arXiv subjects

A. Pilloni

Publications and source records attributed to A. Pilloni.

At least 37 records · Page 2Linked to original sources

arXiv2020

First measurement of direct photoproduction of the $a_2(1320)^0$ meson on the proton

We present the first measurement of the exclusive reaction $γp \rightarrow a_2(1320)^0 \, p$ in the photon energy range $3.5$-$5.5$ GeV and four-momentum transfer squared $0.2<-t<2.0$ GeV$^2$. Data were collected with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The neutral $a_2$ resonance was detected by measuring the reaction $γp \rightarrow π^0 ηp$ and reconstructing the $π^0 η$ invariant mass. The differential cross section $dσ/dt$ was extracted at different beam energies in each $-t$ bin. The most prominent feature of the differential cross section is a dip at $-t\simeq 0.55$ GeV$^2$. This can be well described in the framework of Regge phenomenology, where the exchange degeneracy hypothesis predicts a zero in the reaction amplitude for this value of the four-momentum transfer.

nucl-ex↗

arXiv2020

Diquark Correlations in Hadron Physics: Origin, Impact and Evidence

The last decade has seen a marked shift in how the internal structure of hadrons is understood. Modern experimental facilities, new theoretical techniques for the continuum bound-state problem and progress with lattice-regularised QCD have provided strong indications that soft quark+quark (diquark) correlations play a crucial role in hadron physics. For example, theory indicates that the appearance of such correlations is a necessary consequence of dynamical chiral symmetry breaking, viz. a corollary of emergent hadronic mass that is responsible for almost all visible mass in the universe; experiment has uncovered signals for such correlations in the flavour-separation of the proton's electromagnetic form factors; and phenomenology suggests that diquark correlations might be critical to the formation of exotic tetra- and penta-quark hadrons. A broad spectrum of such information is evaluated herein, with a view to consolidating the facts and therefrom moving toward a coherent, unified picture of hadron structure and the role that diquark correlations might play.

hep-ph↗

arXiv2020

Exclusive tensor meson photoproduction

We study tensor meson photoproduction outside of the resonance region, at beam energies of few GeVs. We build a model based on Regge theory that includes the leading vector and axial exchanges. We consider two determinations of the unknown helicity couplings, and fit to the recent a2 photoproduction data from CLAS. Both choices give a similar description of the a2 cross section, but result in different predictions for the parity asymmetries and the f2 photoproduction cross section. We conclude that new measurements of f2 photoproduction in the forward region are needed to pin down the correct production mechanism. We also extend our predictions to the 8.5 GeV beam energy, where current experiments are running.

hep-ph↗

arXiv2020

$ω\to 3π$ and $ωπ^{0}$ transition form factor revisited

In light of recent experimental results, we revisit the dispersive analysis of the $ω\to 3π$ decay amplitude and of the $ωπ^0$ transition form factor. Within the framework of the Khuri-Treiman equations, we show that the $ω\to 3π$ Dalitz-plot parameters obtained with a once-subtracted amplitude are in agreement with the latest experimental determination by BESIII. Furthermore, we show that at low energies the $ωπ^0$ transition form factor obtained from our determination of the $ω\to 3π$ amplitude is consistent with the data from MAMI and NA60 experiments.

hep-ph↗

arXiv2020

Dalitz-plot decomposition for three-body decays

We present a general formalism to write the decay amplitude for multibody reactions with explicit separation of the rotational degrees of freedom, which are well controlled by the spin of the decay particle, and dynamic functions on the subchannel invariant masses, which require modeling. Using the three-particle kinematics we demonstrate the proposed factorization, named the Dalitz-plot decomposition. The Wigner rotations, that are subtle factors needed by the isobar modeling in the helicity framework, are simplified with the proposed decomposition. Consequently, we are able to provide them in an explicit form suitable for the general case of arbitrary spins. The only unknown model-dependent factors are the isobar lineshapes that describe the subchannel dynamics. The advantages of the new decomposition are shown through three examples relevant for the recent discovery of the exotic charmonium candidate $Z_c(4430)$, the pentaquarks $P_c$, and the intriguing $Λ_c^+\to pK^-π^+$ decay.

hep-ph↗

arXiv2019

Khuri-Treiman equations for $3π$ decays of particles with spin

Khuri-Treiman equations have proven to be a useful theoretical tool in the analysis of 3-body decays, specially into the $3π$ final state. In this work we present in full detail the necessary generalization of the formalism to study the decays of particles with arbitrary spin, parity, and charge conjugation. To this extent, we find it most convenient to work with helicity amplitudes instead of the so-called invariant amplitudes, specially when dealing with the unitarity relations. The isobar expansions in the three possible ($s$-, $t$-, and $u$-) final channels are related with the appropriate crossing matrices. We pay special attention to the kinematical singularities and constraints of the helicity amplitudes, showing that these can be derived by means of the crossing matrix.

