HomeAb initio calculation of the neutronproton mass difference


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 quantumchromodynamics and quantumelectrodynamics computations with four nondegenerate Wilson fermion flavors and computed the neutronproton masssplitting with an accuracy of 300 kilo–electron volts, which is greater than 0 by 5 standard deviations. We also determine the splittings in the Σ, Ξ, D, and Ξ_{cc} isospin multiplets, exceeding in some cases the precision of experimental measurements.^{ } 
Mass splittings of hadrons (here particles composed of quarks and/or antiquarks) in channels that are stable under the strong and electromagnetic interactions. Both of these interactions are fully unquenched in our 1+1+1+1 flavor calculation. The horizontal lines are the experimental values, and the gray shaded regions represent the experimental error. Our results are shown by red dots with their uncertainties. The error bars are the squared sums of the statistical and systematic errors. The results for the neutronproton, Σ^{}Σ^{+} and D^{+}D^{0} meson mass splittings are postdictions, in the sense that their values are known experimentally with higher precision than from our calculation. On the other hand, our calculations yield results for the Ξ^{}Ξ^{0}, and Ξ_{cc}^{++}Ξ_{cc}^{+}, as well as the ColemanGlashow difference (CG), which have either not been measured in experiment or are measured with less precision than obtained here. This feature is represented by a blue shaded region around the label. 
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