A new hadron spectroscopy

Abstract

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 quark 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.

Graphical abstract

Keywords

quarks / charmonium / pentaquarks / di-baryons / baryonium / tetraquarks

References

[1] R. L. Jaffe, Q2Q ¯2 resonances in the baryon-antibaryon system, Phys. Rev. D17, 1444 (1978)

[2] T. Nakano, (LEPS Collaboration), Evidence for a narrow S= +1 baryon resonance in photoproduction from the neutron, Phys. Rev. Lett.91, 012002 (2003)

[3] D. Diakonov, V. Petrov, and M. Polyakov, Exotic antidecuplet of baryons: Prediction from chiral solitons, Z. Phys. A359, 305 (1997)

[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, (CLAS Collaboration), Search for the Θ⁺ pentaquark in the reaction γd→pK⁻K⁺n, Phys. Rev. Lett.96, 212001 (2006)

[6] K. Shirotori, T. N. Takahashi, S. Adachi, M. Agnello, S. Ajimura, , 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)

[7] W.-M. Yao, (Particle Data Group), Review of particle physics, J. Phys. G33(1), 1 (2006). see, in particular, the “Pentaquark Update” by G. Trilling on page 1019.

[8] R. L. Jaffe, Perhaps a stable dihyperon, Phys. Rev. Lett.38, 195 (1977)

[9] H. Takahashi, J. K. Ahn, H. Akikawa, S. Aoki, K. Arai, , Observation of a HeΛ6Λ double hypernucleus, Phys. Rev. Lett.87(21), 212502 (2001)

[10] S. R. Beane, (NPLQCD Collaboration), Evidence for a bound H dibaryon from lattice QCD, Phys. Rev. Lett.106, 162001 (2011)

[11] T. Inoue, (HALQCD Collaboration), Bound H dibaryon in flavor SU(3) limit of lattice QCD, Phys. Rev. Lett.106, 162002 (2011)

[12] B. H. Kim, (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)

[13] J. Beringer, (Particle Data Group), Review of particle physics, Phys. Rev. D 86(1), 010001 (2012)

[14] 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, (BES Collaboration), Observation of a nearthreshold enhancement in the pp‾ mass spectrum from radiative J/ψ→γpp‾ decays, Phys. Rev. Lett. 91, 022001 (2003)

[17] E. Fermi and C. N. Yang, Are mesons elementary particles? Phys. Rev.76(12), 1739 (1949)

[18] S. Godfrey and S. L. Olsen, The exotic XYZ charmoniumlike mesons, Annu. Rev. Nucl. Part. Sci.58(1), 51 (2008)

[19] B. S. Zou and H. C. Chiang, One-pion-exchange final-state interaction and the pp‾ near threshold enhancement in J/ψ→γpp ¯ decays, Phys. Rev. D69(3), 034004 (2004)

[20] A. Sibirtsev, J. Haidenbauer, S. Krewald, U. G. Meisner, and A. Thomas, Near threshold enhancement of the pp‾ mass spectrum in J/ψ decay, Phys. Rev. D71(5), 054010 (2005)

[21] G. J. Ding and M. L. Yan, Proton–antiproton annihilation in baryonium, Phys. Rev. C72(1), 034014 (2005)

[22] M. Ablikim, (BES Collaboration), Observation of a resonance X(1835) in J/ψ→γπ+π−η′, Phys. Rev. Lett.95, 262001 (2005)

[23] M. Ablikim, (BES Collaboration), Spin-parity analysis of pp‾ mass threshold structure in J/ψ and ψ(3686) radiative decays, Phys. Rev. Lett.108, 112003 (2012)

[24] M. Ablikim, (BES Collaboration), Confirmation of the X(1835) and observation of the resonances X(2120) and X(2370) in J/ψ→γπ+π−η′, Phys. Rev. Lett. 106, 072002 (2011)

[25] M. Ablikim, (BES Collaboration), Study of J/ψ decaying into ωpp‾, Eur. Phys. J. C 53, 15 (2008), arXiv: 0710.5369 [hep-ex]

