Why LHC use proton-proton collide instead of proton-antiproton collide?

[magnetar]

Main Question or Discussion Point

Why LHC use proton-proton collide instead of proton-antiproton(like,Tevatron) collide?

 

[malawi_glenn]

more difficult; producing antiprotons and having high luminoisty with them.

 

[magnetar]

more difficult; producing antiprotons and having high luminoisty with them.

What is the advantages use proton-antiproton than proton-proton collision?

 

[malawi_glenn]

you only need one magnet ring, instead of two since the antiproton has opposite charge.

for some processes, the reaction rate is higher for p-bar + p than p+p, but only at "low" energies such as 1-3TeV (which is energy for the Tevatron), but for higher energies, such as 10 (LHC range), this advantage dissapears.

So the advantages is that you only need one set up of magnet rings, and that at lower E, reaction rate is higher for some processes.

Disadvantages is that producing and facilate antiprotons is very diffcult, and that the higher reaction rate is reduced when increasing the energy.

 

[Enthalpy]

Pardon?

Colliding particles circulate in opposite directions. In the same field, identical protons in opposite directions wouldn't stay on the same ring. They need opposite fields.

 

[malawi_glenn]

yes, thats what I said right?

in p-par + p you need only one set of ring, since they will travel in opposite direction due to opposite charge, wheras in p+p collider you need two sets of magnets.


can you point me where I wrote something else?

 

[fermi]

you only need one magnet ring, instead of two since the antiproton has opposite charge.

for some processes, the reaction rate is higher for p-bar + p than p+p, but only at "low" energies such as 1-3TeV (which is energy for the Tevatron), but for higher energies, such as 10 (LHC range), this advantage dissapears.

So the advantages is that you only need one set up of magnet rings, and that at lower E, reaction rate is higher for some processes.

Disadvantages is that producing and facilate antiprotons is very diffcult, and that the higher reaction rate is reduced when increasing the energy.

Yes, all of that is true but you left out the most important advantage of the proton-antiproton colliders. That is the ease of identification and detection of some of the most interesting events. If there are any vector particles to be discovered in the mass range of 2 to 12 TeV, the proton anti-proton collider can produce a clean signal of a particle decaying near at rest, while the proton-proton collider produces the same signal along with 100 tons of other garbage that you have to sort through very very carefully. On one hand the higher luminosity helps with the proton-proton colliders, on the other, the signal to noise ratio at the detectors hurts you. Overall, you win for some processes (probably most I must admit), but you lose for others. Worst of all, for a few select (but important) processes you lose extremely badly (by more than an order of magnitude.)

I consider a proton-antiproton machine a bit of a gamble. Such a machine is more likely not to match the usefulness of a proton-proton collider, but it has a chance to be far more spectacular. I guess CERN played it safe!

 

[Vanadium 50]

If there are any vector particles to be discovered in the mass range of 2 to 12 TeV, the proton anti-proton collider can produce a clean signal of a particle decaying near at rest, while the proton-proton collider produces the same signal along with 100 tons of other garbage that you have to sort through very very carefully.

Are you sure that you mean proton-antiproton and not electron-positron? This is usually given as an argument in favor of e+e-.

 

[malawi_glenn]

Why should there be less "junk" in a p-bar + p collider? You have the same problem there with parton distribution functions... as Vandanium pointed out, you must be talking about e+e- collider...

 

[blechman]

There is another vital reason why LHC is pp and not p-pbar - for TeV-scale physics, the dominant partonic process that would occur in ANY hadronic machine running at 14 TeV CoM energy is gluon fusion. This is quite different than the Tevatron, running only at 2 TeV where the dominant partonic process is quark-antiquark fusion.

So for the Tevatron, you really need p-pbar, otherwise the dominant interactions for TeV-scale physics involve sea-quarks and are pdf-suppressed. However, for the LHC, there is no reason whatsoever to make antiprotons - you don't gain anything (gluon pdfs are the same in both p and pbar), and you loose out from the complications (both in luminosity and cash!) of making antiprotons.

 

[malawi_glenn]

That is the explanation for what I wrote in post #4, that you don't gain anything by using pbar+p at these high energies.

 

[Vanadium 50]

However, for the LHC, there is no reason whatsoever to make antiprotons - you don't gain anything (gluon pdfs are the same in both p and pbar), and you loose out from the complications (both in luminosity and cash!) of making antiprotons.

I disagree with "no reason whatsoever". For very heavy new physics - say a Z' at 10 or 12 TeV - one does better with pbar-p. However, for the vast majority of things we want to study, like you say, there is no physics advantage and a luminosity penalty to go to pbar-p. So it's not difficult to see why this particular design choice was made.

 

[blechman]

malowi_glenn: sorry, you're right. You did say this, I just missed it. Very good.

vanadium_50: I stand corrected. But a Z' at 10 TeV will almost certainly never be seen, at least directly, since you lose out on luminosity by going p-pbar (that's why you would never see a TeV particle at the Tevatron!). At best you might be able to get up to 4 or maybe even 5 TeV, but that's all. At that point, you might as well stick with the pp - it's cheaper, easier, ... If that's where the new physics is, then we're screwed!

Of course, all estimates put the new physics at 1 TeV, so we're probably okay.

 

[malawi_glenn]

nono I meant it was good that you wrote in detail "why", just wanted to stress that so that the OP get the whole picture.