We study possible multiquark contributions to the charm baryon spectrum by considering higher order Fock space components. For this purpose we perform two different calculations. In a first approach we do a coupled-channel calculation of the $ND$ system looking for molecular states. In a second step we allow for the coupling to a heavy baryon--light meson two-hadron system looking for compact exotic five--quark structures. Both calculations have been done within the framework of a chiral constituent quark model. The model, tuned in the description of the baryon and meson spectra as well as the $NN$ interaction, provides parameter-free predictions for charm $+1$ molecular or compact two-hadron systems. Unlike the $N \bar D$ system, no sharp quark-Pauli effects are found. However, the existence of different two-hadron thresholds for the five-quark system will make the coupled-channel dynamics relevant. Only a few channels are candidates to lodge molecular or compact hadrons with a five-quark structure, being specially relevant the $(T)J^P=(0)1/2^-$ and $(T)J^P=(2)5/2^-$ channels. The identification of molecular states and/or compact hadrons with multiquark components either with or without exotic quantum numbers is a challenge of different collaborations like $\bar {\rm P}$ANDA, LHCb, ExHIC or J-PARC.
Exotic charmonium and bottonomium resonances recently discovered are discussed and interpreted as diquark-antidiquark states containing a pair of charm quarks and a pair of light, up and down, quarks. Successes, shortcomings and predictions of the model are illustrated.
Since 2003 many charmonium-like states were observed experimentally. Especially those charged charmonium-like $Z_c$ states and bottomonium-like $Z_b$ states cannot be accommodated within the naive quark model, which are good candidates of either the hidden-charm tetraquark states or molecules composed of a pair of charmed mesons. In 2015, the LHCb Collaboration discovered two hidden-charm pentaquark states, which are also beyond the quark model. In this talk, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark st…
The recent reported charmed-strange resonance at 2.32 GeV/c suggests a possible multiquark state. Three types of multiquark bound states are reviewed. A previous model-independent variational approach considers a tetraquark with two heavy antiquarks and two light quarks as a heavy antidiquark with the color field of a quark bound to the two light quarks with a wave function like that of a heavy baryon. Results indicate that a charmed-strange tetraquark $\bar c \bar s u d$ or a bottom-strange tetraquark $\bar b \bar s u d$ with this "diquark-heavy-baryion" wave function is not bound, in contras…
We assume that the two hidden charm pentaquark discovered at $LHCb$ are built by the three light quarks and the $c \bar{c}$ pair, both colour octets. in relative P-wave for the $\frac{5}{2}^+$ state and by the eigenvectors of the chromomagnetic interaction for the five constituents in S-wave for the four $\frac{3}{2}^-$ state with masses $4360$, $4410$, $4490$ and $4560$ MeV . The "open channel" $p $ $J/ψ$ has large components along the two first states and so appear as the $4380$ resonance and the $Λ_c$ $ \bar{D}^{0*}$ and $Σ_c$ $\bar{D}^*$ are "open channels" for the lower and higher resonan…
In the large $N_c$ limit, one can describe normal heavy baryons $Qqq$ and heavy pentaquarks $\bar Qqqqq$ as bound states of heavy mesons to chiral (anti)solitons. In this picture, the strong and electromagnetic decay parameters of these hadrons can be calculated from those of the constituent heavy mesons and light baryons, while the weak decay form factors can be evaluated analytically.
A review is presented of some discussions about the fully-heavy tetraquarks made of two charmed quarks and two anticharmed antiquarks, and other multiquark states involving some heavy flavors.
