178. Spectroscopy of Mesons Containing Two Heavy Quarks

78. Spectroscopy of Mesons Containing Two Heavy Quarks
Revised March 2024 by J. J. Hernández-Rey (IFIC, Valencia), C. Lourenço (CERN), R.E. Mitchell(Indiana U.), S. Navas (Granada U.) and C. Patrignani (Bologna U.).
A golden age for heavy quarkonium physics dawned at the turn of this century, initiated by the
confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related
experimental activity. The subsequent broad spectrum of breakthroughs, surprises, and continuing
puzzles had not been anticipated. Indeed, CLEO-c, BESIII, and the B-factories, later joined by
ATLAS, CMS and LHCb, have made a series of groundbreaking observations. For an extensive
presentation of the status of heavy quarkonium physics, the reader is referred to several reviews[1–9]. This note focuses on experimental developments in heavy quarkonium spectroscopy withvery few theoretical comments. Possible theoretical interpretations of the states not predicted by
the quark model are presented in the review “Heavy non-q q̄ mesons”. Note that in this review wefollow the new naming scheme for hadrons (see the review “Naming scheme for hadrons” in thecurrent edition of the RPP).
This review covers states discovered since 2003, the year that marked the unexpected discoveryof the X(3872) [10]. The X(3872), now called χc1 (3872), was the first of the mesons containing
two heavy quarks that could not be easily accommodated by the q q̄ quark model. Its discovery
was a watershed event in meson spectroscopy. In earlier versions of this write-up the particles were
sorted according to an assumed conventional or unconventional nature with respect to the quark
model. However, since this classification is not always unambiguous, we here follow Ref. [11] andsort the states into three groups, namely states below (cf. Table 78.1), above (cf. Table 78.2), andnear (cf. Table 78.3) the lowest open-flavor thresholds. Due to the presence of many open-flavor
thresholds, we note that the division of states between “above” and “near” open-flavor thresholds
is not absolute.

78.1 States Below Open-Flavor Threshold
Table 78.1 lists properties of recently observed heavy quarkonium states located below the lowest
open-flavor thresholds. Those are expected to be (at least prominently) conventional quarkonia.
The majority of charmonium (cc̄) and bottomonium (bb̄) states were established prior to 2003.
78.1.1 Charmonium
The hc (1P ) is the 11 P1 charmonium state, the singlet partner of the long-known χcJ triplet 13 PJ .
After being firmly established in 2005 through the process ψ(2S) → π 0 hc (1P ) [12], it has since beenstudied extensively by BESIII using large samples of ψ(2S) decays. Exclusive hadronic decays ofthe hc (1P ), strongly suppressed relative to the dominant radiative transition hc (1P ) → γηc (1S),
were first observed in 2019 [13] and additional decays were found in 2020 [14] and 2022 [15].
Belle reported an observation of the ψ2 (1D) decaying to γχc1 (1P ) with J P C presumed to be−−2[16]. This state is listed in Table 78.1 as ψ2 (3823). Its existence was confirmed with highsignificance by BESIII [17, 18]. While the negative C-parity is indeed established by its observed
decay channel, the assignment of J = 2 was made by matching to the closest quark model state(13 D2 ) and requires experimental confirmation.
The 11 D2 state, or the ηc2 (1D), with a mass expected near 3820 MeV, has not yet been observed.
Recently Belle performed a search in B → ηc2 (1D)K(π) decays in the mass range 3795–3845 MeV
and found no signal [19]. Thus, the ηc2 (1D) remains the only unobserved conventional charmonium
state that does not have open-charm decays.
S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)
31st May, 2024 10:15am

78. Spectroscopy of Mesons Containing Two Heavy Quarks

2

Table 78.1: New states below the open-flavor thresholds in the cc̄, bc̄, andbb̄ regions, ordered by mass. Masses m and widths Γ represent the PDG23
weighted averages with statistical and systematic uncertainties added inquadrature. In the Production column, the state is always denoted by
X. Ellipses (...) indicate inclusively selected event topologies, i.e., additional particles not directly detected by experiment. A question mark(?) indicates an unmeasured value. The Discovery Year column gives thedate of the first measurement cited. The Summary Table column indicates
whether or not the state appears in the summary tables, usually requiring
at least two independent experiments with significance of >5σ. Refer tothe particle listings for references and further information.
m (MeV)Γ (MeV)I G (J P C )Production

Decay3525.37 ± 0.14

0.78+0.30−0.27

0− (1+− )3823.51 ± 0.34

< 2.9

0− (2−− )Bc+6274.47 ± 0.32

stable0(0− )

Bc+ (2S)
ηb (1S)

6871.2 ± 1.0
9398.7 ± 2.0

?
10+5
−4

0(0− )
0+ (0−+ )hb (1P )9899.3 ± 0.8?

0− (1+− )γηc (1S)
hadrons(see listings)
γ χc1 (1P )π + π − J/ψ(1S)
π + J/ψ(1S)(see listings)
Bc+ π + π −hadrons(see listings)γηb (1S)ηb (2S)Υ2 (1D)

9999.0+4.5−4.0
10163.7 ± 1.4< 24?

0+ (0−+ )0− (2−− )hb (2P )10259.8 ± 1.2?

0− (1+− )χb1 (3P )χb2 (3P )10513.42 ± 0.67
10524.02 ± 0.78?
?

0+ (1++ )0+ (2++ )ψ(2S) → π 0 X
pp̄ → Xe+ e− → ππX
B → KXe+ e− → π + π − X
p̄p → X...
pp → X...
pp → X...
Υ (2S, 3S) → γX
hb (1P, 2P ) → γX
Υ (10860) → π + π − X
Υ (3S) → π 0 X
Υ (4S) → ηX
Tbb̄ (10610)+ → π + X
Tbb̄ (10650)+ → π + X
hb (2P ) → γX
Υ (3S) → γγX
Υ (10860) → π + π − X
Υ (10860) → π + π − X
Tbb̄ (10610)+ → π + X
Tbb̄ (10650)+ → π + X
pp → X...
pp → X...

PDG
Namehc (1P )FormerNameψ2 (3823)

X(3823)

Discovery
Year
2004

Summary
Table
YES

2013

YES

2007

YES

2014
2008

YES
YES

2011

YES

hadronsγγΥ (1S)π + π − Υ (1S)γηb (1S, 2S)

2012
2004

NO
YES

2011

YES

γΥ (1S, 2S, 3S)γΥ (3S)

2011
2011

YES
YES

78.1.2 Bottomonium
The ground state of bottomonium, ηb (1S), is well established. After the initial reports from
BaBar in radiative decays of the Υ (3S) (observation) [20] and Υ (2S) (evidence) [21], Belle confirmed
the existence of the ηb (1S) with more than 5σ significance in radiative decays of the newly discoveredhb (1P ) [22,23] and hb (2P ) [22] (see next paragraph), as well as in Υ (2S) radiative decays [24]. Belle
has also reported strong evidence for the ηb (2S) [22], but it still needs confirmation at the 5σ level.
Note that there are hints of tension in the ηb (1S) mass as measured in radiative M1 and E1+2.7+4.5transitions. In the M1 transition Υ (2S) → γηb (1S) Belle measures a mass of 9394.8−3.1−2.7
MeV[24], while in the E1 transitions hb (1P, 2P ) → γηb (1S) Belle measures 9402.4 ± 1.5 ± 1.8 MeV[22]. This tension may point to an incomplete understanding of the ηb (1S) lineshape in different
production mechanisms.
The hb (1P ), the bottomonium counterpart of the hc (1P ), and the next excited state, the hb (2P ),were simultaneously discovered by Belle using dipion transitions from the Υ (10860) [25] (Fig. 78.1).
The same analysis also showed the ΥJ (1D), the lowest-lying D-wave triplet of the bb̄ system, but
31st May, 2024

