arXiv subjects

Francesco Giacosa

Publications and source records attributed to Francesco Giacosa.

At least 73 records · Page 4Linked to original sources

arXiv2020

On the origin of the $Y(4260)$

We study the relation between the $ψ(4160)$ and the $Y(4260)$ within an unitarized effective Lagrangian approach. The $Y(4260)$ arises as a manifestation of the $ψ(4160)$, when a loop-driven decay of the type $ψ(4160)\to D_s^*\bar{D}_s^*\to J/ψf_0(980)$ is enhanced by the proximity of the pole, corresponding to the $ψ(4160)$, to the \emph{almost} closed $D_s^*\bar{D}_s^*$ decay channel. Other $f_0$ resonances that may add a non-negligible contribution, by the same mechanism, are not included for simplicity, but they are not expected to change the main conclusion. Within this picture, the $Y(4260)$ is not, therefore, an independent resonance, but rather a variation of the $ψ(4160)$, which also explains why it is not seen in OZI-allowed decay channels in the experiment.

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arXiv2019

$X(3872)$ as virtual companion pole of the charm-anticharm state $χ_{c1}(2P)$

We study the spectral function of the axial-vector charmonium state $χ_{c1}(2P)$ coupled to $DD^{\ast}$ mesons, by employing a quantum field theoretical approach: a pronounced enhancement close to the $D^{0}% D^{\ast0}$ threshold, to be identified with the $X(3872)$, emerges. In the complex plane we find two poles: a pole for the broad seed state $χ_{c1}(2P)$, and -- in the easiest scenario -- a virtual pole for the $X(3872)$. Thus, our approach describes both the seed state and the dynamically generated $X(3872)$ simultaneously. In particular, it explains the most prominent, both molecular-like and quarkonium-like, features of the $X(3872)$: its very small width (the decay into $D^{0}D^{\ast0}$ is predicted to be about 0.5 MeV), the enhanced radiative decay into $ψ(2S)γ$ w.r.t. $ψ(1S)γ$, and the isospin breaking decay into $J/ψρ$ (thanks to $DD^{\ast}$ loops mediating this decay channel). At the same time, we aim to determine the pole position and the properties of the charmonium seed state: quite interestingly, even if a pole is always present, it is possible that there is no peak corresponding to this state in the spectral function, thus potentially explaining why the corresponding resonance could not yet be seen in experiments.

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arXiv2019

Modelling the inverse Zeno effect for the neutron decay

Beam and trap methods find incompatible results for the lifetime of the neutron: the former delivers a value which is about $8.7\pm2.1$ s longer than the latter. Very recently (1906.10024) it has been proposed that the inverse Zeno effect (IZE) could be responsible for the shorter lifetime in trap experiments. Here, we compare two different models of measurement, one obtained by bang-bang measurements and by a continuous measurement: the IZE turns out to be in both cases very similar, showing that the results do not depend on the details of the measurement process.

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arXiv2019

Role of a four-quark and a glueball state in pion-pion and pion-nucleon scattering

We consider the two-flavor version of the extended linear sigma model (eLSM), which contains (pseudo)scalar and (axial-)vector quark-antiquark mesons, a scalar glueball (predominantly corresponding to $f_{0}(1710)$), as well as the nucleon and its chiral partner. We extend this model by the additional light scalar meson $f_{0}(500)$, predominantly a putative four-quark state. We investigate various interaction terms of the four-quark and glueball states with the other particles, some of which preserve and some of which explicitly break the $U(1)_{\textrm{A}}$ symmetry. We test our model by performing a global fit to masses and decay widths of the scalar resonances and pion-pion scattering lengths. We also discuss the influence of the scalar four-quark state and the glueball on the baryon sector by evaluating pion-nucleon scattering parameters. We find that the inclusion of $f_{0}(500)$ improves the description of pion-pion and pion-nucleon scattering lengths.

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arXiv2019

Companion poles: from the $a_{0}(980)$ to the $X(3872)$

When an unstable ordinary quark-antiquark state couples strongly to other low-mass mesons (such as pions, kaons, $D$-mesons, etc.), the quantum fluctuations generated by the decay products dress the bare `seed' $\bar{q}q$ state and modify its spectral functions. The state is associated to a pole on the complex plane. When the coupling to the decay products is sufficiently large, a remarkable and interesting phenomenon takes place: dynamically generated companion states (or poles) might emerge. Some resonances listed in the PDG , such as the $a_{0}(980),$ the $K_{0}^{\ast}(700),$ and the $X(3872),$ can be well understood by this mechanism, that we briefly review in these proceedings. On the other hand, we show that the $Y(4008)$ and $Y(4260)$ are not independent resonances (or poles), but manifestations of $ψ(4040)$ and $ψ(4160),$ respectively.

