Narrow Results

We present preliminary results on the observation of the η→π⁺π^-e⁺e^- decay in the CELSIUS/WASA experiment. The η mesons were produced from proton-proton interactions at 1360 MeV incident proton energy.

The experimental program on meson production in light-ion collisions at the CELSIUS cooler/storage ring of the The Svedberg Laboratory in Uppsala, is reviewed. Production of pions and eta-mesons have been studied in pp and pd collisions using the PROMICE/WASA detector. This program is now continued at CELSIUS with the WASA4π detector together with studies of meson decays.

The isobar models η-MAID and η′-MAID have been used to analyze new data on quasi-free η photoproduction on the deuteron from Bonn and recent η′ data on the proton from Jlab. In η photoproduction on the neutron a bump around W = 1700 MeV was observed which could possibly arise from a narrow P₁₁ state that is discussed as a non-strange member of the Θ⁺ antidecuplett. In η′ photoproduction on the proton resonance contributions are found that can be attributed to missing resonances in the energy region around W = 1900 MeV.

The production of η mesons at an excess energy of 72 MeV has been studied in the reaction pp → pp(η)γγ. It is shown that a simple model with Pp final states included reproduces observed differential distributions better than the same model restricted to Ss, Sd and Ds final states. The strong influence of the Pp states could be taken as an indication of ρ dominance within an one boson exchange model for the excitation of N*(1535).

Differential cross sections for η production in proton-proton collisions at 40 and 72 MeV above threshold show that higher partial waves are necessary to describe the reaction. A simple model assuming constant production amplitudes and including most partial waves up to two units of angular momentum reproduces the data rather well, and also describes the invariant mass distributions found at 15.5 MeV excess energy.

The inclusive spectrum of the charged particles, $\pi$⁰- and $\eta$-mesons produced in the pp collisions at LHC energies were analyzed by fitting them with exponential functions. It was found the spectra were composed of several p${}_T$ regions, which could be characterized by the length of the regions $L_K^c$ and two free fitting parameters $a_K^c$ and $b_K^c$. The study of the $L_K^c$ dependences of the parameters $a_K^c$ and $b_K^c$ and of the energy dependencies of the $L_K^c$, $a_K^c$ and $b_K^c$ showed that the regions can be classified into two groups depending on the values of the $L_K^c$, $a_K^c$ and $b_K^c$. The values of the $L_K^c$ and $b_K^c$ for the first group don’t depend on colliding energy and the type of the particles (though the values of $a_K^c$ increase linearly with energy) whereas the characteristics in the second group of regions show strong dependencies. It was found that the ratio of the length for the $\eta$-mesons to one for the $\pi$⁰-mesons is approximately equal to the ratio of their mass: $〈L_{\eta }〉:〈L_{{\pi }^0}〉\cong m_{\eta }:m_{{\pi }^0}$. Assuming that the values of the $L_K^c$ are directly proportional to the string tension the result could be considered as evidence in favor of parton string fragmentation dynamics. The increase in the lengths for the $\eta$-mesons’ regions is accompanied by an increase of the values for the parameter $b_k^c$. It can mean that the $\eta$-mesons were produced at smaller values of ${\alpha }_S$ compared with that for $\pi$⁰-mesons. The results show that for the first group of regions the lengths of the regions are $\sim$3–5 times greater than the lengths of neighboring, lower p${}_T$ regions. For the second group of regions the lengths of the regions are $\sim$1–2 times greater than the lengths of neighboring lower p${}_T$ region. In the framework of the string fragmentation and hadronization dynamics, this could mean that the particles in the group $I$ of regions are produced through previous-generation strings decays into $\sim$3–5 strings while those in group $II$ originate from previous-generation strings decays into $\sim$2 strings.