The Li(K, π)ΣH reaction and narrow Σ states

Small-angle cross sections are calculated for ⁶Li(K⁻, π⁺)_Σ⁶H at 720 MeV/c, in order to intepret the data of the preceding paper. Due to the spin-isospin selectivity of ΣN → ΛN conversion, the substitution 1s_N → 1s_Σ leads to a narrow resonance high in the Σ continuum. Further tests of selectivity with ^3,4He targets are suggested. The energy shifts of Σ states which arise from the spin-isospin dependence of the ΣN interaction are discussed.     

Double charge exchange and one-pion production in π He collisions at 1.7 GeV/c

We present the results of a study of the exclusive reactions π^{+ 4}He → π⁻ 4p (double charge exchange, DCE) and π^{+ 4}He → π⁺ π⁻ 3pn (one-pion production). The experiment was performed with the Oxford/RHEL helium bubble chamber irradiated with a 1.7 GeV/c π⁺ beam. A general review of DCE models is presented and experimental results at other energies are discussed in the light of these models. None of the existing theoretical models is compatible with out data at 1.7 GeV/c. We propose a new mechanism for DCE, involving three nucleons, which reproduces the integrated as well as the differential cross sections. Some consequences for other models are also discussed.     

Search for narrow NNπ resonances in exclusive pp→ppππ measurements

Narrow structures in the range of a few MeV have been searched for in ppπ⁺ and ppπ⁻ invariant mass spectra (M_ppπ⁺ and M_ppπ⁻) obtained from exclusive measurements of the pp→ppπ⁺π⁻ reaction at T_p=725,750 and 775 MeV using the PROMICE/WASA detector at CELSIUS. The selected reaction is particularly well suited for the search for dibaryon resonances decoupled from NN and/or NΔ. In the mass range 2020 MeV/c²<m_dibaryon<2085 MeV/c² no narrow structures could be identified on the 3σ level of statistical significance neither in M_ppπ⁻ nor in M_ppπ⁺ giving an upper limit (95% C.L.) for dibaryon production in this reaction of σ<20 nb.     

Pion-transfer (n, d) and (d, He) reactions leading to deeply bound pionic atoms

Theoretical studies are given on the (n, d) and (d, ³He) reactions leading to deeply bound pionic atoms in heavy nuclei of configuration [(nl)_π·j_n⁻¹]J. The cross sections for various pionic and neutron-hole configurations in the case of a ²⁰⁸Pb target are calculated at incident energies 300–1000 MeV/u by using the effective number approach and the eikonal approximation for distortion. The effective number with a pion in the 1s or 2p state and a neutron hole in the orbit peaks around the same incident energy (T_n = 600 MeV) as the elementary cross section n+n→d+π⁻, where the momentum transfer matches the angular-momentum transfer of L = 5–7. The DWIA cross section for (n,d) producing a pion in the 1s or 2p orbit at T_n = 600 MeV is found to be around 42 or 75 μb/sr, respectively. At T_n = 350 MeV, where the momentum transfer is small, quasi-substitutional states of configurations and are preferentially populated with (n, d) cross sections of 95 and 190 μb/sr, respectively. The (d, ³He) cross sections are estimated to be an order of magnitude smaller than the (n, d) cross sections. Thus, the (n, d) and (d, ³He) reactions are found to be suited for the production of deeply bound pionic atoms.     

Self energies of the pion and the Δ isobar from the He(e,e′π)H reaction

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the ³He(e,e′π⁺)³H reaction have been measured in the excitation region of the Δ resonance to determine the longitudinal (L), transverse (T), and the LT interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and Δ-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the π⁺ mass of MeV/c² in the neutron-rich nuclear medium at a density of fm⁻³. Our data are consistent with the Δ self energy determined from measurements of π⁰ photoproduction from ⁴He and heavier nuclei.     

Measurements of interaction cross sections and radii of He isotopes

Secondary beams of ³He, ⁴He, ⁶He, and ⁸He were produced through the projectile fragmentation of an 800 MeV/nucleon ¹¹B primary beam. Interaction cross sections (σ_I) of all He isotopes of 790 MeV/nucleon on Be, C, and Al targets were measured by a transmission-type experiment. The interaction nuclear radii of He isotopes R_I(He) = (σ_I/π)^1/2 − R _I(T) where R_I(T) is the radius of the target nucleus, have been deduced to be R_I(³He) = 1.59 ± 0.06 fm, R_I(⁴He) = 1.40 ± 0.05 fm, R_I(⁶He) = 2.21 ± 0.06 fm, and R_I(⁸He) = 2.52 ± 0.06 fm.     

