PARTICLE PHYSICS 101
What Actually Happens in a Particle Collision?
A Short, Easy to Understand Introduction to a Collision at the LHC.
Have you ever wondered what actually happens inside a particle collider?
Of course you have!
And I bet you’ve Googled it and found hundreds of hits about some sort of man-made Black Hole?
Or maybe you believe that the Illuminati are using it to create anti-matter, despite Tom Hank’s best efforts… (See Angels and Demons for that reference).
Although this would be an extremely cool (albeit very dangerous), Doctor Who-esque thing to be true. The truth is that what actually happens is… We don’t really know… Not completely anyway!
Let me explain…
Let’s imagine we had two marbles to collide together.
We can then imagine having an experimental setup that was accurate enough to ensure we had the exact same collision each time the experiment was run. By this I mean that the marbles would hit each other at the exact same spot with the exact same force each time we ran the experiment.
So we have a complete knowledge of how this experiment will play out… Great!
The Quantum Realm of… Marbles?
Now let’s reduce the size of these marbles to the size of two protons.
At this level, things get weird and we cannot rely on our normal intuitive so called “Classical” Mechanics. We must turn a page and imagine a whole different world named “Quantum” Mechanics.
We’re not going to go through any formal Quantum Mechanics here. But for the purposes of this article I’m just going to say that every result we predict from Quantum Mechanics that we are able to measure experimentally is a probability.
Assuming you haven’t lost yours yet, let’s get back to our marbles for a brief moment…
In our experiment, we were able to say with absolute certainty what was going to happen. Now imagine if these marbles were randomly vibrating as they were rolling along and thus causing fluctuations in our experiment.
This would mean that each time we ran the experiment, the marbles may hit each other at a slightly different point. And in general, we may have a slightly different outcome each time.
Right then… One more thing to mention before we lose the marbles completely, is probability.
So we have some experiment where it seems we have a random outcome each time. BUT… let’s run this Quantum Marble experiment thousands of times.
You have built up a data set with all your results from these experiments. Now imagine you start to see a larger number of experimental outcomes favouring the marbles hitting and recoiling straight back from one another.
This would mean that if we ran the experiment one more time, there would be a higher probability of us seeing the marbles hit and recoiling along the same line.
Although we couldn’t be certain about this, due to the random vibrations.
This is important as it also tells us that if we hadn’t run the experiment thousands of times, we wouldn’t have noticed this behaviour.
Oh Particles, where art thou?
OK… If you have followed those thought experiments, you should be set up to understand some Particle Physics!
Now is the time to lose those marbles…
So what do I mean when I say “We don’t really know what is happening in a Particle collision”?
What I mean is, if you had two protons ready to collide together, we couldn’t predict the outcome with absolute certainty (as with our Quantum Marbles). We would have to say, there is a certain amount of probability that “this” or “that” would happen.
Then given a very large number of proton-proton collisions, we can make accurate predictions (again, as we did with the Quantum Marbles).
OK, OK, so particles vibrate?
Not exactly… This was just a way of introducing the unpredictable nature of protons in a collider.
We actually call this unpredictable nature “non-perturbative” Physics.
More specifically, the theory that deals with the proton is called Quantum Chromodynamics (QCD) and so we actually call this “non-perturbative QCD”.
Don’t worry about this terminology too much, but feel free to use it if you want to sound clever to that friend that thinks they know everything…
The analogy of the marbles “vibrating” was introducing us to not knowing the exact spot on the marbles where the collision would occur. This is similar with the proton, as we do not know what part of the proton will actually be involved in the collision.
OK, so I’m going to try my best to explain this as simply as possible but this is a really complex subject (graduate level Theoretical Particle Physics).
Protons are not fundamental particles. This means they are made up of even smaller particles grouped together in one tiny blob of matter.
These even smaller particles are called quarks… which you may have heard of, but if not then don’t worry!
On the very basic level, the proton is made up of three quarks as in the picture above (this is not strictly accurate, but we will elaborate on this later).
Another thought experiment…
You know those space videos where there’s balls of water just floating in zero gravity? If you don’t then give this a Google.
I want you to imagine two balls of water floating in space with three ice cubes floating within each ball of water.
