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The neutron is heavier than the proton because the down quark’s greater mass outweighs the electromagnetic effect that adds mass to the charged proton. The resulting neutron–proton mass difference is small—about 0.14% of their average mass—but its sign matters for the energetics of familiar atoms and nuclei.
What is the neutron–proton mass difference?
A neutron and a proton are both nucleons, but they contain different combinations of up and down quarks. A neutron is made of two down quarks and one up quark (udd); a proton is made of two up quarks and one down quark (uud). The neutron’s mass exceeds the proton’s by only about 0.14% of their average mass, a proportion reported by Borsanyi and colleagues in their 2014 paper.
That small difference is a net result: two effects push the masses in opposite directions. It is not a simple count of quark masses, because the nucleons’ masses also arise predominantly from strong-interaction energy.
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The down quark adds mass relative to the up quark
The down quark is heavier than the up quark. Because a neutron has two down quarks while a proton has only one, this quark-mass difference tends to make the neutron heavier. The Particle Data Group’s 2023 review gives an illustrative contribution of roughly 2.5 MeV in this direction.
Electromagnetism pushes the other way
The proton carries electric charge, while the neutron is electrically neutral overall. Electromagnetic effects therefore contribute in a way that raises the proton’s mass relative to the neutron. The PDG review describes this contribution as roughly 1 MeV in the proton-heavier direction.
Those rounded figures illustrate the competition, not two independently measured, convention-free pieces. Separating electromagnetic and quark-mass contributions requires a calculation framework, and lattice QED has finite-volume subtleties noted by the PDG. The net result is that the down–up quark mass effect wins, leaving the neutron heavier.
How can physicists calculate such a small splitting?
In an idealized strong-interaction-only treatment where up and down quarks have equal masses, proton and neutron would be degenerate. That symmetry is broken in nature by both the unequal quark masses and electromagnetism. A calculation that omits either ingredient cannot reproduce the full neutron–proton splitting.
Lattice QCD makes numerical calculations of quantum chromodynamics by representing spacetime on a lattice. To calculate this mass difference, physicists combine QCD with QED and allow the up and down quark masses to differ. Borsanyi et al.’s 2014 calculation used lattice QCD+QED with four non-degenerate Wilson fermion flavors. The authors reported 300 keV accuracy for the splitting and a positive result—neutron heavier than proton—by five standard deviations. Those are that study’s reported results, not a claim that it provides the latest or current best precision.
Is the mass difference mainly caused by the Higgs?
No. The Higgs field gives elementary quarks their masses, including the up–down mass contrast relevant to the splitting. But most of a proton’s or neutron’s mass comes from energy associated with the strong interaction, rather than directly from the Higgs-generated masses of its constituent quarks. The neutron–proton difference is a small residual shaped by both the quark-mass contrast and electromagnetism; it is not a direct measurement of the Higgs boson or an explanation for most ordinary matter’s mass.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why does the ordering matter?
The fact that a neutron is heavier than a proton affects the energy available in neutron and proton transformations, and it is one important condition behind the familiar atoms and nuclei around us. Nuclear binding and other physical factors also matter, so the mass ordering alone does not determine whether every nucleus is stable.
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For a technical comparison of calculations, the key questions are whether QED is included, whether up and down quark masses are treated as unequal, how lattice systematics such as finite-volume effects are handled, and what uncertainty is reported. A precision figure for the full splitting should not be confused with an uncertainty on one separated contribution.
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