Study of the ηπ+π− system centrally producedin the reactionpp→p f (ηπ+π−)p s at 300 GeV/c

The reactionpp→p _f (ηπ⁺π^?)p _s where the ηπ⁺π^? system is centrally produced has been studied at 300 GeV/c incident momentum at the CERN Ω spectrometer. The ηπ^± mass spectrum shows a strong δ/a ₀(980) signal having a mass of 984±4 and Γ=95±14 MeV. The ηπ⁺π^? mass spectrum shows η′ andf ₁(1285) signals over little background. A spin-parity analysis of the ηπ⁺π^? system shows evidence for aJ ^PC =1⁺⁺ peak at thef ₁(1285) mass but no evidence for the pseudoscalar states η(1270) andl/η(1440). No evidence is found for the ηππ decay of theE/f ₁(1420) meson for which we set an upper limit BR(E/f ₁(1420)→ηππ)<0.1 at 95% cl.     

3π resonances in 2− partial waves

The J^PC = 2^?+ partial wave intensities and their large phase changes prove the resonant nature of the A₃ meson (mass ≈ 1670 MeV, width ≈ 210 MeV). The decay modes are f⁰π, ?⁰π, and ?⁰π. Evidence is found for a further 2^? enhancement.     

Gluon jets from quarkonia

A detailed analysis is made within perturbative QCD of the decays into gluon jets of P-wave and S-wave quarkonia (J^PC = 0⁺⁺, 1⁺⁺, 2⁺⁺, 1^+? and 0^?+, 1^??). For all those states we compute to leading non-trivial order in α_s the conventional fraction f(?, δ) of the jet-like hadronic final states, characterized by the back-to-back cone of half angle δ, in which all but a small fraction ? of the total energy is emitted. To the same order we give also the average values of thrust momenta and spherocity distributions.     

Dynamics of f 0(980)-resonance production in the reaction γγ → π + π −

The results of the experiment performed by the Belle Collaboration and aimed at observing the f ₀(980) resonance in the reaction γγ → π ⁺ π ^? are analyzed. It is indicated that these results suggest the presence of mechanisms in this reaction that lead to a strong distortion of the f ₀(980)-resonance line shape in relation to the line shape expected for a solitary Breit-Wigner resonance. It is shown that the K ⁺ K ^?-loop mechanism of f ₀(980) coupling to the γγ system and the destructive interference between the background and f ₀(980)-resonance contributions in the region of the π ⁺ π ^? invariant mass below the K ⁺ K ^? threshold are the main factors responsible for the formation of a specific (stepwise) f ₀(980)-resonance line shape in the cross section for the reaction γγ → π ⁺ π ^?.     

On the possibility of observing a 0 0 (980)-ƒ 0(980) mixing in the reaction π − p → ηπ 0 n on a polarized target

It is shown that the spin asymmetry in the reaction π ^? p → a ₀ ⁰ (980)n → (ηπ ⁰)S _n is highly sensitive to the mixing of the a ₀ ⁰ (980) and ? ₀(980) resonances. At low momentum transfers (namely, in any of the intervals 0 ≤ ?t ≤ 0.025, …, 0.1 GeV²), the asymmetry normalized in such a way that it takes values between ?1 and 1must undergo a jump close inmagnitude to unity in the region of the ηπ ⁰ invariant masses between 0.965 and 1.01 GeV. A large jump of the asymmetry is due exclusively to a ₀ ⁰ (980)?? ₀(980) mixing. A very high resolution in the ηπ ⁰ invariant mass is not required for observing the jump of the asymmetry. The energy dependence of the polarization effect is expected to be rather weak; therefore, the polarization effect in question can be studied at any high energy—for example, in the range between 8 and 100 GeV.     

Nuclear structure of 18F: (IV). Negative-parity states

Studies via the ¹⁶O(³He, pγ)¹⁸F, ¹⁴N(α,γ)¹⁸F and ¹⁷O(p, γ)¹⁸F reactions have resulted in new J^π assignments for 11 states or negative parity: E_x(keV) (J^π) = 3791(3^?), 4226(2^(?)), 4398(4^?), 4860(1^(?)), 5502(3^(?)), 5785(2^?), 6097(4^?), 6108(1^(?), 2^(?), 3^(?)), 6241(3^?, T = 1), 6643(2^?, T = 1) and 6878(3^(t-), 4^?). The 6241 keV state is probably isospin mixed. New information for 5 states of positive parity has also been obtained: E_x(keV) (J^π) = 3838(2⁺), 4115(3⁺), 4652(4⁺, T = 1), 4753((0⁺), T = 1) and 4964(2⁺, T = 1). Mean lives, branching and mixing ratios are reported for all states. The results for the negative-parity states are discussed in the framework of the various models available. The states at E_x = 1080(0^?), 2100(2^?) and 4398(4^?) keV are interpreted as the first three members of a K^π = 0^? rotational band.     

