A Jet Model Study of Correlations in Hadron-Hadron Reactions Due to Resonance Production

Secondary particle correlations are discussed as a basic feature of hadron induced multiparticle reactions in the 5 to 1500 GeV energy range. Two introductory sections are devoted to definitions of cross sections and to empirical regularities of multiple production. Many of these regularities can be understood to be consequences of energy-momentum conservation and limited transverse momenta (uncorrelated jet model (UJM)). An extension of the UJM is considered, the correlated jet model (CJM), by introducing dynamical short range correlations between the particles which are assumed to be due to resonance production. Many properties of single-particle distributions and correlation functions can be derived already from kinematics of resonance decay. The CJM is used to investigate in detail the origin of angular correlations (azimuthal, GGLP-effect) and of correlations between neutral and charged particles (〈π°〉n_c, f⁰⁰(n_c)). These correlations are shown to arise almost completely from the production and decay of mainly two-particle resonances like ∂°, K* and Δ, the main evidence coming from the use of the invariant mass as the basic variable where only resonances or reflections of them give remarkable effects. Compared to the UJM, the CJM is the first approximation of a successive correlations analysis of the production amplitude squared, i.e. having separated the effects of energy-momentum and quantum number conservation. This might be also interesting in view of the forthcoming accelerator generation.     

Isospin violation in τ → 3πντ

Isospin violating signals in the τ^? → (3π)^?ν_τ decay mode are discussed. For the τ^? → π^?π^?π⁺ν_τ decay mode, isospin violation arises from the vector current contribution in the τ^? → ωπ^?ν_τ decay with the subsequent isospin violating ω decay into π⁺π^?. We demonstrate that such effects may be observed in presently available data through the measurement of the interference effects of these vector current contributions with the dominating axial vector current,i. e. through a measurement of the structure functionsW _F, WG, WH andW _I. In the case of the τ^? → π⁰π⁰π^?ν_τ decay mode, a vector current contribution is generated by ηπ⁰ mixing in the decay chain τ^? → η?^?ν_τ → π⁰π⁰π^?ν_τ. We find that this effect is rather small, the magnitude of the associated interference terms being too low for present statistics.     

In-medium properties of nucleon resonances

Recent experimental results for the in-medium properties of nucleon resonances are discussed. The experiments were done with the TAPS detector at the tagged photon beam of the MAMI accelerator in Mainz. Measured was the photoproduction of mesons (final states π⁰X, ηX, 2π⁰X and π⁰π^±X) from the nuclei ¹²C, ⁴⁰Ca, ⁹³Nb and ²⁰⁸Pb up to the second resonance region. The results were analyzed in view of the in-medium properties of the P₃₃(1232), the D₁₃(1520), and the S₁₁(1535) resonances.     

Energies and decay channels of negative ion resonances in acetaldehyde

Negative ion mass spectroscopy and electron impact spectroscopy were employed for a detailed study of the electronic structure and decay channels of the acetaldehyde negative ion. The energy-dependence of the vibrational excitation revealed a very broad σ shape resonance peaking at 6.8 eV, in addition to the already known π shape resonance at 1.2 eV. Energy dependence of the excitation of the ³(π, π) state revealed a ²(π, π²) core excited resonance at 8.1 eV. This resonance, besides decaying into its parent valence excited state, also dissociates to form O⁻. Two Feshbach resonances, ²(n3s²) at 6.34 eV and ²(n,3s3p) at 6.64 eV, are observed in the dissociative attachment yields of CH⁻₃ and H⁻, as well as in energy-loss spectra near threshold. Time resolved energy-loss spectra suggest that a fraction of the CH₃⁻ ions are formed in low vibrationally excited states, which subsequently undergo slow autodetachment on the 0.1–1 μs time scale. Study of partially deuterated acetaldehyde showed no 'hydrogen scrambling' prior to dissociation of the Feshbach resonances, and further permitted the assignment of the vibrations excited in the two Feshbach resonances as v₆ (C-H bend) and v₇, (CH₃ deformation).     

Inclusive and semi-inclusive ϱ0 production in π−p interactions at 147 GeV/c

We present data on inclusive and semi-inclusive ?⁰ production in 147 GeV/c π^?p interactions. We find a total cross section of 7.3 ± 1.3 mb. Most of this cross section is found in the lower topology events (? 10 prongs), and in the central and forward rapidity regions. The P_T² dependence of ?⁰ production, 〈:n>_?0 per event, and the ?⁰/π⁺ ratios are also discussed.     

