Physics News Update, Number 337

Physics News Update
The American Institute of Physics Bulletin of Physics News

Number 337 September 18, 1997 by Phillip F. Schewe and Ben Stein

THE SMALLEST NONZERO BRANCHING RATIO EVER MEASURED. Most known particles are highly fragile. Not only do they decay quickly but through a myriad of ways. Calculating and then also measuring the relative likelihoods (branching ratios) of the different forms of particle mortality are important diagnostics for understanding how matter behaves at the most fundamental level. A particularly rare form of decay (a very tiny tributary to the Amazon of expiring particles, as it were) is the decay of the K+ meson into a pi+ meson, a neutrino, and an anti-neutrino, a process sometimes involving the momentary creation of both a charged W boson (the carrier of the weak nuclear force) and its neutral cousin, the Z boson (which itself instantly decays into the two neutrinos), rather than the more common exchange of a single W or Z. A large collaboration at Brookhaven (contact Douglas Bryman,, 604- 222-7338) has examined more than a trillion K decays and, after years of painstaking vigilance, they have finally found one event with the telltale signature. The expected background for this process, for their amount of data, would be 0.08 events. The researchers are therefore confident that they have measured a true K decay branching ratio, with a value of 4.2 x 10-10 (with an uncertainty of +9.7 or -3.5 x 10-10). In the next year or so additional data should settle the issue of whether the observed rare K decay conforms to the standard model of particle physics (in which case the measured decay rate will provide information about related processes, such as the decay of top quarks) or represents evidence of "new physics" outside the current theory. (S. Adler et al., Physical Review Letters, 22 September 1997; see also a figure at Physics News Graphics)

REAL PHOTONS CREATE MATTER. Einstein's equation E=mc2 formulates the idea that matter can be converted into light and vice versa. The vice-versa part, though, hasn't been so easy to bring about in the lab. But now physicists at SLAC have produced electron-positron pairs from the scattering of two "real" photons (as opposed to the "virtual" photons that mediate the electromagnetic scattering of charged particles). To begin, light from a terawatt laser is sent into SLAC's highly focused beam of 47-GeV electrons. Some of the laser photons are scattered backwards, and in so doing convert into high-energy gamma ray photons. Some of these, in turn, scatter from other laser photons, affording the first ever creation of matter from light-on- light scattering of real photons in a lab. (D.L. Burke et al., Physical Review Letters, 1 September 1997.)

DNA-GOLD NANOPARTICLES, employing the talent of DNA strands for recognizing and attaching to complementary strands and gold's electronic and optical properties, operate as a new kind of biosensor. Scientists at Northwestern University glue various "probe" DNA segments onto tiny gold particles (13 nm wide). When a "target" single-stranded DNA introduced into the solution happens to be complementary to DNA already stuck to the particles, the probe and target strands link up, creating a sort of polymer network whose color is different from that of the original solution. Thus recognition of the target DNA is signaled by a color change. The researchers can already use this approach to detect single-strand DNA samples in 10-femtomolar amounts. (Science, 22 August 1997.)

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