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A. Valcarce (Univ. Salamanca) J. Vijande, J.-M. Richard

The Seventh Asia-Pacific Conference on Few-Body Problems in Physics. A few certainties and many uncertainties about multiquarks. A. Valcarce (Univ. Salamanca) J. Vijande, J.-M. Richard. 1.- Introduction: A way forward 2.- Simple systems – Simple models

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A. Valcarce (Univ. Salamanca) J. Vijande, J.-M. Richard

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  1. The Seventh Asia-Pacific Conference on Few-Body Problems in Physics A few certainties and many uncertainties about multiquarks A. Valcarce (Univ. Salamanca) J. Vijande, J.-M. Richard

  2. 1.- Introduction: A way forward 2.- Simple systems – Simple models (All-heavy) tetraquarks: Chromoelectric models String models Dibaryons and baryonia: Chromoelectric models Chromomagnetic models Hidden flavor pentaquarks: AL1 model 3.- Unravelling the pattern of XYZ mesons Molecular states: BCN or CQC Coupled channel effects  Hidden color vectors A quark-model mechanism for the XYZ mesons 4.- Conclusions Outline APFB 2017 / A. Valcarce

  3. SLAC M = 3.695 GeV  = 2.7 MeV Experiment: The first hidden-charm exotic states (1974) M = 3.105 GeV  < 1.3 MeV BNL M = 3.1 GeV   0 MeV SLAC 1.- Introduction APFB 2017 / A. Valcarce

  4. Theory: Predictions 1.- Introduction APFB 2017 / A. Valcarce

  5. T. Barnes et al., Phys. Rev. D72, 054026 (2005) Theory: Predictions Chromoelectric central potential: Chromomagnetic spin-spin term: Chromomagnetic spin-orbit term: 1.- Introduction APFB 2017 / A. Valcarce

  6. X(3872) <2.3 MeV Ds1(2460), JP=1+,  < 3.5 MeV New experimental challenges (2003) Ds0*(2317), JP=0+, <3.8 MeV 1.- Introduction APFB 2017 / A. Valcarce

  7. XYZ mesons LHCb pentaquarks WASA dibaryon Baryonia ………? Are all these resonances (if they really exist!) multiquark states and/or hadron-hadron molecules? Back to 1974: A challenge for theory!! |ccnn |Meson (B=0)  |qq , |qqqq |nnnnc |Baryon (B=1)  |qqq , |qqqqq Predictions: An experimental challenge!! 1.- Introduction APFB 2017 / A. Valcarce

