arXiv subjects

Roberto Bruschini

Publications and source records attributed to Roberto Bruschini.

12 recordsLinked to original sources

arXiv2026

SPARSE: Scattering Poles and Amplitudes from Radial Schrödinger Equations

We introduce an algorithm for the solution of a system of radial Schrödinger equations describing the inelastic scattering of particles with spin in a partial wave with definite total angular momentum. The system of differential equations is approximated as an ordinary linear nonhomogeneous system using the finite difference method. Dirichlet boundary conditions are imposed at the origin and at an arbitrary large radius. The numerical wavefunction is calculated using LAPACK general banded LU decomposition routines. The $K$-matrix for physical energies is calculated from the numerical solutions of the system by least-square fit to the analytical real solutions at large distances. The scattering amplitudes for physical energies is then calculated from the $K$-matrix. Scattering poles in the lower half of the unphysical Riemann sheet are calculated by numerical extrapolation of the scattering amplitudes using the AAA algorithm for rational approximation.

quant-ph↗

arXiv2026

Quarkoniumlike states above open-flavor thresholds in Born-Oppenheimer EFT

Many quarkoniumlike states have been observed above open-flavor thresholds, but their organization and internal structure remain unsettled. We study the isoscalar hidden-charm and hidden-bottom sectors in Born--Oppenheimer effective field theory (BOEFT), between the spin--isospin averaged $S+S$ and $S+P$ thresholds. At leading order, heavy-quark spin decouples, and the quarkonium static potential mixes through string breaking with the lowest tetraquark/open-flavor BO potentials of the same quantum numbers. These potentials are constrained by QCD symmetries, their short- and long-distance behavior, and lattice-QCD data. The only calibrated parameter is the lowest $1^{--}$ adjoint meson mass, fixed from the shallow multiplet associated with the $χ_{c1}(3872)$. Using $T$-matrix, $K$-matrix, and complex-scaling methods, we determine bound states and resonance poles, their masses, pole widths from the included nonstrange $S+S$ channels, normalized pole couplings, and prescription-dependent quarkonium--open-flavor composition measures. Uncoupled hybrid BOEFT multiplets are included as reference levels. The spectrum exhibits a common heavy-quark-spin-symmetry multiplet organization. Most poles are predominantly quarkonium resonances localized at short distances, with the largest open-flavor components closest to threshold. The same equations also generate shallow, spatially extended, open-flavor-dominated states with molecular long-distance characteristics. Their binding energies, radii, and small quarkonium components are highly sensitive to the adjoint meson mass, whereas the higher spectrum is more stable. Together with the hybrid reference levels, the spectrum provides multiplet assignments for most candidates. States not naturally accommodated point to the need for hidden-strange and $S+P$ tetraquark/open-flavor BO sectors and for hybrid--tetraquark and hybrid--quarkonium mixings.

hep-ph↗

arXiv2026

$\boldsymbol{χ_{c1}}(3872)$ and its Partners in the Diabatic Born-Oppenheimer Approximation for QCD

In the Born-Oppenheimer approximation for QCD, the exotic hidden-charm tetraquark meson $χ_{c1}(3872)$ is a near-threshold bound state in Born-Oppenheimer potentials associated with an isospin-0 adjoint meson. The $χ_{c1}(3872)$ is the $1^{++}$ member of a heavy-quark spin-symmetry multiplet whose other members have $J^{PC}$ quantum numbers $0^{++}$, $1^{+-}$, and $2^{++}$. We introduce a simple model for the Born-Oppenheimer potentials that interpolates between the adjoint-meson potential at short distances and the triplet-meson-pair potential at large distances. We take into account the spin splittings of charm mesons nonperturbatively for the first time by solving the diabatic Schrödinger equation. We also take into account the spin splittings of the adjoint meson as well as a narrow avoided crossing with the quarkonium potential. We tune the energy of $χ_{c1}(3872)$ to the $D^* \bar{D}$ threshold and then calculate the spin splittings of the other members of the multiplet and their decay widths into charm-meson pairs. We also calculate the energies and decay widths of the corresponding multiplet of hidden-bottom tetraquarks. These calculations provide a template for the quantitative analysis of all hidden-heavy hadrons using the Born-Oppenheimer approximation for QCD.

hep-ph↗

arXiv2026

Hidden-Heavy Pentaquarks and Where to Find Them

We provide a simple explanation for the observed hidden-charm pentaquarks as bound states in Born-Oppenheimer potentials. We identify $P_{c\bar{c}}(4312)^+$, $P_{c\bar{c}}(4440)^+$, and $P_{c\bar{c}}(4457)^+$ as heavy-quark spin states in a quartet of $c \bar{c}$ pentaquarks with $J^P$ quantum numbers $\frac{1}{2}^-$, $\frac{3}{2}^-$, and $\frac{5}{2}^-$. The quantum numbers of $P_{c\bar{c}}(4457)^+$ differ from most previous predictions. We also predict a fourth $c\bar{c}$ pentaquark with quantum numbers $\frac{3}{2}^-$ near the $Σ_c^\ast\bar{D}$ threshold. We identify $P_{c\bar{c}s}(4338)^0$ and $P_{c\bar{c}s}(4459)^0$ as heavy-quark spin states in a triplet of $c \bar{c}s$ pentaquarks with quantum numbers $\frac{1}{2}^-$ and either $\frac{1}{2}^-$ or $\frac{3}{2}^-$. We also predict a third $c\bar{c}s$ pentaquark with quantum numbers either $\frac{3}{2}^-$ or $\frac{1}{2}^-$ near the $Ξ_c\bar{D}^\ast$ threshold. We explain why the observed hidden-charm pentaquarks have narrow widths.

