How to preserve my constants

[Sigma] = Constant
a = Constant
b = Constant
[Epsilon] = Constant
T[x_] := ([Sigma]/(2*[Epsilon]))[a/(a^2 + x^2)^(1/2) - b/(b^2 + x^2)^(1/2)]

In mathematica this looks way simpler, so I can’t use this function unless I give all the constants some value, but I need them in the form of constants, this is eqn of the magnetic field by a disc with a big hole in it where a is the outer radius and b is inner. What’s the workaround?.
For any value of x, I keep getting output like "[0]/2" , I don’t know what that means. I need to be able to differentiate, integrate and all sorts of other stuff with this formula.