Earlier this year CERN announced the Future Circular Collider study aiming to build on the success of the LHC and extend its research capabilities. Our two-part feature asks, what is it, and is it worth it?
Earlier this year CERN announced the Future Circular Collider (FCC) study, that aims to build on the success of the LHC and extend its research capabilities. The high cost and large infrastructure needed for this machine brings into questioning whether its construction is necessary, or if these resources would be better spent elsewhere.
This article aims to provide some insight into the importance of the LHC and its potential successor by giving its historical context and current performance; perhaps by analysing the past and present of this machine, we will be better equipped to judge the future of fundamental physics with particle colliders.
Let’s start by analysing what it was about the LHC that captivated the general public. Part of it can be attributed to its sheer size. The Large Hadron Collider (LHC) is the biggest accelerator ever built. Straddling the border between France and Switzerland this 27 km-long experiment caught worldwide attention by colliding protons at unprecedented energies.
The right path to achieve cost-effective high energy collisions is almost impossible to predict. Nobody can say for certain that we will find the answers we are looking for
For people that have seen the experiment, whether in pictures or in person it’s difficult not to be amazed by its dimensions, which also somehow makes us aware of its complexity. For accelerator physicists, the large dimensions are simply a bothersome but necessary challenge; the higher the energy to which particles are to be accelerated, the larger the accelerator must be. The current technology limits the maximum energy particles can gain over a given distance, this is overcome by circular colliders by circulating the beam back and reusing the accelerating structures.
However, this results in a second complication: in a circular accelerator like the LHC, dipole magnets are used to keep the particles in a circular orbit, colliding them only in the location of the detectors, but the higher the energy of the particles the less they bend when going through a magnet. With the strength of the magnets also limited by current technology, increasing the size of the tunnel is another way to keep the particles in orbit. The size of the LHC tunnel is enough to circulate protons at 7 TeV; if one wants to circulate particles at higher energy one needs to either increase the strength of the dipoles or increase the size of the tunnel. The FCC study aims do both to circulate protons at 50 TeV.
Not the first, won’t be the last…
The LHC is by no means the only particle accelerator that has existed to study fundamental physics. In fact, its existence has only been possible thanks to a series of predecessors that helped build its physics case and the technological advances necessary for its construction. Some of these accelerators, like the SPS, are still part of the accelerator chain seeding lower energy particles for the LHC to accelerate to the desired energy. Early accelerators first developed in the 1930s such as the cyclotron conceived by Livingston and Lawrence, accelerated protons to 80 keV (103 eV). Fast-forward 70 years and we have TEVATRON in Fermilab which, at the time, was the world’s most powerful, accelerated protons and antiprotons up to 1 TeV (1012 eV). That’s an increase of 9 orders of magnitude!
When considering the evolution of particle accelerators, higher energy is the driving force behind them; this is for two main reasons. Firstly, higher collision energies mean smaller distances can be examined. Secondly, the majority of particles are not stable and decay quickly into more stable particles. To study these evasive particles, the appropriate conditions have to be created. As we know from Einstein’s famous formula (E=mc2) the initial energy of the lighter, colliding particles can be converted into matter; therefore, the higher the energy of the particles we put in, the more exotic, massive particles we can get out.
Imagine now the exhilarating times of the 1970s, when the standard model, aiming to explain the building blocks of nature, was just beginning to take shape, with particle colliders providing the tools to study it with. Theorists conceived the existence of certain particles, accelerator physicists designed the colliders to achieve the necessary energies, and experimental physicists designed the complex detectors and studied the data to prove their existence. Success after success ensued with many new particles found. The importance of these discoveries can be emphasized by the 15 Nobel prices that have been awarded in connection to the development of the standard model. As with any scientific developments however, there were also drawbacks; lessons should be learned from stories like the SSC in the US; this 40 TeV collider was cancelled once its construction had begun and $2 billion already spent, mainly, but not limited to, budgeting and finance problems.
With the discovery of the top quark at TEVATRON in 1995 all the particles of the standard model had been discovered, except the Higgs boson. Perhaps now the hype surrounding the 4th of July of 2012 when physicists and physics-enthusiasts followed the announcement of its discovery, is understandable. Not only was this an incredible achievement by itself, but it also marked the culmination of decades of study and the confirmation of the standard model theory.
