Light Meson Sector Research Articles

Correlations of C and CP violation in η → π0ℓ+ℓ− and η′ → ηℓ+ℓ−

Based on recent progress in the systematic analysis of C and CP violation in the light-meson sector, we calculate the C-odd transition amplitudes η → π0ℓ+ℓ− and η′ → ηℓ+ℓ−. Focusing on long-distance contributions driven by the lowest-lying hadronic intermediate states, we work out the correlations between these beyond-the-Standard-Model signals and the Dalitz-plot asymmetries in η → π0π+π− and η′ → ηπ+π−, using dispersion theory.

News from the light and strange meson sector

Recent high-quality data sets from various experiments grant new insights into the excitation spectrum of light mesons, which are composed of up, down, and strange quarks. In the non-strange light-meson sector, many recent experimental and theory efforts are focused on the search for so-called exotic states, which are states beyond the na¨ıve constituent quark-model. To this end, various potential decay modes of exotic states are studied at experiments such as GlueX or COMPASS. In addition, there is recent progress in the understanding of so-called non-resonant processes, which act as background in searches for light mesons, as well as in the prediction of light mesons from lattice QCD. While the non-strange light-meson spectrum is already mapped out rather well, many predicted strange mesons have not yet been observed experimentally or need further confirmation. Recent high-precision data from experiments such as LHCb and BESIII allow us to study strange mesons in heavy-meson decays. Alternatively, strange mesons are studied in diffractive scattering of high-energy kaon beams. The so far world’s largest data set on the diffractively produced K−π −π + final state was measured by the COMPASS experiment. A recent analysis of this data covers a large variety of strange mesons over a wide mass range in a single, self-consistent analysis

Investigation of the lightest hybrid meson candidate with a coupled-channel analysis of {{bar{p}}p}-, pi ^- p- and {pi pi }-Data

A sophisticated coupled-channel analysis is presented that combines different processes: the channels {pi ^0pi ^0eta }, {pi ^0eta eta } and {K^+K^-pi ^0} from {{bar{p}}p} annihilations, the P- and D-wave amplitudes of the pi eta and pi eta ^prime systems produced in pi ^-p scattering, and data from {pi pi }-scattering reactions. Hence our analysis combines the data sets used in two independent previous analyses published by the Crystal Barrel experiment and by the JPAC group. Based on the new insights from these studies, this paper aims at a better understanding of the spin-exotic pi _1 resonances in the light-meson sector. By utilizing the K-matrix approach and realizing the analyticity via Chew-Mandelstam functions the amplitude of the spin-exotic wave can be well described by a single pi _1 pole for both systems, pi eta and pi eta ^prime . The mass and the width of the pi _1-pole are measured to be (1623 , pm , 47 , ^{+24}_{-75}), mathrm {MeV/}c^2 and (455 , pm 88 , ^{+144}_{-175}), mathrm {MeV}.

Triangle Singularity as the Origin of the a_{1}(1420).

The COMPASS Collaboration experiment recently discovered a new isovector resonancelike signal with axial-vector quantum numbers, the a_{1}(1420), decaying to f_{0}(980)π. With a mass too close to and a width smaller than the axial-vector ground state a_{1}(1260), it was immediately interpreted as a new light exotic meson, similar to the X, Y, Z states in the hidden-charm sector. We show that a resonancelike signal fully matching the experimental data is produced by the decay of the a_{1}(1260) resonance into K^{*}(→Kπ)K[over ¯] and subsequent rescattering through a triangle singularity into the coupled f_{0}(980)π channel. The amplitude for this process is calculated using a new approach based on dispersion relations. The triangle-singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having fewer parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time in the light-meson sector that a resonancelike structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.

