An EU-funded study by physicists in Germany, France and Hungary has demonstrated conclusively that the Standard Model of particle physics, a theory describing the fundamental interactions of the elementary particles that make up all visible matter in the universe, accurately accounts for the mass of protons and neutrons. The results, published online in the journal Science, represent one of the largest computational efforts to calculate particle mass to date and a significant advance in the field of physics. 'More than 99% of the mass of the visible universe is made up of protons and neutrons,' the study says. 'Both particles are much heavier than their quark and gluon constituents, and the Standard Model of particle physics should explain this difference.' Dr Andreas S. Kronfeld of the Fermi National Accelerator Laboratory in the US explained that because atomic nuclei make up almost all of the weight of the world, and because these nuclei are composed of particles called quarks and gluons, 'physicists have long believed that the nucleon's mass comes from the complicated way in which gluons bind the quarks to each other, according to the laws of quantum chromodynamics (QCD)'. The physicists started from the beginning, looking at the basic and established laws of nature through the lens of QCD. QCD is a theory used to describe the 'strong nuclear force', or the interactions between quarks and gluons. However, because the number of interactions and virtual interactions between gluons and quarks is estimated in the trillions, numeric computations are exceedingly difficult (or even impossible) using standard QCD. The researchers used a new approach called lattice QCD, whereby time and space are 'smoothed out' into a kind of lattice, or grid. This approach allowed them to incorporate all of the needed physics, control for numerical approximations, and provide a thorough error budget in their calculation of hadron (e.g. proton, neutron and pion) masses. 'The lattice reduces everything we would want to calculate to integrals that, in principle, can be evaluated numerically on a computer,' noted Dr Kronfeld. Because of this, the authors of the study were able for the first time to include quark-antiquark pairs, one of the more major complexities of the strong nuclear force, into their calculations. According to Dr Kronfeld, the physicists' calculations show that 'even if the quark masses vanished, the nucleon mass would not change much, a phenomenon sometimes called 'mass without mass''. 'Because these accurate calculations agree with laboratory measurements, we now know, rather than just believe, that the source of mass of everyday matter is QCD,' the authors conclude. Their results confirm that the Standard Model correctly describes the origin of hadron masses. As these particles make up most of the visible universe, it can be said that the Standard Model accurately estimates the mass of the Sun, the Earth and all it contains. The study's most important observation is that lattice QCD studies 'have reached the stage where all systematic errors can be fully controlled'. The physicists suggest that lattice QCD 'will play a vital role in unravelling possible new physics from processes that are interlaced with QCD effects'. The way nature generates quark masses is one of the subjects of major interest to the physicists working on the Large Hadron Collider.
Germany, France, Hungary