In view of the renewing experimental interest for searching strangeness $+1$ baryons at J-PARC, we study the existence of light baryon resonances with strangeness +1 generated in the $K$-$(N^*/Δ^*)$ system, where $N^*$ represents either $N^*(1535)$/$N^*(1650)$/$N^*(1700)$, and $Δ^*$ corresponds to $Δ(1620)$. The description of the properties of the aforementioned states requires considering the dynamics involved in the coupled pseudoscalar-baryon and vector-baryon systems with strangeness $S=0$ in the s-wave. For the purpose of our current study, we consider the pseudoscalar-baryon (PB) and ve…
A hadron's multiquark content reflects itself in the quark composition of the interpolator with which it has maximal overlap. The AdS/CFT dictionary translates the anomalous dimension of this interpolator into a mass correction for the corresponding dual mode. Hence such bulk-mass corrections can carry holographic information on multiquark correlations. Two prominent examples are studied by implementing this robust and universal mechanism into AdS/QCD gravity duals. In the baryon sector bulk-mass corrections are used to describe systematic good (i.e. maximally attractive) diquark effects. The …
Based on the color-spin interaction in diquarks, we argue that charmed multiquark hadrons are likely to exist. Because of the appreciable number of charm quarks produced in central nucleus-nucleus collisions at ultrarelativistic energies, production of charmed multiquark hadrons is expected to be enhanced in these collisions. Using both the quark coalescence model and the statistical hadronization model, we estimate the yield of charmed tetraquark meson $T_{cc}$ and pentaquark baryon $Θ_{cs}$ in heavy ion collisions at RHIC and LHC. We further discuss the decay modes of these charmed exotic ha…
A review is presented of past and recent attempts to build multiquark states within current models already describing ordinary mesons and baryons. This includes: coherence in the chromomagnetic interaction, tetraquarks with two heavy quarks, Steiner-tree models of confinement, and hadronic molecules, in particular in the hidden-charm sector. Some emphasis is put on the difficulties encountered when extrapolating toward higher configurations the dynamics of confining forces, starting from the simple case of a quark and an antiquark, or three quarks in a colour singlet.
Why do we see certain types of strongly interacting elementary particles and not others? This question was posed over 50 years ago in the context of the quark model. M. Gell-Mann and G. Zweig proposed that the known mesons were $q \bar q$ and baryons $qqq$, with quarks known at the time $u$ ("up"), $d$ ("down"), and $s$ ("strange") having charges (2/3,-1/3,-1/3). Mesons and baryons would then have integral charges. Mesons such as $qq \bar q \bar q$ and baryons such as $qqqq \bar q$ would also have integral charges. Why weren't they seen? They have now been seen, but only with additional heavy …
I study the Ds(2317) and Ds(2457) discovered at BABAR, CLEO and BELLE, and find that they belong to a class of strange S=-1 tetraquarks and pentaquarks, which is equivalent to the class of kaonic molecules bound by short range attraction. In this class of hadrons a kaon is strongly trapped by a s-wave meson or baryon. To describe this class of multiquarks the Resonating Group Method is applied to a standard quark model with chiral symmetry breaking, and the short range kaon-meson(baryon) interactions are extracted. A criterion is derived to classify the attractive channels. I conclude that the…
The mechanism of string creation for light quarks developed earlier is considered for nonzero quark chemical potential. A strong modification of the confining string due to finite quark density (chemical quark potential $μ$) is observed. As a surprising result in a multiquark system with a common string junction an attractive well appears of radius $μ/σ$ and of an average depth equal to $μ$, which induces formation of multiquark hadrons. Preliminary estimates predict a new phase transition to multiquark hadron phase at rather high densities (in heavy ion collisions or in neutron stars) when ne…
We present a selection of topics with an interplay of hadron and few-body physics. This includes few-nucleon systems, light hypernuclei and quark dynamics for baryons and multiquarks. It is stressed that standard quark models predict very few stable multiquarks.
