Observation of a narrow baryon resonance decaying to pK S 0 in proton-nucleus interactions at 70 GeV/c with the SVD-2 setup

Abstract

SVD-2 experiment data have been analyzed to search for an exotic baryon state, the Θ + -baryon, in a pK 0 s decay mode at 70 GeV /c on IHEP accelerator. The reaction pA → pK 0 s + X with a limited multiplicity was used in the analysis. The pK 0 s invariant mass spectrum shows a resonant structure with M = 1526 ± 3(stat.) ± 3(syst.) M eV /c 2 and Γ < 24 M eV /c 2 . The statistical significance of this peak was estimated to be of 5.6 σ. The mass and width of the resonance is compatible with the recently reported Θ + -baryon with positive strangeness which was predicted as an exotic pentaquark (uudds) baryon state. The total cross section for Θ + production in pN-interactions for XF ≥ 0 was estimated to be (30 ÷ 120) µb and no essential deviation from A-dependence for inelastic events (∼ A 0.7 ) was found.

Vector transition form factors of the and in the chiral quark–soliton model

“…Since Diakonov, Petrov and Polyakov [1] predicted the mass and width of the exotic pentaquark baryon Θ + (with leading Fock-component uudds), the pentaquark baryons have attracted much attention (see also an earlier estimate of the mass by Prasza lowicz in the soliton approach of the Skyrme model [2]). After the LEPS collaboration has reported the evidence of the Θ + first [3], many experiments have announced its existence [4,5,6,7,8,9,10,11], while the Θ + has not been seen in almost all high-energy experiments [12,13,14,15,16,17,18,19,20] (see Ref. [21] for a review of the experimental status until 2005).…”

Present status on experimental search for pentaquarks

“…Since the Θ + has the same quark constituents as the combination of nK + and pK 0 , these two modes are expected as the primary decay channel, and thus are usually used in reconstructions in the experiments. After the first report for the Θ + , the signals were observed by many groups [11,[48][49][50][51][52][53][54][55][56][57][58], and some groups confirmed their results with new data [59][60][61][62][63]. All these results are briefly listed in Table I.…”

“…This does not consist with a narrow resonance. The mass values in LEPS early experiment [11], DIANA experiments [48,[60][61][62], SAPHIR experiment [51] and JINR experiments [55,56] are around 1540 MeV/c 2 , while the values in HERMES experiment [50], SVD early experiment [49], ITEP's analysis on the old neutrino experiments [52,53] and E522 experiment [57] are near 1530 MeV/c 2 . Besides, the LEPS recent experiment [59], the SVD updated experiment [63] and the ZEUS experiment [54] provide even lower mass values close to 1520 MeV/c 2 , and the OBELIX experiment gives a much higher value.…”

“…A resonance of pK 0 S invariant mass spectrum in the reaction pN → pK 0 S X was observed in the data from the SVD-2 experiment [49]. In this experiment, the proton beam with the momentum of 70 GeV/c from the ITEP accelerator was injected into an active target which consisted of five microstrip silicon detectors of 300 µm, a lead foil of 220 µm and a carbon target of 500 µm.…”