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arXiv2019

Thermal contribution of unstable states

Within the framework of the Lee model, we analyze in detail the difference between the energy derivative of the phase shift and the standard spectral function of the unstable state. The fact that the model is exactly solvable allows us to demonstrate the construction of these observables from various exact Green functions. The connection to a formula due to Krein, Friedal, and Lloyd is also examined. We also directly demonstrate how the derivative of the phase shift correctly identifies the relevant interaction contributions for consistently including an unstable state in describing the thermodynamics.

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arXiv2018

The light scalar $K_{0}^{\ast}(700)$ in the vacuum and at nonzero temperature

There is mounting evidence toward the existence of a light scalar kaon $κ\equiv$ $K_{0}^{\ast}(700)$ with quantum numbers $I(J^{P})=\frac{1}{2}(0^{+}).$ Here, we recall the results of an effective model with both derivative and non-derivative terms in which only one scalar kaonic field is present in the Lagrangian (the standard quark-antiquark ,,seed\textquotedblright\ state $K_{0}^{\ast}(1430)$): a second \textquotedblleft companion\textquotedblright\ pole $K_{0}^{\ast}(700)$ emerges as a dynamically generated state. A related question is the role of $K_{0}^{\ast}(700)$ at nonzero $T$: since it is the lightest scalar strange state, one would naively expect that it is relevant for $π$ and $K$ multiplicities. However, a repulsion in the $πK$ channel with $I=3/2$ cancels its effect.

hep-ph↗

arXiv2018

A study of the vector meson $\mathbf{ψ(4040)}$

We investigate the well-known $c\bar{c}$ vector state $ψ(4040)$ in the framework of a quantum field theoretical model. In particular, we study its spectral function and search for the pole(s) in the complex plane. Quite interestingly, the spectral function has a non-standard shape and two poles are present. The role of the meson-meson quantum loops (in particular $DD^*$ ones) is crucial and could also explain the not yet confirmed state $Y(4008)$.

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arXiv2018

Chiral anomaly and strange-nonstrange mixing

As a first step, a simple and pedagogical recall of the $η$-$η'$ system is presented, in which the role of the axial anomaly, related to the heterochiral nature of the multiplet of (pseudo)scalar states, is underlined. As a consequence, $η$ is close to the octet and $η'$ to the singlet configuration. On the contrary, for vector and tensor states, which belong to homochiral multiplets, no anomalous contribution to masses and mixing is present. Then, the isoscalar physical states are to a very good approximation nonstrange and strange, respectively. Finally, for pseudotensor states, which are part of an heterochiral multiplet (just as pseudoscalar ones), a sizable anomalous term is expected: $η_2 (1645)$ roughly corresponds to the octet and $η_2 (1870)$ to the singlet.

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arXiv2018

Can the $ψ(4040)$ explain the peak associated with $Y(4008)$?

We study the well-known resonance $ψ(4040)$, corresponding to a $3^{3}S_{1}$ charm-anticharm vector state $ψ(3S)$, within a QFT approach, in which the decay channels into $DD$, $D^{\ast}D$, $D^{\ast}D^{\ast}$, $D_{s}D_{s}$ and $D_{s}^{\ast}D_{s}$ are considered. The spectral function shows sizable deviations from a Breit-Wigner shape (an enhancement, mostly generated by $DD^{\ast}$ loops, occurs); moreover, besides the $c\bar{c}$ pole of $ψ(4040)$, a second dynamically generated broad pole at $4$ GeV emerges. Naively, it is tempting to identify this new pole with the unconfirmed state $Y(4008).$ Yet, this state was not seen in the reaction $e^{+}e^{-}\rightarrowψ(4040)\rightarrow DD^{\ast}$, but in processes with $π^{+}π^{-}J/ψ$ in the final state. A detailed study shows a related but different mechanism: a broad peak at $4$ GeV in the process $e^{+}e^{-}\rightarrowψ(4040)\rightarrow DD^{\ast }\rightarrowπ^{+}π^{-}J/ψ$ appears when $DD^{\ast}$ loops are considered. Its existence in this reaction is not necessarily connected to the existence of a dynamically generated pole, but the underlying mechanism - the strong coupling of $c\bar{c}$ to \ $DD^{\ast}$ loops - can generate both of them. Thus, the controversial state $Y(4008)$ may not be a genuine resonance, but a peak generated by the $ψ(4040)$ and $D^{\ast}D$ loops with $π^{+}π^{-}J/ψ$ in the final state.