Beam Asymmetries from Light Scalar Meson Production on the Proton at GlueX

The GlueX facility, featuring a linearly polarised 9 GeV real photon beam delivered to a large-acceptance detector system, has recently completed its first phase of running, and analysis efforts of this dataset are well underway. It has been suggested that at GlueX energies, quark systems beyond the three quark and quark-antiquark systems of baryons and mesons, such as hybrid mesons, tetraquarks and glueballs, should exist, and studies of these systems could shed new light on how quarks combine under the strong force, particularly the role played by gluons.Meticulous study of the spectrum of hadronic states is required to understand the strong force in the non-perturbative energy regime, and the light scalar meson sector is an area that remains poorly understood. GlueX data encompasses final states at energies where photoproduction of the a0 (980) and f 0 (980) mesons can provide discriminatory evidence between various models, manifested in experimental observables such as the cross section and beam asymmetry, and performing detailed measurements of these quantities is considered a priority of the ongoing research program.The work presented showcases efforts to measure the beam asymmetry of the reaction γp→pηπ whose mass spectrum encompasses several mesons, including the a 0(980) light scalar, and the a 2(1320) tensor. Future prospects for related analyses in the light scalar meson sector, informed by this measurement, will also be discussed.

Light Meson Decay with Gaussian Confinement in a JKJ Decay Model

A form to describe hadron strong decays is in terms of quark and gluon degrees of freedom in microscopic decay models. Initially we assume that strong decays are driven by the same inter-quark Hamiltonian which determines the spectrum, and that it incorporates gaussian confinement. An [Formula: see text] decay matrix element of the JKJ Hamiltonian involves a pair-production current matrix elements times a scattering matrix element. Diagrammatically this corresponds to an interaction between an initial line and produced pair. In this work we apply the model to the light meson sector and calculate the decay rate, comparing with the experimental values.

Hadron scattering, resonances, and QCD

The non-perturbative nature of quantum chromodynamics (QCD) has historically left a gap in our understanding of the connection between the fundamental theory of the strong interactions and the rich structure of experimentally observed phenomena. For the simplest properties of stable hadrons, this is now circumvented with the use of lattice QCD (LQCD). In this talk I discuss a path towards a rigorous determination of few-hadron observables from LQCD. I illustrate the power of the methodology by presenting recently determined scattering amplitudes in the light-meson sector and their resonance content.

Doubly heavy baryons with chiral partner structure

The spectrum and dominant strong decay properties of the doubly heavy baryons are revisited by using a chiral effective model with the chiral partner structure. By regarding the doubly heavy baryons in the ground states and light angular momentum, jl=1/2, sector of the first orbitally excited states as chiral partners, we estimate their mass splitting arising from the spontaneous breaking of chiral symmetry to be about 430 MeV for baryons including an unflavored light quark and about 350 MeV for that including a strange quark. We point out that, similar to the heavy–light meson sector, the intermultiplet decay from a baryon with negative parity to its chiral partner and a pion is determined by the mass splitting through the generalized Goldberger–Treiman relation. Furthermore, the isospin-violating decay of the Ωcc baryon, ((1/2)−,(3/2)−)s→((1/2)+,(3/2)+)s+π0, through the η–π0 mixing is the dominant decay channel of the doubly heavy baryons including a strange quark.

Baryon states with open charm in the extended local hidden gauge approach

In this paper we examine the interaction of DN and D* N states, together with their coupled channels, by using an extension of the local hidden gauge formalism from the light meson sector, which is based on heavy quark spin symmetry. The scheme is based on the use of the impulse approximation at the quark level, with the heavy quarks acting as spectators, which occurs for the dominant terms where there is the exchange of a light meson. The pion exchange and the Weinberg-Tomozawa interactions are generalized and with this dynamics we look for states generated from the interaction, with a unitary coupled channels approach that mixes the pseudoscalar-baryon and vector-baryon states. We find two states with nearly zero width, which are associated to the Λ c (2595) and Λ c (2625). The lower state, with J P = 1/2−, couples to DN and D* N, and the second one, with J P = 3/2−, to D* N. In addition to these two Λ c states, we find four more states with I = 0, one of them nearly degenerate in two states of J P = 1/2, 3/2. Furthermore we find three states in I = 1, two of them degenerate in J = 1/2, 3/2.

