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Luciano Maiani

Publications and source records attributed to Luciano Maiani.

At least 19 recordsLinked to original sources
arXiv2023

The Pentaquark Spectrum from Fermi Statistics

We study compact hidden charm pentaquarks in the Born-Oppenheimer approximation, previously introduced for tetraquarks, assuming the heavy pair to be in a color octet. We show that Fermi statistics applied to the complex of the three light quarks, also in color octet, requires S-wave pentaquark ground states to consist of three octets of flavour-SU(3)_f, two with spin 1/2 and one with spin 3/2, in line with the observed, strangeness S=0,-1, spectrum. Additional lines corresponding to decays into J/ψ+ Sigma and J/ψ+ Xi are predicted. In the language of non-relativistic SU(6), ground state pentaquarks form either a 56 or a 20 representation, distinguished by presence or absence of pentaquarks decaying in the spin 3/2 decuplet, e.g. in J/ψ+ Delta^++. Observation of a strangeness S=-2 or isospin I=3/2 pentaquarks would be a clear signature of compact, QCD bound pentaquarks.

hep-ph↗
arXiv2022

Doubly Heavy Tetraquarks in the Born-Oppenheimer approximation

Tetraquarks QQq*q* are found to be described remarkably well with the Quantum Chromodynamics version of the Hydrogen bond, as treated with the Born-Oppenheimer approximation. We show the robustness of the method by computing the mass of the observed Tcc tetraquark following two different paths. Relying on this, we provide a prediction for the mass of the expected Tbb particle. The average sizes of tetraquarks are estimated to be approximately 3 - 5 GeV**-1. As a consequence hyperfine separations are not expected to be sizeable. We discussed possible reasons why LHCb has observed only one state in the DD* spectrum.

hep-ph↗
arXiv2022

GGI Lectures on Exotic Hadrons

It is well known that M. Gell-Mann, introducing quarks in 1964 to describe the known mesons and baryons, hinted at the existence of further $qq\bar q\bar q$ mesons (tetraquarks) and $qqqq\bar q$ baryons (pentaquarks). In 1977, R. Jaffe proposed a model of the lightest scalar mesons as diquark-antidiquark pairs and A. de Rujula, H. Georgi and S. Glashow coined the term hadron molecules, to describe possible hadrons made by meson-antimeson pairs bound by the familiar nuclear forces, also an overall tetraquark system. The two alternative pictures have been employed to interpret the unexpected hadron discovered by Belle in 2003, the $X(3872)$, confirmed by BaBar and seen in many other High Energy experiments. Since then, a wealth of Exotic Hadrons have been discovered, mesons and baryons that cannot be described by the classical Gell-Mann, $q\bar q$ and $qqq$, configurations, opening a new chapter of Hadron Spectroscopy.

hep-ph↗
arXiv2022

Substructure of Multiquark Hadrons (Snowmass 2021 White Paper)

In recent years there has been a rapidly growing body of experimental evidence for existence of exotic, multiquark hadrons, i.e. mesons which contain additional quarks, beyond the usual quark-antiquark pair and baryons which consist of more than three quarks. In all cases with robust evidence they contain at least one heavy quark Q=c or b, the majority including two heavy quarks. Two key theoretical questions have been triggered by these discoveries: (a) how are quarks organized inside these multiquark states -- as compact objects with all quarks within one confinement volume, interacting via color forces, perhaps with an important role played by diquarks, or as deuteron-like hadronic molecules, bound by light-meson exchange? (b) what other multiquark states should we expect? The two questions are tightly intertwined. Each of the interpretations provides a natural explanation of parts of the data, but neither explains all of the data. It is quite possible that both kinds of structures appear in Nature. It may also be the case that certain states are superpositions of the compact and molecular configurations. This Whitepaper brings together contributions from many leading practitioners in the field, representing a wide spectrum of theoretical interpretations. We discuss the importance of future experimental and phenomenological work, which will lead to better understandingof multiquark phenomena in QCD.

hep-ph↗
arXiv2022

From the lineshape of the $X(3872)$ to its structure

From a study of the lineshape of the $X(3872)$, the LHCb collaboration measures a sizeable negative effective range. This cannot be reconciled with a shallow $D\bar{D}^*$ bound state hypothesis. Based on Weinberg's compositeness criterion, together with a theorem by Smorodinsky, it follows that the $X$ has to have a compact hidden charm structure, most likely a tetraquark, interacting with unbound $D\bar{D}^*$ pairs via short-distance color forces. This conclusion is strengthened by the general pattern recently emerging for exotic mesons.

hep-ph↗
arXiv2021

The new resonances Z_{cs}(3985) and Z_{cs}(4003) (almost) fill two tetraquark nonets of broken SU(3)_f

