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arxiv: 1406.4088 · v2 · pith:NYIWP6JDnew · submitted 2014-06-16 · ✦ hep-lat · hep-ph· nucl-th

Ab initio calculation of the neutron-proton mass difference

classification ✦ hep-lat hep-phnucl-th
keywords differencemassisospinneutron-protonaccuracyatomsaveragebreaking
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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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