When I replace the line
TF1 *fitHits = new TF1("fitHits", "gaus(0)");
by
TF1 *fitHits = new TF1("fitHits", "[0] * exp(-(x-[1])*(x-[1])/2./[2]/[2])");
it starts working. However, according to the documentation, it should be the same. May it be a bug?