CBELSA/TAPS at ELSA

The ELSA accelerator delivers a beam of either polarized or unpolarized electrons. The maximum energy reached is 3.5 GeV. In a bremsstrahlung process, the electrons produce the photon-beam needed for the photoproduction experiments. Using polarized electrons,
a beam of circularly polarized photons is produced, using unpolarized electrons, either unpolarized photons or linearly polarized photons produced by coherent bremsstrahlung of a diamond crystal become
available.

Central to the experiment is the ability to measure cross sections as well as polarization observables. The latter is made possible by the available polarized beam and the polarized target. Without the measurement of such polarization observables, an unambiguous extraction of the bound states of the strong interaction, the baryon resonances, from the data is not possible.

The CBELSA/TAPS experiment is very well suited to measure neutral mesons decaying into photons, due to its electromagnetic calorimeters the Crystal Barrel calorimeter with its 1320 CsI(Tl)-crystals as well as the TAPS-detector in forward direction with 216 BaF₂ crystals. For charged particle identification a three-layer scintillating fibre detector surrounding the target and scintillators in front of the crystals in forward direction are used. The detector system covers 96% of the complete solid angle.

The excellent resolution and signal to background ratio becomes e.g. visible in the γ-γ versus γ-γ invariant mass for γp → p 4γ events. Events of the type γp → p 2π⁰ and γp → pπ⁰η are clearly observed. Based on the polarization data from ELSA combined with data from other facilities worldwide, new particles resonances) have been discovered and entered the review of particle properties (PDG) .