If you’re the kind of person who wants to
figure out the ultimate rules of the universe, the first thing you need to do is to figure
out what tool you can use. Now some people use telescopes, while others
locate their detectors a kilometer or two underground. But, I’m a particle physicist, which means
that I usually choose to employ a particle accelerator. But the word “particle accelerator” is
pretty vague. There are lots of considerations. Do you collide two beams together? Or run one beam into a stationary target? Do you care about collision energy? Or number of collisions per second? Those are important questions and I’ve made
videos that address both of those questions. But another question is “even if you have
decided that you want to collide beams together, what particles do you use to make your beams?” There are many options. You could collide two beams of electrons or
two beams of protons. You could collide an electron beam with an
antimatter electron beam, or a proton with an antiproton one. You could even collide bare nuclei of atoms
together. Each of these choices makes sense, depending
on what questions you want to answer. Colliding two beams of electrons is something
you would do only in the most specialized of cases, so we can ignore that possibility. There are two main issues we need to think
about. First we need to understand the difference
between an electron and a proton. And, just to be clear, I don’t care about
matter versus antimatter for that question. The second question is whether you want two
matter beams or a matter and an antimatter one. So let’s talk about the nature of an electron
versus the nature of a proton. An electron is a point-like particle that
doesn’t have any structure or components or anything inside of it. That means when you give energy to an electron,
you know exactly the energy the electron has. In contrast, protons are messy beasts. In the simplest of models, protons contain
three quarks, but the reality is more complicated. Protons not only carry three quarks, but they
also contain gluons, which are the particles that govern the strong force. In addition, gluons can temporarily convert
into pairs of quarks and antiquarks. And the components of the proton are constantly
changing, with quarks and antiquarks being created and destroyed and gluons being emitted
and absorbed. If we had a super fast camera, we could take
pictures of what a proton looks like at any particular time. Here is when it has just three quarks. Here is another time when it contains quarks
and gluons. And here is another time when it contains
quarks, gluons and antimatter quarks. And that’s just the sad truth of protons. They’re constantly changing what’s inside
them. In very high energy collisions, when you collide
two protons, the collision doesn’t occur between the protons themselves, but rather
from the building blocks of the protons. Basically, one quark or antiquark or gluon
from one proton hits a quark or antiquark or gluon from the other proton. But because the constituents of the protons
are constantly in flux, you can’t know in advance what any particular collision will
entail. Further, since the constituents can swap energy
back and forth, you can never know in advance the energy involved in the collision. That all might sound kind of confusing, so
let’s hang some numbers on it to help make it clearer. Suppose you repeatedly collide pairs of protons
head on and, further, each proton has exactly 100 units of energy. Given that, what sorts of collisions can you
expect? Well the first collision might involve a gluon
with 3 units of energy from one proton colliding with a quark with 40 units of energy. This collision has a combined energy of 43
units. But there are other possibilities. You might collide a quark with 22 units of
energy with another quark with 16 units of energy, which adds to 38 units. The third collision might be between two gluons,
one with 17 units of energy and another with 21 units of energy, which is also 38 units
of energy, but with different particles involved. Each collision involves a randomly selected
pair of particles with a randomly selected amount of energy. It’s really quite a mess. Every collision is unique and you can’t
know in advance what any particular collision will be. Combine that with the collision rate at a
modern particle accelerator like the LHC, which is about a billion collisions per second,
and you have a whole ton of confusion. Contrast this with collider that has an electron
and antimatter electron beams. By the way, the name for antimatter electron
is “positron.” The first collision involves an electron and
positron with 200 units of energy. The second collision involves an electron
and positron with 200 units of energy. The third collision involves an electron and
positron with 200 units of energy. The fourth collision- well, I bet you’ve
figured out the pattern. Physicists have called an electron/positron
collider a scalpel, while a proton/proton collider equivalent to colliding two garbage
cans. So why would anyone every make a proton collider? There are two reasons. The first is that the complicated mix of collisions
in a proton/proton collider is not only a curse, it is also a blessing. By colliding protons, you can explore a vast
range of possible collisions. You can look at high energy collisions and
low energy collisions. You can look at the collisions of all sorts
of combinations of particles. A proton/proton collider lets you explore
a lot of configurations more or less for free. In contrast, an electron/positron collider
does one thing and one thing well. On the other hand, it has limitations. If you have an accelerator that provides electron/positron
collisions at 200 units of energy, you’d completely miss seeing some cool physics that
happens at an energy of 149 units. You’d just never see it. For that reason, proton colliders are usually
thought of as discovery machines, while electron-positron machines are used for precise measurements. Historically, the CERN SPS, S-p-pbar-S, the
Fermilab Tevatron and the CERN LHC are or were all proton or proton/antiproton colliders
and made discoveries, while the CERN LEP accelerator was an electron/positron collider tuned to
exactly the energy to make Z bosons. The LEP accelerator allowed scientists to
make incredibly precise measurements of Z bosons. Now, if you were listening carefully, I said
that the LEP accelerator collided electrons and positrons. And I didn’t say, but it’s true, that
the S-p-pbar-S and the Tevatron collided protons and antimatter protons. Why would you use an antimatter beam? The reason we make that choice is tied up
in Einstein’s equation E = mc2. While many understand that equation to say
that mass and energy can be converted to one another, the reality is a bit more complex. What really happens is that matter and antimatter
can come together and annihilate and make energy. Thus, the annihilation energy of an electron
and positron in the LEP accelerator is what made it possible to routinely make Z bosons. For the Fermilab Tevatron, which collided
protons and antimatter protons, the real advantage was that antiprotons are far more likely to
have high energy antiquarks. Thus quarks and antiquarks can merge and make
very high energy particles. That’s a recipe for a discovery. Now, as it happens, the CERN LHC is a proton/proton
collider. No antiprotons are involved. So why would the world’s premier particle
accelerator not use an antiproton beam, when I just said that’s the easiest way to make
the highest energy collisions? Well, it’s because the LHC was built for
many reasons, one of which was to find Higgs bosons. Higgs bosons are created by a complicated
merging of gluons and ordinary protons have tons of gluons. So there was no need to go to the effort of
making antiprotons. And making antiprotons is hard. By deciding to use two proton beams, the LHC
is able to generate collisions at rates that are a hundred times higher than the Fermilab
Tevatron could. And, as I’ve said in the video that compares
luminosity and energy, more collisions, means more likelihood that there will be the discovery
of a rare object. So that gives you the basics. But I think the takeaway message is the following. Colliders with protons or antiprotons are
for discovery, while colliders with electrons and/or positrons are for precision. And remember that you need both to understand
the secrets of the universe.
