The Rise and Fall of Pentaquarks in Experiments

Experimental evidence for and against the existence of pentaquarks has accumulated rapidly in the last three years. If they exist, they would be dramatic examples of hadronic states beyond our well-tested and successful particle models. The positive evidence suggests existence of baryonic objects with widths of at most a few MeV, some displaying exotic quantum numbers, such as baryons with strangeness $S = +1$. The non-observations of these states have often come from reaction channels very different from the positive evidence channels, making comparisons difficult. The situation has now been largely clarified, however, by high-statistics repetitions of the positive sightings, with the result that none of the positive sightings have been convincingly reproduced. The most recent unconfirmed positive sightings suffer again from low statistics and large backgrounds. It seems that a kind of ``bandwagon'' effect led to the overly-optimistic interpretation of numerous experiments in the earlier reports of exotic pentaquarks.Comment: Presented at Particles and Nuclei International Conference (PANIC'05), Santa Fe, N.M. October 28, 2005; 9 pages, 1 figur

Strangeness Production Experiments at Jefferson Lab

Experimental results for photo- and electro-production of open strangeness from the Thomas Jefferson National Accelerator Facility are discussed. The results are from work completed by mid-2003 on elementary $KY$ production, nuclear targets, and the exotic $\Theta^+$ state. It is shown how the increases in intensity and precision of JLab experiments over earlier work have allowed new phenomena to become measurable.Comment: 14 pages, 10 figures, For proceedings of SENDAI03: Electrophoto-production of Strangeness on Nucleons and Nucle

Observation of γp→ΛΛˉp\gamma p \to \Lambda \bar{\Lambda} p with GlueX at Jefferson Lab

For the first time, baryon-antibaryon photoproduction in the reaction $\gamma p \to \Lambda \bar{\Lambda} p$ has been observed at photon energies from threshold near 4.9 GeV to 11.6 GeV. The measurements are in progress with the GlueX spectrometer in Hall D at Jefferson Lab. We describe here the apparatus and methods used to make these measurements and outline the physics goals of the work. Some of the newly-seen reaction phenomenology is presented.Comment: From an invited talk to the "13th International Conference on Hypernuclear and Strange Particle Physics (HYP2018), 8 pages, 8 figures; replaced to update Acknowledgement

Properties of the Lambda(1405) Measured at CLAS

The nature of the Lambda(1405), and its place in the baryon spectrum has remained uncertain for decades. Theoretical studies have shown that it may possess strong dynamical components which are not seen in other well-known baryons. Using the CLAS detector system in Hall B at Jefferson Lab, we have measured the photoproduction reaction gamma+p->K+ Lambda(1405) with high statistics and over different Sigma pi decay channels. The reconstructed invariant mass distribution (lineshape) has been measured, as well as the differential cross sections for the Lambda(1405), Sigma(1385), and Lambda(1520). Our analysis method is discussed and our near-final results for the Lambda(1405) lineshape and differential cross section are presented.Comment: Proceedings for The 8th International Workshop on the Physics of Excited Nucleons (NSTAR11), May 17-20, 2011 held at Thomas Jefferson National Accelerator Facility Newport News, Virginia US

Measurement of Excited Hyperons in Photoproduction at CLAS

Measurement results of photoproduced excited hyperon states using the CLAS detector at Jefferson Lab are shown. The invariant mass distribution of the Lambda(1405) has recently been shown to be different for each of the three Sigma pi channels that it decays to, showing that there is prominent interference between the isospin I=0 and I=1 isospin amplitudes. Measurements of the differential and total cross sections of the three hyperons Lambda(1405), Sigma0(1385), and Lambda(1520) are presented and compared. Prospects of future studies using a 12 GeV beam with the GlueX detector are briefly given.Comment: 6 pages, 4 figures, proceedings for THE 9th International workshop on the Physics of Excited Nucleons, "NSTAR2013" 9th International workshop on the Physics of Excited Nucleon

Electroproduction of Λ\Lambda(1405)

The electroproduction of $K^+ \Lambda$ (1405) was studied by analyzing the E1F data set collected in Hall B at Jefferson Lab. The analysis utilized the decay channel $\Sigma^+ \pi^-$ of the $\Lambda$ (1405) and $p \pi^0$ of the $\Sigma^+$. Simulations of background, $\Lambda$ (1405) and $\Lambda$ (1520) production according to PDG values were performed by using standard CLAS analysis tools adapted for the E1F run. Fits of the acceptance-corrected simulations were made to the acceptance-corrected data to determine contributions from signal and background processes. The line shape of $\Lambda$ (1405) varies with the four momentum transfer, $Q^2$, and does not match the line shape based on PDG resonance parameters. It corresponds approximately to predictions of a recent two-pole meson-baryon picture of this state.Comment: 4 pages, 6 figures, Contribution to the Proceedings of "NSTAR2011 - The 8th International Workshop on the Physics of Excited Nucleons," Thomas Jefferson National Accelerator Facility, Newport News, Virginia USA, 17-20 May 201