The invariant mass spectra of $\phi \to K^{ +}K^{ -}$ are measured in 12 GeV $p+A$ reactions in order to search for the in-medium modification of $\phi$ mesons. The observed $K^{ +}K^{ -}$ spectra are well reproduced by the relativistic Breit-Wigner function with a combinatorial background shape in three $\beta\gamma$ regions between 1.0 and 3.5. The nuclear mass-number dependence of the yields of the $K^{ +}K^{ -}$ decay channel is compared to the simultaneously measured $e^{ +}e^{ -}$ decay channel for carbon and copper targets. We parameterize the production yields as $\sigma (A) = \sigma_0 \times A^\alpha$ and obtain $\alpha_{ \phi\to K^+K^-} - \alpha_{ \phi\to e^+e^-}$ to be 0.14 $\pm$ 0.12. Limits are obtained for the partial decay widths of $\phi$ mesons in nuclear matter.
Categories: nucl-ex