The cusp effect in <Emphasis Type="Italic">η</Emphasis>′→<Emphasis Type="Italic">η</Emphasis><Emphasis Type="Italic">π</Emphasis><Emphasis Type="Italic">π</Emphasis> decays

Strong final-state interactions create a pronounced cusp in η′→η π ⁰ π ⁰ decays. We adapt and generalize the non-relativistic effective field theory framework developed for the extraction of π π scattering lengths from K→3π decays to this case. The cusp effect is predicted to have an effect of more than 8% on the decay spectrum below the π ⁺ π ⁻ threshold.     

Deviation from the standard model in the decay <Emphasis Type="Italic">π</Emphasis><Superscript>+</Superscript> → <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">νγ</Emphasis>

A precise investigation of radiative pion decay (π⁺ → e⁺νγ) in a pion beam from the meson factory of the Paul Scherrer Institute (Switzerland) was performed by the PIBETA Collaboration with the aid of the PIBETA detector. This resulted in finding 41 601 events of radiative pion decay in three kinematical regions. The absolute values of the branching ratio for radiative pion decay were determined in each of these regions. To a precision approximately four times higher than that known previously, the ratio of the axial-vector to the vector form factor was found to be γ ≡ F _A /F _V = 0.443 (15), the latter being fixed at F _V = 0.0259. The number of events found in the kinematical region specified by photon energies of E_γ > 55.6 MeV, positron energies of E _e > 20.0 MeV, and angles of θ_{γ, e} > 40° between the momenta of the corresponding particles (B region) was 5233. In region B, the measured branching ratio for radiative pion decay, R_π→evγ(expt) = 11.6(3) × 10^?8, proved to be smaller by eight standard deviations than that which follows from the Standard Model, R_π→evγ (theor) = 14.34(1) × 10^?8.     

Measurement of nuclear-modification factors for <Emphasis Type="Italic">π</Emphasis><Superscript>0</Superscript>, <Emphasis Type="Italic">η</Emphasis>, and <Emphasis Type="Italic">ϕ</Emphasis> mesons and protons in heavy-ion interactions in the PHENIX experiment

Light hadrons provide a convenient tool for studying the properties of hot and dense media formed in central collisions of relativistic heavy nuclei. The results obtained in the PHENIX experiment at the relativistic heavy-ion collider (RHIC, Brookhaven National Laboratory, USA) by measuring nuclearmodification factors for light hadrons in various colliding systems (pp, dAu, CuCu, and AuAu) at the c.m. energies of √s _NN = 62.4 and 200 GeV are presented.     

Estimating of CP-violation in <Emphasis Type="Italic">B</Emphasis><Superscript>0</Superscript>→ψ(2<Emphasis Type="Italic">S</Emphasis>)π<Superscript>0</Superscript> decay

I present estimates of CP-violating asymmetries in the non-leptonic charmonium two-body B⁰→ψ(2S)π⁰ decay and the same decays of B⁺→ψ(2S)π⁺ and B⁺→ψ(2S)K⁺ These estimates are based on QCD and improved QCD factorization approach making use of next-to-leading order (NLO) contributions. The CP-violating asymmetry for B⁰→ψ(2S)π⁰ decay is not available, according to the same calculations, it is expected if it can be measured in the future its value will be S_ψ(2S)π0(B⁰ → ψ(2S)π⁰)= 0.662 ± 0.197 and C_ψ(2S)π0(B⁰ → ψ(2S)π⁰)= 0.024 ± 0.007.     

On mass corrections to the decay <Emphasis Type="Italic">P</Emphasis> → <Emphasis Type="Italic">l</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">l</Emphasis><Superscript>−</Superscript>

The Mellin-Barnes representation is used to improve the theoretical estimate of mass corrections to the width of a light pseudoscalar meson decay into a lepton pair, P → l ⁺ l ⁻. The full resummation of the terms ln(m _l ²/Λ²)(m _l ²/Λ²) ⁿ and (m _l ²/Λ²) ⁿ to the decay amplitude is performed, where m _l is the lepton mass and Λ ≈ m _ρ is the characteristic scale of the P → γ*γ* form factor. The total effect of the mass corrections for the e ⁺ e ⁻ channel is negligible and, for the μ⁺μ⁻channel, its order is of a few percent. The text was submitted by the authors in English.     