3

78. Spectroscopy of Mesons Containing Two Heavy Quarks

Figure 78.1: From Belle [25], the mass recoiling against π + π − pairs, Mmiss , in e+ e− collision datataken near the peak of the Υ (10860). The smooth combinatorial and KS0 → π + π − backgroundcontributions have been subtracted. The fit to the various labeled signal contributions is overlaid(curve). The fit is performed separately in three regions with boundaries indicated by the verticaldashed lines.
did not resolve the J = 1, 2, 3 components. The search for the hb (1P ) was directly inspired by
a CLEO result [26], which found a surprisingly copious production of e+ e− → π + π − hc (1P ) as
well as an indication that ψ(4230) → π + π − hc (1P ) occurs at a comparable rate with the signaturemode ψ(4230) → π + π − J/ψ(1S). The presence of Υ (nS) peaks in Fig. 78.1 at rates two orders ofmagnitude larger than expected, along with separate studies with exclusive decays Υ (nS) → µ+ µ− ,allow precise calibration of the π + π − recoil mass spectrum and very accurate measurements of thehb (1P ) and hb (2P ) masses. Both corresponding hyperfine splittings are consistent with zero within
an uncertainty of about 1.5 MeV (lowered to 1.1 MeV for the hb (1P ) in Ref. [27]). Belle later
observed the transition Υ (4S) → hb (1P )η [23] and the corresponding 1P hyperfine splitting was
also found to be compatible with zero at a similar precision level.
Just before Christmas 2011, ATLAS offered the world a beautiful gift, in the form of the
discovery of the χb (3P ) quarkonium state [28], observed by combining dimuons from Υ (1S) orΥ (2S) decays with photons emitted in the radiative χb (3P ) decays (Fig. 78.2, bottom left panel).
The new resonance, with a mass of 10 530 ± 5(stat) ± 9(syst) MeV, was soon confirmed by D0 [29].
Also LHCb observed the χb (3P ) peak, using the full Run 1 event sample, corresponding to an
integrated luminosity of 3 fb−1 [30] (Fig. 78.2, middle left panel). Finally, CMS used 80 fb−1 of
13 TeV pp collisions, collected in 2016 and 2017, to show two well-resolved χb1 (3P ) and χb2 (3P )peaks [31], separated by a mass difference of 10.60 ± 0.64(stat) ± 0.17(syst) MeV (Fig. 78.2, top left
panel). The remarkable precision of the individual mass measurements, with relative uncertainties
as small as 50 ppm, shows that the LHC experiments can provide important results in the fieldof hadron spectroscopy, especially in the case of heavy particles, which require very high collisionenergies and large event samples.
78.1.3 Bc System
The Bc± family is quite special because these (charged) quarkonium states consist of two heavy
quarks of different flavor. Among other interesting properties, this means that they cannot annihilate into gluons, the excited states only decaying to the pseudoscalar ground state, Bc± , via
electromagnetic and pionic transitions.
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78. Spectroscopy of Mesons Containing Two Heavy Quarks

4

60

b2

0

20
10.45

LHCb

10.5

10.55

(1S )

(3P )

-1

L = 3.0 fb
s = 7, 8 TeV

10.6

(2S )

b

600

500

Background10.4

Events / 6 MeV

Bc(2S)+

10

40

30

L = 8.5 fb
s = 7, 8, 13 TeV

20

(3S )

Comb. backg.

Bc*(2S)+

-1

30

(3P )

-1

L = 80 fb
s = 13 TeV

LHCb

Events / 3 MeV

80

(3P )

b1

CMS

Events / 10 MeV

Events / 3 MeV

100

Bc*(2S)+

CMS

60

700

Bc(2S)+-1

L = 143 fb
s = 13 TeV

40

Backgrounds++

Bc ( J/ψ K ) π +π −Comb. backg.

20
20

10

200

Events / 25 MeV

160

10.4

ATLAS

10.5

10.6

10.7

10.8

(1S )

-1

(2S )

L = 4.4 fb
s = 7 TeV

Background

120

(3P )

b

80

0

500

400

30

Events / 20 MeV

0

ATLAS
L = 19.2 fb
s = 8 TeV

20

600

700

800

-1

-1

L = 4.9 fb
s = 7 TeV

Comb. backg.

10

40
0

9.6

9.8

10.0

10.2

10.4

10.6

m ( μ μ ) − m ( μ μ ) + m (n S) (GeV)
+ −

+ −

0

300

400

500

600

700

800

m (B+c π +π - ) − m (B+c) (MeV)

900

Figure 78.2: (Left Column) Invariant mass distributions measured by the ATLAS [28] (bottom),
LHCb [30] (middle) and CMS [31] (top) experiments in their searches for the χb (3P ) states throughtheir radiative decays to one of the S-wave bottomonia. (Right Column) Invariant mass distributions measured by the ATLAS [32] (bottom), CMS [33] (middle) and LHCb [34] (top) experimentsin their searches for Bc± excited states decaying to the Bc± ground state with the emission of two
charged pions.

On the basis of an event sample collected in the Run 1 of the LHC, corresponding to an
integrated luminosity of 24 fb−1 , adding the 7 and 8 TeV data, the ATLAS Collaboration observeda resonance in the Bc+ π + π − invariant mass spectrum [32] (Fig. 78.2, bottom right panel). Thispeak, observed with a significance of 5.2 standard deviations and a mass of 6842 ± 4(stat) ±5(syst) MeV, was immediately recognized as the Bc (2S)± state, the first radial excitation in the
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578. Spectroscopy of Mesons Containing Two Heavy Quarks

Bc± family. Profiting from the much larger Run 2 event sample, collected in the 2015, 2016, 2017and 2018 running periods and corresponding to 143 fb−1 of 13 TeV pp collisions, as well as from ameasurement resolution of around 6 MeV, the CMS Collaboration could observe two well-resolved
peaks, separated by 29.1 ± 1.5(stat) ± 0.7(syst) MeV [33] (Fig. 78.2, middle right panel). The
existence of two peaks, rather than a single one, is established with a significance of 6.5 standard
deviations. The “right peak” has a mass of 6871.0 ± 1.2(stat) ± 0.8(syst) ± 0.8(Bc+ ) MeV, where thelast term is the uncertainty in the Bc+ mass, and is identified as the Bc (2S)± state, which decays
directly to the Bc± , emitting two (easy to detect) pions. The CMS observation, reported a couple ofmonths after the end of the LHC Run 2, was soon followed by the corresponding LHCb result [34](Fig. 78.2, top right panel), which confirmed the existence of the two states and reported a second
measurement of the Bc (2S)± mass, 6872.1 ± 1.3(stat) ± 0.1(syst) ± 0.8(Bc+ ) MeV.
The “left peak” is interpreted as being the Bc∗ (2S)± signal. It is observed at a mass lower than
the real value because the experiments are unable to detect the low-energy photon emitted in the
decay chain, Bc∗ (2S)± → Bc∗± π + π − followed by Bc∗± → Bc± γ (Fig. 78.3). Its energy, expected
to be in the range 40–80 MeV, leads to a very small probability that the photon converts intoan e+ e− pair and the two electrons are reconstructed. The relative ordering of the two peaks
is based on a generally-agreed assumption: the M (Bc∗± ) − M (Bc± ) mass difference is larger thanthe M (Bc∗ (2S)± ) − M (Bc (2S)± ) difference. Naturally, these observations provide evidence for the
existence of the Bc∗ (1S)± state. They also provide measurements of two interesting mass differences,
between the masses of the pseudoscalar mesons, M (Bc (2S)± ) − M (Bc (1S)± ) = 596.1 MeV, and of
the vector mesons, M (Bc∗ (2S)± ) − M (Bc∗ (1S)± ) = 567.0 MeV (Fig. 78.3).