Hadron physics at the COMPASS experiment

Quantum Chromodynamics (QCD), the theory of strong interactions, in principle describes the interaction of quark and gluon fields. However, due to the self-coupling of the gluons, quarks and gluons are confined into hadrons and cannot exist as free particles. The quantitative understanding of this confinement phenomenon, which is responsible for about 98\% of the mass of the visible universe, is one of the major open questions in particle physics. The measurement of the excitation spectrum of hadrons and of their properties gives valuable input to theory and phenomenology. In the Constituent Quark Model (CQM) two types of hadrons exist: mesons, made out of a quark and an antiquark, and baryons, which consist of three quarks. But more advanced QCD-inspired models and Lattice QCD calculations predict the existence of hadrons with exotic properties interpreted as excited glue (hybrids) or even pure gluonic bound states (glueballs). The COMPASS experiment at the CERN Super Proton Synchrotron has acquired large data sets, which allow to study light-quark meson and baryon spectra in unprecedented detail. The presented overview of the first results from this data set focuses in particular on the light meson sector and presents a detailed analysis of three-pion final states. A new $J^{PC} = 1^{++}$ state, the $a_1(1420)$, is observed with a mass and width in the ranges $m = 1412-1422\,\mathrm{MeV}/c^2$ and $\Gamma = 130-150\,\mathrm{MeV}/c^2$.

Exotic $$J^{PC}=1^{-+}$$ J P C = 1 - + mesons in a holographic model of QCD

Mesons with quantum numbers $J^{PC}=1^{-+}$ cannot be represented as simple quark-antiquark pairs. We explore hybrid configurations in the light meson sector comprising a quark, an antiquark and an excited gluon, studying the properties of such states in a phenomenological model inspired by the gauge/gravity correspondence. The computed mass, compared to the experimental mass of the $1^{-+}$ candidates $\pi_1(1400)$, $\pi_1(1600)$ and $\pi_1(2015)$, favous $\pi_1(1400)$ as the lightest hybrid state. An interesting result concerns the stability of hybrid mesons at finite temperature: they disappear from the spectral function (i.e. they melt) at a lower temperature with respect to other states, light vector and scalar mesons, and scalar glueballs.

Baryon states with open beauty in the extended local hidden gauge approach

In this paper we examine the interaction of \bar B N, \bar B \Delta, \bar B^* N and \bar B^* \Delta states, together with their coupled channels, using a mapping from the light meson sector. The assumption that the heavy quarks act as spectators at the quark level automatically leads us to the results of the heavy quark spin symmetry for pion exchange and reproduces the results of the Weinberg Tomozawa term, coming from light vector exchanges in the extended local hidden gauge approach. With this dynamics we look for states dynamically generated from the interaction and find two states with nearly zero width, which we associate to the \Lambda_b(5912) and \Lambda_b(5920) states. The states couple mostly to \bar B^* N, which are degenerate with the Weinberg Tomozawa interaction. The difference of masses between these two states, with J=1/2, 3/2 respectively, is due to pion exchange connecting these states to intermediate \bar B N states. In addition to these two \Lambda_b states, we find three more states with I=0, one of them nearly degenerate in two states of J=1/2,3/2. Furthermore we also find eight more states in $I=1$, two of them degenerate in J=1/2, 3/2, and other two degenerate in J=1/2, 3/2, 5/2.

Radial and angular-momentum Regge trajectories: a systematic approach

We present the analysis of Ref.[1] of the radial (n) and angular-momentum (J) Regge trajectories for all light-quark meson states listed in the Particle Data Tables. The parameters of the trajectories are obtained with linear regression, with weight of each resonance inversely proportional to its half-width squared, (\Gamma/2)^2. The joint analysis in the (n,J,M^2) Regge plane indicates, at the 4.5 standard deviation level, that the slopes in n are larger from the slopes in J. Thus no strict universality of slopes occurs in the light non-strange meson sector. We also extend our analysis to the kaon sector.

