1

8. Naming Scheme for Hadrons

8. Naming Scheme for Hadrons
Revised August 2023 by C. Amsler (Stefan Meyer Inst.), E. Pianori (LBNL), D.J. Robinson (LBNL)
and R.L. Workman (George Washington U.).
In the 1986 edition [1], the Particle Data Group introduced an extended and systematizednaming scheme for mesons and baryons. The extensions were necessary in order to name the newparticles containing c or b quarks that were then rapidly being discovered. The 1986 naming scheme
has been retained in all subsequent editions, including this one. However, a number of particles
were since discovered that could not be accommodated in the 1986 scheme. These include, forexample the meson state originally named Zc (3900)+ (now Tcc̄1 (3900)+ ) containing cc̄ but withisospin I = 1 [2, 3]. Because of such discoveries, in the 2017 edition [4] the 1986 naming scheme
was extended to cover cc̄ and bb̄ states with I = 1, including the Zc and Zb states.
The further discovery of various exotic hadron states—including in particular tetraquarks andpentaquarks containing qq q̄ q̄ and q q̄qqq minimal quark content, respectively—has rendered the2017 extension insufficient. These include, for example: the meson state Tcc (3875)+ containing
two charm quarks and with unknown isospin [5]; and the baryon state originally named Pc (4312)+(now Pcc̄ (4312)+ ) containing minimal quark content cc̄uud [6]. In this edition, the 2017 scheme isrevised and extended to cover all experimentally known states.
The naming scheme as a whole provides a name for each hadron, driven primarily by consideration of its quantum numbers, its minimal quark content—the smallest combination of quarks
required to produce a color singlet with the required isospin, parities, strangeness, and open or
hidden-heavy flavor—and its mass. While much of the naming scheme has been developed within
the context of the naive quark model, a hadron name does not designate the internal structureof a state per se: we make no attempt, for example, to distinguish meson states that are “mostlygluonium” from those that are “mostly q q̄.” (Other reviews within the RPP discuss our current
understanding of the nature of various hadron states). While the naming scheme conventions aim
to be comprehensive, exceptions to the scheme are occasionally made for historical reasons (e.g.
J/ψ, p, or n) or in cases where the spectroscopic identity of a state is considered established (e.g.
Υ (1S)). Note also the mass label used in particle names is chosen using the best information available when a name is assigned. A more accurate value of a particle mass may become available ata later time. PDG decides on a case-by-case basis whether to revise the mass label, taking into
account the updated information.

8.1 Light unflavored, cc̄, and bb̄ mesons
Unflavored mesons—also referred to as “neutral-flavor” mesons—have zero strangeness and all
heavy-flavor quantum numbers are equal to zero. Their minimal quark content is denoted q q̄, withthe understanding that for q = u or d, such states may carry unit isospin: the “light unflavored”
nomenclature is understood to encompass all states furnishing an isospin-1 multiplet that contains
a q q̄ neutral meson. Mesons composed of cc̄ or bb̄ are said to have hidden-heavy flavor.
Within the naive quark model, a neutral q q̄ bound state has quantum numbers P = (−1)L+1
and C = (−1)L+S , where S(= 0, 1) and L are the spin and orbital angular momentum of the q q̄system, respectively. The total angular momentum J satisfies |L − S| ≤ J ≤ L + S as usual. Thereis thus a selection rule between the angular-momentum state 2S+1 LJ and the P C configuration,
S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)
31st May, 2024 10:17am

8. Naming Scheme for Hadrons

2such thatPC=− +,+ −,− −,+ +,(8.1)

11

3

3(8.2)

corresponds to
2S+1LJ=(L even)L ,(L odd)L ,(L even)|L−1|≤J≤L+1 ,(L odd)L−1,L,L+1 .

Table 8.1 describes the naming scheme for unflavored mesons. The scheme is categorized byisospin and hidden-heavy flavor (rows) and parity/charge-conjugation configurations (columns); thefirst few allowed J P C combinations for each column are shown in the header. Note C-parity is only
well-defined for neutral unflavored mesons; the states furnishing an isospin-1 multiplet instead havea well-defined G-parity= (−1)L+S+I ; see the review on the quark model. The angular momentum
J is included as a subscript to the base symbol for most unflavored mesons, but is omitted forvarious pseudoscalar, vector, and axial vector mesons whenever disambiguation by the J subscript
is deemed superfluous. The mass is added in parentheses for mesons that decay strongly. However,for some of the familiar mesons (e.g. η 0 , φ, ω), we omit the mass.
Table 8.1: Base symbols for unflavored mesons (zero strangeness andheavy-flavor quantum numbers equal to zero). Each base symbol isuniquely determined by the isospin/hidden-heavy flavor and P C configuration.
(JPC =Minimal quark content¯ uū − dd,¯ dū (I = 1)ud,¯
dd + uū, ss̄
and/or gluons (I = 0)cc̄ (I = 0)bb̄ (I = 0)∗0−+2−+..
.

