Jean-Marc Richard

Issues and opportunities in exotic hadrons

The last few years have been witness to a proliferation of new results concerning heavy exotic hadrons. Experimentally, many new signals have been discovered that could be pointing towards the existence of tetraquarks, pentaquarks, and other exotic configurations of quarks and gluons. Theoretically, advances in lattice field theory techniques place us at the cusp of understanding complex coupled-channel phenomena, modelling grows more sophisticated, and effective field theories are being applied to an ever greater range of situations. It is thus an opportune time to evaluate the status of the field. In the following, a series of high priority experimental and theoretical issues concerning heavy exotic hadrons is presented.

Exotic Hadrons: Review and Perspectives

The physics of exotic hadrons is revisited and reviewed, with emphasis on flavour configurations which have not yet been investigated. The constituent quark model of multiquark states is discussed in some detail, as it can serve as a guide for more elaborate approaches.

Isovector and hidden-beauty partners of the X(3872)

Article history: The isovector partners of the X(3872), recently found at BES III, Belle and CLEO-c were predicted in a simple model based on the chromomagnetic interaction among quarks. The extension to the hidden- beauty sector is discussed.

Relation between Tcc,bb and Xc,b from QCD

Abstract We have studied, using double ratio of QCD (spectral) sum rules, the ratio between the masses of T c c and X ( 3872 ) assuming that they are respectively described by the D − D ⁎ and D − D ¯ ⁎ molecular currents. We found (within our approximation) that the masses of these two states are almost degenerate. Since the pion exchange interaction between these mesons is exactly the same, we conclude that if the observed X ( 3872 ) meson is a D D ¯ ⁎ + c . c . molecule, then the D D ⁎ molecule should also exist with approximately the same mass. An extension of the analysis to the b -quark case leads to the same conclusion. We also study the SU(3) breakings for the T Q Q s / T Q Q mass ratios. Motivated by the recent Belle observation of two Z b states, we revise our determination of X b by combining results from exponential and FESR sum rules.

Elusive multiquark spectroscopy

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.

Stable heavy pentaquarks in constituent models

Abstract It is shown that standard constituent quark models produce ( c ¯ c q q q ) hidden-charm pentaquarks, where c denotes the charmed quark and q a light quark, which lie below the lowest threshold for spontaneous dissociation and thus are stable in the limit where the internal c ¯ c annihilation is neglected. The binding is a cooperative effect of the chromoelectric and chromomagnetic components of the interaction, and it disappears in the static limit with a pure chromoelectric potential. Their 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 hadrons. These pentaquark states could be searched for in the experiments having discovered or confirmed the hidden-charm meson and baryon resonances.

Non-Abelian dynamics and heavy multiquarks, Steiner-tree confinement in hadron spectroscopy

A brief review is first presented of attempts to predict stable multiquark states within current models of hadron spectroscopy. Then a model combining flip–flop and connected Steiner trees is introduced and shown to lead to stable multiquarks, in particular for some configurations involving several heavy quarks and bearing exotic quantum numbers.

Level rearrangement in exotic-atom-like three-body systems

Abstract We study systems of three bosons bound by a long-range interaction supplemented by a short-range potential of variable strength. This generalizes the usual two-body exotic atoms where the Coulomb interaction is modified by nuclear forces at short distances. The energy shift due to the short-range part of the interaction combines two-body terms similar to the ones entering the Trueman–Deser formula, and three-body contributions. A sudden variation of the energy levels is observed near the coupling thresholds of the short-range potential. But the patterns of rearrangement are significantly modified as compared to the two-body case.

Possibility of a new neutral hypernucleus 4ΛΛn = (n,n, Λ, Λ)

The status of the light hypernuclei is reviewed, and discussed with models based either on the Nijmegen-RIKEN baryon-baryon interaction or on recent studies using chiral effective field theory. The latter suggests a significantly shorter range for the ΛΛ interaction, and this favours the formation of a Borromean state made of two neutrons and two hyperons. Various corrections are discussed, in particular the coupling of NΛ to NΣ, or of ΛΛ to NΞ, and the role of tensor forces. The new nucleus 4ΛΛn = (n,n, Λ, Λ) could be produced in various reactions, in particular deuteron-deuteron scattering with the simultaneous production of two charged kaons, for which an estimate of the cross section is provided.

BARYON SPECTROSCOPY AND HEAVY QUARKS

Two topics are briefly reviewed in this talk: the decay of flavored hadrons or quarkonium states involving a baryon--antibaryon pair, and the spectroscopy of heavy baryons containing one, two or three heavy quarks. Some prospects for exotic heavy baryons are also discussed.