Soft supersymmetry breaking adds operators of positive mass dimension that break supersymmetry without reintroducing new ultraviolet quadratic divergences.
Soft supersymmetry breaking removes quadratic ultraviolet sensitivity but can leave scalar-mass corrections proportional to the soft squared masses and logarithms of scale ratios. The high-momentum difference of a paired scalar and fermion propagator falls as their squared-mass splitting divided by momentum to the fourth power. Heavy soft partners can therefore still create a tuning problem through logarithmic and finite thresholds even though the original quadratic hierarchy problem has been controlled.
A sparticle is the supersymmetric partner of a Standard Model particle. It differs in spin by one half and has the opposite R-parity.
In the MSSM, electroweak symmetry breaking mixes the neutral bino, wino and two neutral higgsinos into four neutral Majorana spinor mass eigenstates, the neutralinos. A neutralino can be the lightest supersymmetric particle; with conserved R-parity it then provides a stable possible dark matter candidate.
The lightest supersymmetric particle is the least massive sparticle in a given spectrum. Exact R-parity makes it stable: a lighter final state cannot contain another odd particle, while a final state of ordinary particles is even. A neutral weakly interacting lightest supersymmetric particle is a possible dark matter constituent, but its cosmological abundance must be computed separately.
A slepton is a scalar sparticle partner of a lepton. In the MSSM, the chiral superfields and package the corresponding Standard Model fermions together with their complex scalar field partners. Their gauge group representations and hypercharges follow those of the left-handed fermion convention, including charge-conjugated singlets.
A higgsino is the Weyl spinor partner of a Higgs scalar field in a chiral superfield. After a supersymmetric Higgs mechanism, its combination along a broken gauge direction mixes with the gaugino in the resulting massive N=1 vector multiplet.
A gaugino is the spin-one-half superpartner of a gauge boson and transforms in the adjoint representation of its gauge group. Gaugino loops modify gauge-coupling beta functions above the supersymmetry-breaking threshold.
The bino is the spin-one-half gaugino of the hypercharge gauge group. Its adjoint representation is the singlet , so it has no hypercharge of its own. After electroweak symmetry breaking, it can mix with the neutral wino and higgsinos into neutralinos.
A wino is a spin-one-half gaugino of the weak gauge group. Before electroweak symmetry breaking, its three adjoint components transform as and can be described by a triplet of Majorana spinors. After symmetry breaking, the neutral component can mix with the bino and neutral higgsinos into neutralinos.
The gluino is the spin-one-half sparticle paired with a gluon. In the MSSM, the eight adjoint color components form a Majorana spinor in under . Its gauge representation is real, so the opposite-helicity antiparticle states are already included in the adjoint multiplet.
A squark is the spin-zero supersymmetric partner of a quark. It carries the same colour, weak-isospin and hypercharge quantum numbers as the corresponding quark chirality.
The top squark, or stop, is the scalar superpartner of the top quark. Its mass controls the leading residual top-sector correction to the Higgs mass after soft supersymmetry breaking.
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