In the past decade many charmonium-like states were observed experimentally. Especially those charged charmonium-like $Z_c$ states and bottomonium-like $Z_b$ states can not be accommodated within the naive quark model. These charged $Z_c$ states are good candidates of either the hidden-charm tetraquark states or molecules composed of a pair of charmed mesons. Recently, the LHCb Collaboration discovered two hidden-charm pentaquark states, which are also beyond the quark model. In this work, we review the current experimental progress and investigate various theoretical interpretations of these …
A number of candidate multiquark hadrons, i.e., particle resonances with substructures that are more complex than the quark-antiquark mesons and three-quark baryons that are prescribed in the textbooks, have recently been observed. In this talk I present: some recent preliminary BESIII results on the near-threshold behavior of sigma(e+e- --> Lambda Lambda-bar) that may or may not be related to multiquark mesons in the light- and strange-quark sectors; results from Belle and LHCb on the electrically charged, charmoniumlike Z(4430)^+ --> pi^+ psi ' resonance that necessarily has a four-quark sub…
We use a bag model to study flavoured mesonic $(Qq\bar q\bar q)$ and baryonic $({\overline Q}qqqq)$ states, where one heavy quark $Q$ is associated with light quarks or antiquarks, and search for possible stable multiquarks. No bound state is found. However some states lie not too high above their dissociation threshold, suggesting the possibility of resonances, or perhaps bound states in improved models.
Within a theoretical framework that accounts for all open-charm mesons, including the $D_0^*(2308)$, the $D_{sJ}^*(2317)$ and the $D_{sJ}(2460)$, we analyze the structure and explore possible quantum number assignments for the $D_{sJ}(2860)$ and the $D_{sJ}(2700)$ mesons reported by BABAR and Belle Collaborations. The open-charm sector is properly described if considered as a combination of conventional quark-antiquark states and four--quark components. All negative parity and $2^+$ states can be understood in terms only of $q\bar q$ components, however the description of the $0^+$ and $1^+$ m…
This talk reviews the last applications of the covariant quark model for studying the properties of the multiquark states: B_s-meson (quark-antiquark state), light baryons (three-quark states) and tetraquark (four-quark state). The form factors of the B(B_s)-> P(V)-transitions are evaluated in the full kinematical region of momentum transfer squared. The widths of some B_s-nonleptonic decays are calculated. The static properties of the proton and neutron, and the Λ-hyperon (magnetic moments and charge radii) and the behavior of the nucleon form factors at low momentum transfers are described. …
The decays of the antisymmetric dibaryon octet $D(8_F)$ into two baryon octets are considered. Sum rules for these decays in first order broken SU(3) are given. An SU(4) extension of the analysis is commented upon. Possibilities for the experimental observation of multibaryon and anti-multibaryon states is pointed out.
We study the nature of the new charm and charmonium like states observed in the last few years, using a constituent quark model with an screened confinement potential. In the open charm sector we discuss different options for the $D_{sJ}(2860)$ including tetraquark configurations. In the hidden charm sector the new Y(4360) and Y(4660) are interpreted as $1^{--}$ states. The influence in the puzzling 3940 MeV region of multiquark configuration is discussed.
Phases of nuclear matter are crucial in the determination of physical properties of neutron stars~(NS). In the core of NS, the density and pressure become so large that the nuclear matter possibly undergoes phase transition into a deconfined phase, consisting of quarks and gluons and their colour bound states. Even though the quark-gluon plasma has been observed in ultra-relativistic heavy-ion collisions\cite{Gyulassy, Andronic}, it is still unclear whether exotic quark matter exists inside neutron stars. Recent results from the combination of various perturbative theoretical calculations with…
The possibility of the existence of mesons with two or more quark-antiquark pairs is investigated with a new application of the Thomas-Fermi (TF) statistical quark model. Quark color couplings are treated in a mean field manner similar to a previous application to baryons, and short and concise expressions for energies are derived. We find that, on average, quarks only interact with antiquarks in such systems. The TF differential equation is constructed and systems with heavy-light quark content are examined. Three types of mesonic systems are defined. In the case of charm quarks, multi-charmo…
I review some selected aspects of the phenomenology of multiquark states discovered in high energy experiments. They have four valence quarks (called tetraquarks) and two of them are found to have five valence quarks (called pentaquarks), extending the conventional hadron spectrum which consists of quark-antiquark $(q\bar{q})$ mesons and $qqq$ baryons. Multiquark states represent a new facet of QCD and their dynamics is both challenging and currently poorly understood. I discuss various approaches put forward to accommodate them, with emphasis on the diquark model.
