The quark model was an intellectual revolution for physics. Physicists were faced with an ever-growing zoo of unstable particles that seemed to play no part in the universe around us. Quarks explained all that through a (at least superficially) simple set of rules that built all these particles through combinations of two or three quarks.
While that general overview may seem simple, the rules by which particles called “gluons” hold the quarks together are devilishly complex, and we don’t always know their limits. Are there reasons why particles seem to stop at sets of three quarks?
With the advent of increasingly powerful particle accelerators, we’ve found some evidence that the answer is “no.” Reports of four-quark and even five-quark particles have appeared in several experiments. But questions remain about the nature of the interactions in these particles. Now, CERN has announced a new addition to a growing family of tetraquarks, a collection of two charm quarks and two anti-charm quarks.
How do you put that together?
The quarks-based particles we are most familiar with, the proton and neutron, are composed of three of the lightest quarks tightly bound together via gluons. We’ve also discovered heavier versions of these known particles, where one of the up or down quarks has been replaced by a heavier quark, such as an alien or bottom. In addition, there is a large collection of unstable particles, collectively called mesons, involving two quarks of different masses, also held together by gluons.
So, what happens when you try to cram in more quarks? We’re not quite sure. Two options are being considered. In one case, the new high quark particles are made in the same way as the familiar ones: gluons bind them tightly together into a single particle. An alternative, however, is that the large number of quarks arises because two more well-known particles are closely connected. So a tetraquark can simply be a close association of a pair of two-quark particles. A pentaquark would be composed of a two-quark meson that associates with a three-quark particle.
Unfortunately, we found it difficult to distinguish between these two options. These high quark number particles tend to decay extremely quickly into known particles, and it’s generally only the decay of those last particles that we can track. That makes it a challenge to determine exactly what is going on further back. So the more ways we can look at these things, the better. And that brings us to the latest results from CERN, in which a team of scientists analyzed data from the LHC’s first few runs.
The data comes from the LHCb experiment, a detector that specializes in particles containing the very heavy bottom (or beauty) quark. But it is capable of picking up heavier quarks in general. And the new particle has much heavier quarks.
Needs more charm
So far, all the high quark number particles we’ve found have been a mix of mostly lighter up and down quarks, with a few of their heavier ilk. But the CERN team was interested in looking for combinations where all quarks were either charm or anti-charm. Charm quarks are middle-generation quarks; charm and strange are heavier than above or below, but much lighter than above or below.
How would we find such a thing? Conveniently, a particle with four charms decays 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 they both gave it have stuck.) Since we know how J/ψ particles decay, we can easily look for pairs of decays that arises 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 originating from a single location. (Muons can be thought of as heavier, unstable cousins of the electron.) Since there should be two of the J/ψ particles, we need to look for two pairs of muon tracks in the aftermath of a collision.
So the researchers scanned a range of energies for an excess of these events. And they find one in the right energy range for a four-charm particle that is five standard deviations from the expected background noise. That means it meets the standard for discovery in particle physics.
The new particle, currently unnamed, is the first to have more than three quarks composed entirely of one type of quark, and the first to be composed entirely of heavier quarks. And since the particular quark is called “charm,” its existence opens up huge puns — and that’s without even going into the fact that the technical term for the entire family of particles containing these quarks is “charmonium.” ‘ is.
What is going on?
But the big question has not yet been answered: what is the nature of the new particle? We don’t know if it’s just two J/ψ particles closely connected or if there’s a single particle made up of four charm quarks. The answer is quite important, as it would provide information about the strong force that governs all quark-gluon interactions.
But the more of these particles we have to study, the more likely we are to determine the details of what it takes to build them. So while it would have been nice for this particle to immediately announce the solution to an open question, its discovery will likely represent progress toward a solution.
The arXiv. Summary Number: 2006.16957 (About the arXiv).