Multipole analysis of pion photoproduction based on fixed t dispersion relations and unitarity

We have analyzed pion photoproduction imposing constraints from fixed t dispersion relations and unitarity. Coupled integral equations for the S and P wave multipoles were derived from the dispersion relations and solved by the method of Omnès and Muskhelishvili. The free parameters were determined by a fit to the most recent data for π⁺ and π⁰ production on the proton as well as π⁻ production on the neutron, in the energy range 160 MeV ⩽ E_γ ⩽ 420 MeV. The lack of high precision data on the neutron and of polarization observables leads to some limitations of our results. Especially the multipole M₁₋ connected with the Roper resonance P₁₁ (1440) cannot be determined to the required precision. Our predictions for the threshold amplitudes are in good agreement with both the data and chiral perturbation theory. In the region of the Δ(1232) we have determined the ratio of electric quadrupole and magnetic dipole excitation. The position of the resonance pole is obtained in exellent agreement with pion-nucleon scattering, and the complex residues of the multipoles are determined with the speed-plot technique.     

Elastic and transition form factors of light pseudoscalar mesons from QCD sum rules

We revisit F _π(Q ²) and F _Pγ(Q ²), P = π, η, η′, making use of the local-duality (LD) version of QCD sum rules. We give arguments that the LD sum rule provides reliable predictions for these form factors at Q ² ≥ 5–6 GeV², the accuracy of the method increasing with Q ² in this region. For the pion elastic form factor, the well-measured data at small Q ² give a hint that the LD limit may be reached already at relatively low values of momentum transfers, Q ² ≈ 4–8 GeV²; we therefore conclude that large deviations from LD in the region Q ² = 20–50 GeV² seem very unlikely. The data on the (η, η′) → γγ* form factors meet the expectations from the LD model. However, the BaBar results for the π ⁰ → γγ* form factor imply a violation of LD growing with Q ² even at Q ² ≈ 40 GeV², at odds with the η, η′ case and with the general properties expected for the LD sum rule.     

Effective Lagrangian Approach to pion photoproduction from the nucleon

We present a pion photoproduction model on the free nucleon based on an Effective Lagrangian Approach (ELA) which includes the nucleon resonances (Δ(1232), N(1440), N(1520), N(1535), Δ(1620), N(1650), and Δ(1700)), in addition to Born and vector meson exchange terms. The model incorporates a new theoretical treatment of spin-3/2 resonances, first introduced by Pascalutsa, avoiding pathologies present in previous models. Other main features of the model are chiral symmetry, gauge invariance, and crossing symmetry. We use the model combined with modern optimization techniques to assess the parameters of the nucleon resonances on the basis of world data on electromagnetic multipoles. We present results for electromagnetic multipoles, differential cross-sections, asymmetries, and total cross-sections for all one pion photoproduction processes on free nucleons. We find overall agreement with data from threshold up to 1 GeV in laboratory frame.     

Measurement of the relative probability of the decay φ → η′γ in the channel η′ → ηπ0π0, η → γγ

The process e ⁺ e ^? → η′γ, η′ → η π⁰π⁰, η → γγ is investigated by an SND detector in experiment on a VEPP-2M equipment with colliding electron-positron beams. The analysis of experimental data yields a value of B(? → η′γ)=(6.7 _?4.2 ^+5.0 ±1.5)×10^?5 for the decay probability. Taking into account a previous measurement of this quantity by an SND detector in the decay channel η′ → π⁺π^?η, one finally obtains B(?→η ′γ)=(6.7 _?2.4 ^+2.8 ±0.8)×10^?5.     

Instanton-induced flavour mixing in mesons

The U_L(N_f)×U_R(N_f) chiral symmetric version of the Nambu-Jona-Lasinio model is extended by the 't Hooft determinant and bosonized for an arbitrary number of flavours N_f. The resulting effective meson lagrangian is explicitly calculated to leading order in the derivatives for three flavours. The 't Hooft determinant induces flavour mixing of the mesons with diagonal flavour content (π⁰, η, η′, and their scalar chiral partners δ⁰, S, ϵ) and pushes up the physical η′-mass. The η-η′ mixing angle is found to be −31°.     