We discuss whether the bound nature of multiquark states in quark models could benefit from relativistic effects on the kinetic energy operator. For mesons and baryons, relativistic corrections to the kinetic energy lead to lower energies, and thus call for a retuning of the parameters of the model. For multiquark states, as well as their respective thresholds, a comparison is made of the results obtained with non-relativistic and relativistic kinetic energy. It is found that the binding energy is lower in the relativistic case. In particular, $QQ\bar q\bar q$ tetraquarks with double heavy fla…
In this work we study the radiative decay of dynamically generated $J^P=\oh^-$ charm baryons into the ground state $J^P=\oh^+$ baryons. Since different theoretical interpretations of these baryonic resonances, and in particular of the $Λ_c(2595)$, give different predictions, a precise experimental measurement of these decays would be an important step for understanding their nature.
The hidden-charm tetraquarks with strangeness, $c\bar{c}s\bar{q}$ $(q=u,\,d)$, in $J^P=0^+$, $1^+$ and $2^+$ are systematically investigated in the framework of real- and complex-scaling range of a chiral quark model, whose parameters have been fixed in advance describing hadron, hadron-hadron and multiquark phenomenology. Each tetraquark configuration, compatible with the quantum numbers studied, is taken into account; this includes meson-meson, diquark-antidiquark and K-type arrangements of quarks with all possible color wave functions in four-body sector. Among the different numerical techn…
Exotic multi-quark states are examined in the quark model and in QCD. Current status of theoretical studies of the pentaquark Theta^+ is reported. We show recent analyses of multi-quark components of baryons. A novel method to extract a compact excited state in the lattice QCD is applied. We also discuss how to determine mixings of different Fock components in the hadron in the QCD sum rule approach.
Starting from an effective Hamiltonian modelling a baryon made of $N$ identical quarks in the large-$N$ approach of QCD, we obtain analytical formulas allowing to estimate the contributions of multiquark interactions to the baryon mass. The cases of vanishing (mass spectrum) and non-vanishing (baryon melting) temperatures are treated.
We have investigated the charmonium survival probability, in a high baryon density parton plasma, expected to be produced in nuclear collisions at FAIR. Charmonia are assumed to undergo complete dissociation by color screening, if the in-medium Debye radius becomes comparable to the spatial size of the corresponding bound state. Results indicate a non-trivial dependence of the suppression pattern on the plasma evolution dynamics. A much larger magnitude of suppression is foreseen induced by cold nuclear matter compared to that due to plasma screening.
We recall and update, both theoretically and phenomenologically, our (nearly) forty-years-old proposal of a string-junction as a necessary complement to the conventional classification of hadrons based just on their quark-antiquark constituents. In that proposal single (though in general metastable) hadronic states are associated with "irreducible" gauge-invariant operators consisting of Wilson lines (visualized as strings of color flux tubes) that may either end on a quark or an antiquark, or annihilate in triplets at a junction $J$ or an anti-junction $\bar{J}$. For the junction-free sector …
The multiquark confining potential is proportional to the total distance of the fundamental strings linking the quarks and antiquarks. We address the computation of the total string distance an of the Fermat points where the different strings meet. For a meson (quark-antiquark system) the distance is trivially the quark-antiquark distance. For a baryon (three quark system) the problem was solved geometrically from the onset, by Fermat and by Torricelli. The geometrical solution can be determined just with a rule and a compass, but translation of the geometrical solution to an analytical expres…
We study the mass spectrum of baryons with two and three charmed quarks. For double charm baryons the spin splitting is found to be smaller than standard quark-model potential predictions. This splitting is not influenced either by the particular form of the confining potential or by the regularization taken for the contact term of the spin-spin potential. We consistently predict the spectra for triply charmed baryons.
