Sunday, December 23, 2007

Why a proton-proton vs. electron-positron collider

The LHC will be a proton-proton. The next proposed collider,the ILC , will be an electron-positron collider. The natural question to ask is: why would you want to make one collider verses the other?

Short Answer
  1. Proton-Proton for discovering new physics.
  2. Electron-Positron for precisely measuring known physics.
Slightly longer answer:

A proton has three fundamental particles that make it up: three quarks. The quarks are the objects that do the interacting. Lets say you smash two protons together at 14 TeV as will be done at the LHC. The 14 TeV will be distributed among the 6 interacting quarks in completely random ways.

So lets say you want to find a Higgs and you know there is a large energy range where it could show up. Proton-Proton colliders are great since the 6 particles divide the 14 Tev up randomly so that after millions of interactions a wide range of energies are explored. You will have some low-energy, mid-energy and high energy interactions continually taking place and if the Higgs is in that wide energy range you will hopefully detect it.

On the other hand, lets say you spot the Higgs at about 115 GeV. Then proton-proton won't help you learn precisely about it because you can't force the random interactions to always be at 115 Gev. Furthermore, doing 6 particle interaction physics is messy and gives lots of uncertainty.

Now electron-positron colliders come to the rescue. There are only 2 particles interacting and you can give them the exact energy you are wanting to measure. Additionally, two particle interactions have a lot less uncertainty.

So again, lets take the 115 GeV Higgs. We can set the electron-positron collider on precisely 115 GeV and let it run until we have very precise measurements on the Higgs.

However, discovering the Higgs when you don't know its energy scale is hard with a electron-positron collider since you don't get a wide range of random energies represented in each reaction. So in short we need both colliders to do good physics.


  1. I will show my ignorance of particle physics and ask a question. --- What is it that makes the energy distribution among the quarks random? I have an idea of what the answer is, but I want to ask the question because it leads to another question. --- At high energies would there necessarily be a random distribution of energy or could it be possible that it would not be random? If so, what would have to be the case (i.e. what kind of physics would we be looking at) for it not to be a random distribution?

  2. Good question. To be honest I don't know know a lot of the technical details, except that I believe it is as random as any other statistical mechanics problem. Ie, still follows a distribution.

    As I understand, the proton moves through space and all kinds of crazy things are going on. What makes it seem random is the quarks are passing the energy back and forth.

    Think of tying a bunch of marbles together with a rubber band and throw them. As they move through space they will pass energy back and forth to each other. At least that is how I understand it.

    I said rubber band because they tug on each other harder the further away they are. The strong force is weird like that. After they have escaped a certain distance the strong force is so strong that quarks literally pull virtual quarks out of the vacuum and sail off in their new partnerships. This is why there are no single quarks caught.

    Perhaps at some scale it is no longer random, I don't know.


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