New data from BESIII and LHCb show the existence of resonances with strangeness filling multiplets of the broken SU(3)_f symmetry, with the pattern predicted by the quark model. This is the case of the newly discovered Z_{cs} (3985) and Z_{cs}(4003), which have a natural accommodation in the tetraquark picture, as shown in this note. The quasi-degeneracy between Z_{cs} (3985) and Z_{cs}(4003) reproduces, in the strange sector, the situation observed with X(3872) and Z_c(3900). This represents a significative score in favor of the tetraquark scheme.

hep-ph↗
arXiv2021

Radiative and meson decays of $Y(4230)$ in flavor SU(3)

The charmonium-like exotic states $Y(4230)$ and the less known $Y(4320)$, produced in $e^+e^-$ collisions, are sources of positive parity exotic hadrons in association with photons or pseudoscalar mesons. We analyze the radiative and pion decay channels in the compact tetraquark scheme, with a method that proves to work equally well in the most studied $D^*\to γ/π+D$ decays. The decay of the vector $Y$ into a pion and a $Z_c$ state requires a flip of charge conjugation and isospin that is described appropriately in the formalism used. Rates however are found to depend on the fifth power of pion momentum which would make the final states $πZ_c(4020)$ strongly suppressed with respect to $πZ_c(3900)$. The agreement with BES III data would be improved considering the $πZ_c(4020)$ events to be fed by the tail of the $Y(4320)$ resonance under the $Y(4230)$. These results should renovate the interest in further clarifying the emerging experimental picture in this mass region.

hep-ph↗
arXiv2020

Tetraquark Interpretation and Production Mechanism of the Belle $Y_b (10750)$-Resonance

Recently, the Belle Collaboration has updated the analysis of the cross sections for the processes $e^+ e^- \to Υ(nS)\, π^+ π^-$ ($n = 1,\, 2,\, 3$) in the $e^+ e^-$ center-of-mass energy range from 10.52 to 11.02~GeV. A new structure, called $Y_b (10750)$, with the mass $M (Y_b) = (10752.7 \pm 5.9^{+0.7}_{-1.1})$~MeV and the Breit-Wigner width $Γ(Y_b) = (35.5^{+17.6 +3.9}_{-11.3 -3.3})$~MeV was observed. We interpret $Y_b (10750)$ as a compact $J^{PC} = 1^{--}$ state with a dominant tetraquark component. The mass eigenstate $Y_b (10750)$ is treated as a linear combination of the diquark-antidiquark and $b \bar b$ components due to the mixing via gluonic exchanges shown recently to arise in the limit of large number of quark colors. The mixing angle between $Y_b$ and $Υ(5S)$ can be estimated from the electronic width, recently determined to be $Γ_{ee} (Y_b) = (13.7 \pm 1.8)$~eV. The mixing provides a plausible mechanism for $Y_b (10750)$ production in high energy collisions from its $b \bar b$ component and we work out the Drell-Yan and prompt production cross sections for $p p \to Y_b (10750) \to Υ(nS)\, π^+ π^-$ at the LHC. The resonant part of the dipion invariant mass spectrum in $Y_b (10750) \to Υ(1S)\, π^+ π^-$ and the corresponding angular distribution of $π^+$-meson in the dipion rest frame are presented.

hep-ph↗
arXiv2020

The X(3872) tetraquarks in B and B_s decays

We discuss how the latest data on X(3872) in B and B_s decays speak about its tetraquark nature. The established decay pattern, including the up to date observations by CMS, are explained by the mixing of two quasi-degenerate, unresolvable, neutral states. The same mechanism also explains isospin violations in X decays and strongly suggests that the lurking charged partners are required to have very small branching fractions in J/psi rho^pm, well below the current experimental limits. In addition, a new prediction on the decay into J/psi omega final states is attained. The newest experimental observations are found to give thrust to the simplest tetraquark picture and call for a definitive, in-depth study of final states with charged rho mesons.