Radiative decays and quark content of f 0(980) and φ(1020)

The quark structure of φ(1020) and f ₀(980) is studied on the basis of data on the radiative decays φ(1020) → γπ ⁰, γη, γη′, γ a ₀(980), γ f ₀(980) and f ₀(980) → γγ. The partial widths are calculated under the assumption that all the mesons under consideration are $\bar qq$ states: φ(1020) is a dominantly $s\bar s$ state ( $\eta ' = n\bar nsin\theta + s\bar scos\theta $ component contributes not more than 1%); η, η′, and π ⁰ are standard $q\bar q$ states, $\eta = n\bar ncos\theta - s\bar ssin\theta $ and $\eta ' = n\bar nsin\theta + s\bar scos\theta $ with θ?37°; and f ₀(980) is a e5 meson with the flavor wave function $n\bar ncos\varphi + s\bar ssin\varphi $ . The transition φ → γπ ⁰ specifies the admixture of the $n\bar n$ component in the φ meson: it is on the order of 0.5%. We argue that this order of $n\bar n$ value does not contradict data on the decay φ(1020) → γ a ₀(980). The partial widths calculated for the decays φ → γη, γη? are in reasonable agreement with experimental data. The measured branching-ratio value Br(φ→γf ₀(980))=(3.4±0.4 _?0.5 ^+1.5 ×10^?4) requires 25°≤|?|≤90°. For the decay f ₀(980) → γγ, the agreement with data, Γ(f ₀(980) → γγ)=0.28 _?0.13 ^+0.09 keV, is attained at either ?=85°±8° or ?=?46°±8°. A simultaneous analysis of the decays φ(1020) → γ f ₀(980) and f ₀(980) → γγ favors the solution with the negative mixing angle of ?=?48°±6°, setting f ₀(980) very close to the flavor octet (? _octet=±54.7°).     

The reaction Pomeron-Pomeron →π + π − and an unusual production mechanism for thef 2 (1270)

Data are presented on Pomeron-Pomeron interactions which produce a centralπ ⁺ π ^? system in proton-proton collisions at \(\sqrt s = 62 GeV\) at the CERN Intersecting Storage Rings. This process may favor the production of gluonic bound states. A partial-wave analysis of theπ ⁺ π ^? system shows evidence for the production of the statesf ₀(975),f ₀(1400), andf ₂(1270). The fitted mass for thef ₂(1270) is about 50 MeV below the world average. In addition, the production mechanism for thef ₂(1270) is uniquely different from that for the other final states in that there is a correlation between the outgoing protons. this is consistent with a picture of two-gluon exchange with thef ₂(1270) produced by gluon fusion, and could indicate that thef ₂(1270) has a glueball component.     

New meson configuration in the bag model (I). First order energy spectrum of qqg states

Perturbative QCD in the bag is applied to unconventional configuration of quarks and gluons. The meson spectrum is computed to second order in the strong coupling. Exotic states with J^PC = 1^?+, 0^+?, 0^?? are found in the energy range 1.57 ? E ? 1.93 GeV.     

Study of the π+π+π-π- system centrally 0 produced by incident π+ andp beams at 85 GeV/c

The reactions π⁺ p→π⁺(π⁺π⁺π^-π^-)p andpp→p(π⁺π⁺π^-π^-)p, where the (π⁺ π⁺ π^- π^-) system is centrally produced have been studied at 85 GeV/c. The π⁺π⁺π^-π^- mass spectrum shows evidence for thef ₁ (1285) meson with a mass of 1278±2 MeV and width 41±12 MeV which decays mainly through ρ⁰(770)π⁺π^-. Thef ₁(1285) is also observed in the ηπ⁺π^- channel. There is no significant evidence for ηππ or 4π decay modes of thef ₁(1420). The ρ⁰ρ⁰ production is found to be small. A Dalitz plot analysis of the 3π system shows evidence fora ₂ (1320) production and for a large contribution of theJ ^PC=1⁺⁺ ρπ wave.     

Exchange,production and decay of the Z meson (JPC = 2−−)

Recent high-statistics data for π^?p→ω⁰n indicate the presence of an amplitude with exchange quantum numbers in the series J^PC = 2^?? 4^??,… Previous phenomenological and theoretical estimates of reggeised Z exchange J^PC = 2^??) are shown to be in good agreement with data. Implications for observing the exchange of its EXD partner (the much sought A₁) and other unnatural parity states are discussed with reference to a with a quark model estimate of its branching ratio, we predict the cross section for production of the Z and its subsequent decay into ωπ. The prospects for a partial-wave analysis of the ωπ enhancement observed at 1700 MeV are discussed.     

Electroexcitation of isovector magnetic dipole and quadrupole transitions in 18O and their relation to radiative pion capture

In a high-resolution ¹⁸O(e, e′) experiment sharp states at E_X=16.38±0.01 MeV (J^π=2^?, ground state analogue of and at E_X=18.86±0.01 MeV (J^π=1⁺) and clustering of strength (possibly with J^π=1^?, 2^?) at E_X≈23.7 MeV have been observed and compared to theoretical predictions. The transition strengths for the narrow states are B(M2) ↑ = 58± 8 μ_K² fm² and B(M1) ↑ = 0.28± 0.04 μ_K², respectively. These results agree qualitatively with the resu lts from the analogous ¹⁸O(π^?, γ)¹⁸N reaction.     

