Pion and kaon SIDIS multiplicities from COMPASS

We present preliminary COMPASS results on pion and kaon multiplicities produced in semi- inclusive deep inelastic scattering of 160 GeV muons off an isoscalar ($^6$LiD) target. The results constitute an impressive data set of more than 600 points for pions and 600 for kaons, covering a large $x, Q^2$ and $z$ domain in a fine binning. The data are available for QCD fits to extract quark fragmentation functions. We show results of a first leading order fit performed to extract the favored and unfavored quark fragmentation functions into pions $D_{\mathrm{fav}}$ $^{\pi}$ and $D_{\mathrm{unfav}}$ $^{\pi}$. The results from the sum of the $z$-integrated multiplicities $M(\pi^+) + M(\pi^-)$ and $M(K^+) +M(K^-)$ are presented versus $x$ and compared to previous experiments. At high $x$, the kaon data indicate a value of the non-strange quark fragmentation function significantly larger than obtained by the earlier QCD DSS fit. Overall, the new kaon data will strongly impact the flavour decomposition of kaon fragmentation functions. Recent additional data were added at high $z$ on the ratio of $K^-$ to $K^+$ multiplicities. For $z>0.8$, the ratio falls well below expectations from LO and NLO pQCD calculations. The ratio also shows a strong and unexpected dependence on the missing mass of the single kaon production process. This suggest to take into account the phase space available for hadronisation in further theoretical calculations.