There are lots of ways one can do particle physics research, but the most common way
is to accelerate particles to very high energy.
But then the question becomes “what do you do with them?”
I mean, you could make a beam and smash them into a stationary target.
In fact, we have a name for that.
Rather unimaginatively, we call it fixed target.
Alternatively, we could do something different.
We could take two counter rotating beams and collide them head on.
We call this collider mode.
These two distinct methods each have their place.
So I thought I’d make a video telling you the pros and cons of the two options.
When do we use one and when do we use the other?
So what are the two biggest parameters a scientist must consider while designing an experiment?
They are called energy and luminosity.
I made a video on those two topics, but briefly energy is related to the potential for an
object to do something.
In the world of the familiar, more energy comes with faster motion.
The damage done to a car that crashes at low speed is smaller than the damage done at high
speed.
In the world of particle physics, you have to take into account relativity and the fact
that things can’t go faster than light, but the basic idea is similar.
Energy is useful because of Einstein’s equation E = mc2.
C is just the speed of light, which is a constant, so that says that energy is equal to mass
times a constant.
Consequently, more energy means that you can make higher mass particles.
There’s a second reason why energy is useful.
You’ve probably heard that light has a wavelength.
The wavelength of red light is 700 nanometers, which is about a tenth of the size of a red
blood cell.
In contrast, deep blue, which is on the other end of the rainbow, has a wavelength of 400
nanometers.
It turns out that if you want to see something, you need to use light with a shorter wavelength
than the object being inspected.
Blue light has a shorter wavelength than red and so it can see smaller things.
That’s the reason that a Blu-ray disk can store more information or movies with higher
definition than a regular DVD.
It’s all about the wavelength of the light.
In 1924, French physicist Louis deBroglie postulated that all particles had a wave and
this has subsequently been proven to be true.
And this is why I bring it up.
The higher energy the particle, the shorter the wavelength.
So a higher energy accelerator allows you to look at smaller things.
One of the goals of modern particle accelerators is to see if quarks and leptons, which are
the building blocks of current theories, are composed of smaller objects still.
If so, that would mean we rewrite the textbooks.
So that’s why energy is important.
In contrast, luminosity is related to the number of particles undergoing collisions.
The reason that’s important is that more collisions means a better chance of seeing
something rare.
So if energy and luminosity are the important parameters, how do fixed target and collider
mode come into play?
Well, I’ll give you the punchline first.
Colliders give lots of energy and fixed target gives lots of collisions.
The lots of collisions is easier to see, so let’s talk about that first.
In a collider, you shoot a collection of particles through another collection of particles.
We call these collections “bunches.”
Now a bunch might have 10 to the 11th particles, which is the scientific way to say 100 billion.
Two bunches is two hundred billion particles, and some of those particles collide.
But if you shoot a bunch into a solid target, things are way different.
The number of particles in a bunch are the same, but the target is, well, solid.
And that means tons of particles.
A single cubic centimeter of water contains about a trillion trillion particles- that’s
10 to the 24th particles for my scientifically minded viewers.
And, once the bunch passes through one cubic centimeter, it can pass through more and more
and more.
With so many targets, the chances of a collision is way, way higher.
The energy advantage of a collider is a little harder to see.
But you can picture it if you imagine what happens when you shoot a watermelon.
The bullet will destroy the watermelon, but after the impact, all of the watermelon guts
will go flying off in the direction that the bullet was traveling.
That means that the energy of the moving bullet partially went into moving gobs of watermelon.
Now imagine two cars hitting head on.
If they’re the same size and have the same velocity, they will stop dead in their tracks.
Since they’re not moving after the collision, all of the energy can go into damaging the
cars and the occupants in them.
That’s why head on collisions are so dangerous.
And the difference is huge.
Naively, you’d think that a head on collision between two cars would do double the damage
of one of those same cars, traveling at the same speed, but hitting a wall instead, but
it’s not true.
As an example, you can consider the proton beam at the Large Hadron Collider.
At design energy, the beam will have 7 trillion electron volts of energy.
If two beams hit each other, the energy available to do research is a whopping 14 trillion electron
volts.
But if you took a proton from one of those beams and collided it with a stationary target,
the useful energy wouldn’t be 7 trillion electron volts.
It would only be 0.114 trillion electron volts, or less than 1 percent what you get when the
two beams collided head on.
With such a big difference, it’s clear that colliding is the way to go.
And there are ways to get higher luminosity by making the beam go in a circle and have
the bunches pass through one another again and again.
But steering the beam in a collider is much harder.
To give a sense of scale, as two bunches of protons enter one of the detectors to collide,
they have to be aimed with incredible precision.
Scaled up to more familiar sizes, it takes the same degree of accuracy as if you were
to take two ordinary sewing needles, separate them by ten kilometers (that’s six miles
for my American viewers), and shoot them towards one another and have them collide in the middle.
It’s pretty tough.
In contrast, in fixed target, it’s more like shooting a needle at a wall.
It’s kind of hard to miss.
And, if you’re a lousy shot, you just get a bigger wall.
So that’s the difference between fixed target and collider.
Fixed target is easier and can make lots of collisions.
Collider is way harder, but it’s the way to go if you want to make new and undiscovered
particles and look at smaller things.
Now actually doing these things is really hard and I’d like to give a shout out to
the technical staff that does the real work.
Without them, none of the discoveries made at CERN or Fermilab or any other particle
accelerator lab would be possible.
Our engineers, technicians and computer professionals totally rock.