hep-ph↗

arXiv2019

Moments of angular distribution and beam asymmetries in $ηπ^0$ photoproduction at GlueX

In the search for exotic mesons, the GlueX collaboration will soon extract moments of the $ηπ^0$ angular distribution. In the perspective of these results, we generalize the formalism of moment extraction to the case in which the two mesons are produced with a linearly polarized beam, and build a model for the reaction $\vec γp \to ηπ^0 p$. The model includes resonant $S$-, $P$-, $D$-waves in $ηπ^0$, produced by natural exchanges. Moments of the $ηπ^0$ angular distribution are computed with and without the $P$-wave, to illustrate the sensitivity to exotic resonances. Although little sensitivity to the $P$-wave is found in moments of even angular momentum, moments of odd angular momentum are proportional to the interference between the $P$-wave and the dominant $S$- and $D$-waves. We also generalize the definition of the beam asymmetry for two mesons photoproduction and show that, when the meson momenta are perpendicular to the reaction plane, the beam asymmetry enhances the sensitivity to the exotic $P$-wave.

hep-ph↗

arXiv2019

Interpretation of the LHCb Pc(4312) Signal

We study the nature of the new signal reported by LHCb in the J/psi p spectrum. Based on the S-matrix principles, we perform a minimum-bias analysis of the underlying reaction amplitude, focusing on the analytic properties that can be related to the microscopic origin of the Pc(4312) peak. By exploring several amplitude parametrizations, we find evidence for the attractive effect of the Sigma_c+ D0bar channel, which is not strong enough, however, to form a bound state.

hep-ph↗

arXiv2019

On the Equivalence of Three-Particle Scattering Formalisms

In recent years, different on-shell $\mathbf{3}\to\mathbf{3}$ scattering formalisms have been proposed to be applied to both lattice QCD and infinite volume scattering processes. We prove that the formulation in the infinite volume presented by Hansen and Sharpe in Phys.~Rev.~D92, 114509 (2015) and subsequently Briceño, Hansen, and Sharpe in Phys.~Rev.~D95, 074510 (2017) can be recovered from the $B$-matrix representation, derived on the basis of $S$-matrix unitarity, presented by Mai {\em et al.} in Eur.~Phys.~J.~A53, 177 (2017) and Jackura {\em et al.} in Eur.~Phys.~J.~C79, 56 (2019). Therefore, both formalisms in the infinite volume are equivalent and the physical content is identical. Additionally, the Faddeev equations are recovered in the non-relativistic limit of both representations.

hep-ph↗

arXiv2019

Three-body scattering: Ladders and Resonances

We discuss unitarity constraints on the dynamics of a system of three interacting particles. We show how the short-range interaction that describes three-body resonances can be separated from the long-range exchange processes, in particular the one-pion-exchange process. It is demonstrated that unitarity demands a specific functional form of the amplitude with a clear interpretation: the bare three-particle resonances are dressed by the initial- and final-state interaction, in a way that is consistent with the considered long-range forces. We postulate that the resonance kernel admits a factorization in the energy variables of the initial- and the final-state particles. The factorization assumption leads to an algebraic form for the unitarity equations, which is reminiscent of the well-known two-body-unitarity condition and approaches it in the limit of the narrow-resonance approximation.

hep-ph↗

arXiv2019

Nucleon resonance contributions to unpolarised inclusive electron scattering

The first CLAS12 experiments will provide high-precision data on inclusive electron scattering observables at a photon virtuality $Q^2$ ranging from 0.05 GeV$^2$ to 12 GeV$^2$ and center-of-mass energies $W$ up to 4 GeV. In view of this endeavour, we present the modeling of the resonant contributions to the inclusive electron scattering observables. As input, we use the existing CLAS electrocoupling results obtained from exclusive meson electroproduction data off protons, and evaluate for the first time the resonant contributions based on the experimental results on the nucleon resonance electroexcitation. The uncertainties are given by the data and duly propagated through a Monte Carlo approach. In this way, we obtain estimates for the resonant contributions, important for insight into the nucleon parton distributions in the resonance region and for the studies of quark-hadron duality.