Multiquark states have been advocated to explain recent experimental data in the heavy-light sector, and there are already speculations about multiquarks containing only heavy quarks and antiquarks. With a rigorous treatment of the four-body problem in current quark models, full-charm $(cc\bar c\bar c)$ and full-beauty $(bb\bar b\bar b)$ tetraquarks are found to be unbound. Thus their stability should rely on more subtle effects that are not included in the simple picture of constituent quarks. The case of $(bc\bar b\bar c)$ might be more favorable if the naive color-additive model of confinem…
With the one-boson-exchange model we have studied the possible existence of the very loosely bound hidden-charm molecular baryons composed of anti-charmed meson and charmed baryon. Our numerical results indicate that there exist $Σ_c\bar{D}^*$ states with $I(J^P)=1/2(1/2^-), 1/2(3/2^-), 3/2(1/2^-), 3/2(3/2^-)$ and $Σ_c\bar{D}$ state with $3/2(1/2^-)$. But the $Λ_c \bar{D}$ and $Λ_c \bar{D}^*$ molecular states do not exist.
We review the spectroscopy and some properties of hadrons containing two charmed quarks, or more generally, two heavy quarks. This includes heavy baryons such as $(bcu)$, and possible exotic multiquark states.
We study the discovered Ds(2317) at BABAR, CLEO and BELLE, and find that it belongs to a class of strange multiquarks, which is equivalent to the class of kaonic molecules bound by hard core attraction. In this class of hadrons a kaon is trapped by a s-wave meson or baryon. To describe this class of multiquarks we apply the Resonating Group Method, and extract the hard core kaon-meson(baryon)interactions. We derive a criterion to classify the attractive channels. We find that the mesons f0(980), Ds(2457), Bs scalar and axial, and also the baryons with the quantum numbers of Lambda, Xi_c, Xi_b …
In recent years there has been a rapidly growing body of experimental evidence for existence of exotic, multiquark hadrons, i.e. mesons which contain additional quarks, beyond the usual quark-antiquark pair and baryons which consist of more than three quarks. In all cases with robust evidence they contain at least one heavy quark Q=c or b, the majority including two heavy quarks. Two key theoretical questions have been triggered by these discoveries: (a) how are quarks organized inside these multiquark states -- as compact objects with all quarks within one confinement volume, interacting via …
The relativistic six-quark amplitudes of the nonstrange baryonia with the open charm are calculated. The poles of these amplitudes determine the masses of baryonia. 9 masses of baryonia are predicted.
In this paper the masses and decay processes of several new strange, charmed and bottomed exotic pentaquarks are predicted. Multiquarks are studied microscopically in a standard quark model. In pure ground-state pentaquarks the short-range interaction is computed and it is shown to be repulsive. The long-range and medium-range interactions are not expected to provide sufficient attraction. An additional quark-antiquark pair is then considered, and this is suggested to produce a narrow linear molecular system. The quarks assemble in three hadronic clusters, and the central hadron provides stabi…
It is proved that the nonleptonic decay amplitudes of antitriplet charmed baryons to octet baryons do not depend on the representation of the octet baryons in the quark diagram scheme. Relations among various representations are derived, and correct decay amplitudes of the charmed baryons are given.
The physics of charm has become one of the best laboratories exposing the limitations of the naive constituent quark model and also giving hints into a more mature description of meson spectroscopy, beyond the simple quark--antiquark configurations. In this talk we review some recent studies of multiquark components in the charm sector and discuss in particular exotic and non-exotic four-quark systems.