[26] S. B. Athar, (CLEO Collaboration), Radiative decays of the ϒ(1S) to a pair of charged hadrons, Phys. Rev. D73, 032001 (2006)

[27] M.-Z. Wang, (Belle Collaboration), Observation of B⁺ →pp‾π⁺, B⁰ →pp‾ K⁰, and B⁺ →pp‾ K^∗+, Phys. Rev. Lett.92, 131801 (2004)

[28] See, for example, M. Ablikim, (BESIII Collaboration), Observation of a structure at 1.84 GeV/c² in the 3(π⁺π⁻) mass spectrum in J/ψ → γ3(π⁺π⁻) decays, Phys. Rev. D88, 091502(R) (2013)

[29] For recent reviews see N. Brambilla, S. Eidelman, B. K. Heltsley, R. Vogt, G. T. Bodwin, , Heavy quarkonium: Progress, puzzles, and opportunities, Eur. Phys. J. C71(2), 1534 (2011)

[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, (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, (Belle Collaboration), Production of new charmoniumlike states in e⁺e⁻ →J/ψD^∗D‾ at s≈10.6 GeV, Phys. Rev. Lett.100, 202001 (2008)

[33] K.-T. Chao, Interpretations for the observed in the double charm production at B factories, Phys. Lett. B661(5), 348 (2008)

[34] J.-Z. Bai, (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)

[35] M. Ablikim, (BES Collaboration), Determination of the ψ(3770), ψ(4040), ψ(4160) and ψ(4415) resonance parameters, Phys. Lett. B660(4), 315 (2008)

[36] S. Godfrey and N. Isgur, Mesons in a relativized quark model with chromodynamics, Phys. Rev. D32(1), 189 (1985)

[37] B. Aubert, (BaBar Collaboration), Observation of a broad structure in the π+π−J/ψ mass spectrum around 4.26 GeV/c₂, Phys. Rev. Lett. 95, 142001 (2005)

[38] B. Aubert, (BaBar Collaboration), Evidence of a broad structure at an invariant mass of 4.32 GeV/c₂ in the reaction e⁺e⁻ → π⁺π⁻ψ(2S) measured at BABAR, Phys. Rev. Lett.98, 212001 (2007)

[39] C. Z. Yuan, (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)

[40] X. L. Wang, (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)

[41] G. Pakhlova, (Belle Collaboration), Measurement of the near-threshold e⁺e⁻ →D^(∗)±D^(∗)∓ cross section using initial-state radiation, Phys. Rev. Lett.98, 092001 (2007)

[42] G. Pakhlova, (Belle Collaboration), Observation of the ψ(4415)→DD ¯2*(2460) decay using initial-state radiation, Phys. Rev. Lett.100, 062001 (2008)

[43] G. Pakhlova, (Belle Collaboration), Observation of a near-threshold enhancement in the e⁺e⁻→Λc+Λc− cross section using initial-state radiation, Phys. Rev. Lett.101, 172001 (2008)

[44] G. Pakhlova, (Belle Collaboration), Measurement of the e⁺e⁻ →D⁰D^*−π⁺ cross section using initial-state radiation, Phys. Rev. D80, 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. B640(4), 182 (2006)

[46] See, for example, S.-L. Zhu, The possible interpretations of Y(4260), Phys. Lett. B625(3−4), 212 (2005)

[47] E. Kou and O. Pene, Suppressed decay into open charm for the being a hybrid, Phys. Lett. B631(4), 164 (2005)

[48] Q. Wang, C. Hanhart, and Q. Zhao, Decoding the riddle of Y(4260) and Z_c(3900), Phys. Rev. Lett.111(13), 132003 (2013)

[49] M. Ablikim, (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, (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)

[51] M. Ablikim, (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)

[52] M. Ablikim, (BESIII Collaboration), Observation of a charged charmoniumlike structure in e⁺e⁻ →(DD‾)^±π^∓ at s = 4.26 GeV, Phys. Rev. Lett.112, 132001 (2014), arXiv: 1308.2760 [hep-ex]

[53] R. Mizuk, (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, (Belle Collaboration), Observation of a resonancelike structure in the π±ψ′ mass distribution in exclusive B→Kπ±ψ′ decays, Phys. Rev. Lett. 100, 142001 (2008)

