$K$-series X-ray yield measurement of kaonic hydrogen atoms in a gaseous   target
  We measured the $K$-series X-rays of the $K^{-}p$ exotic atom in the SIDDHARTA experiment with a gaseous hydrogen target of 1.3 g/l, which is about 15 times the $\rho_{\rm STP}$ of hydrogen gas. At this density, the absolute yields of kaonic X-rays, when a negatively charged kaon stopped inside the target, were determined to be 0.012$^{+0.004}_{-0.003}$ for $K_{\alpha}$ and 0.043$^{+0.012}_{-0.011}$ for all the $K$-series transitions $K_{tot}$. These results, together with the KEK E228 experiment results, confirm for the first time a target density dependence of the yield predicted by the cascade models, and provide valuable information to refine the parameters used in the cascade models for the kaonic atoms. 
M. Bazzi, G. Beer, G. Bellotti, C. Berucci, A.M. Bragadireanu, D.   Bosnar, M. Cargnelli, C. Curceanu, A.D. Butt, A. d'Uffiz...
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