arXiv subjects

Francesco Giacosa

Publications and source records attributed to Francesco Giacosa.

At least 91 records · Page 5Linked to original sources

arXiv2017

Excited vector mesons: phenomenology and predictions for a yet unknown vector $s\bar{s}$ state with a mass of about 1.93 GeV

The firm understanding of standard quark-antiquark states (including excited states) is necessary to search for non-conventional mesons with the same quantum numbers. In this work, we study the phenomenology of two nonets of excited vector mesons, which predominantly correspond to radially excited vector mesons with quantum numbers $n$ $^{2S+1}L_{J}=2$ $^{3}S_{1}$ and to orbitally excited vector mesons with quantum numbers $n$~ $^{2S+1}L_{J}=1$ $^{3}D_{1}$. We evaluate the decays of these mesons into two pseudoscalar mesons and into a pseudoscalar and a ground-state vector meson by making use of a relativistic quantum field theoretical model based on flavor symmetry. Moreover, we also study the radiative decays into a photon and a pseudoscalar meson by using vector meson dominance. We compare our results to the PDG and comment on open issues concerning the corresponding measured resonances. Within our approach, we are also able to make predictions for a not-yet discovered $s\bar{s}$ state in the $n$ $^{2S+1}L_{J}=1$ $^{3}D_{1}$ nonet, which has a mass of about 1.93 GeV. This resonance can be searched in the upcoming GlueX and CLAS12 experiments which take place at the Jefferson Lab.

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arXiv2017

Toward a QFT treatment of nonexponential decay

We study the properties of the survival probability of an unstable quantum state described by a Lee Hamiltonian. This theoretical approach resembles closely Quantum Field Theory (QFT): one can introduce in a rather simple framework the concept of propagator and Feynman rules, Within this context, we re-derive (in a detailed and didactical way) the well-known result according to which the amplitude of the survival probability is the Fourier transform of the energy distribution (or spectral function) of the unstable state (in turn, the energy distribution is proportional to the imaginary part of the propagator of the unstable state). Typically, the survival probability amplitude is the starting point of many studies of non-exponential decays. This work represents a further step toward the evaluation of the survival probability amplitude in genuine relativistic QFT. However, although many similarities exist, QFT presents some differences w.r.t. the Lee Hamiltonian which should be studied in the future.

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arXiv2017

Strong and radiative decays of excited vector mesons and predictions for a new $ϕ(1930)$ resonance

We study the phenomenology of two nonets of excited vector mesons, $\{ρ(1450)$, $K^{\ast}(1410)$, $ω(1420)$, $ϕ(1680)\}$ and $\{ρ(1700),$ $K^{\ast}(1680),ω(1650),$ $ϕ(???)\},$ which (roughly) correspond to radially excited $2^{3}S_{1}$ and to orbitally excited $1^{3}D_{1}$ vector mesons. We evaluate the strong and radiative decays of these mesons into pseudoscalar and ground-state vector mesons by using an effective relativistic QFT model based on flavour symmetry. We compare decay widths and branching ratios with various experimental results listed in the PDG. An overall agreement of theory with experiment reinforces the standard quark-antiquark assignment of the resonances mentioned above. Predictions for not yet measured quantities are also made. In particular, we shall also make predictions for the not-yet discovered $s\bar{s}$ state of the $1^{3}D_{1}$ nonet, denoted as $ϕ(???)$. Its mass can be estimated to be about $1930$ MeV, hence we shall call this putative state $ϕ(1930).$ Its main decays are into $KK^{\ast}(892)$ (about $200$ MeV) and $KK$ (about $100$ MeV). Since this state couples also to $γη,$ it can be searched in the near future in the photoproduction-based experiments GlueX and CLAS12 at Jefferson Lab.

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arXiv2017

The $f_0(1790)$ and $a_0(1950)$ Resonances as Excited $\bar{q}q$ States in the Extended Linear Sigma Model

A decade ago, BES Collaboration reported the discovery of a new scalar isosinglet resonance denoted as $f_0(1790)$. The finding was subsequently confirmed by LHCb. Recently, the existence of the corresponding isotriplet state -- the $a_0(1950)$ resonance -- has been claimed by BABAR. We investigate whether these resonances can be described as excited $\bar{q}q$ states. To this end, a comprehensive Lagrangian containing ground-state $\bar{q}q$ mesons as well as their first excitations is constructed in accordance with symmetries of the strong interaction. Both $f_0(1790)$ and $a_0(1950)$ emerge as compatible with $\bar{q}q$ excitations; however, tension appears to arise between the simultaneous interpretation of $f_0(1790)$/$a_0(1950)$ and pseudoscalar mesons $η(1295)$, $π(1300)$, $η(1440)$ and $K(1460)$ as excited $\bar{q}q$ states.

