Progress in experimental high‐energy physics is limited in practice by two complementary aspects: the types of beam particles available with useful intensities and energies, and the characteristics of the detection techniques available for measuring needed information about collisions of interest and their subsequent reaction products. Most impressively, advances in accelerator design over the last three decades have led to an increase in beam energies of nearly three orders of magnitude, and the advent of colliding‐beam machines has brought a comparable increase to the center‐of‐mass energy available. The diversity of useful beam species has now grown to include essentially all known particles with lifetimes greater than 10−11seconds.

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