COMPASS measurement of pion and kaon multiplicities and extraction of quark fragmentation functions into pions

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 400 points in p and 400 in K, covering a large x,Q$^{2}$ and z domain in a fine binning, which will be used in future QCD fits at next to leading order 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 $D^{\pi}_{fav}$ and $D^{\pi}_{unfav}$.