The neutron and proton weigh in, theoretically

Adding electromagnetic effects to quantum chromodynamics calculations helps theorists achieve a leap in accuracy.

The mass difference between the neutron and proton—about 0.14%—is known experimentally with an impressive precision of 3 parts in 10 million. But calculating that difference from scratch via quantum chromodynamics, the theory of the strong force that binds quarks through the exchange of gluons, is another matter altogether. That’s because the up and down “valence” quarks, the commonly given constituents of neutrons and protons (up, up, down for protons and up, down, down for neutrons), are awash in a roiling, virtual sea of gluons and quark–antiquark pairs, as illustrated in this sketch. To complicate matters, those sea quarks and antiquarks, like the valence quarks, also interact electromagnetically. Theorists have dealt with the resulting computational difficulties by resorting to such approximations as using only two or three of the six types of quarks, assigning equal mass to the up and down quarks, or ignoring electromagnetic effects. Now an international collaboration led by Zoltán Fodor at the University of Wuppertal has made the most accurate calculation of the neutron–proton mass splitting to date. The researchers were able to do so by using four quarks: up, down, strange, and charm, each with a different mass. They also developed new computational techniques to incorporate quantum electrodynamics in their simulations. The new calculation, with its 0.03% precision, isn’t just a computational feat. The results reveal in detail how the neutron–proton mass splitting results from a competition between electromagnetic effects and the mass difference between the up and down quarks. (S. Borsanyi et al., Science 347, 1452, 2015.) Source: The neutron and proton weigh in, theoretically

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