Ρπ System Research Articles

LOW Q2, HIGH ν NEUTRINO PHYSICS (CVC, PCAC, HADRON DOMINANCE)

A critical review is given of the main theoretical bases and of the experimental results of neutrino physics at low four-momentum transfer (Q2≤1 GeV 2) and high energy transfer (ν≥a few GeV). The theoretical predictions for the vector current are first presented, using the CVC hypothesis and the hadron dominance model of Piketty and Stodolsky. The predictions for the axial current at very small Q2 values are discussed on the basis of the PCAC hypothesis (Adler’s theorem), and extended for Q2≲1 GeV 2 in the lines of hadron dominance; the structure of the longitudinal component of the axial current is particularly discussed. Experimental data on neutrino and antineutrino interactions on nucleons are reviewed, in particular the total cross sections, which provide good tests of the PCAC hypothesis and of the model of Piketty and Stodolsky; also reviewed are the data on diffractive production of π, ρ and a1 mesons. The observation of shadowing, from the comparison of the total cross sections of neutrinos and antineutrinos on neon and deuterium nuclei, is discussed in detail with emphasis on the predictions of the PCAC hypothesis, in the framework of the Glauber-Gribov model. Finally a review is given of the results on coherent neutrino and antineutrino interactions on atomic nuclei: • π meson production, by charged and neutral currents in several experiments, providing a detailed test of the PCAC hypothesis; • ρ meson production, providing a test of the CVC hypothesis in weak interactions; • a1 meson or nonresonant ρπ system production, allowing the study of the weak axial current structure.

Isovector electromagnetic exchange currents in the chiral approach

The problem studied in the paper concerns the structure of the isovector e.m. MEC operators of the A1−, ϱ- and π ranges in the interval of intermediate energies. The two main dynamic principles which we invoke in our considerations are the current conservation and the gauge chiral invariance. Respecting them consistently allows us to describe correctly the interaction of NA1 ϱπ system with the external e.m. field. Our main results are as follows: (i) We verified that our Lagrangian approach is consistent with the prediction of the low energy theorem at threshold. (ii) We showed explicitly the continuity equation which the longitudinal parts of our currents obey. (iii) We proved the equivalence relation for the MEC operator of the pion range and demonstrated the existence of the seagull current in the MEC operator built up using PS πN coupling. This new term influences strongly the exchange charge density.

Diffraction dissociation, the deck mechanism and diffractive resonance production

We have constructed a model amplitude for the diffractive production of resonant states in the presence of Deck amplitudes: rescattering corrections to the Deck amplitudes are included. We have found that gross distortion of the resonance may occur and also that the phase of the diffractively produced system may vary very slowly, despite the existence of resonances: the only requirement is that the phase of the Deck amplitude leads the production phase of the resonance by ≈40°. Our model simultaneously accommodates the well established lack of phase variation in the diffractively produced 1 + s-wave (A1) ϱπ system and the details of the variation of intensity with mass, with resonance parameters M A1 ≈ 1.3 GeV/ c 2 and Γ A1 ≈ 240 MeV/ c 2. The analogous Kππ (Q) diffractive enhancement fits satisfactorily into the same framework. Our model also accounts for a number of features of diffractive N ∗ production.

A 1 and A 2 production in π−p → π−π−π+p and π−p → π−π0π0p reactions at 7.0 GeV/ c

The A 1 and A 2 mesons as well as a ϱπ enhancement at 1230 MeV/ c, the A (1230) are investigated. Both the A 1 and A(1230) appear as small signals on a large Deck background, and like the background are produced very peripherally suggesting a diffractive process. Their J P are also consistent with diffractive production, the A 1 being J P = 1 +(ℓ = 0,2) with non-negligible D wave, and the A(1230) being consistent with either a J P = 1 +(ℓ = 0), or 2 −(ℓ = 1) assignment. Further, our data is inconsistent with the A 1 being a kinematical enhancement as is sometimes asserted. Studying next the general characteristics of the non-resonant ϱπ background we establish it to be mostly J P = 1 +(ℓ = 0) with a small J P = 2 −(ℓ = 1) component which becomes larger at higher ϱπ mass and Δ pp 2. Finally, the ϱ 0 π −/ ϱ − π 0 ratio is found to be consistent with unity, the value expected for an I = 1 ϱπ system for the ϱπ mass region of 1.1−1.4 GeV/ c 2. We next study the A 2, and find it to be a J P = 2 + object. Its non-peripheral production mechanism is qualitatively in agreement with ϱ or natural parity exchange, and is quite different from the diffractive background on which it rides. We conclude by studying the A 2 νπ decay and separately establish it as a J P = 2 + object produced via natural parity exchange. The νπ/ ϱπ branching ratio of the A 2 is found to be 20%.