hep-ph↗
arXiv2020

Interpretation of $Y_b (10750)$ as a tetraquark and its production mechanism

Recently, the Belle Collaboration has updated the analysis of the cross sections for the processes $e^+ e^- \to Υ(nS)\, π^+ π^-$ ($n = 1,\, 2,\, 3$) in the $e^+ e^-$ center-of-mass energy range from 10.52 to 11.02 GeV. A new structure, called here $Y_b (10750)$, with the mass $M (Y_b) = (10752.7 \pm 5.9^{+0.7}_{-1.1})$ MeV and the Breit-Wigner width $Γ(Y_b) = (35.5^{+17.6 +3.9}_{-11.3 -3.3})$ MeV was observed \cite{Abdesselam:2019gth}. We interpret $Y_b (10750)$ as a compact $J^{PC} = 1^{--}$ state with a dominant tetraquark component. The mass eigenstate $Y_b (10750)$ is treated as a linear combination of the diquark-antidiquark and $b \bar b$ components due to the mixing via gluonic exchanges shown recently to arise in the limit of large number of quark colors. The mixing angle between $Y_b$ and $Υ(5S)$ can be estimated from the electronic width, recently determined to be $Γ_{ee} (Y_b) = (13.7 \pm 1.8)$ eV. The mixing provides a plausible mechanism for $Y_b (10750)$ production in high energy collisions from its $b \bar b$ component and we work out the Drell-Yan and prompt production cross sections for $p p \to Y_b (10750) \to Υ(nS)\, π^+ π^-$ at the LHC. The resonant part of the dipion invariant mass spectrum in $Y_b (10750) \to Υ(1S)\, π^+ π^-$ and the corresponding angular distribution of $π^+$-meson in the dipion rest frame are presented as an example.

hep-ph↗
arXiv2018

Tetraquarks, Pentaquarks and Dibaryons in the large $N$ QCD

We study the multiquark hadrons in large $N$ QCD under the 't-Hooft limit, extending Witten's picture of the baryons. We explore the decay widths of tetraquarks, pentaquarks and dibaryons. Based on the decay behaviors, we point out in the $N\to \infty$ limit decay widths of tetraquarks stay constant, while those of pentaquarks and dibaryons above certain thresholds can diverge. In the large $N$ limit, we find that the ground states of the three spectroscopic series are stable or narrow and that the excited states of pentaquarks and dibaryons above the indicated thresholds are not observables. We compare our results with those obtained in a different large $N$ generalization of tetraquarks.

hep-ph↗
arXiv2018

Tetraquarks in the 1/N Expansion: a New Appraisal

We discuss the necessary, albeit not sufficient, conditions for tetraquark poles to occur in the 1/N expansion of QCD and find the minimum order at which such poles may appear. Assuming tetraquark poles, we find a new non-planar solution with the minimal number of topologies and tetraquark species. The solution implies narrow states. Mixing with quarkonium states is allowed so that P-wave tetraquarks with J^PC=1^-- would couple to e^+e^-.

hep-ph↗
arXiv2017

A New Look at the $Y$ Tetraquarks and $Ω_c$ Baryons in the Diquark Model

We analyze the hidden charm $P$-wave tetraquarks in the diquark model, using an effective Hamiltonian incorporating the dominant spin-spin, spin-orbit and tensor interactions. We compare with other $P$-wave system such as $P$-wave charmonia and the newly discovered $Ω_c$ baryons, analysed recently in this framework. Given the uncertain experimental situation on the $Y$ states, we allow for different spectra and discuss the related parameters in the diquark model. In addition to the presently observed ones, we expect many more states in the supermultiplet of $L=1$ diquarkonia, whose $J^{PC}$ quantum numbers and masses are worked out, using the parameters from the currently preferred $Y$-states pattern. The existence of these new resonances would be a decisive footprint of the underlying diquark dynamics.

hep-ph↗
arXiv2017

The LHC Timeline: A personal recollection (1980-2012). Oral History Interview

The object of this interview is the history of the Large Hadron Collider in the LEP tunnel at CERN, from first ideas to the discovery of the Brout-Englert-Higgs boson, seen from the point of view of a member of CERN scientific committees, of the CERN Council and a former Director General of CERN in the years of machine construction

physics.hist-ph↗
arXiv2017

The Charm of Theoretical Physics (1958-1993)

Personal recollections on theoretical particle physics in the years when the Standard Theory was formed. In the background, the remarkable development of Italian theoretical physics in the second part of the last century, with great personalities like Bruno Touschek, Raoul Gatto, Nicola Cabibbo and their schools.

physics.hist-ph↗
arXiv2014

Selected Topics in Majorana Neutrino Physics

Starting from the original Majorana's article of 1937, the see-saw mechanism is illustrated, first for one and later for three neutrino generations, and neutrinoless double beta decay is considered. Neutrino mixing and oscillations in three flavors are described. The Yukawa couplings to the Higgs field of quarks and leptons are considered, their transformation properties under the corresponding flavor groups are spelled out and the principle of Minimal Flavor Violation is illustrated, in connection with possible new physics beyond the Standard Theory. The idea that the Yukawa couplings may be the vacuum expectation value of some new fields is introduced and natural extrema of potentials which are invariant under quark and lepton flavor groups are characterized. A recent result indicating large mixing of almost degenerate neutrinos is derived from the heavy lepton invariance under flavor ${\cal O}(3)$.

hep-ph↗