Production and decay of a W particle in the ZEUS detector at DESY, the German particle physics lab in Hamburg. The computer display shows a cross- section of the central region of the cylindrical detector, blocked out in red and blue. An unseen electron and proton, accelerated in DESY's HERA collider, experience a glancing collision at centre. This creates a W particle, which lives too briefly to be seen but is revealed when it decays to a single electron (track & yellow block) and an invisible neutrino. The two W particles, positive & negative, along with the neutral Z particle, are the carrier particles of the weak nuclear force.