B+c (2S)
567.0 ± 1.0 MeV

2S

B*c + (2S)

596.1 ± 1.4 MeV

ππ

1S

B*c +

γ
B+c

Figure 78.3: Diagram showing the decays mentioned in the text.

78.2 States Above Open-Flavor Threshold
Many states have been discovered both above and near the lowest open-flavor thresholds. They
are displayed in Tables 78.2 and 78.3, respectively. With the exception of the ψ3 (3842) and thetensor state located at 3930 MeV (now called χc2 (3930)), which have properties consistent withthose expected for the ψ3 (13 D3 ) and χc2 (23 P2 ), respectively, none of these states can easily be
assigned a place in the quark model spectrum of the charmonium or bottomonium families. The
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6

78. Spectroscopy of Mesons Containing Two Heavy Quarks

theoretical interpretation of these states remains under discussion.
78.2.1 Charmonium
Using proton-proton collisions, LHCb observed a narrow state, the ψ3 (3842) resonance, in the
decay modes ψ3 (3842) → D0 D̄0 and D+ D− [35]. The mass and width of this state are measured
to be 3842.71 ± 0.16 ± 0.12 MeV and 2.79 ± 0.51 ± 0.35 MeV, respectively. The observed mass andnarrow width are consistent with expectations for the spin-3 ψ3 (13 D3 ) charmonium. Accordingly,
the state is given the name ψ3 (3842) in the listings, with the remark that the quantum numbers
are fixed from the quark model and need to be confirmed.
The χc2 (3930), which is a natural candidate for the χc2 (23 P2 ) quark model state, was originallyseen by Belle [36] and later confirmed by BaBar [37] in the γγ process e+ e− → e+ e− DD̄. This
interpretation was strengthened by the more recent LHCb observation of the χc2 (3930) alongsidethe ψ3 (3842) in proton-proton collisions [35].
Unlike the χc2 (23 P2 ), the identification of the χc0 (23 P0 ) quark model state remains controversial. The original candidate was the χc0 (3915), discovered by Belle in the γγ process e+ e− →e+ e− ωJ/ψ(1S) [38]. In a subsequent measurement by BaBar, its quantum numbers were determined to be J P C = 0++ [39]. However, its identification as the χc0 (23 P0 ) quark model state wassoon challenged [40, 41]. In addition, it was pointed out in Ref. [42] that if the assumption ofhelicity-2 dominance is abandoned and, instead, one allows for a sizeable helicity-0 component, a
J P C = 2++ assignment is possible. This could imply that it is the same as the χc2 (3930), but toexplain the large helicity-0 component a sizable portion of non-q q̄ is necessary [42]. A more recent
LHCb amplitude analysis of the process B + → D+ D− K + finds distinct 0++ and 2++ components
decaying to D+ D− [43], which are currently identified in the listings as the χc0 (3915) and χc2 (3930),
respectively.
An alternative candidate for the χc0 (23 P0 ) (here referred to as the χc0 (3860)) was reported inRef. [44] with properties more consistent with expectation: its mass is close to the potential model
expectations, it decays to DD̄, and the preferred quantum numbers are J P C = 0++ (this hypothesis
is favored over the 2++ one with a 2.5σ significance).
In the excited vector charmonium spectrum, the ψ(4040), ψ(4160), and ψ(4415) are prominent
in the inclusive e+ e− hadronic cross section and are naturally identified as the 33 S1 , 23 D1 , and 43 S1
cc̄ quark model states, respectively. In addition to these long-established states, however, anotherset of peaks has been found in exclusive e+ e− cross sections. Unlike conventional vector charmonia,
they do not appear in the inclusive hadronic cross section and they apparently do not decay to DD̄.
The PDG summary table currently lists the ψ(4230), ψ(4360), and ψ(4660) within this category.
The first of these to be discovered was originally known as the Y (4260) (now the ψ(4230)), seen by
BaBar [45] and Belle [46,47] in e+ e− → π + π − J/ψ(1S) using initial state radiation. In a more recent
high-statistics scan of e+ e− → π + π − J/ψ(1S), BESIII demonstrated that the lineshape in thismass range is highly non-trivial [48]. The latter observation was interpreted by the authors as thepresence of two states. However, this lineshape is also consistent with other possible interpretations,such as one assuming a molecular structure for the ψ(4230) [49]. The data of Ref. [48] also called
for a significant downward shift of the mass of what was originally called the Y (4260), making it
consistent with peaks in other exclusive cross sections, such as hc (1P )ππ [50]. We thus merged theoriginal Y (4260) (or, more formally, the ψ(4260)) with the ψ(4230) in the listings.
BESIII observed the χc1 (3872), also known as X(3872), in e+ e− → γχc1 (3872) in the ψ(4230)
mass range [51], which could allow for additional insight into the structure of both states (see thereview on heavy non-q q̄ mesons). BESIII also performed a recent study of the process e+ e− →π + π − ψ(2S) and found evidence for a lower mass state, possibly the ψ(4230), in addition to themore dominant ψ(4360) [52].

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78. Spectroscopy of Mesons Containing Two Heavy Quarks

7

Table 78.2: As in Table 78.1, but for states above the first open-flavor
thresholds in the cc̄ and bb̄ regions, ordered by mass.
PDG
Name
ψ3 (3842)χc0 (3860)χc0 (3915)
χc2 (3930)
X(3940)Tcc̄ (4050)
Tcc̄ (4055)

FormerName(s)X(3915),
Y (3940)χc2 (2P ),Z(3930)Z1 (4050)
X(4050)Zc (4055)
X(4055)X(4100)Y (4140)m (MeV)Γ (MeV)I G (J P C )Production

Decay3842.71 ± 0.20
3862+48−35
3922.1 ± 1.8

2.79 ± 0.62
201+177−106
20 ± 40− (3−− )∗0+ (0++ )0+ (0/2++ )3922.5 ± 1.0

35.2±2.2

0+ (2++ )pp → X...
e+ e− → J/ψ(1S)XB → KXe+ e− → e+ e− X
e+ e− → e+ e− X

DD̄DD̄ωJ/ψ(1S)DD̄
DD̄

3942+9−84051+24
−43

37+27
−17
82+51
−28

?? (??? )1− (??+ )e+ e− → J/ψ(1S)X
B̄ 0 → K − X

4054 ± 3

45 ± 13

1+ (??− )4096+27−30
4146.5 ± 3.04153+23−21

152+83−68
19+7−5
136+60−35

1− (??? )0+ (1++ )?? (??? )Discovery
Year
2019
2017
2004

Summary
Table
YES
NO
YES

2005

YES

π+χ

DD̄∗
c1 (1P )

2007
2008

NO
NO

e+ e− → π − X

π + ψ(2S)

2015

NO

2018
2009
2007

NO
YES
NO

2014
2015

NO
YES

2014

NO

Tcc̄1 (4200)
ψ(4230)Zc (4200)Y (4230)
Y (4260)

4196+35−32
4222.1 ± 2.3

370+99−149
49 ± 7

1+ (1+− )0− (1−− )B̄ 0 → K − X
B+ → K +X
e+ e− → J/ψ(1S)X
B+ → K +X
B̄ 0 → K − X
e+ e− → X

Tcc̄0 (4240)

Zc (4240)
Rc0 (4240)
Z2 (4250)
X(4250)Y (4274)

4239+48−21

220+118−88

1+ (0−− )B̄ 0 → K − X

π + ηc (1S)
φJ/ψ(1S)D∗ D̄∗
φJ/ψ(1S)
J/ψ(1S)π +π + π − J/ψ(1S)ωχc0 (1P )π + π − hc (1P )(see listings)
π + ψ(2S)

