arXiv subjects

Francesco Giacosa

Publications and source records attributed to Francesco Giacosa.

At least 37 records · Page 2Linked to original sources

arXiv2025

Chiral oscillations in finite time quantum field theory

We demonstrate how chiral oscillations of a massive Dirac field can be described within quantum field theory using a finite-time interaction picture approach, where the mass term in the Lagrangian is treated as a perturbative coupling between massless fields of definite chirality. We derive the formula for chiral oscillations at the fourth order in the perturbative expansion, obtaining a result consistent with the formula derived by means of other methods. Furthermore, we illustrate how the perturbative framework of chiral oscillations can effectively describe production processes where an electron must exhibit both left chirality and positive helicity, as in decay $π^- \to e^- + {\bar ν}_e$. Finally, we argue that, in this perturbative view, chiral oscillations are also essential for detecting the decay products in such processes.

hep-ph↗

arXiv2025

Anomalous $U(1)_A$ couplings and the Columbia plot

When the quark masses are lighter than those in QCD, the standard lore is that a chiral transition of first order must emerge for three, light flavors. Recently, however, numerical simulations on the lattice suggest that the chiral transition is of second order in the chiral limit. Using an extended linear sigma model in the mean field approximation, we study the relation between terms which break the anomalous, $U(1)_A$ symmetry and the order of the chiral phase transition, especially how a chiral transition of second order can arise for three, massless flavors. We note that in an (unphysical) region of the "Columbia" phase diagram, when the strange quark mass is light and negative, corresponding to topological angle $θ=π$, the $CP$ symmetry is spontaneously broken.

hep-ph↗

arXiv2024

Scalar field with a time-independent classical source, not trivial after all: from vacuum decay to scattering

Historically it has been believed that a time-independent classical source has no effect on the scattering of relativistic uncharged field, in contrast with single particle quantum mechanics. In this work we show that the dynamics is not trivial. We solve exactly for the scattering amplitudes and find that a key ingredient is the production of particles from the unstable vacuum, conceptually similar to the Schwinger mechanism. We compute exactly the probabilities for the vacuum to decay in $n$ particles. The time dependence of such probabilities displays interesting properties such as the quantum Zeno effect and in particular has no regime where the exponential decay law is a good approximation. We show that the trivial scattering found in the past is the byproduct of the adiabatic switching of the interaction. In fact, it is not possible to switch off the interaction (adiabatically or otherwise) at distant times and recover the exact results. Finally, this non trivial vacuum behavior is a source of particle production. We argue that such non-perturbative calculations can be phenomenologically relevant for the production processes that are suppressed at the lower orders in perturbation theory, for instance dilaton production in a medium.

hep-th↗

arXiv2024

Nonexponential decay law of the 2P-1S transition of the H-atom

We evaluate numerically the survival probability $P(t)$ for the unstable 2P excited state of the hydrogen atom, which decays into the ground-state 1S emitting one photon ($τ\sim 1.595$ ns), thus extending the analytic study of Facchi and Pascazio, Physics Letters A 241 (1998). To this end, we first determine the analytic expression of the spectral function of the unstable state, which allows for an accurate evaluation of $P(t)$. As expected, for short and long times $P(t)$ shows deviations from the exponential law: a `Zeno' region occurs at extremely short times (up to $\sim 0.3$ attosec, followed by a longer `anti-Zeno' domain (up to $\sim 50$ attosec); at long times above $125 τ$, the decay law scales as $t^{-4}$.

quant-ph↗

arXiv2024

Isospin-symmetry breaking in kaon production: the role of charge-symmetry

An excess of charged versus neutral kaons has been recently reported by the NA61/SHINE collaboration. Similar excesses were also present in previous experiments, even if with larger errors. Models for hadron productions in heavy ion collisions systematically underestimate the measured charge-to-neutral kaon ratio. In this report, we comment on the role of charge-symmetry as a specific isospin transformation. In particular, we show that the ensemble of the initial colliding nuclei being charge-symmetric is sufficient for producing an equal number of charged and neutral kaons in the isospin-symmetry exact limit.