Hadronic decays of η and η′ in the large-N limit

Working in the large-N approximation (N being the number of colors), we relate thestrong η′→ηππ processes to theweak K _L→πππ decays. Chiral corrections are crucial to reproduce the experimental data. The isospin-violating η(η′)→πππ and the weakη→K ⁺π^- decays are also treated in this framework. We comment on the predictions of a “QCD-inspired” approach which determines in principle the chiral symmetry-breaking scales considered, and consequently the Skyrme coupling.     

In search of the ηc

We argue that the X(2.88) detected in π^?p → γγ + n at 40 GeV/c cannot be the η_c. We discuss the possibility that a qq?cc? state is produced there, probably the same state discovered in radiative decay of the J/ψ. Only at much higher energies is the η_c expected to dominate over qq?cc? in the π^?p interaction. We also discuss coherent photoproduction and find that four quark state production would dominate over η_c.     

Multipole analysis of π+ and π0 photoproduction from threshold to Eγ = 330 MeV

We present results of an energy independent multipole analysis of π⁺ and π⁰ photoproduction on protons for photon energies between 160 MeV and 330 MeV. Each isospin part of the l ? 1 amplitudes has been parametrized; the 1 ? 2 amplitudes have been approximated by the Born terms. Unambiguous solutions for all the multipoles in the whole range of E_γ have been obtained. The continuity of the solutions as functions of the energy is very good.     

Neutral pion photoproduction off nucleons revisited

We investigate threshold pion photoproduction in the framework of heavy baryon chiral perturbation theory. We give the expansion of the electric dipole amplitude E ₀₊ to three orders in μ the ratio of the pion to nucleon mass, and show that it is slowly converging. We argue that this observable is not a good testing ground for the chiral dynamics of QCD. In contrast, we exhibit new and fastly converging low-energy theorems in the P-waves which should be used to constrain the data analysis. We also discuss the importance of polarization observables to accurately pin down certain multipoles and give predictions for the reaction γ n → π⁰ n.     

Implications of a JPC exotic

Recent experimental data from BNL on the isovector J^PC = 1⁻⁺ exotic at 1.6 GeV in ϱπ indicate the existence of a non-quarkonium state consistent with lattice gauge theory predictions. We discuss how further experiments can strengthen this conclusion. We show that the ϱπ, η′π and ηπ couplings of this state qualitatively support the hypothesis that it is a hybrid meson, although other interpretations cannot be eliminated.     

Non-perturbative treatment of hyperfine interaction in charmonium; a clue to understanding the pseudoscalars?

We give arguments that the wave function of η _c is relativistic and strongly localized while that of η _c , though non-relativistic, looks quite different from what is naively expected. This has consequences for the overlap integrals with ψ and ψ′ and may solve the problems associated with magnetic transitions. The hadronic width of η _c is expected around 20 MeV, that of η _c ^′ can become very small.     

Eta and omega meson production in medium energy π+-p and K−p interactions

The production of η(550) and ω(780) mesons is studied in π^±p interactions at 16 GeV/c and K^?p interactions at 10 and 16 GeV/c. Cross sections for exclusive channels are presented, and for the π⁺p data differential cross sections are given for quasi-inclusive production where the η or ω is required to be accompanied by charged particles only. Close similarities are observed between η, ω and also ?⁰(770) production in terms of longitudinal and transverse variables. By a rough estimate, the η, ω and ?⁰ inclusive yields are found to be in the ratio 0.32 : 0.85 : 1, respectively, for 16 GeV/cπ⁺p collisions. For non-peripheral production we estimate this same ratio to be 0.34 : 0.9 : 1.     

