Exotic hadrons brought into order by LHCb

5 September 2022
LHCb’s latest tetraquarks

With so many new hadronic states being discovered at the LHC (67 and counting, with the vast majority seen by LHCb), it can be difficult to keep track of what’s what. While most are variations of known mesons and baryons, LHCb is uncovering an increasing number of exotic hadrons, namely tetraquarks and pentaquarks. A case in point is its recent discovery, announced at CERN on 5 July, of a new strange pentaquark (with quark content ccuds) and a new tetraquark pair: one constituting the first doubly charged open-charm tetraquark (csud) and the other a neutral isospin partner (csud). The situation has prompted the LHCb collaboration to introduce a new naming scheme. “We’re creating ‘particle zoo 2.0’,” says Niels Tuning, LHCb physics coordinator. “We’re witnessing a period of discovery similar to the 1950s, when a ‘zoo’ of hadrons ultimately led to the quark model of conventional hadrons in the 1960s.” 

While the quark model allows the existence of multiquark states beyond two- and three-quark mesons and baryons, the traditional naming scheme for hadrons doesn’t make much allowance for what these particles should be called. When the first tetraquark candidate was discovered at the Belle experiment in 2003, it was denoted by “X” because it didn’t seem to be a conventional charmonium state. Shortly afterwards, a similarly mysterious but different state turned up at BaBar and was denoted “Y”. Subsequent exotic states seen at Belle and BESIII were dubbed “Z”, and more recently tetraquarks discovered at LHCb were labelled “T”. 

Complicating matters further, the subscripts added to differentiate between the various states lack consistency. For example, the first known tetraquark states contained both charm and anticharm quarks, so a subscript “c” was added. But the recent discoveries of tetraquarks and pentaquarks containing a single strange quark require an extra subscript “s”. On top of all of that, explains LHCb’s Tim Gershon, who initiated the new naming scheme, tetraquarks discovered by LHCb in 2020 contain a single charm quark. “We couldn’t assign the subscript ‘c’ because we’ve always used that to denote states containing charm and anticharm, so we didn’t know what symbols to use,” he explains. “Things were starting to become a bit confusing, so we thought it was time to bring some kind of logic to the naming scheme. We have done this over an extended period, not only within LHCb but also involving other experiments and theorists in this field.”  

Helpfully, the new proposal labels all tetraquarks “T” and all pentaquarks “P”, with a set of rules regarding the necessary subscripts and superscripts. In this scheme, the two different spin states of the open-charm tetraquarks discovered by LHCb in 2020 become Tcs0(2900)0 and Tcs1(2900)0 instead of X0(2900)0 and X1(2900)0, for example, while the latest pentaquark is denoted PΛψs(4338)0. The collaboration hopes that the new scheme, which can be extended to six- or seven-quark hadrons, will make it easier for experts to communicate while also helping newcomers to the field. 

The new scheme could make it easier to spot patterns that might have been missed before

Importantly, it could make it easier to spot patterns that might have been missed before, perhaps shedding light on the central question of whether exotic hadrons are compact tightly bound multi-quark states or more loosely bound molecular-like states. The new LHCb scheme might even help researchers predict new exotic hadrons, just as the multiplets arising from the quark model made it possible to predict new mesons and baryons such as the Ω. 

“Before this new scheme it was almost like a Tower of Babel situation where it was difficult to communicate,” says Gershon. “We have created a document that people can use as a kind of dictionary, in the hope that it will help the field to progress more rapidly.” 

Further reading

LHCb Collab. 2022 arXiv:2206:15233.

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