A new look at the KLOE data on the decay <Emphasis Type="Italic">φ</Emphasis>→<Emphasis Type="Italic">ηπ</Emphasis><Superscript>0</Superscript><Emphasis Type="Italic">γ</Emphasis>

We present an analysis of recent high-statistical KLOE data on φ → ηπ⁰γ decay. This decay mainly goes through the a₀γ intermediate state, which makes it possible to investigate properties of the a₀. It is shown that KLOE data prefer a higher a0 mass and a considerably larger a0 coupling to the K\(\bar K\) than those obtained in the analysis of the KLOE group.     

Near-threshold radiative 3<Emphasis Type="Italic">π</Emphasis> production in <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> annihilation

We consider the π⁺π^?π₀γ final state in electron-positron annihilation at c.m.s. energies not far from the threshold. Both initial-and final-state radiations of the hard photon are considered, but without interference between them. The amplitude for the final-state radiation is obtained by using the effective Wess-Zumino-Witten Lagrangian for pion-photon interactions valid for low energies. In real experiments, energies are never so small that ρ and ω mesons would have a negligible effect. So a phenomenological Breit-Wigner factor is introduced in the final-state radiation amplitude to account for the vector mesons' influence. Using radiative 3π production amplitudes, a Monte Carlo event generator was developed which could be useful in experimental studies.     

Electromagnetic corrections in <Emphasis Type="Italic">η</Emphasis>→3<Emphasis Type="Italic">π</Emphasis> decays

We re-evaluate the electromagnetic corrections to η→3π decays at next-to-leading order in the chiral expansion, arguing that effects of order e ²(m _u −m _d ) disregarded so far are not negligible compared to other contributions of order e ² times a light-quark mass. Despite the appearance of the Coulomb pole in η→π ⁺ π ⁻ π ⁰ and cusps in η→3π ⁰, the overall corrections remain small.     

Role of the <Emphasis Type="Italic">ππ</Emphasis> interaction in pairwise photoproduction of <Emphasis Type="Italic">π</Emphasis><Superscript>0</Superscript> mesons on a proton

Photoproduction of two neutral pions on a proton is considered. Qualitative analysis of the available experimental data demonstrates that a linear dependence of the total cross section on the energy testifies to the dominating contribution of s-waves for E _γ  < 800 MeV. Since in the tree approximation, the s-wave is almost completely absent from the π ⁰ π ⁰ channel, the effects of π ⁺ π ⁻ meson rescattering into the neutral channel are included in calculations. Owing to the high intensity of π ⁺ π ⁻ pair formation, the influence of rescattering appears considerable. Its consideration yields a better agreement with the available experimental data.     

Elliptic flow of ν ands <Emphasis Type="Italic">π</Emphasis><Superscript>0</Superscript> mesons in heavy-ion collisions at 2 <Emphasis Type="Italic">A</Emphasis> GeVmesons in heavy-ion collisions at 2 <Emphasis Type="Italic">A</Emphasis> GeV

Elliptic flow of ν and π ⁰ mesons emitted at midrapidity are studied in collisions of 1.9 A GeV ⁵⁸Ni+⁵⁸Ni and 2 A GeV ⁴⁰Ca+^natCa. The observed anisotropy corresponds to a negative elliptic flow signal for ν mesons, indicating a preferred emission perpendicular to the reaction plane. In contrast, only small azimuthal anisotropies are observed for π ⁰ mesons. This may indicate that ν mesons freeze out earlier from the central interaction region than pions.     

The role of the intermediate resonance <Emphasis Type="Italic">a</Emphasis><Subscript>0</Subscript>(980) in the decay <Emphasis Type="Italic">η</Emphasis> → <Emphasis Type="Italic">πγγ</Emphasis>

It is shown that if the a₀(980) boson is the scalar chiral partner of a π meson, its contribution to the transition η → πγγ has a value comparable to that originated by vector meson exchange. Together, these two mechanisms give the decay probability consistent with the experimental observation.     

