Abstract
Ultra-peripheral heavy ion collisions constitute an ideal setup to look for exotic hadrons because of their low event multiplicity and the possibility of an efficient background rejection. We propose to look for four-quark states produced by photon-photon fusion in these collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.5~{\rm TeV}$. In particular, we focus on those states that would represent a definite smoking gun for the compact tetraquark model. We show that the $X(6900)$, a likely $cc\bar{c}\bar{c}$ compact state, is a perfect candidate for this search, and estimate a production cross section ranging from around $250$ nb to $1150$ nb, depending on its quantum numbers. Furthermore, we discuss the importance of ultra-peripheral collisions to the search for the scalar and tensor partners of the $X(3872)$ predicted by the diquarkonium model, and not yet observed. The completion of such a flavor-spin multiplet would strongly speak in favor of the compact tetraquark model.
Highlights
The existence of hadrons with more than three valence constituents is well assessed [1,2,3,4], but the understanding of their nature remains a long standing problem of low-energy QCD
We propose to look for fourqpuaiarrkpsffisffitffiNaffiffiNtffieffi s1⁄4pr5o.5duTceeVd .bIynpphaorttoicnu-lpahr,owtoen fusion in focus on these those collisions states that at the center-of-mass energy per nucleon would represent a definite smoking gun for the compact tetraquark model
We discuss the importance of ultraperipheral collisions to the search for the scalar and tensor partners of the Xð3872Þ predicted by the diquarkonium model, and not yet observed
Summary
The existence of hadrons with more than three valence constituents is well assessed [1,2,3,4], but the understanding of their nature remains a long standing problem of low-energy QCD Are these states extended hadronic molecules arising from color neutral interactions? Xð3872Þ in prompt pp collisions [41,42] The observation of these states in UPCs would be another indication of the existence of compact tetraquarks in the spectrum of short distance QCD, alongside a recently emerging pattern which includes the observation of the hidden charm and strange states [43,44,45] and the study of the lineshape of the. Xð3872Þ [46,47]
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