85% of the matter in our universe is a mystery.
We don’t know what it’s made of, which is why we call it dark matter.
But we know it’s out there because we can observe its gravitational attraction
on galaxies and other celestial objects.
We’ve yet to directly observe dark matter,
but scientists theorize that we may actually be able to create it
in the most powerful particle collider in the world.
That’s the 27 kilometer-long Large Hadron Collider, or LHC,
in Geneva, Switzerland.
So how would that work?
In the LHC, two proton beams move in opposite directions
and are accelerated to near the speed of light.
At four collision points, the beams cross and protons smash into each other.
Protons are made of much smaller components called quarks and gluons
In most ordinary collisions, the two protons pass through each other
without any significant outcome.
However, in about one in a million collisions,
two components hit each other so violently,
that most of the collision energy is set free
producing thousands of new particles.
It’s only in these collisions that very massive particles,
like the theorized dark matter, can be produced.
The collision points are surrounded by detectors
containing about 100 million sensors.
Like huge three-dimensional cameras,
they gather information on those new particles,
including their trajectory,
electrical charge,
and energy.
Once processed, the computers can depict a collision as an image.
Each line is the path of a different particle,
and different types of particles are color-coded.
Data from the detectors allows scientists to determine
what each of these particles is,
things like photons and electrons.
Now, the detectors take snapshots of about a billion of these collisions per second
to find signs of extremely rare massive particles.
To add to the difficulty,
the particles we’re looking for may be unstable
and decay into more familiar particles before reaching the sensors.
Take, for example, the Higgs boson,
a long-theorized particle that wasn’t observed until 2012.
The odds of a given collision producing a Higgs boson are about one in 10 billion,
and it only lasts for a tiny fraction of a second
before decaying.
But scientists developed theoretical models to tell them what to look for.
For the Higgs, they thought it would sometimes decay into two photons.
So they first examined only the high-energy events
that included two photons.
But there’s a problem here.
There are innumerable particle interactions
that can produce two random photons.
So how do you separate out the Higgs from everything else?
The answer is mass.
The information gathered by the detectors allows the scientists to go a step back
and determine the mass of whatever it was that produced two photons.
They put that mass value into a graph
and then repeat the process for all events with two photons.
The vast majority of these events are just random photon observations,
what scientists call background events.
But when a Higgs boson is produced and decays into two photons,
the mass always comes out to be the same.
Therefore, the tell-tale sign of the Higgs boson
would be a little bump sitting on top of the background.
It takes billions of observations before a bump like this can appear,
and it’s only considered a meaningful result
if that bump becomes significantly higher than the background.
In the case of the Higgs boson,
the scientists at the LHC announced their groundbreaking result
when there was only a one in 3 million chance
this bump could have appeared by a statistical fluke.
So back to the dark matter.
If the LHC’s proton beams have enough energy to produce it,
that’s probably an even rarer occurrence than the Higgs boson.
So it takes quadrillions of collisions combined with theoretical models
to even start to look.
That’s what the LHC is currently doing.
By generating a mountain of data,
we’re hoping to find more tiny bumps in graphs
that will provide evidence for yet unknown particles, like dark matter.
Or maybe what we’ll find won’t be dark matter,
but something else
that would reshape our understanding of how the universe works entirely.
That’s part of the fun at this point.
We have no idea what we’re going to find.