[55] B. Aubert, (BaBar Collaboration), Search for the Z(4430)⁻ at BABAR, Phys. Rev. D 79, 112001 (2009)

[56] R. Mizuk, (Belle Collaboration), Dalitz analysis of B→Kπ±ψ′ decays and the Z(4430)⁺, Phys. Rev. D80, 031104(R) (2009)

[57] K. Chilikin, (Belle Collaboration), Experimental constraints on the spin and parity of the Z(4430)⁺, Phys. Rev. D 88, 074026 (2013)

[58] R. Aaij, (LHCb Collaboration), Observation of the resonant character of the Z(4430)⁻ state, Phys. Rev. Lett. 112, 222002 (2014)

[59] See, for example, P. Pakhlov, Charged charmonium-like states as rescattering effects in decays, Phys. Lett. B 702(2−3), 139 (2011)

[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, (BaBar Collaboration), Measurements of the absolute branching fractions of B^± →K^±Xcc‾, Phys. Rev. Lett.96, 052002 (2006)

[62] K. Chilikin, (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)

[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. B591(1−2), 313 (2000)

[65] M. Beneke, G. Buchelle, M. Neubert, and C. Sachrajda, QCD factorization in B → πK, ππ decays and extraction of Wolfenstein parameters, arXiv: hep-ph/0104110 (2001)

[66] J. P. Lees, (BaBar Collaboration), Search for the Z₁(4050)⁺ and Z₂(4250)⁺ states in B‾ → χ_c1K⁻π⁺ and B⁺ → χ_c1Ks0π⁺, Phys. Rev. D85, 052003 (2012)

[67] C. Zhang, Studies on BEPC upgrade from pretzel to doublering, Sci. China G 53(11), 2084 (2010)

[68] M. Ablikim, (BESIII Collaboration), Design and construction of the BESIII detector, Nucl. Instrum. Methods A614, 345 (2010), arXiv: 0911.4960 [physics.ins-det]

[69] M. Ablikim, (BESIII Collaboration), Observation of a charged (DD‾)^± mass peak in e⁺e⁻ → πDD‾ at s=4.26 GeV, Phys. Rev. Lett.112, 022001 (2014)

[70] M. Ablikim, (BESIII Collaboration), Observation of e⁺e⁻ → π⁰π⁰h_c and a neutral charmoniumlike structure Z_c(4020)⁰, (in preparation) (2014)

[71] S. K. Choi, (Belle Collaboration), Observation of a near-threshold ωJ/ψ mass enhancement in exclusive B→KωJ/ψ decays, Phys. Rev. Lett.94, 182002 (2005)

[72] P. del Amo Sanchez, (BaBar Collaboration), Evidence for the decay X(3872)→J/ψω, Phys. Rev. D82, 011101(R) (2010)

[73] B. Aubert, (BaBar Collaboration), Observation of Y(3940)→J/ψω in B→J/ψωK at BABAR, Phys. Rev. Lett.101, 082001 (2008)

[74] S. Uehara, (Belle Collaboration), Observation of a charmoniumlike enhancement in the γγ → ωJ/ψ process, Phys. Rev. Lett.104, 092001 (2010)

[75] J. P. Lees, (BaBar Collaboration), Study of X(3915)→J/ψω in two-photon collisions, Phys. Rev. D 86, 072002 (2012)

[76] T. Barnes, S. Godfrey, and E. S. Swanson, Higher charmonia, Phys. Rev. D72(5), 054026 (2005)

[77] F. K. Guo, and U. G. Meissner, Where is the χ_c0(2P)? Phys. Rev. D86(9), 091501 (2012)

[78] S. Uehara, (Belle Collaboration), Observation of a χc2′ candidate in γγ →DD‾ production at Belle, Phys. Rev. Lett.96, 082003 (2006)

[79] B. Aubert, (BaBar Collaboration), Observation of the χ_c2(2P) meson in the reaction γγ →DD‾ at BABAR, Phys. Rev. D81, 092003 (2010)

[80] J. Brodzicka, (Belle Collaboration), Observation of a new DsJ meson in B⁺ →D‾DK decays, Phys. Rev. Lett.100, 092001 (2008)

[81] B. Aubert, (BaBar Collaboration), Study of resonances in exclusive B decays to D⁻(^*)D(^*)K, Phys. Rev. D77, 011102(R) (2008)

[82] This is the weighted average of results reported in Refs. [74] and [75].

