Chiral Constituent Quark Model : Low Energy Hadron Matrix Elements

Quantum Chromodynamics (QCD) views hadrons as bound states of quarks and gluons. However, hadron structure studies involve non-perturbative aspects of QCD- confinement and chiral symmetry breaking which makes it a challenging task to understand. Considerable progress has been achieved through Non- Relativistic Quark Model which is based on very simple assumptions and gives a remarkable fit to many of the hadron spectroscopy data. However, starting with the observations in 1988 by the European Muon Collaboration, in the last decade or so extremely important information pertaining to spin and flavor structure of the proton have been discovered in the deep inelastic scattering experiments. The present experimental information is in contradiction with the predictions of Non- Relativistic Quark Model which referred to as "proton spin problem". The chiral constituent quark model probes the regime between the confinement scale and the chiral symmetry breaking scale. Several current experimental initiatives in this direction indicate the importance of the problem for high energy physics community.