arXiv subjects

Davide Germani

Publications and source records attributed to Davide Germani.

7 recordsLinked to original sources

arXiv2026

Isosinglet-isotriplet mixing and the $X(3872)$ lineshape

We investigate the lineshapes of the $X(3872)$ in $B^+$ decays production within a framework that incorporates two underlying QCD configurations: a compact isosinglet state $X_S$ and the neutral component of a molecular isotriplet $X_T^0$. The physical signal is interpreted as arising from the mixing of these states, induced by strong isospin breaking. The decay amplitude is constructed in a factorized form, separating short-distance production, non-relativistic propagation and final-state interactions. This setup allows for a unified description of both $DD^*$ and $J/\psi\,+$ pions final states. We show that the interplay between the two components and their mixing can qualitatively reproduce several nontrivial experimental features. In particular, interference effects can enhance the charged $DD^*$ channel relative to the neutral one despite phase-space suppression, and generate distinctive structures in the $J/\psi \pi^+\pi^-$ and $J/\psi \pi^+\pi^-\pi^0$ lineshapes, including the possibility of strong distortions near threshold.

hep-ph↗

arXiv2026

Positivity of the effective range for finite range attractive potentials with a repulsive core

In the phenomenological study of exotic hadrons, the sign of the effective range, $r_0$, is invoked as a criterion to distinguish between compact multiquark configurations (associated with $r_0 < 0$) and loosely bound hadronic molecules ($r_0 > 0$). Motivated by this, we investigate the fundamental constraints on the sign of the effective range for single-channel local interactions. We rigorously prove that for finite-range potentials, characterized by an inner repulsive core and an outer attractive tail, the effective range remains strictly positive provided that the scattering length is greater than the range of the potential ($a > R$).

hep-ph↗

arXiv2025

A short review on the compositeness of the $X(3872)$

The $X(3872)$ could be a shallow $D\bar D^*$ bound state, a compact four-quark state, or a partially composite particle, i.e. a superposition of the two. We will review how these hypotheses could be tested experimentally, examining especially the cases in which the $X$ is a pure bound state or a pure compact tetraquark. Data on $X\to D\bar D\pi$ decays are compared with the analysis of the $X$ lineshape. The pure bound state hypothesis corresponds to a well-defined region in parameter space defined by the width of the $D^*$ versus the binding energy of the $X$. As for the $X$ lineshape, we observe that the currently available experimental analysis tests the compatibility with the compact hypothesis for the $X$. We propose how to extend the analysis to examine the molecular or the partially composite hypotheses. We also review the analysis on the radiative decays of the $X$ including pion corrections confirming some conclusions reached in the literature on the use of the universal wave function description for the molecular $X$.

hep-ph↗

arXiv2025

Tetraquarks in the Born-Oppenheimer approximation

The conventional loosely bound molecule interpretation of the $X(3872)$ is not compatible with the recent LHCb experimental measurement of the ratio of branching fractions $\mathcal{R}=\text{Br}(X\to\psi^\prime\gamma)/\text{Br}(X\to\psi\gamma)$. We systematically determine the entire tetraquark spectrum for $J=0,1,2$ and refine the calculation of $\mathcal{R}$ in an improved Born-Oppenheimer description of the $X(3872)$ compact tetraquark. This refinement yields a significantly better agreement with experimental data on ${\cal R}$ and on the spectroscopy of the states themselves. Extending the diquark-antidiquark paradigm to encompass tetraquarks that are linear superposition of open charm singlets and color octets, we discover that these exotic resonances manifest as compact shallow bound states of quarks in color force potentials.

hep-ph↗

arXiv2025

On the Atomki nuclear anomaly after the MEG-II result

Recent experimental results from the Atomki collaboration have reported the observation of anomalous effects in Beryllium, Helium and Carbon nuclear transitions that could hint at physics beyond the Standard Model. However, the MEG-II experiment has recently found no significant anomalous signal in the Beryllium transition ${^8}\text{Be}^\star\to{^8}\text{Be}+e^+e^-$. In view of this result, we critically re-examine the possible theoretical interpretations of the anomalies observed by the Atomki experiment in terms of a new boson $X$ with mass around $17\;$MeV. The present work aims to study the phenomenology of a spin-2 state and revisit the possibility of a pure CP-even scalar, which was initially dismissed due to its inability to explain the Beryllium anomalous signal. Our analysis shows that a spin-2 state is highly disfavoured by the SINDRUM constraint while a scalar boson could explain the Helium and Carbon anomalies while being compatible with other experimental constraints.

hep-ph↗

arXiv2024

A Simple Model of Pentaquarks

We describe pentaquarks as baryo-charmonia with a color octet $c\bar{c}$ core bonded to a color octet three-quark system. Fermi statistics of the light quark cloud allows to describe two pentaquark triplets: a lower one, well supported by experiment, and a higher one with strangeness. For the time being, the lowest line of the strange triplet has been experimentally identified in a $3σ$ peak. Data also suggest (at least) two different production mechanisms for pentaquarks. We show how this can be described in the proposed scheme.

hep-ph↗

arXiv2023

The role of the pion in the lineshape of the $X(3872)$

We determine the contribution of long-range pion interactions to the $X(3872)$ dynamics, assuming it is a loosely bound $D^0 \bar{D}^{*0}$ molecule. Our result is based on the distorted wave Born approximation in non-relativistic quantum mechanics. Despite their long-range nature, we find that pion interactions cannot produce a large and negative effective range. Nonetheless, they introduce imaginary parts. In particular, they contribute to the total decay width of the $X(3872)$ with a term associated with, but not precisely corresponding to, the $D^*$ width. Our approach can also be applied to the recently discovered $T_{cc}^+$ states.

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