Atomic target mass dependence of φ-meson production in proton-nucleus collisions

The modification of the vector meson properties in a strongly interacting environment is currently a hot subject in terms of spontaneous chiral symmetry breaking and partial restoration of this symmetry in nuclear matter. The dropping of the vector-meson mass by about 20% has been predicted already at normal nuclear density. However, according to theoretical investigations the φ-meson mass shift in matter is small and the main medium effect on the φ is a sizable increase of its width up to an order of magnitude compared to the vacuum value of 4.3 MeV/c2. The in-medium width of a meson is related to the imaginary part of the nuclear optical potential which is responsible for the meson absorption in nuclear matter. Therefore, information about the φ-meson width can be obtained by analysing the target mass dependence of the φ-meson production cross sections. An ANKE experiment aimed at the measurement of the φ width in the nuclear matter of normal density in proton-nucleus collisions at maximum COSY energy of 2.83 GeV. The φ-mesons were detected in the K+K- decay channel. The total amount of φ's collected from carbon, copper, silver and gold targets is about 20000. In the talk the measured A-dependence of the φ production cross sections is compared with available theoretical predictions.