Design and testing of a fast beam position monitor
The University of Maryland Electron Ring (UMER) group is currently exploring the physics of space-charge dominated beams. Seventeen Beam Position Monitors (BPMs) will be used to determine the beam centroid for steering correction purposes to within 0.5 mm. Since the pulse length is relatively long (100 ns), the BPMs can also be used for temporal beam profiling. These features are extremely useful for perturbation and longitudinal dynamics studies. For these uses the BPM needs a temporal resolution better than 2 ns. We report on the final design and testing as well as other unique features of this device.

title: The Extinction toward the GRB 970228 Field
abstract: We determine the local Galactic extinction toward the field of gamma-ray burst GRB 970228 using a variety of methods. We develop a maximum-likelihood method for measuring the extinction that compares galaxy counts in different bands in the field of interest to those in a field of known extinction, and we apply this method to the GRB 970228 field. We also measure the extinction by comparing the observed broadband colors of stars in the GRB 970228 field to the colors of library spectra of the same spectral type. Finally, we estimate the extinction using the neutral hydrogen column density and the amount of infrared dust emission toward this field. Combining the results of these methods, we find a best-fit Galactic extinction in the optical of AV = 1.09, which implies that the intrinsic afterglow of GRB 970228 is significantly brighter and bluer than it appears.

title: Evidence for reionization at z ∼ 6: Detection of a gunn-peterson trough in a z = 6.28 quasar
abstract: We present moderate-resolution Keck spectroscopy of quasars at z = 5.82, 5.99, and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly? absorption in the spectra of these quasars evolves strongly with redshift. To z ~ 5.7, the Ly? absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest-redshift object, SDSSp J103027.10+052455.0 (z = 6.28), the average transmitted flux is 0.0038 ? 0.0026 times that of the continuum level over 8450 ? 20, on the optical depth to Ly? absorption at z = 6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly? forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z ~ 5 to 6, and the universe is approaching the reionization epoch at z ~ 6.

title: Evidence that gamma-ray bursts repeat and are Galactic in origin
abstract: We describe work which has provided evidence that there are two distinct classes of γ-ray bursts, that the bursts repeat, and that they are Galactic in origin.

title: High-energy observations of XRF 030723: Evidence for an off-axis gamma-ray burst?
abstract: We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-ray Monitor (WXM) and French Gamma Telescope observations of XRF 030723 along with observations of the XRF afterglow made using the 6.5 m Magellan Clay telescope and Chandra. The observed peak energy E of the νFν burst spectrum is found to lie within (or below) the WXM 2-25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best-fit value is E = 8.4 keV. The ratio of X-ray to γ-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy Eiso and Epk satisfy the relation discovered by Amati et al. (2002), a redshift of z = 0.38 can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that the X-ray fade may actually trace the optical fade. If this is the case, the late-time rebrightening observed in the optical cannot be due to a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium that is due to a progenitor wind.

title: Likelihood methods and classical burster repetition
abstract: We develop a likelihood methodology which can be used to search for evidence of burst repetition in the BATSE catalog, and to study the properties of the repetition signal. We use a simplified model of burst repetition in which a number Nr of sources which repeat a fixed number of times Nrep are superposed upon a number Nnr of non‐repeating sources. The instrument exposure is explicitly taken into account. By computing the likelihood for the data, we construct a probability distribution in parameter space that may be used to infer the probability that a repetition signal is present, and to estimate the values of the repetition parameters. The likelihood function contains contributions from all the bursts, irrespective of the size of their positional errors—the more uncertain a burst’s position is, the less constraining is its contribution. Thus this approach makes maximal use of the data, and avoids the ambiguities of sample selection associated with data cuts on error circle size. We present the results ...

title: Measurement of dijet electroproduction at small jet separation
abstract: Deep-inelastic scattering data in the range 150 < Q^2 < 35000 GeV^2 are used to investigate the minimum jet separation necessary to allow accurate description of the rate of dijet production using next-to-leading order perturbative QCD calculations. The required jet separation is found to be small, allowing about 1/3 of DIS data to be classified as dijet, as opposed to approximately 1/10 with more typical jet analyses. A number of precision measurements made using this dijet sample are well described by the calculations. The data are also described by the combination of leading order matrix elements and parton showers, as implemented in the QCD based Monte Carlo model RAPGAP.

title: Search for compositeness, leptoquarks and large extra dimensions in eq contact interactions at HERA
abstract: The reaction e + p → e + X is studied with the H1 detector at Hera . The data cover momentum transfers Q 2 between 200 GeV 2 and 30 000 GeV 2 and correspond to an integrated luminosity of 35.6 pb −1 . The differential cross section d σ /d Q 2 is compared to the Standard Model expectation for neutral current scattering and analysed to search for ( e e)( q q) contact interactions. No evidence for new phenomena is observed. The results are used to set limits on scales within models of electron–quark compositeness, quark form factors and the exchange of virtual heavy leptoquarks. A search for gravitational effects mediated through the exchange of virtual gravitons which propagate into large extra dimensions is presented.

title: Measurement of di-jet cross-sections in photoproduction and photon structure
abstract: The production of hard di-jet events in photoproduction at HERA is dominated by resolved photon processes in which a parton in the photon with momentum fraction x γ is scattered from a parton in the proton. These processes are sensitive to the quark and gluon content of the photon. The differential di-jet cross-section d σ /dlog( x γ ) is presented here, measured in tagged photoproduction at HERA using data taken with the H1 detector, corresponding to an integrated luminosity of 7.2 pb −1 . Using a restricted data sample at high transverse jet energy, E T,jet >6 GeV, the effective parton density f γ ,eff (x γ )= q(x γ )+ q (x γ )+9/4 g(x γ ) in the photon in leading order QCD is measured down to x γ = 0.05 from which the gluon density in the photon is derived.

title: Measurement of deeply virtual Compton scattering at HERA
abstract: The cross-section for deeply virtual Compton scattering in the reaction ep → egammap has been measured with the ZEUS detector at HERA using integrated luminosities of 95.0 pb(-1) of e(+) p and 16.7 pb(-1) of e(-) p collisions. Differential cross-sections are presented as a function of the exchanged- photon virtuality, Q(2), and the centre-of-mass energy, W, of the gamma*p system in the region 5 lt Q(2) lt 100 GeV2 and 40 lt W lt 140 GeV. The measured cross-sections rise steeply with increasing W. The measurements are compared to QCD-based calculations.

title: Search for leptoquark bosons in ep collisions at HERA
abstract: A search for scalar and vector leptoquarks coupling to first generation fermions is performed using the e(+) p and e(-) p scattering data collected by the H1 experiment between 1994 and 2000. The data correspond to a total integrated luminosity of 117 pb(-1). No evidence for the direct or indirect production of such particles is found in data samples with a large transverse momentum final state electron or with large missing transverse momentum. Constraints on leptoquark models are established. For leptoquark couplings of electromagnetic strength, leptoquarks with masses up to 275-325 GeV are ruled out at 95% confidence level. These limits improve and supercede earlier H1 limits based on subsamples of the data used here.