LIGHT MESON DECAY IN THE ${\rm C}\,{}^{3}\!P_{0}$ MODEL

Having its origin in a successful mapping technique, the Fock–Tani formalism, the corrected [Formula: see text] model [Formula: see text] retains the basic aspects of the [Formula: see text] predictions with the inclusion of bound-state corrections. Evaluation of the decay amplitudes has been performed for open-flavor strong decays in the light meson sector. The bound-state corrections introduce a fine-tuning for the former [Formula: see text] model, in particular, the adjustment of the D/S ratios in b1→ωπ, a1→ρπ and h1→ρπ decays.

Tetraquark Spectroscopy: A Symmetry Analysis

We present a detailed analysis of the symmetry properties of a four-quark wave function and its solution by means of a variational approach for simple Hamiltonians. We discuss several examples in the light and heavy-light meson sector.

ISOVECTOR COMPONENTS OF LIGHT SCALAR MESON AND NUCLEAR MATTER PROPERTIES

We have predicted (contribution to this issue) an isovector component of the light scalar meson sector by using the chiral symmetry transformation formalism. On the basis of this result, we study dense hadronic matter in a generalized relativistic mean field approach with σ, ω and ρ mesons as well as nonlinear self-couplings of the I = 1 component of a light scalar meson field and compare its predictions for neutron star properties with results from different models for nuclear matter found in the literature.

SIGNATURES OF ISOVECTOR COMPONENTS OF A LIGHT SCALAR MESON IN PULSARS

In a previous work, we have predicted an isovector component of the light scalar meson sector by using the chiral symmetry transformation formalism. On the basis of this result, we study dense hadronic matter in a generalized relativistic mean field approach with σ, ω and ρ mesons as well as nonlinear self-couplings of the I = 1 component of a light scalar meson field and compare its predictions for neutron star properties and with results from different models for nuclear matter found in the literature.

Experiments Needed in Meson and Baryon Spectroscopy

Three (or four) straightforward experiments would contribute greatly to completing the spectroscopy of baryons and light mesons. In the baryon sector, data are needed on inelastic reactions from a polarised target withπ±andK±beams up to∼3 GeV/c. Similar data are needed in the light meson sector forp¯pinteractions in the momentum range 0.3–2 GeV/c. In both cases, valuable information is to be obtained from longitudinal (L) and sideways (S) target polarisations as well as the conventional normal (N) polarisation. Thirdly,3S1and3D1mesons in the mass range 1–2.4 GeV could probably be separated either by diffractive dissociation of transversely polarised photons or bye+e−radiative return experiments using transversely and longitudinally polarised electrons.

3S1 and ¹S0 meson spectra in a renormalized QCD-inspired model

In this work we study an extension of a light-cone QCD-inspired model, where the interacting part of the mass squared operator consists of a harmonic oscillator potential as confinement, a Coulomb-like interaction and a zero-range spin interaction acting on the ³S0 states. The renormalization of the model is performed by using as a input the pion mass to fix the strength of the spin interaction. We apply the extended model to study the splitting of the ³S1 and ³S0 ground state mesons and the spectrum in the light meson sector. We show that the experimental values of the splitting between the masses of ³S1 and ³S0 ground state mesons as a function of the ground state pseudoscalar mass is reproduced by the model. In the botonium case our result is consistent with other theoretical estimates.

Electromagnetic Properties of Diquarks

Diquark correlations play an important role in hadron physics. The properties of diquarks can be obtained from the corresponding bound state equation. Using a model for the effective quark-quark interaction that has proved successful in the light meson sector, we solve the scalar diquark Bethe-Salpeter equations and use the obtained Bethe-Salpeter amplitudes to compute the diquarks' electromagnetic form factors. The scalar ud diquark charge radius is about 8% larger than the pion charge radius, indicating that these diquarks are somewhat larger in size than the corresponding mesons. We also provide analytic fits for the form factor over a moderate range in Q^2, which may be useful, for example, in building quark-diquark models of nucleons.