Virtual particles, including matter-antimatter pairs, allow what would have been infinite “bare” charges and masses to be renormalized and replaced with measured masses. However, a mass problem still remains since the correct masses have to be inserted “by hand.”  Fundamental considerations indicate that masses should still be many orders of magnitude larger than observed. A solution that has worked before might work again, so it was suggested to double the number of particles by having the distinction between bosons and fermions be a broken symmetry. Then all fermions should have boson counterparts, and vice versa. However, these “superpartners” have never been observed, although most experts think they almost certainly should have shown up at energy levels already probed. When the LHC came back on-line at double the previous center-of-momentum energy, and much greater luminosity, many physicists thought that this would be the last chance to confirm supersymmetry. No superpartners were seen and it is looking very much as if it will probably be necessary to abandon this idea or modify it beyond recognition.

The biggest problem resulting from the failure of supersymmetry is that, as we shall see later, supersymmetry is intimately embedded in String Theory. Strings are inherently bosonic in nature, and so the only way to get String Theory to describe fermions is via making String Theory supersymmetric! But essentially ALL suggested approaches going beyond the Standard Model also incorporate supersymmetry. That's not good... in any way.