Why don't protons decay?

© 2025 Kieron Conway - All rights reserved.

Protons (the uud quark configuration of the lightest of all baryons) are the most stable of all the hadrons. Experiments show that the proton’s lifetime must be greater than 10 ^34 years, far exceeding the age of the universe. So if protons do decay, it happens so rarely that it is effectively invisible to us.

So, how come they are so stable when hadrons in general, are not: even the neutron (ddu quark composition) is unstable when on its own outside a nucleus. There is no baryon lighter than a proton for them to decay into, but why can't they decay into leptons?

It all stems from what happened in the very early stages of the universe, when hypothetical X and Y bosons existed. It's to these bosons, that the proton owes its longevity.

What Are X and Y Bosons?

Here's a Section from Chapter 1 in Part 2 of a Journey into Modern Physics, discussing the start of the universe just as gravity separated from the grand unified force:

This would have been a very strange time. Without the Higgs field, any matter formed should have had no mass as we understand it. However, speculation using the model of the Grand Unified Theory (GUT) produced by Georgi and Glashow (we came across Sheldon Glashow of B-W1-W2-W3 bosons fame, in part1), indicates that the X and Y bosons had immense mass. These bosons mediated the interactions between early forms of quarks and leptons, which coupled together under the influence of what is sometimes called the electronuclear interaction.

The masses of the X and Y bosons are thought to have been higher than anything that the LHC is capable of producing in its proton-proton collisions. Presumably, during this epoch, mass was made available through a mechanism involving the intense energy density of the universe in some way as there was no Higgs field yet. The electronuclear force allowed early quarks and leptons to couple in esoteric particles, although this is pure, theoretical speculation.

The Birth of the X and Y Bosons

At the very beginning (or TIME = 0 as it was defined in the Journey into Modern Physics), all four fundamental forces; gravity, the strong nuclear force, the weak nuclear force, and electromagnetism, are thought to have been united into a single grand unified force (the GUT force from the Grand Unified Theory). As the universe expanded and cooled, gravity was the first to separate, leaving behind what is sometimes referred to as the electronuclear force (the combined strong nuclear and electroweak forces).

This separation of gravity from the GUT force triggered a major symmetry breaking. In physics, symmetry breaking is when a perfectly balanced system falls into a lower-energy state, creating distinctions where none existed before as the perfect balance is destroyed. This is where the X and Y bosons, saviours of the proton's stability, enter the picture.

The breaking of the GUT symmetry produced a new kind of Higgs-type field, not the Higgs field that we know today, but an earlier, vastly more powerful cousin! This field had a colossal vacuum expectation value (the hump in the middle of the Mexican hat potential from part 1), and through it the X and Y bosons acquired their extraordinary masses: estimated to be about 10^15 GeV each, far beyond the collision energy possible from the existing Large Hadron Collider.

So, while the Higgs field of today had not yet appeared, the early universe already had a Higgs-like super-mechanism at work, operating at a much higher energy scale and only on the two mediating bosons.

What Could X and Y Bosons Do?

The X and Y bosons were no ordinary force carriers. Unlike the photon, gluon, or even the W and Z bosons of today, they carried both colour charge (like gluons) and electroweak charge. This allowed them to transform quarks into leptons, and vice versa.

At that time, quarks and leptons themselves were still massless, they had to wait until later, when the electroweak symmetry broke and the familiar Higgs field of today made its appearance. But the X and Y bosons obtained their gigantic masses, thanks to the earlier mechanism of the super Higgs-type field created from GUT symmetry breaking.

Proton Decay — And Why It Hasn’t Happened

The most dramatic consequence of X and Y bosons is the possibility that they could create proton decay. A proton is made of three quarks (uud) and if an X or Y boson were exchanged, one of those quarks could transform into a lepton, causing the proton to decay into lighter particles such as positrons and mesons. For example;

P+(uud) → e+ + π0

Where a massless proton decays into a massless positron (lepton anti-matter) and a massless meson (u+anti-u or d+anti-d), an interaction mediated by an X or Y boson.

Another theoretical interaction is;

P+(uud) → anti-neutrino + π+

Where a massless proton decays into a massless anti-neutrino (lepton) and a massless meson (u+anti-d), mediated by an X or Y boson.

There are plenty of other possible decay channels for a proton involving all three generations of leptons and the X and Y boson mediators.

The more knowledgable amongst you may be wondering what's happened to lepton conservation as only one lepton is produced in each of the above examples. Well, standard GUT theory of proton decay doesn't require lepton conservation, which only came into being with the standard model of particle physics at energies much lower than prevalent in the GUT epoch.

The fundamental point about proton decay

If the X and Y bosons had not been so super-heavy, this process of changing a quark to a lepton and meson would have been very common. The legacy of such a common proton decay in this strange high-energy environment, would mean that proton decay in today's low-energy environment would also be common.

This would be disastrous, as the matter that makes up suns, planets, atoms and even us, is totally dependent on protons being stable. Fortunately, because the predicted masses of the X and Y bosons are so huge, proton decay is suppressed to an almost negligible level and has never been encountered experimentally.

Why X and Y Bosons Matter

Even though no experiment can detect X or Y bosons, they remain a crucial prediction of Grand Unified Theories. They offer a glimpse of a time when the universe was hotter, denser, and stranger than anything we can recreate on earth.

For now, the X and Y bosons remain hypothetical. But their story shows how modern physics tries to peer back into the earliest instants of the universe, connecting the tiniest particles with the grandest cosmic events like the separation of gravity from the GUT force and the breaking of a major symmetry.

So, to sum up; we have the theoretical X and Y force mediators of the very early universe to thank for the stability of today's protons.

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