A proton is a hadron composed of two up quarks and one down; a neutron consists of two downs and one up.) Fermions also include neutrinos, which, somewhat unnervingly, stream through our bodies at the rate of trillions per second.
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The L.H.C., Doser explained, relies on much the same design, and, in fact, makes use of the tunnel originally dug for LEP. Instead of electrons and positrons, however, the L.H.C. will send two beams of protons circling in opposite directions. Protons are a good deal more massive than electrons—roughly eighteen hundred times more—which means they can carry more energy. For this reason, they are also much harder to manage.

“Basically, what you must have to accelerate any charged particles is a very strong electric field,” Doser said. “And the longer you apply it the more energy you can give them. In principle, what you’d want is an infinitely long linear structure, in which particles just keep getting pushed faster and faster. Now, because you can’t build an infinitely long accelerator, you build a circular accelerator.” Every time a proton makes a circuit around the L.H.C. tunnel, it will receive electromagnetic nudges to make it go faster until, eventually, it is travelling at 99.9999991 per cent of the speed of light. “It gets to a hair below the speed of light very rapidly, and the rest of the time is just trying to sliver down this hair.” At this pace, a proton completes eleven thousand two hundred and forty-five circuits in a single second.