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Merit of different collision types

Posted: Sun Dec 19, 2010 9:26 pm
by amiso
- The actual LHC collides protons with protons.
- Its predecessor(LEP), used to collide electrons with protons.
- The Fermilab (tevatron) collides protons with antiprotons.
- The future generation or linear accelerators (LIC and CLIC) will collide electrons and positrons.

Can someone explain what are the advantages of the different types of collisions ?

Re: Merit of different collision types

Posted: Mon Dec 20, 2010 12:26 am
by JNW
Actually, LEP also used electron-positron collisions.

With electron-positron collisions the particles annihilate and nearly all the energy is available for the collision products. Also, the amount of energy is fairly consistent and known. And there is no special requirement imposed on the products, such as lepton number (an electron-electron collision would force the products to have a net lepton number of two). The downside is that electrons have a very small rest mass, so they emit a lot of synchrotron radiation in a circular collider. This gets much worse as the energy increases. LEP was near the limit of what's practical. For higher energies something linear like CLIC will be needed.

At higher energies the amount of energy lost in an electron-positron collision due to bremsstrahlung increases. (Bremsstrahlung is radiation emitted by an accelerating charge, and colliding electrons are certainly accelerating.) This energy loss is somewhat random, so the energy of the collision isn't known as precisely as it might be. Because of this, some people have suggested using muon-antimuon collisions. The muon suffers far less from bremsstrahlung because its rest mass is much greater than the electron. The muons have to be manufactured, however, and they are short lived, so they have to be made and collided very quickly.

Protons are heavier, so they don't have much problem with synchrotron radiation. Thus they can be accelerated in a circular collider to much higher energies. But protons are composite particles made out of valence quarks, gluons, sea particles, etc. So a proton-proton collision is really a collision between a particle in one proton and a particle in another proton. That means the energy of the actual collision is much smaller than the energy of the protons. The energy is also not known, as the amount of momentum carried by each part of a proton is rather random.

Proton-antiproton collisions can be better because sometimes valence quarks will annihilate. This requires the extra step of manufacturing the antiprotons, though. Also, it's much easier to use a proton-proton machine for ions, as ions and protons are both positively charged.

Some people have suggested colliding magnetic monopoles. The biggest problem is that monopoles haven't been discovered yet. They would also have to be manufactured. Once made, however, they would be stable and could be used in a circular accelerator. All their energy, plus their rest mass, would be available in the collision (they're fundamental particles, not composite). And best of all, they have a charge that's much stronger than an electron, so they could be accelerated to a couple of orders of magnitude higher energy than protons for a given machine size. If the MoEDAL experiment at LHC actually discovers monopoles, we will hear more about this idea.

Re: Merit of different collision types

Posted: Thu Dec 23, 2010 12:35 pm
by amiso

Thank you for the answer.

I read somewhere that the synchroton radiation is inversely proportional to m^4, so the linear accelerators would be apealing to those who want to accelerate electrons or positrons.

The anti- and short-lived- particles were not very atractive for a high luminosity machine such as the LHC, I guess.

You mention that the "simple" elementary particle colisions such as the electron and positron would be "cleaner" and in a sense better understood. I have a very naive question about this:

Is it possible that collisions between two very elementary particle such a the electron and positron, would be too "simple" and would not yeld any interresting (new) particles such as the (in-) famous Higgs?

Thanks any way.

Re: Merit of different collision types

Posted: Sat Dec 25, 2010 3:58 am
by JNW
amiso wrote: Is it possible that collisions between two very elementary particle such a the electron and positron, would be too "simple" and would not yeld any interresting (new) particles such as the (in-) famous Higgs?
When an electron and positron annihilate, they can produce a virtual photon or virtual Z0 (which is sort of a heavy photon). A virtual photon can then split into any charged particle antiparticle pair. A virtual Z0 can create pairs of particles that interact through the weak force (such as a neutrino anti-neutrino pair). The Z0 can also interact with the higgs (which is why the Z0 is so massive). So electron positron collisions are great for making higgs bosons. (Assuming there's enough energy to make them, and that they exist.)

The virtual photon or Z0 can also split into quark anti-quark pairs (as quarks have charge). These quarks can then undergo further interactions that produce other particles. So an electron positron collider has access to the same zoo of particles as a proton collider, although the probability of getting any particular particle will be different.

However, a particle that interacts only through gravity can't be produced in a collider.