DEEP in the heart of every atom lurk protons, tiny particles from which the chemical elements were forged, first in the searing heat of the big bang and then in the nuclear furnaces of stars. The number of protons in an atom determines whether it is hydrogen, carbon, oxygen or uranium. They make up more than 86 per cent of the visible matter in the universe by mass, and they are fundamental to our existence. Yet we still don’t really understand them.
It was just over a century ago that Ernest Rutherford demonstrated that protons are one of the basic building blocks of all atomic nuclei. Yet despite our best efforts in the intervening years, much about this ubiquitous particle remains shrouded in mystery.
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Whether protons live forever, how big they are and what they are really made from are just some of the questions physicists continue to grapple with. Finding the answers won’t just change how we think about the particles themselves. It could alter our understanding of the universe and the fundamental laws that govern it. Here are five of the biggest unanswered questions about the proton.
1. What are protons made of?
The simple, oft-repeated story is that the proton is made of three quarks – two up quarks and one down quark – locked together by the vice-like grip of the strong nuclear force that binds atomic nuclei. However, when physicists started looking at higher and higher resolution, they discovered that around these three “valence” quarks in a proton is a churning, quantum-mechanical sea of other particles that pop in and out of existence.
Most of the time, when particle physicists smash protons together at the Large Hadron Collider (LHC) at the CERN particle physics lab near Geneva, Switzerland, we don’t see quark-on-quark collisions, but collisions between particles called gluons, the carriers of the strong force that bind the proton’s three valence quarks together. Gluons are responsible for the majority of the proton’s mass. But experiments at even higher energies reveal that these gluons, too, are constantly splitting into fleeting quark-antiquark pairs, including so-called strange and charm quarks.
The charm quark is about 35 per cent heavier than the proton, so you might very reasonably wonder how a proton could contain one. It would be like opening a 1-kilogram package to discover the stuff inside weighs 1.3kg. The answer is that all particles, including charm quarks, can be thought of as tiny vibrations in underlying quantum fields. This means it is possible to have a contribution from the charm quark field that doesn’t correspond to a well-defined particle, but is more akin to a low-energy quantum-mechanical echo. It is as if just a part of a charm quark is there.
Beyond the short-lived quark-antiquark pairs, there has been a long-running investigation into whether charm quarks, or at least a taste of them, may loiter inside the proton on a more permanent basis.
This is known as an “intrinsic” charm component and it would even when viewed at low resolution.
In 2022, at Free University of Amsterdam in the Netherlands and his colleagues used a combination of large, experimental datasets and machine learning to discern what they called “unambiguous” evidence of just such an intrinsic charm contribution to the proton. Crucially, they claim their technique can distinguish between a genuine intrinsic charm contribution and the short-lived one from gluons splitting into charm quark-antiquark pairs. The evidence they found doesn’t meet the gold-standard 5-sigma threshold of statistical significance usually required to claim an unambiguous discovery in particle physics. However, their claim got a boost from the LHCb experiment at the LHC, which separately . That said, the jury is still out, for now at least.
Understanding what the proton is really made of is crucial for interpreting the results of collider experiments, such as those at the LHC, that search for signs of new particles. It is only by resolving mysteries like whether the proton contains charm quarks that physicists will ultimately be able to make progress on some of the biggest outstanding questions in physics.
2. How big is a proton?
This is such a basic question that you might assume physicists would have a clear answer, and until relatively recently, they did. More or less everyone agreed that the proton’s radius was around 0.88 femtometres – 0.88 trillionths of a millimetre. In other words, you could line up around a trillion protons across the full stop at the end of this sentence.
Then, in 2010, a new measurement blew this consensus apart. A team based at the in Switzerland performed an experiment that showed protons were smaller than previously believed. The new result was just 0.04 femtometres smaller, but the measurements were so precise that this discrepancy represented a yawning chasm. The only ways to bridge the gap would be if there were errors in one or more of the experiments or, tantalisingly, there were new particles outside of the standard model, our best theory of particle physics.
Earlier measurements of the proton’s size had used two methods. The first involved firing electrons at protons and measuring how often they bounced off and at what angles. The second studied quantum jumps taking place within hydrogen atoms, as electrons were excited between different orbits. Crucially, both techniques – electron scattering and atomic transitions – gave an answer of around 0.88 femtometres.
However, the 2010 measurement used a variant of the quantum jumps approach where the electron in orbit around the proton in hydrogen was replaced with a muon, a heavy cousin of the electron, to form muonic hydrogen. Since muons are about 200 times heavier than electrons, they orbit the proton much closer and are more likely to be found inside it. This means the size of the proton has a far stronger effect on the energy levels of muonic hydrogen than for the garden-variety version of this element, making the new experiment significantly more sensitive.
Since the original 2010 result, other experiments using muonic hydrogen have confirmed that the proton appears smaller than previously believed. One exciting possibility was that an unknown force was acting on muons and electrons with different strengths, causing the proton to appear a different size depending on which particle was used. To rule out a mistake, physicists went on a deep dive to see if they could improve the earlier techniques.
This culminated in new, improved measurements using electron scattering and ordinary hydrogen transitions, released in 2019, that agreed with the smaller proton radius result from the muonic hydrogen experiments. Then, just last year, researchers at the University of Bonn and the Technical University of Darmstadt, Germany, reanalysed old collision data with a new technique, .
So is the proton’s size solved? Not quite. There is still no clear explanation for the larger values reported by earlier studies. Ultimately, new experiments will be needed to confirm once and for all that the proton really is a little bit tinier than previously thought.
