We examine the negative $3\ensuremath{\pi}$ final state produced in association with ${\ensuremath{\Delta}}^{++}(1232)$ in the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\Delta}}^{++}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}$ at an incident photon energy of 19.3 GeV. The most prominent enhancement in the $3\ensuremath{\pi}$ spectrum occurs at a mass and with a width consistent with the parameters of the ${a}_{2}(1320)$. This identification is confirmed by the various angular distributions. The ${a}_{2}$ production cross section, corrected for efficiencies and alternate ${a}_{2}$ decay modes, is 0.45\ifmmode\pm\else\textpm\fi{}0.05 \ensuremath{\mu}b.

The Mark II detector has been upgraded in preparation for its role as the first detector to take data at the Stanford Linear Collider. The new detector components include the central drift chamber, the time-of-flight system, the coil, the endcap electromagnetic calorimeters and the beam energy and luminosity measuring devices. There have also been improvements in detector hermeticity. All of the major components were installed for a test run at the PEP storage ring (√s = 29 GeV) in 1985. This paper describes the upgraded detector, including its trigger and data acquisition systems, and gives performance figures for its components. Future improvements are also discussed.

The low-mass ..omega pi../sup 0/ enhancement in ..gamma..p..-->..p..omega pi../sup 0/ has been of considerable interest in the past due to its suggested vector nature and possible role in the spectroscopy of the rho-meson radial recurrences. We have measured the properties of this photoproduced ..omega pi../sup 0/ system using the SLAC Hybrid Facility. The experimental data consists of 306 785 usable hadronic events for which excellent ..gamma..-ray detection is provided by the large lead-glass array located behind the bubble-chamber. The photon beam had a 52% polarization. We have examined in detail the angular distributions of the 274 events from the reaction ..gamma..p..-->..p..omega pi../sup 0/. The angular distribution of the production plane relative to the polarization vector shows a structure inconsistent with an s-channel helicity-conserving process. We have extracted the moments of the decay angular distribution. Our data favors a B(1235) interpretation of the ..omega pi../sup 0/ state over a vector-meson interpretation.

Fast forward particles photoproduced in 20-GeV interactions on a hydrogen target are shown to be preferentially positive, the asymmetry increasing with transverse momentum and Feynman x. Evidence is given that this effect is not due to forward-going target fragments. A model in which production from the photon of a forward-going spectator u is preferred over a ū, due to a higher probability for interactions of antiquarks with the proton constituents, is shown to be qualitatively consistent with the data.Received 7 July 1986DOI:https://doi.org/10.1103/PhysRevD.36.1©1987 American Physical Society

Seventy-one events containing charmed-particle decays have been observed in an experiment using the SLAC Hybrid Facility exposed to a backward-scattered photon beam. Several improvements were made to the apparatus since the previous experiment on charm photoproduction. Results for the charmed-meson lifetimes are consistent with the published results from the previous experiment and the two data samples have been combined yielding a total sample of 136 charm events. After imposing rigorous cuts, 50 neutral, 48 charged, and 2 charged/neutral ambiguous decays remain. From these, the charmed-meson lifetimes are measured to be =(8.6\ifmmode\pm\else\textpm\fi{}${1.3}_{\mathrm{\ensuremath{-}}0.3}^{+0}$ $^{.7}$)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ sec, =(6.1\ifmmode\pm\else\textpm\fi{}0.9\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}10 $^{\mathrm{\ensuremath{-}}13}$ sec, and their ratio =1.4\ifmmode\pm\else\textpm\fi{}${0.3}_{\mathrm{\ensuremath{-}}}$ $_{0.1}^{+0.2}$. The total charm cross section at a photon energy of 20 GeV has been measured to be (62\ifmmode\pm\else\textpm\fi{}${8}_{\mathrm{\ensuremath{-}}10}^{+15}$) nb. There is evidence for both DD\ifmmode\bar\else\textasciimacron\fi{}X and D\ifmmode\bar\else\textasciimacron\fi{}${\ensuremath{\Lambda}}_{c}^{+}$X production with ${\ensuremath{\sigma}}_{D\ifmmode\bar\else\textasciimacron\fi{}{\ensuremath{\Lambda}}_{c}^{+}X}$/${\ensuremath{\sigma}}_{\mathrm{charm}}$=(71\ifmmode\pm\else\textpm\fi{} 11\ifmmode\pm\else\textpm\fi{}6)%.

