What is high-energy physics?
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High-energy physics deals with high energies. We can achieve the highest energies in particle accelerators.
The sole purpose of heavy ion collision experiments for example is creating high energy environments.
Why?
Think of a universe thats very cold. So call all the water is ice. Liquid water cant exist naturally. So people in that universe decide to collide little ice crystals at very high speeds. The energy of the collision melts the ice and turns it into a liquid for a small amount of time. You cant directly measure that but we know that the water as it expands cools down and tiny ice crystals form that we measure and deduce the properties of liquid water.
Now replaced ice with atoms and liquid water with quark gluon plasma (QGP). The analogy is good because QGP behaves like a fluid. The energy of a collision is so high that the hadrons (particles made of quarks) melt into that QGP, that fluid will have some idiotic shape, it expends and cools. After it cools enough hadrons start to form this is called freeze out.
These particles then fly away through our detectors, identifying the particles and their paths, momenta… allows us to reconstruct the properties of the QGP. Of course this QGP forms on scale of femtometers.
Why we would want to do this is simple. This is basically an environment really colse to the big bang. Looking into whats going on at high energies allows us to experiment with near big bang conditions. This leads to better or at least verified models and the hopes are that we will find new physics that will help us understand nature more fundamentally.
How high energies we need, or better yet energy density? Well infinite basically but realistically its hard to say whether a bigger accelerator will bring new results. The general opinion is that theoretically we did everything we could think of so right now we need some new results to kink us down the right path.
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