Hi, experts I got a very naive question. I read the following sentence in a paper: "New results on the production of charged pions in p+p interactions are presented. The data come from a sample of 4.8 million inelastic events obtained with the NA49 detector at the CERN SPS at 158 GeV/c beam momentum". (see the abstract of DOI:10.1140/epjc/s2005-02391-9) Then it says that the the centre of mass energy is 17.3 GeV. It is really hard for me to get this conclusion. Could sb. gives me some hint? Best wishes W.