Search

In the study of rapidity distributions of charged pions produced ine+e- annihilation at 29 GeV, we observe a significant dip in the rapidity distributions aty˜0 for events with low sphericity. A comparison of data with several QCD-based phenomenological models strongly suggests that this structure is related to the emission of multiple soft and/or collinear gluons.

Inclusive production cross sections for photons and p0's ine+e- annihilation at a center of mass energy of 29 GeV have been measured. The p0 production spectrum agrees with a corresponding measurement for p±. The ratio of the p0 inclusive rate to the average for p± is 0.92±0.14. The fractions of the total energy carried by photons and p0's are 0.244±0.016 and 0.217±0.033, respectively. The fraction of total energy carried by all stable hadrons, prompt leptons and photons is determined to be 0.938±0.045, leaving 0.062±0.045 for neutrinos.

We have measured the inclusive prompt electron cross section over a wide momentum range (P>0.5 GeV/c) with the PEP-4 TPC detector. The semielectronic branching fractions of thec andb quarks are (9.1±0.9 (stat.)±1.3 (syst.))% and (11.0±1.8±1.0)%, respectively. Theb quark fragmentation function peaks at highz with b>=0.74±0.05±0.03. The axial couplings to the neutral current areac=2.3±1.4±1.0 for thec quark andab=-2.0±1.9±0.5 for theb quark.

Reynolds &38; Begelman have recently proposed a model in which the broad and extremely redshifted iron line seen during a deep minimum of the light curve of the Seyfert 1 galaxy MCG–6‐30‐15 originates from matter spiralling into a Schwarzschild black hole, contrary to previous claims that the black hole may be spinning rapidly. Here we calculate in detail the X‐ray spectrum produced by their model using the full reflected continuum emission, including absorption features. This calculation takes into account the Doppler and relativistic effects. For the range of parameters that we consider, we find that the spectrum should show a large photoelectric absorption edge of iron, which is not seen in the data. The absorption edge is a consequence of the line‐emitting matter within the innermost stable orbit being highly ionized, and is largely independent of the parameters chosen for the model. If we restrict our attention to the 3–10 keV band we may effectively remove this absorption edge by fitting a steeper power law, but this results in a significant underprediction of the 0.4–0.5 keV flux. We conclude that the data on MCG–6‐30‐15 are more consistent with the Kerr than the Schwarzschild model.

Cultured endothelial cells secrete a platelet-derived growth factor-like molecule (PDGFc). We examined the effects of purified human alpha-thrombin on the production of PDGFc in cultures of human umbilical vein endothelial cells (HUVE) using a specific radioreceptor assay for PDGF. Addition of physiologically relevant concentrations of alpha-thrombin (0.1 to 10 U/ml) induced a time- and dose-dependent increase in the release of PDGFc into the culture medium. Significant stimulation of PDGFc release was observed as early as 1.5 h after addition of alpha-thrombin (10 U/ml) with a 4.9 +/- 1.1 fold increase at 24 h (mean +/- SEM of nine experiments, P less than 0.01). alpha-Thrombin treatment of HUVE did not affect cell viability as assessed by trypan blue dye exclusion. The receptor binding of PDGFc secreted by HUVE in response to alpha-thrombin was inhibited by monospecific antibody to purified human PDGF indicating that the molecule(s) is closely related to PDGF. alpha-Thrombin inactivated with diisopropylfluorophosphate was without stimulatory effect. Lysis of HUVE by repeated cycles of freeze/thaw released minimal PDGFc (less than 0.3 ng per 10(6) cells) compared to levels of PDGFc released into supernatant medium in response to alpha-thrombin (greater than 5.0 ng per 10(6) cells after a 24-h incubation with 10 U/ml alpha-thrombin). Moreover, incubation of freeze/thaw lysates of HUVE with alpha-thrombin failed to release PDGFc. Over a 3-h time course, however, alpha-thrombin-induced secretion of PDGFc was not prevented by cycloheximide. We conclude that alpha-thrombin induces secretion of PDGFc from HUVE by a nonlytic mechanism requiring the serine esterase activity of the enzyme. Although this effect does not initially require de novo protein synthesis, it does require cell-mediated conversion of PDGFc from an inactive to an active form.

