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ß isn't beta. It's the (german) sharp s. The greek beta looks like this β.

The fastest method probably is:

  • enable the greek keyboard layout permamently in System Preferences -> Keyboard -> Input Sources
  • defining and enabling a free keyboard shortcut in System Preferences - Keyboard -> Shortcuts -> Input Sources -> Select next source in input menu (e.g ShiftAltSpacebar)

To type α/β, you just have to switch the keyboard layout with ShiftAltSpacebar, hit A/B and switch back to your common keyboard layout with ShiftAltSpacebar.