3. How stretchy is a proton?
You can think of a proton as a messy bag of quarks, gluons and other ephemeral particles that are constantly flitting in and out of existence. Expose a proton to an electromagnetic field and its innards get pulled this way and that depending on their electric charge and magnetism. This means that protons have an inherent stretchiness in response to electric and magnetic fields – technically called the electric and magnetic polarisability. But it is up for debate just how stretchy they really are.
The standard model of particle physics predicts that as you zoom in on a proton and study it in smaller and smaller pieces, it should appear less and less stretchy. It is a little like how an orange is squashy, but its pips are hard. However, in 2000, an experiment carried out using a particle accelerator in Mainz, Germany, revealed that as you zoom in, the proton does indeed get harder, but at a certain scale becomes briefly stretchier again, before once again becoming more rigid as you continue to zoom in.
Physicists have long doubted that this effect is genuine, particularly given those experiments were rather imprecise. However, in 2022, researchers working at appeared to at roughly the same scale as before. They measured the proton’s electric and magnetic polarisabilities by firing a beam of electrons at a liquid hydrogen target. As an electron whizzed past a proton, a photon was produced, distorting the proton in the process. By watching how the proton and electron recoiled, they were able to measure how much the proton was stretched by the interaction.
While this new measurement was more precise than the original Mainz result, the effect they measured was half as strong. Overall, the purported anomaly is still too weak for us to be confident that it is more than a statistical fluke or systematic bias, but it remains intriguing. If confirmed by further experiments, it would suggest that our theoretical model of the proton is missing some crucial effects, with significant implications for how we think about the strong force that binds it together.
4. How often do protons collide inside a nucleus?
Something strange appears to be happening inside atomic nuclei. Protons are banging into each other more often than expected, which suggests there are things about the strong nuclear force that we don’t understand.
This force binds protons and particles called neutrons together inside atomic nuclei. But it isn’t always a force of attraction. As it pulls two nucleons – a catch-all term for protons and neutrons – together inside a nucleus, the attractive force between them gets stronger, accelerating the particles towards each other. But when they are around a quadrillionth of a metre apart, the force between them suddenly becomes repulsive, causing them to bounce off each other and whizz apart at high speed.
Previous studies of the nuclei of elements such as carbon and lead had shown that the vast majority of these head-butts happened between a proton and a neutron, with only 3 per cent being between two protons. However, in 2022, a team of physicists announced the result of an experiment at Jefferson Lab using a helium isotope made of two protons and one neutron, called helium-3, and a radioactive form of hydrogen made of one proton and two neutrons, called tritium. What they found was surprising.
The work involved firing a beam of electrons at helium-3 and tritium and watching how they were scattered. In doing so, the team could estimate how often the electron bounced off a recoiling nucleon from a proton-proton or proton-neutron interaction. Rather than the 3 per cent measured in earlier experiments, proton-proton interactions were involved 20 per cent of the time.
The researchers suspect the difference may be related to the fact that protons and neutrons are much less tightly packed in helium-3 and tritium than in much larger carbon and lead nuclei. This suggests that the strong nuclear force behaves in ways we don’t currently understand at very short distances. If this is the case, it will have significant implications for nuclear physics and even the study of astronomical bodies such as neutron stars, which are, in effect, giant, city-sized atomic nuclei.
5. Are protons eternal?
Nothing lasts forever, or so the saying goes, but perhaps protons do. Every other composite particle we know of lives a short and uncertain life. A solitary neutron, for instance, survives for a mere 15 minutes on average before decaying. Yet, despite decades of searching, no one has ever seen a proton snuffing it.
The question of whether protons decay is more than a bit of scientific bookkeeping. It is connected to one of the great quests of modern science: the search for a grand unified theory (GUT). Such a theory would unite the strong, weak and electromagnetic forces, the three different forces at the heart of the standard model, into a single entity. It may also explain many of the apparently arbitrary features of the standard model. The catch is that new particles predicted by GUTs are so heavy as to be out of reach of any conceivable collider, making the theories extremely difficult to test. But there is one way they might be probed: GUTs usually result in new forces that allow protons to decay.
The problem is that even if such forces exist, the proton’s average lifetime would still be billions upon trillions of times longer than the age of the universe. But gather enough protons together in one place and wait for long enough, and you might just spy a handful of decays.
In the 1980s, when GUTs were all the rage, two teams of physicists attempted just that. Under Lake Erie in the US and in a mine in Hida, Japan, physicists placed vast subterranean tanks filled with thousands of tons of ultra-pure water, each surrounded by detectors. A flicker of light emerging from the dark water would herald the decay of a proton, but, after years of watching and waiting, both experiments drew a blank. In 1996, a souped-up version of the Japanese experiment, more than 10 times the size and named Super-Kamiokande, continued the search for proton decay, but has so far come up empty handed.
From this outcome, physicists were able to conclude that the average lifetime of the proton must be longer than 1034 years, a truly inconceivable span of time, a trillion trillion times longer than the age of the universe. In the process, they also ruled out many of the most popular GUTs.
However, undeterred, physicists are now planning an even larger detector called Hyper-Kamiokande. Its tank will contain 260,000 tons of water and it will begin taking data in 2027. Ultimately, Hyper-K should be able to spot protons decaying, even if their average lifetime is up to 10 times longer than the current lower limit for this.
Not only would this provide the first evidence that the forces of nature really do unify at high energies, it could even challenge theories of quantum gravity, which aim to overcome the problem of our seemingly disparate descriptions of the quantum realm and gravitation by uniting them. In April this year, theorists at the University of Southern Denmark and Shouryya Ray at the Technical University of Dresden in Germany showed that well beyond the reach of even Hyper-K. So, if we do witness the death of a proton in the coming years, it could spell trouble for one of the most confounding problems in physics.
Harry Cliff is a particle physicist at the University of Cambridge
