Ab initio calculation of the neutron-proton mass difference

Borsányi, Szabolcs and Durr, S. and Fodor, Zoltán and Hoelbling, C. and Katz, Sándor (2015) Ab initio calculation of the neutron-proton mass difference. Science, 347 (6229). pp. 1452-1455. ISSN 0036-8075 (print) 1095-9203 (online)

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Abstract

The existence and stability of atoms rely on the fact that neutrons are more massive than protons. The measured mass difference is only 0.14% of the average of the two masses. A slightly smaller or larger value would have led to a dramatically different universe. Here, we show that this difference results from the competition between electromagnetic and mass isospin breaking effects. We performed lattice quantum-chromodynamics and quantum-electrodynamics computations with four nondegenerate Wilson fermion flavors and computed the neutron-proton mass-splitting with an accuracy of 300 kilo-electron volts, which is greater than 0 by 5 standard deviations. We also determine the splittings in the Sigma, Xi, D, and Xi(cc) isospin multiplets, exceeding in some cases the precision of experimental measurements.

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