[83] Y. Jiang, G. L. Wang, T. H. Wang, and W. L. Ju, Why X(3915) is so narrow as a χ_c0(2P) state, arXiv: 1310.2317 [hep-ph] (2013)

[84] Y. C. Yang, Z. Xia, and J. Ping, Are the X(4160) and X(3915) charmonium states? Phys. Rev. D81(9), 094003 (2010)

[85] T. Abe, (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. D80(11), 114013 (2009)

[87] T. Aaltonen, (CDF Collaboration), Evidence for a narrow near-threshold structure in the J/ψφ mass spectrum in B⁺ →J/ψφK⁺ decays, Phys. Rev. Lett. 102, 242002 (2009)

[88] T. Aaltonen, (CDF Collaboration), Observation of the Y (4140) structure in the J/ψφ mass spectrum in B^± → J/ψφK^± decays, arXiv: 1101.6058 (2011)

[89] R. Aaij, (LHCb Collaboration), Search for the X(4140) state in B⁺ →J/ψφK⁺ decays, Phys. Rev. D85, 091103(R) (2012)

[90] S. Chatrchyan, (CMS Collaboration), Observation of a peaking structure in the J/ψφ mass spectrum from B^± → J/ψK^± decays, Phys. Lett. B734, 261 (2014)

[91] J. P. Lees, (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. B699(5), 341 (2011)

[93] K. Yi, Experimental review of structures in the J/ψφ mass spectrum, Int. J. Mod. Phys. A28(18), 1330020 (2013)

[94] C. P. Shen, (Belle Collaboration), Evidence for a new resonance and search for the Y (4140) in the γγ → φJ/ψ process, Phys. Rev. Lett.104, 112004 (2010)

[95] S. K. Choi, (Belle Collaboration), Observation of a narrow charmoniumlike state in exclusive B^±→K^±π⁺π⁻J/ψ Decays, Phys. Rev. Lett.91, 262001 (2003)

[96] D. Acosta, (CDF II Collaboration), Observation of the narrow state X(3872)→J/ψπ⁺π⁻ in PP‾ collisions at s=1.96 TeV, Phys. Rev. Lett.93, 072001 (2004)

[97] V. M. Abazov, (D0 Collaboration), Observation and properties of the X(3872) Decaying to J/ψπ⁺π⁻ in PP‾ Collisions at s=1.96 TeV, Phys. Rev. Lett.93, 162002 (2004)

[98] B. Aubert, (BaBar Collaboration), Study of the B⁻→J/ψK⁻π⁺π⁻ decay and measurement of the B⁻→X(3872)K⁻ branching fraction, Phys. Rev. D71, 071103 (2005)

[99] R. Aaij, (LHCb Collaboration), Observation of X(3872) production in pp collisions at s = 7 TeV, Eur. Phys. J. C72, 1972 (2012)

[100] S. Chatrchyan, (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, (Belle Collaboration), Bounds on the width, mass difference and other properties of X(3872)→ π⁺π⁻J/ψ decays, Phys. Rev. D84, 052004 (2011)

[103] R. Aaij, (LHCb Collaboration), Determination of the X(3872) meson quantum numbers, Phys. Rev. Lett.110, 222001 (2013)

[104] A. Abulencia, (CDF Collaboration), Analysis of the quantum numbers JPC of the X(3872) particle, Phys. Rev. Lett. 98, 132002 (2007)

[105] D. Acosta, (CDF Collaboration), Measurement of the dipion mass spectrum in X(3872)→J/ψπ⁺π⁻ decays, Phys. Rev. Lett.96, 102002 (2006)

[106] B. Aubert, (BaBar Collaboration), Search for a charged partner of the X(3872) in the B meson decay B→X⁻K, X⁻ →J/ψπ⁻π⁰, Phys. Rev. D71, 031501 (2005)

