Enlarge /. Traces of particles from the LHCb detector.
The quark model was an intellectual revolution for physics. The physicists were faced with an ever-growing zoo of unstable particles that didn't seem to matter in the universe around us. Quarks explained all of this through a (at least superficially) simple set of rules that built all of these particles through combinations of two or three quarks.
While this general outline appears simple, the rules by which particles called "gluons" hold quarks together in particles are devilishly complex, and we don't always know their limits. Are there reasons why particles remain in collections of three quarks?
With the advent of increasingly powerful particle colliders, we have found some evidence that the answer is "no". Reports of four-quark and even five-quark particles have appeared in various experiments. However, questions remain about the nature of the interactions in these particles. CERN has now announced a new addition to the growing family of tetraquarks, a collection of two charm quarks and two anti-charm quarks.
How do you put it together?
The quark-based particles that we know best, the proton and the neutron, consist of three of the lightest quarks that are closely linked by gluons. We have also discovered heavier versions of these known particles, in which one of the top or bottom quarks is replaced by a heavier quark, such as a strange or a lower one. In addition, there is a large collection of unstable particles, collectively called mesons, that comprise two quarks of different masses, which are also held together by gluons.
So what happens when you try to stuff more quarks? We are not quite sure. There are two options that can be considered. In one case, the new particles with a high quark number are produced in exactly the same way as the known ones: gluons bind them tightly together to form a single particle. However, an alternative is that the large number of quarks is created because two more well-known particles are closely linked. A tetra quark could simply be a close connection between a pair of two quark particles. A pentaquark would be composed of a two-quark meson associated with a three-quark particle.
Unfortunately, we found it difficult to distinguish between these two options. These high quark number particles tend to disintegrate extremely quickly into known particles, and in general we can only follow the decay of these latter particles. This makes it difficult to determine exactly what is going on behind. The more options we have to look at these things, the better. And that brings us to the latest results from CERN, in which a team of scientists analyzed the data from the first LHC runs.
The data comes from the LHCb experiment, a detector that specializes in particles containing the very heavy bottom (or beauty) curd. But it is able to absorb heavier quarks more generally. And the new particle has many heavier quarks.
Needs more charm
So far, all particles with a high quark count were a mixture of mostly lighter up and down quarks, into which some of their heavier colleagues were thrown. However, the CERN team was interested in looking for combinations where all the quarks were either charm or anti-charm. Charm quarks come from the middle generation of quarks. Charm and strange are heavier than the top or bottom, but much lighter than the top or bottom.
How would we find something like that? Conveniently, a particle with four charms should decay through an intermediate state involving a pair of particles with two charms. And we know this very well as the J / ψ particle. (Two groups found this particle at about the same time, and in a rare moment of compromise, the names that both gave it remained.) Since we know how J / ψ particles decay, we can simply search for pairs of them Decays from a single proton-proton collision.
The decay of J / ψ particles can in turn be recognized by the appearance of a muon-antimuon pair that comes from a single location. (Muons can be considered heavier, unstable cousins of the electron.) Since there should be two of the J / ψ particles, we have to look for two pairs of muon traces after a collision.
Therefore, the researchers searched a number of energies for an excess of these events. And they find one in the appropriate energy range for a four-charm particle that has five standard deviations from the expected background noise. That is, it meets the standard for discovery in particle physics.
The new particle, which is currently unnamed, is the first with more than three quarks that is made entirely from one type of quark, and the first to be made entirely from heavier quarks. And given that the specific curd is called "charm," its existence opens up tremendous opportunities for wordplay – without even considering the fact that the term for the entire family of particles that contain these curds, "Charmonium "is.
However, the big question was not answered: what is the nature of the new particle? We do not know whether there are simply two J / ψ particles in close association or whether there is a single particle that consists of four charm quarks. The answer is quite important as it would provide information about the strong force that controls all of the quark-gluon interactions.
However, the more of these particles we need to study, the greater the likelihood that we can determine the details of what is required to build them. While it would have been nice if this particle immediately announced the solution to an open question, its discovery is probably an advance towards a solution.
The arXiv. Abstract number: 2006.16957 (About the arXiv).