The discovery of a new Tetraquark in a nutshell
Ayush Kumar. 11/07/2020
Particle physics is such a unique and interesting topic, but first, let’s explore the fundamentals!
Quarks are the particles which constitute the protons and neutrons and were discovered after the invention of particle accelerators. They are always found in clusters bound to each other through means of strong nuclear force (one of the fundamental forces of the universe, which affects a very small region in the nuclei of atoms). For example, protons are made up of two up and one down quark, while neutrons are made up of two down and one up quark.
There are six types of quarks, also referred to as flavors, which differ from each other in terms of mass and charge characteristics. These can be grouped into three pairs: up and down, strange and charm, and top and bottom. Quarks are also further divided into antiquarks, or counterparts of quarks. When combined, quarks and antiquarks form hadrons. Quarks are also classified into colors, even though they do not possess any color in reality. Different colors indicate the source of their strong nuclear force which binds them together. Now let’s learn about tetra-quarks.
The discovery
So, now to understand the importance and unusualness of this discovery, let’s learn about the usual characteristics associated with quarks. Simply put, quarks are the fundamental particles of the universe that constitute matter, mostly three-quark particles— protons, neutrons, mesons, etc. These particles do not contain more than two quarks of the same type, specifically not more than two heavy quarks (quarks with comparatively large masses).
But this newly discovered particle actually has four quarks (which is rare to be found), two charms and two anti-charms. This means that all the quarks are of the heaviest type! This discovery baffles scientists and has made scientists realize that there is much more to learn about quarks.
This four quark particle has been named X6900 and was discovered at Large Hadron Collider (LHC), near Geneva, Switzerland. The European Organization for Nuclear Research (CERN) scientists reported the discovery on June 30, 2020. The first four-quark particle was discovered in 2014 and a year later, penta-quarks (five-quark particles) were discovered by LHC beauty (LHCb) Collaboration. Outgoing spokesperson of the LHCb Collaboration Dr. Giovanni Passaleva states, “Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type, specifically two charm quarks and two charm anti-quarks.” He also mentioned that the LHCb and other experiments had only observed tetra-quarks with two heavy quarks at most and not a single with more than two quarks of the same type or with four heavy quarks.
The LHC data showed that this four-quark particle decayed into two J/ψ particles each containing a charm and a charm anti-quark. X6900 can be a single entity with four quarks bundled together, or a pack of two-quark particles loosely bound to each other.
Impact of discovery on research
It will be easier for scientists to track observations for X6900 over any other four-quark particle consisting of lighter quarks, as it is quite difficult to accurately predict the complex behavior of light quark particles. Thus, it will be easier for scientists to understand various unknown properties of quarks. This heavy quark particle will also answer some of the most important questions in particle-physics, such as how quarks constructed matter and how strong-nuclear forces interact with various quarks.