[107] K. Abe, (Belle Collaboration), Evidence for X(3872) → γJ/ψ and the sub-threshold decay X(3872) → ω J/ψ, arXiv: 0505037 [hep-ex] (2005)

[108] T. Aushev, (Belle Collaboration), Study of the B→X(3872)(→D*0D‾0)K decay, Phys. Rev. D 81, 031103(R) (2010)

[109] G. Gokhroo, (Belle Collaboration), Observation of a near-threshold D0D‾0π0 enhancement in B→D0D‾0π0K decay, Phys. Rev. Lett.97, 162002 (2006)

[110] B. Aubert, (BABAR Collaboration), Study of resonances in exclusive B decays to D^(*)D ¯(*)K, Phys. Rev. D77, 011102(R) (2008)

[111] E. Braaten and M. Lu, Line shapes of the X(3872), Phys. Rev. D76(9), 094028 (2007)

[112] E. Braaten and H. W. Hammmer, Universality in few-body systems with large scattering length, Phys. Rep.428(5−6), 259 (2006)

[113] S. Coito, G. Rupp, and E. van Beveren, X(3872) is not a true molecule, Eur. Phys. J. C 73(3), 2351 (2013)

[114] N. A. Törnqvist, Isospin breaking of the narrow charmonium state of Belle at 3872 MeV as a deuson, Phys. Lett. B590(3−4), 209 (2004)

[115] M. Ablikim, (BESIII Collaboration), Observation of e⁺e⁻ → γX(3872) at BESIII, Phys. Rev. Lett.112, 092001 (2014)

[116] N. A. Törnqvist, From the deuteron to deusons, an analysis of deuteronlike meson-meson bound states, Z. Phys. C61(3), 525 (1994)

[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. B578(1−2), 119 (2003)

[119] C. Y. Wong, Molecular states of heavy quark mesons, Phys. Rev. C69(5), 055202 (2004)

[120] S. Pakvasa and M. Suzuki, On the hidden charm state at 3872 MeV, Phys. Lett. B579(1−2), 67 (2004)

[121] E. Braaten and M. Kusunoki, Production of the X(3870) at the Υ(4S) by the coalescence of charm mesons, Phys. Rev. D69(11), 114012 (2004)

[122] E. S. Swanson, Short range structure in the X(3872), Phys. Lett. B588(3−4), 189 (2004)

[123] M. B. Voloshin, Heavy quark spin selection rule and the properties of the X(3872), Phys. Lett. B604(1−2), 69 (2004)

[124] S. Fleming, M. Kusunoki, T. Mehan, and U. van Kolck, Pion interactions in the X(3872), Phys. Rev. D76(3), 034006 (2007)

[125] E. Braaten and M. Lu, Line shapes of the X(3872), Phys. Rev. D76(9), 094028 (2007)

[126] S. L. Zhu, New hadron states, Int. J. Mod. Phys. E17(02), 283 (2009)

[127] D. Gamermann and E. Oset, Isospin breaking effects in the X(3872) resonance, Phys. Rev. D80(1), 014003 (2009)

[128] D. Gamermann and E. Oset, Couplings in coupled channels versus wave functions: Application to the X(3872) resonance, Phys. Rev. D81(1), 014029 (2010)

[129] O. Zhang, C. Meng, and H. Q. Zheng, Ambiversion of X(3872), Phys. Lett. B680(5), 453 (2009)

[130] 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)

[132] A. De Rujula, H. Georgi, and S. L. Glashow, Molecular charmonium: A new spectroscopy? Phys. Rev. Lett.38(7), 317 (1977)

[133] A. V. Manohar and M. B. Wise, Exotic states in QCD, Nucl. Phys. B339(1), 17 (1993)

[134] CDF note 7159,

[135] S. Chatrchyan, (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, (BaBar Collaboration), Evidence for X(3872)→ ψ(2S)γ in B^± →X(3872)K^± decays and a study of B→cc‾γK, Phys. Rev. Lett.102, 132001 (2009)

