# Why proton?

1. ### Nick.M

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Hi
why scientists picked up proton for LHC?

### Staff: Mentor

It's charged which means it can be accelerated across an electric potential and deflected by a magnetic field, unlike a neutron which is neutral.

It is a baryon, as opposed to a lepton.

It is easier to get it up to TeV than say a deuteron, He-3, He-4, . . . nuclei. There are experiments with accelerated nuclei.

It's relatively simple - a single particle. Multinucleon nuclei would produce many different types of reactions, and comparatively a proton-proton collision is pretty clean.

3. ### humanino

I expect accelerating protons to be easier than accelerating antiprotons, but I don't really know the technical reasons. Is it because of the vacuum requirements, or is it because a "clean beam" is more difficult ? And what about the physics : is there a difference between proton-proton and proton-antiproton collision above the TeV ?

4. ### Vanadium 50

17,442
Staff Emeritus
Accelerating antiprotons is no harder than accelerating protons (it's actually incrementally easier). The issue is that you have to make the antiprotons: the Tevatron luminosity is already limited by antiproton production. This shortage of pbars would be even more acute at the LHC.

### Staff: Mentor

I believe accelerating anti-protons is the same as protons, however there is ready access to protons, but anti-protons have to be made by proton-proton collisions then separated from the protons.

I'm not up on the high energy anti-proton/proton interactions, but I would imagine they are different than proton-proton.
I think the number of charged particles coming of an proton-antiproton interaction is greater than for a proton-proton interacting at the same energies - http://www.phys.ufl.edu/~rfield/cdf/chgjet/chgjet_intro.html

Some interesting papers here - http://hep-www.px.tsukuba.ac.jp/research/thesis_d.html

The single proton has the highest charge to mass ratio when compared to composite particles of nucleon, i.e. deuteron, triton and atomic nuclei of the other elements.

This might be of interest - http://www-bd.fnal.gov/public/antiproton.html

and this - Search for Electroweak Single Top Quark Production
http://www-cdf.fnal.gov/thesis/cdf8013_thesis_berndstelzer.pdf

6. ### humanino

Is accelerating antiproton incrementally easier because of free electrons in the cavities, or for some other reasons ? For the remainder of the technical difficulties, I gather that only the production is more challenging, especially if you are interested in luminosity.

The links above indeed state that at a few TeV, there is no more difference in $pp$ and $p\bar{p}$. I would be extremely interested in the very rare events where the 3 quarks annihilate each other. But independantly of whether BH will be produced, this (say for instance) $p\bar{p}\rightarrow\gamma\gamma$ would be impossible to see, and probably dominated by FFs anyways...

7. ### Vanadium 50

17,442
Staff Emeritus
It's because the emittance - essentially, the size of the beam - is smaller for antiprotons than for protons. Antiprotons sit in an accumulator for ~24 hours, continually being cooled, but protons come out of a bottle. So you have better initial beam quality for the antiprotons, and that is (like I said, incrementally) helpful.

8. ### humanino

Ah, I see what you meant, thank you for the precision. However, nothing in principle (except cost of course) prevents you from doing the same with protons !

9. ### Vanadium 50

17,442
Staff Emeritus
That's right, it doesn't have to be that way. It just is.

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