the eLSM. Our results show that the inclusion of hadronic loops does not modify much the previously obtained tree-level results. Moreover the $a_{0}(980)$ cannot be found as a propagator pole generated by hadronic loop contributions.

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arXiv2014

Meson production in proton-proton scattering within an extended linear sigma model

We study the production of non-strange mesons in proton-proton scattering reactions within an effective model of mesons and baryons with global chiral symmetry. The model includes the nucleon ($N$) and its chiral partner ($N^{\ast}$) and their couplings to the $N_{f}=2$ multiplets of (pseudo-)scalar and (axial-)vector mesons. Results for the production of $ω$ and $η$ mesons are presented.

nucl-th↗

arXiv2014

Mirror versus naive assignment in chiral models for the nucleon

In the framework of chiral model(s) based on the linear realization of chiral symmetry, we investigate the low-energy phenomenological properties of the nucleon and its chiral partner (identified with either $N(1535)$ or $N(1650)$) in the naive assignment and compare the results with the ones obtained in the mirror assignment. We find that, within the naive assignment, we are not able to reproduce the experimental value of the isospin-odd scattering length, while the mirror assignment is in good agreement with it for both choices $N(1535)$ and $N(1650)$. The isospin-even scattering length is not yet conclusive in either assignment because it depends crucially on the poorly known scalar mesonic sector. The decay with $Γ_{N(1535)\rightarrow Nη}$ turns out to be far too small in both the naive and mirror assignments, while $Γ_{N(1650)\rightarrow Nη}$ is described correctly by both of them. In conclusions, the mirror assignment with $N(1650)$ as the chiral partner of the nucleon is the favored configuration.

hep-ph↗

arXiv2014

Pulsed and continuous measurements of exponentially decaying systems

We study the influence of a detector on the decay law of a quantum state whose "undisturbed" survival probability is purely exponential. In particular, we consider a detector with a finite energy band of detection, i.e. it interacts only with decay products having an energy within a certain range of values. In one case, we assume that the detector performs many repeated measurements at short time intervals in all of which a collapse of the wave function occurs (bang-bang or pulsed-type measurements). In the second case, we assume a continuous measurement which preserves unitarity. We confirm the slowing down of the decay in presence of a measuring apparatus, the Quantum Zeno effect, but the outcomes of the detector are in general qualitatively and quantitatively different in the two cases. In turn, this implies that the so-called Schulman relation (the equivalence of pulsed and continuous measurements) does not hold in general and that it is in principle possible to experimentally access how a certain detector performs a measurement.

quant-ph↗

arXiv2013

Resonances f0(1370), f0(1500) and f0(1710) within the extended Linear Sigma Model

In the framework of the extended Linear Sigma Model (eLSM) we investigate the masses and decays of the three scalar-isoscalar resonances $f_{0}(1370)$, $f_{0}(1500)$ and $f_{0}(1710)$. The degrees of freedom of the eLSM are (pseudo)scalars and (axial)vectors as well as the scalar glueball. Although still preliminary, present results, based on the physical masses of the above mentioned resonances, show that $f_{0}(1710)$ is the predominantly glueball state. However, acceptable decays for this resonance can be obtained only for a (very) large gluon condensate.

hep-ph↗

arXiv2013

Non-exponential decay in Quantum Mechanics and Quantum Field Theory

We describe some salient features as well as some recent developments concerning short-time deviations from the exponential decay law in the context of Quantum Mechanics by using the Lee Hamiltonian approach and Quantum Field Theory by using relativistic Lagrangians. In particular, the case in which two decay channels are present is analyzed: the ratio of decay probability densities, which is a constant equal to the ratio of decay widths in the exponential limit, shows in general sizable fluctuations which persist also at long times.

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arXiv2013

Energy uncertainty of the final state of a decay process

We derive the expression for the energy uncertainty of the final state of a decay of an unstable quantum state prepared at the initial time $t=0$. This expression is function of the time $t$ at which a measurement is performed to determine if the state has decayed and, if yes, in which one of the infinitely many possible final states. For large times the energy spread is, as expected, given by the decay width $Γ$ of the initial unstable state. However, if the measurement of the final state is performed at a time $t$ comparable to (or smaller than) the mean lifetime of the state $1/Γ$, then the uncertainty on the energy of the final state is much larger than the decay width $Γ$. Namely, for short times an uncertainty of the type $1/t$ dominates, while at large times the usual spread $Γ$ is recovered. Then, we turn to a generic two-body decay process and describe the energy uncertainty of each one of the two outgoing particles. We apply these formulas to the two-body decays of the neutral and charged pions and to the spontaneous emission process of an excited atom. As a last step, we study a case in which the non-exponential decay is realized ad show that for short times eventual asymmetric terms are enhanced in the spectrum.

quant-ph↗

arXiv2013

Phenomenology of axial-vector and pseudovector mesons: decays and mixing in the kaonic sector

