TUESDAY, 18 JULY 2023Close to the beautiful Jura mountains, on the border between France and Switzerland, we can find the Large Hadron Collider (LHC): the largest and most powerful particle accelerator ever built. The LHC is operated by the European Centre for Nuclear Physics (CERN), and it consists of a 27-kilometre ring buried in a tunnel 100 metres underground, in which beams of subatomic particles collide front to front at almost the speed of light. Trillions of protons are set in motion with electromagnets 100,000 times more powerful than the Earth’s magnetic field and go around the tunnel 11,000 times per second. At the LHC, with these magnitudes that challenge the imagination, high energy physicists are trying to understand the laws of the smallest, most fundamental constituents of matter.
High energy physics is the branch of physics that deals with elementary particles and their interactions through the four fundamental forces of Nature: electromagnetism (which describes the behaviour of charged particles and light), the strong force (which binds elementary particles to form protons and neutrons), the weak force (responsible for the radioactive decay of unstable particles), and gravity. Except for gravity (which is too weak to be probed at the LHC), all of these forces can be described by the Standard Model (SM) of particle physics. In the SM, these fundamental forces have carriers – called gauge bosons – and there are twelve particles of matter that interact through these carriers: six quarks (sensitive to the strong force) and six leptons (not sensitive to the strong force). The last piece of the SM is the Higgs boson, which is responsible for giving mass to the other elementary particles.
The SM is one of the most successful scientific theories ever devised, but we know that it cannot be the whole story. There are still many big unanswered questions that the SM cannot explain, like how to describe gravity as a quantum theory, what is dark matter, why is there matter at all in the Universe, and many others.
At the LHC, high energy physicists test the SM by using particle accelerators. Charged particles (protons, specifically) are accelerated through very powerful electric and magnetic fields, and then are collided into each other at immense energies–the higher the energy of the collision, the better resolution we have to study the process. The LHC hosts four different main experiments that analyse these collisions: ATLAS, CMS, ALICE, and LHCb. They provide measurements for different particle processes to tackle different problems. ATLAS and CMS are general purpose detectors as they study a very large range of subareas in particle physics, including different sectors of the SM and the properties of the Higgs boson, and also dark matter, extra dimensions, and physics beyond the SM. ALICE is specialised in the study of heavy-ion collisions that result in quark-gluon plasma: the state in which matter would have been just after the Big Bang at the beginning of the Universe. Finally, the LHCb experiment is dedicated to study the differences between matter and antimatter. These four experiments can be understood as extremely powerful ‘microscopes’ that help us understand the physics that is realised at the scales of a millionth part of a millionth part of a millimetre.
Although the SM has been permanently tested, it has not been clearly broken so far. We have not found convincing signals of supersymmetry (which proposes that every fundamental particle has a so-called superpartner with the same properties but different spin), signatures of dark matter, or extra dimensions.
Still, as history has taught us, the ‘absence’ of discovery is discovery itself. Consider, for example, that Einstein’s theory of special relativity would not exist without the results of the Michelson-Morley experiment, which did not detect any signals of aether – a supposed medium for the propagation of light.
The importance of the LHC for the development and progress of particle physics since it started operating in September of 2008 cannot be overstated. Its most famous discovery has been the detection of the Higgs boson: the final piece of the SM. This particle, postulated almost 50 years before its detection by Peter Higgs, François Englert, and Robert Brout, is extremely important as it is responsible for giving mass to the other elementary particles. The experiments of the LHC provided definitive evidence to confirm its existence, but their discoveries do not end there. They have also found 59 new hadrons: new composite states of elementary particles (much like the well- known proton or the neutron, but of far higher energy). The LHC has also studied the charge-parity asymmetry, which describes how particles would behave if we swap their electric charges (and other additional features, known as quantum numbers) and we look at them from an opposite orientation.
The LHC has been fundamental in the construction and consolidation of theories of fundamental interactions over the past decade. However, particle physics is not just about the past, but about the future as well. The LHC will continue to break records in the coming years. Now, after a three year long hiatus for maintenance and upgrades, it is back in operation. The so-called ‘Run 3’ has begun, and it will last for four years. In this time, particle collisions will be generated at the unprecedented energy of 13.6 trillion electronvolts, allowing us to probe smaller spatial scales and expand our knowledge about potential heavier particles and effects that become visible at the higher energy collisions. Data will be collected in numbers never seen before: the experiments of the LHC will obtain more measurements in this individual Run than in all the previous runs combined, which will allow us to gain very robust statistics to probe our theories. But the history does not end there either. After Run 3, another upgrade, even more powerful, will take place: the High Luminosity LHC phase. It will increase the amount of data we collect from particle collisions by a factor of 10 and, once the era of the High Luminosity LHC is over, particle physicists even discuss the idea of building a completely new collider: the Future Circular Collider (FCC). It would be a ring of 100 kilometres in diameter and would run at energies ten times higher than the LHC in its High Luminosity phase.
Pushing boundaries at the LHC is not only useful in high energy physics, but also for the betterment of wider society. Intrinsic human curiosity comes with extra benefits other than just the pleasure of discovery and understanding, such as the development of new technology.
Particle accelerators are extremely complicated, and the sophisticated machines and experiments at the LHC, obviously, cannot order their equipment from a catalogue but have to manufacture it themselves. These curiosity driven processes end up with some of the most advanced technology on Earth that has then been taken, developed, and made available by industries many times before. For example, this has happened with technology developed at the LHC for the manufacture of semiconductors used in laptops and supercomputers, as well as extremely powerful big data processing and storage techniques. Electron and proton beam therapy, used all over the world, has also been developed at the LHC. Furthermore, if you are reading this online, you are benefiting from the World Wide Web, a system originally developed by CERN to share data.
Pushing the physical and technical frontiers at the LHC has meant the discovery of some of the most fundamental facts of our Universe and the access to cutting-edge technology that would have been impossible to obtain otherwise. Let us keep testing and pushing the boundaries of what we know since, as the great polar adventurer Sir Ernest Shackleton would have said, ‘It is in our nature to explore, to reach out into the unknown. The only true failure would be not to explore at all’. We can only guess at what wonders may lie ahead.
Manuel Morales Alvarado is a second year PhD student in the High Energy Physics group of the Department of Applied Mathematics and Theoretical Physics and member of the PBSP collaboration. Illustration by Caroline Reid.