CERN’s Ambitious Plan to Build the Largest Particle Smasher Ever

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The Large Hadron Collider is the largest and
most powerful atom smasher in the world. Built to hunt for new particles and probe
the fundamental forces of nature, this massive machine is a 27 kilometer underground loop
filled with supercooled magnets and massive detectors that whip particles at the highest
speeds possible, to eventually collide into each other. And during one famous sprint in 2012, particles
collided, and the Higgs Boson was officially discovered. “I would like to add my congratulations to
everybody involved in this tremendous achievement.” The Higgs is a special particle. Its presence confirmed the existence of an
invisible quantum field that’s responsible for giving particles their mass. This field permeates the universe, leading
some to suspect that the Higgs may play an important role in the origin of everything. But at this point, the Large Hadron Collider
and the community that built it are at a crossroads. Physicists haven’t found the super symmetry
particles they were hoping to see. If they did, it would have solved some open
mysteries we have about the Higgs and the inner workings of the universe. This has created a huge international debate
over what to do next. For many at CERN, the institution that runs
the Large Hadron Collider, the next step in the hunt for new physics is to build an even
bigger machine. People expected for 40 years before the Higgs
was discovered that the Higgs could not be a lonely elementary particle. It would have to come along with a lot of
other things in order to give a coherent, rational explanation for the origin of its
mass. And the big surprise since July 4, 2012, when
the Higgs was triumphantly discovered, is that has not happened. So that’s really four decades of a certain
paradigm for what’s going on in physics associated with the Higgs that has not worked out the
way that theorists had imagined that it would. And that’s kind of fascinating. I think the last time something of this degree
of surprise happened in theoretical physics was a little over 100 years ago. What nature has in mind for what the Higgs
is about is something different than what theorists had in mind. While theorists are very confused about it,
the program for experimentalists is completely clear. When you run into a kind of elementary particle
you’ve never seen before, you’ve never seen anything like it in physics before you just
put the damn thing under a microscope and you study it to death. It’s pretty remarkable that we need to build
enormous machines that produce an incredible amount of energy to probe the smallest things
in the universe. And the push towards higher collision energies
to discover new particles is connected to Albert Einstein’s famous equation, e=mc^2. There’s an equivalence here between energy
on one side and mass on the other side. When we collide two particles, we gain access
to the kinetic energy they carry. And out of this kinetic energy, new particles
can be made, according to Einstein’s relation. And of course, the higher the energy that
we bring into this collision, the higher the mass of a particle that is forming out of
this energy can be. To get more juice out of the machine, CERN
shut the LHC down for performance upgrades. They’re working on cranking up the luminosity. Luminosity is a measure for the quality of
a collider. And in some sense, it tells you how many collisions
per second this collider can provide. When two of the elementary particles have
a head on collision, you can tell that happened because the result of those collisions come
out at larger angles relative to the beams. But it’s still an incredibly messy, kind of
complicated environment and even when we produce new elementary particles like the Higgs, they
don’t come out wearing a name tag saying I am a Higgs, they decay in a blink of an eye. It’s the results of those decays that experimental
colleagues have to sift through like they’re looking for a needle in a haystack in order
to actually see the evidence. This luminosity upgrade would ultimately produce
more collisions and would make measurements of particles like the Higgs even more accurate. Once completed in 2026, it’ll produce an
estimated 15 million Higgs per year, compared to the 3 million in 2017. It will be very beneficial to operate this
infrastructure until about 2035 or 2040. By then, we will have collected such a huge
amount of data from the collisions that we somehow saturate the knowledge that can be
provided by this machine. Operating it five years longer or 10 years
longer will not give significantly more information, which means for particle physicists that the
useful time of life of this accelerator will be reached. These time scales seem way out in the future,
but to put this in perspective: planning for the Large Hadron Collider began back in the
1980s, construction was approved in 1994 and the first runs didn’t start until 2008. So to prepare for what comes next, teams are
delivering conceptual designs for next generation particle machines. There are proposals for an International Linear
Collider, which Japan just backed out on, China has a circular collider project, and
there’s one from CERN. I’m in charge of the Future Circular Collider
Study. What we’re working on is really not an upgrade
of the LHC machine. It’s really new machines to come after the
LHC era, so from 2040 onwards. It’ll take international collaboration,
billions of dollars, and scientists to invent tools that don’t even exist yet. First thing’s first though, CERN wants a bigger
tunnel. On a map, you can imagine you have a circle,
which is the LHC, and then you would put a new circle that is roughly four times larger. The whole existing CERN accelerator complex,
including the LHC, would serve as a pre-accelerator for this future 100 km machine. Like the gearbox in a car, if you want to
drive very fast you must have several gears. You start in a small gear at low velocity,
and once you accelerate, you go to the second gear, third gear, fourth gear, fifth gear. This thing is very similar. We would start with small accelerators at
low energy, and then we go larger, larger, larger, and to higher energy, higher energy,
higher energy. The CERN study presents a path forward to
achieve these energy gear shifts. There’s a new lepton collider, which collides
electrons and positrons, a more advanced hadron collider, which collides protons and protons