We present a measurement of the photon structure functionF2γ in the reactionee→eeX forQ2 in the range 0.22<7 GeV2, using 9,200 multihadron events obtained with the TPC/Two-Gamma detector at PEP. The data have been corrected for detector effects using a regularized unfolding procedure and are presented as a function ofx andQ2. The structure function shows scaling in the region 0.32<1.6 GeV2,x<0.3 and rises for higherQ2. AtQ2=5.1 GeV2 the results are compared with QCD and, within the scheme of Antoniadis and Grunberg, rather conservative bounds for the QCD scale parameter of 133±50< <268±98 MeV are obtained. A study of the final state structure shows that the rise ofF2γ is consistent with being entirely due to the pointlike component of the photon.

An experimental search for dark matter particle candidates using cryogenic detectors requires a low radioactive background environment. We discuss the status of a cryogenic dark matter experiment to be performed in the Stanford Underground Facility. The detectors will be cooled in a specially designed cryostat connected to a modified side access Oxford 400 dilution refrigerator. Details of the cryostat design and its operating performance are presented. The effectiveness of the multi-level shield surrounding the cryostat, as well as the background levels we expect to achieve in the pilot experiment are discussed. Finally, we examine the limits which can be set on dark matter candidates with such an experiment.

We have developed low noise voltage and low noise charge sensitive preamplifiers for use in dilution refrigerator experiments. The amplifiers use a JFET at 140K as the front end. Each JEET dissipates about 7 mW on the 4K heat sink. The flat noise of the voltage amplifier is 0.9 nV/Hz1/2, with the 1/f knee below 100 Hz. The charge amplifier noise is 500 eV with a total input capacitance of approximately 90 pF.

We have tested a new 60 gram germanium phonon and ionization detector to study incomplete charge collection effects in cryogenic detectors. An understanding of such effects is necessary to further improve the nuclear vs. electronic recoil discrimination capability of cryogenic detectors. The detector is operated at ˜ 20 mK. It has three concentric ionization channels, and four electric field shaping structures on its sides. Phonons are sensed using NTD Ge thermistors. First results obtained using this device and moveable radioactive sources of241Am and57Co will be presented.

Three currently used fragmentation models are tested by studying the distribution of particles between jet axes in the 3-jet events ofe+e− annihilation, using data collected by the Time Projection Chamber at PEP. These three models — the Lund String model, the Webber Cluster model and an Independent Fragmentation model (IF) — each implement different Lorentz-frame structures for the fragmentation process of quarks and gluons into hadrons. The Lund model provides a good description of the data, while the IF model does not. The Webber model, which is untuned, does not describe the absolute particle densities between jets, but correctly predicts the ratios of those densities, which are less sensitive to the tuning.

Identifying the spatial distribution and magnitude of seepage flux across the groundwater‐surface water (GW‐SW) interface is critical for assessing potential impairments and restoration alternatives for water bodies adjacent to sites with groundwater contamination. Measurement of the vertical distribution and time‐varying characteristics of temperature in sediments provides an indirect way to map out spatial and temporal patterns of seepage flux into surface water. Two spreadsheet‐based calculation tools are introduced that implement four one‐dimensional analytical solutions to calculate the magnitude and direction of seepage flux based on measurement of steady‐state vertical temperature profiles or transient diel temperature signals at two depths within sediment. Performance of these calculation tools is demonstrated for a pond receiving contaminated groundwater discharge from an adjacent landfill. Transient versus steady‐state model performance is compared, and limitations of transient models are illustrated for a situation with unfavorable sediment characteristics and inadequate sensor spacing. The availability of a range of analytical solutions implemented within Microsoft Excel® is intended to encourage practitioners to explore use of this seepage flux characterization method and develop greater insight into best practices for model selection and use. Open‐access spreadsheet‐based calculation tools are provided to make use of sediment temperature for estimating the magnitude and direction of seepage flux between groundwater and surface water. The performance of the different models coded in these tools is illustrated for a study site influenced by varying weather conditions. Open‐access spreadsheet‐based calculation tools are presented for estimating vertical seepage flux from sediment temperaturesData from a study site is used to compare performance of various one‐dimensional analytical modelsThe influence of monitoring network design and precipitation events on the usability of model output is demonstrated