a Measurement of the Branching Ratio for the Weak Radiative Decay of the Lambda Hyperon.

In a measurement performed at the Brookhaven National Laboratory we have measured the branching ratio for the weak radiative decay of the Lambda hyperon. Our result, {{rm rate} (Lambdato n + gamma) over {rm rate} (Lambda to anything)} = 1.77 +/- _sp{0.25 }{0.27}times 10^{-3} is the second, but most precise, measurement of this quantity. It is in slight disagreement with the earlier measurement. Using a thick copper degrader, kaons of initial momentum 680 MeV/c were brought to rest in a liquid hydrogen target. Absorption of these kaons on target protons produced a Lambda, either directly via the reaction K^- ptoLambda+pi ^0, or indirectly via the reaction K ^- p to Sigma ^0+pi^0 with subsequent decay, Sigma^0toLambda+gamma. The occurrence of each of these processes was established by observing the pi^0 decay photons in a modular NaI detector, the Crystal Box. This detector was used to measure the energy and position of the final state particles, and hence allowed the topology of the event to be reconstructed. A wide range of theoretical techniques have been employed to estimate hyperon weak radiative decay amplitudes, including pole models, symmetry arguments, PCAC, and direct quark level calculations. These calculations must combine facets of the strong, weak and electromagnetic interactions. Hence, hyperon decays provide a testing ground for theorists to measure their ability to bring together these components in a relatively simple system. We discuss the implications of our result for a representative selection of theoretical estimates.