In nuclear fusion, energy is produced by the rearrangement of protons and neutrons. The discovery of an analogue of this process involving particles called quarks has implications for both nuclear and particle physics. See Letter p.89
Stars are powered by nuclear fusion, in which two or more atomic nuclei have a close encounter and form one or more different nuclei1. A key aspect of nuclear fusion is that the rearrangement of protons and neutrons between initial- and final-state nuclei releases energy. Protons and neutrons are made of elementary particles called quarks, and a long-standing question has been whether systems of quarks can undergo a reaction similar to nuclear fusion. On page 89, Karliner and Rosner2 identify such a process and, in doing so, predict the existence of a tetraquark — an exotic particle comprising four quarks.
When two quarks are sufficiently close together, they exchange massless particles called gluons. These interactions, known as strong interactions, bind the quarks together. Quarks come in six types, whimsically called flavours: up, down, strange, charm, beauty (also known as bottom) and top. Each quark also has an antimatter partner, known as an antiquark. Up and down quarks form the main content of protons and neutrons, but the central players of the current study are the much heavier charm and beauty quarks.
More than 40 years ago, the Λc particle (containing one up, one down and one charm quark) was discovered3. And earlier this year, experimentalists working at the Large Hadron Collider near Geneva, Switzerland, detected the Ξcc++ particle, which comprises one up and two charm quarks4. The Ξcc++ particle is stable, which implies that its two charm quarks are strongly bound.
Karliner and Rosner report that this strong binding allows a quark-rearrangement process to occur, in which two Λc particles interact to produce a Ξcc++ particle and a neutron (Fig. 1). The authors show that the rearrangement generates a substantial amount of energy — about 12 million electronvolts (MeV). The process is analogous to a nuclear-fusion reaction in which two helium-3 nuclei combine to form a helium-4 nucleus and two protons, releasing a similar amount of energy (12.86 MeV).
Charm quarks can form a bound state because they are relatively heavy — their low kinetic energy is overcome by the attractive effect of gluon exchange. Consequently, Ξcc++ is bound, whereas a system in which the charm quarks in Ξcc++ are replaced by lighter strange quarks would be unbound. Beauty quarks are about three times heavier than charm quarks, and, because a larger mass implies a stronger binding, Karliner and Rosner previously predicted the existence of the Ξbb0 particle, which contains one up and two beauty quarks5. The authors show that, should such a particle exist, the quark-fusion reaction that produces Ξbb0 would release about ten times more energy than the equivalent process involving charm quarks.
There is another important consequence of the binding of heavy quarks. Karliner and Rosner predict that a tetraquark comprising two beauty quarks, an up antiquark and a down antiquark would be bound (see also ref. 6). The original quark model developed in the 1960s was postulated to, and did successfully, explain all the particles known at the time7,8,9. Such particles were made of either two or three quarks, raising the question of why systems of four or even five quarks had not yet been found. The authors' prediction is therefore remarkable in its own right, because it provides a possible answer: the quarks that make up ordinary matter are too light to bind together to form four- or five-quark systems.
Assuming the tetraquark exists, the authors point out that reactions are possible in which two particles containing beauty quarks fuse to form the tetraquark. Such processes are analogous to the fusion of a proton and a neutron to form a nucleus of the hydrogen isotope deuterium. The authors show that the energy released from the tetraquark reactions is of the order of 200 MeV — more than 10 times that produced in nuclear fusion.
At present, the quark-fusion processes discovered by Karliner and Rosner have no practical applications. This is because the particles containing heavy quarks (such as Λc and Ξcc++) are stable only with respect to the strong and electromagnetic interactions. They decay with a lifetime of about 10−10 seconds through weak interactions — those responsible for radioactive decay. Although this seems an extremely short time, it is about 100 billion times longer than the typical time that it takes for a quark to move from one end of a particle to the other. In other words, such particles can be considered to be stable in the context of the time taken for fusion to occur.
Other implications of the authors' findings include the possible existence of nuclei containing two charm quarks or two beauty quarks. These nuclei could be produced in particle collisions in which at least one of the particles is a heavy ion (such as a lead ion). Going even further, a previously undiscovered form of stable matter consisting mainly of beauty quarks could exist. Such matter might have important consequences for cosmology10. The authors' new work, along with that of others, shows that there is still much to learn about strong interactions.Footnote 1
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