When two protons collide, why does one up quark switch to a down quark?

It just depends upon what kinds of particles result as the debris of the collision of two protons... it's just chance. Overall, the particles that result from a collision of two protons must add up to the same as what the two protons had in terms of spin, charge, and mass. It doesn't matter if the material of the protons rearrange into different combinations of quarks, as long as the sum-total charge, spins, and mass are the same.

The spin of a top (up) quark is -1/2 and it's charge is +2/3 The spin of a down (bottom) quark is -1/2 and it's charge is -1/3 That just means that in somewhere else in the debris will be a particle (not a quark... a hadron or lepton of some sort) with a charge of +1.

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