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Particle Colliders on the Horizon

Emily ThompsonSep 25, 2016

Click to Enlarge ImageOn June 3rd, 2015: At the same time that the Large Hadron Collider (LHC) successfully starts colliding protons at its maximal design energy, I’m 4,000 miles away packing my bags in preparation to visit CERN, the home of the LHC and the Mecca of particle physics. Fast forward a week and I’m sitting among 300 fellow enthusiastic young particle physicists in the iconic CERN auditorium where the discovery of the Higgs Boson was announced only three years prior. The room is filled with excitement and anticipation for the start of that year’s summer student program and for what the LHC will discover in its most powerful run yet. But mixed among these emotions is a lingering question: What will come next after the era of the LHC has ended?

The basic idea behind any particle collider is simple: Smash really small particles together at really high energies—and see what happens. The byproducts of these collisions tell physicists about the structure of elementary particles and the physical laws that govern them. The higher the energy, the more interesting the byproducts are. The LHC is a circular proton-proton collider with a maximal collisional energy of 13 teraelectron volts, making it the most powerful collider ever built. To put this into perspective, 1 teraelectron volt is approximately equivalent to the energy of a flying mosquito. The collisions at the LHC, however, are much more enlightening than a few mosquitos bumping into each other, because the LHC squeezes the energy into the size of a proton, a million million times smaller than a mosquito. The LHC has achieved many of its scientific goals, including the discovery of the Higgs Boson in 2012. But still it has left many fundamental questions in particle physics unanswered.

What is the dark matter and dark energy that presumably comprises 96 percent of our universe? Why is gravity so much weaker than the other three fundamental forces? Does our universe contain extra dimensions? To answer these big questions, we need higher energy particle collisions. When two protons collide, the result is a spray of different particles, both usual ones that make up ordinary matter, as well as interesting particles that only existed moments after the Big Bang. These interesting particles are often much heavier than the usual matter that surrounds us. Thus, according to Einstein's famous equation, E = mc2, for a certain massive particle to result from a collision, there must be sufficient energy in the collision to be converted into that mass.

But increasing the collisional energy isn’t as easy as turning up a dial in the LHC control room. Indeed, it’s impossible to do with the LHC we have now. The world needs a new particle collider, and scientists across the globe are working hard to deliver the blueprints that will receive the world’s support and funding.

One exciting option is the International Linear Collider (ILC). The ILC will be a 30- to 50-kilometer linear collider with a maximal collisional energy of 500 gigaelectron volts, with a possible upgrade to 1 teraelectron volt. Contrary to the LHC, the ILC will be a lepton collider, which means it will collide electrons and their antimatter counterpart, the positron. Instead of accelerating the particles in a circle like the LHC, the ILC will simply accelerate them in a straight line in opposite directions. While the ILC’s maximum collisional energy (1 teraelectron volt) is less than what we are colliding now at the LHC (13 teraelectron volts), its measurements will be much more accurate because collisions between elementary particles, such as leptons, are much simpler than collisions between protons, which are made up of even smaller constituent parts. The ILC’s estimated cost: $7.8 billion (according to 2013 estimates), plus 23 million person hours of labor. The timeline of operation: If construction started tomorrow, the machine wouldn’t be ready until after 2026. The location? Likely Japan, but CERN (on the French-Swiss border) and Fermilab (in Batavia, IL) are also secondary options.

Click to Enlarge ImageOther scientists have even bigger aspirations: the Compact Linear Collider (CLIC). CLIC is a study for a future collider that would collide electrons and positrons at an energy of 3 teraelectron volts, making it three times more powerful than the ILC. The goal is to do this with a linear accelerator of manageable size, specifically about 48 kilometers long. To do this, it must accelerate particles at a rate of 100 megavolts per meter, 20 times faster than the LHC. CLIC is exploring new accelerating technologies to make this possible. Traditionally, colliders have used superconducting radiofrequency cavities to accelerate particles. However, superconducting technology has so far been unable to achieve such high acceleration rates. Therefore, CLIC is spearheading a novel approach to accelerating particles: two-beam acceleration (TBA). Essentially, TBA uses special radio frequency devices called Power Extraction and Transfer Structures to transfer energy from a low-energy and intense beam of electrons to a higher energy and less-dense beam of electrons. Because there are fewer electrons in the second beam, it gains more energy per electron and is able to achieve very high acceleration. The estimated cost: currently unknown. The timeline of operation: construction will begin after the LHC completes complete data collection, which is predicted to be in 2030. The location: It likely will be built underground near CERN.

At first, I only heard other physicists talking about potential new linear colliders, and I started to wonder whether another circular collider like the LHC might also offer advantages. The Future Circular Collider Study (FCC) is investigating just this prospect, and is looking into colliding either hadrons (like the LHC), leptons (like the ILC), or colliding hadrons with leptons, something never done before. The FCC is looking into a wealth of new technologies, such as superconducting magnets twice as strong as the LHC's magnets, radio frequency systems that can accelerate particles much faster than current systems, and more sustainable ways to keep the accelerating superconducting magnets at their ideal temperature: –456 degrees Fahrenheit (No, this is not a typo). The details of their proposed blueprints are still unknown, but a conceptual design report is due to be published in 2018.

Needless to say, the coming years in particle physics will be exciting ones. But the proposal of such time-consuming and expensive projects won’t be without critics. Recently, I visited my old high school to give a talk about my time working at the LHC. Afterwards, one girl raised her hand and asked, “What is the point of spending all this time and money to build another particle accelerator? Why does it matter?” Particle physicists constantly struggle to convey the answer to this age-old question. The truth is that the direct applications of fundamental research are seldom immediately evident.

Nevertheless, fundamental science has led to a myriad of influential applications in society. For example, Tim Berners-Lee invented the World Wide Web in 1989 while working at CERN. Similarly, Carl Anderson discovered the positron in 1932, and had no foresight of its heavy use now in the medical field via positron emission tomography (PET scans). The moral is that we won’t know the impact that building a new accelerator will have on society until many years after we use it. In the meantime it is guaranteed to foster peaceful international collaboration and continue to inspire the imagination and hope of future generations of scientists.

During my time at CERN, I worked as a part of the ATLAS collaboration, one of the groups working to analyze the collisions from the LHC. I’m optimistic that I will go back soon and continue working with the LHC. But in the far future, I’m excited to hear which of these accelerators the world will agree to construct, and which will likely determine the focus of my future career.

This post is published in Macroscope

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