Unconventional Baryonic and Mesonic States

The search for and investigation of unconventional baryonic and mesonic states is a topical issue in current hadron physics, for example [1–5] and references therein. This is much focused on the sector of c and b quarks, but there are also well known issues in the light quark sector [6], and there are similarities between c and s sectors. The interesting parallels between c and s sectors include the tetra-quark X(3872), a possible DD* configuration, and pentaquark Pc(4380/4450) candidates [7], possibly (D*Σc) or (D*Λ*c) configurations. The mesonic and baryonic states could be considered analogues of each other when a color antitriplet in the meson (q) is replaced by another one in the baryon (qq). This appears expandable upon the strange-sector: in the meson sector with the f1(1285) as a bound state of K and K* close to threshold, in the baryon sector the N*(2030) and N*(2080), two states earlier listed in the PDG and suggested in [8] to be associated with the cusp-like structure observed in K⁰SΣ⁺ photoproduction at the K* threshold [9]. This situation is compiled in Table 1.

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Tab. 1: Overview of possible parallels between meson and baryon states in (hidden) c and s quark sectors.

The interpretation of Ref. [8] leads to a remarkable prediction. The destructive interference of amplitudes driven by intermediate K*Λ and K*Σ channels is magnified by the presence of the N*(2030) resonance, and thereby causes the cusp-like structure in the γp →K⁰SΣ⁺ cross section around 2 GeV. Contrary, for the reaction γn →K⁰Σ⁰, this interference is mitigated by the dominance of the amplitude that drives the intermediate K*Σ channel, and gives rise to a resonance peak in the spectrum. This would be a smoking gun signal for these meson-baryon states to exist.

Recently the idea was brought up in addition that, as a dominant K*⁻Σ structure, this same N*(2030) state, could drive the photoproduction of K⁺Λ(1405) via a triangle singularity [12], such that not only the Λ(1405) in itself represents a possible object of interest in the given context, but also the mechanism of photoproduction to shed more light on the molecular type interactions in the vicinity of the K*Y thresholds.
Generally, meson-baryon interactions could be expected to show up not only at the K* threshold, but at every threshold when the relative momenta are sufficiently small. Fig. 1 shows photoproduction thresholds and indicates peculiarities which are experimentally observed and might be associated with meson-baryon interaction effects. It also illustrates that the region of the K* threshold is of particular interest, due to the numerous close-by thresholds within an energy interval of only 12 MeV.

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Fig. 1: Overview of production/decay thresholds.

Our results hitherto obtained indeed provide support for such unconventional structures to exist. Highlights include:

  • Measurements of K⁺Λg.s. photoproduction at very forward K⁺angles. The results extend the kinematic coverage of earlier experiments and are consistent with partial wave analyses based on those results. The significant normalisation discrepancy among previous measurements is resolved.
  • Λ(1405) line shape in the pure isospin I = 0 π⁰Σ⁰ decay and its mechanism of photoproduction. In both we find general agreement with previous measurements. New results include the direct indication of a possible double peak structure in the pure I = 0 line shape and, based on the extended kinematic coverage, the involvement of a triangle singularity in the production process which in itself is driven by the above mentioned N*(2030) possibly pentaquark type state.
  • A cusp-like structure is revealed in K⁺Σ⁰g.s. photoproduction off the proton. It is shown that the effect becomes more pronounced as the K+ goes extremely forward, i.e. the momentum transfer to the hadronic system remains minimal. A preliminary partial wave analysis of the Bonn-Gatchina PWA group indicates this might be associated with a previously unobserved narrow JP = (5/2)⁻ isospin 3/2 structure of under 60 MeV width. The cusp in forward direction is consistent in transverse momentum with the kinematics of the Sphinx experiment [13], where indication of an X(2000) state was reported. This was hypothesised a pentaquark candidate but hitherto remained unseen in photoproduction experiments.
  • For the first time the smoking gun experiment in K⁰SΣ⁰ photoproduction off the neutron has been successfully conducted. Within the still limited statistics a peak structure is indeed indicated at the K* threshold, consistent with the prediction of Ref. [8] of a structure similar to the pentaquarks observed at LHCb. Firm conclusions require improved statistics, however.
[1] R. Aaij et al. (LHCb Collaboration), Phys. Rev. Lett. 122 (2019) 222001, e-print: 1904.03947 [hep-ex]
[2] R. Aaij et al. (LHCb Collaboration), Phys. Rev. Lett. 115 (2015) 072001, e-print: 1507.03414 [hep-ex]
[3] H.-X. Chen et al., Phys. Rept. 639 (2016) 1
[4] S.-K. Choi et al. (Belle Collaboration), Phys. Rev. Lett. 91 (2003) 262001, e-print: hep-ex/0309032 [hep-ex]
[5] Y.-R. Liu et al., Prog. Part. Nucl. Phys. 107 (2019) 237, e-print: 1903.11976 [hep-ex]
[6] L.Ya. Glozman and D.O. Riska, Physics Reports 268 (1996) 263
[7] T.J. Burns, Eur. Phys. J. A 51 (2015) 152
[8] A. Ramos and E. Oset, Phys. Lett. B 727 (2013) 287
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