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arXiv2018

Line-shape and poles of the $ψ(3770)$

We study the non-Breit-Wigner line-shape of the $ψ(3770)$ resonance, predominantly a $1\ ^{3}D_{1}$ $\bar{c}c$ state, using an unitarized effective Lagrangian approach, including the one-loop effects of the nearby thresholds $D^+D^-$ and $D^0\bar{D}^0$. A fit of the theoretical result to the total cross-section $e^{+}e^{-}\rightarrow D\bar{D}$ is performed, leading to a good description of data ($χ^{2}/d.o.f.\sim 1.03$). The partial cross sections $e^{+}e^{-}\rightarrow D^0\bar{D}^0$ and $e^{+}e^{-}\rightarrow D^+D^-$ turn out to be separately in good agreement with the experiment. We find a pole at $3777-i12$ MeV, that is within the Particle Data Group (PDG) mass and width estimation for this state. Quite remarkably, we find an additional, dynamically generated, companion pole at $3741-i19$ MeV, which is responsible for the deformation on the lower energy side of the line-shape. The width for the leptonic decay $ψ(3770)\rightarrow e^{+}e^{-}$ is 112 eV, hence smaller than the PDG fit of $262\pm 18$ eV, yet in agreement with a recent experimental study.

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arXiv2018

Wheeler-DeWitt Universe Wave Function in the presence of stiff matter

We study the Wheeler-DeWitt (WDW) equation close to the Big-Bang. We argue that an interaction dominated fluid (speed of sound equal to the speed of light), if present, would dominate during such an early phase. Such a fluid with $p=ρ\propto1/a^{6}$ generates a term in the potential of the wave function of the WDW equation proportional to $-1/a^{2}.\ $This very peculiar quantum potential, which embodies a spontaneous breaking of dilatation invariance, has some very remarkable consequences for the wave function of the Universe: $Ψ(a)$ vanishes at the Big-Bang: $Ψ(0)=0$; the wave function $Ψ(a)$ is always real; a superselection rule assures that the system is confined to $a\geq0$ without the need of imposing any additional artificial barrier for unphysical negative $a$. These results do not depend on the operator-ordering problem of the WDW equation.

hep-th↗

arXiv2018

How the axial anomaly controls flavor mixing among mesons

It is well known that because of the axial anomaly in QCD, mesons with $J^P = 0^-$ are close to $SU(3)_{\mathrm{V}}$ eigenstates: the $eta^\prime(958)$ meson is largely a singlet, and the $η$ meson an octet. In contrast, states with $J^P = 1^-$ are flavor diagonal: \textit{e.g.}, the $ϕ(1020)$ is almost pure $\bar{s} s$. Using effective Lagrangians, we show how this generalizes to states with higher spin, assuming that they can be classified according to the unbroken chiral symmetry of $G_{\mathrm{fl}} = SU(3)_{\mathrm{L}} \times SU(3)_{\mathrm{R}}$. We construct effective Lagrangians from terms invariant under $G_{\mathrm{fl}}$, and introduce the concept of \textit{hetero-} and \textit{homo}chiral multiplets. Because of the axial anomaly, only terms invariant under the $Z(3)_{\mathrm{A}}$ subgroup of the axial $U(1)_{\mathrm{A}}$ enter. For heterochiral multiplets, which begin with that including the $η$ and $η^\prime(958)$, there are $Z(3)_{\mathrm{A}}$ invariant terms with low mass dimension which cause states to mix according to $SU(3)_{\mathrm{V}}$ flavor. For homochiral multiplets, which begin with that including the $ϕ(1020)$, there are no $Z(3)_{\mathrm{A}}$ invariant terms with low mass dimension, and so states are diagonal in flavor. In this way we predict the flavor mixing for the heterochiral multiplet with spin one, as well as for hetero- and homochiral multiplets with spin two and spin three.

