1. Hadron 2015 / A. Valcarce1/19 Unraveling the pattern of XYZ mesons. Phys. Lett. B 736, 325 (2014) Phys. Rev. Lett. 103, 222001 (2009); Phys. Rev. D 79, 074010 (2009); Phys. Lett. B 699, 291 (2011); Phys. Rev. D 76, 094022 (2007); Phys. Lett. B 709, 358 (2012) A. Valcarce (Univ. Salamanca) J. Vijande (Univ. Valencia) On the origin of the XYZ mesons Jefferson Lab Virginia, USA Hadron 2015

2. Hadron 2015 / A. Valcarce Outline 2/19 1.- Introduction: Experiment vs. Theory Experiment: The november revolution (1974) Theory & Experiment: A quiet period (1974-2003) Experiment: The beginning of a new era (2003) 2.- Methodology Solving the four-quark problem 3.- Results for four-quark states Non-exotic states: Charmonium Exotic states: Predictions 4.- Looking for molecules Non-exotic states: Charmonium Exotic states: Completeness of quark-model calculations 5.- Too many X’s, Y’s and Z’s? 6.- Unraveling the pattern of XYZ mesons... and beyond 7.- Conclusions

3. Hadron 2015 / A. Valcarce Introduction 3/19 Experiment: The november revolution (1974) BNL SLAC M = 3.1 GeV   0 MeV M = 3.105 GeV  < 1.3 MeV SLAC M = 3.695 GeV  = 2.7 MeV

4. Hadron 2015 / A. Valcarce Introduction 4/19 Theory

5. Hadron 2015 / A. Valcarce Introduction 5/19 Spin-orbit interaction: Spin-spin interaction: Central potential: T. Barnes et al., Phys. Rev. D72, 054026 (2005) Theory & Experiment: A quiet period (1974-2003) S. Godfrey and N. Isgur, Phys. Rev. D32, 189 (1985)

6. Hadron 2015 / A. Valcarce Introduction 6/19 Experiment: The beginning of a new era (2003) D s0 * (2317), J P =0 +,  <3.8 MeV D s1 (2460), J P =1 +,  < 3.5 MeV X(3872)  <2.3 MeV

7. Hadron 2015 / A. Valcarce Introduction 7/19 X(3872) Z(3930) D sJ (2317) D 0 (2308) D sJ (2700) D sJ (2860) Y(3940) X(4160) X(4260) Y(4350) Y(4660) Z(4430) Z 1 (4040) Z 2 (4240) D sJ (3040) D sJ (2460) X(4008) X(3940) Are all these resonances (if they really do exist!) four-quark states and/or meson-meson molecules? R.L. Jaffe, Phys. Rev. D15, 267 (1977) = 0 ccnn This is an experimental challenge!! …… This is again a challenge for theory!!

8. Hadron 2015 / A. Valcarce Methodology 8/19 C-parity is a good symmetry Identical quarks → Pauli 11 22 33 1 2 3 4 1,2  Q3,4  n ccnn 11 22 33 1 2 3 4 1,2  Q3,4  n cncn Solving the Schrödinger equation for a 4q system: VM and HH

9. Hadron 2015 / A. Valcarce Results for four-quark states 9/19 Energías del sistema 4q J.V., A.V. et al., Phys. Rev. D76, 094022 (2007) System: ccnn. Model: BCN M 1 M 2 threshold 4q energies There are no non-exotic deeply four-quark bound states (compact)

10. Hadron 2015 / A. Valcarce Results for four-quark states 10/19 E ( M e V ) 0 + (2 8 ) 1 + (24) 2 + (30) 0  (21) 1  (21) 2  (21) 0 + (28) 1 + (24) 2 + (30) 0  (21) 1  (21) 2  (21) I=1I=0 4q energies M 1 M 2 threshold J.V., A.V., N.B., Phys. Rev. D79, 074010 (2009) System: cncn. Model: CQC One compact state in the ccnn system (J P =1 + )

11. Hadron 2015 / A. Valcarce Looking for molecules 11/19 We solved the scattering of two-meson systems in a coupled-channel approach by means of the Lippmann-Schwinger equation, looking for attractive channels. Molecular states, how to look for them? The meson-meson interacting potential is obtained from the same quark-quark interaction used in the HH and VM methods, by means of the adiabatic approximation. (I) D D c n We study the consequences of allowing for the rearrangement of quarks (I) or not (II). D D c n DD  D * D * (II)

12. Hadron 2015 / A. Valcarce Looking for molecules 12/19 There are no charged partners of the X(3872) [ diquark-antidiquark ] J PC (I)=1 ++ (0) (I) DD * (II) DD * – J/   X(3872) T. F.-C., A.V., J.V., Phys. Rev. Lett. 103, 222001 (2009) Hidden flavor sector: Charmonium