The reaction γp→ρfast0pπ+π− has been studied with the linearly polarized 20-GeV monochromatic photon beam at the SLAC Hybrid Facility to test the prediction of s-channel helicity conservation in inelastic diffraction for t’<0.4 (GeV/c)2. In a sample of 1934 events from this reaction, the ρ0 decay-angular distributions and spin-density-matrix elements are consistent with s-channel helicity conservation, the π+π− mass shape displays the same skewing as seen in the reaction γp→pπ+π−, and the pπ+π− mass distribution compares well and scales according to the vector dominance model with that produced in π±p→πfast±pπ+π−.Received 25 March 1985DOI:https://doi.org/10.1103/PhysRevD.32.2288©1985 American Physical Society

Cross sections are presented for the inclusive photoproduction of K/sub S//sup 0/, ..lambda.., Lambda-bar, ..xi../sup -/, Xi-bar /sup -/, ..sigma../sup 0/, and ..sigma../sup asteriskplus-or-minus/(1385) at 20 GeV. An upper limit to ..cap omega../sup -/ production is also given. The data come from 284 000 hadronic events photoproduced in the SLAC 1-m hydrogen-bubble-chamber hybrid facility exposed to a nearly monochromatic, polarized 20-GeV backscattered photon beam. A comparison of the K/sub S//sup 0/, ..lambda.., Lambda-bar, and ..xi../sup -/ rates per inelastic event to ..pi../sup + -/p data show that ..gamma..p rates are consistent with being higher than the ..pi../sup + -/p rates, providing evidence of an ss-bar component of the photon. The pair cross sections for K/sub S//sup 0/K/sub S//sup 0/, K/sub S//sup 0/..lambda.., K/sub S//sup o/Lambda-bar, and ..lambda..Lambda-bar are presented. The x/sub F/ distributions of the ..lambda.., Lambda-bar, and ..xi../sup -/ are compared to a quark-diquark fusion model, giving information on strange-baryon photoproduction mechanisms.

Results of a search for a predicted enhancement of several microbarns in the charm-photoproduction cross section just above threshold are reported. No charm decays were detected, from which an upper limit to the charm cross section of 94 nb (90% C.L.) at ${E}_{\ensuremath{\gamma}}\ensuremath{\simeq}10$ GeV was obtained. Upper limits in the range 270 to 450 nb were also obtained for the peak cross sections for threshold enhancements in $\ensuremath{\gamma}p\ensuremath{\rightarrow}{D}^{\ensuremath{-}}{\ensuremath{\Sigma}}_{c}^{++}$ and similar channels.

Sixty-two charm events have been observed in an exposure of the SLAC Hybrid Facility toa backward sacttered laser beam. Based on 22 neutral and 21 charged decays we have measured the charmed-meson lifetimes to be τD0=(6.8+2.3−1.8)×10−13 sec, τD±=(7.4+2.3−2.0)×10−13 sec and their ratio τD±τD0=1.1+0.6−0.3. The inclusive charm cross section at a photon energy of 20 GeV has been measured to be 56+24−23 nb. Evidence is presented for a non-D¯D component to charm production, consistent with (35±20)% Λ+c production and some D*± production. We have found no unambiguous F decays.Received 27 December 1983DOI:https://doi.org/10.1103/PhysRevD.30.1©1984 American Physical Society

We have observed the ..pi../sup +/..pi../sup -/ decay of the rho'(1600) in the production reaction ..gamma..p..-->..rho'p at 20 GeV. Using a calculation which takes into account the interference of the rho' with the rho(770) and a Drell background, we find good evidence that this resonance is a radial excitation of the rho(770). The background interference strongly distorts the angular distributions predicted by a purely s-channel helicity-conserving production mechanism. We measure m/sub 0/ = (1.55 +- 0.07) GeV/c/sup 2/ and GAMMA/sub 0/ = (0.28/sub -0.08//sup +0.03/) GeV/c/sup 2/.

We have studied inclusive production of ${K}_{S}^{0}$, $\ensuremath{\Lambda}$, and $\overline{\ensuremath{\Lambda}}$ particles in 20-GeV $\ensuremath{\gamma}p$ interactions and have found features similar to those observed in both hadronic and leptonic interactions. The production cross sections, charged-particle multiplicities, and average $\ensuremath{\Lambda}$ polarization are reported. Inclusive distributions of $x$ and ${p}_{T}$ are shown and discussed in terms of quark fragmentation models. Production cross sections for ${K}^{*}(890)$ and ${\ensuremath{\Sigma}}^{*}(1385)$ are also reported.

We have observed the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ decay of the ${\ensuremath{\rho}}^{\ensuremath{'}}(1600)$ in the production reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\rho}}^{\ensuremath{'}}p$ at 20 GeV. Using a calculation which takes into account the interference of the ${\ensuremath{\rho}}^{\ensuremath{'}}$ with the $\ensuremath{\rho}(770)$ and a Drell background, we find good evidence that this resonance is a radial excitation of the $\ensuremath{\rho}(770)$. The background interference strongly distorts the angular distributions predicted by a purely $s$-channel helicity-conserving production mechanism. We measure ${m}_{0}=(1.55\ifmmode\pm\else\textpm\fi{}0.07)$ GeV/${\mathit{c}}^{2}$ and ${\ensuremath{\Gamma}}_{0}=({0.28}_{\ensuremath{-}0.08}^{+0.03})$ GeV/${\mathit{c}}^{2}$.