The Higgs boson’s most important puzzle remains unsolved

In some ways, the deepest question one can ask about our physical reality is simply, “What is the Universe made of?” In the early 1800s, we thought we knew the answer: atoms. By the early 1900s, the answer had grown much richer: atoms themselves were composed of atomic nuclei and electrons, which were capable of both emitting and absorbing photons: particles of light. However, it was the 1932 discovery of the neutron — along with the puzzle of radioactive beta decay — that would kick off the series of events that led to our modern theoretical picture of particles in the Universe: the Standard Model of elementary particles, including quarks, leptons, and all of the force-carrying bosons.

Even though the last of the undiscovered particles of the Standard Model was finally found in 2012 with the announcement of the Higgs boson, our understanding of what the Universe is made of is still not complete. From a holistic perspective, there’s much that we still don’t know, including: While the connection might not seem obvious, it’s that last question that scientists studying the Higgs boson might be on the cusp of answering, if only they can obtain the critical data. Here’s why the puzzle of baryogenesis is so captivating, and why the undiscovered properties of the Higgs field may hold the key to solving it.

Based on the masses of the top quark and the Higgs boson, we could either live in a region where the quantum vacuum is stable (true vacuum), metastable (false vacuum), or unstable (where it cannot stably remain). The evidence suggests, but does not prove, that we are in a region of false vacuum, which might be a hint that the Standard Model is possibly not all that there is to the Universe of accessible particles. . Sci, 2018 Inside every atomic nucleus, only two species of composite particle can be found: the proton and neutron.

Protons can easily and stably exist on their own, with a mass of 938.272 MeV. They are composed of two up quarks and one down quark, all bound together by gluons. Neutrons, on the other hand, cannot stably exist on their own, but only in three states: The reason for this is that a neutron is not only slightly more massive than a proton, with a rest mass of 939.565 MeV, but slightly more massive than a proton, electron, and antineutrino combined.

So why do neutrons not only decay, but why do their decays take so long: 15 minutes for a free neutron, and anywhere from a fraction-of-a-second to more than billions of years for a neutron-rich nucleus to decay? This diagram shows how a free neutron (or antineutron) decays at the subatomic level. A down quark (or antiquark) within a neutron (or antineutron), shown on the left in red, emits a virtual W-(or W+) boson, transforming into an up quark (or antiquark).

The W-(or W+) boson forms an electron/electron antineutrino (or positron/electron neutrino) pair, while the up quark (or antiquark) recombines with the original remnant up-and-down quarks (or antiquarks) to form a proton (or antiproton). This is now known to be the process behind all beta decays in the Universe. h Center, Lawrence Livermore National Laboratory In order to explain the existence of this decay, we would need to both hypothesize a new force (the weak nuclear force) and also a new particle: the W-boson, which must be many times heavier than either the proton or neutron itself to make the numbers work out right. This was bizarre from a theoretical perspective, because as far as we understood particle physics, force-carrying particles (i.e., bosons) had to be massless.