When physicists at CERN’s Large Hadron Collider, one of the most ambitious experimental machines ever built, provided affirmation of the famously termed ‘God Particle’ and along with it the greatest scientific discovery of the decade, one may have expected that they would rest easy for a while. They did not.
Using the LHCb experiment (which complements the ATLAS detector of Higgs Boson fame) at the world’s largest particle accelerator, an entirely new form of matter known as the tetraquark has been confirmed. Enigmatically named ‘Z(4330)’, the particle is four times as massive as a proton and has a negative electromagnetic charge.
Particle physics is the study of subatomic particles and the forces that they exert on each other. We know that atoms are made of negatively charged electrons orbiting a tiny, heavy nucleus, which contains positively charged protons and neutrons (particles with no charge). But which particles are truly fundamental? Which particles are the smallest building blocks in nature? It turns out that many particles, including protons and neutrons, are made of combinations of quarks.
Protons and neutrons are hadrons, each comprising three quarks. Protons comprise up-up-down quarks, while neutrons comprise up-down-down. All hadrons are held together by the strong nuclear force. (Credit: Swinburne Astronomy Online)
Quarks are subatomic particles that are unique in having fractional electromagnetic charges of 1/3 or 2/3 (compared to electrons and protons, which have an integer charge of -1 and +1 respectively), as well as exhibiting another characteristic that physicists call colour-charge. While there are just two types of electromagnetic charge – positive and negative – the property of colour-charge is slightly more complex. There are the red, green and blue colours, as well as their opposites: anti-red, anti-green and anti-blue. (Almost all particles have a corresponding “anti-particle”.)
The labelling of a quark’s colour has nothing to do with our eye’s perception of visible light. Instead it describes the tendency of quarks to combine in neutral states. Just as visible light’s colours mix to produce white light, red, green and blue quarks will form a colour-neutral particle (just as the combination of a positively and negatively charged particle will result in a charge-neutral particle). When quarks combine, the resulting particle will be colour-neutral.
Quarks - which are never found in isolation – are held together by the strong nuclear force, governed by the what is called the “theory of quantum chromodynamics” (or QCD) that fits into the Standard Model of particle physics. The Standard Model attempts to describe the fundamental particles and how they interact, and is able to explain nearly all of the interactions we know about: electromagnetic, weak and strong nuclear forces… but not gravity. Within the Standard Model, bound quarks form the family of particles known as hadrons, which all feel the strong nuclear force.
Until a week ago hadrons were known to come in two distinct varieties: baryons, which are made of three quarks and include protons and neutrons, and mesons, which are made of two quarks (a quark-antiquark pair). There is a new form of hadron: tetraquarks, which contain four quarks.
Tetraquarks comprise two colour-anticolour pairs, thus obeying the ‘colour neutrality’ of quantum chromodynamics. The vast majority of its mass comes from the energy required to hold these quarks together via the strong force. The interesting thing about the tetraquark is that it provides an insight into the mechanism that binds quarks together. By smashing particles together and observing vast numbers of particles as they flash in and out of existence inside the LHCb, particle physicists can replicate conditions similar to that of the energetic early Universe. In the very early Universe, just after the Big Bang, there was a period known as the “quark epoch”, when particle collisions were too energetic to allows quarks to combine to form baryons or mesons. Understanding how quarks bind may help us understand the quark epoch of the early Universe.
The existence of the long theorised tetraquark may also change our perception and description of neutron stars, which are the remnants of massive stars left over after a supernova explosion. The basic model of a neutron star assumes that is comprised almost entirely of neutrons. Because of this, the density of their interior is immense, with a teaspoon of neutron star material weighing around a billion tonnes, and harbouring some of the strongest gravitational fields in the Universe. The existence of a tetraquark is proof that nature is able to form matter with more complex quark configurations than have previously been found. The colossal energy of particle interactions inside the neutron star may allow tetraquarks and even higher quark configurations, such as the hypothesised pentaquark (five quarks) and hexaquark (six quarks), to be produced. This means astrophysicists must revisit the model of a neutron star and the assumptions that dictate its internal structure. If it is possible for these complex quark states to exist, then this opens up the possibility of quarks interacting individually, creating a free quark plasma and giving tantalising hints to the existence of the long sought after quark star.
Neutron stars, comprising neutrons, compared with a (theorised) quark star, comprising free quarks. (Credit: Univirse Today)
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[Toby Brown & Sarah Maddison]