13. Hadron 2015 / A. Valcarce Looking for molecules 13/19 (I) J P = (0) 1 + We have analyzed all positive parity channels until J P =2 + Only one channel is attractive: (I) J P = (0) 1 + Explicit flavor sector: Exotics J P (I)=1 + (0) (I) T.F.C., A.V., J.V., Phys. Lett. B 699, 291 (2011) (I) (II) Formalisms based on meson-meson configurations and those considering explicitly four-quark states are equivalent if (and only if) a full basis is considered. P DD* P D*D* P DD Four-quark states Meson – Meson

14. Hadron 2015 / A. Valcarce Too many X's, Y's and Z's? 14/19 Too many X’s, Y’s and Z’s? T.F.C., A.V., J.V., Phys. Lett. B 709, 358 (2012) c n c n J/J/  c n c n D D

15. Hadron 2015 / A. Valcarce Unraveling the pattern of XYZ mesons... 15/19 Unraveling the pattern of XYZ mesons (QnQn) (QQ)(nn) (Qn)(nQ) M QQ + M nn  M Qn + M nQ (QQ)(nn) (Qn)(nQ) CentralSpin-spin (bnbn) L=0,S=1,C=+1,P=+1,I=0 L=0,S=1,C=+1,P=+1,I=1 (Qn)(nQ)  Qn 

16. Hadron 2015 / A. Valcarce Unraveling the pattern of XYZ mesons... 16/19 X axis  =E[(Qn)(nQ)] – E[(QQ)(nn)] Y axis E K=22 [(QnQn)]/[E(M 1 )+E(M 2 )]  =0  E[(Qn)(nQ)] = E[(QQ)(nn)] : Degeneracy  >0  E[(Qn)(nQ)] > E[(QQ)(nn)] : Normal ordering  <0  E[(Qn)(nQ)] < E[(QQ)(nn)] : Reversed levels Normal ordering Reversed levels Degeneracy =1  E[(QnQn)] = [E(M 1 )+E(M 2 )] : Threshold >1  E[(QnQn)] > [E(M 1 )+E(M 2 )] : Continuum state <1  E[(QnQn)] < [E(M 1 )+E(M 2 )] : Bound state Continuum state Threshold Bound state L=0,S=1,C=+1,P=+1,I=0

17. A possible example of this mechanism: Four quark systems T. Barnes, F.E. Close, E.S. Swanson Belle Collaboration Hadron 2015 / A. Valcarce... and beyond 17/19 4482 MeV D(1S) D * (2S) 4433 MeV D * (1S) D(2S)  50 MeV D(1S) D * (2S)  D(1S) D * (2S)  V OPE = 0 D * (1S) D(2S)  D * (1S) D(2S)  V OPE = 0 D(1S) D * (2S)  D * (1S) D(2S)  V OPE ≠ 0 P-wave pion exchange model D D  D*D* D  Forbidden Allowed S-wave  J P =1 + 00 11 c n n c 1S 2S

18. Another possible example: Five quark systems (QnQn)  (Qnnnn) (Qnn)(nn) (nnn)(Qn) M Qnn + M nn  M nnn + M Qn Diquark hypothesis: Q  nn R.M. Albuquerque, S.H. Lee, M. Nielsen BABAR Collaboration Hadron 2015 / A. Valcarce... and beyond 18/19

19. Hadron 2015 / A. Valcarce Conclusions Increasing interest in hadronic spectroscopy due to the advent of a large number of experimental data of difficult explanation: XYZ mesons. These data provide with the best laboratory for studying QCD in what N. Isgur called the strong limit. We have the methods, so we can learn about the dynamics. Hidden flavor components (unquenching the quark model) offer a possible explanation of new experimental data and old problems in the meson and baryon spectra. There is not a proliferation of multiquarks, they are very rare. We have presented a plausible mechanism explaining the origin of the XYZ mesons: When there is an attractive interaction characterizing the (Qn)(nQ) upper system combined with the vicinity of the two allowed thresholds, a four-quark bound state may emerge. Experimentalists: Exotic charmed four-quark states may exist if our understanding of the low-energy QCD dynamics does not hide some important information. We do not find evidence for charged and bottom partners of the X(3872). To answer these questions is a keypoint to advance in the study of hadron spectroscopy. Theorists: We have seen many different approaches to explain the new charmonium states. It would be great to extend these models to other sectors making predictions that may be tested in the near future. Conclusions 19/19

20. Hadron 2015 / A. Valcarce Too many X's, Y's and Z's? 20/20 Hidden flavor Explicit flavor There should not be a partner of the X(3872) in the bottom sector There should be a J P =1 + bound state in the exotic bottom sector