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.