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Sidebar: Hidden Symmetry That Shapes Our World

If there were no Higgs mechanism, what a different world it would be! Elementary particles of matter such as quarks and electrons would have no mass. Yet that does not mean the universe would contain no mass. An underappreciated insight from the Standard Model is that particles such as the proton and neutron represent matter of a novel kind. The mass of a proton, in contrast to macroscopic matter, is only a few percent of its constituent masses. (In fact, quarks account for not more than 2 percent of the proton’s mass.) Most of the mass arises through the original form of Albert Einstein’s famous equation, m = E/c2, from the energy stored up in confining the quarks in a tiny volume. In identifying the energy of quark confinement as the origin of proton and neutron mass, we explain nearly all the visible mass of the universe, because luminous matter is made mostly of protons and neutrons in stars.

Quark masses do account for an important detail of the real world: that the neutron is slightly more massive than the proton. One might expect the proton to be the more massive one, because its electric charge contributes to its intrinsic energy—a source of self-energy the neutron lacks. But quark masses tip the balance the other way. In the no-Higgs zone, the proton would outweigh the neutron. Radioactive beta decay would be turned on its head. In our world, a neutron sprung from a nucleus decays into a proton, electron and antineutrino in about 15 minutes, on average. If quark masses were to vanish, a free proton would decay into a neutron, positron and neutrino. Consequently, hydrogen atoms could not exist. The lightest “nucleus” would be one neutron rather than one proton.

In the Standard Model, the Higgs mechanism differentiates electromagnetism from the weak force. In the absence of the Higgs, the strong force among quarks and gluons would induce the distinction. As the strong interaction confined the colored quarks into colorless objects like the proton, it would also act to distinguish the weak and electromagnetic interactions, giving small masses to the W and Z bosons while leaving the photon massless. This manifestation of the strong force would not give any appreciable mass to the electron or the quarks. If it, rather than the Higgs, operated, beta decay would operate millions of times faster than in our world.

Some light nuclei would be produced in the early no-Higgs universe and survive, but they would not form atoms we would recognize. An atom’s radius is inversely proportional to the electron’s mass, so if the electron has zero mass, atoms—less than a nanometer across in our world—would be infinitely big. Even if other effects gave electrons a tiny mass, atoms would be macroscopic. A world without compact atoms would be a world without chemistry and without stable composite structures like our solids and liquids. —C.Q.

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