Evidence for a Narrow S = +1 Baryon Resonance in Photoproduction from the Neutron

The γ n → K⁺ K⁻ n reaction on ¹²C has been studied by measuring both K⁺ and K⁻ at forward angles. A sharp baryon resonance peak was observed at 1.54 ± 0.01 GeV/c² with a width smaller than 25 MeV/c² and a Gaussian significance of 4.6σ. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic five-quark state (u u d d s-bar) that decays into a K⁺ and a neutron. The resonance is consistent with the lowest member of an antidecuplet of baryons predicted by the chiral soliton model.

Matter is made of quarks, as it is widely known. For instance, protons have two Up quarks and one Down quark and neutrons have two Down quarks and one Up quark. So far only baryons made of three quarks (q q q) and mesons (such as pions) made of one quark and one anti-quark (q q-bar) have been found as bound-quark states. However, other configurations like 5-quark (q q q q q-bar) states are not forbidden by quantum chromodynamics (QCD) which is the theory of how quarks interact each other.

Figure: The n(γ,K⁻)X missing mass spectrum for the events
satisfied all the selection criteria (solid histogram) and
the estimated background spectra (dotted histogram).

For the better understanding of how quarks form matter, searching for baryon resonances with the strangeness quantum number S = +1 is especially intersting because the states must contain an anti-strange quark and cannot be formed by three quarks. In fact, the experimental search for S = +1 baryon resonances has a long and interesting history but past searches have come to no definite conclusion.
At the Laser Electron Photon facility at SPring-8 (LEPS) , we searched the S = +1 resonances (Θ⁺) in the reaction of

γ + n → K⁻ + Θ⁺ → K⁻ + K⁺ + n

in the mass region around 1.5 GeV/c² where the past experimental searches have not been carried out.
In the experiment, a γ-ray beam with energies of 1.5 - 2.4 GeV was used and a plastic scintillator which contains ¹²C was used as a target. K⁻ mesons generated at the target were detected with the LEPS spectrometer. The "missing" mass for the K⁻ measurement can be calculated and it gives the mass of Θ⁺ resonance when the above reaction occurred. Detecting K⁺ mesons at the same time, we removed the "background" events that K⁻ are produced via the different reactions. Most of the K⁻ K⁺ pairs were produced via the photoproduction of the φ meson and they were excluded by using the K⁻ K⁺ invariant mass. In order to eliminate reactions on protons in ¹²C and ¹H at the target, the recoiled protons were also detected. For the events satisfied all these selection criteria, we obtained the missing mass spectrum as shown in the figure. A sharp baryon resonance peak has been found at 1.54 ± 0.01 GeV/c² above the background. The Gaussian significance of the peak is 4.6 standard deviations, or it is unlikely that the peak is just a statistical fluctuation. The width of the resonance is estimated to be smaller than 25 MeV/c². It strongly indicates the existence of an S = +1 resonance which may be attributed to the molecular meson-baryon resonance or alternatively as an exotic five-quark baryon. The mass and width are close to the prediction of a five-quark Θ⁺ resonance by the chiral soliton model by Diakonov, Petrov and Polyakov [Z. Phys. A 359, 305 (1997)].

Scientific Paper:
Physical Review Letters, 91, 012002 (2003), 4 July 2003
hep-ex/0301020 at e-Print Archive

Evidence for the S = +1 resonance reported from other laboratories:

DIANA collaboration at ITEP (hep-ex/0304040) K⁺ Xe → K⁰ p Xe'
CLAS collaboration at Jefferson Lab. (hep-ex/0307018) γ d → K⁺ K⁻ p n
SAPHIR collaboration at ELSA, Bonn (hep-ex/0307083) γ p → n K⁰ K⁺
CLAS collaboration at Jefferson Lab. (hep-ex/0307088) γ p → π⁺ K⁻ K⁺ n

Citation Search for the PRL, 91, 012002 (2003) paper

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Last modified: Fri Aug 1 16:19:08 JST 2003