| Summary: | Since 2003, a new family of states without a clear theoretical interpretation has been measured in the heavy quarkonium spectrum, the so-called X Y Z states. While the nature of these states is so far unclear, the experimental search is one of the most active fields in the intensity frontier. In this review, the most important and representative results obtained in this field in recent years are going to be illustrated, providing insights into the nature of these exotic states. The focus will be on the experimental side, describing how it is possible to investigate their exotic nature with respect to the conventional quarkonium states.
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