nucl-th↗

arXiv2024

Introductory visual lecture on QCD at large-$N_{c}$: bound states, chiral models, and phase diagram

In these lectures, we present the behavior of conventional $\bar{q}q$ mesons, glueballs, and hybrids in the large-$N_{c}$ limit of QCD. To this end, we use an approach based on rather simple NJL-like bound-state equations. The obtained large-$N_{c}$ scaling laws are general and coincide with the known results. A series of consequences, such as the narrowness of certain mesons and the smallness of some interaction types, the behavior of chiral and dilaton models at large-$N_{c},$ and the relation to the compositeness condition and the standard derivation of large-$N_{c}$ results, are explained. The bound-state formalism shows also that mesonic molecular and dynamically generated states do not form in the large-$N_{c}$ limit. The same fate seems to apply also for tetraquark states, but here further studies are needed. Next, following the same approach, baryons are studied as bound states of a generalized diquark ($N_{c}-1$ antisymmetric object) and a quark. Similarities and differences with regular mesons are discussed. All the standard scaling laws for baryons and their interaction with mesons are correctly reproduced. The behavior of chiral models involving baryons and describing chirally invariant mass generation is investigated. Finally, properties of QCD in the medium at large-$N_{c}$ are studied: the deconfinement phase transition is investigated along the temperature and the chemical potential directions, respectively. Within the QCD phase diagrams, the features of different models at large-$N_{c}$ are reviewed and the location of the critical endpoint is discussed. In the end, the very existence of nuclei and the implications of large-$N_{c}$ arguments for neutron stars are outlined.

hep-ph↗

arXiv2024

Anomalous interactions between mesons with nonzero spin and glueballs

Topologically nontrivial fluctuations control the anomalous interactions for the $η$ and and $η\prime$ pseudoscalar mesons. We consider the anomalous interactions for mesons with higher spin, the heterochiral nonets with $J^{P C} = 1^{+ -}$ and $2^{- +}$. Under the approximation of a dilute gas of instantons, the mixing angle between non-strange and strange mesons decreases strongly as $J$ increases, and oscillates in sign. Anomalous interactions also open up new, rare decay channels. For glueballs, anomalous interactions indicate that the $X(2600)$ state is primarily gluonic.

hep-ph↗

arXiv2023

A QFT scalar toy model analogous to positronium and pion decays

In the framework of a scalar QFT, we evaluate the decay of an initial massive state into two massless particles through a triangle-shaped diagram in which virtual fields propagate. Under certain conditions, the decaying state can be seen as a bound state, thus it is analogous to the neutral pion (quark-antiquark pair) and to the positronium (electron-positron pair), which decay into two photons. While the pion is a relativistic composite object, the positronium is a non-relativistic compound close to threshold. We exam similarities and differences between these two types of bound states.

hep-ph↗

arXiv2023

Sill distribution: genesis and salient features

We present the so-called Sill distribution, both in the nonrelativistic and relativistic cases, as a natural and simple way to include the effect of threshold(s) on the energy line shapes of resonances. The Sill is correctly normalized (even for broad states), is continuous at threshold(s), does not require any modification to the `mass part', is easily extendable to the multichannel case, and can be applied to both mesons and baryons. Here, as a novel example, we employ the Sill to describe the resonance $ψ(3770)$.

