Physicists at Cern have discovered a plethora of exotic new particles being created in collisions produced by the Large Hadron Collider in recent years. So many have been found, in fact, that our collaboration (LHCb), which has discovered 59 of the 66 recent particles, has come up with a new naming scheme to help us impose some order on the growing particle zoo.
Particle physicists have a pretty checkered history when it comes to naming things. As more and more particles were discovered throughout the 20th century, the nomenclature became increasingly confusing. For example, in the lepton group, we have electrons, muons, and then tau, but no tauons.
And when two rival teams in the 1970s couldn’t agree on whether a new particle consisting of two quarks (the smallest building blocks of matter) they had just discovered should be named J or ψ (psi), they ended up clumsily mixing the two names. together to get J/ψ.
Even today, physicists can’t agree on whether to call the fifth heaviest quark “bottom” or “beauty” and thus use the two interchangeably. And let’s not even get started on the terrifyingly named particle bestiary predicted by the theory known as “supersymmetry,” which suggests that every particle we know of also has a (yet-undiscovered) super partner: weird. [sic], squark, smuon, or gluino anyone? Frankly, it’s better if they don’t seem to exist.
complex hadrons – The LHC has been a treasure trove for new types of particles called hadrons. These are subatomic particles made up 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 up of three quarks. Mesons, on the other hand, are made of a quark paired with an antiquark (every 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 them make up hadrons, there are a large number of possible combinations. In the 1980s, particle physicists devised a naming scheme for the hadron zoo, with a symbol for each particle that made it easy to discern its quark content, such as the Greek letter Π (pi) to denote pions, the smallest mesons. light.
Until recent years, all newly discovered particles fitted nicely into that scheme as baryons or mesons. But scientists eventually realized that more complicated hadrons with more than three quarks might also be possible: so-called tetraquarks, made up of two quarks and two antiquarks; and the pentaquarks, made up of four quarks and one antiquark (or the other way around).
The first clear candidate tetraquarks were discovered by the Belle and BESIII collaborations and labeled Zc states (this was a random choice; X and Y had already been used to label other states). This was followed by the spectacular discovery of pentaquark states, called Pc, by the LHCb collaboration. Since around 2019, the rate of discovery has accelerated, with names like X, Zcs, Pcs, and Tcc being assigned on a more or less ad-hoc basis, 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. The particular problem was that the subscript “c” in the symbols Zc and Pc implied that these hadrons contained both charm and anti-charm quarks (sometimes called “hidden charm”). In contrast, the subscript “s” in the symbols Zcs and Pcs implies that these hadrons also contain a strange quark (“open strangeness”). So how should states containing both open charm (a charm quark alone) and strangeness, as recently discovered by the LHCb collaboration, be named?
As the array of new states and their assigned names risked becoming even more puzzling, we and colleagues in the LHCb collaboration decided it was time to try to restore some semblance of order, at least to the newly discovered particles. Our new naming scheme follows a few guiding principles. First of all, the basic idea should be simple enough for non-experts to follow, achieved with a base symbol of T for tetraquarks and P for pentaquarks.
The scheme must also allow all possible combinations to be distinguished; this is done by adding superscripts and subscripts to the basis to indicate which quarks each particle is made of and other quantum information. But these should be consistent with the existing scheme for conventional mesons and baryons, achieved by reusing existing symbols.
However, the current names of the exotic hadrons would need to be changed. For example, the states Zcs and Pcs mentioned above will be known as Tψs and Pψs, respectively (the particle J/ψ contains hidden charm), solving the problem of distinguishing hidden charm 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 broader particle physics community. Although the LHCb collaboration has discovered most of the new particles, which traditionally gives us some naming rights, there are other current 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 are working hard to interpret the measurements that are being made.
Both the general principles and the details of the new naming scheme have been discussed with these different groups, with positive and constructive feedback incorporated into our final version.
A naming scheme is an important part of the language used to communicate between people working on particle physics. We hope that this new scheme will help in the ongoing quest to understand how the so-called strong force confines quarks within hadrons, for example, a feature that defies deep mathematical understanding.
New experimental results, including discoveries of new hadrons, are driving improvements in theoretical understanding. Other discoveries could one day lead to a breakthrough. Ultimately, though, the success of the new scheme will be judged by how often conversations include the phrase, “Remind me, what’s that?”
This article was originally published on The conversation by harry cliff at Cambridge University Y Tim Gerson in the University of Warwick. Read the original article here.