showing that in this limit the numerics are under control . For the rescaled equation ( eq.14 ) we can study the lowest mass meson as a function of the quark mass . Figure 3 shows the value of the first meson mass as a function of the quark mass . The important point to note here is that there appears to be a mass gap in the m → 0 limit for l = 0 . The numerics make this calculation difficult , though at m = 10 −10 the value of M 1 is 0.28 . Note that in contrast to the equation of motion with l = 0 , the D7 - brane equation is perfectly well behaved in this limit and has discrete eigenvalues . This will be shown analytically in section 4.2 . The scale is set by the AdS radius R which can be tuned by hand to compare with lattice data .