1+−3+−..
.

1−−2−−..
.

0++
1++
..
.

πb

ρ

aη,η 0h,h0ω,φf ,f 0ηcηbhchbψ∗ΥχcχbWith the exception of the J/ψ.

For light unflavored mesons—i.e. those unflavored mesons formed from u, d, and s quarks—
there can be two isospin-0 mesons: a prime is used to distinguish one from the other (e.g. η andη 0 ), with the exception that vector mesons comprised of uū + dd¯ and ss̄ (ideal mixing) are labeledω and φ, respectively. As usual, we assign the spectroscopic name (e.g. Υ (1S)) as the primaryname to most of those ψ, Υ , and χ states whose spectroscopic identity is known. We use the form
Υ (9460) as an alternative, and as the primary name when the spectroscopic identity is not known.
Determination of the quark content (where relevant) and the quantum numbers I, J, P , andC (or G) of an unflavored meson thus determine its symbol. For example, the light unflavoredisospin-1 2−− meson would be the ρ2 . Conversely, these properties may be inferred unambiguously
from the symbol. The name X is used for states with still unknown quantum numbers. In somecases names are assigned when quantum numbers have not been strictly measured but have instead
been inferred from a broader context. These cases typically include an explanation in the header,such as “Quantum numbers shown are quark-model predictions.” One example is the ψ2 (3823), for31st May, 2024

8. Naming Scheme for Hadrons

3

which the I G (J P C ) = 0− (2−− ) assignment needs confirmation. Since the top quark is so heavythat it decays too rapidly to form bound states, no name is assigned to structures like tt̄.
The selection rules in Eq. (8.2) impose restrictions on the J P C quantum numbers that areallowed for q q̄ states. These do not apply to more complicated structures such as tetraquarks,
hybrid mesons, or glueballs. In particular, unflavored mesons with quantum numbers J P C = 0−− ,0+− , 1−+ , 2+− , 3−+ , . . . cannot be q q̄ bound states, but might be glueballs or q q̄g etc. (This isdistinct from light unflavored mesons with I ≥ 2 or hidden-heavy flavor mesons with I ≥ 1, which
could be interpreted as tetraquarks). For such a “manifestly exotic” unflavored meson, the naming
scheme nonetheless assigns the same base symbol as solely determined by the isospin/hidden-heavyflavor and P C in Tab. 8.1. For example, an isospin-0 light unflavored 1−+ meson would be denotedη1 while an isospin-1 0−− light unflavored meson would be denoted ρ0 . The exotic nature of themeson can be inferred from its P C quantum numbers—given by the symbol—and the spin J—givenby the subscript.

8.2 Remarks on “neutral-flavor” mesons with hidden charm or bottom not classified as qq
In the heavy-quark sector, there are several states with properties – such as masses, decay
patterns, and widths – that are in disagreement with predictions from the naive quark model.
For example, the vector state at 4230 MeV (the ψ(4230)) apparently does not decay into DD,
although within the naive quark model its quantum numbers would call for this decay channel
to be dominant. These states were originally named X, Y , or Z, with their masses added inparentheses. This nomenclature differs from the conventions outlined in the previous section, since
the meson names are not related to their quantum numbers. However, these states have properties
in conflict with the naive quark model and therefore deserve some special labeling.
Since their original discovery, the properties of some of these states have become better understood and it has become possible to include them in the Listings as well as Summary Tables using
the name assigned according to this review. The X-, Y -, or Z-based names used at the time of thediscovery are also reported in the Listings and Summary Tables from the 2018 edition onwards asa sub-header (listed are only some examples of the particles that appear in the Summary Tables),• the state originally named X(3872) appears as ‘χc1 (3872) also known as X(3872)’;• the state originally named Zc (3900) appears1 as ‘Tcc̄1 (3900) also known as Zc (3900)’;• the state originally named Y (4260) appears2 as ‘ψ(4230) also known as Y (4230)’;
In addition, states with quantum numbers allowed by the naive quark model but showing somepeculiarities, such as an unusual decay pattern, will have the following information in the header:This state shows properties different from a conventional qq state. A candidate for anexotic structure. See the review on hname of the proper reviewi.

