hree papers this period, #1, #8 and #10, could hold the key to one of the greatest puzzles in astrophysics, the solar neutrino problem. A solution involving startling new physics could be in sight.

   Neutrinos easily pass through matter, so a detector for solar neutrinos is a telescope looking at physics deep in the core of the Sun. Solar neutrinos, released from the dense matter that is the central nuclear furnace, pass unhindered through the Sun and journey 1.5 X 10⁸ km to Earth where the flux is 10¹⁴ m^-2 s^-1. Four different experiments have now detected neutrinos from the Sun, and they all record a lower flux than that predicted by standard solar models. For 30 years this failure of theory and observation to converge has acted as the showstopper to a fuller understanding of nuclear energy generation in stars. Astrophysicists long suspected that the solution would emerge from new discoveries in neutrino physics rather than revisions to the solar models. It now looks as if those views are correct, thanks to data from the Super-Kamiokande collaboration (#1, 10).

   Super-Kamiokande (SK) is a second generation solar neutrino experiment 2.7 km underground. Its detector includes 50 kiloton of imaging water, and two cylindrical arrays of 13,000 photomultiplier tubes (PMT). In the water, neutrino-electron scattering produces relativistic electrons which emit a cone of Cherenkov light, effectively from the point of interaction. SK can reconstruct the interaction, including the direction of the incoming neutrino, from the charge and timing data of the triggered PMTs.

   In #1 (see Science Watch, 10[6]: 6, Nov/Dec 1999) Yoji Fukuda’s collaboration reported evidence that neutrinos from atmospheric cosmic-ray collisions appear to oscillate between electron and muon forms. Since the detection techniques used for solar experiments are specific to electron or muon neutrino, neutrino oscillations will cause the detector to miss part of the solar flux. Paper #1 has now been cited more than 185 times.

   The new paper from SK, #10, spanning 300 days of data from 1996-97, includes ~ 3 x 10⁸ hits. After elimination of background events and noise, the effective solar neutrino detection rate is 7.6 (events/day) /kiloton, which translates to a flux from ^8B solar neutrinos of 2.42 X 10⁶ cm^-2 s^-1. There is no significant difference between daytime and night-time fluxes, nor are there seasonal variations. The new result, which is consistent with earlier Kamiokande claims, represents just 36% of the predictions of the standard solar model. This low count strongly suggests that neutrino oscillations are present.

   At the 195th meeting of the American Astronomical Society (Atlanta, GA, January 2000), Yoji Totsuka (University of Tokyo) gave an upbeat assessment of the astronomical prospects for Super-Kamiokande, which can be used for supernova alerts. No supernova has been sighted in the Milky Way galaxy since the invention of the telescope; statistically one is long overdue and is eagerly awaited. SK can act as an alarm because the neutrino pulse from a collapsed star arrives in advance of photon flood. As an early warning system SK can pinpoint the location of a galactic supernova to within 5° and within 30 minutes alert professional and amateur astronomers worldwide over the Internet.

   The theme of cosmic events setting the pace for physics continues with newcomer #9, whose authors now have a trio (#4, 7, 9) in the Physics Top Ten. They have a scheme for solving the hierarchy problem of the fundamental forces by bringing quantum gravity up to the realm of accessibility. This is achieved by invoking 2 new spatial dimensions of sub-mm scale. Gravity is posited to operate in these new dimensions, but the Standard Model forces are shut out. Paper #9 looks at whether anything we already know in physics and cosmology rules out the scheme. It explores extreme astrophysical environments such as 10⁸ TeV cosmic rays, extragalactic supernova explosions, and benchtop observables. The good news is that nothing rules out a 6-dimensional Planck scale. So an exciting vista of new physics beckons, including an experimental probing of quantum gravity.