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arXiv2018

QFT derivation of the decay law of an unstable particle with nonzero momentum

We present a quantum field theoretical derivation of the nondecay probability of an unstable particle with nonzero three-momentum $\mathbf{p}$. To this end, we use the (fully resummed) propagator of the unstable particle, denoted as $S,$ to obtain the energy probability distribution, called $d_{S}^{\mathbf{p} }(E)$, as the imaginary part of the propagator. The nondecay probability amplitude of the particle $S$ with momentum $\mathbf{p}$ turns out to be, as usual, its Fourier transform: $a_{S}^{\mathbf{p}}(t)=\int_{\sqrt{m_{th} ^{2}+\mathbf{p}^{2}}}^{\infty}dEd_{S}^{\mathbf{p}}(E)e^{-iEt}$ ($m_{th}$ is the lowest energy threshold in the energy frame, corresponding to the sum of masses of the decay products). Upon a variable transformation, one can rewrite it as $a_{S}^{\mathbf{p}}(t)=\int_{m_{th}}^{\infty}dmd_{S} ^{\mathbf{0}}(m)e^{-i\sqrt{m_{th}^{2}+\mathbf{p}^{2}}t}$ [here, $d_{S} ^{\mathbf{0}}(m)\equiv d_{S}(m)$ is the usual spectral function (or mass distribution) in the rest frame]. Hence, the latter expression, previously obtained by different approaches, is here confirmed in an independent and, most importantly, covariant QFT-based approach. Its consequences are not yet fully explored but appear to be quite surprising (such as the fact that usual time-dilatation formula does not apply), thus its firm understanding and investigation can be a fruitful subject of future research.

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arXiv2018

Formation and Deformation of the $ψ(3770)$

The form of resonance line-shapes unveils information about its nonperturbative properties and formation mechanisms. Here, we study the non-Breit-Wigner energy distribution of the resonance $ψ(3770)$ using an unitarized effective Lagrangian approach, that includes the effect of the nearby threshold $D^{+}D^{-}$. Two poles are found in the second Riemann sheet near the resonance amplitude. We discuss the setting of the free parameters and possible effects contributing to the signal.

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arXiv2017

A study of the excited radial vector meson $ρ$

We study the strong and radiative decays of the radial excitation of the $ρ$ meson, with quantum numbers $n$ $^{2S+1}L_{J}=2$ $^{3}S_{1},$ $I=1$, and denoted as $ρ_{E}$, by making use of an effective model. We test different masses: $1.45$ GeV, corresponding to well known assignment $ρ_{E}\equivρ(1450)$ state; $1.25$ GeV for the assignment $ρ_{E}\equivρ(1250)$ (a resonance whose existence has not yet been confirmed), and $1.35$ GeV, which lies just in the middle between them in order to study the dependence of the results on the mass. The decay widths for different decay processes and two branching ratios of the radially excited meson $ρ_{E}$ were determined and compared to the experimental data reported in the Particle Data Group.

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arXiv2017

Revisiting the axial anomaly for light mesons and baryons

The axial anomaly is responsible for the masses and mixing of the mesons $η$ and $η'$. An open question is if (and to what extent) it affects also other hadrons. We show that anomalous terms can be important to understand the spectroscopy of the pseudotensor mesons $η_2 (1645)$ and $η_2 (1870)$. In fact, pseudotensor mesons belong to a so-called heterochiral multiplet, for which a quadratic mixing term between nonstrange and strange isoscalar members arises. On the contrary, for so-called homochiral multiplets, such as the ground-state (axial-)vector and tensor mesons, this mixing is not possible, hence one can easily understand why the isoscalar members of these multiplets are almost purely nonstrange and strange, respectively. Moreover, the axial anomaly can be also coupled to baryons (within the mirror assignment), and thus it helps to explain the large decay width $ N^{\ast }(1535)\rightarrow Nη$ and to clarify which baryons are chiral partners.

hep-ph↗

arXiv2017

Influence of the axial anomaly on the decay $N(1535) \rightarrow Nη$

The decay width of $N(1535)\rightarrow Nη$ is as large as that of $N(1535)\rightarrow Nπ$. This is in evident conflict with simple expectations based on flavor symmetry and phase space. Similarly, the decay width of $Λ(1670) \rightarrow Λ(1116) η$ is larger than predicted by flavor symmetry. In this work, we propose that the axial $U(1)_{A}$ anomaly is responsible for an enhanced coupling of (some) excited baryons to the $η$ meson. We test this idea by including a new, chirally symmetric but $U(1)_{A}$ anomalous, term in an effective hadronic model decribing baryons and their chiral partners in the mirror assignment. This term enhances the decay of the chiral partners into baryons and an $η$ meson, such as $N(1535)\rightarrow Nη$. Moreover, a strong coupling of $N(1535)$ to $N η^\prime $ emerges (this is important for studies of $η^\prime$ production processes). Our approach shows that $N(1535)$ is predominantly the chiral partner of $N(939)$, and $Λ(1670)$ the chiral partner of $Λ(1116)$. Finally, our formalism can be used to couple the pseudoscalar glueball $\tilde{G}$ to baryons. We expect a large cross section for the reaction $\bar{p}p \rightarrow \tilde{G} \rightarrow \bar{p}p(1535)$, which can be experimentally tested in the future PANDA experiment.

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