3 π and 4 π meson production in np interactions at intermediate energies

The study of multipion production in NN collisions is one way to obtain information about the NN, πN and ππ states, including: dibaryons (including I=2 in ppπ), dipions (narrow σ0 meson, ππ state with I=2), pentaquarks (I=5/2, S=+1), missing resonances, etc. Also the important task is the test of various models of pion production in NN interaction, such as, for example, Valencia model [1], Xu Cao model [2] and OPER model [3, 4].


Introduction
The study of multipion production in NN collisions is one way to obtain information about the NN, πN and ππ states, including: dibaryons (including I=2 in ppπ + ), dipions (narrow σ 0 meson, ππ state with I=2), pentaquarks (I=5/2, S=+1), missing resonances, etc.Also the important task is the test of various models of pion production in NN interaction, such as, for example, Valencia model [1], Xu Cao model [2] and OPER model [3,4].

Experiment
The neutron-proton interactions were studied using neutron beam and liquid hydrogen bubble (target) at the JINR Synchrophasotron [5].The unique in fullness and precision data were obtained.It allowed to carry out the detailed study of inelastic np interactions in in a wide range of energies using the quasimonochromatic neutrons with P 0 <2.5% under condition of 4π geometry.
The accuracy of the momentum and scattering angle reconstruction for the secondary charged particles was σ p /p ∼ 2% and σ Θ ∼ 10 mrad, respectively.The separation of the reaction channels were carried out by the standard χ 2 procedure using corresponding constraint equations [6].Finally, the numbers of events that were selected for the further study: 390 events 743 events 5790 events np → ppπ + π − π − π 0 66 events 215 events 2666 events np → npπ + π + π − π − 83 events 344 events 4530 events The cross sections of the considered reactions were presented in [7].

Data analysis
This reaction is characterized by: -plentiful production of the Δ resonance, both from direct production and from Δ * and N * decays through the mode Δπ, -large peripherality of the secondary nucleons.The model of reggeized π meson exchange suggested in ITEP [3] was taken to describe the experimental distributions of the considered reactions.The advantages of this model are: -small number of free parameters (3 in our case), -wide range of the described energies (2÷200 GeV), -calculated values are automatically normalized to the reaction cross section.The following diagrams were taken into account to calculate the characteristics of the reactions np → NN 3π and np → NN 4π:

Reaction np
The results of the calculations using OPER model for the reaction np → ppπ + π − π − are shown in figure 3 for the data at P 0 = 5.2 GeV/c and in figure 4 for the data at P 0 = 4.42 GeV/c and P 0 = 3.83 GeV/c.One can see a good agreement between the experimental data and theoretical calculations.

Reaction np
The results of the calculations using OPER model for the reaction np → ppπ + π − π − π 0 are shown in figure 5 for the data at P 0 = 5.20 GeV/c.One can see a good agreement between the experimental data and theoretical calculations except the region of η 0 and ω 0 mesons at the masses of π + π − π 0 combinations.As far as it concerned the description of η 0 and ω 0 mesons it is necessary to take into account the diagrams of the type that are showed in figure 6.
The results at P 0 =4.42 GeV/c and P 0 =3.83GeV/c are not presented due to a small statistics.But OPER model also described satisfactorily the experimental distribution and the signal of η 0 and ω 0 mesons production was also observed at these energies.

Reaction np
The results of the calculations using OPER model for the reaction np → npπ + π + π − π − are shown in figure 7 for the data at P 0 = 5.2 GeV/c and in figure 8 for the data at P 0 = 4.42 GeV/c and P 0 = 3.83 GeV/c.
One can see a good agreement between the experimental data and theoretical calculations.

Conclusion
Multi π mesons production in np interaction is provided by the excitation and Nπ and Nππ decays of Δ * and N * resonances (taken from PWA and GIM).
The large peripherality of the secondary hadrons leads to the idea to use some exchange models (π, P etc. exchange).
It was shown that there are no noticeable signal of ρ meson production in the considered reactions.OPER model allows to obtain a good description of the characteristics of 3 and 4 pions production in np interactions.
To get a better description of the reaction np → ppπ + π − π − π 0 it is necessary take into account η 0 and ω 0 production.

Figure 1 .
Figure 1.The momentum distributions of the incident neutrons at P 0 = 3.83, 4.42 and 5.20 GeV/c

Figure 2 .
Figure 2. Diagrams of OPER model for the reactions of 3π and 4π meson production in np interactions.