Study of process e + e − → π+π−π0π0 at energies √s < 1 GeV with the spherical neutral detector

The cross section for the process e ⁺ e ^? → π⁺π^?π⁰π⁰ has been measured in an experiment with a spherical neutral detector, with the cross section at energies √s < 920 MeV having been measured for the first time. The model of vector dominance describes well the data obtained if the ρ, ρ′, and ρ″ resonances are taken into account. The probability of the decay ρ → π⁺π^?π⁰π⁰ is B(ρ → π⁺π^?π⁰π⁰) = (1.60 ± 0.74 ± 0.18) × 10^?5. The upper limit for the decay ω → π⁺π^?π⁰π⁰ has been improved by two orders of magnitude compared to previous measurements and is B(ω → π⁺π^?π⁰π⁰) < 2 × 10^?4 at a 90% confidence level.     

A Partial Wave Analysis of the Reaction π+p→ (π+π−π0) Δ++ at 16 GeV/c

A partial wave analysis of the 3π-system has been performed for the reaction π⁺p→ (π⁺π^?π⁰) Δ⁺⁺ (1232) at 16 GeV/c. Beside the well-established A₂⁰ (1300), the resonant state ω* with isospin I = 0 and spin-parity J^P = 3^? decaying mainly into (?π) has been found. Its mass and width have been determined to be M = (1.71 ± 0.03) GeV and Γ = (0.22 ± 0.10) GeV. The cross section for the reaction π⁺p→ ω* (1700) Δ⁺⁺ (1232) is σ = (12 ± 6) μb.     

Observation of structure in p̄p → π0π0

We present a partial wave amplitude analysis of the differential cross sections for p?p → π⁰π⁰ at 25 energies from 2.12 to 2.43 GeV. The results suggest the presence of a J^PCI^G = 2⁺⁺0⁺ resonance at 2.15 GeV. An isospin decomposition of p?p → π⁺π^- is also presented.     

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 \(\Gamma _{f_2 \to \gamma \gamma } (m_{f_2 }^2 ) \approx 3.8 keV\).     

A partial-wave analysis of the 3π system in the reaction K−p → π+π−π0Λ at 4.2 GeV/c

A partial-wave analysis of the (3π)⁰ system produced peripherally in the reaction K^?p → π⁺π^?π⁰Λ at 4.2 GeV/c is presented. The observation of the weak Λ decay allows a determination of all the transversity production amplitudes except for two phases. The production of known resonances having decay modes other than 3π is used to test the isobar model ansatz. Significant ω(783), φ(1020) and A₂(1310) production is observed. The spin parity of the $\text{ω}{}^1\text{(1675)}$ is established as 3^?. No evidence for production of other resonances, such as axial vector-mesons, is found.     

Meson pair production in γγ scattering and implications for scalar mesons

We describe a simple analytic unitary model which provides with a parametrization of low energy (from threshold up to 1.4GeV) two photon production of ππ andK \(\bar K\) pairs in term of a few parameters related to the γγ width of the scalar and tensor resonances. It can be useful for analysing data because of the large continuum found in γγπ⁺π^? production besides thef ₀ (1,270) peak in recent experiments. We show that the present data already provides a strong evidence for the existence of a broad low mass scalar state, the old σ or ε (600) with a γγ width of 5 to 9KeV, and that it will be difficult to get more information on the controversialS ^* parameters from ππ pair production only. In thef ₀ range we explain the downwards shift of the π⁺π^? peak but we expect the π⁰π⁰ peak around 1.27–1.29 GeV which could raise the question either of systematic errors in mass calibration of crystal Ball experiment or of the precise value of thef ₀ mass.     

A partial wave analysis of the (Kππ) system in the reaction π− p→(K +π−π0)Λ at 3.95 GeV/c

Results from a partial wave analysis of theKππ system produced in the hypercharge exchange reaction π^? p→(K ⁺π^?π⁰)Λ at 3.95 GeV/c are presented. SignificantQ ₁ production is observed but no evidence is found forQ ₂ production thus confirming the results obtained in a previous decay Dalitz plot analysis of the same reaction. The relative phase behaviour of the 1⁺(K?) partial waves obtained with the present analysis provides additional confirmation of the resonance interpretation of theQ ₁ enhancement. Information is also obtained about its production properties and the role played by the Λ polarization in the hypercharge exchange reaction. This is compared with the results obtained in the reactionK ^? p→(π⁺π^?π⁰)Λ at 4 GeV/c.     

Isospin symmetry violating effects and scattering length extraction from kaon decays

The isospin symmetry breaking effects in the charged kaons decays to two or three pions are considered. In semileptonic decay K ^± → π ⁺ π ^?e^± ν (called K _e4) these effects turn out to be crucial for correct extraction of ππ scattering lengths. Taking in account electromagnetic interaction between the pions in the final state and isospin symmetry breaking due to different masses of charged and neutral pions allows to adjust the values of scattering lengths obtained from experimental data on K _e4 decay and predictions of Chiral Perturbation Theory (ChPT). Final state interactions of pions in the decay K ^± → π ^± π ⁰ π ⁰ leading to the anomaly (cusp) in the π ⁰ π ⁰ invariant mass distribution in the vicinity of charged pions’ threshold are discussed and recent results of accounting of the electromagnetic interaction among charged pions leading to π ⁺ π ^? bound states (pioniumatom) just under the charged pions’ threshold are presented.     

