Superstring theory describes the particles of the Standard Model through "vibrating strings" whose ends are attached to "membranes" (also called "D-branes"). Some of them, like photons, are "open strings," meaning that their ends are not merged; while others, like gravitons, are "closed strings" (forming closed loops). In the most "simple" solution of string theory, open strings are connected to D3-branes (three-dimensional D-branes), whereas closed strings propagate through all dimensions of space (9 in number). The former acquire mass through interactions with the latter.

Physicists have calculated these masses and have determined a close relationship between fermions (particles with half-integral spin) and their "super-symmetric partners," particles with a zero or integral spin (bosons). It turns out that the sums of the squares of the super-symmetric boson and of the fermion masses are equal. Under these conditions, it seems highly unlikely that such bosons could all have significant mass—and none of them has been spotted by the LHC thus far.