Two basic methods are used for experiments in particle physics: Two high energetic particle beams in opposite directions are collided with one another head-on; or a single particle beam is fired at a stationary target.
But there is still a third possibility: Two particle beams moving in the same direction. Between the highly excited particles will take place various interactions.
I'm not the experimental physicist so will not give an answer on the practical realization of such experiment. Still I'm quite sceptical on the technical realization of two very near beams moving in the same direction. After all if you are interested in such collimation it is much more reasonable to use ... a single beam.
But let's look at the problem from a little more general perspective. Why you collide particles at all ? You do this to analyse the particles' inner structure and interaction mechanisms (which at quantum level also has a particle interpretation). Now, the smaller distances you want to probe and the finer particle properties you want to study the larger relative momenta must be used (the shorter de Broglie waves).
Given the short distance nature of the particle interactions (apart from e-m interactions) you must guarantee that particles collide with very small collision parameters (otherwise nothing happens).
So what really matters (apart from good collimation which guaranties that interactions are frequent enough to collect sufficient statistics) is to set up the experiment in such a way as to operate with relative momenta adjusted to the length scale we want to probe.
Now return to your two-beams-in-the-same-direction example. If the momenta of particles in two beams are more-less the same then their relative momenta are small - which is equivalent to low energy scattering, be it on the fixed target (there were such experiments, see. eg. LEAR @ CERN) or with two colliding low energy beams.
If two beams have different energies then the particles' relative momenta are large but this is equivalent to high energy colliding or fixed target scattering.
In my opinion there are no new physical insights from such experiment apart from those already present in fixed target and colliding beam experiments.
As Łukasz Bibrzycki said I can't think of something useful I can use two collinear beams for that a single beam can't do because to have interactions taking place you have to get them close enough which make them look like a single beam anyway.
Thank you for the answers!
The technical problems are smaller than by other experiments. In my opinion there are some interesting insights to interact two particle baems moving in the same direction. In this case you can especially investigate the interactions between high energetic particles. If particles are colliding the structures are destroyed and the particle-energy is transfomed in extra particles and structures. (So you can see sometime the particles whichever the theory has invented.)
Thank you for the answer, Christian.
One particle beam, e.g. protons emit many other particles (Bremsstrahlung), you know.
The crux of the question is: positive and negative elementary charge carriers interacting over a longer period. The reaction products can be separated or fractionated more easily and they are not distorted or destroyed by other interactions.
"The energy of particles must be available in the center of mass frame" - this is the unanimous opinion since the first accelerators were constructed. Sorry, I name this as ton ideology in the particle physics.
It's highly incredibly that the fusions in fixed stars only started if two or more particles are colliding in the center of mass frame.
Probably, you will find an interesting aspect, related to your question, looking at the literature on the Bose-Einstein correlations (starting with Wikipedia). These correlations present a quantum-mechanical ``interaction" (coherence) between identical particles moving in the same direction.
Good luck.
Thanks for the answers.
Bremsstrahlung is not only electromagnetic radiation, it's also particles. The most energy is at last Bremsstrahlung at the LHC.
Vassili, I will do it. But "Quantum mechanics" is a notion I cannot support unreserved.
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