On July 4, 2012, the European Organization for Nuclear Research (CERN) announced the discovery of the Higgs boson. While this was big news in fundamental physics, it was not surprising. The existence of the Higgs boson, or something like it, was necessary for the consistency of the Standard Model of particle physics, established in the 1970s and now supported by an extraordinary amount of experimental data. Finding the Higgs was central to confirming the Standard Model. However, despite the well-deserved attention accorded to the discovery of the Higgs, this was not the biggest news. The biggest recent news in fundamental physics is what has NOT been discovered: superparticles.
The theory of supersymmetry—that all existing particles are matched by “superpartners” having opposite spins—was introduced in the 1970s, soon after the Standard Model was established, and quickly became a darling of theoretical physicists. The theory helped solve a fine tuning problem in the Standard Model, with superparticles balancing the quantum contributions to existing particle masses and making the observed masses more natural. (Although, why particles have precisely the masses they do remains the largest open question in fundamental physics.) Also, for proponents of Grand Unified Theories (GUTs), the three strengths of the known Standard Model forces more perfectly converge to one value at high energies if superparticles exist. (Although a similar convergence can be achieved more simply by adding a handful of non-super bosons.) And, finally, supersymmetry (SUSY) became a cornerstone of, and necessary to, superstring theory—the dominant speculative theory of particle physics.
One of the strongest motivations for constructing the Large Hadron Collider (LHC), along with finding the Higgs, was to find superparticles. In order for SUSY to help the Standard Model’s naturalness problem, superparticles should exist at energies reachable by the LHC. During the collider’s first run, the anticipation of superparticles at CERN was palpable—it felt as if a ballroom had been set up, complete with a banner: “Welcome home SUSY!” But superparticles have not shown up to the party. The fact that the expected superparticles have not been seen puts many theorists, including string theorists, in a scientifically uncomfortable position.
Imagine that you had a very vivid dream last night that you saw a unicorn, probably in your backyard. The dream was so vivid that the next day you go into your backyard and look around, expecting to find your unicorn. But it’s not there. That is the position string theorists and other SUSY proponents now find themselves in. You may claim, correctly, that even though you now know there is no unicorn in the backyard, the Bayesian expectation that the unicorn is actually hiding in the closet has increased! You are “narrowing in” on finding your unicorn! This is precisely the argument SUSY proponents are presenting now that the LHC has failed to find superparticles near the electroweak energy scale. Yes, it is correct that the probability the unicorn is in the closet, and that superparticles might be found during the current LHC run, has gone up. But do you know what has gone up more? The probability that the unicorn and superparticles do not exist at all. A unicorn would be a wonderful and magical animal, but maybe it, and SUSY, and superstrings, really just don’t exist, and it’s time to think about other animals.