We study the decays of the lightest axial-vector and pseudovector mesons, interpreted as quark-antiquark states, into a vector and a pseudoscalar meson. We show that the quarkonium assignment delivers a good description of the decays and allows also to make further testable predictions. In the kaonic sector, the physical resonances $K_{1}(1270)$ and $K_{1}(1400)$ emerge as mixed objects of an axial vector state $K_{1,A}$ and a pseudovector state $K_{1,B}$. We determine the mixing angle as $\left\vert θ_{K}\right\vert=\left(33.6\pm4.3\right) ^{\circ}$, a value compatible with previous studies but with a smaller uncertainty. This result may be helpful for testing models beyond the Standard Model of particle physics in which decays into the kaonic resonances $K_{1}(1270)$ and $K_{1}(1400)$ are investigated.

hep-ph↗

arXiv2013

The lightest scalar meson in a simple approach

We study basic properties of scalar hadronic resonances within a quantum field theoretical toy model. In particular, we focus on the spectral function, the mass and the decay width of the resonance $f_{0}(500)$. In this work, this meson is understood as a seed state in an effective Lagrangian which couples to pions. With such a setup we use the position of the pole on the second Riemann sheet in order to obtain its spectral function. We confirm that $f_{0}(500)$ cannot be described by an ordinary Breit--Wigner function, and that a more complicated structure is needed.

hep-ph↗

arXiv2013

Spectral function of a scalar boson coupled to fermions

We present the calculation of the spectral function of an unstable scalar boson coupled to fermions as resulting from the resummation of the one loop diagrams in the scalar particle self energy. We work with a large but finite high-energy cutoff: in this way, the spectral function of the scalar field is always correctly normalized to unity, independently on the value of the cutoff. We show that this high energy cutoff affects the Breit-Wigner width of the unstable particle: the larger the cutoff, the smaller is the width at fixed coupling. Thus, the existence of a high energy cutoff (alias minimal length), and for instance the possible opening of new degrees of freedom beyond that energy scale, could then be in principle proven by measuring, at lower energy scales, the line shape of the unstable scalar state. Although the Lagrangian here considered represents only a toy-model, we discuss possible future extensions of our work which could be relevant for particle physics phenomenology.

hep-ph↗

arXiv2013

Thermodynamics of the O(3) model in 1+1 dimensions: lattice vs. analytical results

A detailed study of the thermodynamics of the O(N=3) model in 1+1 dimensions is presented, employing a two-particle-irreducible resummation prescription as well as fully nonperturbative finite-temperature lattice simulations. The analytical results are computed using the Cornwall-Jackiw-Tomboulis (CJT) formalism and the auxiliary field method to one- and to two-loop order. The lattice results are obtained through Monte Carlo simulation for various lattice spacings. The analytical and lattice results for pressure, trace anomaly, and energy density, resembling closely those of four-dimensional Yang-Mills theories, are compared with each other. We find that to one-loop order there is a good correspondence between the CJT formalism and the lattice study for low temperatures. However, at high T the two-loop calculation fares better, correcting for the overestimation from the former approximation.

hep-ph↗

arXiv2013

The O(2) model in polar coordinates at nonzero temperature

We study the restoration of spontaneously broken symmetry at nonzero temperature in the framework of the O(2) model using polar coordinates. We apply the CJT formalism to calculate the masses and the condensate in the double-bubble approximation, both with and without a term that explicitly breaks the O(2) symmetry. We find that, in the case with explicitly broken symmetry, the mass of the angular degree of freedom becomes tachyonic above a temperature of about 300 MeV. Taking the term that explicitly breaks the symmetry to be infinitesimally small, we find that the Goldstone theorem is respected below the critical temperature. However, this limit cannot be performed for temperatures above the phase transition. We find that, no matter whether we break the symmetry explicitly or not, there is no region of temperature in which the radial and the angular degree of freedom become degenerate in mass. These results hold also when the mass of the radial mode is sent to infinity.

nucl-th↗

arXiv2013

Decay of the pseudoscalar glueball into scalar and pseudoscalar mesons

We study a chiral Lagrangian which describes the two- and three-body decays of a pseudoscalar glueball into scalar and pseudoscalar mesons. The various branching ratios are a parameter-free prediction of our approach. We compute the decay channels for a pseudoscalar glueball with a mass of 2.6 GeV, as predicted by Lattice QCD in the quenched approximation, which is in the reach of the PANDA experiment at the upcoming FAIR facility. For completeness, we also repeat the calculation for a glueball mass of 2.37 GeV which corresponds to the mass of the resonance X(2370) measured in the BESIII experiment.

hep-ph↗

arXiv2013

Propagator poles and an emergent stable state below threshold: general discussion and the E(38) state

In the framework of a simple quantum field theory describing the decay of a scalar state into two (pseudo)scalar ones we study the pole(s) motion(s) of its propagator: besides the expected pole on the second Riemann sheet, we find -- for a large enough coupling constant -- a second, additional pole on the first Riemann sheet below threshold, which corresponds to a stable state. We then perform a numerical study for a hadronic system in which a scalar particle couples to pions. We investigate under which conditions a stable state below the two-pion threshold can emerge. In particular, we study the case in which this stable state has a mass of 38 MeV, which corresponds to the recently claimed novel scalar state E(38). Moreover, we also show that the resonance $f_{0}(500)$ and the stable state E(38) could be two different manifestation of the same `object'. Finally, we also estimate the order of magnitude of its coupling to photons.

hep-ph↗

arXiv2013

Eta, Eta' and eLSM

We discuss the eta-eta' mixing in the Extended Linear Sigma Model (eLSM).

hep-ph↗