On June 30th, an experiment at the LHC in Geneva (LHCb) reported on arXiv what could be a particle made up of four quarks. The tetraquark particle, named X(6900), is thought to be made of two charm quarks and two of their antimatter counterparts, anticharm quarks. This is the first time that a particle has been discovered to be made up of more than two heavy quarks, as the charm quark is the heaviest known quark.
Typically, particles contain either three quarks, like protons and neutrons, or a quark-antiquark pair, such as pions. However, for decades theorists predicted the existence of four and five quark particles. The newly found tetraquark particle contains four quarks of the same type, something which has never been observed before. Previously, there have been exotic particles found, such as quark quartets and quintets, but they have usually been made of lighter particles and never contain more than two heavy quarks.
The particle-hunting method the LHCb experiment used was to look for an excess of collision events over a smooth background of events, which would result in a bump. By using the first run data (taken from 2009-2013) and second run data (taken from 2015-2018), researchers were able to find a bump in the mass distribution of a pair of J/Ψ particles. Both of which contain a combination of a charm and anticharm quarks. The bump discovered has a statistical significance of more than five standard deviations, what is usually considered the threshold for discovering a new particle, and this bump corresponds to the tetraquark.
With a mass of 6900 million electron-volts, it is one of the heaviest tetraquark particles discovered and thus could be very useful in studying what is known as “strong interaction” or “strong force”. This strong force is what binds quarks together and, therefore, also binds neutrons, protons, and atomic nuclei together.
The discovery of this particle provides a good test for theorists’ current understanding of quantum chromodynamics, the theory of strong interaction. As the particle is heavy, it provides an extreme and yet theoretically simple case, which models can be tested against, and can also explain ordinary matter particles such as neutrons and protons. It is easier to study heavier tetraquark particles because lighter four quark particles usually have more complex, fast-moving behaviour.
It may be too good to be true though. There is a chance that X(6900) could actually be a pair of quark-antiquark particles that are weakly bonded together. Therefore, more data will be needed in order to find out more about the newly discovered particle, from the LHC, and other particle collision experiments.
The discovery of X(6900) could open up a new, long awaited test of the Standard Model of particle physics. Since quarks make up the most fundamental particles, neutrons and protons, studying how four quarks bind together will be able to give us insights to the very basics of particle physics. The strong force is being tested and it will be very interesting to see how it will hold up, or whether anything unexpected will come up instead.
Written by Jessie Hammond and edited by Tara Wagner-Gamble.
Jessie’s thoughts… Personally, I wasn’t expecting this discovery and neither was I expecting it to have such ramifications into the strong force. For me, anything that could potentially challenge what we already know is exciting within physics, for the reason that it opens so many possibilities!
I do wonder how and why particles usually have three or two quarks. The discovery of a four quark particle could help to figure out why this is, and opens up the future to more discoveries of these types of particles. I am also really intrigued to see whether five quark particles could be found that are similar to the tetraquark found and what the limits of quarks making particles are. Can you have a five quark particle with five charm quarks? Is there a more common combination of a five quark or four quark particle?
The strong interaction aspect this discovery has is also really intriguing. As someone who is interested in all four fundamental forces and how they may be intertwined, I am really interested to see how well theorists’ models hold up to the X(6900) discovery and what can be found from this discovery.
From this one particle there are so many questions. What does this mean for the Standard Model? What does it mean for our understanding of quarks and the strong interaction? I cannot wait to see the ramifications of this discovery.