Wonderful World of Pentaquarks: Technical Information
From Decuplets to Anti-Decuplets and Quarks to Pentaquarks
Figure 3: Invariant mass of the nK+ system, which has strangeness S=+1, showing a peak at the mass of about 1.54 GeV/c2 a) Data from the exclusive reaction on a deuterium target from Jefferson Lab, and b) data from the inclusive reaction on a carbon target from SPring-8.
The first experimental observation of the Θ+ state came from the LEPS collaboration working at the SPring-8 facility in Japan [10]. This experiment used a high-energy photon beam incident on a carbon target producing the pentaquark state in association with a K- particle. The Θ+ decays almost immediately into a neutron and a K+ as indicated schematically in Figure 1. (This state can also decay via Θ → pK0. The pentaquark is identified as a peak in the invariant mass spectrum of its decay products as shown in Figure 3b. The strangeness of the Θ+ is determined using the charge of the kaons in the reaction.
Confirmation of this result came quickly from other laboratories around the world. To date there have been seven experimental observations of a narrow exotic S=+1 baryon state at a mass of approximately 1.54 GeV [10, 11,13, 14,15, 16,17]. The measured masses of the states are shown in Figure 4. The exotic state has been observed in photon, neutrino, and proton reactions with both nuclear and proton targets. In the reactions that contain neutral kaons, these can be identified using the Ks → Π +Π- decay, but these do not uniquely determine the strangeness. Both of the Θ+ decay modes have been observed, although not in the same experiment, as indicated in Figure 4. The existence of this state is given considerable support by its observation with different probes and under very different experimental conditions.
Figure 4: The mass of the Θ- is given for each of the experimental observations. The world average +/-1 standard deviation is shown as a green band [16]. On the right we give the reaction which was used for the measurement with the Θ- decay mode given in parenthesis.
Photoproduction measurements of the Θ+ on both deuteron and proton targets with the CLAS detector at Jefferson Lab [12] has provided important confirmation for this exotic baryon state and shed light on the production mechanisms which may be responsible for its production. The measurement with a deuteron target produced the Θ+ in the reaction γd → nK+K-p, detecting all three charged particles, and reconstructed the neutron using the missing-mass technique. The complete reconstruction of the final state provided a way of reconstructing the Θ+ using just momentum and energy conservation. The mass spectra for a S=+1 baryon final state is shown in Figure 4a along with the data from SPring-8.
The anti-decuplet family also contains two more exotic states, denoted by Ξ5-- and Ξ5+, which have S = -2 and charge Q = -2 and Q = +1, respectively. The subscript "5" indicates the five-quark (pentaquark) nature of the states. The NA49 collaboration at CERN has reported the only observation of the Ξ5-- at a mass of 1.86 GeV [18]. Clearly, confirmation of this new particle is highly desired. The third exotic state, Ξ5+, not yet observed, must be found at a mass close to that of the Ξ5-- in order to tie down the three corners of the anti-decuplet triangle.