I've been looking at a few papers in experimental physics (from the ATLAS collaboration, for example) and I've often run across phrases such as "high-$p_T$ electron." What exactly is $p_T$? Is it simply momentum, but with the component parallel to the main beam projected off?
Also, why is $p_T$, as opposed to $p$, an important characteristic of a particle in a collision process?
Answer
The component of momentum transverse (i.e. perpendicular) to the beam line.
It's importance arises because momentum along the beamline may just be left over from the beam particles, while the transverse momentum is always associated with whatever physics happened at the vertex.
That is, when two protons collide, they each come with three valence quarks and a indeterminate number of sea quarks and gluons. All of those that don't interact keep speeding down the pipe (modulo Fermi motion and final state interaction).
But the partons that react do so on average{*} at rest in the lab frame, and so will on average spray the resulting junk evenly in every direction. By looking at the transverse momentum you get a fairly clean sample of "stuff resulting from interacting partons" and not "stuff resulting from non-interacting partons".
There are also advantages related to the engineering of the detector.
{*} Only on average. Indivudual events may involve high Bjorken $x$ particles and be a long way from at rest in the lab frame.
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