Resonances in the ωω system

Results of a partial-wave analysis for the reaction π ^? → ωωn studied at the VES spectrometer (Institute for High Emergy Physics, Protvino) are presented. The behavior of the J ^PC = 2⁺⁺ amplitudes in the ωω system is described by the f ₂(1565) and f ₂(1910) resonances, whose parameters were fixed at M = 1.590 ± 0.010 GeV and Γ = 0.140 ± 0.011 GeV for the former and at M = 1.890 ± 0.010 GeV and Γ = 0.165 ± 0.019 GeV for the latter. The decay f ₄(2050) → ωω was observed at parameters values of M = 1.960 ± 0.015 GeV, Γ = 0.290 ± 0.020 GeV.     

Study of the centrally produced ωρ0 and ωω systems inpp interactions at 300 GeV/c

We have performed a search for vector-vector final states centrally produced in proton proton interactions at 300 GeV/c using the CERN Ω spectrometer. Evidence is found for ωρ⁰ production in the reactionpp→p _f (2π⁺2π^?2π⁰)p _s and for ωω production in the reactionpp→p _f (2π⁺2π^?2π⁰)p _s . However no evidence is found for ωø production in the reactionpp→p _f (K ⁺ K ^?π⁺π^?π⁰)p _s .     

Sum rules for the decays of the c-even charmonium states

Sum rules for the decays of the C-even charmonium levels (¹S₀, ³P₀, ³P₂, ¹D₂) are derived. These rules are based on the asymptotic freedom of the quantum chromodynamics at small distances and on the analyticity. They refer to the various vacuum amplitudes involving products of charmed quark currents: electromagnetic current as well as currents with quantum numbers J^PC=0^?+,0⁺⁺,2⁺⁺,2^?+. The contribution of the continuum to some of the sum rules is small, and they are saturated by the contribution of the lowest charmonium levels. In this way we predict the widths of the two-photon decays of the charmonium states and estimate their total hadronic widths.     

Lightest scalar resonances and the dynamics of the γγ → ππ reactions

The high-statistics Belle data on the γγ → π⁺π^? and γγ → π⁰π⁰ reactions have been jointly analyzed. The main dynamical mechanisms of these reactions for energies below 1.5 GeV have been revealed. It has been shown that the direct coupling constants of the σ(600) and f ₀(980) resonances with a γγ pair are small and that the σ(600) → γγ and f ₀(980) → γγ decays are four-quark transitions due primarily to π⁺π^? and K ⁺ K ^?-loop mechanisms, respectively. The role of the chiral shielding of the σ(600) resonance is emphasized. The widths of the f ₀(980) → γγ and σ(600) → γγ decays averaged over the resonance mass distributions, as well as the width of the f ₂(1270) → γγ decay, are estimated as \(\langle \Gamma _{f_0 \to \gamma \gamma } \rangle _{\pi \pi } \approx 0.19 keV, \langle \Gamma _{\sigma \to \gamma \gamma } \rangle _{\pi \pi } \approx 0.45 keV\), and \(\Gamma _{f_2 \to \gamma \gamma } (m_{f_2 }^2 ) \approx 3.8 keV\).     

QQG hybrid mesons in the MIT bag model

We suggest that hybrid (q$\text{q}$g) mesons could exist with rather light masses. The spectrum of the ground state nonets, J^PC = (0, 1, 2)^?+; 1^?? is calculated in the MIT bag model including O(α_s) energy shifts. We discuss hadronic transitions among these states, consider their possible production at LEAR and SPEAR and indicate some interesting decay signatures.     

A meson-theoretical explanation of the f 0 (980)-production puzzle in the reaction π− p → π0π0 n

A meson-theoretical model for the reaction π^? p → π⁰π⁰ n for large π^? momenta is developed which treats the final-state interaction between the two produced pions microscopically. For small momentum transfers, the squares of the S-wave amplitudes of the produced two-pion system show a dip for invariant twopion masses in the vicinity of the f ₀(980) meson, while for momentum transfers ?t>0.2(GeV/c)², the f ₀(980) meson appears as a bump. The model provides a microscopic explanation of the long-standing puzzle seen by both the GAMS and the BNL experimental collaborations.     

e + e − annihilation to π 0 π 0 γ and π 0 η γ as a source of information on scalar and vector mesons

We present a general framework for the model-independent decomposition of the fully differential cross section of the reactions e ⁺ e ^?→γ ^?→π ⁰ π ⁰ γ and e ⁺ e ^?→γ ^?→π ⁰ η γ, which can provide important information on the properties of scalar mesons: f ₀(600), f ₀(980) and a ₀(980). For the model-dependent ingredients in the differential cross section, an approach is developed, which relies on Resonance Chiral Theory with vector and scalar mesons. Numerical results are compared to data. The framework is convenient for development of a Monte Carlo generator and can also be applied to the reaction e ⁺ e ^?→γ ^?→π ⁺ π ^? γ.