4248+185− 45

177+321− 72

1− (??+ )B̄ 0 → K − X

π + χc1 (1P )

2008

NO

51 ± 713+18−10118±120+ (1++ )0+ (??+ )0− (1−− )B+ → K +X
e+ e− → e+ e− X

Y (4360)

4286+8−94350.6+4.7−5.1
4374 ± 7

2011
2009
2007

YES
NO
YES

Tcc̄1 (4430)

Zc (4430)

4478+15−18

181 ± 31

1+ (1+− )B̄ 0 → K − X

2007

YES

χc0 (4500)
X(4630)ψ(4660)X(4500)4474 ± 44626+24−111
4641 ± 10

77+12
−10
174+137
−78
72+13
−11

0+ (0++ )0+ (??+ )0− (1−− )B+ → K +X
B+ → K +X
e+ e− → X

2017
2021
2007

NO
NO
YES

Υ (5S)

4684+15−174694+16−56899 ± 1210752.7+5.9−6.010885.2+2.6−1.6

126+40
−44
87+18
−10
153 ± 2936+18
−12
37 ± 40+ (1++ )0+ (0++ )0+ (??+ )?? (1−− )0− (1−− )B+ → K +X
B+ → K +X
pp → X...
e+ e− → Xe+ e− → X

2021
2017
2020
2019
1985

NO
NO
NO
NO
YES

Υ (6S)

11000 ± 4

24+8
−6

0− (1−− )e+ e− → X

φJ/ψ(1S)
φJ/ψ(1S)π + π − ψ(2S)π + π − J/ψ(1S)
π + ψ(2S)
π + J/ψ(1S)
φJ/ψ(1S)
φJ/ψ(1S)π + π − ψ(2S)
−
Λ+
c Λ̄c
+
Ds Ds1 (2536)
φJ/ψ(1S)
φJ/ψ(1S)
J/ψ(1S)J/ψ(1S)
ππΥ (1S, 2S, 3S)(∗) (∗)
B(s) B̄(s) (π)ππΥ (1S, 2S, 3S)
π + π − hb (1P, 2P )ηΥ (1S, 2S)
π + π − Υ (1D)(see listings)(∗) (∗)
B(s) B̄(s) (π)ππΥ (1S, 2S, 3S)
π + π − hb (1P, 2P )(see listings)

1985

YES

Tcc̄ (4100)
χc1 (4140)
X(4160)Tcc̄ (4250)χc1 (4274)
X(4350)ψ(4360)χc1 (4685)χc0 (4700)
Tccc̄c̄ (6900)
Υ (10753)
Υ (10860)

Υ (11020)∗

Y (4660),
X(4660)X(4700)X(6900)Quantum numbers fixed from the quark model and need confirmation.

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e+ e− → X

78. Spectroscopy of Mesons Containing Two Heavy Quarks

8

Another interesting question is whether a heavier π + π − ψ(2S) state, the ψ(4660), discovered−by Belle [53, 54] and confirmed by BaBar [55], is identical to the Λ+c Λ̄c resonance observed by−Belle with a nearby mass and width [56]. Most probably it is, the Λ+c Λ̄c being one more decay
mode of the ψ(4660) (see the review on heavy non-q q̄ mesons for more details). Note that this is
the interpretation adopted in the particle listings. In addition, Belle reported the first observationof a vector charmonium-like state decaying to Ds+ Ds1 (2536) with a significance of 5.9σ [57]. Its
+6.2
+13.9
measured mass and width are 4625.9−6.0±0.4 MeV and 49.8−11.5±4.0 MeV, respectively, consistent+
with those of the ψ(4660). Therefore, Ds Ds1 (2536) appears as an additional decay mode of theψ(4660) in the listings.
A series of isovector states1 containing cc̄ have been found in B decays to Kπ(cc̄), where
the isovector state decays to π(cc̄) and (cc̄) stands for J/ψ(1S), ψ(2S), or χc1 (1P ). They are
manifestly non-q q̄. The Tcc̄1 (4430) (originally called the Zc (4430)), decaying to πψ(2S), is the mostwell established. Based on a full amplitude analysis of B 0 → K + π − ψ(2S) decays, Belle determined
the spin-parity of the Tcc̄1 (4430) to be J P = 1+ [58]. From their study of B 0 → K + π − J/ψ(1S)
decays, Belle also found evidence for the decay mode Tcc̄1 (4430) → πJ/ψ(1S) [59], which has anorder of magnitude lower branching fraction than the discovery mode Tcc̄1 (4430) → πψ(2S). In thesame analysis, Belle reported evidence for one more charged state, dubbed Tcc̄1 (4200), decaying to
πJ/ψ(1S). The observation of the Tcc̄1 (4430) in πψ(2S), as well as its quantum number assignments,
were confirmed by LHCb [60] with a much larger data sample, leading to improved mass and width
values, consistent with earlier measurements; the experiment even reports a resonant behavior of
the Tcc̄1 (4430) amplitude. The Tcc̄1 (4430) was not confirmed (or excluded) by BaBar [61].
Belle also reported an observation of two charged states decaying to πχc1 (1P ) in an analysis of
B 0 → K + π − χc1 (1P ) decays [62]. These were originally called Z1 (4050)± and Z2 (4250)± , but arereferred to in Table 78.2 as Tcc̄ (4050) and Tcc̄ (4250). These states were not confirmed by BaBar [63].
Belle observes signals with 5.0σ significance for both the Tcc̄ (4050) and Tcc̄ (4250), whereas BaBar
reports 1.1σ and 2.0σ effects, respectively, setting upper limits that are not inconsistent with Belle’s
measured rates. The situation remains unresolved.
The decay B + → K + φJ/ψ(1S) appears to be especially rich in resonant substructure. The
Y (4140) (now the χc1 (4140)), decaying to φJ/ψ(1S), was first observed in 2008 by CDF [64,65], andconfirmed by D0 and CMS [66, 67]. However, a second structure, the Y (4274) (now the χc1 (4274)),
could not be established unambiguously. Neither of the two states was seen in B decays at Belle [68],
LHCb [69] and BaBar [70], or in γγ collisions at Belle [71]. The real breakthrough happened when
LHCb performed a full amplitude analysis of B + → K + φJ/ψ(1S) with J/ψ(1S) → µ+ µ− , φ →K + K − decays and showed that the data cannot be described in a model that contains only excitedkaon states decaying into K + φ [72, 73]. They observe two 1++ states with masses close to thoseoriginally reported by CDF (the χc1 (4140) and χc1 (4274)), but the width of the one at 4140 MeV
is much larger. In addition, they find two significant 0++ structures at 4500 and 4700 MeV (theχc0 (4500) and χc0 (4700)). The LHCb analysis was extended even further in Ref. [74] with a factorof six increase in statistics: the χc1 (4140), χc1 (4274), χc0 (4500), and χc0 (4700) were confirmed, andtwo new states, the X(4630) and χc1 (4685) were reported, also decaying to φJ/ψ(1S).
78.2.2 Resonances in the J/ψ(1S)-pair mass spectrum√
Based on proton-proton event samples collected at s = 7, 8 and 13 TeV, corresponding toan integrated luminosity of 9 fb−1 , the LHCb Collaboration reported the observation of a narrow
structure in the invariant mass distribution of J/ψ(1S) pairs [75]. The new resonance, denoted as
Tccc̄c̄ (6900) (also X(6900)), is clearly seen at a mass of 6.9 GeV in the bottom panel of Fig. 78.4, as
indicated by the red curve. The ATLAS Collaboration has also observed the Tccc̄c̄ (6900) resonance
1

These isovector states were originally called Zc . They are now referred to as Tcc̄ . See the naming scheme review.