hep-ph↗

arXiv2023

Higgs-Higgs scattering and the (non-)existence of the Higgsonium

We study the Higgs-Higgs scattering process and the possible emergence of a Higgs-Higgs bound state (Higgsonium) in any Higgs potential with the vacuum expectation value and second derivative matching the corresponding values from the Standard Model (SM). From the tree-level Higgs-Higgs scattering amplitude, we construct the unitarized amplitude using two different unitarization schemes (the well-known on-shell and N/D methods). We reproduce the known result that there is no Higgsonium state in the SM and, in addition, we determine the S-, D-, and G-wave SM scattering lengths, both at tree-level and upon unitarization. In doing so, we refine previous results by checking the convergence of the N/D approach. Next, we extend the calculation for non-SM potentials and investigate under which conditions a formation of a bound state close to the Higgs-Higgs threshold is possible. In this way, the assumption that no Higgsonium exist, imposes certain bounds on the values of the self-interaction parameters that complement those imposed by the vacuum stability condition.

hep-ph↗

arXiv2023

Radiative production and decays of the exotic $η_1^\prime(1855)$ and its siblings

We study the production of the light $J^{PC}=1^{-+}$ hybrid isoscalars $η_1^\prime(1855)$ and the yet undiscovered $η_1^{hyb}(1660)$ as intermediate states in the radiative decays of the charmonium ($J/ψ$) to two conventional mesons using a flavor symmetric Lagrangian. For this purpose, we use the $J/ψ\toγη_1^\prime(1855)\toγηη^\prime$ process as the reference. We find that some of the decay channels have branching ratios similar to or larger than that of the $γηη^\prime$ channel and are sensitive to the mixing between the hybrid isoscalars. We propose that relatively stable $γηf_1(1285)$ channel be explored for the presence of the light hybrid isoscalar $η_1^{hyb}(1660)$. We also exploit the strong decay channels containing at least one vector meson to study the radiative decays of the whole hybrid nonet $\{π_1(1600),K_1^{hyb}(1750),η_1^{hyb}(1660),η_1'(1855)\}$. We find that the hybrids cannot radiatively decay into the $I=0$ pseudoscalars. Furthermore, the vector decay channels ($(ρ/ω/ϕ)γ$) of the hybrid isoscalars are sensitive to the strangeness content of the hybrids. We also provide estimates for the branching fractions for the radiative production and partial widths for the radiative decays of the hybrids.

hep-ph↗

arXiv2023

Neutrino oscillations in the interaction picture

We study the mixing of different kind of fields (scalar in 0+1D, scalar in 3+1D, fermion in 3+1D) treating the mixing term as an interaction. To this aim, we employ the usual perturbative series in the interaction picture. We find that expression for flavor changing probability exhibits corrections with respect to the usual quantum mechanical (e.g. neutrino) oscillation formula, in agreement with the result previously obtained in the non-perturbative flavor Fock space approach.

hep-ph↗

arXiv2023

The case for an EIC Theory Alliance: Theoretical Challenges of the EIC

We outline the physics opportunities provided by the Electron Ion Collider (EIC). These include the study of the parton structure of the nucleon and nuclei, the onset of gluon saturation, the production of jets and heavy flavor, hadron spectroscopy and tests of fundamental symmetries. We review the present status and future challenges in EIC theory that have to be addressed in order to realize this ambitious and impactful physics program, including how to engage a diverse and inclusive workforce. In order to address these many-fold challenges, we propose a coordinated effort involving theory groups with differing expertise is needed. We discuss the scientific goals and scope of such an EIC Theory Alliance.

hep-ph↗

arXiv2023

Is $f_2(1950)$ the tensor glueball?

Glueballs remain an experimentally undiscovered expectation of QCD. Lattice QCD (As well as other theoretical approaches) predicts a spectrum of glueballs, with the tensor ($J^{PC}=2^{++}$) glueball being the second lightest, behind the scalar glueball. Here, using a chiral hadronic model, we compute decay ratios of the tensor glueball into various meson decay channels. We find the tensor glueball to primarily decay into two vector mesons, dominated by $ρρ$ and $K^*K^*$ channels. These results are compared to experimental data of decay rates of isoscalar tensor mesons. Based on this comparison, we make statements on the eligibility of these mesons as potential tensor glueball candidates: the resonance $f_2(1950)$ turns out to be, at present, the best match as being predominantly a tensor glueball.