8.3 Mesons with nonzero S, C and/or B
Mesons with nonzero strangeness S or heavy flavor C and/or B are not eigenstates of charge
conjugation, and in each of them one of the quarks is heavier than the other (as above, states
containing top quarks are not considered). The rules have been and remain:
1. The main symbol is an upper-case italic letter indicating the heavier quark as follows:
s→K

c→D

b → B,We use the convention that the flavor quantum number and the charge of a quark have the1
2The name Tcc̄1 (3900) derives from the scheme outlined in a later section.
This is one example where the mass label needed to be shifted given improved experimental information.

31st May, 2024

8. Naming Scheme for Hadrons

4same sign. Thus the strangeness of the s quark is negative, the charm of the c quark is
positive, and the bottomness of the b quark is negative. The effect of this convention is asfollows: any flavor carried by a charged meson has the same sign as its charge. Thus the K + ,D+ , and B + have positive strangeness, charm, and bottomness, respectively, and all havepositive I3 . The Ds+ has positive charm and strangeness. Furthermore, the ∆(flavor) = ∆Q
rule, best known for the strange kaons, applies to every flavor.
2. If the lighter quark is not a u or a d quark, its identity is given by a subscript. The Ds+ is anexample.
3. When the spin-parity is in the natural series, J P = 0+ , 1− , 2+ , · · · , a superscript “∗ ” is added.
4. The spin is added as a subscript except for pseudoscalar or vector mesons.

8.4 Baryons with ordinary quantum numbers
All baryons having quantum numbers consistent with a minimal quark content of three quarks
are denoted by the symbols N , ∆, Λ, Σ, Ξ, and Ω introduced more than 50 years ago. Thesesymbols are followed by J P signifying their spin J and parity P . For those where the minimal
content involves one or more heavier quarks than the light (u, d, and s) quarks, subscripts areadded to their symbols, (c and b) as appropriate. The rules are:
1. Baryons with minimal content of three u and/or d quarks are N ’s (isospin 1/2) or ∆’s (isospin3/2).
2. Baryons with two u and/or d quarks are Λ’s (isospin 0) or Σ’s (isospin 1). If the third quark
is a c or b quark, its identity is given by a subscript.
3. Baryons with one u or d quark are Ξ’s (isospin 1/2). One or two subscripts are used if one
or both of the remaining quarks are heavy: thus Ξc , Ξcc , Ξb , etc.3
4. Baryons with no u or d quarks are Ω’s (isospin 0), and subscripts indicate any heavy-quark
content.
5. A baryon that decays strongly has its mass in parentheses. Examples are the ∆(1232) 3/2+ ,Σ(1385) 3/2+ , N (1440) 1/2+ , Ξc (2645) 3/2+ .
6. If individual states of isospin multiplets are addressed, the electric charge is specified as asuperscript. The electric charge is not necessary for isospin-0 states (Λ and Ω), as there is noambiguity, but may still be included.
7. Antibaryons are labeled by an overline (bar) on the base symbol. For example, p̄ for the+antiproton, or Λ̄−c for the antiparticle of Λc . Alternatively, particle and antiparticle can bedistinguished by including only a charge-label superscript, so long as the flavor structure of
the baryon is defined solely by the base symbol (and subscripts). Thus one may also write−++−+Λ−c or p for the antiparticles of the Λc and p , but the Σ is not the antiparticle of Σ ,−which is called Σ̄ .
In short, the minimal number of u plus d quarks together with the isospin determine the main
symbol, and subscripts indicate any content of heavy quarks. A Σ always has isospin 1, an Ωalways has isospin 0, etc.

8.5 Hadrons with a minimal quark content of qq q̄ q̄ or q q̄qqq
The naming scheme described above does not accommodate various qq q̄ q̄ mesons and q q̄qqq
baryons. In such cases, the 1986 naming scheme must be extended. Following discussion initiatedby the proposal made by the LHCb collaboration [7], we adopt the notation(∗)

TquarksJ (mass)q3for tetraquarks,

See the "Charmed and Bottom Baryons" section of the "Quark Model" review.

