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Collisions Matter

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The Ultimate Building Blocks of Matter

David Cassel

David Cassel

For more than 25 years CESR and the CLEO experiment have maintained the Laboratory for Elementary-Particle Physics (LEPP) at the forefront of the field of elementary-particle physics, the study of the ultimate building blocks of matter. With CESR, electrons and their antiparticles, positrons, collide at high energies in the CLEO detector to produce other elementary particles. Some of these elementary particles are fundamental particles called leptons (the electron is a lepton), while others—called mesons and baryons—are composed of yet more fundamental particles called quarks and antiquarks. Quarks and leptons are collectively the smallest particles known to physicists today and are the constituents of all observed matter.

Creation through Annihilation

When electrons and positrons collide, they can annihilate each other, producing an unstable state with energy equal to the total energy of the two particles before the collision. This state disappears extremely quickly and produces new particles— mostly, but not always, mesons. Many of these particles live long enough to travel uninterrupted through the CLEO detector, leaving a trail of signals that can be utilized to observe them and measure properties such as their directions and energies. Particles that decay in the detector produce longer-lived particles that can also be observed in the same manner. The signals are produced in about 125,000 individual detector elements; most of them are contained in a cylindrical volume that is about 10 feet in diameter and 11 feet long. Within this volume is a very strong magnetic field, about 2,400 times the earth’s magnetic field. Approximately 1,000 tons of iron—the CLEO detector is a 20-foot cube—shape and contain the field.

The six types of quarks that exist have been given fanciful names: up (u), down (d), strange (s), charm (c), bottom (b), and top (t). All except the t quark, which is too heavy, have been produced routinely by CESR. For their first 20 years, CESR and CLEO dominated the study of the b quark, which is heavier than all others except the t quark.


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