lawlz, can we do LaTex online??
Since alt's should really be no longer than 100 characters, only add an alt if you can get a mathematician to concisely explain that formula. It looks possible. alt="A mathematical forumla showing..."
But if this is like on Wikipedia where people are expected to have the ability to actually read or even copy-paste it, you'll need to see if you can make it real text somehow. Frankly, even with Ruby annotation (which wouldn't be correct usage), you're not going to get it.
Check out Wikipedia's solution which uses sup and sub tags, with an image overlaid (view page with images blocked to see the formulas). This might be the best you can do, and you may need a mathematician to write it for you or make sure you write it correctly. You can use numerical character entities to type the Greek letters. Someone without those fonts is just screwed. In any case if you do Gilder-Levin with real forumlas underneath, you'll have a CSS background and no alt text, and alt won't be needed either, so, cool.