[138] V. Bhardwaj, (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 DD‾ molecular picture, Phys. Rev. D86(11), 113007 (2012)

[140] E. S. Swanson, Diagnostic decays of the X(3872), Phys. Lett. B598(3−4), 197 (2004)

[141] R. Aaij, (LHCb Collaboration), Evidence for the decay X(3872) → ψ(2S)γ, Nucl. Phys. B886, 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. D71(1), 014028 (2005)

[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. D87, 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. D89(11), 114010 (2014)

[145] D. Horn and H. Mandula, Model of mesons with constituent gluons, Phys. Rev. D17(3), 898 (1978)

[146] L. Liu, (Hadron Spectrum Collaboration), Excited and exotic charmonium spectroscopy from lattice QCD, J. High Energy Phys.07, 126 (2012)

[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

[149] K. K. Seth, The quintessential exotic X(3872), Prog. Part. Nucl. Phys.67(2), 390 (2012)

[150] W. S. Hou, Searching for the bottom counterparts of X(3872) and Y(4260) via π⁺π⁻ϒ, Phys. Rev. D74(1), 017504 (2006)

[151] K.-F. Chen, (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. D82, 091106(R) (2010)

[152] K.-F. Chen, (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 bb‾ tetraquarks based on e⁺e⁻ → bb‾ cross section between s = 10.54 and 11.20 GeV, Phys. Lett. B684(1), 28 (2010)

[154] I. Adachi, (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)

[155] A. Bondar, (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, (Belle Collaboration), First observation of the Zb(10610) in a Dalitz analysis of ϒ(10860)→ ϒ(nS)π⁰π⁰, Phys. Rev. D88, 052016 (2013)

[158] A. Garmash, (Belle Collaboration),Amplitude analysis of e⁺e⁻ → ϒ(nS)π⁺π⁻ at s = 10.865 GeV, arXiv: 1403.0992 [hep-ex] (submitted for publication in Phys. Rev. D)

[159] I. Adachi, (Belle Collaboration), Observation of two charged bottomonium-like resonances, arXiv: 1105.4583 (2011)

[160] I. Adachi, (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, (CDF Collaboration), Precision measurement of the X(3872) mass in J/ψπ⁺π⁻ decays, Phys. Rev. Lett.103, 152001 (2009)

[163] B. Aubert, (BaBar Collaboration), Study of the exclusive initial-state-radiation production of the DD‾ system, Phys. Rev. D 76, 111105(R) (2007)

[164] G. Pakhlova, (Belle Collaboration), Measurement of the near-threshold e⁺e⁻ →DD‾ cross section using initialstate radiation, Phys. Rev. D77, 011103(R) (2008)

[165] B. Aubert, (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, (CLEO Collaboration), Confirmation of the Y(4260) resonance production in initial state radiation, Phys. Rev. D74, 091104(R) (2006)

[167] T. E. Coan, (CLEO Collaboration), Charmonium decays of Y(4260), ψ(4160), and ψ(4040), Phys. Rev. Lett.96, 162003 (2006)

[168] G. Pakhlova, , Belle Collaboration, Observation of a near-threshold enhancement in the e⁺e⁻ → Λc+Λc− cross section using initial-state radiation, Phys. Rev. Lett.101, 172001 (2008)

[169] P. del Amo Sanchez, (BaBar Collaboration), Measurement of the B→D‾(*)D(*)K branching fractions, Phys. Rev. D83, 032004 (2011)

[170] P. del Amo Sanchez, (BaBar Collaboration), Observation of new resonances decaying to Dπ and D*π in inclusive e⁺e⁻ collisions near s = 10.58 GeV, Phys. Rev. D82, 111101(R) (2010)

[171] S. Chatrchyan, (CMS Collaboration), Search for a new bottomonium state decaying to ϒ(1S)π⁺π⁻ in pp collisions at s = 8 TeV, Phys. Lett. B727, 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. D85(5), 054011 (2013)

[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. C73(2), 2298 (2013)

[175] W. Erni, (PANDA Collaboration), Physics performance report for PANDA: Strong interaction studies with antiprotons, arXiv: 0903.3905 [hep-ex] (2009)

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