by Clark Hickman
On July 4, 2012, hundreds of people gathered in a room in Switzerland to celebrate. These weren’t American expats congregating for Independence Day though; instead it was a group of scientists from all over the world eagerly awaiting the announcement of one of the greatest scientific discoveries of the 21st century. The room burst into applause with the declaration that the data collected from the Large Hadron Collider (LHC) confirmed the existence of the Higgs boson, the so-called “God particle” that gives all other fundamental particles their mass and creates the structure of the universe as we know it. First proposed in 1964, the search for the Higgs had become one of the largest scientific undertakings ever, with thousands of scientists and engineers building careers around the project. Confirmation of its existence was rejoiced throughout the scientific community, led to a Nobel Prize, and validated the Standard Model – the current theory underlying all of particle physics – which had been created assuming the existence of the Higgs particle.
To many physicists, the Standard Model (SM) is considered one of the most successful scientific theories of all time. First proposed in the 1950s and 60s, the SM not only predicted the Higgs particle but several other new particles as well. With the discovery of the Higgs, the model is now “complete,” but this doesn’t mean there isn’t more for physicists to discover. In fact, for all of its success, the SM had some glaring holes. For one, gravity is not explained by the model at all. Additionally, we know from astronomical observations that the majority of the universe is made up of dark matter and dark energy; however, this missing material is “dark” specifically because it doesn’t interact with the known particles of the SM, and is therefore very difficult to detect. This has led to an entire field of science looking for physics “Beyond the Standard Model” (BSM) that might answer these discrepancies.
Finding fundamental particles is no easy feat. The “easiest” particles to discover were the electron and the proton; these are relatively large particles that make up regular matter and have a charge, which means that they can be manipulated with electromagnetic fields. However, in the era of the SM, detection of new particles has required a lot more ingenuity, and energy. Because energy is proportional to mass (E = mc2), increasingly large particles such as the Higgs (250,000 times heavier than the electron) can only be produced in manufactured environments that generate enough energy. This is done by accelerating charged particles (protons or electrons) to near the speed of light and then smashing them together to observe the particle shrapnel that is produced. With enough data (the LHC produces a billion such collisions each second), scientists can sort through the results to recreate the collision products to see if these large fundamental particles could explain the measurements. However, because energy is always conserved, particles can only be created if they have less energy than the pieces being smashed together. Therefore, each collider has an energy limit – based on its size and design – past which potential new physics is inaccessible.
Figure 1: Scale and location of proposed FCC. The FCC would be roughly four times the circumference of the LHC but would use many of the facilities already in place at CERN. Image source.
Therefore, colliders at higher energies are a great method to investigate possible BSM physics. At CERN, home of the LHC, a proposal for the next-generation collider is currently being reviewed with hopes of running once the LHC has completed its experiments. Dubbed the Future Circular Collider (FCC) (Figure 1), the proposed beam of protons would travel four times farther than they do at the LHC, producing collisions with eight times more energy (around 105 GeV, or 100 TeV; see Figure 2). At this range, scientists think they can learn a lot more about interactions between the Higgs and the other particles, make more precise measurements regarding other known heavy particles, and even explore the frontier of possible dark matter candidates. Different types of colliders can probe even more physics, as they explore different possible interactions. For example, the Compact Linear Collider would operate at lower energies but bring together electrons (or electron-like particles). Because electrons are fundamental particles – unlike protons, they do not break down into any smaller particles – they create much cleaner collisions in which possible new products are much easier to parse out.
Figure 2: The energy of colliders has risen over the decades, leading to more discoveries of fundamental particles. Future colliders will continue to raise the energy scale, including the proposed FCC-hh, which hopes to reach 105 GeV in a proton-proton (hadron) collision sometime after 2060. Figure made by the author.
High-energy particle colliders are not the only path forward when searching for BSM physics. In fact, given that the potential energy of possible dark matter candidates is almost infinite, some physicists feel as though continuing to build particle colliders with higher and higher energies is an exercise with no end. Rather than look at greater energies, other experiments aim to measure properties of known particles with greater and greater precision. This is a great way to look for indirect evidence of potential new physics, as finding values different than those predicted by the SM can help guide theorists to a better theory that can explain existing and new observations. Even results where no new interactions are found are useful in this regime, as they constrain these same theories. Ideally, both precision experiments and high-energy colliders can work in concert to unlock the next generation of scientific achievement.
So why do physicists want to keep building bigger colliders? Ultimately it boils down to their curiosity and the human desire to understand as much about the universe as possible. The 20th century featured many incredible discoveries in particle physics, which led to countless progress in other fields of science as well. Yet there are still outstanding questions about some of the most fundamental properties of the universe. If we’re lucky, the next generation of colliders might help answer some of them.
