Meson Interactions Research Articles

ρρinteraction in the hidden gauge formalism and thef0(1370)andf2(1270)resonances

We have studied the interaction of vector mesons within the hidden gauge formalism and applied it to the particular case of the $\ensuremath{\rho}\ensuremath{\rho}$ interaction. We find a strong attraction in the isospin, spin channels $I$, $S=0$, 0 and 0, 2, which is enough to bind the $\ensuremath{\rho}\ensuremath{\rho}$ system. We also find that the attraction in the $I$, $S=0$, 2 channel is much stronger than in the 0, 0 case. The states develop a width when the $\ensuremath{\rho}$ mass distribution is considered, and particularly when the $\ensuremath{\pi}\ensuremath{\pi}$ decay channel is turned on. Using a regularization scheme with cutoffs of natural size, we obtain results in fair agreement with the mass and the width of the ${f}_{0}(1370)$ and ${f}_{2}(1270)$ meson states, providing a natural explanation of why the tensor state is more bound than the scalar and offering a new picture for these states, which would be dynamically generated from the $\ensuremath{\rho}\ensuremath{\rho}$ interaction or, in simpler words, $\ensuremath{\rho}\ensuremath{\rho}$ molecular states.

RECENT DEVELOPMENTS IN CHIRAL UNITARY DYNAMICS OF RESONANCES

In this talk I summarize recent findings made on the description of axial vector mesons as dynamically generated states from the interaction of pseudoscalar mesons and vector mesons, dedicating some attention to the two K1(1270) states. Then I review the generation of open and hidden charm scalar and axial states. Finally, I present recent results showing that the low lying 1/2+ baryon resonances for S=−1 can be obtained as bound states or resonances of two mesons and one baryon in coupled channels dynamics.

Transition form factors of theN*(1535)as a dynamically generated resonance

We discuss how electromagnetic properties provide useful tests of the nature of resonances, and we study these properties for the ${N}^{*}(1535)$ that appears dynamically generated from the strong interaction of mesons and baryons. Within this coupled-channels chiral unitary approach, we evaluate the ${A}_{1/2}$ and ${S}_{1/2}$ helicity amplitudes as a function of ${Q}^{2}$ for the electromagnetic ${N}^{*}(1535)\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}N$ transition. Within the same formalism we evaluate the cross section for the reactions $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\eta}N$. We find a fair agreement for the absolute values of the transition amplitudes, as well as for the ${Q}^{2}$ dependence of the amplitudes, within theoretical and experimental uncertainties discussed in the article. The ratios obtained between the ${S}_{1/2}$ and ${A}_{1/2}$ for the neutron or proton states of the ${N}^{*}(1535)$ are in qualitative agreement with experiment and there is agreement on the signs. The same occurs for the ratio of cross sections for the $\ensuremath{\eta}$ photoproduction on neutron and proton targets in the vicinity of the ${N}^{*}(1535)$ energy. The global results support the idea of this resonance as being dynamically generated, hence, largely built up from meson baryon components. However, the details of the model indicate that an admixture with a genuine quark state is also demanded that could help obtain a better agreement with experimental data.

Charge renormalization in quantum field theory by the unitary clothing transformation method

The clothing procedure in several lowest orders in the coupling constant has been implemented using the unitary clothing transformation method. Within a simple field theory model, including interaction of charged spinless nucleons and scalar mesons, an expression for the charge shift is obtained, which is determined by the operators beyond the energy shell.

Radiative decay intoγPof the low lying axial-vector mesons

We evaluate the radiative decay into a pseudoscalar meson and a photon of the whole set of the axial-vector mesons dynamically generated from the vector-pseudoscalar meson interaction. We take into account tree level and loop diagrams coming from the axial-vector decay into a vector and a pseudoscalar meson. We find a large span for the values of the radiative widths of the different axial-vector mesons. In particular, we evaluate the radiative decay into K{gamma} of the two K{sub 1}(1270) states, recently claimed theoretically, and discuss the experimental values quoted so far on the assumption of only one state.

