A search for πNN bound systems

The d(π⁺, π⁻)T⁺⁺⁺ and the d(π⁻, π⁺) T⁻ reactions were studied at an incident pion energy of 260 MeV. Outgoing negative pions were detected at 35° and 55° in the first reaction, and the positive pions at 45° in the second. Indications for structures corresponding to a (π⁺pp) system with a mass of 2003.0±1.0 MeV and a (π⁻nn) system with a mass of 200.1±1.5 MeV were observed with marginal statistical significance. Upper limits for the cross sections are obtained.     

Observation of the (π−, π−n) and (π+, π+p) reactions at 165 MeV

The (π^?, π^?n) and the (π⁺, π⁺p) reactions were studied on C, Fe, and Bi at 165 MeV by coincidence measurement o outgoing particles. The nucleon-knockout reaction is the dominant inelastic process at backward pion-scattering angles at this energy. The quasifree (π^?, π^?n) and (π⁺, π⁺p) cross sections were determined from the π-N an correlations. Their σ_qf(π^?, π^?n)/σ_qf(π⁺, π⁺p) ratio is consistent with unity for C and is larger than the N/Z ratio for the heavier nuclei.     

Investigation of the double diffraction dissociation reaction pp → (pπ+π−) (pπ+π−) at 24 GeV/c

Data from a high-statistics bubble chamber experiment at 24 GeV/c are used to study the double diffraction dissociation reaction pp→(pπ⁺π^?) (pπ⁺π^?) where both protons of the initial state dissociate into pπ⁺π^? systems. Cross section, mass spectra and dσ/dt' distributions have been determined and are compared with the results found for the single diffraction dissociation reaction pp → p(pπ⁺π^?), which have been obtained from the same experiment. All our findings agree well with what is expected from the factorization hypothesis.     

Partial-wave analysis of backwardly produced three-pion systems in K−p interactions at 4.2 GeV/c

We report on the results of a partial-wave analysis of the 3π system produced by baryon exchange in the reaction K^?p→Σ^?π⁺π⁺π^? at 4.2 GeV/c. We confirm the existence of an enhancement in the 1⁺S(?π) wave as previously established from a Dalitz plot analysis of the same data. The phase variation of this wave is found to be consistent with that expected for a resonance and thus the enhancement is identified with A₁ production. No clear signal for this state is found in either the reaction K^?p→Σ⁺π⁺π⁺π^?π^? or K^?p→Λπ⁺π^?π⁰. We also find production via baryon exchange of the A₂ in all three reactions and the ω and $\text{ω}{}^1$ (1975) in the third reaction.     

S-wave ππ scattering from the reaction π+p → π+pπ0π0 at 3.5 GeV/c

We have studied the 2π⁰ system produced in the reaction π⁺p → π⁺pπ⁰π⁰ at 3.5 GeV/c, using the heavy-liquid bubble chamber technique. The cross section of the reaction π⁺π^? → π⁰π⁰ saturates the S-wave unitarity limit around 750 MeV/c² and near threshold is indicative of an I=0 S-wave scaterring length larger than expected.     

The reaction γp → pπ+π−π+π− for photon energies from 25 to 70 GeV

Measurements of the reaction γp → pπ⁺π^?π⁺π^? are presented, in which π⁺π^?π⁺π^? systems with masses up to 3 GeV are produced from fragmentation of the incident photon. The reaction is dominated by production of the large peak of the ?′(1600) meson and, at higher masses ?2 GeV, y production of jet-like 4π systems. The ?′(1600) meson is produced by a predominantly s-channel helicity conserving mechanism. At higher masses there are also indications of ?π peaks, of masses 1.3 GeV (the A₂ meson) and 1.75 GeV, produced with a recoiling π meson by a mechanism consistent with the Deck effect.     

Coherent two pion photoproduction on 12C

We develop the formalism for coherent two pion photoproduction in nuclei and perform actual calculations of cross sections for π⁻π⁺ and π⁰π⁰ photoproduction on ¹²C. We find that due to the isospin symmetry the cross section for π⁰π⁰ production is very small and has a maximum when the pions propagate together. However, the kinematical region where the energies and polar angles of the two π⁰ mesons are equal and their relative azimuthal angle Ø = 180° is forbidden. Conversely in the π⁻π⁺ production the pions prefer to have a relative azimuthal angle 180° and the production of the pions propagating together is suppressed. The dominant one-body mechanism in both channels is related to the excitation of the Δ isobar. Hence the reaction can serve as a source of information about Δ's properties in nucleus. We have found that the reaction is sensitive to effects of the pion and Δ renormalization in the nuclear medium, similar to those found in the coherent (γ, π⁰) reaction, but magnified because of the presence of the two pions.     

A partial wave analysis of the (3π)0 system from the charge exchange reaction π+n→π+π−πop AT 4 GeV/c

From a partial wave analysis of the (3π)^o state in the charge exchange reaction π⁺n→π⁺π^?π^op, we observe strong unnatural parity as well as natural parity production. The observed strong unnatural parity states are identified with well-established resonances. The unnatural parity production is consistent with Reggeized Deck model predictions, with the exception of the I = 1, JP = 1⁺ state. Here there is no evidence for A₁ production at ～1.1 GeV, but the data could support resonance production at higher masses.     

ππ scattering in the I = 2 channel

I=2π⁺π⁺ scattering is studied in π⁺p reactions at 5.45 GeV/c. The π⁺π⁺ cross sections and phase shifts have been determined from the reaction π⁺p → π⁺π⁺n using first a pure and then a modified Chew-Low extrapolation to the pion pole.     