31st May, 2024

978. Spectroscopy of Mesons Containing Two Heavy Quarks

Candidates / 25 MeV√
in the di-J/ψ(1S) mass spectrum, using 140 fb−1 of pp collisions at s = 13 TeV [76], as can beseen in the middle panel of Fig. 78.4. More recently, also the CMS Collaboration reported theobservation of the Tccc̄c̄ (6900) in the J/ψ(1S)-pair mass distribution, using 135 fb−1 of 13 TeV pp
collisions [77]. Thanks to its good mass resolution and relatively large event yields, CMS determined
the mass and width of the Tccc̄c̄ (6900) with the best precision among the three measurements.
Furthermore, another resonance is seen with a significance exceeding 5 standard deviations, at amass of 6.55 GeV, as shown in the top panel of Fig. 78.4. The measured values of the masses
and widths of the new states depend on whether the model used to fit the distributions includes
interference effects between the resonances and the underlying continuum (and also among theresonances). The larger event samples presently being collected by the LHC experiments will
surely lead to significantly improved measurements, as well as to complementary results (including
determinations of spin-parity quantum numbers), so that one can expect near-future progress in
our understanding of the nature of these “narrow structures”.

-1

Data
Fit
Resonances
Background

120
80
40
0

6.5

7

7.5

8
-1

160

Events / 40 MeV

CMS

L = 135 fb
s = 13 TeV

160

9

ATLAS

L = 140 fb
s = 13 TeV

Data
Fit
Background

120
80
40
0

Weighted candidates / 28 MeV

8.5

6.5

7

7.5

8

8.5

L = 9 fbs = 7, 8, 13 TeV

LHCb

-1

160
120

9

Data
Fit
Resonance

80
40
0

6.5

7

7.5

8

m (J/ψ J/ψ) (GeV)

8.5

9Figure 78.4: J/ψ(1S)-pair invariant mass distributions measured by the LHCb [75] (bottom),
ATLAS [76] (middle), and CMS [77] (top) experiments.

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10

78. Spectroscopy of Mesons Containing Two Heavy Quarks

78.2.3 Bottomonium
Belle reported a new measurement of the e+ e− → Υ (nS)π + π − (n = 1, 2, 3) cross sections atenergies from 10.52 to 11.02 GeV [78]. They observed, with a 5.2σ significance, a new structurein the energy dependence of the cross sections. If described by a Breit–Wigner function, its mass
+17.6+3.9
and width are 10752.7 ± 5.9+0.7−1.1 MeV and 35.5−11.3−3.3 MeV, respectively. The new structure could
have a resonant origin and correspond to the not yet observed Υ (3D) state, provided S − D mixing
is enhanced, or an exotic state, e.g., a compact tetraquark or hadrobottomonium. It could also bea non-resonant effect due to rescattering. The Υ (10750) was confirmed in a global K-matrix fit to
exclusive and inclusive e+ e− → bb̄ cross sections in Ref. [79].
We no longer mention a hypothetical Yb (10888) state since a new analysis of the Υ (10860)
energy range does not show evidence for an additional state with a mass different from that of theΥ (10860) [80]. After the mass of the ηb (1S) was shifted upwards by about 10 MeV based on the
Belle measurements [22,23], all of the bottomonium states mentioned above fit into their respective
spectroscopies, roughly where expected. An independent experimental confirmation of the shiftedmasses came from the Belle observation of Υ (4S) → ηhb (1P ) decays [23]. This process turns out
to be the strongest observed transition of the Υ (4S) to lower bottomonium states.

78.3 States Near Open-Flavor Threshold
A number of states, listed in Table 78.3, appear near open-flavor thresholds, which is likely an
important factor in their theoretical interpretation [81].
78.3.1 Charmonium
The χc1 (3872), also known as X(3872), is widely studied and seen in many transitions — see
Table 78.3. Yet its interpretation remains unsettled (see the heavy non-q q̄ review). Its uniqueexperimental features include: it has J P C = 1++ [82, 83], yet it is too light to be the χc1 (23 P1 )quark model state; its mass is within 200 keV of the D0 D̄0∗ threshold; it shows substantial isospinbreaking in its decays to ρJ/ψ(1S) and π 0 χc1 (1P ); and it is extremely narrow. Using a large sample
of inclusively produced χc1 (3872) decaying to π + π − J/ψ(1S), LHCb recently determined the decay
width of the χc1 (3872) under two different assumptions [84]. Assuming a Flatté-inspired line shapeand exploiting the strong coupling of the χc1 (3872) to D0 D̄∗0 , LHCb performed the first explorationof the pole structure of the χc1 (3872), finding a FWHM width of 0.22+0.06+0.25−0.08−0.17 MeV. On the other
hand, assuming a Breit–Wigner line shape, its width was found to be 1.39 ± 0.24 ± 0.10 MeV. While
the former analysis has a more firm theoretical foundation, the LHCb detector resolution did notallow for a distinction between the different line shapes.
In addition to the Tcc̄ (also known as Zc ) states found in B decays, discussed above, several
isovector states with masses near DD̄∗ and D∗ D̄∗ thresholds appear to be unique to e+ e− annihilation. In 2013, a state named Tcc̄1 (3900) (originally Zc (3900)) was unearthed in the charmonium
region at BESIII [86] and Belle [47] in the process e+ e− → π ∓ Tcc̄1 (3900)± with Tcc̄1 (3900)± →π ± J/ψ(1S). The corresponding spectrum from BESIII is shown in Fig. 78.5. An analysis of CLEO
data [87] confirmed this finding and also provided evidence for a neutral partner. A nearby signal
was also seen in the DD̄∗ channel [88] whose quantum numbers were fixed to 1+− . BESIII reported
its neutral partner in both J/ψ(1S)π 0 [89] and DD̄∗ [90] decay modes. The masses extracted
from these experiments in different decay modes agree within 2σ. However, since the extraction of
the mass and width parameters did not allow for an interference with the background and Breit–
Wigner line shapes are used, which is not justified near thresholds, there might be some additional
systematic uncertainty in the mass values. Therefore, in the RPP listings as well as in Table 78.3
both structures appear under the name Tcc̄1 (3900). BESIII also reported the observation of anothercharged state, the Tcc̄ (4020)± (originally called Zc (4020)± ), in two decay modes: hc (1P )π ± [91] and(D∗ D̄∗ )± [92]. The neutral partners have also been observed by BESIII in the hc (1P )π 0 [93] and31st May, 2024

1178. Spectroscopy of Mesons Containing Two Heavy Quarks
Table 78.3: As in Table 78.1, but for states near the first open-flavor
thresholds in the cc̄ and bb̄ regions, ordered by mass. Updated from Ref.
[85] with kind permission, copyright (2011), Springer, and from Ref. [11]with kind permission from the authors.
FormerName
X(3872)m (MeV)Γ (MeV)I G (J P C )Production

3871.64±0.06

1.19 ± 0.21

0+ (1++ )B → KXpp̄ → X...
pp → X...
e+ e− → γX

Tcc (3875)
Tcc̄1 (3900)

0.410+0.172−0.17528.4 ± 2.6?(?? )

Zc (3900)

3874.83 ± 0.11
3887.1 ± 2.6

1+ (1+− )pp → X...
ψ(4230) → π − X
ψ(4230) → π 0 X

Tcc̄s̄1 (4000)

Zcs (4000)

3980 − 4010

5 − 150

1 +
2 (1 )

Tcc̄ (4020)

Zc (4020)
X(4020)4024.1 ± 1.9

13 ± 51+ (??− )Tcc̄s̄1 (4220)
Tbb̄1 (10610)