Intra-Nuclear Cascade (INC) code for simulation of meson photoproduction in nuclei

η meson photoproduction in the 14N nucleus was studied using simulations done with INC (Intra-Nuclear Cascade) code. The probability for mesons and nucleons to escape the nucleus on different steps of the reaction was evaluated. These results predict the possibility of studying elastic and inelastic η meson interactions with intra-nuclear nucleons using the recoil proton as a tag for the case of slow η meson and fast recoil proton.

Systematic study on QCD interactions of heavy mesons with ρ meson

The strong interactions of the negative-parity heavy mesons with ρ meson may be described consistently in the context of an effective Lagrangian, which is invariant under isospin SU(2) transformation. Four coupling constants gHHρ, fH∗Hρ, gH∗H∗ρ and fH∗H∗ρ enter the effective Lagrangian, where H(H∗) denotes a pseudoscalar bottom or charm meson (the corresponding vector meson). Using QCD light cone sum rule (LCSR) method and, as inputs, the hadronic parameters updated recently, we give an estimate of gH∗H∗ρ and fH∗H∗ρ, about which little was known before, and present an improved result for gHHρ and fH∗Hρ. Also, we examine the heavy quark asymptotic behavior of these nonperturbative quantities and assess the two low energy parameters β and λ of the corresponding effective chiral Lagrangian.

Quantum loops in radiative decays of the [formula omitted] and [formula omitted] axial-vector mesons

A previous model where the low-lying axial-vector mesons are dynamically generated, implementing unitarity in coupled channels in the vector–pseudoscalar (VP) meson interaction, is applied to evaluate the decay widths of the a1(1260) and b1(1235) axial-vector mesons into πγ. Unlike the case of the a1, the b1 radiative decay is systematically underestimated at tree level. In this work we evaluate for the first time the loop contribution coming from an initial VP vertex. Despite the large superficial divergence of the loops, the convergence of the relevant loops can be established by using arguments of gauge invariance. The partial decay widths obtained agree with the experimental values within uncertainties, and we show that the loop contribution is crucial in the b1 case and also important for the a1 case.

Cross sections for meson–meson nonresonant reactions

Meson–meson nonresonant reactions governed by the quark-interchange mechanism are studied in a potential that is derived from QCD. S-wave elastic phase shifts for I = 2 ππ and I = 3 / 2 Kπ scattering are obtained with wave functions determined by the central spin-independent term of the potential. The reactions include inelastic scatterings of two mesons in the ground-state pseudoscalar octet and the ground-state vector nonet. Cross sections for reactions involving π, ρ, K and K * indicate that mesonic interactions in matter consisting of only K and K * can be stronger than mesonic interactions in matter consisting of only π and ρ and the reaction of I = 3 / 2 π K * → ρ K is most important among the endothermic nonresonant reactions. By the quark-interchange mechanism we can offer s -dependences of ϕ absorption cross sections in collisions with π and ρ and relevant average cross sections what are very small for the reaction of I = 1 π ϕ → K * K * and remarkably large for the reaction of I = 1 ρ ϕ → K * K * . It is found from the s -dependences of cross sections that ρ and K * creation cross sections can be larger than their absorption cross sections, respectively.

Loop corrections and naturalness in a chiral effective field theory

The loop expansion is applied to a chiral effective hadronic Lagrangian; with the techniques of Infrared Regularization, it is possible to separate out the short-range contributions and to write them as local products of fields that are already present in our Lagrangian. (The appropriate field variables must be re-defined at each order in loops.) The corresponding parameters implicitly include short-range effects to all orders in the interaction, so these effects need not be calculated explicitly. The remaining (long-range) contributions that must be calculated are nonlocal and resemble those in conventional nuclear-structure calculations. Nonlinear isoscalar scalar ( σ ) and vector ( ω ) meson interactions are included, which incorporate many-nucleon forces and nucleon substructure. Calculations are carried out at the two-loop level to illustrate these techniques at finite nuclear densities and to verify that the coupling parameters remain natural when fitted to the empirical properties of equilibrium nuclear matter. Contributions from the ωN tensor coupling are also discussed.

