Number 91, August 12, 1992 by Phillip F. Schewe and Ben Stein|
MEASUREMENTS OF MULTI-JET PRODUCTION RATES in deep inelastic muon-proton scattering support the theory of quantum chromodynamics (QCD). Stealing a march on HERA---the electron-proton collider in Hamburg, where proton-lepton (electrons instead of muons) scattering will be the exclusive concern---scientists at Fermilab (contact: Jorge Morfin, 708-840-4561) have studied what happens when a beam of 490-GeV muons is smashed into a stationary hydrogen target. Quite often the muon can be thought of as scattering not from the proton as a whole but from individual quarks within the proton. Some of these quark-muon interactions will be highly inelastic; that is, much of the collision energy will be converted into new particles, some of which emerge from the collision vertex in jets. QCD holds that in certain reactions, gluons (the carriers of the strong nuclear force between quarks) may also spawn jets through a sort of radiative process, much like photon bremsstrahlung. Fermilab has not only seen such multi-jet (in particular, 3-jet) events, but has for the first time measured their rate of production, a value which agrees with the QCD prediction. (M.R. Adams et al., 17 August issue of Physical Review Letters.)
THE FIRST EXTRAGALACTIC GAMMA RAYS in the TeV energy range have been observed by a group of astronomers at the Whipple Observatory in Arizona. The gamma source, the galaxy Markarian 421 (400 million light years away), had previously been seen at radio, visible, and x-ray wavelengths, and at gamma energies into the GeV range by the Gamma Ray Observatory. The Whipple measurements, made with Cerenkov counters that detect showers engendered by the gammas hitting our atmosphere, show that the gamma flux from Mk421 is only a factor of three lower than that of the Crab Nebula (which, sitting in our own galaxy, serves as the standard candle for TeV gammas) even though it is 100,000 times further away. Mk421 is considered to be a "blaser," a category of active galaxy---possibly containing a black hole---that emits light strongly across the whole electromagnetic spectrum. (Nature, 6 August.)
BUCKYBALL CRYSTALS HAVE DIELECTRIC PROPERTIES that require the existence of a permanent, non-symmetrical distribution of electric charge in the material at lower temperatures. Below 260 K, C-60 crystals undergo a phase change in which the lattice structure changes and the individual molecules assume a fixed orientation instead of rotating freely in the lattice. Theorists previously believed that the molecules in the low temperature phase were oriented in such a way as to ensure a zero electric dipole. However, Glenn B. Alers and Brage Golding (517-355-9708) of Michigan State University and their co-workers at AT&T Bell Labs found that single C-60 crystals subjected to low-frequency electric fields exhibited a dielectric response containing frequency-dependent contributions that require the existence of permanent dipoles in the material. The researchers propose that the dipoles are induced by misoriented C-60 molecules which maintain their own spherical symmetries but break the charge symmetry of the lattice. These imperfections in the C-60 lattice would have implications for the optical and nonlinear properties of buckyball films, and may mean that the crystal can interact with a wider variety of electromagnetic frequencies than previously thought. (Science, 24 July 1992).