Zcs (4220)
Zb (10610)

4216+49−38
10607.2 ± 2.0

233+110−90
18.4 ± 2.4

1 +
2 (1 )+
1 (1+− )B + → φXΥ (10860) → π − X
Υ (10860) → π 0 X

Tbb̄1 (10650)

Zb (10650)

10652.2 ± 1.5

11.5 ± 2.2

1+ (1+− )Υ (10860) → π − X

67&89:#(#505;#%&'()"

PDG
Name
χc1 (3872)

Decaye+ e− → KX
B + → φXψ(4230, 4360) → π − X
ψ(4230, 4360) → π 0 X

DiscoveryYearπ + π − J/ψ(1S)2003
3πJ/ψ(1S)D∗0 D̄0γJ/ψ(1S)γψ(2S)
π 0 χc1 (1P )D0 D0 π2022+
π J/ψ(1S)2013
π 0 J/ψ(1S)(DD̄∗ )+
(DD̄∗ )0
Ds D∗ + Ds∗ D
2021
K + J/ψ(1S)
π + hc (1P )2013
π 0 hc (1P )(D∗ D̄∗ )+(D∗ D̄∗ )0
K + J/ψ(1S)2021
π + Υ (1S, 2S, 3S) 2011
π 0 Υ (1S, 2S, 3S)
π + hb (1P, 2P )(B B̄ ∗ )+π + Υ (1S, 2S, 3S) 2011
π + hb (1P, 2P )(B ∗ B̄ ∗ )+Summary
Table
YES

NO
YES

NO
YES

NO
YES

YES

=,9,

;55

>?9,@#AB9C,)DEF?G8H#AB925

IJKI#.L
KBH&M,8H

<5
45
"5
5

/01

/02

.+,-

/03

405

$π±*(ψ!#$%&'()"!

Figure 78.5: The π ± J/ψ(1S) invariant mass distribution from BESIII [86] e+ e− collision datataken at a center-of-mass energy near 4260 MeV.

(D∗ D̄∗ )0 [94] final states. The Tcc̄ states show some remarkable similarities to the Tbb̄ states (dis31st May, 2024

12

78. Spectroscopy of Mesons Containing Two Heavy Quarks

cussed below), e.g. they decay dominantly to D(∗) D̄∗ channels. However, current analyses suggestthat the mass of the Tcc̄1 (3900) might be somewhat above the DD̄∗ threshold. If confirmed, thisfeature would challenge a possible DD̄∗ –molecular interpretation with S-wave interactions only —prominent D-waves can shift molecular poles above threshold (see the discussion in Sec. 78.3.3).
Finally, 3.5σ evidence for one more charged charmonium-like state at 4055 MeV decaying intoψ(2S)π ± was reported by Belle in their analysis of the process e+ e− → ψ(2S)π + π − [54]. The sameprocess was studied by BESIII, where there appears to be complications in the Dalitz plot requiring
further investigation [52].
An isospin-1/2 state with open strangeness, the Tcc̄s̄1 (4000), has been reported near Ds D∗
threshold by both BESIII [95, 96] and LHCb [74]. BESIII observes the charged Tcc̄s̄1 (4000) in theprocess e+ e− → K + (Ds− D∗0 + Ds∗− D0 ) [95] and the neutral Tcc̄s̄1 (4000) in the process e+ e− →KS (Ds− D∗+ + Ds∗− D+ ) [96]. LHCb observes the charged version in B + → φ(K + J/ψ(1S)) [74],although with a much larger decay width. The same LHCb analysis also finds a higher mass Tcc̄s̄
0 near D ∗ D ∗ threshold was
candidate, the Tcc̄s̄1 (4220). A search by BESIII for a heavier partner Tcc̄s̄
s
negative [97].

70
60

LHCb
9 fb−1

50
40
30
20

•
+

Yield/(200 keV/c2 )Yield/(500 keV/c2)

78.3.2 Double Charm
The most striking recent result, however, is due to the observation by LHCb [98,99] of a doubly
charmed state, the Tcc (3875)+ , in the inclusive cross section of D∗+ D0 → D0 D0 π + (see Fig. 78.6).
¯ and the width
The statistical significance is overwhelming, the minimal quark content is ccūd,
is approximately what would be expected for an isoscalar ground state having J P = 1+ . The
integrated luminosity used in this study was 9 fb−1 . The larger integrated luminosity of the event
sample to be collected by the end of the LHC Run 3, together with improved analysis techniques,
might bring the Tbc states within reach.
40
35
30
25
20
15
10

5
0
Data
0 0 +
T+
cc → D D π
Background
Total
D∗+ D0 thresholdD∗0 D+ threshold

3.874

3.876

mD0 D0 π+ [GeV/c2 ]

10
0
3.87

3.88

3.89

mD0D0π+3.9
GeV/c2

Figure 78.6: The D0 D0 π + invariant mass distribution from LHCb [98, 99]
78.3.3 Bottomonium
New results on the ηb , hb , and Tbb̄ mostly come from Belle [22, 23, 25, 27, 80, 100–106], all fromanalyses of 121.4 fb−1 of e+ e− collision data collected near the peak of the Υ (10860) resonance,
as well as from an additional 25 fb−1 of data collected during the scans of the c.m. energy range
10.63–11.05 GeV. The ηb , hb , and Tbb̄ appear in the decay chains Υ (10860) → π − Tb+b̄ , Tb+b̄ → π + (bb̄),
and, when the bb̄ forms an hb (1P ), frequently decaying as hb (1P ) → γηb .
Belle soon noticed that, for events in the peaks of Fig. 78.1 corresponding to the processes
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78. Spectroscopy of Mesons Containing Two Heavy Quarks

Figure 78.7: From Belle [100] e+ e− collision data taken near the peak of the Υ (10860) for eventswith a π + π − -missing mass consistent with an Υ (2S) → µ+ µ− , (a) the maximum of the two possible
single π ± -missing-mass-squared combinations vs. the π + π − -mass-squared; and (b) projection of
the maximum of the two possible single π ± -missing-mass combinations overlaid with a fit (curve).
Events to the left of the vertical line in (a) are excluded from the amplitude analysis. The hatchedhistogram in (b) corresponds to the combinatorial background. The two horizontal stripes in (a)and two peaks in (b) correspond to the two Tbb̄ states.
e+ e− → π + π − Υ (1S, 2S, 3S) and π + π − hb (1P, 2P ), there seemed to be two intermediate charged
states in the π ± Υ and π ± hb channels, called the Tbb̄ (10610) and the Tbb̄ (10650). For example,
Fig. 78.7 shows a Dalitz plot for events restricted to the Υ (2S) region of π + π − recoil mass, with
Υ (2S) → µ+ µ− [100]. The two bands observed in the maximum of the two M [π ± Υ (2S)]2 values also
appear in the Υ (1S), Υ (3S), hb (1P ), and hb (2P ) samples. Belle fits all subsamples to resonant plus
non-resonant amplitudes, allowing for interference (notably, between π − Tb+b̄ and π + Tb−b̄ ), and finds
consistent pairs of Tbb̄ masses for all bottomonium transitions, and comparable strengths of the twostates. A recent angular analysis assigned J P = 1+ for both Tbb̄ states [101], which must also havenegative G-parity. Transitions through Tbb̄ to the hb (nP ) saturate the observed π + π − hb (nP ) crosssections. While the two masses of the Tbb̄ states as extracted from Breit–Wigner fits for the various
channels are just a few MeV above the B ∗ B̄ and B ∗ B¯∗ thresholds, more refined analyses using only
S-waves find pole locations right below the corresponding thresholds either on the physical [107] or
the unphysical [108] sheet. Once D-waves are included, the pole of the Tbb̄ (10650) moves above theB ∗ B̄ ∗ threshold [109]. Regardless of their proximity to the corresponding thresholds, both states
predominantly decay into these open-flavor channels [103,110] with branching fractions that exceed
80% and 70%, respectively, at 90% CL. This feature provides strong evidence for their molecularnature.

