Weighty Matters for Particle Physics
The HPQCD collaboration: (from Left to right) Eduardo Follana, Greg Millar, Ian Allison, Craig McNeile, Emel Gulez, Junko Shigemitsu, Peter Lepage, Elvira Gamiz, Howard Trottier, Ron Horgan, Kent Hornbostel, Christine Davies, Iain Kendall, Eric Gregory.
[This is an invited article based on a recently published work of the High Precision Quantum Chromodynamics (HPQCD) collaboration. -- 2Physics.com]
Author: Christine Davies
Affiliation: Department of Physics and Astronomy,
University of Glasgow, UK
Link to HPQCD Collaboration >>
Particle physicists at the Fermilab Tevatron and at the CERN Large Hadron Collider are engaged in an exciting race to be the first to discover the Higgs particle, a 'smoking gun' remnant of the mechanism that we believe gives mass to the other fundamental particles. Particles interact with the Higgs field pervading all of space and, rather like moving through molasses, gain a mass as result.
Meanwhile an important question is: what are these masses? A recent paper  by the High Precision QCD (HPQCD) collaboration answers this question accurately for up, down and strange quarks for the first time.
The masses of the electron and its cousins, the muon and tau, are very well-known since these particles can be studied by the clear tracks they leave in particle detectors.
The masses of the quarks are much less well determined. The reason for this is that the strong force interactions never allow quarks to be seen as free particles. Only their bound states called hadrons (of which the proton is an example) can be produced and studied in particle physics experiments. The quark masses must then be inferred by matching experimental results for the masses of hadrons to those obtained from theoretical calculations using the theory of the strong force, Quantum Chromodynamics (QCD). The quark mass is a parameter in the theory and so matching theory and experiment allows the quark mass to be determined.
This could only be done rather approximately for many years, particularly for the lightest up, down and strange quarks. As Figure 1 (history of the strange quark mass) shows, improvements have been very slow. Recently, however, a technique known as lattice QCD has enabled theorists to calculate the masses of some hadrons very accurately and establish mastery over QCD at last .
Fig.1 History of the strange quark mass: This figure shows the new result compared to earlier evaluations of the strange quark mass in the Particle Data Tables. The mass is given in units of MeV/c2 - for comparison the proton mass is 938 MeV/c2.
The High Precision QCD (HPQCD) Collaboration has now determined the mass of the strange quark to an accuracy of better than 2%, which improves on the evaluation of previous results given in the Particle Data Tables  by a factor of 10.
The technique used by HPQCD has been to determine the ratio of the mass of the charm quark to that of the strange quark. This can be done more accurately than determining the strange quark mass on its own, and gives the breakthrough in precision that has been achieved. Determining this ratio had not been possible before because previous methods had large systematic errors for the relatively heavy charm quarks, which HPQCD have now been able to overcome.
Because the charm mass is already known to 1% from several calculations, including an earlier calculation by HPQCD and others, this then allows an accurate determination of the strange quark mass. Similarly a determination of the ratio of the strange quark mass to that of the up and down quarks provided by the MILC collaboration, allows HPQCD to cascade the accuracy that they have for the charm quark mass down to all of the light quarks.
Fig. 2: Summary of quark mass values from this paper: a comparison of the new lattice QCD results for the masses of the up, down and strange quarks (from this paper) and the charm quark (from an earlier paper) to the current evaluations in the Particle Data Tables.
With this improvement in the masses of the light quarks shown in Figure 2, we now have values for the masses of all 6 quarks at the level of a few percent and a much clearer and more complete picture of what the Higgs particle has done for the quarks.
 C. T. H. Davies, C. McNeile, K. Y. Wong, E. Follana, R. Horgan, K. Hornbostel, G. P. Lepage, J. Shigemitsu, and H. Trottier (HPQCD Collaboration), "Precise Charm to Strange Mass Ratio and Light Quark Masses from Full Lattice QCD", Phys. Rev. Lett. 104:132003 (2010). Abstract.
 C. Davies, "Colourful calculations", Physics World 19N12:20 (2006).
 Particle Data Group, http://pdg.lbl.gov/
 I. Allison, E. Dalgic, C. T. H. Davies, E. Follana, R. R. Horgan, K. Hornbostel, G. P. Lepage, C. McNeile, J. Shigemitsu, H. Trottier, R. M. Woloshyn, K. G. Chetyrkin, J. H. Kühn, M. Steinhauser, and C. Sturm (HPQCD Collaboration), "High-precision charm-quark mass and QCD coupling from current-current correlators in lattice and continuum QCD", Phys. Rev. D78:054513 (2008) Abstract; K. G. Chetyrkin, J. H. Kühn, A. Maier, P. Maierhöfer, P. Marquard, and M. Steinhauser, C. Sturm, "Charm and bottom quark masses: An update", Phys. Rev. D80:074010 (2009) Abstract.
 C. Aubin, C. Bernard, C. DeTar, J. Osborn, Steven Gottlieb, E. B. Gregory, D. Toussaint, U. M. Heller, J. E. Hetrick, R. Sugar (MILC collaboration), "Light pseudoscalar decay constants, quark masses, and low energy constants from three-flavor lattice QCD", Phys. Rev. D70:114501 (2004) Abstract.
Labels: Elementary Particles 2