Modern compact star observations and the quark matter equation of state

We present a hybrid equation of state (EoS) for dense matter that satisfies phenomenological constraints from modern compact star (CS) observations which indicate high maximum masses (M∼2M⊙) and large radii (R>12 km). The corresponding isospin symmetric EoS is consistent with flow data analyses of heavy-ion collisions and a deconfinement transition at ∼0.55 fm−3. The quark matter phase is described by a 3-flavor Nambu–Jona-Lasinio model that accounts for scalar diquark condensation and vector meson interactions while the nuclear matter phase is obtained within the Dirac–Brueckner–Hartree–Fock (DBHF) approach using the Bonn-A potential. We demonstrate that both pure neutron stars and neutron stars with quark matter cores are consistent with modern CS observations. Hybrid star configurations with a CFL quark core are unstable within the present model.

NUCLEON PROPERTIES FROM MODIFIED SIGMA MODEL

Birse and Banerjee model is extended to include higher-order mesonic interactions. The field equations have been solved in the mean-field approximation and a good agreement with the data for the nucleon properties has been obtained. The agreement is better than that obtained by the original model of Birse and Banerjee which indicates the important of the inclusion of higher-order meson correlations.

Properties of nuclear and neutron matter in a nonlinear σ– ω– ρ mean-field approximation with self- and mixed-interactions

Properties of nuclear and neutron matter are discussed in a nonlinear σ– ω– ρ mean-field approximation with self- and mixed-interactions of mesons and baryons. The nonlinear interactions of mesons are naturally renormalized as effective masses of mesons by thermodynamic consistency required by the theory of conserving approximations. The nonlinear mean-field approximation results in a thermodynamically consistent approximation that maintains Hugenholtz–Van Hove theorem and Landau's requirement of quasiparticles, and it is generally proved that the nonlinear approximation is equivalent to the Hartree approximation with effective masses of baryons and mesons. The approximation is applied to nuclear and neutron matter, which suggests that the lower bound of nuclear compressibility, K ∼ 150 MeV , be required to be consistent with the binding energy ( − 15.75 MeV at k F = 1.30 fm −1 ) and the maximum mass of neutron stars ( M max ⩾ 2.00 M ⊙ ). Since the contributions of nonlinear interactions are strictly confined as the effective masses of mesons and baryons, properties of nuclear and neutron matter will enable us to study the validity of nonlinear interactions of mesons. The implication of thermodynamic consistency to the hypothesis of naturalness is discussed. The density-dependence of the compressibility, symmetry energy together with effective masses of hadrons will be important constraints for nonlinear mean-field approximations, and the accumulating data together with accurate measurements of observables in high density and energy region will supply significant information in order to testify theoretical consistency of nuclear models.

Weak axial nuclear heavy meson exchange currents and interactions of solar neutrinos with deuterons

Starting from the axial heavy meson exchange currents, constructed earlier in conjunction with the Bethe–Salpeter equation, we first present the axial ρ-, ω- and a 1 -meson exchange Feynman amplitudes that satisfy the partial conservation of the axial current. Employing these amplitudes, we derive the corresponding weak axial heavy meson exchange currents in the leading order in the 1 / M expansion ( M is the nucleon mass), suitable for the nuclear physics calculations beyond the threshold energies and with wave functions obtained by solving the Schrödinger equation with one-boson exchange potentials. The constructed currents obey the nuclear form of the partial conservation of the axial current. We apply the space component of these currents in calculations of the cross sections for the disintegration of deuterons by low energy (anti)neutrinos. The deuteron and the final state nucleon–nucleon wave functions are derived (i) from a variant of the OBEPQB potential, and (ii) from the Nijmegen 93 and Nijmegen I nucleon–nucleon interaction. The extracted values of the constant L 1 , A , entering the axial exchange currents of the pionless effective field theory, are in a reasonable agreement with its value predicted by the dimensional analysis.

Bounding effective parameters in the chiral Lagrangian for excited heavy mesons

We use recent experimental data on charmed mesons to constrain three coupling constants in the effective Lagrangian describing the interactions of excited heavy-light mesons with light pseudoscalar mesons at order mQ−1. Predictions in the beauty sector are also derived.