We present a systematic investigation of the possible molecular states composed of a pair of doubly charmed baryons ($Ξ_{cc}Ξ_{cc}$) or one doubly charmed baryon and one doubly charmed antibaryon $(Ξ_{cc}\barΞ_{cc})$ within the framework of the one-boson-exchange-potential model. For the spin-triplet systems, we take into account the mixing between the ${}^3S_1$ and ${}^3D_1$ channels. For the baryon-baryon system $Ξ_{cc}Ξ_{cc}$ with $(R,I) = (\bar{3}, 1/2)$ and $(\bar{3}, 0)$, where $R$ and $I$ represent the group representation and the isospin of the system, respectively, there exist loosely…
We evaluate exclusive semileptonic decays of ground-state spin-1/2 doubly heavy charmed baryons. The decays are driven by a $c\to s,d$ transition at the quark level. Our form factors are consistent with Heavy Quark Symmetry constraints. The latter are valid in the limit of infinitely heavy quark mass at zero recoil.
The measured masses of the three charge states of the charmed $Σ_c$ baryon are found to be in disagreement with a sum rule based on the quark model, but relying on no detailed assumptions about the form of the interaction. This poses a significant problem for the charmed baryon sector of the quark model. Other relations among charmed baryon masses are also discussed.
The high production rate and $\langle x_{F} \rangle > 0.33$ of the doubly charmed baryons measured by the SELEX experiment is not amenable to perturbative QCD analysis. In this paper we calculate the production of the doubly heavy baryons with the double intrinsic charm Fock states whose existence is rigorously predicted by QCD. The production rate and the longitudinal momentum distribution are both reproduced. We also show that the production rates of the doubly charmed baryons and double $J/ψ$ production observed by NA3 collaboration are comparable. Recent experimental results are reviewed. …
We use the method of QCD Sum Rules to estimate the masses of the charm baryon, $Λ^+_c$, bottom baryon $Λ^0_b$, strange baryon $Λ^0_s$ and compare them to their experimental values.
The masses of heavy quark baryons are studied in an expansion in 1/N_c, SU(3) flavor symmetry breaking, and heavy-quark symmetry breaking. Very accurate model-independent mass relations are obtained for charm and bottom baryons.
In this talk, we show our recent theoretical results for three-body systems in the charm sector which are made of three hadrons and contain one nucleon, one $D$ meson and in addition another meson, $\bar{D}$, $K$ or $\bar{K}$.
We describe work being done at Baylor University investigating the possibility of new states of mesonic matter containing two or more quark-antiquark pairs. To put things in context, we begin by describing the lattice approach to hadronic physics. We point out there is a need for a quark model which can give an overall view of the quark interaction landscape. A new application of the Thomas-Fermi (TF) statistical quark model is described, similar to a previous application to baryons. The main usefulness of this model will be to detect systematic energy trends in the composition of the various …
In the framework of quark-hadron duality, the cross section values of charmonium and doubly charmed baryon production in $e^+e^-$-annihilations have been estimated at interaction energy $\sqrt{s}=10.6$ GeV. It has been shown that the approach under discussion allows to resolve some contradictions between the experimental data and pQCD predictions.
Lorentz covariant wave functions for meson and baryon supermultiplets are simply derived by boosting $SU(2)_J$ representations corresponding to multiquark systems at rest.
The masses of negative parity $(70,1^-)$ charmed nonstrange baryons are calculated in the relativistic quark model. The relativistic three-quark equations of the $(70,1^-)$ charmed baryon multiplet are found in the framework of the dispersion relation technique. The approximate solutions of these equations using the method based on the extraction of leading singularities of the amplitude are obtained. The calculated mass values of the $(70,1^-)$ charmed baryons are in good agreement with the experimental data.
We propose that the charged-particle multiplicity dependence of the charm-baryon-to-meson ratio observed in high-energy $pp$ collisions can be explained by canonical treatment of quantum charges in the statistical hadronization model (SHM). Taking the full particle listings of PDG complemented by additional charm-baryon states from relativistic quark model predictions, we evaluate the canonical partition function and the charm-hadron chemical factors that measure the canonical suppression arising from the requirement of strict conservation of quantum charges. We demonstrate that, while charm n…