31st May, 2024

(8.3a)

8. Naming Scheme for Hadrons

5Pquarks (mass)q J P

for pentaquarks,(8.3b)

according to the following rules:
1. The basename T or P is chosen in cases where the minimal quark content of a state isqq q̄ q̄ (tetraquark) or q q̄qqq (pentaquark), respectively.
2. The bottom, charm, and strange quark content (b, b̄, c, c̄, s, s̄) is explicitly listed as a
subscript, in order from the heaviest quarks and antiquarks to the lightest and listing quarks
before antiquarks. The hidden-flavor cc̄ and bb̄ combinations are listed explicitly when theirpresence can be inferred from production or decay patterns.
3. The approximate mass of a state, in MeV/c2 , is given in parentheses.
4. The electric charge (q) can be specified as a superscript after the mass, for example whenreferring to a specific member of an isospin multiplet.
5. Antibaryons are marked with an overline (P ). The antiparticle of a meson can be specified
either with an overline or by the quark content label (e.g. the charge conjugate of Tcc (3875)+is T cc (3875)− , which is equivalent to Tc̄c̄ (3875)− ).
6. For pentaquarks, as in the case of ordinary baryons, when known the spin-parity, J P , of thestate is included as a suffix.
7. For tetraquarks, similarly to the naming scheme for mesons, the spin J is appended as asubscript after the quark content. When the spin-parity is in the natural series, J P =0+ , 1− , 2+ , · · · , a superscript “∗ ” is added.
Table 8.2 summarizes PDG name changes with respect to the 2022 edition [8]. We acknowledge
that experiments might choose to add I (G) J P (C) information to hadron names using experimentspecific conventions that differ from the one utilized by the PDG.
Table 8.2: Names used from the 2024 edition onward compared to thosein the 2022 edition. Whenever the C-parity of a charged tetraquark isspecified in the Particle Listings, it refers to the C-parity of the prospective/observed neutral isospin partner. Only one member per isospin multiplet is shown (see the Listings for more information).
2024 Name∗ (2870)0
Tcs0∗ (2900)0
Tcs1∗ (2900)++
Tcs̄0
Tcc (3875)+
Tcc̄1 (3900)+
Tcc̄ (4020)+
Tcc̄ (4050)+Tcc̄ (4055)+
Tcc̄ (4100)+Tcc̄1 (4200)+Tcc̄0 (4240)+Tcc̄ (4250)+

minimal quark
contentcsūd¯csūd¯cs̄ud¯ccūd¯cc̄ud¯cc̄ud¯cc̄ud¯cc̄ud¯cc̄ud¯cc̄ud¯cc̄ud¯cc̄ud¯I (G) (J P (C) )2022 Name

2024 Name?(0+ )?(1− )
1(0+ )?(?? )+1 (1+− )
1+ (??− )1− (??+ )1+ (??− )1− (??+ )1+ (1+− )1+ (0−− )1− (??+ )X0 (2900)
X1 (2900)

Tcc̄1 (4430)+Tcc̄s̄1 (4000)+Tcc̄s̄1 (4220)+Tbs̄ (5568)+Tbb̄1 (10610)+Tbb̄1 (10650)+
Tccc̄c̄ (6900)0
Pcc̄ (4312)+Pcc̄ (4380)+
Pcc̄ (4440)+Pcc̄ (4457)+
Pcc̄s (4459)0

Zc (3900)+
X(4020)+X(4050)+X(4055)+X(4100)+Zc (4200)+
Rc0 (4240)+
X(4250)+31st May, 2024

minimal quark
contentcc̄ud¯cc̄us̄
cc̄us̄
bs̄ūdbb̄ud¯bb̄ud¯ccc̄c̄
cc̄uudcc̄uudcc̄uudcc̄uudcc̄uds

I (G) (J P (C) )2022 Name

1+ (1+− )1/2(1+ )1/2(1+ )1(?? )+1 (1+− )1+ (1+− )0+ (??+ )1/2(?? )1/2(?? )1/2(?? )1/2(?? )0(?? )

Zc (4430)+
Zcs (4000)+Zcs (4220)+
X(5568)+Zb (10610)+
Zb (10650)+
X(6900)Pc (4312)+
Pc (4380)+
Pc (4440)+Pc (4457)+

6

8. Naming Scheme for Hadrons

References[1] M. Aguilar-Benitez et al. (Particle Data Group), Phys. Lett. 170B, 1 (1986).
[2] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 110, 252001 (2013), [arXiv:1303.5949].
[3] Z. Q. Liu et al. (Belle), Phys. Rev. Lett. 110, 252002 (2013), [Erratum: Phys.Rev.Lett. 111,019901 (2013)], [arXiv:1304.0121].
[4] M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 3, 030001 (2018).
[5] R. Aaij et al. (LHCb), Nature Phys. 18, 7, 751 (2022), [arXiv:2109.01038].
[6] R. Aaij et al. (LHCb), Phys. Rev. Lett. 122, 22, 222001 (2019), [arXiv:1904.03947].
[7] LHCb (2022), [arXiv:2206.15233].
[8] R. L. Workman and Others (Particle Data Group), PTEP 2022, 083C01 (2022).

31st May, 2024