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Register a new userHeavy Holographic Exotics: Tetraquarks as Efimov States
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We provide a holographic description of non-strange multiquark exotics as compact topological molecules by binding heavy-light mesons to a tunneling configuration in D8-D$bar 8$ that is homotopic to the vacuum state with fixed Chern-Simons number. In the tunneling process, the heavy-light mesons transmute to fermions. Their binding is generic and arises from a trade-off between the dipole attraction induced by the Chern-Simons term and the U(1) fermionic repulsion. In the heavy quark limit, the open-flavor tetraquark exotics $QQbar qbar q$ and $bar Qbar Q qq$, emerge as bound Efimov states in a degenerate multiplet $IJ^pi=(00^+ , 01^+)$ with opposite intrinsic Chern-Simons numbers $pm frac 12$. The hidden-flavor tetraquark exotics such as $Qbar Q qbar q$, $QQbar Qbar q$ and $QQbar Qbar Q$ as compact topological molecules are unbound. Other exotics are also discussed.
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In this work, we systematically study the mass spectrum of the fully heavy tetraquark in an extended chromomagnetic model, which includes both color and chromomagnetic interactions. Numerical results indicate that the energy level is mainly determined by the color interaction, which favors the color-sextet $ket{(QQ)^{6_{c}}(bar{Q}bar{Q})^{bar{6}_{c}}}$ configuration over the color-triplet $ket{(QQ)^{bar{3}_{c}}(bar{Q}bar{Q})^{3_{c}}}$ one. The chromomagnetic interaction mixes the two color configurations and gives small splitting. The ground state is always dominated by the color-sextet configuration. We find no stable state below the lowest heavy quarkonium pair thresholds. Most states may be wide since they have at least one $S$-wave decay channel into two $S$-wave mesons. One possible narrow state is the $1^{+}$ $bbbar{b}bar{c}$ state with a mass $15719.1~text{MeV}$. It is just above the $eta_{b}bar{B}_{c}$ threshold. But this channel is forbidden because of the conservation of the angular momentum and parity.
In this work we study the mass spectra of the fully-heavy tetraquark systems, i.e. $ccbar{c}bar{c}$, $bbbar{b}bar{b}$, $bbbar{c}bar{c}/ccbar{b}bar{b}$, $bcbar{c}bar{c}/ccbar{b}bar{c}$, $bcbar{b}bar{b}/bbbar{b}bar{c}$, and $bcbar{b}bar{c}$, within a potential model by including the linear confining potential, Coulomb potential, and spin-spin interactions. It shows that the linear confining potential has important contributions to the masses and is crucial for our understanding of the mass spectra of the fully-heavy tetraquark systems. For the fully-heavy tetraquarks $Q_1Q_2bar{Q}_3bar{Q}_4$ our explicit calculations suggest that no bound states can be formed below the thresholds of any meson pairs $(Q_1bar{Q}_3)$-$(Q_2bar{Q}_4)$ or $(Q_1bar{Q}_4)$-$(Q_2bar{Q}_3)$. Thus, we do not expect narrow fully-heavy tetraquark states to be existing in experiments.
In this article a systematic quantitative analysis of the isoscalar bosonic states is performed in the framework of supersymmetric light front holographic QCD. It is shown that the spectroscopy of the $eta$ and $h$ mesons can be well described if one additional mass parameter -- which corresponds to the hard breaking of chiral $U(1)$ symmetry in standard QCD -- is introduced. The mass difference of the $eta$ and $eta$ isoscalar mesons is then determined by the strange quark mass content of the $eta$. The theory also predicts the existence of isoscalar tetraquarks which are bound states of diquarks and anti-diquarks. The candidates for these exotic isoscalar tetraquarks are identified. In particular, the $f_0(1500)$ is identified as isoscalar tetraquark; the predicted mass value 1.52 GeV agrees with the measured experimental value within the model uncertainties.
Alerted by the recent LHCb discovery of exotic hadrons in the range (6.2 -- 6.9) GeV, we present new results for the doubly-hidden scalar heavy $(bar QQ) (Qbar Q)$ charm and beauty molecules using the inverse Laplace transform sum rule (LSR) within stability criteria and including the Next-to-Leading Order (NLO) factorized perturbative and $langle G^3rangle$ gluon condensate corrections. We also critically revisit and improve existing Lowest Order (LO) QCD spectral sum rules (QSSR) estimates of the $({ bar Q bar Q})(QQ)$ tetraquarks analogous states. In the example of the anti-scalar-scalar molecule, we separate explicitly the contributions of the factorized and non-factorized contributions to LO of perturbative QCD and to the $langlealpha_sG^2rangle$ gluon condensate contributions in order to disprove some criticisms on the (mis)uses of the sum rules for four-quark currents. We also re-emphasize the importance to include PT radiative corrections for heavy quark sum rules in order to justify the (ad hoc) definition and value of the heavy quark mass used frequently at LO in the literature. Our LSR results for tetraquark masses summarized in Table II are compared with the ones from ratio of moments (MOM) at NLO and results from LSR and ratios of MOM at LO (Table IV). The LHCb broad structure around (6.2 --6.7) GeV can be described by the $overline{eta}_{c}{eta}_{c}$, $overline{J/psi}{J/psi}$ and $overline{chi}_{c1}{chi}_{c1}$ molecules or/and their analogue tetraquark scalar-scalar, axial-axial and vector-vector lowest mass ground states. The peak at (6.8--6.9) GeV can be likely due to a $overline{chi}_{c0}{chi}_{c0}$ molecule or/and a pseudoscalar-pseudoscalar tetraquark state. Similar analysis is done for the scalar beauty states whose masses are found to be above the $overlineeta_beta_b$ and $overlineUpsilon(1S)Upsilon(1S)$ thresholds.
We investigate non-linear extensions of the holographic soft wall model proposed by Karch, Katz, Son and Stephanov [1] including non-minimal couplings in the five-dimensional action. The non-minimal couplings bring a new parameter $a_0$ which controls the transition between spontaneous and explicit symmetry breaking near the limit of massless quarks (the chiral limit). In the physical region (positive quark mass), we show that above a critical value of the parameter $a_0$ the chiral condensate $langle bar{q} q rangle$ is finite in the chiral limit, signifying spontaneous chiral symmetry breaking. This result is supported by the lightest states arising in the spectrum of the pseudoscalar mesons, which become massless in the chiral limit and are therefore intrepreted as Nambu-Goldstone bosons. Moreover, the decay constants of the pseudoscalar mesons also support this conclusion, as well as the Gell-Mann-Oakes-Renner (GOR) relation satisfied by the lightest states. We also calculate the spectrum of scalar, vector, and axial-vector mesons with their corresponding decay constants. We describe the evolution of masses and decay constants with the increasing of the quark mass and for the physical mass we compare our results against available experimental data. Finally, we do not find instabilities in our model for the physical region (positive quark mass).
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