EXTENDED LINEAR SIGMA MODEL IN HIGHER ORDER MESONIC INTERACTIONS

The Gell-Mann and Levy model, as well as the Birse and Banerjee model, describe quark interactions via the exchange of σ- and π-mesons. We extend these models to include higher order mesonic interactions. The field equations were solved in the mean-field approximation and good agreement with the data for nucleon properties was obtained. Our agreement is better than that obtained by the original model of Birse and Banerjee and by other models. This indicates the importance of including higher order meson correlations.

Di-electron bremsstrahlung in intermediate-energy pn and Dp collisions

Invariant mass spectra of di-electrons stemming from bremsstrahlung processes are calculated in a covariant diagrammatical approach for the exclusive reaction D p → p sp n p e + e − with detection of a forward spectator proton, p sp . We employ an effective nucleon–meson theory for parameterizing the subreaction n p → n p e + e − and, within the Bethe–Salpeter formalism, derive a factorization of the cross section in the form d σ D p → p sp n p e + e − / d M = d σ n p → n p e + e − / d M × kinematical factor related solely to the deuteron ( M is the e + e − invariant mass). The effective nucleon–meson interactions, including the exchange mesons π, σ, ω and ρ as well as excitation and radiative decay of Δ ( 1232 ) , have been adjusted to the process p p → p p e + e − at energies below the vector meson production threshold. At higher energies, contributions from ω and ρ meson excitations are analyzed in both, NN and Dp collisions. A relation to two-step models is discussed. Subthreshold di-electron production in Dp collisions at low spectator momenta is investigated as well. Calculations have been performed for kinematical conditions envisaged for forthcoming experiments at HADES.

Life of µ: The Observation of the Spontaneous decay of Mesotrons and its Consequences, 1938–1947

Summary The mesotrons, or mesons, were the first elementary particles observed to be inherently unstable. This essay offers a reconstruction of the stream of researches related to mesotron decay, and examines how these researches shaped some of the basic concepts and practices of the emerging field of particle physics. Mass measurements could not settle the question of whether the mesons were a homogeneous kind of particles or an assortment of particles with different masses. The assumption of a single mass prevailed not on experimental grounds but because the mesons were identified tentatively with the carriers of the nuclear force according to a theory formulated by Hideki Yukawa. The identification gained currency because it entailed the prediction of meson decay, and thereby upheld the promise of a unified explanation of nuclear and cosmic-ray phenomena. In turn, the observation of decay and the measurement of the mean lifetime created the conditions for investigating the nuclear interactions of mesons at rest. Interest in these interactions was heightened, immediately after WWII, by the prospect of building and using accelerators to acquire knowledge about fundamental nuclear processes. Using decay to study nuclear capture, however, led to the realization that there exist not only different kinds of mesons but also two nuclear forces.

SU(2)×SU(2) chiral quark model with nonlocal interaction

Masses and interactions of light mesons are described in the framework of the model with the chiral invariant SU(2)×SU(2) four-quark interaction. The nonlocal kernel of the interaction is chosen in the form that ensures the absence of ultraviolet divergences in the Feynman diagrams and poles in the quark propagator. Within this model, we demonstrate that, in the chiral limit, the pion mass equals zero and the Goldberger-Treiman relation is fulfilled. The sigma-meson mass and the widths of strong decays σ → ππ, ρ → νν are estimated.

Effects of Perturbative Exchanges in a QCD String Model

The QCD string model for baryons derived by Yu. A. Simonov and used for the calculation of baryon magnetic moments in a previous paper is extended to include also perturbative gluon and meson exchanges. The mass spectrum of the baryon multiplet is studied. For the meson interaction, either pseudoscalar or pseudovector coupling is used. Predictions are compared with the experimental data. Besides these exchanges, the influence of excited quark orbitals on the baryon ground state are considered by performing a multichannel calculation. The nucleon-Δ splitting increases due to the mixing of higher quark states, while the baryon magnetic momenta decrease. The multichannel calculation with perturbative exchanges is shown to yield reasonable magnetic moments, while the mass spectrum is close to experiment.