  8. J.-M.R., A.V., J.V., Phys. Rev. D 95, 054019 (2017) (All-heavy) tetraquarks • Manyspeculationsaboutthestability of (Q1Q2Q3Q4): (cccc), (bbcc), (bccc), … • Extrapolation of quarkoniumdynamics to higherconfigurations • New color substructures: 33 and 66 • 3- or 4-body forces: many-bodyconfiningforces • Role of antisymmetry: maypenalizefavourablecomponents Chromoelectric (CE) limit (Two-body forces and color as a global operator) Limit of very heavy constituents: Neglect chromomagnetic terms  (mi mj)-1  Atomic physics • Atomicphysics • (e+ e+ e- e-)  Ps2positroniummolecule, stablealthoughwithtinybinding • (p pe- e-)  H2hydrogenmolecule, stablewith a comfortablebinding • Stabilitydependscriticallyonthemassesinvolved: (M+ M+ m- m-) more stablethan(m+ m+ m- m-) • But (M+ m+ M- m-) unstableif M/m  2.2 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  9. Symmetry breaking • QM  Min (p2 + x2 + lx) [E=1-l2/4]< Min (p2 + x2)  Min (Heven + Hodd) < Min (Heven) • Min (M+ M+ m- m-) < Min (m+m+m-m-),, 2m-1=M-1+m-1 • Theyhavethesamethreshold • Min (HC-even + HC-odd) < Min (HC-even) • H2is more stable tan Ps2 • Thus(M+ m+ M- m-) more stablethan(m+m+m-m-) ???? • Symmetrybreakingbenefits more to (M+M-)+(m+m-) • Onemayexpectsomekind of metastabilitybelow(M+m-)+(m+M-) • In short: (Un)favourablesymmetrybreaking can (spoil)generatestability Breakingparticlesymmetry No!! 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  10. Symmetry breaking • Equal-mass case: Asymmetry in thepotentialenergy •  Highestenergy: equalgij (rigorous) •  Lowestenergy: broaderdistribution of gij • Multiquarks • Tetraquarks are penalizedbythe non-Abeliannature of color • Ps2 favoredcompared to quark models  (QQQQ) unstable in naive CE limit • Likewise H2, (QQqq) wouldstableforlarge M/m ratio • (QqQq) stronglydependentonthepresence of twothresholds • Delicatefour-bodyproblem: Mixingeffects do nothelpmuch • Approximationsmay favor bindingartificially × 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  11. Improved chromoelectric model: Many-body confining forces A  String model with |T B  String model with |T and |M C  Pairwise with |T and |M D  Adiabatic limit of C Bound (M+ M+ m- m-)  QQqq Unbound Unbound (M+ m+ M- m-)  QQqq Bound Flip-flop B  u Butterfly A,D (adiabatic) bound !! BUT B,C (color+antisymmetry) unbound J.V., A.V., J.-M.R., Phys. Rev. D 76, 114013 (2007), Phys. Rev. D 87, 034040 (2013) 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  12. All-heavy tetraquarks • (bbbb) and (cccc) are unstable in coherent 4-body estimates in naiveCE models. Theyfollowthetrends of (++) in atomicphysics, butlessfavourabledue to the non-Abelian algebra of color charges. • (bcbc) mighthavesomeopportunities as compared to (bbbb) and (cccc) • (bc)(cb)  MM • BUT (bcbc) there are twodifferentthresholds • (bb)(cc)   J • THUSitmaypresentmetastabilitybelowthe MMthreshold (degeneracy!!) • (bbcc) although more delicate: bestcandidate to be stable • Benefitsfrom C-conjugationbreaking • Thereis a single threshold • (QQqq) favored in the CE limitdue to thestriking M/m dependence • For non-identical quarks and antiquarks, stringpotentialsoffergoodopportunities 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  13. Similar findings: Baryonia (Q3q3) and dibaryons (Q3q3) Dibaryons  Baryonia  J.V., A.V., J.-M.R., Phys. Rev. D 85, 014019 (2012) 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  14. Chromomagnetic term: Dibaryons (qqqq'QQ') J.V., A.V., J.-M.R., P.S., Phys. Rev. D 94, 034038 (2016) Color + Antisymmetry JP=0+ Conflict between CE and CM It goes against binding 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  15. Hidden flavor pentaquarks: (QQqqq) AL1 Three color vectors for each Jacobi coordinate arrangement Two of them with hidden color: octet-octect • Basic contribution of hidden-color components: 15 color-spin vectorsfor J=1/2 or 12 for J=3/2 • The color octet (qqq) configurationreceives a more favourablechromomagneticenergythanthe color singlet in thethreshold: • Color chemistry: H. Hogaasen, P. Sorba, • Nucl. Phys. B145, 119 (1978) Stable! Below S- and D-wave thresholds Perhapsstillnarrow! Below S-wave thresholdbutabove D-wave one. 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  16. Thebindingis a cooperativeeffect of CE and CMeffects, disappearing in the CE limit • The wave function contains color sextet and color octet configurations for the subsystems and can hardly be reduced to a molecular state made of two interacting color-singlet hadrons  CE CE + CM  CE CE + CM 2.- Simple systems – Simple models APFB 2017 / A. Valcarce

  17. Molecular QnQn states: BCN and CQC Q  c Molecular How the molecular QnQn states are formed in a quark model framework? 3.- Unravelling the pattern of XYZ mesons APFB 2017 / A. Valcarce

  18. c c n c n n w J/ c n D D Coupled channel effect  Hidden color vectors T.F.C., A.V., J.V., Phys. Lett. B 709, 358 (2012) 3.- Unravelling the pattern of XYZ mesons APFB 2017 / A. Valcarce

  19. There should not be a partner of the X(3872) in the bottom sector There should be a JP=1+ bound state in the exotic bottom sector QnQn T.F.C., A.V., J.V., Phys. Lett. B 699, 291 (2011) T.F.C., A.V., Phys. Lett. B 758, 244 (2016) Q  b Q  c QQnn 3.- Unravelling the pattern of XYZ mesons APFB 2017 / A. Valcarce

  20.  aQn (Qn)(nQ) A quark-model mechanism for XYZ mesons (bnbn) (QnQn) L=0,S=1,C=+1,P=+1,I=0 (QQ)(nn) (Qn)(nQ) Central Spin-spin MQQ + Mnn MQn + MnQ (bnbn) L=0,S=1,C=+1,P=+1,I=1 (Qn)(nQ) (QQ)(nn) 3.- Unravelling the pattern of XYZ mesons 20/21

  21. Conclusions • In multiquark studies, both CE and CM effects have to be included, color may generate conflicts between the preferred configurations. • All-heavy tetraquarks are unstable in naive CE models. • (QqQq) might have some opportunities of metastability below the MM threshold, which is extremely important for the existence of the X(3872) in the charm sector. • (QQqq) are definitively the best candidates for stable multiquark states due to the striking M/m dependence in the CE limit. Besides, for non-identical quarks and antiquarks, string potentials offer good opportunities. • Hidden heavy flavor pentaquarks are predicted if there is chromomagnetic interaction due to hidden-color component 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: based on coupled-channel effects. • We do not find evidence for charged and bottom partners of the X(3872). To answer this question is a keypoint to advance in the study of hadron spectroscopy. 4.- Conclusions 21/21 APFB 2017 / A. Valcarce

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