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arXiv2017

Heavy glueballs: status and large-$N_{c}$ width's estimate

Glueballs, an old and firm prediction of various QCD approaches (lattice QCD, bag models, AdS/QCD, effective models, etc.), have not yet been experimentally confirmed. While for glueballs below $2.6$ GeV some candidates exist, the situation for heavy glueballs (above $2.6$ GeV) is cloudy. Here, after a brief review of scalar, tensor, and pseudoscalar glueballs, we present predictions for the decays of a putative pseudotensor glueball with a lattice predicted mass of $3.04$ GeV and a putative vector glueball with a lattice predicted mass of $3.81$ GeV. Moreover, we discuss in general the width of heavy glueballs by using large-$N_{c}$ arguments: we obtain a rough estimate according to which the width of a glueball (such as the vector one) is about $10$ MeV. Such a width would be narrow enough to enable measurement at the future PANDA experiment.

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arXiv2017

Time evolution of an unstable quantum system

After reviewing the description of an unstable state in the framework of Lee Hamiltonians (valid both for Quantum Mechanics (QM) and Quantum Field Theory (QFT)), we consider some theoretical aspects of non-exponential decays: the case of two decay channels, the broadening of the energy spectrum at short times, the effect of an imperfect measurement, the link to QFT, and the decay of an unstable moving particle with definite momentum. All the presented effects were not confirmed in experiments, hence are at the present stage predictions.

quant-ph↗

arXiv2017

Neutron stars in the large-$N_{c}$ limit

We study the phase transition from dense baryonic matter to dense quark matter within the large-$N_{c}$ limit. By using simple constant speed of sound equations of state for the two phases, we derive the scaling with $N_{c}$ of the critical quark chemical potential $μ_{c}^{\mathrm{crit}}$ for this phase transition. While quark matter is strongly suppressed at large $N_{c}$, the phase transition at a large but finite density could nevertheless be important to determine the maximum mass of compact stars. In particular, in the range $3\leq N_{c}\lesssim5.5$ the quark phase would take place in compact stars and would lead to the formation of an unstable branch of hybrid stars. As a consequence, the maximum mass is restricted to the range $2.1M_{\odot}<M_{max}<3M_{\odot}$. For larger value of $N_{c}$, the phase transition would occur at densities too high to be reached in the core of compact stars. However, the very requirement that it occurs (although at a very large density) translates into interesting constraints on the stiffness of the baryonic phase: its speed of sound must exceed $\sqrt{1/3}$.

nucl-th↗

arXiv2017

Excited Scalar and Pseudoscalar Mesons in the Extended Linear Sigma Model

We present an in-depth study of masses and decays of excited scalar and pseudoscalar $\bar{q}q$ states in the Extended Linear Sigma Model (eLSM). The model also contains ground-state scalar, pseudoscalar, vector and axial-vector mesons. The main objective is to study the consequences of the hypothesis that the $f_0(1790)$ resonance, observed a decade ago by the BES Collaboration and recently by LHCb, represents an excited scalar quarkonium. In addition we also analyse the possibility that the new $a_0(1950)$ resonance, observed recently by BABAR, may also be an excited scalar state. Both hypotheses receive justification in our approach although there appears to be some tension between the simultaneous interpretation of $f_0(1790)$/$a_0(1950)$ and pseudoscalar mesons $η(1295)$, $π(1300)$, $η(1440)$ and $K(1460)$ as excited $\bar{q}q$ states.

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arXiv2017

Decays of the vector glueball

We calculate two- and three-body decays of the (lightest) vector glueball into (pseudo)scalar, (axial-)vector, as well as pseudovector and excited vector mesons\textbf{ }in the framework of a model of QCD. While absolute values of widths cannot be predicted because the corresponding coupling constants are unknown, some interesting branching ratios can be evaluated\textbf{ }by setting the mass of the yet hypothetical vector glueball to $3.8$ GeV as predicted by quenched Lattice QCD\textbf{.} We find that the decay mode $ωππ$ should be one of the largest (both through the decay chain $\mathcal{O}\rightarrow b_{1}π\rightarrow$ $ωππ$ and through the direct coupling $\mathcal{O}\rightarrowωππ$)$.$ Similarly, the (direct and indirect) decay into $πKK^{\ast}(892)$ is sizable. Moreover, the decays into $ρπ$ and $K^{\ast}(892)K$ are, although subleading, possible and could play a role in explaining the $ρπ$ puzzle of the charmonium state $ψ(2S)$ thank to a (small) mixing with the vector glueball. The vector glueball can be directly formed at the ongoing BESIII experiment as well as at the future PANDA experiment at the FAIR facility. If the width is sufficiently small ($\lesssim100$ MeV) it should not escape future detection. It should be stressed that the employed model is based on some inputs and simplifying assumptions: the value of glueball mass (at present, the quenched lattice value is used), the lack of mixing of the glueball with other quarkonium states, and the use of few interaction terms. It then represents a first step toward the identification of the main decay channels of the vector glueball, but shall be improved when corresponding experimental candidates and/or new lattice results will be available.

