On March 25, the Large Hadron Collider (LHC) experiment switched on for experiments in 2016. Even without physicists eagerly anticipating more information about a possible new fundamental particle first sighted late last year, the LHC has its tasks cut out as the world’s premier particle physics probe.
Its reopening comes at the end of a 13-week maintenance run, during which engineers checked to see if all of its instruments, as well as the four attending detectors, were performing normally. The checks are necessary because, until June 2015, the LHC had been shut for major upgrades that almost doubled the energy at which it performs its experiments and the number of particles it produces in every collision. Then, it ran for a short while in 2015 before shutting for maintenance over the winter.
The LHC accelerates two beams of protons, each containing billions of the particles, to close to the speed of light and then smashes them head on. The accelerated particles contain a large amount of energy (each particle potentially contains about 6,000-times the energy it has when at rest). Right after the collision, the surplus energy manifests as rarely observed particles. What kinds of particles do or don’t manifest, and at what rates, comprise the data logged by detectors. Physicists compare this data against their theoretical models to understand more about the fundamental properties of nature.
The new particle, suspicions of whose existence were announced in December 2015, is a good example. It weighs about 750 GeV/c2, and is not predicted by theory. But if the LHC finds one such particle in its 2016 run, then physicists will have a broken theory on their hands. And their – our – understanding of fundamental physics will have been wrong in some way. This isn’t all bad because physicists are looking for just such a thing.
One dominant theory that has proved remarkably successful in the last four decades is the Standard Model of particle physics. Its predicted suite of fundamental particles have all been found, mostly according to its exact specifications, making it the preeminent model used by physicists to understand new phenomena. But try as they might, some of these new phenomena don’t seem reconcilable within the limits of the model. One famous example is dark matter. There is observational evidence to suggest that such a kind of matter exists and makes up up to 26.8% of all matter in the universe. However, physicists have not yet zeroed in on its particulate constituents.
Some suspect the 750-GeV particle to be that constituent. Others think the particle could be the graviton, the long-sought mediator of the gravitational force. Even others think it could be a particle fitting the predictions of a more expansive yet unproven theory called supersymmetry. Whatever it is, the first shape of the answer will emerge out of the LHC, and hopefully in 2016.