78.4 Concluding Remarks
The discovery of the χc1 (3872) (also known as the X(3872)) in 2003 ushered in an era of tremendous progress in experimental heavy quark meson spectroscopy, even though many issues remain
unsettled. As shown in Tables 78.1 to 78.3, more than 40 new states have been reported during
this period, many of which were unanticipated. While the states below open-flavor thresholds(Table 78.1) appear to be well-explained by the conventional q q̄ quark model, a thorough understanding of the suite of states above (Table 78.2) and near (Table 78.3) open-flavor thresholds
remains elusive. After nearly two decades, experimental progress remains rapid with the continuation of BESIII, the commencement of the Belle II program, and the imminent accumulation of
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78. Spectroscopy of Mesons Containing Two Heavy Quarks

additional data at the LHC.
References[1] N. Brambilla et al., Phys. Rept. 873, 1 (2020), [arXiv:1907.07583].
[2] Y.-R. Liu et al., Prog. Part. Nucl. Phys. 107, 237 (2019), [arXiv:1903.11976].
[3] S. L. Olsen, T. Skwarnicki and D. Zieminska, Rev. Mod. Phys. 90, 1, 015003 (2018),[arXiv:1708.04012].
[4] A. Ali, J. S. Lange and S. Stone, Prog. Part. Nucl. Phys. 97, 123 (2017), [arXiv:1706.00610].
[5] F.-K. Guo et al., Rev. Mod. Phys. 90, 1, 015004 (2018), [arXiv:1705.00141].
[6] R. F. Lebed, R. E. Mitchell and E. S. Swanson, Prog. Part. Nucl. Phys. 93, 143 (2017),[arXiv:1610.04528].
[7] A. E. Bondar, R. V. Mizuk and M. B. Voloshin, Mod. Phys. Lett. A 32, 04, 1750025 (2017),[arXiv:1610.01102].
[8] H.-X. Chen et al., Rept. Prog. Phys. 86, 2, 026201 (2023), [arXiv:2204.02649].
[9] M. Mai, U.-G. Meißner and C. Urbach, Phys. Rept. 1001, 1 (2023), [arXiv:2206.01477].
[10] S. K. Choi et al. (Belle), Phys. Rev. Lett. 91, 262001 (2003), [hep-ex/0309032].
[11] N. Brambilla et al., Eur. Phys. J. C74, 10, 2981 (2014), [arXiv:1404.3723].
[12] J. L. Rosner et al. (CLEO), Phys. Rev. Lett. 95, 102003 (2005), [hep-ex/0505073].
[13] M. Ablikim et al. (BESIII), Phys. Rev. D 99, 7, 072008 (2019), [arXiv:1810.12023].
[14] M. Ablikim et al. (BESIII), Phys. Rev. D 102, 112007 (2020), [arXiv:2010.12092].
[15] M. Ablikim et al. (BESIII), JHEP 05, 108 (2022), [Erratum: JHEP 03, 022 (2023)],[arXiv:2203.10439].
[16] V. Bhardwaj et al. (Belle), Phys. Rev. Lett. 111, 3, 032001 (2013), [arXiv:1304.3975].
[17] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 115, 1, 011803 (2015), [arXiv:1503.08203].
[18] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 129, 10, 102003 (2022), [arXiv:2203.05815].
[19] K. Chilikin et al. (Belle) (2020), [arXiv:2003.08335].
[20] B. Aubert et al. (BaBar), Phys. Rev. Lett. 101, 071801 (2008), [Erratum: Phys.Rev.Lett.
102, 029901 (2009)], [arXiv:0807.1086].
[21] B. Aubert et al. (BaBar), Phys. Rev. Lett. 103, 161801 (2009), [arXiv:0903.1124].
[22] R. Mizuk et al. (Belle), Phys. Rev. Lett. 109, 232002 (2012), [arXiv:1205.6351].
[23] U. Tamponi et al. (Belle), Phys. Rev. Lett. 115, 14, 142001 (2015), [arXiv:1506.08914].
[24] B. G. Fulsom et al. (Belle), Phys. Rev. Lett. 121, 23, 232001 (2018), [arXiv:1807.01201].
[25] I. Adachi et al. (Belle), Phys. Rev. Lett. 108, 032001 (2012), [arXiv:1103.3419].
[26] T. K. Pedlar et al. (CLEO), Phys. Rev. Lett. 107, 041803 (2011), [arXiv:1104.2025].
[27] I. Adachi (Belle), in “8th International Workshop On Heavy Quarkonium (QWG2011) Darmstadt, Germany, October 3-7, 2011,” (2011), [arXiv:1110.3934].
[28] G. Aad et al. (ATLAS), Phys. Rev. Lett. 108, 152001 (2012), [arXiv:1112.5154].
[29] V. M. Abazov et al. (D0), Phys. Rev. D 86, 031103 (2012), [arXiv:1203.6034].
[30] R. Aaij et al. (LHCb), JHEP 10, 088 (2014), [arXiv:1409.1408].
[31] A. M. Sirunyan et al. (CMS), Phys. Rev. Lett. 121, 092002 (2018), [arXiv:1805.11192].
[32] G. Aad et al. (ATLAS), Phys. Rev. Lett. 113, 212004 (2014), [arXiv:1407.1032].
31st May, 2024

15

78. Spectroscopy of Mesons Containing Two Heavy Quarks[33] A. M. Sirunyan et al. (CMS), Phys. Rev. Lett. 122, 132001 (2019), [arXiv:1902.00571].
[34] R. Aaij et al. (LHCb), Phys. Rev. Lett. 122, 232001 (2019), [arXiv:1904.00081].
[35] R. Aaij et al. (LHCb), JHEP 07, 035 (2019), [arXiv:1903.12240].
[36] S. Uehara et al. (Belle), Phys. Rev. Lett. 96, 082003 (2006), [hep-ex/0512035].
[37] B. Aubert et al. (BaBar), Phys. Rev. D 81, 092003 (2010), [arXiv:1002.0281].
[38] K. Abe et al. (Belle), Phys. Rev. Lett. 94, 182002 (2005), [hep-ex/0408126].
[39] J. P. Lees et al. (BaBar), Phys. Rev. D86, 072002 (2012), [arXiv:1207.2651].
[40] F.-K. Guo and U.-G. Meissner, Phys. Rev. D86, 091501 (2012), [arXiv:1208.1134].
[41] S. L. Olsen, Phys. Rev. D91, 5, 057501 (2015), [arXiv:1410.6534].
[42] Z.-Y. Zhou, Z. Xiao and H.-Q. Zhou, Phys. Rev. Lett. 115, 2, 022001 (2015),[arXiv:1501.00879].
[43] R. Aaij et al. (LHCb), Phys. Rev. D 102, 112003 (2020), [arXiv:2009.00026].
[44] K. Chilikin et al. (Belle), Phys. Rev. D95, 112003 (2017), [arXiv:1704.01872].
[45] B. Aubert et al. (BaBar), Phys. Rev. Lett. 95, 142001 (2005), [hep-ex/0506081].
[46] C. Z. Yuan et al. (Belle), Phys. Rev. Lett. 99, 182004 (2007), [arXiv:0707.2541].
[47] Z. Q. Liu et al. (Belle), Phys. Rev. Lett. 110, 252002 (2013), [arXiv:1304.0121].
[48] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 118, 9, 092001 (2017), [arXiv:1611.01317].
[49] M. Cleven et al., Phys. Rev. D90, 7, 074039 (2014), [arXiv:1310.2190].
[50] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 118, 9, 092002 (2017), [arXiv:1610.07044].
[51] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 112, 9, 092001 (2014), [arXiv:1310.4101].
[52] M. Ablikim et al. (BESIII), Phys. Rev. D96, 3, 032004 (2017), [erratum:
Rev.D99,no.1,019903(2019)], [arXiv:1703.08787].