hep-ph↗

arXiv2025

The Pattern of Exotic Hidden-Heavy Hadrons Revealed

For more than twenty years, theory has failed to explain the pattern of the exotic heavy hadrons. We illustrate a simple solution to this longstanding puzzle using the Born-Oppenheimer approximation for QCD. Exotic hidden-heavy hadrons are bound states and resonances in potentials that are repulsive at short range and cross a heavy-hadron--pair threshold before approaching it. This explains the proximity of the exotic hidden-heavy hadrons to heavy-hadron--pair thresholds, identifies the thresholds that support bound states or resonances, and prevents an explosion in the number of predicted states. We also discuss the fine tunings of QCD that are responsible for the remarkable properties of some of the exotic hidden-heavy mesons.

hep-ph↗

arXiv2025

Exotic Hidden-heavy Hadrons and Where to Find Them

The Born-Oppenheimer potentials for QCD with light quarks include adjoint-hadron potentials that are repulsive at short distances and heavy-hadron-pair potentials that approach thresholds at large distances. The adjoint-hadron potentials must connect smoothly to the heavy-hadron-pair potentials at intermediate distances. We identify exotic hidden-heavy hadrons as bound states and resonances in adjoint-hadron potentials that cross below a heavy-hadron-pair threshold before approaching it. This explains why many exotic hidden-charm and hidden-bottom hadrons have energies near heavy-hadron-pair thresholds. The remarkable properties of some exotic hidden-heavy mesons can be explained by fine tunings of adjoint-meson energies in QCD.

hep-ph↗

arXiv2024

The ${^{3}\!}P_{0}$ model reloaded

We revisit the phenomenological ${^{3}\!}P_{0}$ model for the decay of quarkonium $\left( Q\bar{Q}\right) $ into two open flavor mesons ($\bar{\mathfrak{M}}\mathfrak{M}% $). We take the heavy-quark limit and derive a transition rate between $Q\bar{Q}$ and $\bar{\mathfrak{M}}\mathfrak{M}$ to be compared with the one calculated in studies of string breaking using lattice QCD. This comparison allows to fit the creation amplitude of a light quark-antiquark pair in the ${^{3}\!}P_{0}$ model to the string-breaking transition rate in QCD.

hep-ph↗

arXiv2023

Charm-Meson $t$-channel Singularities in an Expanding Hadron Gas

We study the time evolution of the numbers of charm mesons after the kinetic freezeout of the expanding hadron gas produced by the hadronization of the quark-gluon plasma from a central heavy-ion collision. The $πD$ reaction rates have contributions from a $D^\ast$ resonance in the $s$ channel. The $πD^\ast$ reaction rates are enhanced by $t$-channel singularities from an intermediate $D$. The contributions to reaction rates from $D^\ast$ resonances and $D$-meson $t$-channel singularities are sensitive to thermal mass shifts and thermal widths. In the expanding hadron gas, the $t$-channel singularities are regularized by the thermal $D$ widths. After kinetic freezeout, the thermal $D$ widths are dominated by coherent pion forward scattering. The contributions to $πD^\ast$ reaction rates from $t$-channel singularities are inversely proportional to the pion number density, which decreases to 0 as the hadron gas expands. The $t$-channel singularities produce small but significant changes in charm-meson ratios from those predicted using the known $D^\ast$-decay branching fractions.

hep-ph↗

arXiv2022

Evolution of charm-meson ratios in an expanding hadron gas

We study the time evolution of the numbers of charm mesons after the kinetic freeze-out of the hadron gas produced by a central heavy-ion collision. The $πD^\ast \to πD^\ast$ reaction rates have $t$-channel singularities that give contributions inversely proportional to the thermal width of the $D$. The ratio of the $D^0$ and $D^+$ production rates can differ significantly from those predicted using the measured $D^\ast$ branching fractions.

hep-ph↗

arXiv2021

Strong decays of the lowest bottomonium hybrid within an extended Born-Oppenheimer framework

We analyze the decays of the theoretically predicted lowest bottomonium hybrid $H(1P)$ to open bottom two-meson states. We do it by embedding a quark pair creation model into the Born-Oppenheimer framework which allows for a unified, QCD-motivated description of bottomonium hybrids as well as bottomonium. A new $^{1}\!P_{1}$ decay model for $H(1P)$ comes out. The same analysis applied to bottomonium leads naturally to the well-known $^{3}\!P_{0}$ decay model. We show that $H(1P)$ and the theoretically predicted bottomonium state $Υ(5S)$, whose calculated masses are close to each other, have very different widths for such decays. A comparison with data from $Υ(10860)$, an experimental resonance whose mass is similar to that of $Υ(5S)$ and $H(1P)$, is carried out. Neither a $Υ(5S)$ nor a $H(1P)$ assignment can explain the measured decay widths. However, a $Υ(5S)$-$H(1P)$ mixing may give account of them supporting previous analyses of dipion decays of $Υ(10860)$ and suggesting a possible experimental evidence of $H(1P)$.

hep-ph↗

arXiv2019

Quark model description of $ψ(4260)$

From lattice indications we follow a Born-Oppenheimer approximation to build a quark-antiquark static potential for $J^{PC}=1^{--}$ charmonium states below their first S- wave meson-meson threshold. We show that a good description of the mass and decay properties of the experimentally well established $ψ(4260)$ resonance is feasible.

hep-ph↗

arXiv2019

A plausible explanation of $Υ(10860)$

We show that a good description of the $Υ(10860)$ properties, in particular the mass, the $e^+ e^-$ leptonic widths and the $π^{+}π^{-}Υ(ns)$$\ (n=1,2,3)$ production rates, can be obtained under the assumption that $Υ(10860)$ is a mixing of the conventional $Υ(5s)$ quark model state with the lowest $P-$ wave hybrid state.

hep-ph↗