Cern physicists have discovered an abundance of new exotic particles in recent years from the collisions created by the Large Hadron Collider. In fact, so many have been found that our collaboration (LHCb), which discovered 59 of the 66 recent particles, come up with a new naming scheme to help us bring some order to the growing particle zoo
Particle physicists have a rather checkered history when it comes to naming things. As more and more particles were discovered over the course of the 20th century, the nomenclature became more and more confusing. For example, in the group of leptons we have electrons, muons and then taus, but no tauons.
And when two rival teams in the 1970s couldn’t agree on whether new particle composed of two quarks (the smallest building blocks of matter) that they had just discovered should be called J or ψ (psi), they ended up clumsily fusing the two names together to get J/ψ.
Even today, physicists disagree on whether to call the fifth heaviest quark “soil” or “beauty” — which is why they use the two interchangeably. And let’s not even get started on the horribly named bestiary of particles, predicted by the theory known as “supersymmetry,” suggesting that every particle we know of also has a (yet undiscovered) superpartner: strange [sic], squark, smuon or gluino anyone? Honestly, it’s a good thing they don’t seem to exist.
The LHC has been a treasure trove for new types of particles called hadrons. These are subatomic particles made of two or more quarks. Conventionally, these come in two types. Baryons, like the protons and neutrons that make up the atomic nucleus, are made of three quarks. Mesons, on the other hand, are made of a quark combined with an antiquark (each fundamental particle has an antiparticle with the same mass but opposite charge).
Although there are only six different types of quarks, and only five of these hadrons, there is a huge number of possible combinations. In the 1980s, particle physicists came up with a naming scheme for the hadron zoo, with a symbol for each particle that made it easy to distinguish the quark content, such as the Greek letter Π (pi) to denote pions, the lightest mesons.
Until recently, all newly discovered particles fit nicely into that scheme as baryons or mesons. But scientists eventually realized that more complicated hadrons with more than three quarks could also be possible: so-called tetraquarks, composed of two quarks and two antiquarks; and pentaquarks, composed of four quarks and one antiquark (or vice versa).
The first clear tetraquark candidates were discovered by the Belle and BESIII collaborations, and labeled Zc states (this was a random choice, X and Y were already used to label other states). This was followed by the spectacular discovery of pentaquark stateslabeled Pc, through the LHCb collaboration. Since about 2019, the speed of discovery has accelerated, with names like X, Zcspcs and Tcc is assigned in a more or less ad-hoc manner, leading to an alphabet soup of particles.
The absence of logic underlying the names given to the new particles led, perhaps inevitably, to some confusion. A particular problem was that the subscript “c” in the Zc and Pc symbols was meant to imply that these hadrons contain both charm and anticharm quarks (sometimes called “hidden charm”). In contrast, the subscript “s” in the Zcs and Pcs symbols implies that these hadrons also contain a strange quark (“open strangeness”). So what then should be called states that contain both open charm (just a charm quark) and strangeness, as recently found by the LHCb collaboration?
As the array of new states and their assigned names threatened to get even more confusing, we and colleagues in the LHCb collaboration decided it was time to try and restore some semblance of order — at least for the newly discovered particles. Our new naming scheme follows some guiding principles. First, the basic idea should be simple enough for non-experts to follow, achieved with a basic symbol of T for tetraquarks and P for pentaquarks.
The scheme must also make it possible to distinguish all possible combinations; this is done by adding superscripts and subscripts to the base to indicate which quarks each particle is made of and other quantum information. But these must be consistent with the existing scheme for conventional mesons and baryons – achieved by reusing existing symbols.
However, the current names for exotic hadrons should be changed. For example the Zcs and Pcs above states will be known as Ts and Psrespectively (the J/ particle contains hidden charm), solving the problem of distinguishing hidden from open charm by reusing ψ for the former and c for the latter.
The final guiding principle behind the scheme is that it must be accepted by the wider particle physics community. While the LHCb collaboration has discovered most of the new particles, which traditionally gives us some naming rights, there are other ongoing and planned experiments in this area, and their contributions are essential to the progress of the field. Of course, there are also many theorists around the world who work hard to interpret the measurements that are taken.
Both the general principles and the details of the new naming scheme have been discussed with these various groups, with positive and constructive feedback being incorporated into our final version.
A naming scheme is an important part of the language used to communicate between people working in particle physics. We hope this new scheme will aid in the ongoing quest to understand how the so-called strong force traps quarks in hadrons, for example — a feature that defies deep mathematical comprehension.
New experimental results, including the discoveries of new hadrons, provide improvements in theoretical understanding. Further discoveries could one day lead to a breakthrough. Ultimately, though, the success of the new schema will be judged by how often conversations contain the phrase, “Remember, which one is that again?”