Now in a minute we’re going to throw these at each other, but I want to set up the experiment first… So let’s contain our excitement for a second!
The ball of water = The proton
The ice cubes = The quarks inside the proton
OK, go ahead and throw these at each other.
Now for this first experiment we’re going to ignore any collision that is water-water and focus only on any ice cube-ice cube collision.
If you do this, there is actually a lot of times where no ice cubes collide. This is very similar to the LHC where sometimes protons seemingly pass right through each other.
But, every now and then you’re going to get two ice-cubes colliding… which is where things get interesting!
OK so back to our three quark picture of the proton… Two of these are one type of quark (the up quark) and one of them is a different type (the down quark). Do not worry about the difference between these, just remember that they are different.
So let’s imagine these ice cubes are secretly labeled “up” and “down”. Where we have two “up’s” and one “down”.
We can now say that when these do collide, we expect to see more “up” labelled ice cubes involved in these collisions than “down” ones.
If you think about this, this scenario is similar to our quantum marble situation earlier. We cannot predict exactly which ice cubes will collide, only that after running the experiment a large number of times, there is a higher probability of the “up” ice cubes being involved than the “down” ice cubes (since there are more of them available).
Finally, coming back to our particle collision, we have varying probabilities of the type of collision we will see depending on the composition of the protons.
So in the image to the side, we are more likely to have an up quark colliding with an up quark than a down quark colliding with a down quark. But this kind of statement is all we can really say!
Can we not just look inside?
The simple answer is no, unfortunately. We can’t actually see inside the boundaries of the proton.
The reason for this is that quarks simply cannot exist outside of the environment of a particle. So much so that if they find themselves too far away from the other quarks in a proton, they break off and out of thin air spawn the existence of two other quarks to form their own particle (or find another lonely quark that is too far from its friends).
Once quarks are involved in a particle collision, this essentially means the protons are completely obliterated. Meaning everything inside these protons goes flying all over the place. So if you are a quark in this situation, you’re in trouble.
You’re thinking “OMG where the **** are my quark mates?”, or something like that.
In this situation, these quarks seemingly randomly form new particles by converting some of the energy from the collision into new quarks and/or gathering together safely into, for example, a new proton.
This makes it extremely difficult for us to reconstruct what actually happened at the point of collision and which quarks were involved, since they have formed new particles and along the way produced a host of other products (which are all the lines you can see in the title image of this article).
What about the water?
This is an excellent question, and is one of the main reasons why Particle Physics is such a complicated subject.
Before I mentioned that the “three quark” picture of the proton is not strictly true. This is essentially because there are loads of other particles floating around in there too. These “other” particles form what we call the “sea” and are constantly interacting with each other (see below).
The sea actually includes a different sort of particle called a gluon (named this way because it is essentially responsible for gluing the proton together), you can think of this in the same way that you think of quarks at the moment but if you go into this any further then you should know they have very different properties. In the picture above (and almost everywhere) the gluon is shown by the spirally lines.
If you have any experience with Chemistry you’ll know that the valence electrons of an element give it it’s properties. This is the similar with protons, the three quarks we had previously determine the protons properties and are called the “valence quarks”. These are denoted by the large spheres in the image above, whereas the sea quarks are the smaller spheres.
So, we have the valence quarks and the sea, which are composed of quarks and gluons. As a collective term we call quarks and gluons, the partons. And the probability functions that predict their abundance inside the proton are called the Parton Distribution Functions (PDFs).
For this article you can imagine that the valence quarks are the most abundant quarks in the proton and this is why we only considered them in our earlier experimental picture.
However this again is not completely correct so maybe I will tackle this in a further article… watch this space!
So… What Actually Happens?
The question we may be able to shed some light on finally.
Step by step what happens is:
- Two protons are collided.
- A parton from each proton interacts in a super complicated way.
- This produces loads of extra material inside the collider (seen by all the lines showing the particle tracks).
- Physicists use theoretical predictions to attempt to explain the experimental data seen (after lots and lots of experiments have been run (Remember our Quantum Marbles!)).
- Any deviations from the theory could be a sign of New Physics!
OK, we made it! Hopefully you have a little bit more of an idea as to what goes on inside a Particle Collider now. But if you don’t or if you have any suggestions/questions or comments then please let me know!
