What would be the result of the collision of two down quarks? Even if we can't have single quarks in nature because of the charge colour, what would be the result of the collision of two down quarks at high velocities (0,99% c) at high energies, like the ones of LHC proton's collision..?
 A: 99% of the speed of light generates a Lorentz factor of only
$$ \gamma = \left[ 1 - (.99)^2 \right]^{-1/2} \approx 7 $$
which means that you have only about 14 times the mass of a down-quark to make additional particles. The PDG puts the bare mass of the down quark in the neighborhood of 5 MeV, so $14 \times 5\,\mathrm{MeV} = 70\,\mathrm{MeV}$ isn't enough energy to create any pair except an electron--positrons.
So, here are some possible outcomes (taking into account that you must be doing baryon collisions and just looking at cases where you have collisions between constituent down quarks


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*Elastic collision. Your baryons go in and come out with different moment but otherwise unchanged.

*Electron-positron pair creation, but the baryons still come out unchanged.

*(If you have at least on nucleus rather than a bare nucleon in the input state) nuclear excitation.


In other words: nothing much. That is a result of your specifying a very low energy regime.
Some things you don't have enough energy to do:


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*Muon--antimuon creation.

*Meson creation (not even a $\pi^0$).

*Nucleon excitation (assuming a nucleon or nuclear beam).

*Most meson excitations (assuming a meson beam for at least one of the quarks)



BTW--Particle physicists rarely talk about speed in this kind of context, because most speeds approach $c$ (often very closely). Instead we generally talk about energy and/or momentum. The exception when we do talk about speeds is when we are using Cerenkov detectors.
