Naming Exotic Hadrons

The LHCb collaboration at the Large Hadron Collider has developed a new set of rules for naming composite particles bound by the strong force (i.e. hadrons) that are contemplated by existing rules for naming them.

The paper also neatly summarizes the new hadrons that have been discovered at the LHC, some of which have made this naming convention necessary (although others were predicted to exist in advance):

Many new exotic hadrons, that do not fit into the existing naming scheme for hadrons, have been discovered over the past few years. A new scheme is set out, extending the existing protocol, in order to provide a consistent naming convention for these newly discovered states, and other new hadrons that may be discovered in future.

LHCb collaboration, "Exotic hadron naming convention" arXiv:2206.15233 (June 30, 2022) (LHCb-PUB-2022-013).

The proposal is as follows (which is explained at greater length in the fourteen page pdf):

1. As in the current scheme, symbols are assigned based on measured quantum numbers, rather than speculation about the degrees of freedom within the hadron. 

2. There is no change to the existing scheme for any state that does not unambiguously have four- or five-quark content. 

3. States with minimum four-quark content are labelled T; states with minimum five-quark content are labelled P. T states are mesons, while P states are baryons.<2> 

<2> The current scheme is limited to states with minimum four- or five-quark content. It should, however, be possible to extend later to states with minimum six- or seven-quark content, if necessary. It is also limited to the allowed isospin values of conventional hadrons, although in principle value of I up to 2 for T states and 5/2 for P states are possible. Extension of the scheme at a later date to cover more isospin values will also be possible if necessary.

4. A superscript is added to indicate the isospin, and (where appropriate) parity and G-parity. The superscripts follow existing conventions for labelling these properties, which differ for the cases of mesons with zero or non-zero net strangeness and heavy flavour quantum numbers, and differ again for baryons. 

(a) For T states with zero net strangeness and heavy flavour quantum numbers, the symbols from the first two rows of Table 1 are used, to cover the eight possibilities of I = 0, 1, and even or odd P and G-parity. The symbol ω is used (instead of φ) for the case I = 0, P and G odd. 

(b) For T states with non-zero strangeness, charm or beauty quantum numbers, the G-parity is not defined, and therefore the superscript encodes only the isospin and parity. The labels η, τ, π are used for I = 0, 1/2 , 1 with P odd, and f, θ, a are used for I = 0, 1/2 , 1 with P even.<3> 

<3> The symbols for I = 1/2 refer to the historical “τ–θ” puzzle, in which decays of the kaon to three or two pions, which are respectively parity-odd and parity-even final states, were denoted by τ and θ. 

(c) For P states, the superscript indicates the isospin only: it is Λ, N, Σ, ∆ for I = 0, 1/2 , 1, 3/2. These superscripts are summarised in Table 4. It should be stressed that the superscript does not convey the spin (see below); the distinction between π and ρ is thus G-parity rather than spin, as is also formally the case for conventional mesons in the PDG convention. Note also that there is no need to add ∗ to denote natural spin-parity mesons.  

5. Subscripts Υ, ψ and φ are added to denote hidden beauty, charm and strangeness. These should be in order of mass, where more than one is needed, and can be repeated if necessary. Since ss¯ content may mix with uu¯ + d ¯d content, the φ subscript should only be applied where the ss¯ content is clear. It must be stressed that this reuse of the symbols for vector mesons conveys information only about the quark content, not about their arrangement or other quantum numbers.

6. Subscripts b, c and s are added to denote open flavour quantum numbers. These should be in order of mass, where more than one is needed, and can be repeated if necessary. They indicate the quark content, rather than the flavour quantum number. If only one such symbol is needed, it should always be that of the quark (b, c, s) rather than the antiquark (¯b, c¯, s¯). If more than one is needed, the symbols can be those of quark or antiquark but the first should always be a quark. If subscripts are needed to indicate both hidden and open flavour, the symbols for hidden flavour should appear first. 

7. For T states an additional subscript should be added to indicate the spin J, while for P states, the spin-parity should be added after the name. These labels are thus added in the same way as in the current naming schemes for mesons and baryons, respectively. This information is omitted when it is not yet known. 

8. For neutral T states with open flavour quantum numbers, the particle and antiparticle are distinguished by the addition of an overline to the latter. The “particle” is chosen to be that with the quark content specified in the subscript.4 Overlines are never used for charged T states, as in the current scheme for mesons. P states with baryon number +1 (−1) are always written without (with) an overline, as in the current scheme for baryons. 

<4> The choice of which neutral meson is particle and which is antiparticle does not have any physical significance, it is simply whether the symbol does not or does include an overline.

9. For all hadrons, the mass, in units of MeV/c 2, should be added in parentheses, and the charge superscript should be added where appropriate. 

10. In practice it is anticipated that some labels will be omitted where they are considered unnecessary.

The new scheme renames a number of existing exotic hadrons: