QCD-motivated models for hadrons predict an assortment of “exotic” hadrons that have structures that are more complex than the quark-antiquark mesons and three-quark baryons of the original quark-parton model. These include pentaquark baryons, the six-quark H -dibaryon, and tetraquark, hybrid and glueball mesons. Despite extensive experimental searches, no unambiguous candidates for any of these exotic configurations have been identified. On the other hand, a number of meson states, one that seems to be a proton-antiproton bound state, and others that contain either charmed-anticharmed quark pairs or bottom-antibottom q uark pairs, have been recently discovered that neither fit into the quark-antiquark meson picture nor match the expected properties of the QCD-inspired exotics. Here I briefly review results from a recent search for the H -dibaryon, and discuss some properties of the newly discovered states –the proton-antiproton state and the so-called XYZ mesons– and compare them with expectations for conventional quark-antiquark mesons and the predicted QCD-exotic states.
T. Nakano , et al. (LEPS Collaboration), Evidence for a narrow S = +1 baryon resonance in photoproduction from the neutron , Phys. Rev. Lett. 91 , 012002 ( 2003 ) https://doi.org/10.1103/PhysRevLett.91.012002
3
D. Diakonov , V. Petrov , and M. Polyakov , Exotic antidecuplet of baryons: Prediction from chiral solitons , Z. Phys. A 359 , 305 ( 1997 ) https://doi.org/10.1007/s002180050406
4
A review of the events during this period together with references to the experimental work is provided in: R. A. Schumacher, The rise and fall of pentaquarks in experiments , arXiv: nucl-ex/0512042 ( 2005 )
5
See, for example, B.McKinnon , et al. (CLAS Collaboration), Search for the Θ + pentaquark in the reaction γd→pK − K + n , Phys. Rev. Lett. 96 , 212001 ( 2006 ) https://doi.org/10.1103/PhysRevLett.96.212001
6
K. Shirotori , T. N. Takahashi , S. Adachi , M. Agnello , S. Ajimura , et al., Search for the Θ + pentaquark via the π − p→K − X reaction at 1.92 GeV/c , Phys. Rev. Lett. 109 ( 13 ), 132002 ( 2012 )(and references cited therein) https://doi.org/10.1103/PhysRevLett.109.132002
7
W.-M. Yao , et al. (Particle Data Group), Review of particle physics , J. Phys. G 33 ( 1 ), 1 ( 2006 ). see, in particular, the “Pentaquark Update” by G. Trilling on page 1019.
H. Takahashi , J. K. Ahn , H. Akikawa , S. Aoki , K. Arai , et al., Observation of a H e Λ 6 Λ double hypernucleus , Phys. Rev. Lett. 87 ( 21 ), 212502 ( 2001 )
T. Inoue , et al. (HALQCD Collaboration), Bound H dibaryon in flavor SU (3) limit of lattice QCD , Phys. Rev. Lett. 106 , 162002 ( 2011 ) https://doi.org/10.1103/PhysRevLett.106.162002
12
B. H. Kim , et al. (Belle Collaboration), Search for an Hdibaryon with a mass near 2m Λ in ϒ (1S) and ϒ (2S) decays , Phys. Rev. Lett. 110 , 222002 ( 2013 )
Reported by T. E. Barnes, HADRON05 summary: Heavy quark hadrons and theory , arXiv: hep-ph/0510365 ( 2005 )
15
R. L. Jaffe , Exotica , arXiv: hep-ph/0409065v2 ( 2004 )
16
J. Z. Bai , et al. (BES Collaboration), Observation of a nearthreshold enhancement in the p p ‾ mass spectrum from radiative J / ψ → γ p p ‾ decays , Phys. Rev. Lett. 91 , 022001 ( 2003 )
B. S. Zou and H. C. Chiang , One-pion-exchange final-state interaction and the p p ‾ near threshold enhancement in J / ψ → γ p p ¯ decays , Phys. Rev. D 69 ( 3 ), 034004 ( 2004 ) https://doi.org/10.1103/PhysRevD.69.034004
20
A. Sibirtsev , J. Haidenbauer , S. Krewald , U. G. Meisner , and A. Thomas , Near threshold enhancement of the p p ‾ mass spectrum in J / ψ decay , Phys. Rev. D 71 ( 5 ), 054010 ( 2005 ) https://doi.org/10.1103/PhysRevD.71.054010
M. Ablikim , et al. (BES Collaboration), Observation of a resonance X(1835) in J / ψ → γ π + π − η ′ , Phys. Rev. Lett. 95 , 262001 ( 2005 )
23
M. Ablikim , et al. (BES Collaboration), Spin-parity analysis of p p ‾ mass threshold structure in J / ψ and ψ (3686) radiative decays , Phys. Rev. Lett. 108 , 112003 ( 2012 ) https://doi.org/10.1103/PhysRevLett.108.112003
24
M. Ablikim , et al. (BES Collaboration), Confirmation of the X(1835) and observation of the resonances X(2120) and X(2370) in J / ψ → γ π + π − η ′ , Phys. Rev. Lett. 106 , 072002 ( 2011 )
25
M. Ablikim , et al. (BES Collaboration), Study of J/ ψ decaying into ω p p ‾ , Eur. Phys. J. C 53 , 15 ( 2008 ), arXiv: 0710.5369 [hep-ex] https://doi.org/10.1140/epjc/s10052-007-0467-4
26
S. B. Athar , et al. (CLEO Collaboration), Radiative decays of the ϒ (1S) to a pair of charged hadrons , Phys. Rev. D 73 , 032001 ( 2006 )
27
M.-Z. Wang , et al. (Belle Collaboration), Observation of B + → p p ‾ π + , B 0 → p p ‾ K 0 , and B + → p p ‾ K ∗+ , Phys. Rev. Lett. 92 , 131801 ( 2004 ) https://doi.org/10.1103/PhysRevLett.92.131801
28
See, for example , M. Ablikim , et al. (BESIII Collaboration), Observation of a structure at 1.84 GeV/c 2 in the 3(π + π − ) mass spectrum in J/ ψ → γ3(π + π − ) decays , Phys. Rev. D 88 , 091502(R) ( 2013 )
29
For recent reviews see N. Brambilla , S. Eidelman , B. K. Heltsley , R. Vogt , G. T. Bodwin , et al., Heavy quarkonium: Progress, puzzles, and opportunities , Eur. Phys. J. C 71 ( 2 ), 1534 ( 2011 ) https://doi.org/10.1140/epjc/s10052-010-1534-9
30
G. T. Bodwin , E. Braaten , E. Eichten , S. L. Olsen , T. K. Pedlar , and J. Russ , Quarkonium at the frontiers of high energy physics: A snowmass white paper , arXiv: 1307.7425 [hep-ph] ( 2013 )
31
K. Abe , et al. et al. (Belle Collaboration), Observation of a charmoniumlike state produced in association with a J/ ψ in e + e − annihilation at s ≈ 10.6 GeV , Phys. Rev. Lett. 98 , 082001 ( 2007 )
32
P. Pakhlov , et al. (Belle Collaboration), Production of new charmoniumlike states in e + e − →J/ ψ D ∗ D ‾ at s ≈10.6 GeV , Phys. Rev. Lett. 100 , 202001 ( 2008 )
J.-Z. Bai , et al. (BES Collaboration), Measurements of the cross section for e + e − → hadrons at center-of-mass energies from 2 to 5 GeV , Phys. Rev. Lett. 88 , 101802 ( 2002 ) https://doi.org/10.1103/PhysRevLett.88.101802
35
M. Ablikim , et al. (BES Collaboration), Determination of the ψ (3770), ψ (4040), ψ (4160) and ψ (4415) resonance parameters , Phys. Lett. B 660 ( 4 ), 315 ( 2008 ) https://doi.org/10.1016/j.physletb.2007.11.100
36
S. Godfrey and N. Isgur , Mesons in a relativized quark model with chromodynamics , Phys. Rev. D 32 ( 1 ), 189 ( 1985 ) https://doi.org/10.1103/PhysRevD.32.189
37
B. Aubert , et al. (BaBar Collaboration), Observation of a broad structure in the π + π −J/ ψ mass spectrum around 4.26 GeV/c 2 , Phys. Rev. Lett. 95 , 142001 ( 2005 ) https://doi.org/10.1103/PhysRevLett.95.142001
38
B. Aubert , et al. (BaBar Collaboration), Evidence of a broad structure at an invariant mass of 4.32 GeV/c 2 in the reaction e + e − → π + π − ψ (2S) measured at BABAR , Phys. Rev. Lett. 98 , 212001 ( 2007 ) https://doi.org/10.1103/PhysRevLett.98.212001
39
C. Z. Yuan , et al. (Belle Collaboration), Measurement of the e + e − → π + π − J/ ψ cross section via initial-state radiation at Belle , Phys. Rev. Lett. 99 , 182004 ( 2007 ) https://doi.org/10.1103/PhysRevLett.99.182004
40
X. L. Wang , et al. (Belle Collaboration), Observation of two resonant structures in e + e − → π + π − ψ (2S) via initial-state radiation at Belle , Phys. Rev. Lett. 99 , 142002 ( 2007 ) https://doi.org/10.1103/PhysRevLett.99.142002
41
G. Pakhlova , et al. (Belle Collaboration), Measurement of the near-threshold e + e − →D ( ∗ ) ± D ( ∗ ) ∓ cross section using initial-state radiation , Phys. Rev. Lett. 98 , 092001 ( 2007 ) https://doi.org/10.1103/PhysRevLett.98.092001
42
G. Pakhlova , et al. (Belle Collaboration), Observation of the ψ (4415)→ D D ¯ 2 * (2460) decay using initial-state radiation , Phys. Rev. Lett. 100 , 062001 ( 2008 )
43
G. Pakhlova , et al. (Belle Collaboration), Observation of a near-threshold enhancement in the e + e − → Λ c + Λ c − cross section using initial-state radiation , Phys. Rev. Lett. 101 , 172001 ( 2008 )
44
G. Pakhlova , et al. (Belle Collaboration), Measurement of the e + e − →D 0 D *− π + cross section using initial-state radiation , Phys. Rev. D 80 , 091101(R) ( 2009 )
45
X. H. Mo , G. Li , C. Z. Yuan , K. L. He , H. M. Hu , J. H. Hu , P. Wang , and Z. Y. Wang , Determining the upper limit of Γ ee for the Y(4260) , Phys. Lett. B 640 ( 4 ), 182 ( 2006 ) https://doi.org/10.1016/j.physletb.2006.07.060
46
See, for example , S.-L. Zhu , The possible interpretations of Y(4260) , Phys. Lett. B 625 ( 3−4 ), 212 ( 2005 )
M. Ablikim , et al. (BESIII Collaboration), Observation of a charged charmoniumlike structure in e + e − → π + π − J/ ψ at s =4.26 GeV , Phys. Rev. Lett. 110 , 252001 ( 2013 )
50
Z. Q. Liu , et al. (Belle Collaboration), Study of e + e − → π + π − J/ ψ and observation of a charged charmoniumlike state at Belle , Phys. Rev. Lett. 110 , 252002 ( 2013 ) https://doi.org/10.1103/PhysRevLett.110.252002
51
M. Ablikim , et al. (BESIII Collaboration), Observation of a charged charmoniumlike structure Z c (4020) and search for the Z c (3900) in e + e − → π + π − hc , Phys. Rev. Lett. 111 , 242001 ( 2013 ) https://doi.org/10.1103/PhysRevLett.111.242001
52
M. Ablikim , et al. (BESIII Collaboration), Observation of a charged charmoniumlike structure in e + e − →( D D ‾ ) ± π ∓ at s = 4.26 GeV , Phys. Rev. Lett. 112 , 132001 ( 2014 ), arXiv: 1308.2760 [hep-ex]
53
R. Mizuk , et al. (Belle Collaboration), Observation of two resonance-like structures in the π + χ c1 mass distribution in exclusive B ‾ →K − π + χ c1 decays , Phys. Rev. D 78 , 072004 ( 2008 )
54
S.-K. Choi , et al. (Belle Collaboration), Observation of a resonancelike structure in the π ± ψ ′ mass distribution in exclusive B→ K π ± ψ ′ decays , Phys. Rev. Lett. 100 , 142001 ( 2008 )
R. Mizuk , et al. (Belle Collaboration), Dalitz analysis of B→ K π ± ψ ′ decays and the Z(4430) + , Phys. Rev. D 80 , 031104(R) ( 2009 )
57
K. Chilikin , et al. (Belle Collaboration), Experimental constraints on the spin and parity of the Z(4430) + , Phys. Rev. D 88 , 074026 ( 2013 ) https://doi.org/10.1103/PhysRevD.88.074026
58
R. Aaij , et al. (LHCb Collaboration), Observation of the resonant character of the Z(4430) − state , Phys. Rev. Lett. 112 , 222002 ( 2014 ) https://doi.org/10.1103/PhysRevLett.112.222002
59
See, for example , P. Pakhlov , Charged charmonium-like states as rescattering effects in decays , Phys. Lett. B 702 ( 2−3 ), 139 ( 2011 ) https://doi.org/10.1016/j.physletb.2011.06.079
60
P. Pakhlov and T. Uglov , Charged charmonium-like Z + (4430) from rescattering in conventional B decays , arXiv: 1408.5295 [hep-ph] ( 2014 )
61
B. Aubert , et al. (BaBar Collaboration), Measurements of the absolute branching fractions of B ± →K ± X c c ‾ , Phys. Rev. Lett. 96 , 052002 ( 2006 )
62
K. Chilikin , et al. (Belle Collaboration), Observation of a new charged charmonium-like state in B→ J/ ψ K π decays , arXiv: 1408.6457 ( 2014 ) (submitted for publication in Phys. Rev. D)
63
For discussions of factorization in heavy quark decays see, for example , M. Beneke , G. Buchelle , M. Neubert , and C. Sachrajda , QCD factorization for B → ππ decays: Strong phases and CP violation in the heavy quark limit , Phys. Rev. Lett. 83 ( 10 ), 1914 ( 1999 ) https://doi.org/10.1103/PhysRevLett.83.1914
64
M. Beneke , G. Buchelle , M. Neubert , and C. Sachrajda , QCD factorization for exclusive non-leptonic-meson decays: General arguments and the case of heavy-light final states , Nucl. Phys. B 591 ( 1−2 ), 313 ( 2000 ) https://doi.org/10.1016/S0550-3213(00)00559-9
65
M. Beneke , G. Buchelle , M. Neubert , and C. Sachrajda , QCD factorization in B → π K, ππ decays and extraction of Wolfenstein parameters , arXiv: hep-ph/0104110 ( 2001 )
66
J. P. Lees , et al. (BaBar Collaboration), Search for the Z 1 (4050) + and Z 2 (4250) + states in B ‾ → χ c1 K − π + and B + → χ c1 K s 0 π + , Phys. Rev. D 85 , 052003 ( 2012 )
M. Ablikim , et al. (BESIII Collaboration), Design and construction of the BESIII detector , Nucl. Instrum. Methods A 614 , 345 ( 2010 ), arXiv: 0911.4960 [physics.ins-det]
69
M. Ablikim , et al. (BESIII Collaboration), Observation of a charged ( D D ‾ ) ± mass peak in e + e − → π D D ‾ at s =4.26 GeV , Phys. Rev. Lett. 112 , 022001 ( 2014 )
70
M. Ablikim , et al. (BESIII Collaboration), Observation of e + e − → π 0 π 0 h c and a neutral charmoniumlike structure Z c (4020) 0 , ( in preparation ) ( 2014 )
71
S. K. Choi , et al. (Belle Collaboration), Observation of a near-threshold ω J/ ψ mass enhancement in exclusive B→K ω J/ ψ decays , Phys. Rev. Lett. 94 , 182002 ( 2005 ) https://doi.org/10.1103/PhysRevLett.94.182002
72
P. del Amo Sanchez , et al. (BaBar Collaboration), Evidence for the decay X(3872)→J/ ψω , Phys. Rev. D 82 , 011101(R) ( 2010 )
S. Uehara , et al. (Belle Collaboration), Observation of a charmoniumlike enhancement in the γγ → ω J/ ψ process , Phys. Rev. Lett. 104 , 092001 ( 2010 ) https://doi.org/10.1103/PhysRevLett.104.092001
75
J. P. Lees , et al. (BaBar Collaboration), Study of X(3915)→J/ ψω in two-photon collisions , Phys. Rev. D 86 , 072002 ( 2012 ) https://doi.org/10.1103/PhysRevD.86.072002
T. Abe , et al. (BelleII Collaboration), Belle II technical design report , arXiv: 1011.0352 [hep-ex] ( 2010 )
86
R. Molina and E. Oset , Y(3940), Z(3930), and the X(4160) as dynamically generated resonances from the vector-vector interaction , Phys. Rev. D 80 ( 11 ), 114013 ( 2009 ) https://doi.org/10.1103/PhysRevD.80.114013
87
T. Aaltonen , et al. (CDF Collaboration), Evidence for a narrow near-threshold structure in the J/ ψφ mass spectrum in B + →J/ ψφ K + decays , Phys. Rev. Lett. 102 , 242002 ( 2009 ) https://doi.org/10.1103/PhysRevLett.102.242002
88
T. Aaltonen , et al. (CDF Collaboration), Observation of the Y (4140) structure in the J/ ψφ mass spectrum in B ± → J/ ψφ K ± decays , arXiv: 1101.6058 ( 2011 )
89
R. Aaij , et al. (LHCb Collaboration), Search for the X (4140) state in B + →J/ ψφ K + decays , Phys. Rev. D 85 , 091103(R) ( 2012 )
90
S. Chatrchyan , et al. (CMS Collaboration), Observation of a peaking structure in the J/ ψφ mass spectrum from B ± → J/ ψ K ± decays , Phys. Lett. B 734 , 261 ( 2014 ) https://doi.org/10.1016/j.physletb.2014.05.055
91
J. P. Lees , et al. (BaBar Collaboration), Study of B ±,0 → J/ ψ K + K − K ±,0 and search for B0 → J/ ψφ at BABAR , arXiv: 1407.7244 , 2014 (submitted for publication in Phys. Rev. D )
92
X. Liu , Z. G. Luo , and S. L. Zhu , Novel charmoniumlike structures in the invariant mass spectra , Phys. Lett. B 699 ( 5 ), 341 ( 2011 ) https://doi.org/10.1016/j.physletb.2011.04.024
C. P. Shen , et al. (Belle Collaboration), Evidence for a new resonance and search for the Y (4140) in the γγ → φ J/ ψ process , Phys. Rev. Lett. 104 , 112004 ( 2010 ) https://doi.org/10.1103/PhysRevLett.104.112004
95
S. K. Choi , et al. (Belle Collaboration), Observation of a narrow charmoniumlike state in exclusive B ± →K ± π + π − J/ ψ Decays , Phys. Rev. Lett. 91 , 262001 ( 2003 ) https://doi.org/10.1103/PhysRevLett.91.262001
96
D. Acosta , et al. (CDF II Collaboration), Observation of the narrow state X(3872)→J/ ψπ + π − in P P ‾ collisions at s =1.96 TeV , Phys. Rev. Lett. 93 , 072001 ( 2004 )
97
V. M. Abazov , et al. (D0 Collaboration), Observation and properties of the X(3872) Decaying to J/ ψπ + π − in P P ‾ Collisions at s =1.96 TeV , Phys. Rev. Lett. 93 , 162002 ( 2004 )
98
B. Aubert , et al. (BaBar Collaboration), Study of the B − →J/ ψ K − π + π − decay and measurement of the B − →X(3872)K − branching fraction , Phys. Rev. D 71 , 071103 ( 2005 ) https://doi.org/10.1103/PhysRevD.71.071103
99
R. Aaij , et al. (LHCb Collaboration), Observation of X (3872) production in pp collisions at s = 7 TeV , Eur. Phys. J. C 72 , 1972 ( 2012 )
100
S. Chatrchyan , et al. (CMS Collaboration), Measurement of the X (3872) production cross section via decays to J/ ψππ in pp collisions at s = 7 TeV , J. High Energy Phys. 154 , 1304 ( 2013 ); see also arXiv: 1302.3968 [hep-ex]
101
S. Eidelman , Talk at the φ to ψ symposium , Rome , 2013 -Sep. -9–12
102
S.-K. Choi , et al. (Belle Collaboration), Bounds on the width, mass difference and other properties of X(3872)→ π + π − J/ ψ decays , Phys. Rev. D 84 , 052004 ( 2011 ) https://doi.org/10.1103/PhysRevD.84.052004
A. Abulencia , et al. (CDF Collaboration), Analysis of the quantum numbers JPC of the X (3872) particle , Phys. Rev. Lett. 98 , 132002 ( 2007 ) https://doi.org/10.1103/PhysRevLett.98.132002
105
D. Acosta , et al. (CDF Collaboration), Measurement of the dipion mass spectrum in X(3872)→J/ ψπ + π − decays , Phys. Rev. Lett. 96 , 102002 ( 2006 ) https://doi.org/10.1103/PhysRevLett.96.102002
106
B. Aubert , et al. (BaBar Collaboration), Search for a charged partner of the X(3872) in the B meson decay B→X − K, X − →J/ ψπ − π 0 , Phys. Rev. D 71 , 031501 ( 2005 ) https://doi.org/10.1103/PhysRevD.71.031501
107
K. Abe , et al. (Belle Collaboration), Evidence for X(3872) → γ J/ ψ and the sub-threshold decay X(3872) → ω J/ ψ , arXiv: 0505037 [hep-ex] ( 2005 )
108
T. Aushev , et al. (Belle Collaboration), Study of the B→X(3872)(→ D * 0 D ‾ 0 )K decay , Phys. Rev. D 81 , 031103(R) ( 2010 )
109
G. Gokhroo , et al. (Belle Collaboration), Observation of a near-threshold D 0 D ‾ 0 π 0 enhancement in B→ D 0 D ‾ 0 π 0 K decay , Phys. Rev. Lett. 97 , 162002 ( 2006 )
110
B. Aubert , et al. (BABAR Collaboration), Study of resonances in exclusive B decays to D (*) D ¯ ( * ) K , Phys. Rev. D 77 , 011102(R) ( 2008 )
N. A. Törnqvist , Isospin breaking of the narrow charmonium state of Belle at 3872 MeV as a deuson , Phys. Lett. B 590 ( 3−4 ), 209 ( 2004 ) https://doi.org/10.1016/j.physletb.2004.03.077
N. A. Törnqvist , From the deuteron to deusons, an analysis of deuteronlike meson-meson bound states , Z. Phys. C 61 ( 3 ), 525 ( 1994 ) https://doi.org/10.1007/BF01413192
117
N. A. Törnqvist , Comment on the narrow charmonium state of Belle at 3871.8 MeV as a deuson , arXiv: hep-ph/0308277 ( 2003 )
118
See, for example , F. E. Close , and P. R. Page , The threshold resonance , Phys. Lett. B 578 ( 1−2 ), 119 ( 2003 )
S. Fleming , M. Kusunoki , T. Mehan , and U. van Kolck , Pion interactions in the X(3872) , Phys. Rev. D 76 ( 3 ), 034006 ( 2007 ) https://doi.org/10.1103/PhysRevD.76.034006
D. Gamermann and E. Oset , Couplings in coupled channels versus wave functions: Application to the X(3872) resonance , Phys. Rev. D 81 ( 1 ), 014029 ( 2010 ) https://doi.org/10.1103/PhysRevD.81.014029
M. B. Voloshin and L. B. Okun , Hadron molecules and charmonium atom , JETP Lett. 23 , 333 ( 1976 )
131
M. Bander , G. L. Shaw , P. Thomas , and S. Meshkov , Exotic mesons and e + e − annihilation , Phys. Rev. Lett. 36 ( 13 ), 695 ( 1976 ) https://doi.org/10.1103/PhysRevLett.36.695
132
A. De Rujula , H. Georgi , and S. L. Glashow , Molecular charmonium: A new spectroscopy ? Phys. Rev. Lett. 38 ( 7 ), 317 ( 1977 ) https://doi.org/10.1103/PhysRevLett.38.317
S. Chatrchyan , et al. (CMS Collaboration), Measurement of the X(3872) production cross section via decays to J/ ψππ in pp collisions at s = 7 TeV , J. High Energy Phys. 1304 , 154 ( 2013 )
136
C. Bignamini , B. Grinstein , F. Piccinini , A. D. Polosa , and C. Sabelli , Is the X(3872) production cross section at s =1.96 TeV compatible with a hadron molecule interpretation ? Phys. Rev. Lett. 103 ( 16 ), 162001 ( 2009 )
137
B. Aubert , et al. (BaBar Collaboration), Evidence for X(3872)→ ψ (2S) γ in B ± →X(3872)K ± decays and a study of B→ c c ‾ γ K , Phys. Rev. Lett. 102 , 132001 ( 2009 )
138
V. Bhardwaj , et al. (Belle Collaboration), Observation of X(3872)→J/ ψγ and search for X(3872)→ ψ ′ γ in B decays , Phys. Rev. Lett. 107 , 091803 ( 2011 )
139
F. Aceti , R. Molina , and E. Oset , X(3872)→J/ ψγ decay in the D D ‾ molecular picture , Phys. Rev. D 86 ( 11 ), 113007 ( 2012 )
R. Aaij , et al. (LHCb Collaboration), Evidence for the decay X(3872) → ψ (2S) γ , Nucl. Phys. B 886 , 665 ( 2014 )
142
L. Maiani , F. Piccinini , A. D. Polosa , and V. Riguer , Diquark-antidiquark states with hidden or open charm and the nature of X(3872) , Phys. Rev. D 71 ( 1 ), 014028 ( 2005 ) https://doi.org/10.1103/PhysRevD.71.014028
143
L. Maiani , V. Riguer , R. Faccini , F. Piccinini , A. Pilloni and A. D. Polosa , J PC =1 ++ charged resonance in the Υ(4260)→ π + π − J/ ψ decay ? Phys. Rev. D 87 , 111102(R) ( 2013 )
144
L. Maiani , F. Piccinini , A. D. Polosa , and V. Riguer , Z(4430) and a new paradigm for spin interactions in tetraquarks , Phys. Rev. D 89 ( 11 ), 114010 ( 2014 ) https://doi.org/10.1103/PhysRevD.89.114010
L. Liu , et al. (Hadron Spectrum Collaboration), Excited and exotic charmonium spectroscopy from lattice QCD , J. High Energy Phys. 07 , 126 ( 2012 ) https://doi.org/10.1007/JHEP07(2012)126
147
M. B. Voloshin , Z c (3900)-what is inside ? arXiv: 1304.0380 ( 2013 )
148
M. Takizawa and S. Takeuchi , X(3872) as a hybrid state of the charmonium and the hadronic molecule , Prog. Theor. Exp. Phys . 9 , 093D01 , ( 2013 ), arXiv: 1206.4877
W. S. Hou , Searching for the bottom counterparts of X(3872) and Y(4260) via π + π − ϒ , Phys. Rev. D 74 ( 1 ), 017504 ( 2006 )
151
K.-F. Chen , et al. (Belle Collaboration), Observation of an enhancement in e + e − → ϒ (1S) π + π − , ϒ (2S) π + π − , and ϒ (3S)π + π − production near s =10.89 GeV , Phys. Rev. D 82 , 091106(R) ( 2010 )
152
K.-F. Chen , et al. (Belle Collaboration), Observation of anomalous ϒ (1S) π + π − and ϒ (2S) π + π − production near the ϒ (5S) Resonance , Phys. Rev. Lett. 100 , 112001 ( 2008 )
153
A. Ali , C. Hambrock , I. Ahmed , and M. J. Aslam , A case for hidden b b ‾ tetraquarks based on e + e − → b b ‾ cross section between s = 10.54 and 11.20 GeV , Phys. Lett. B 684 ( 1 ), 28 ( 2010 )
154
I. Adachi , et al. (Belle Collaboration), First observation of the P-wave spin-singlet bottomonium states h b (1P) and h b (2P) , Phys. Rev. Lett. 108 , 032001 ( 2012 ) https://doi.org/10.1103/PhysRevLett.108.032001
155
A. Bondar , et al. (Belle Collaboration), Observation of Two Charged Bottomoniumlike Resonances in ϒ (5S) Decays , Phys. Rev. Lett. 108 , 122001 ( 2012 )
156
A. Bondar , Talk at the φ to ψ symposium , Rome , Sep. 9–12, 2013
157
P. Krokovny , et al. (Belle Collaboration), First observation of the Z b (10610) in a Dalitz analysis of ϒ (10860)→ ϒ ( n S) π 0 π 0 , Phys. Rev. D 88 , 052016 ( 2013 )
158
