R. A. Schumacher

Observation of an Exotic S = +1 Baryon in Exclusive Photoproduction from the Deuteron

In an exclusive measurement of the reaction gammad-->K(+)K(-)pn, a narrow peak that can be attributed to an exotic baryon with strangeness S=+1 is seen in the K(+)n invariant mass spectrum. The peak is at 1.542+/-0.005 GeV/c(2) with a measured width of 0.021 GeV/c(2) FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is (5.2+/-0.6)sigma. The mass and width of the observed peak are consistent with recent reports of a narrow S=+1 baryon by other experimental groups.

The CLAS drift chamber system

Abstract Experimental Hall B at Jefferson Laboratory houses the CEBAF Large Acceptance Spectrometer, the magnetic field of which is produced by a superconducting toroid. The six coils of this toroid divide the detector azimuthally into six sectors, each of which contains three large multi-layer drift chambers for tracking charged particles produced from a fixed target on the toroidal axis. Within the 18 drift chambers are a total of 35,148 individually instrumented hexagonal drift cells. The novel geometry of these chambers provides for good tracking resolution and efficiency, along with large acceptance. The design and construction challenges posed by these large-scale detectors are described, and detailed results are presented from in-beam measurements.

Possible violation of the Delta I = 1/2 rule in non-mesonic hypernuclear weak decay

The weak decays of strange mesons and free hyperons strongly favor {Delta}I=1/2 amplitudes over {Delta}I=3/2 amplitudes. It is now known to what extent this rule applies to the non-mesonic interactions of the type {Lambda}p {yields} np and {Lambda}n {yields} nn. This paper examines existing data on non-mesonic decays of light hypernuclei in order to obtain a quantitative estimate of the relative strength of the two isospin channels., The data show that a pure {Delta}I=1/2 decay amplitude is ruled out at the 1.6{sigma} level, and favor a solution where either: (1) both isospin channels contribute about equally, or (2) the {Delta}I=3/2 channel dominates by an order of magnitude.

Search for double-Λ hypernuclei and the H-dibaryon in the (K−,K+) reaction on 12C

Abstract A search for double- Λ hypernuclei ( 12 ΛΛ Be) and H -dibaryons using the 12 C( K − , K + ) reaction was performed at the BNL-AGS using a high-intensity 1.8 GeV/ c K − beam. A missing-mass analysis below the end point of the quasi-free Ξ − production was used to investigate these S =−2 systems. The upper limit obtained for the forward-angle cross section of 12 ΛΛ Be production is 6 to 10 nb/sr. This is the first search for the direct production of double- Λ hypernuclei to reach the sensitivity required to observe the signal predicted by theoretical calculations. For the H -production cross section, we have obtained an upper limit in the range of a few nb/sr to 10 nb/sr for the H mass below 2100 MeV/ c 2 . This upper limit is the most sensitive H search result to date for a tightly bound H .

Properties of the Λ(1405) Measured at CLAS

Abstract Using the CLAS detector system in Hall B at Jefferson Lab, we have done a high statistics measurement of the photoproduction of K + Λ ( 1405 ) using a proton target. The reconstructed invariant mass seen in the various Σ π decay modes has been measured, as well as the differential cross section d σ d t . The nature of the Λ ( 1405 ) with its peculiar mass distribution is not well understood, and studies have shown that it may posess strong dynamical components, which could be extracted from our data. Various aspects of our analysis are discussed, as well as future prospects to further enhance the precision of the results.

Scaling and resonances in elementary K+Λ photoproduction

Recent cross-section data for the reaction {gamma}+p{yields}K{sup +}+{Lambda} are examined for evidence of scaling in both the low-t Regge domain and in the high-{radical}(s) and -t domain where constituent counting may apply. It is shown that the reaction does scale in both regimes. At large center-of-mass angles, s{sup -7} scaling appears to hold at essentially all -t but with angle-dependent oscillations. The scaled data show particularly strong evidence for s-channel resonances for -t below 2 GeV{sup 2} and for W below about 2.3 GeV. The dominant contributions are consistent with an N{sup *}S{sub 11} resonance at 1690 MeV, a P{sub 13} resonance at 1920 MeV, and a D{sub 13} resonance at 2100 MeV, which interfere to give the observed strong angular dependence.

LARGE-SOLID-ANGLE STUDY OF PION ABSORPTION ON 3HE

Measurements have been made of [pi][sup +] absorption on [sup 3]He at [ital T][sub [pi]][sup +]=118, 162, and 239 MeV using the Large Acceptance Detector System. The nearly 4[pi] solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The total absorption cross section is reported. In addition, the total cross section is divided into components in which only two or all three nucleons play a significant role in the process. These are the first direct measurements of the total and three nucleon absorption cross sections.

A time-of-flight array for 1 to 2 GeV/c particles

A time-of-flight detector array has been developed for an experiment searching for the strangeness −2 H-particle. The array consists of 40 logs of plastic scintillator with dimensions 2.00 × 0.085 × 0.050 m3. The photomultiplier tubes are coupled to the scintillators without the use of light guides, and the mounting of the bars is designed for easy adjustment and servicing. The average intrinsic time resolution was found to be 110 ps σ.

Weak decays of He-4(Lambda)

We measured the lifetime and the mesonic and non-mesonic decay rates of the 4He-Lambda hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K- beam on a liquid 4He target by the reaction 4He(K-,pi-)4He-Lambda. The 4He-Lambda lifetime was directly measured using protons from Lambda p -> n p non-mesonic decay (also referred to as proton-stimulated decay) and was found to have a value of tau = 245 +/- 24 ps. The mesonic decay rates were determined from the observed numbers of pi-s and pi0s as Gamma_pi-/Gamma_tot = 0.270 +/- 0.024 and Gamma_pi0/Gamma_tot = 0.564 +/- 0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Gamma_p/Gamma_tot = 0.169 +/- 0.019 and Gamma_n/Gamma_tot <= 0.032 (95% CL), respectively. The effects of final-state interactions and possible 3-body Lambda N N decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the non-mesonic branching fractions. The implications of the results of this analysis were considered for the empirical Delta I = 1/2 rule and the decay rates of the 4H-Lambda hypernucleus.

A TPC in the context of heavy-ion collisions

Abstract The design of a TPC for a high-multiplicity environment is described. The sense plane is composed of a matrix with 40 rows of short anode wires. The end-cap is completed with rod and wire electrodes to establish the amplification region and to minimize space charge effects. The digital readout of more than 6000 channels is performed by a combination of purpose-built comparators and FASTBUS TDCs. The performance of the chamber is briefly described, and some ideas to improve the hit efficiency and resolution in a new end-cap design are discussed.