Phys.

[53] X. L. Wang et al. (Belle), Phys. Rev. Lett. 99, 142002 (2007), [arXiv:0707.3699].
[54] X. L. Wang et al. (Belle), Phys. Rev. D91, 112007 (2015), [arXiv:1410.7641].
[55] J. P. Lees et al. (BaBar), Phys. Rev. D89, 11, 111103 (2014), [arXiv:1211.6271].
[56] G. Pakhlova et al. (Belle), Phys. Rev. Lett. 101, 172001 (2008), [arXiv:0807.4458].
[57] S. Jia et al. (Belle), Phys. Rev. D100, 11, 111103 (2019), [arXiv:1911.00671].
[58] K. Chilikin et al. (Belle), Phys. Rev. D88, 7, 074026 (2013), [arXiv:1306.4894].
[59] K. Chilikin et al. (Belle), Phys. Rev. D90, 11, 112009 (2014), [arXiv:1408.6457].
[60] R. Aaij et al. (LHCb), Phys. Rev. Lett. 112, 22, 222002 (2014), [arXiv:1404.1903].
[61] B. Aubert et al. (BaBar), Phys. Rev. D79, 112001 (2009), [arXiv:0811.0564].
[62] R. Mizuk et al. (Belle), Phys. Rev. D78, 072004 (2008), [arXiv:0806.4098].
[63] J. P. Lees et al. (BaBar), Phys. Rev. D85, 052003 (2012), [arXiv:1111.5919].
[64] T. Aaltonen et al. (CDF), Phys. Rev. Lett. 102, 242002 (2009), [arXiv:0903.2229].
[65] T. Aaltonen et al. (CDF), Mod. Phys. Lett. A32, 26, 1750139 (2017), [arXiv:1101.6058].
[66] V. M. Abazov et al. (D0), Phys. Rev. D89, 1, 012004 (2014), [arXiv:1309.6580].
[67] S. Chatrchyan et al. (CMS), Phys. Lett. B734, 261 (2014), [arXiv:1309.6920].
[68] J. Brodzicka, Conf. Proc. C0908171, 299 (2009), [,299(2009)].
[69] R. Aaij et al. (LHCb), Phys. Rev. D85, 091103 (2012), [arXiv:1202.5087].
[70] J. P. Lees et al. (BaBar), Phys. Rev. D91, 1, 012003 (2015), [arXiv:1407.7244].
31st May, 2024

16

78. Spectroscopy of Mesons Containing Two Heavy Quarks[71] C. P. Shen et al. (Belle), Phys. Rev. Lett. 104, 112004 (2010), [arXiv:0912.2383].
[72] R. Aaij et al. (LHCb), Phys. Rev. D95, 1, 012002 (2017), [arXiv:1606.07898].
[73] R. Aaij et al. (LHCb), Phys. Rev. Lett. 118, 2, 022003 (2017), [arXiv:1606.07895].
[74] R. Aaij et al. (LHCb), Phys. Rev. Lett. 127, 8, 082001 (2021), [arXiv:2103.01803].
[75] R. Aaij et al. (LHCb), Sci. Bull. 65, 1983 (2020), [arXiv:2006.16957].
[76] G. Aad et al. (ATLAS) (2023), [arXiv:2304.08962].
[77] A. Hayrapetyan et al. (CMS) (2023), [arXiv:2306.07164].
[78] R. Mizuk et al. (Belle), JHEP 10, 220 (2019), [arXiv:1905.05521].
[79] N. Hüsken, R. E. Mitchell and E. S. Swanson, Phys. Rev. D 106, 9, 094013 (2022),[arXiv:2204.11915].
[80] D. Santel et al. (Belle), Phys. Rev. D93, 1, 011101 (2016), [arXiv:1501.01137].
[81] F.-K. Guo, X.-H. Liu and S. Sakai, Prog. Part. Nucl. Phys. 112, 103757 (2020),[arXiv:1912.07030].
[82] R. Aaij et al. (LHCb), Phys. Rev. Lett. 110, 222001 (2013), [arXiv:1302.6269].
[83] R. Aaij et al. (LHCb), Phys. Rev. D92, 1, 011102 (2015), [arXiv:1504.06339].
[84] R. Aaij et al. (LHCb), Phys. Rev. D 102, 9, 092005 (2020), [arXiv:2005.13419].
[85] N. Brambilla et al., Eur. Phys. J. C71, 1534 (2011), [arXiv:1010.5827].
[86] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 110, 252001 (2013), [arXiv:1303.5949].
[87] T. Xiao et al., Phys. Lett. B727, 366 (2013), [arXiv:1304.3036].
[88] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 112, 2, 022001 (2014), [arXiv:1310.1163].
[89] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 115, 11, 112003 (2015), [arXiv:1506.06018].
[90] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 115, 22, 222002 (2015), [arXiv:1509.05620].
[91] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 111, 24, 242001 (2013), [arXiv:1309.1896].
[92] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 112, 13, 132001 (2014), [arXiv:1308.2760].
[93] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 113, 21, 212002 (2014), [arXiv:1409.6577].
[94] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 115, 18, 182002 (2015), [arXiv:1507.02404].
[95] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 126, 10, 102001 (2021), [arXiv:2011.07855].
[96] M. Ablikim et al. (BESIII), Phys. Rev. Lett. 129, 11, 112003 (2022), [arXiv:2204.13703].
[97] M. Ablikim et al. ((BESIII)„ BESIII), Chin. Phys. C 47, 3, 033001 (2023), [arXiv:2211.12060].
[98] R. Aaij et al. (LHCb), Nature Phys. 18, 7, 751 (2022), [arXiv:2109.01038].
[99] R. Aaij et al. (LHCb), Nature Commun. 13, 1, 3351 (2022), [arXiv:2109.01056].
[100] A. Bondar et al. (Belle), Phys. Rev. Lett. 108, 122001 (2012), [arXiv:1110.2251].
[101] A. Garmash et al. (Belle), Phys. Rev. D91, 7, 072003 (2015), [arXiv:1403.0992].
[102] P. Krokovny et al. (Belle), Phys. Rev. D88, 5, 052016 (2013), [arXiv:1308.2646].
[103] I. Adachi et al. (Belle) (2012), [arXiv:1209.6450].
[104] K. F. Chen et al. (Belle), Phys. Rev. Lett. 100, 112001 (2008), [arXiv:0710.2577].
[105] P. Krokovny (Belle Collab.), talk given at Les Rencontres de Physique de la Vallee d’Aoste,
La Thuile, Aosta Valley, Italy, 2012.
[106] A. Abdesselam et al. (Belle), Phys. Rev. Lett. 117, 14, 142001 (2016), [arXiv:1508.06562].
[107] M. Cleven et al., Eur. Phys. J. A47, 120 (2011), [arXiv:1107.0254].
31st May, 2024

17

78. Spectroscopy of Mesons Containing Two Heavy Quarks[108] F. K. Guo et al., Phys. Rev. D93, 7, 074031 (2016), [arXiv:1602.00940].
[109] Q. Wang et al., Phys. Rev. D 98, 7, 074023 (2018), [arXiv:1805.07453].
[110] A. Garmash et al. (Belle), Phys. Rev. Lett. 116, 21, 212001 (2016), [arXiv:1512.07419].

31st May, 2024