▁first ▁such ▁particles ▁discovered : ▁X ( 3 8 7 2 ), ▁Y ( 4 2 6 0 ) ▁and ▁Z c ( 3 9 0 0 ), ▁although ▁the ▁symbols ▁X ▁and ▁Y ▁have ▁since ▁been ▁deprecated ▁by ▁the ▁Part icle ▁Data ▁Group . ▁ ▁The oret ical ▁significance ▁Since ▁ 2 0 0 3 ▁a ▁front ier ▁for ▁the ▁Standard ▁Model ▁( SM ) ▁has ▁emer ged ▁at ▁low ▁energ ies ▁through ▁X Y Z ▁particle ▁discover ies . ▁The ▁well - est ab lished ▁theory ▁of ▁Quant um ▁Ch rom od ynam ics ▁( Q CD ) ▁is ▁tested ▁by ▁many ▁ex otic ▁char mon ium ▁discover ies ▁since ▁the ▁X ( 3 8 7 2 ) ▁was ▁first ▁identified ▁at ▁the ▁Belle ▁experiment ▁in ▁ 2 0 0 3 . ▁The ▁basic ▁model ▁of ▁had ron ▁physics ▁is ▁the ▁assemb ling ▁of ▁qu arks ▁into ▁groups ▁of ▁ 3 ▁( b ary ons ) ▁or ▁a ▁qu ark ▁and ▁anti - qu ark ▁pair ▁( mes ons ). ▁A ▁mes on ▁with ▁a ▁charm ▁qu ark ▁and ▁an ▁anti - ch arm ▁qu ark ▁is ▁called ▁char mon ium , ▁and ▁the ▁same ▁par alle ls ▁with ▁the ▁bottom ▁qu ark ▁and ▁bottom on ium . ▁More ▁than ▁two ▁dozen ▁previously ▁un pred icted ▁char mon ium - ▁and ▁bottom on ium - like ▁states ▁have ▁been ▁discovered , ▁and ▁the ▁understanding ▁of ▁heavy ▁qu ark on ium ▁physics ▁is ▁und et erm ined . ▁Pre viously ▁post ulated