Conservation of momentum.

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P = m x v

P = momentum

m= mass

v = velocity

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P t = P 1 x P 2

Total momentum = Momentum 1 X Momentum 2

Total momentum = ( mass x velocity of the first object ) x ( mass x velocity of the second object )

It doubles. Momentum (p) is the product of velocity (v) and mass (m). For a given mass, if you double the velocity, you'll double the momentum. Velocity and momentum are said to be directly proportional. p = m x v

Simply put, four-momentum transfer is the special relativistic spacetime analog of classical (three-) momentum transfer. In classical physics, two bodies can interact and exchange momentum in three spacial dimensions. In particle physics, strictly spatial momentum vectors do not suffice. Instead we use four-momentum, a Lorentz vector.

Four-momentum transfer is often referred to as Q^2 is particle physics literature. An interaction that transfer a large amount of four-momentum is a high Q^2 interaction.