Combining the recent lattice calculation of $a_0(1450)$ and $\sigma(600)$ mesons with the overlap fermion in the chiral regime with the pion mass less than $300 {\rm MeV}$, the quenched lattice calculation of the scalar glueball, and the phenomenological s

2K.F.Liu

Theradiativedecayφ→a0(980)γ,whichcannotproceedifa0(980)isaqq¯state,canbenicelydescribedinthekaonloopmechanism.4)ThissuggestsaconsiderableadmixtureoftheK

Kmolecularstates.7)Weshallrefer

themgenericallyastetraquarkmesoniums,nottobeconcernedwiththeirpossibleclusteringstructure.Withthefourquarkcontent,itisrelativelyeasytounderstandthedegeneracyofa0(980)andf0(980)andtheirnarrowwidths.Sincetheyhavethe¯)ss¯threshold,theydonothavemuchphasequarkcontentu(d)¯u(d¯andsitatKK¯(a0(980)decaytoηπissuppressedbyhavingtogothroughspacetodecaytoKK

thess¯inη);whereas,σ(600)andκ(800)arerelativelyfarabovetherespectiveππandπKthresholdsandhencehavemuchlargerwidths.

Recentexperimental ndingofσ(600)inD+→π+π π+2)andJ/Ψ→ωπ+π 2)andthedispersionanalysisofππscatteringwiththeRoyequation8)whichfound+18aresonanceat441+16 8MeVwithawidthof544 25MeVhavehelpedestablishtheexistenceofthebroadσresonance.Besidesthelow-lyingscalarmesons,othercandidatesfortetraquarkmesoniumsincludethosevectormesonspairsproducedinγγreactions9)andhadronicproductions10)andtherecentlydiscoveredcharmednarrowresonances.11)

Giventhatthespectrumbelow1GeVisbetterunderstood,manyquestionsaboutclassi cationofscalarmesonsabove1GeVarestilloutstanding.Forexample: (1430),whichisaqq TheK0¯stateinallthemodels,1)lieshigherthantheaxial-