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arXiv2016

Phenomenology of pseudotensor mesons and the pseudotensor glueball

We study the decays of the pseudotensor mesons $[ π_{2}(1670) , K_{2}(1770) , η_{2}(1645) , η_{2}(1870) ]$ interpreted as the ground-state nonet of $1^1 D_{2}$ $\bar{q}q$ states using interaction Lagrangians which couple them to pseudoscalar, vector, and tensor mesons. While the decays of $π_2 (1670)$ and $K_2 (1770)$ can be well described, the decays of the isoscalar states $η_2 (1645)$ and $η_2 (1870)$ can be brought in agreement with experimental data only if the mixing angle between nonstrange and strange states is surprisingly large (about $-42^\circ$, similar to the mixing in the pseudoscalar sector, in which the chiral anomaly is active). Such a large mixing angle is however at odd with all other conventional quark-antiquark nonets: if confirmed, a deeper study of its origin will be needed in the future. Moreover, the $\bar{q}q$ assignment of pseudotensor states predicts that the ratio $[ η_2 (1870) \rightarrow a_2 (1320) π]/[η_2 (1870) \rightarrow f_2 (1270) η]$ is about $23.5$. This value is in agreement with Barberis et al., ($20.4 \pm 6.6$), but disagrees with the recent reanalysis of Anisovich et al., ($1.7 \pm 0.4$). Future experimental studies are necessary to understand this puzzle. If Anisovich's value shall be confirmed, a simple nonet of pseudoscalar mesons cannot be able to describe data (different assignments and/or additional state, such as an hybrid state, will be needed). In the end, we also evaluate the decays of a pseudoscalar glueball into the aforementioned conventional $\bar{q}q$ states: a sizable decay into $K^\ast_2 (1430) K$ and $a_2 (1230) π$ together with a vanishing decay into pseudoscalar-vector pairs [such as $ρ(770) π$ and $K^\ast (892) K$] are expected. This information can be helpful in future studies of glueballs at the ongoing BESIII and at the future PANDA experiments.

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arXiv2016

Anomalies in weak decays of H-like ions

We investigate the emergence of oscillations in the decay law of unstable systems. We discuss in particular the case of the so-called GSI anomaly seen in the electron capture decays of H-like ions and prove that such oscillations cannot be explained by neutrino oscillations. We then discuss how such anomalies could be intimately related to the decay law of unstable systems in the case in which their spectral function deviates from a Breit-Wigner shape.

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arXiv2016

Role of scalar dibaryon and $f_0(500)$ in the isovector channel of low-energy neutron-proton scattering

We calculate the total and the differential cross section for $np$ scattering at low energies in the isospin $I=1$ channel within the so-called extended Linear Sigma Model. This model contains conventional (pseudo)scalar and (axial--)vector mesons, as well as the nucleon and its chiral partner within the mirror assignment. In order to obtain good agreement with experimental data analysis results we need to consider two additional resonances: the lightest scalar state $f_{0}(500)$ and a dibaryon state with quantum numbers $I=1,$ $J^{P}=0^{+}$ (also known as $^{1}S_{0}$ resonance). The resonance $f_{0}(500)$ is coupled to nucleons in a chirally invariant way through the mirror assignment and is crucial for a qualitatively correct description of the shape of the differential cross section. On the other hand, the dibaryon is exchanged in the $s$--channel and is responsible of the large cross section close to threshold. We compare our results to data analysis results performed by the SAID program of the CNS Data Analysis Center (in the following "SAID results").

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arXiv2016

Modelling glueballs

Glueballs are predicted in various theoretical approaches of QCD (most notably lattice QCD), but their experimental verification is still missing. In the low-energy sector some promising candidate for the scalar glueball exist, and some (less clear) candidates for the tensor and pseudoscalar glueballs were also proposed. Yet, for heavier gluonic states there is much work to be done both from the experimental and theoretical points of view. In these proceedings, we briefly review the current status of research of glueballs and discuss future developments.