hep-ph↗

arXiv2019

Regge phenomenology of the $N^$ and $Δ^$ poles

We use Regge phenomenology to study the structure of the poles of the $N^*$ and $Δ^*$ spectrum. We employ the available pole extractions from partial wave analysis of meson scattering and photoproduction data. We assess the importance of the imaginary part of the poles (widths) to obtain a consistent determination of the parameters of the Regge trajectory. We compare the several pole extractions and show how Regge phenomenology can be used to gain insight in the internal structure of baryons. We find that the majority of the states in the parent Regge trajectories are compatible with a mostly compact three-quark state picture.

hep-ph↗

arXiv2019

Determination of the pole position of the lightest hybrid meson candidate

Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, pi1(1400) and pi1(1600), which couple separately to eta pi and eta' pi. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the eta(') pi system by the COMPASS collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the S-matrix. We provide a robust extraction of a single exotic pi1 resonant pole, with mass and width 1564 +- 24 +- 86 MeV and 492 +- 54 +- 102 MeV, which couples to both eta(') pi channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the a2(1320) and a2'(1700).

hep-ph↗

arXiv2019

Pole position of the $a_1(1260)$ from $τ$-decay

We perform an analysis of the three-pion system with quantum numbers $J^{PC}=1^{++}$ produced in the weak decay of $τ$ leptons. The interaction is known to be dominated by the axial meson $a_1(1260)$. We build a model based on approximate three-body unitarity and fix the free parameters by fitting it to the ALEPH data on $τ^-\to π^-π^+π^-\,ν_τ$ decay. We then perform the analytic continuation of the amplitude to the complex energy plane. The singularity structures related to the $ππ$ subchannel resonances are carefully addressed. Finally, we extract the $a_1(1260)$ pole position $m_p^{(a_1(1260))}-iΓ_p^{(a_1(1260))}/2$ with $m_p^{(a_1(1260))} = (1209 \pm 4^{+12}_{-9})\text{MeV}$, $Γ_p^{(a_1(1260))} = (576 \pm 11 ^{+89}_{-20})\text{MeV}$}.

hep-ph↗

arXiv2018

Review of phenomenological analyses of $η^{(\prime)} π$ resonances

We present a robust analysis of the $η^{(\prime)} π$ system in COMPASS data. We fit the extracted relative phases and intensities with a coupled-channel formalism enforcing both unitarity and analyticity. We provide a robust extraction of a single exotic $π_1(1600)$ decaying to both $η^{(\prime)} π$ final states, and the resonance parameters of the $a_2(1320)$ and $a'_2(1700)$. We find no evidence for a second exotic state, which is compatible with recent Lattice QCD estimates.

hep-ph↗

arXiv2018

Phenomenology of Relativistic $\mathbf{3} \to \mathbf{3}$ Reaction Amplitudes within the Isobar Approximation

Further progress in hadron spectroscopy necessitates the phenomenological description of three particle reactions. We consider the isobar approximation, where the connected part of the $\mathbf{3}\to\mathbf{3}$ amplitude is first expressed as a sum over initial and final pairs, and then expanded into a truncated partial wave series. The resulting unitarity equation is automatically fulfilled by the $B$-matrix solution, which is an integral equation for the partial wave amplitudes, analogous to the $K$-matrix parameterization used to describe $\mathbf{2}\to\mathbf{2}$ amplitudes. We study the one particle exchange and how its analytic structure impacts rescattering solutions such as the triangle diagram. The analytic structure is compared to other parameterizations discussed in the literature. We briefly discuss the analogies with recent formalisms for extracting $\mathbf{3}\to\mathbf{3}$ scattering amplitudes in lattice QCD.

hep-ph↗

arXiv2018

What is the right formalism to search for resonances? II. The pentaquark chain

We discuss the differences between several partial-wave analysis formalisms used in the construction of three-body decay amplitudes involving fermions. Specifically, we consider the decay Lambda_b -> psi p K- , where the hidden charm pentaquark signal has been reported. We analyze the analytical properties of the amplitudes and separate kinematical and dynamical singularities. The result is an amplitude with the minimal energy dependence compatible with the S-matrix principles.

hep-ph↗

arXiv2018

Structure of Pion Photoproduction Amplitudes

We derive and apply the finite energy sum rules to pion photoproduction. We evaluate the low energy part of the sum rules using several state-of-the-art models. We show how the differences in the low energy side of the sum rules might originate from different quantum number assignments of baryon resonances. We interpret the observed features in the low energy side of the sum rules with the expectation from Regge theory. Finally, we present a model, in terms of a Regge-pole expansion, that matches the sum rules and the high-energy observables.

hep-ph↗