Physicists launch charm offensive

The subatomic world is undergoing a revolution and a relatively exotic denizen, the charm quark, is the latest target.

A team of theoretical physicists from Glasgow, Cornell and the Ohio State University has calculated key properties of particles known as D mesons that contain the charm quark with an accuracy improved by a factor of 4 over previous calculations and existing experimental results.

Experimental numbers are expected to improve significantly in the next year and this, with the new theoretical result, will provide a stringent test of the Standard Model of particle physics in a new arena.

This is an important part of the worldwide programme exploring all areas of particle physics for clues to a more complete theory than our current Standard Model. 

The theory of how quarks interact is well understood in principle, but in practice is very difficult to handle because the interaction is so strong. It can be tackled in numerical calculations on a computer using a technique known as lattice QCD. This underwent a revolution five years ago when new methods and increased computer power came together to make accurate  calculations possible for the first time. The new methods focussed on the up, down and strange quarks that make up the 'everyday' subatomic world, being constituents of, for example, the proton and the pi meson. These quarks are very light, the up and down quarks in particular weighing almost nothing.

The more exotic charm quark has a mass heavier than the proton, and this makes it much harder to work with in lattice QCD. Initial calculations used different methods for the charm quark than for up, down and strange and produced less accurate results because of this.

Now Professor Christine Davies and collaborators have produced results by improving the method for up, down and strange so that it also works well for charm quarks. This gives not only hugely improved accuracy but also increased predictive power. For example, they have been able to determine the mass of the D meson and its cousin, the D_s, for the first time.

They obtain an accuracy of better than 0.5%, and good agreement with experiment. They have also calculated the decay constants for the D, D_s and the pi meson in the same calculation.

The decay constant measures the probability of the quark and antiquark that make up the meson being in the same place so that they can annihilate.

The rate of annihilation can be measured by experiment and is well known for the pi meson, where the theorists agree to 2%, but not yet very well known for the D or D_s. Early results have appeared from the CLEO-c experiment at Cornell, the BaBar experiment at Stanford and the Belle experiment at KEK in Japan, and they will be working hard to reduce their errors to the 2% level
that theorists have now achieved. 

Professor Christine Davies said: "It is very exciting to be able to do such accurate calculations at last for charm quarks. It has taken several years of work to develop this method but it has paid off handsomely. We have lots of further calculations to do now that will allow even tougher tests against experiment for the theory of particle physics."

The theorists will now go on to more complicated calculations that can provide additional stringent tests of the Standard Model `jigsaw', searching for the piece that does not fit.

Further information:

Professor Christine Davies,
Department of Physics and Astronomy
University of Glasgow
Tel: 0141 330 4710

Brief Glossary:

quark - the smallest constituents so far known of the protons and neutrons that make up the atomic nucleus, as well as a host of other particles seen in particle physics experiment. Quarks themselves are never seen since it is impossible for them to exist in isolation.

antiquark - the antimatter version of the quark. Has the same mass as its corresponding quark but the opposite of other properties such as electric charge.

meson - a particle whose principal constituents are a quark and an antiquark.
They can be studied in particle detectors and many different mesons are known. The pi meson has as constituents an up quark and down antiquark or vice versa. The D meson has a charm quark and a down antiquark, the D_s has a charm quark and a strange antiquark.

Standard Model - our current theory of particle physics that describes the most fundamental particles currently known and their interactions by the strong, weak or electromagnetic forces of Nature. The strong force, by which quarsk interact, is believed to be described by the theory of Quantum Chromodynamics (QCD). 

lattice QCD - a method of calculating the effects of the strong force using numerical techniques on a computer to solve the theory of Quantum Chromodynamics (QCD). 

First published: 27 February 2008