Physicists at Cern have found a plethora of new exotic particles becoming developed in the collisions manufactured by the Big Hadron Collider around the earlier couple of yrs. So numerous have been identified in actuality, that our collaboration (LHCb), which has identified 59 out of 66 modern particles, has appear up with a new naming scheme to assist us impose some get on the rising particle zoo
Particle physicists have a fairly chequered history when it will come to naming matters. As far more and additional particles were identified around the training course of the 20th century, the nomenclature became significantly befuddling. For occasion, in the team of leptons we have electrons, muons and then taus, but not tauons.
And when two rival groups in the 1970s were being not able to concur whether or not a new particle consisting of two quarks (the smallest building blocks of make any difference) they had just discovered should really be identified as J or ψ (psi), they ended up awkwardly smooshing the two names with each other to get J/ψ.
Even nowadays, physicists are not able to concur irrespective of whether to phone the fifth heaviest quark “bottom” or “beauty” – and as a result use the two interchangeably. And let’s not even get started off on the appallingly named bestiary of particles predicted by the concept identified as “supersymmetry”, which implies each and every particle we know also has a (but undiscovered) super spouse: sstrange [sic], squark, smuon or gluino anyone? Frankly, it’s just as well they never seem to exist.
The LHC has been a treasure trove for new sorts of particles called hadrons. These are subatomic particles produced from two or more quarks. Conventionally, these occur in two forms. Baryons, such as the protons and neutrons which make up the atomic nucleus, are built of three quarks. Mesons, on the other hand, are made of a quark paired with an antiquark (every single basic particle has an antiparticle with the similar mass but reverse demand).
Whilst there are only six different kinds of quarks, and only five of these kind hadrons, there are a large quantity of probable combinations. In the 1980s, particle physicists devised a naming scheme for the hadron zoo, with a symbol for just about every particle that made it straightforward to discern its quark content, this sort of as the Greek letter Π (pi) to denote pions, the lightest mesons.
Till modern years, all freshly uncovered particles fitted properly into that plan as both baryons or mesons. But researchers at some point realised that more complex hadrons with more than 3 quarks could also be doable: so-named tetraquarks, composed of two quarks and two antiquarks and pentaquarks, composed of four quarks and just one antiquark (or the other way close to).
The 1st crystal clear tetraquark candidates ended up found by the Belle and BESIII collaborations, and labelled Zc states (this was a random option, X and Y had presently been utilized to label other states). This was adopted by the stunning discovery of pentaquark states, labelled Computer system, by the LHCb collaboration. Due to the fact all-around 2019, the price of discovery has accelerated, with names these as X, Zcs, Pcs and Tcc remaining assigned in a additional-or-fewer advertisement-hoc trend, top to an alphabet soup of particles.
The absence of logic underlying the names provided to the new particles led, potentially inevitably, to some confusion. A specific challenge was that the subscript “c” in the Zc and Pc symbols was meant to indicate that these hadrons contain equally charm and anticharm quarks (often identified as “hidden charm”). By distinction, the subscript “s” in the Zcs and Pcs symbols indicates that these hadrons also have a unusual quark (“open strangeness”). So then what must states that include both open up attraction (a charm quark by yourself) and strangeness, as uncovered lately by the LHCb collaboration, be named?
Dominguez, Daniel/Cern, CC BY-NC
As the assortment of new states and their assigned names risked starting to be more perplexing, we and colleagues in the LHCb collaboration made a decision it was time to test to restore some semblance of get – at least for the freshly uncovered particles. Our new naming plan, follows some guiding concepts. For starters, the essential idea should be easy enough for non-industry experts to follow, obtained with a base symbol of T for tetraquarks and P for pentaquarks.
The plan should really also let for all feasible mixtures to be distinguished this is accomplished by addition of superscripts and subscripts to the base to denote which quarks every single particle is built from and other quantum details. But these must be dependable with the current scheme for common mesons and baryons – reached by reusing existing symbols.
Latest names for exotic hadrons would need to have to be improved, nevertheless. For instance, the Zcs and Pcs states stated over will develop into identified as Tψs and Pψs, respectively (the J/ψ particle incorporates hidden attraction), fixing the challenge of distinguishing concealed from open up appeal by reusing ψ for the former and c for the latter.
The final guiding principle at the rear of the plan is that it should really be recognized by the wider particle physics group. Though the LHCb collaboration has learned most of the new particles, which customarily offers us some naming legal rights, there are other recent and planned experiments in this space, and their contributions are necessary for the progress of the area. There are also, of course, quite a few theorists throughout the world working hard to interpret the measurements that are remaining built.
Each the normal principles and the particulars of the new naming plan have been mentioned with these distinct groups, with favourable and constructive suggestions included into our last model.
A naming scheme is an essential section of the language utilized to communicate involving men and women working in particle physics. We hope that this new scheme will enable in the ongoing quest to comprehend how the so-termed strong power confines quarks inside of hadrons, for instance – a aspect that defies deep mathematical comprehension.
New experimental effects together with the discoveries of new hadrons are fuelling enhancements in theoretical knowing. Even more discoveries could just one working day lead to a breakthrough. Eventually, though, the achievement of the new scheme will be judged by how frequently conversations incorporate the phrase: “Remind me, which 1 is that again?”
Harry Cliff gets funding from the Science and Technological innovation Facilities Council (Uk).
firstname.lastname@example.org isles receives funding from the Science and Engineering Services Council (United kingdom).