and then heavy ions and then a third option, an electron-proton collider. The big difference between an electron and
the proton, which are the two particles that we have for these colliders, is essentially
that the electron and its anti-particle positron are point-like particles that to our present
knowledge have no substructure. When we say the electron looks point-like
and the proton does not, it actually means if you bounce things off the electron you
see that the way photons bounce off of it, you’ll see that the electron has no substructure
of any sort. Who knows, if we’re probing things with microscopes
that are a million times stronger than anything we’ve seen in some alien civilization that’s
a million times stronger than the LHC, maybe we would see some substructure to the electron
too. Or, if you believe string theorists, if we
look at ridiculously short distances, everything is made out of some little loop of string. In what sense are things elementary or composite? But that’s a story for another day. The Higgs is kind of point like, the Higgs
is sort of point liken and that’s just not good enough to sort of really settle this
theoretically dramatic question. We can try to measure all the known particles
like the Higgs particle, the W, and the set particle in the top quark with the best precision
possible. And for this, you will build this lepton collider,
because the lepton collider could produce exactly these particles in a very clean environment,
in huge numbers. The electrons are super clean for collisions,
but we cannot reach extremely high energies. The protons are a bit more dirty in the collision,
but we can accelerate them to far, far higher energies. Unlike the electron, a proton is not an elementary
particle. The proton is kind of a big messy object that’s
made up out of these smaller constituents known as quarks that are held together inside
the proton by the imaginatively named gluons. When we smash protons into each other at incredibly
high energies, one set is going this way at .9999999 the speed of light, the others are
going the other way the same number of 9s times the speed of light, and when they smash
into each other, mostly they go splat. And the debris of the collisions goes into
the direction of the beams that were coming in. The next generation Hadron Collider would
smash protons together like the LHC, except it’d reach energies of 100 trillion electron
volts. The Hadron Collider would provide much higher
collision energies that would allow direct creation of, today not known particles. This boosted machine could be used as a tool
to search for theoretical particles like WIMPS, which are connected to dark matter. It’s one of the most abundant and mysterious
forms of matter in the universe, and we haven’t detected it directly yet. We might be able to, and answer other big
questions, by upping the power and tweaking the detector’s precision. A factor of 100 in precision is what we need
to decisively settle the question of whether the Higgs looks more point-like than anything
we’ve seen before as far as its probes interact with other particles, factor of 10 higher
in energy will let us produce billions of Higgs. 100 TV is what we need to settle this question
of the simplest model of weakly interacting particles. The natural sequence is clearly to start with
a lepton collider, which is also a machine that is today technically ready for construction. And in parallel to the operation and physics
analysis of this machine, you can use the time to develop the very high field superconducting
magnets that you need for the successor machine. The magnets that we have presently operating
in the LHC tunnel can only reach eight or nine Tesla, which is the magnetic field strength. So we want to double this to 16 or even higher. Magnets, is in this case the really big challenge
for such a project. All these things need to be addressed from
the very beginning in small setups because you do not want to build 15 meter long heavy
magnets every time to test something new. While this project is an incredible scientific
endeavor, the price tag is very steep. These future colliders could cost over $25
billion dollars and would need investment from the international community to even get
off the ground. For this decision process, there are several
aspects, of course. There’s a scientific political one, there
is an economical one. There is of course also a physics community
process. And this is exactly what started out as a
bottom-up opinion making process, which is taking place in Europe in the coming year. While the discussions continue, some have
even questioned whether an investment like this is even the right course forward for
the particle physics community. There are questions over whether the science
case is as strong, if investing in this project is worth the cost compared to other global
issues, and how we can be so sure a machine of this magnitude can answer these big questions. There’s a spectrum of possibilities for what
could be out there theoretically and so we can’t know until we look. What’s definitely true, is that no one who
is arguing for building these next machines is now saying we should build them because
we expect to see particle x, we should build them because supersymmetry is around the corner
or extra dimensions are around the corner, or anything like that. If you believe that the purpose of doing these
experiments is making new particles, it’s definitely time to take your ball and go home
and do something else with your life because it cannot be guaranteed at all. I think it’s one of the more profound things
that there is to say about this human adventure of science. Period. Which is that everyone who works in fundamental
science has the sense that we’re exploring something that’s out there. And something that’s much much larger than
each one of us individually. So there’s this gigantic structure in the
universe, it knows vastly more about the laws of nature than we do. It is nature. By studying it, we put ourselves in the neighborhood
of something that’s vastly more powerful, vastly deeper than any of us are individually. The only method that we know of to access
this tremendous power and depth, far beyond what any of us have individually, is to interact
with it. And I think that’s the ultimate source of
real magic that goes well beyond what humans are capable of now, is out there in the structure
of the universe. And we only can find what it is by interacting
with it. And studying it.