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.
