  Attempts to extract the order of the chiral transition of QCD at zero
chemical potential, with two dynamical flavors of massless quarks, from
simulations with progressively decreasing pion mass have remained inconclusive
because of their increasing numerical cost. In an alternative approach to this
problem, we consider the path integral as a function of continuous number
$N_\text{f}$ of degenerate quarks. If the transition in the chiral limit is
first-order for $N_\text{f} \ge 3$, a second-order transition for
$N_\text{f}=2$ then requires a tricritical point in between. This in turn
implies tricritical scaling of the critical boundary line between the
first-order and crossover regions as the chiral limit is approached.
Non-integer numbers of fermion flavors are easily implemented within the
staggered fermion discretization. Exploratory simulations at $\mu=0$ and
$N_\text{f} = 2.8, 2.6, 2.4, 2.2, 2.1$, on coarse $N_\tau = 4$ lattices, indeed
show a smooth variation of the critical mass mapping out a critical line in the
$(m,N_\text{f})$-plane. For the smallest masses the line appears consistent
with tricritical scaling, allowing for an extrapolation to the chiral limit.
