Making pions with lasers

Of the 140 or so two-quark composites known as mesons, pions are the lightest. First discovered from tracks left by cosmic rays in photographic emulsions, pions are brought fleetingly to life whenever high-energy protons slam into other particles. Besides Earth’s atmosphere, pions are also produced in particle accelerators, supernova explosions, and the interstellar medium. Now an international team led by Karl Krushelnick of the University of Michigan has demonstrated a new way to make pions: with short, intense pulses of laser light. The method depends on laser wakefield acceleration (LWFA). When a plasma is hit with a laser pulse, the electrons respond far faster than the sluggish, heavier ions do. As the pulse propagates, it is followed by a wake—a wave of charge separation—whose electric field gradient exceeds those in conventional accelerators by at least 1000 times. In their implementation of LWFA, Krushelnick and his collaborators used 43 fs pulses...