Interaction of low energy (5–10 keV) argon atoms and ions with a Cu(100) surface; Ion fraction,ionization and neutralization

The ion fractions η⁺ of low energy (5–10 keV) argon particles scattered from a Cu(100) surface, are measured with a time of flight spectrometer. Neutral as well as charged projectiles are used. The scattering angle θ is 30°. The results for different angles of incidence ψ and crystal directions are reported. For scattering in the 〈100〉 direction, with a ψ-value of 15° and a primary energy E₀ of 5 and 10 keV, the ion fractions for the quasi single scattering peak, η⁺_QS, are 1.5 and 6.1% respectively. When E₀ is between 5 and 10 keV a reionization process with a constant reionization probability occurs during the violent interaction. This process, but also neutralization along the outgoing trajectory, determines η⁺_QS. With ions as projectiles, an energy difference of about 16 eV is observed between the quasi single scattering peaks in the spectra of all scattered particles and of ions only. The ion fraction for the quasi double scattering peak, η⁺_QD. depends largely upon E₀, indicating that the efficiency of the reionization process increases with E₀. A qualitative discussion of the data is given, using the reionization process and the interatomic neutralization processes along the trajectory of the scattered particles.     

Spectral function sum rules for gluonic currents

We determine the masses and the decay amplitudes of the 2⁺⁺, 0^?+ and 0⁺⁺ gluonia in anSU(3) _c Yang-Mills theory. We estimate the so-called topological susceptibility of theU(1) sector. We discuss theSU(3) _F breaking effects to the η′(896) massrelation from current algebra and predict the η′ parameter values.     

Near-Threshold Photo- and Electroproduction of Neutral Pions on Protons

The state of the art of the problem on threshold neutral-pion production on protons by photons and electrons is examined. New experimental data on the threshold of pion production are analyzed within the phenomenological model based on the gauge invariance, the hypothesis of partial conservation of the axial-vector current, and allowance for obvious chiral-symmetry breaking. The amplitudes of the p-wave threshold multipoles and the photon asymmetry are derived for the pion photoproduction on protons with a photon energy of 159.5 MeV. The differential cross sections are calculated as functions of the pion emission angle and amplitudes of the s- and p-wave multipoles for the threshold pion electroproduction on protons at the squared momentum transfer k ² = 0.05 GeV²/c². The total reaction cross sections are also obtained as functions of k ². The results obtained are compared with new experimental data and available theoretical predictions.     

Relative strengths of the competing multipoles in the K and L series x-ray spectra in theNR limit

For most of the transitions of theK andL series x-ray spectra the ratio of the contributions of the competing multipoles, δ², is independent of the radial matrix element in the non-relativistic limit. In the present paper calculations of δ² are made in the non-relativistic limit which give the relative strengths of the two closely competing electric and magnetic multipoles. The dominant mode in the emission processes of the transitions in theK andL series x-ray spectra has been assigned on the basis of these calculations.     

The electromagnetic interaction in chiral perturbation theory

We investigate electromagnetic effects in the framework of chiral perturbation theory. Using a completely independent method, we confirm Urech’s results for the divergences of the one-loop functional in the electromagnetic sector. We perform a one-loop analysis of allP _t2 (P=π, K, η) and theK _t3 form factors $f_ + ^{K^ + \pi ^o } (0),f_ + ^{K^o \pi ^ - } (0)$ , including a systematic treatment of theO(e ² p ²) contributions in the mesonic part. We illustrate our results by several numerical estimates.     

Low energy (E0 < 10 keV) atom and ion scattering of neon from a Cu(100) surface; Ionization and neutralization

The ion fractions η⁺ of low energy (5–10 keV) neon particles scattered from a Cu(100) surface are measured with a time of flight spectrometer. These fractions are obtained for neutral as well as charged projectiles and for different crystal directions. The scattering angle θ was 30°. For a primary energy E₀ of 5 keV neutral projectiles have a value for η⁺ which is 30 times lower than for charged projectiles; these values are 0.15 and 4.5% respectively. For E₀ = 10 keV the values of η⁺ are about the same (~22%). Energy differences up to 22 eV, depending on E₀, are observed between the single scattering peaks in the ion spectra of charged and neutral projectiles but also between the single scattering peak in the spectra of all scattered particles and of ions, with ions as projectiles. A qualitative discussion of these data is given, involving charge transfer processes of noble gas particle and target atom. The data suggest that these neutralization processes can be described more adequately with interatomic neutralization processes along the trajectory than with Auger neutralization by conduction electrons.