It is hard to overstate the anticipation that preceded the opening of the Large Hadron Collider (LHC) 10 years ago. Smashing protons together at energies well above those produced at any previous particle accelerator, the LHC seemed capable of vindicating the most fanciful speculations of theoretical physicists, from curled-up extra dimensions to microscopic black holes to a hidden realm of new particles mirroring the particles that we know.
A decade on, particle physicists find themselves in what some at the time called the “nightmare scenario”: discovery of the Higgs boson and nothing else. The triumphant discovery of the Higgs in 2012 confirmed theoretical notions about the generation of particle masses introduced in the 1960s with the Standard Model of particle physics, which describes three of the four fundamental forces of nature (gravity being the exception). The absence of new physics at the LHC so far comes as a snub to many of the speculative ideas for physics beyond the Standard Model that have been advanced since the 1960s and ’70s. This development (which still could be overturned by future analyses at the LHC) has invigorated discussion about the status of a central idea in modern elementary particle physics called the naturalness principle, which served as the basis for the prediction that “new physics”—experimental hints of more fundamental patterns beyond the Standard Model—would be found at the LHC.