Doctoral thesis : Cédric Lorcé


Exotic and non-exotic baryon properties on the light-cone 


We have used Chiral Quark-Soliton Model to study the structure and static properties of light baryons, i.e. those made of u, d and s quarks, in their fundamental state. This model has already demonstrated is efficiency by predicting numbers for physical quantities quite close to experimental values, and requiring only a few parameters. This is in fact the only model based on the idea of constituent quark, respecting approximate chiral symmetry of QCD and Lorentz invariance, and providing in some way an interpolation picture between Naive Quark Model (no meson
field) and Skyrme Model (no quarks).
The usual approach to this model is to consider that the (self-consistent) mean chiral field (soliton) binding quarks is slowly rotating. While this approximation is justified for ordinary baryons, i.e. those that can be described by three valence quarks only, it becomes questionable for exotic ones, in particular pentaquarks, i.e. those that need at least five quarks to be described. These exotic baryons seems to have, from a theoretical point of view, very unusual properties, e.g. a very small decay width. From the experimental point of view, the question is quite involved and rather fuzzy and no definitive conclusion can be inferred. That is why it was necessary to check if the theoretical prediction for such a small width was not an artefact due to theapproximation.
Two years ago, Chiral Quark-Soliton Model has been formulated on the light cone. This compeltely new approach allows one to study the
model within different approximations. For example, It was possible to consider exact rotations for the chiral soliton. In compensation, the contribution coming from the distortion of Dirac sea due to this chiral soliton can no more be treated exactly. This contribution is expanded in Fock space, i.e. we have considered wave functions with explicitly three, five and even seven quarks. Within this approach, we have studied static properties and structure of light baryons. For this purpose, we have systematically computed all vector, axial and tensor charges, and magnetic moments. These quantities provide us with valuable information concerning the nature and distribution of effective degrees of freedom within
baryons. They also account for the distribution of angular momentum on the different degrees of freedom and for the baryon shapes.

The light-cone approach consists of determining and computing the shape of baryon wave functions. Chiral Quark-Soltion Model provides a particularly elegant description since, beside respecting fundamental symmetries of QCD, it describes collectively all light baryons as collective rotational excitations of the very same object: chiral soltion. Once these wave functions are known, the different charges are obtained by sandwiching the corresponding current operator between baryon states. Furthermore, the model allows one to split charges into individual flavor, valence and non-valence contributions. Numerous result have therefore been obtained. Despite the invoked approximation of flavor SU(3) symmetry, many of these results are in excellent agreement with experimental data. Disagreements seem mainly due to the fact that flavor SU(3) symmetry is in fact broken in nature. The study of these charges being done systematically, lots of prediction have been obtained concerning properties and structure of light baryons which have not been measured hitherto.
Among the conclusions of these researches, we have confirmed that pseudoscalar mesons and relativity are essential ingredients to describe correctly static properties and structure of light baryons. We have also observed some explicit signature indicating the quadrupolar distortion of baryons due to pseudoscalar mesons. This signature is directly observable is some transitions, e.g. nucleon-to-Delta photoexcitation, i.e. quadrupolar electric
transition. Pentaquark width has been estimated to a few MeV, confirming thus previous estimations. Moreover, a qualitative explanation for such a small width is proposed and is based on the fact that on the light cone and in the Drell-Yan-West frame, i.e. current four-vector has not time component), a
pentaquark cannot decay in the dominant three-quark nucleon sector. This relation between the weight associated to the dominant three-quark nucleon sector and pentaquark width has been tested and confirmed in various approximations.
Finally, let us note that for baryon wave functions projected on Fock components to be computed, the spin-flavor part has been computed. Since in the model baryons are generated by the quantization of rotations of the mean chiral field, this spin-flavor structure is uniquely fixed by the soliton symmetry. For the very first time, a spin-flavor wave function for three, five and seven quarks has been obtained with Clebsh-Gordan coefficients fixed. The three quark spin-flavor wave function matches exactly to the well-known non-relativistic SU(6) one. Fock states with higher number of quarks do not respect SU(6) symmetry and are required for a relativistic description. We have observed that these higher Fock contributions improve further the agreement between theoretical prediction and experimental knowledge.

gzipped postscript (1.6 Mb)

pdf (983 kb)