Evidence forf 2 (1720) production in the reaction pomeron-pomeron→π+π−π+π−

Data are presented on pomeron-pomeron interactions which produce a central π⁺π^?π⁺π^? system in proton-proton collisions at √s=62 GeV at the CERN Intersecting Storage Rings. A spin-parity analysis of the π⁺π^?π⁺π^? system shows evidence for the production of the statef ₂ (1720) with decay to ρ⁰π⁺π^?. Since pomeron-pomeron interactions are expected to favor the production of gluonic bound states, observation of thef ₂ (1720) supports earlier interpretations of it as a glueball. In addition, enhancements near threshold give indication of the statef ₂ (1270) decaying to ρ⁰ρ⁰ and the statef ₀ (1400) decaying to ρ⁰π⁺π^?.     

Study of a possible exotic 1.5 GeV meson decaying into ϕπ0

New data have been obtained on a resonance in the ϕπ⁰ system, the C meson, which is formed in the π⁻p→ϕπ⁰n charge-exchange reaction. The experiment has been performed at the Serpukhov 70 GeV accelerator. The mass and the width of the resonance are measured to be M=1480±40 MeV, Γ=130±60 MeV. The production cross section is determined at a π⁻ momentum of 32.5 GeV/c:σ(π⁻p→Cn)·BR(C→ϕπ⁰)=40±15nb. The C(1480) meson has an isospin I=1 and spin-parity J^PC=1^{− −}. It is strongly coupled to the ϕπ⁰ channel and is considered as a possible exotic meson.     

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.     

Limits on the production of large transverse momentum direct photons deduced from the measurement of low-mass electron pairs

The hadronic production of electron pairs with masses between 200 and 500 MeV and large transverse momentum has been measured at the CERN Intersecting Storage Rings (ISR). The expected relation between low-mass electron pairs and real photons is used to determine the direct hadronic production of photons. Contrary to indications from some previous experiments, the observed spectrum is consistent with expectations from the decay of known mesons, and leads to a value for the ratio of direct photons to π⁰ of γ/π⁰ = (0.55 ± 0.92)% for 2 < p_T < 3 GeV and 〈√s〉 = 55 GeV.     

Two-pion spectra for the reaction π− p → π0π0 n at a pion momentum of 38 GeV/c and combined analysis of GAMS, Crystal Barrel, and BNL data

We perform a K-matrix analysis of IJ ^PC =00⁺⁺, 10⁺⁺, 02⁺⁺, and 12⁺⁺ meson partial waves using GAMS data on π^? p → π⁰π⁰ n, ηηn, ηη'n that are supplemented with BNL data on π^? p → K $\bar K$ n and Crystal Barrel data on p $\bar p$ (at rest) → π⁰π⁰π⁰, π⁰ηη, π⁰π⁰η. The positions of the amplitude poles (physical resonances) are determined, together with the positions of the K-matrix poles (bare states) and the values of barestate couplings to two-meson channels. The nonet classification of the bare states found in the present analysis is discussed.     

Measurement of the f1(1285) → π+π−π0 decay

The isospin symmetry-breaking (ISB) decay f ₁(1285) → π⁺π^?π⁰ has been studied at the VES facility. The branching ratio is measured: $\frac{{BR\left( {f_1 \to \pi ^ + \pi ^ - \pi ^0 } \right)}} {{BR\left( {f_1 \to \eta \pi ^ + \pi ^ - } \right)}} = 0.86 \pm 0.16_{ - 0.20}^{ + 0.70} \%Low-energy resonances in ³¹P(p, α ²⁸Si and ³⁵Cl(p, α)³²S were studied directly in order to gain a better understanding of reaction cycling in the Si-Ar region in novae. New resonance strengths at E _c.m. = 600 and 622 keV in ³¹P(p, α)²⁸Si were measured (ωγ _{p, α} = (2.2#x00B1;0.7) × 10^?2 eV and ωγ _{p, α} = (0.99±0.08) eV, respectively) as well as the E _c.m. = 610 keV resonance in ³⁵Cl(p, α)³²S [ωγ _{p, α} = (1.2±0.2) × 10^?2 eV], the lowest energy that any resonance in this reaction has been observed, directly or indirectly. The strengths of these resonances were found to be lower than previously determined, resulting in even weaker cycling in the Si-Ar region.