GrAVitAtionAl WAVE dEtEctors

uncertainty from having a discrete spacetime has been reduced by a factor of three. This correspondingly reduces the error on extrapolating from the unphysical lattice world to the real world. Until now, this uncertainty has been a problem for the charm quark because its relatively large mass gives it a shortwavelength quantum field that can ‘see’ the lattice spacing. This improvement also helps for light quarks: the pion and kaon decay constants calculated by Follana et al. have uncertainties of 1.5% and agree with experiment. In another significant check, which was not possible previously, the computed masses, with no free parameters, of the D meson and the Ds meson (charm quark, anti-strange quark) also agree with experiment. The LQCD calculation of fD agrees with the experimental result5 from the CLEO-c ‘charm factory’ — within the errors of about 8% for experiment and 2% for theory. However, there is a fly in the ointment. For the Ds meson, the most precise measurement6,7 of its decay constant, fDs (analogous to fD), sits about three standard deviations above the calculation by Follana and colleagues1. The discrepancy is a surprise. Follana et al. are checking their calculation by computing other related quantities that have already been well measured. CLEO-c will have an updated result from a larger data set by the summer. Then we will be in a better position to judge: either the discrepancy will disappear, or it will have been established with greater statistical significance. What could be causing it? If the discrepancy is real, it could be due to new physics. If a particle exists that is not in the standard model but couples predominantly to leptons (electrons, muons and taus) and charm quarks, but not to down quarks, then the decays of the D+ meson would be unaffected by it; but the decays of Ds mesons would. The Ds decays could be more sensitive to new physics than any other process explored so far8. On the other hand, if the discrepancy disappears and the data validate the LQCD calculation of fDs, this would inspire confidence in LQCD calculations of fB. The result would be improved measurements of the sides of the unitarity triangle, and hence an increased sensitivity in CP-violation experiments to new physics. It’s a win–win situation, just what particle physicists like best.