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In collider experiments usually heavier and heavier mass resonances of particles are searched for. Examples: production of W and Z bosons at CERN $Sp\overline{p}S$ or production of the Higgs at LHC. When selecting candidate events for the production of these resonances, the most important "cut" is on transverse momentum. Only high transverse momentum events are selected and we talk about "high $p_t$ physics".

Question: is there a particular reason why these heavy resonances should decay into products with large transverse momentum (at least a few GeV)?

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In interactions between two particles, Lorenz invarianece ensures that the interactions look the same in p-transverse, in all frames of a moving center of mass system, and was the reason for the cut in linear accelerator experiments.

In colliders the lab frame is the center of mass frame, but still , the direction of the beams defines an angle where resonances with large mass will spread their decay products, whereas small mass and simple scatters, the majority, are close to the beam line. By making a p-transvers cut one ensures that the event chosen contains the decays of large massive resonances, if they exist, or jets.

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Well, in an $e^+e^-$ collider, where the center-of-mass frame is the lab frame, so an $s$-channel $Z$ is made at rest in the lab frame and can decay any which way. Scattering ($t$-channel) usually falls as some power of $t$, so high $p_T$ events are supressed.

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  • $\begingroup$ Than why do they introduce a cut on the transverse momentum? See here for example at page 10. $\endgroup$ Commented Nov 7, 2020 at 20:28

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