A critical discussion of a prism plot analysis of the four-body reaction π−p → π−pπ+π− at 3.93 GeV/c

A multidimensional analysis of the reaction π^?p → π^?pπ⁺π^? at 3.93 GeV/c is presented. Its results are compared to those obtained with conventional methods and its limitations are discussed.     

Analysis of the reaction np → dπ+π− below 3.5 GeV/c

The reaction np → dπ⁺π^? was studied from threshold up to 3.5 GeV/c neutron beam momentum. Cross-section values for this reaction are evaluated as a function of the beam momentum. The deuteron production features and the enhancement observed in the dπ mass spectrum at 2.17 GeV are adequately described in terms of an OPE model and nucleon-nucleon final-state interactions.     

Photoproduction of charged pions on deuterium in the first resonance region

Photoproduction of π⁺ and π^? on deuterium has been measured in the photon energy range from 240 to 400 MeV and for pion c.m. angles between 15° and 180°. The pions were analysed in angle and momentum by a magnetic spectrometer. From the measured π^?/π⁺ ratio, corrected for Coulomb interactions in the final state, differential cross sections of the reaction γ+n→π^?+p were calculated. Together with the π⁺ photoproduction our data show no isotensor contribution. Comparison of our data with the recent experiments done on the inverse reaction shows no evidence of a violation of time reversal invariance. With the measured π⁺ photoproduction on deuterium, a test of the spectator model has been made. Using the closure-approximation of Chew and Lewis our data agree within a range of ±10%.     

Measurement of the reaction π+ p→π+π+ n near threshold

The reaction π⁺ p→π⁺π⁺ n was studied in the vicinity of the reaction threshold at ten incident pion beam momenta from 297 MeV/c to 480 MeV/c. From data angular distributions, invariant mass spectra and integrated cross-sections were deduced. The chiral symmetry breaking parameter as determined by this reaction equals to ξ=1.56±0.26±0.39, where the first error is experimental, while the latter reflects the uncertainty in the ansatz used in the extrapolation to the reaction threshold. A comparison with the other reaction channels of the reaction πp→ππN indicates that a single parameter (ξ) is not sufficient to describe low energy ππ interactions.     

Study of π−π− correlations in the reaction pp → π−π−+ anything at 28.5 GeV/c

Correlations between π^?π^? pairs are studied in the reaction pp → π^?π^?+ anything at 28.5 GeV/c. We find that if we examine the data for each multiplicity separately, the major portion of the data (90%) shows no significant two-particle correlations.     

A study of the reaction K+n → K+π−p at 2–3 GeV/c

Results are given from a study of 15 518 events of the reaction K⁺d → K⁺π^?pp. The K⁺π^? spin density matrix and the constraints imposed on it by positivity have been studied. Analyses of K⁺π^? → K⁺π^? elastic scattering have been carried out using methods developed by Estabrooks and Martin and Ochs and Wagner for the analogous case of ππ scattering. Results are found to be in agreement with earlier Kπ scattering studies using the reaction K⁺p → K⁺π^?Δ⁺⁺ at much higher energies. The S-wave scattering length is found to be in agreement with the prediction of current algebra.     

A study of s - wave π π scattering in the reaction πn → ππp at 2.15 GeV/c

We have studied the 2_π⁰ final states in the reaction π⁺d → π⁰π⁰p(p) at 2.15 GeV/c in a 2 million picture exposure of the PPA rapid cycling deuterium bubble chamber. Two tantalum plates were added to the bubble chamber to convert γ rays which were kinematically constrained to a 2π⁰ hypothesis. The 2π⁰ mass spectrum is observed to saturate s-wave unitarity in the ππ mass region between 0.6 and 0.9 GeV/c², clearly favoring the ‘up-down’ or broad resonance solution for s-wave, I = 0, ππ scattering.     

Strangeness at J-PARC

Strangeness nuclear physics is one of the main fields to be explored at the J-PARC Hadron Hall. The K ^?? and π ^± beams at the K1.8 beam line will be used mostly for hypernuclear experiments. The γ-spectroscopy of Λ hypernuclei using the K ^??,π ^?? reaction and the spectroscopy of neutron-rich hypernuclei via the π ^??,K ^?+? reaction are planned to be studied. As for Strangeness =???2 systems, experiments for Ξ hypernuclear spectroscopy via the K ^??,K ^?+? reaction, double Λ hypernuclei by the hybrid emulsion method, and Ξ^?-atomic X rays will be carried out.     

On the calculation of spin dependence in the absorption model for π-exchange

A generalization of the absorption model of Williams done for the reaction π^?p → π^?π⁺n is presented for arbitrary π⁺B final states in the reaction Bp → Bπ⁺n. For small values of the π⁺B crossing angle we can express the results in terms of the π⁺B scattering amplitude instead of the π⁺B partial-wave amplitudes. By this a considerable simplification of the general formalism is achieved. Some of the predictions which can be made turn out to be independent of the π⁺B amplitudes. This allows a sensitive test also in the case of B = π or K, where the πB amplitudes are unknown.     

Measurement of the π−n→π−π−p cross section near threshold

The cross section for the reaction π^?n→π^?π^?p has been measured in a deuterium bubble chamber. The cross section at E_π,lab = 226, 250 and 312 MeV was found to be equal to 9.4 ± 2.3, 25.0 ± 5.3 and 57 ± 10 μb, respectively.