This month is a time of celebration. CERN has just announced the discovery of four brand new particles at the Large Hadron Collider (LHC) in Geneva. This means that the LHC has now found a total of 59 new particles, in addition to the Nobel Prize-winning Higgs boson, since it started colliding with protons – particles that make up the atomic nucleus with neutrons – in 2009. Exciting, while some of these new particles were expected based on our established theories, some were quite more surprising.
The LHC’s goal is to explore the structure of matter at the shortest distances and highest energies ever tested in the laboratory – testing our best current theory of nature: the Standard Model of particle physics. And the LHC delivered the goods – it allowed scientists to discover the Higgs boson, the last missing piece of the model. That said, the theory is still far from being fully understood.
One of its most troublesome features is its depiction of the strong force that holds the atomic nucleus together. The nucleus is made up of protons and neutrons, which in turn are each made up of three tiny particles called quarks (there are six different types of quarks: high, low, charm, strange, high, and low). If we turned off the mighty force for a second, all matter would immediately disintegrate into a loose quark soup – a state that existed for a fleeting moment at the beginning of the universe.
Make no mistake: the theory of strong interaction, pretentiously called “quantum chromodynamics”, has a very solid foundation. It describes how quarks interact through the strong force by exchanging particles called gluons. You can think of gluons as analogues of the more familiar photon, the particle of light and carrier of electromagnetic force.
However, the way gluons interact with quarks causes the strong force to behave very differently from electromagnetism. While the electromagnetic force weakens when you separate two charged particles, the strong force actually becomes stronger when you separate two quarks. As a result, quarks are forever locked inside particles called hadrons – particles made up of two or more quarks – which include protons and neutrons. Unless, of course, you open them at incredible speeds, like we do at CERN.
To complicate matters further, all particles in the Standard Model have antiparticles that are almost identical to themselves but with the opposite charge (or some other quantum property). If you remove a quark from a proton, the force will eventually be strong enough to create a quark-antiquark pair, with the newly created quark entering the proton. You end up with a proton and a brand new “meson,” a particle made up of a quark and an antiquark. It might sound strange, but according to quantum mechanics, which governs the universe on the smallest of scales, particles can emerge from empty space.
This has been shown repeatedly by experiments – we have never seen a single quark. A nasty feature of the strong force theory is that the calculations of what would be a simple process in electromagnetism can end up being incredibly complicated. We therefore cannot (yet) theoretically prove that quarks cannot exist by themselves. Worse yet, we can’t even calculate which combinations of quarks would be viable in nature and which would not.
When quarks were first discovered, scientists realized that several combinations should be possible in theory. This included pairs of quarks and antiquarks (mesons); three quarks (baryons); three antiquarks (antibaryons); two quarks and two antiquarks (tetraquarks); and four quarks and one antiquark (pentaquarks) – as long as the number of quarks minus the antiquarks in each combination was a multiple of three.
For a long time, only baryons and mesons were observed in experiments. But in 2003, the Belle experiment in Japan discovered a particle that doesn’t fit anywhere. It turned out to be the first in a long line of tetraquarks. In 2015, the LHCb experiment at the LHC discovered two pentaquarks. The four new particles we recently discovered are all tetraquarks with a pair of charm quarks and two other quarks. All of these objects are particles in the same way that the proton and the neutron are particles. But these are not fundamental particles: quarks and electrons are the real building blocks of matter.
Charming new particles
The LHC has now discovered 59 new hadrons. These include the most recently discovered tetraquarks, but also new mesons and baryons. All of these new particles contain heavy quarks such as “charm” and “background”.
These hadrons are interesting to study. They tell us what nature considers acceptable as a bound combination of quarks, even if only for very short times. They also tell us what nature doesn’t like. For example, why do all tetra and pentaquarks contain a charm-quark pair (with one exception)? And why are there no corresponding particles with strange quark pairs? There is currently no explanation.
Another mystery is how these particles are linked together by powerful force. A school of theorists considers them as compact objects, like the proton or the neutron. Others claim that they are like “molecules” formed by two weakly bound hadrons. Each newly found hadron allows experiments to measure its mass and other properties, which tell us something about the behavior of the strong force. This helps bridge the gap between experience and theory. The more hadrons we can find, the better we can adapt the models to the experimental facts.
These models are essential to achieve the ultimate goal of the LHC: to find physics beyond the Standard Model. Despite its successes, the Standard Model is certainly not the last word in understanding particles. It is for example incompatible with the cosmological models describing the formation of the universe.
The LHC is looking for new fundamental particles that could explain these differences. These particles could be visible at the LHC, but hidden in the background of particle interactions. Or they could appear as small quantum mechanical effects in known processes. In either case, a better understanding of the strong force is needed to find them. With each new hadron, we improve our knowledge of the laws of nature, leading us to a better description of the most basic properties of matter.
This article by Patrick Koppenburg, particle physics researcher, Netherlands National Institute for Subatomic Physics and Harry Cliff, particle physicist, University of Cambridge is republished by The Conversation under a Creative Commons license. Read the original article.