Supersymmetry

Supersymmetry (SUSY) is a proposed symmetry of nature that pairs every fermion (a matter particle with half-integer spin) with a boson (a force-type particle with integer spin), and vice versa. It is the most-studied extension of the Standard Model, attractive because a single idea would address several of the model's deepest puzzles at once. Despite decades of searches, no superpartner has yet been found.

Superpartners

SUSY doubles the particle content. Each fermion gains a bosonic partner named with an "s-" prefix (the electron → selectron, the quark → squark), and each boson gains a fermionic partner named with an "-ino" suffix (the photon → photino, the gluon → gluino, the Higgs → Higgsino). Partners differ in spin by exactly ½. If the symmetry were exact, each superpartner would have the same mass as its twin — which is experimentally ruled out, so SUSY must be spontaneously broken, pushing the superpartners to higher, as-yet-unobserved masses.

The hierarchy problem

SUSY's strongest motivation is the hierarchy problem. In the Standard Model, quantum corrections to the Higgs boson's mass are enormous — naively driving it up toward the Planck scale, some 10¹⁷ times heavier than its measured value of 125 GeV. Keeping it light requires an extraordinary, unexplained cancellation. Supersymmetry supplies that cancellation automatically: the correction from each particle is cancelled by an equal-and-opposite correction from its superpartner, because bosons and fermions enter the calculation with opposite signs. This natural stabilisation is why SUSY was, for decades, the favoured route beyond the Standard Model.

Dark matter and unification

SUSY offers two further bonuses. In many versions the lightest superpartner — often the neutralino — is stable, electrically neutral, and weakly interacting, making it an excellent candidate for dark matter. And when the strengths of the electromagnetic, weak, and strong forces are extrapolated to high energy, adding superpartners makes the three couplings meet at almost exactly one point, hinting at a grand unification the Standard Model alone narrowly misses.

The experimental status

The Large Hadron Collider has searched intensively for superpartners and found none, pushing the lower mass limits on squarks and gluinos above roughly 1–2 TeV. These null results strain the "naturalness" argument: the heavier the superpartners must be, the less effectively they solve the hierarchy problem. Supersymmetry is therefore not ruled out — its broken form allows a wide range of masses — but its simplest, most natural versions are now disfavoured, and the field is actively debating whether the idea survives in some form or should be set aside.