The affine ensemble: determinantal point processes associated with the $ax + b$ group

. We introduce the affine ensemble, a class of determinantal point processes (DPP) in the half-plane C + associated with the ax + b (affine) group, depending on an admissible Hardy function ψ . We obtain the asymptotic behavior of the variance, the exact value of the asymptotic constant, and non-asymptotic upper and lower bounds for the variance on a compact set Ω ⊂ C + . As a special case one recovers the DPP related to the weighted Bergman kernel. When ψ is chosen within a finite family whose Fourier transform are Laguerre functions, we obtain the DPP associated to hyperbolic Landau levels, the eigenspaces of the finite spectrum of the Maass Laplacian with a magnetic field.