Absorption cross sections for antiprotons on carbon, aluminum and copper at 485 and 597 MeV/c

Absorption cross sections for antiprotons on C, Al and Cu have been measured by a transmission method at 485 and 597 MeV/c. Nuclei are shown to be black to antiprotons at these momenta. Using the available data up to 280 GeV/c, the momentum dependence of the absorption cross sections is investigated. Empirically, a simple expression σ_abs = γ(p/p₀)^−β, with β and γ constants and p₀ = 1 GeV/c, gives a good account of the data in a remarkably wide momentum range, 0.5–280 GeV/c.     

Determination of the K(1800) spin parity

A spherical harmonic moment analysis of the reactions K⁻p → K⁻π⁺n and K⁺p → K⁺π⁻Δ⁺⁺ at 13 GeV/c demonstrates the existence of a broad K^* state with mass in the vicinity of 1800 MeV and spin parity 3⁻.     

π Meson interaction on He at 120, 145 and 165 MeV

The total cross sections and the differential cross sections of π⁺-⁴He elastic and inelastic reactions at E_π=120, 145 and 165 MeV have been measured using a 38 cm diffusion cloud chamber in a magnetic field exposed to the Frascati Laboratories' pion beam. Total π⁺ track lengths of (2141 ± 10) × 10³ cm, (3435 ± 10) × 10³ cm and (2413 ± 10) × 10³ cm were measured at the three considered energies, respectively. The elastic cross-section data are in good agreement with the results of the Dubna-Torino collaboration. The total inelastic cross sections have been obtained taking into account the contributions from all the inelastic channels. The analysis of the various inelastic processes has allowed us to distinguish five main reaction mechanisms, which compare reasonably with the existing data and with the models for pion-light-nuclei interactions.     

Inelastic electron scattering from He

The spectra of electrons scattered inelastically from ⁴He have been measured at incident energies from 150 MeV to 400 MeV for scattering angles from 38° to 90°. Through the use of a liquid ⁴He target, a high momentum resolution (≈0.25%) was obtained, and the 20 MeV 0⁺ state of the -particle was observed for the first time in electron scattering. The excitation energy and the total width of this state were determined and are in good agreement with the results from other experimental methods. It was found that the total disintegration cross section appears be smaller than 2(σ_p+σ_n) by more than an order of magnitude at the lowest q²(≈ 0.33 fm⁻²). In earlier work the assumption of a total cross section of 2(σ_p+σ_n) was found to be quite accurate for higher q². As a new result, the partial radiative width for the 0⁺ state is determined to be Γ_rad = 1.1±0.3 meV.     

The weak electroproduction of Σ in electron–proton scattering

We obtain differential cross sections for the reaction e⁻+p→Σ⁰+ν_e, for incident electron energies from 0.5 GeV to 6.0 GeV. This calculation is phenomenologically based and makes use of SU(3) and SU(2) relations. We obtain contributions of the individual form factors to the differential cross section and show that the vector current form factor dominates in the region of observability. We also obtain reaction rates for a standard set of conditions. Finally we compare this case with that for Λ production and discuss what might be learned from this reaction concerning the structure of the weak, strangeness changing, hadronic current.     

Hadronic production by ee collisions at the energy 990 MeV with the Orsay storage ring

Data were taken at the energy 2E = 990 MeV to search for multibody events, with the same large solid angle detector which has been used for the measurement of the , ω andφ production by e⁺e⁻ annilations. Assuming a π⁺π⁻π⁰π⁰ production by the quasi two-body process e⁺e⁻ → → ωπ⁰ we give the correspondi ng cross section σ(e⁺e⁻ → π⁺π⁻π⁰π⁰) = (1.1 ± 0.5) 10⁻³² cm². Since no events with 3 and 4 charged pions have been observed σ(e⁺e⁻ → π⁺π⁻π⁰π⁻) 1.5 × 10⁻³³ cm².     

Magnetic moment of the Δ (1232) resonance

The reaction γp → π^°γ′p has been measured with the TAPS BaF₂ calorimeter at the Mainz Microtron accelerator facility MAMI for energies between √2 = 1221–1331 MeV. Cross sections differential in angle and energy have been determined for the photon γ′ in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the Δ⁺(1232) resonance and, for the first time, a value of μ_Δ+ = (2.7^+1.0_−1.3(stat) ± 1.5(syst) ± (theor)) π_N has been extracted.     

