Have QuasinuclearNN¯Bound States Been Discovered?

Abstract

It is suggested that a broad structure ^°(1480) recently observed in the reaction pn -* TT~X° -> 3TT~2TT + may be interpretable in terms of an unstable quasinuclear bound state of the (complex) nucleon-antinucleon (NN) potential. The observed mass, production branching ratio, and preference for the X°-* p°p° rather than TT + TT' decay mode are consistent with a quantum-number assignment /* c (/ c )»2 + + (0 + ). Prospects for finding the lower-lying 0 + + (0 + ) and 1'"(()-) members of the expected natural-parity quasinuclear band are discussed.PACS numbers: 14.40. Cs, 12.40.Qq, 13.25.+m, 13.75.Cs It has long been recognized that the AW system offers in principle an ideal entrance channel for the production of new types of jnesons, in addition to the usual quark-antiquark (QQ) configurations. These new mesons could include, Jor instance, quasinuclear (QN) bound .states of the AW potential, 1 "barycmium" states of Q 2 Q 2 structure, 2 "hybrids," i.e., QQg mesons 3 containing a dynamical gluon g, or glueballs.^ The search for new narrow mesons coupled to the AW system has been intense, but earlier evidence for the existence of such objects has not been confirmed in recent experiments 5 with intense p beams at LEAR (Low Energy Antiproton Ring at CERN).Unless there is a topological selection rule 6 suppressing direct decay of "baryonium" into mesons, on£ generally expects broad states. For instance, for AW quasinuclear mesons, the typical scale of width 1 is r =as 100 MeV, although narrower structures may occur in particular cases. 7 ' 8 New mesons, if they are highly unstable, are generally difficult to extract in a convincing way from AW annihilation data. Even if one picks a particular pion multiplicity n, there will often be numerous quasi two-body (QTB) reactions AW -• M X M 2 , leading directly to ordinary s= (77,17, p,a>} and p sss [€,8,A\,A2,D,f,B,H} mesons M x and M 2 . This "background" of QTB modes, interesting in its own right because of the connection between the underlying quark-gluon dynamics and observed selection rules, 9 masks the production of new mesons X in reactions such as NN-+ TTXOT yX.To minimize the QTB^and other backgrounds, one can choose the initial AW state and the particular final pion charge states in an optimum way. For instance, the pn or Tip initial states are more favorable in this regard than pp, since we thereby restrict attention to entrance channel isospin /=1, and thus eliminate / = 0 QTB modes. If we then choose n even or odd, i.e., G parity -(-)", we select 2/+1 ' 25+1 £ J = 33 S 1 or 31 S 0 initial AW states, respectively. We here assume that annihilation takes place from L = 0 states. Finally, we focus on annihilation modes containing the minimum number of 7r 0, s, since these contain the fewest QTB configurations. By explicit enumeration of QTB modes for each «, we have found that the reaction pn -• 3IT "~ 27r + is optimal for the study of new mesons X° via the process pn-+7T~X°. Note that X° can have isospin 0, 1, or 2. On the other hand, pp-> TT ± X T can only be u...

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