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arXiv2016

A covariant nonlocal Lagrangian for the description of the scalar kaonic sector

Mesons are extended objects, hence their interaction can be described by utilizing form factors. At the Lagrangian level, one can use nonlocal interaction terms. Here we describe two possible nonlocal Lagrangians leading to a 3D form factor: the first one is simple but does not fulfill covariance (if one insists on a 3D cutoff), the second extension is more involved but guarantees covariance. Such form factors are useful when calculating mesonic loops. As an important example, we discuss the scalar kaonic sector, $I(J^{P})=\frac{1}{2}(0^{+})$. The Lagrangian contains a single scalar kaon (the well-establish state $K_{0}^{\ast}(1430)$), but through loops $K_{0}^{\ast}(800)$ emerges as a dynamically generated companion pole (which disappears in the large-$N_{c}$ limit).

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arXiv2016

Excited Scalar Mesons and the Search for Glueballs

We discuss the importance of mixing between glueballs -- bound states of gluons -- and excited $\bar{q}q$ states for the glueball search. A preliminary study of the excited states in the Extended Linear Sigma Model (eLSM) suggests their masses in the scalar channel to be in the vicinity of the scalar-glueball mass found in Lattice QCD. This could have implications for future glueball searches.

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arXiv2016

$K_{0}^{\ast}(800)$ as a companion pole of $K_{0}^{\ast}(1430)$

We study the light scalar sector up to $1.8$ GeV by using a quantum field theoretical approach which includes a single kaonic state in a Lagrangian with both derivative and non-derivative interactions. By performing a fit to $πK$ phase shift data in the $I=1/2,$ $J=0$ channel, we show that $K_{0}^{\ast}(800)$ (or $κ$) emerges as a dynamically generated companion pole of $K_{0}^{\ast }(1430)$. This is a result of investigating quantum fluctuations with one kaon and one pion circulating in the loops dressing $K_{0}^{\ast}(1430)$. We determine the position of the poles on the complex plane in the context of our approach: for $K_{0}^{\ast}(1430)$ we get $(1.413\pm0.002)-i{0.02cm}(0.127\pm0.003)$ (in GeV), while for $κ$ we get $(0.746\pm0.019)-i{0.02cm}(0.262\pm0.014)$ (in GeV). The model-dependence of these results and related uncertainties are discussed in the paper. A large-$N_{c}$ study confirms that $K_{0}^{\ast}(1430)$ is predominantly a quarkonium and that $K_{0}^{\ast}(800)$ is a molecular-like dynamically generated state.

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arXiv2016

A study of the resonances $K_{0}^{}(800)$ and $K_{0}^{}(1430)$

We study the scalar kaonic states $K_{0}^{\ast}(800)$ and $K_{0}^{\ast}(1430)$ by using a relativistic QFT Lagrangian in which only a single kaonic field corresponding to the well-established scalar state $K_{0}^{\ast}(1430)$ is considered and in which both derivative and non-derivative interaction terms are taken into account. Even if the scalar spectral function shows a unique peak close to $1.4$ GeV, we find two poles in the complex plane: $1.413\pm0.002-i(0.127\pm0.003)$ GeV, which is related to the seed quark-antiquark state $K_{0}^{\ast}(1430),$ and $0.746\pm0.019-i(262\pm0.014)$ GeV, which is an additional companion pole related to $K_{0}^{\ast}(800)$. As a further investigation for increasing $N_{c}$ confirms, $K_{0}^{\ast}(800)$ emerges as a dynamically generated four-quark object as a consequence of pion-kaon loops.

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arXiv2016

Large-$N_{c}$ pole trajectories of the vector kaon $K^{\ast}(892) $ and of the scalar kaons $K_{0}^{\ast}(800)$ and $K_{0}^{\ast}(1430)$

We study the spectral functions, the poles and their trajectories for increasing $N_{c}$ of the vector kaon state $K^{\ast}(892),$ characterized by $I(J^{P})=\frac{1}{2}(1^{-})$, and of the scalar kaons $K_{0}^{\ast}(800)$ and $K_{0}^{\ast}(1430),$ characterized by $I(J^{P})=\frac{1}{2}(0^{+})$. To this end, we use relativistic QFT's Lagrangians with both derivative and non-derivative terms. In the vector kaonic sector the spectral function is well approximated by a Breit-Wigner function: there is one single peak and, correspondingly, a single pole in the complex plane. On the contrary, in the scalar sector, although the Lagrangian contains only one scalar kaonic field, we find two poles, one corresponding to a standard quark-antiquark ,,seed\textquotedblright\ state $K_{0}^{\ast}(1430),$ and one to a \textquotedblleft companion\textquotedblright\ dynamically generate pole $K_{0}^{\ast}(800)$. The latter does not correspond to any peak in the scalar kaonic spectral function, but only to an enhancement in the low-energy regime.

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