On the pp → ppη(η′) reactions near threshold

The production rate for η′ in pp → ppη′ at rest is calculated in a covariant one boson exchange model, previously applied to study π⁰ and η production in NN collisions. The transition amplitudes for the elementary BN → η′N processes with B being the meson exchanged (B = π, σ, η, , ω and a₀) are taken to be the sum of s- and u-channels with a nucleon in the intermediate states, and an a₀ meson pole in a t-channel. The couplings of the η′ to hadrons are a factor 0.4 weaker than the respective η-hadron couplings, as suggested by a quark model and a singlet-octet mixing angle θ = −23°. The model reproduces near threshold cross sections for the quasielastic processes π⁻p → nη(η′) and pp → ppη(η′) reactions.     

Single and double dissociation in πp collisions at 11 and 16 GeV/c

The reactions π⁻p → 2π⁻π⁺p, π⁻p → 2π⁻π⁺π^op and π⁻p → 2π⁻2π⁺n are analysed at 11 and 16 GeV/c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π⁻p → 2π⁻π⁺p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π⁻p → (2π⁻π⁺)p, and of the proton, π⁻p → π⁻(π⁻π⁺p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π⁻p → (2π)(πp) decrease with increasing energy.

In the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π⁻p → π⁻(π⁻π⁺π^op); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π⁻p → π⁻(π⁻2π⁺)n. Another strong maximum corresponds to the pion dissociation π⁻p → (2π⁻π⁺π^o)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π⁻p → (2π⁻2π⁺)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π⁻p → (2π⁻π⁺)(π^op) and π⁻p → (2π⁻π⁺)(π⁺n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed.

An appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy.     

Pion-He scattering length

We present relativistic three-body calculations for the real part of the π-⁴He scattering length Re a. Nucleon binding, exclusion principle and Fermi motion are all included in a very careful manner. Because of our careful evaluation of the single-scattering process the remaining discrepancies with the experimental data can be attributed to higher-order mechanisms involving more than one nucleon. In particular for the πA S-wave interaction at threshold, one expects that these higher-order contributions arise largely from the absorption effect. We have performed our calculations for four current models of the input πN t-matrix. Using a πN t-matrix which reproduces the low energy πN phase shifts correctly leads to Re a_abs -Im a_abs, if we attribute the discrepancy of the calculated value with the accurately determined empirical value of Re a to pion absorption. We determine the strength parameter B₀ in a p² optical potential term from an adjustment to the π-⁴He scattering length. Finally some cross sections are calculated for T^lab_π = 10 MeV and T^lab_π = 15 MeV.     

π-electroproduction on hydrogen near threshold at four-momentum transfers of 0.2, 0.4 and 0.6 GeV

The reaction e + p → e + π⁰ + p was measured near the one pion threshold, detecting the final electron and proton in coincidence for values of q² = 0.2, 0.4 and 0.6 GeV². The slope of the cross section at threshold is determined. The data are compared with those of the e + p → e + π⁺ + n reaction, measured simultaneously, and with the results of pseudovector Born approximation and with dispersion theoretical calculations.     

DWBA and CCBA analysis of selected transitions in the C(d,τ) B reaction at Ed = 80 MeV

Angular distribution of the reaction ¹²C(d,τ) ¹¹B leading to the 3/2⁻ ground state and the 4.44 MeV excited (5/2⁻ state ¹¹B have measured at E_d = 80 MeV up to about 70° and 40°, respectively. The inclusion of second-order processes in the framework of CCBA calculations gives a good reproduction of the 3/2⁻ as well as of the DWBA forbidden 3/2⁻ angular distribution. The strong dependence of the cross section on the sign of the deformation supports a positive β₂ value for ¹¹B.     

Experimental observation of the Σ hypernuclei, ΣH and ΣC

The hypernuclei _Σ⁶H and _Σ¹⁶C were observed by the (K⁻, π⁺) reaction on targets of ⁶Li and liquid O, respectively, at 713 MeV/c incident K⁻ momentum. Structure is seen in _Σ⁶H which may be interpreted in terms of particle-hole excitations similar to the previously observed states in _Λ⁶Li. The inablitity to resolve individual Σ hypernuclear levels in _Σ¹⁶C, due in part to the excitation of non-coherent states as a result of the large momentum transfer of about 130 MeV/c, precludes the extraction of the Σ-nucleus spin-orbit potential strength. The Σ-nucleus well depth appears to be 7 to 10 MeV less than that for the Λ.     

Photoproduction of Θ and other pentaquark states

We present constraints on the relative photoproduction cross sections of positive parity pentaquark states, Σ₅, Λ₅, and N₅, based on a minimum phenomenology gained in and their baryon–meson couplings as in the work of Close and Dudek. The possibility of anomalous signals in γp→K⁰_S(Θ⁺;Σ_d⁺) is discussed. We emphasize the importance of comparing with “conventional” states such as γN→KΣ(1660).