Chapter 1 Introduction 1.1 Motivation for Research 1 Nearly all hadrons that have been observed to date can be classified according to the quark model. The quark model was introduced in Refs. [51, 131] to bring some order to the already large number of hadrons that were known at the time. The model introduces two families of hadrons: baryons such as the neutron and proton that are fermions, and mesons such as the pion that are bosons. Both baryons and mesons are composite particles composed of fundamental particles, called quarks. Baryons are composed of three quarks and In 2003, the X (3872) was discovered by the Belle collaboration [36]. The discovery was sub- sequently confirmed by the Babar [13], CDF [8], D0 [4] and LHCb collaborations [3]. The decays of this particle that have been observed to date clearly indicate that it is a hadron, that is, a composite particle composed of quarks that experiences the strong nuclear force. This particle was found within the mass region occupied by a well understood family of hadrons known as charmonia. However, the properties of the X (3872) make it difficult to interpret it as a member of the charmonium spectrum [114]. Since 2003, more hadrons have been discovered in the charmonium mass region that are difficult to interpret as charmonium states. A few anomalous hadrons have also been found within the closely related bottomo- nium spectrum. These anomalous particles are called heavy quarkonium-like, or XYZ states. Table 1.1 includes basic information for experiments that have discovered XYZ states and Table 1.2 lists the XYZ states that have been confirmed by more than one experiment at a high level of statistical significance. Ref. [20] provides a more complete list that includes particles that have only been observed by a single experiment and particles that have been observed at a lower level of statistical significance.