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Two of the major questions left unanswered by the Standard Model of particle physics have to do with hierarchies of mass scales. The first is the flavor problem: what determines the masses of the quarks and leptons, and why do they span such a large range, e.g. why is the top quark 3 × 10 times heavier than the electron? The second is the gauge hierarchy problem: why is the weak scale seventeen orders of magnitude smaller than the Planck scale?
— Nima Arkani-Hamed