A. Garmash , et al. (Belle Collaboration),Amplitude analysis of e + e − → ϒ ( n S) π + π − at s = 10.865 GeV , arXiv: 1403.0992 [hep-ex] (submitted for publication in Phys. Rev. D )
159
I. Adachi , et al. (Belle Collaboration), Observation of two charged bottomonium-like resonances , arXiv: 1105.4583 ( 2011 )
160
I. Adachi , et al. (Belle Collaboration), Study of three-body Y(10860) decays , arXiv: 1209.6450 ( 2012 )
161
S. Eidelman , B. K. Heltsley , J. J. Hernandez-Rey , S. Navas , and C. Patrignani , Developments in heavy quarkonium spectroscopy , arXiv: 1205.4189 [hep-ex] ( 2012 )
162
T. Aaltonen , et al. (CDF Collaboration), Precision measurement of the X(3872) mass in J/ ψπ + π − decays , Phys. Rev. Lett. 103 , 152001 ( 2009 ) https://doi.org/10.1103/PhysRevLett.103.152001
163
B. Aubert , et al. (BaBar Collaboration), Study of the exclusive initial-state-radiation production of the D D ‾ system , Phys. Rev. D 76 , 111105(R) ( 2007 )
164
G. Pakhlova , et al. (Belle Collaboration), Measurement of the near-threshold e + e − → D D ‾ cross section using initialstate radiation , Phys. Rev. D 77 , 011103(R) ( 2008 )
165
B. Aubert , et al. (BaBar Collaboration), Study of the π + π − J/ ψ mass spectrum via initial-state radiation at BABAR , arXiv: 0808.1543v2 [hep-ex] ( 2008 )
166
Q. He , et al. (CLEO Collaboration), Confirmation of the Y(4260) resonance production in initial state radiation , Phys. Rev. D 74 , 091104(R) ( 2006 )
167
T. E. Coan , et al. (CLEO Collaboration), Charmonium decays of Y(4260), ψ (4160), and ψ (4040) , Phys. Rev. Lett. 96 , 162003 ( 2006 ) https://doi.org/10.1103/PhysRevLett.96.162003
168
G. Pakhlova , et al., Belle Collaboration, Observation of a near-threshold enhancement in the e + e − → Λ c + Λ c − cross section using initial-state radiation , Phys. Rev. Lett. 101 , 172001 ( 2008 )
169
P. del Amo Sanchez , et al. (BaBar Collaboration), Measurement of the B→ D ‾ ( * ) D ( * ) K branching fractions , Phys. Rev. D 83 , 032004 ( 2011 )
170
P. del Amo Sanchez , et al. (BaBar Collaboration), Observation of new resonances decaying to D π and D* π in inclusive e + e − collisions near s = 10.58 GeV , Phys. Rev. D 82 , 111101(R) ( 2010 )
171
S. Chatrchyan , et al. (CMS Collaboration), Search for a new bottomonium state decaying to ϒ (1S) π + π − in pp collisions at s = 8 TeV , Phys. Lett. B 727 , 57 ( 2013 )
172
A. Ali , C. Hambrock , and W. Wang , Tetraquark interpretation of the charged bottomonium-like states Z± b (10610) and Z± b (10650) and implications , Phys. Rev. D 85 ( 5 ), 054011 ( 2013 ) https://doi.org/10.1103/PhysRevD.85.054011
173
A. Ali , C. Hambrock , and M. J. Aslam , Tetraquark interpretation of the BELLE data on the anomalous ϒ (1S) π + π − and ϒ (2S) π + π − production near the ϒ (5S) resonance , Phys. Rev. Lett. 104 ( 16 ), 162001 ( 2010 )
174
T. Friedmann , No radial excitations in low energy QCD (I): Diquarks and classification of mesons , Eur. Phys. J. C 73 ( 2 ), 2298 ( 2013 ) https://doi.org/10.1140/epjc/s10052-013-2298-9
175
W. Erni , et al. (PANDA Collaboration), Physics performance report for PANDA: Strong interaction studies with antiprotons , arXiv: 0903.3905 [hep-ex] ( 2009 )