Investigation of the I = 2 ϱπ system in [formula omitted

The I = 2 ϱπ system produced in the reaction π −n → ϱ − π − p is investigated within the framework of the Reggeized charge exchange Deck mechanism. The diagram in which a π and ϱ are exchanged gives rise to an enhancement in the ϱπ effective mass at 1.1 GeV which is nearly independent of beam energy. Experimental evidence for such a state in π −n → ϱ − π −p, and in the related processes π +n → ϱ 0 π 0p and π −p → ϱ − π − Δ ++ is discussed. The interpretation of this enhancement as an exotic state on the basis of the duality argument made by Chew and Pignotti is considered.

Spin-parity analysis of the 3π system in the reaction π−p → p π−π+π− at 3.9 GeV/ c

A spin-parity ( J P ) analysis of the 3π system is presented for a sample of events of the type π −p → p π − π + π −. Using the maximum-likelihood fitting procedure, the behaviour of different J P states for the intermediate ρπ system has been investigated as a function of the 3 π effective mass. The analysis gives a clear indication for a J P = 2 + ρπ resonance at the mass of the A 2.

Consistency condition for the A 1ϱπ system from unitarized current algebra

Using recent attempts to incorporate unitarity into the current-algebra approach additional information on the A 1 ϱπ system is obtained. When the resulting consistency condition is applied to the original hard-pion model of Schnitzer and Weinberg, the parameter δ, which is related to the magnetic moment of the A 1 meson, can be determined. The result turns out to depend crucially on the exact value of gϱ, the coupling constant of the isovector current to the ϱ-meson.

Analysis of the reaction π±d → d π±π+π− at 5 GeV/ c interpretation with a reggeised π-exchange model

With a simple of 1304 events that fit the reaction π ±d → d π ± π + π − at 5 GeV/ c, we find a total cross section of 342 ± 25 μb and that the final state has 31% d ∗ϱ 0 , 6% d ∗f 0, 14% d ∗(ππ) IZ=2 and 43% of d πϱ. We are able to interpret the production cross sections and differential distributions in terms of a reggeised π-exchange mechanism and we show that for the ϱπ system the model predicts distributions indistinguishable from the ones obtained from the A 1(1 +) hypothesis. The experimental width of the ϱ 0 and f 0 are smaller than usually observed in strong interaction experiments.

The four-prong annihilation of 1.2 GeV/ c antiprotons in hydrogen (I): Four-pion final state

Approximately 17 000 four-prong annihilations of 1.2 GeV/ c antiprotons in hydrogen have been analyzed. The cross section for annihilation into four prongs is 31.6 ± 1.0 mb and the partial cross section for the 4 π final state 2.9 ± 0.2 mb. The 4 π state, which is reported here, shows production of ϱ 0, f 0, A 2 ± mesons and an enhancement in the 3 π and/or ϱπ system at the mass of the A 1 meson. The frequencies of various resonance channels have been determined. Two-body intermediate states account for 40–60% of the 4 π final state. Centre-of-mass angular distributions of the pions are presented. A change in the momentum dependence of the forward-backward asymmetry of the pions is observed at a pion momentum corresponding to the production of A 2 ± π +.

ϱπ resonances in π+π−πo system

Results on the analysis of neutral ϱπ system from π+d interactions at 5.1 GeV/c are presented. Evidence for a new isoscalar resonance (ϱπ) is reported. Its mass, width and cross section are M = 1.636 GeV, Γ = 0.112 GeV and σ = 53 μb, respectively. There is no evidence for H and A1o resonance production.