It is however, unfair to a machine like the LHC, to attribute its success only to this discovery. The LHC has produced collisions at unprecedented energies since 2009, resulting in incredible amounts of new physics, analysed by thousands of particle physicists, either related to the Higgs boson or to other studies. Furthermore, the complexity of every part of the accelerator and detectors is difficult to overestimate, pushing technological developments in a multitude of areas, thanks to the work of thousands of engineers, physicists and technicians. Most of these technological advances have already been transferred to industry, medical imaging and radiotherapy for cancer treatment for example. This technological transfer can also have unexpected consequences: one of the greatest inventions of the last century, the world wide web, was invented at CERN in 1989 as a way to improve data sharing between users.
It is also important not to underestimate the social impact of such large experiment; more than 500 doctoral degrees are awarded each year for work related to the LHC; most of these highly skilled people go on to do research at CERN, other institutions or move to industry. CERN has also done a particularly good job in involving the general public, receiving more than 90,000 visitors a year, most of which are students from schools around Europe. The LHC, as well as other high profile projects like LIGO, which announced the first observation of gravitational waves in 2016, or the landing of the rosetta at a distant comet in 2014, have helped inspire a new generation of students that grew up feeling part of major scientific achievements, even if a complete understanding and implication of these discoveries is difficult to grasp.
Building the future
Now is the moment to think about the future. As mentioned earlier the confirmation of the standard model is complete. But there are still unanswered questions that cannot be explained with the current theory, such as: What is dark matter made of? What causes the imbalance of matter and antimatter? Given the success that particle accelerators have enjoyed in exploring the unknown, it is perhaps understandable that many scientists want to continue building bigger and better accelerators and one way to move forward is to explore building up on the LHC infrastructure.
There are currently several projects being considered for the future of LHC and CERN. Currently, plans to upgrade the machine to increase the luminosity are already underway with the High-Luminosity experiment envisioned to be taking data within the next decade. But this higher luminosity (increase in amount of collisions) is no assurance that physicists will find the answers they are looking for. In order to explore new areas, colliding particles at higher energies might be one, and potentially the only, way to go. The FCC aims to achieve that by using magnets twice as strong as the LHC in a 100 km circumference. Another possibility is the HE-LHC, a kind of middle way which will use the same strong magnets of the FCC but in the current tunnel of the LHC. The LHeC proposes using the LHC proton beam but to collide with electrons instead; CLIC on the other hand is the proposal to have electrons colliding with positrons but accelerated linearly rather than in a ring.
The high cost and large infrastructure for these machines is of course an issue, but so far, an unavoidable one. This is motivating research in alternative acceleration techniques; the AWAKE project at CERN studies such an alternative by accelerating electrons using a wakefield generated by protons zipping through a plasma.
The right path to achieve cost-effective high energy collisions is almost impossible to predict. Nobody can say for certain that we will find the answers we are looking for. In most cases however, such as the search for dark matter, it is accurate to say that a higher energy means a higher possibility of finding these answers. However, this has brought into questioning the high price to pay for these discoveries, and if money should not be spent instead on other projects. Thing is, that several projects are indeed currently searching for dark matter but so far the search has proven unsuccessful; it is therefore the duty of physicists in different areas to exhaust the methods to find it. Furthermore, despite the high price of this project, there are likely to be incredible benefits outside of its core function that the supporting technology can bring. We have already witnessed the benefits, often unforeseen, that the technology needed for particle accelerators can bring – developing cancer treatment or improving the way we communicate just to name a few. Given the scientific, technological and cultural impact of accelerators, is only fair to consider the high cost against the high value that these experiments bring back to society.
The way I see it as an early researcher in accelerator physics, the study of the feasibility of the FCC and other possible colliders is already and achievement on itself. If history is anything to go by collaboration and sufficient planning have proven to be extremely valuable for the success of the large accelerators. The next big collider is likely to be a worldwide effort, rather than a single country or a continental one. In terms of planning, even when these accelerators are not likely to happen in decades to come, now is the best time to start studying them, when the benefits of the LHC are still tangible and while we still have the experience of the last generation of physicists that was involved in the construction of large colliders, ensuring continuity and knowledge transfer.
The decision of what, if any, the next collider at CERN will be is a difficult one. The LHC is a great example of a group of nations that decided to bet on fundamental research; and perhaps, the generation that grew up witnessing incredible achievements of science will do the same for the next collider and, if they do, they have the FCC study to help them build it in the best possible way.
Dr Emilia Cruz Alaniz is a postdoctoral researcher in Accelerator Physics at the University of Oxford and the John Adams Institute.?As of 1st of March she moved to a postdoc position at the University of Liverpool but based at CERN.