Particle Colliders on the Horizon
On 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
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.
Other 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