Strong decay Δ<Superscript>++</Superscript>→<Emphasis Type="Italic">p</Emphasis><Emphasis Type="Italic">π</Emphasis> with light-cone QCD sum rules

We calculate the strong coupling constant g _{ΔN π} and study the strong decay Δ⁺⁺→p π with light-cone QCD sum rules. The numerical value of the strong coupling constant g _{ΔN π} is consistent with the experimental data. The small discrepancy may be due to the failure to take into account perturbative corrections.     

Independent <Emphasis Type="Italic">π</Emphasis><Superscript>−</Superscript>-meson production in <Emphasis Type="Italic">pp</Emphasis> interactions

It is shown that experimental data on the multiparticle production of negatively charged pions in proton-proton interactions at \(\sqrt s \leqslant 30 GeV\) do not involve significant indications of the existence of anycorrela tions between negatively charged pions, apart from those that are associated with momentum conservation and with interference. The multiplicity distributions in rapidity intervals, forward-backward correlations, and two-particle rapidity and transverse-momentum correlations do not contradict the independent production of negatively charged pions. No constraints on the multiplicity distributions of product particles follow from their independent production.     

Higher <Emphasis Type="Italic">η</Emphasis><Subscript>c</Subscript>(<Emphasis Type="Italic">nS</Emphasis>) and <Emphasis Type="Italic">η</Emphasis><Subscript>b</Subscript>(<Emphasis Type="Italic">nS</Emphasis>) mesons

The hyperfine splittings in heavy quarkonia are studied in a model-independent way using experimental data on dielectron widths. Relativistic correlations are taken into account together with the smearing of spin-spin interaction. The radius of smearing is fixed by known G/ψ−η _c(1S), ψ(2S)−η _c′(2S) splittings, which appears to be small, r _ss ≅ 0.06 fm. Nevertheless, even with such a small radius, substantial suppression of hyperfine splittings (∼50%) is observed in bottomonium. For nS b states (n = 1, 2, ..., 6), our predicted splittings (in MeV) are 28, 12, 10, 5, 6, 3. For the 3S and 4S charmonium states, the splittings 16(2) and 12(4) MeV are obtained. The text was submitted by the authors in English.     

Measurement of the cross sections for <Emphasis Type="Italic">π</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">π</Emphasis><Superscript>−</Superscript> production in <Emphasis Type="Italic">pp</Emphasis> collisions at energies below 1 GeV

The cross sections for the reaction pp → ppπ ⁺ π ^? are measured at several values of the incident-proton energy. The resulting values are compared with other results, including values obtained previously for a deuteriumtarget, as well as with the predictions of a theoreticalmodel. This comparison reveals that, at energies below 850 MeV, the use of the impulse approximation in extracting the cross sections for double pion production on a free nucleon from data obtained with deuteron targets is illegitimate.     

Understanding jet shapes with <Emphasis Type="Italic">π</Emphasis><Superscript>0</Superscript>–<Emphasis Type="Italic">h</Emphasis><Superscript>±</Superscript> correlations

Angular correlations involving energetic particles associated with partonic jet fragmentation provide an important opportunity to study the hot nuclear matter produced in A+A collisions, particularly when compared against a p+p reference. Recent results from di-hadron correlations suggest that jet observables from A+A and p+p differ considerably at low p _T but approach comparable values as p _T increases above 7 GeV/c, providing clues on the nature of partonic energy loss and the medium response. The most recent π ⁰-triggered jet correlation results from the PHENIX experiment are presented, as well as a quantitative study of the jet-peak widths and shapes.     

The nature of low-lying <Emphasis Type="Italic">K</Emphasis><Superscript><Emphasis Type="Italic">π</Emphasis></Superscript> = 0<Superscript>+</Superscript> bands in nuclei

Low-lying K ^π = 0⁺ bands are one of the most fundamental excitation modes in the spectra of deformed nuclei; however, very little is known about the nature of these excitations. We report on some new lifetime measurements using the GRID technique at ILL, and high-precision (p, t) reactions to elucidate the character of low-lying K ^π = 0⁺ bands in deformed nuclei. We also present results from a recent calculation on the nature of 0⁺ states using the projected shell model.