AFTER decades of trying, physicists have used the theory of the strong nuclear force to correctly predict the mass of a particle. The powerful computer method used to solve the theory鈥檚 staggeringly difficult equations could now help discover new particles and even challenge the standard model of particle physics.
Quantum chromodynamics (QCD) is the theory that models the strong force, which binds quarks into protons, neutrons and other heavy particles. In most cases the equations of QCD are intractable 鈥� even for simple particles called mesons, made from just two quarks.
Physicists attempt approximate solutions by slicing up space and time into a four-dimensional lattice, then letting massive computers crunch away at the simplified equations. But even using this approach, calculations have always lagged behind experimental methods of determining a particle鈥檚 mass. 鈥淧eople thought it was going to be easier than it turned out to be,鈥� says Christine Davies of the University of Glasgow in the UK. 鈥淭hey thought that the next computer, the next set of new techniques, would solve the problem 鈥� but it didn鈥檛. It鈥檚 amazing we have kept going.鈥�
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But techniques have gradually improved, and now Davies鈥檚 team has calculated the mass of the Bc meson, a particle formed of two heavy quarks called a charm and a bottom. Earlier attempts to predict the Bc meson鈥檚 mass failed because researchers had neglected lighter quarks, called up, down and strange, which appear briefly inside the meson. 鈥淲e hadn鈥檛 got the basic soup in which the bottom and charm quarks were living,鈥� Davies says.
The properties of this soup were finally determined this year by a US group, the MILC collaboration. It required a vast computing effort: machines carrying out 500 billion calculations per second took two years to complete the job. Davies used their results to predict the Bc meson鈥檚 mass at 6304 卤 22 megaelectronvolts, which agrees nicely with the value of 6287 卤5 megaelectronvolts measured recently at the Fermilab particle accelerator near Chicago.
With lattice QCD validated, physicists will now be able to calculate masses for hypothesised particles such as the glueball, making it easier to find them in experiments. It will also allow them to probe the weak nuclear force, and thus help to explain why our universe is full of matter rather than antimatter or just radiation. Examining the weak force might also reveal the limits of the standard model of physics and point to a new, more fundamental theory of particles.