hep-ph↗

arXiv2023

Role of bound states and resonances in scalar QFT at nonzero temperature

We study the thermal properties of quantum field theories (QFT) with three-leg interaction vertices $gφ^{3}$ and $gSφ^{2}$ ($φ$ and $S$ being scalar fields), which constitute the relativistic counterpart of the Yukawa potential. We follow a non-perturbative unitarized one-loop resummed technique for which the theory is unitary and well-defined for a large range of values of the coupling constant $g$. Using the partial wave decomposition of two-body scattering we calculate the phase shifts, whose derivatives are used to infer the pressure of the system at nonzero temperature by using the so-called phase shift formalism. A $φφ$ bound state is formed when the coupling $g$ is larger than a certain critical value. As the main outcomes of this work, we estimate the influence of particle interaction on the pressure (both without and with the bound state), and we demonstrate that the latter is always continuous as a function of the coupling constant $g$ (no sudden jumps occurs when the bound state forms), and we show that the contribution of the bound state to the pressure does not count as \textit{one} state in the thermal gas, since a cancellation with the residual $φφ$ interaction occurs. The amount of this cancellation depends on the details of the model and its parameters and a variety of possible scenarios is presented. %Moreover, even when no bound state occurs, we estimate the role of the interaction (including a resonance in the $gSφ^{2}$ theory), which is in general non-negligible. We also show how the overall effect of the interaction, including eventual resonances and bound states, can be formally described by a unique expression that makes use of the phase shift continued below the threshold.

hep-ph↗

arXiv2022

Thermodynamics of the Glueball Resonance Gas

We study the thermodynamic properties -- pressure, entropy and trace anomaly -- of a gas of glueballs that includes the glueball states obtained by various lattice simulations. We show that this model, called Glueball Resonance Gas (GRG) approach, describes well the thermal properties of the Yang-Mills sector of QCD below the critical temperature $T_c$, provided that $T_c$ is properly matched to the corresponding determination of the glueball masses, obtaining $T_c \sim 320 \pm 20$ MeV. The inclusion into the GRG of heavier glueballs not yet seen on the lattice, assuming that glueballs follow Regge trajectories as quark-antiquark states do, leads only to a small correction. We consider the contribution to the pressure of the interactions between scalar-scalar and tensor-tensor glueballs, which turn out to be also negligible.

hep-ph↗

arXiv2022

Conventional mesons below 2 GeV

We briefly review the status of various conventional quark-antiquark mesons below 2 GeV and outline some open questions: the status of the strange-antistrange orbitally excited vector meson, the status of the nonet of axial-tensor mesons (chiral partners of the well known tensor mesons), and the isoscalar mixing angle in the pseudotensor sector, which can eventually represent a novel manifestation of the chiral anomaly.

hep-ph↗

arXiv2022

The fate of the critical endpoint at large $N_c$

The phase diagram of QCD is investigated by varying number of colors $N_c$ within a Polyakov loop quark-meson chiral model. In particular, our attention is focused on the critical point(s): the critical point present for $N_c=3$ moves toward the $μ_q$-axis and disappears as soon as the number of color is increased. Yet, a distinct critical point emerges along the temperature axis for $N_c = 53$ and moves toward finite density when increasing $N_c$ further. Thus, the phase diagram at large $N_c$ looks specular w.r.t. the $N_c = 3$ results, with the first order transition in the upper-left and crossover in the down-right regions of the the $(μ_q,T)$-plane. The pressure is also evaluated in dependence of $N_c$, showing a scaling with $N_c^0$ in the confined and chirally broken phase and with $N_c^2$ in the deconfined one. Moreover, the presence of a chirally symmetric but confined `quarkyonic phase' at large density and moderate temperature with a pressure proportional to $N_c$ is confirmed.

hep-ph↗