7762463
Diffractive Jet Production in Deep-Inelastic $e^{+}p$ Collisions at HERA
A measurement is presented of dijet and 3-jet cross sections in low-\(|t|\) diffractive deep-inelastic scattering interactions of the type \(ep \rightarrow eXY\), where the system X is separated by a large rapidity gap from a low-mass baryonic system Y. Data taken with the H1 detector at HERA, corresponding to an integrated luminosity of 18.0 pb\(^{-1}\), are used to measure hadron level single and double differential cross sections for \(4 4 \rm GeV\). The energy flow not attributed to jets is also investigated. The measurements are consistent with a factorising diffractive exchange with trajectory intercept close to 1.2 and tightly constrain the dominating diffractive gluon distribution. Viewed in terms of the diffractive scattering of partonic fluctuations of the photon, the data require the dominance of \(q\overline{q}g\) over \(q\overline{q}\) states. Soft colour neutralisation models in their present form cannot simultaneously reproduce the shapes and the normalisations of the differential cross sections. Models based on 2-gluon exchange are able to reproduce the shapes of the cross sections at low \(x_\mathbb{P}\) values.
1490101
Measurement of deeply virtual Compton scattering at HERA
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. (C) 2003 Elsevier B.V. All rights reserved
111380664
Inelastic photoproduction of J/psi mesons at HERA
An analysis of inelastic photoproduction of \(J/\psi\) mesons is presented using data collected at the ep collider HERA corresponding to an integrated luminosity of above 80 pb-1. Differential and double differential cross sections are measured in a wide kinematic region: \(60 < W_{\gamma p} < 260\) GeV,\(1 < p_{t, \psi}^2 < 60 {\rm GeV}^2\) and 0.05<z<0.9, where z is the fraction of the energy of the exchanged photon transferred to the \(J/\psi\) meson in the rest frame of the target proton. Cross sections at \(z\lesssim 0.3\) are presented for the first time. Theoretical calculations within the Colour Singlet Model at NLO for direct photon processes are shown to give a good description of the data in the medium z region (0.3<z<0.9) up to the highest \(p_{t, \psi}^2\) values. A calculation using a kt factorisation approach in LO in the Colour Singlet Model is also able to describe these data. The data in the full z range are also compared to LO calculations within a non-relativistic QCD framework including colour octet and colour singlet contributions for direct and resolved photons. It seems possible to reconcile data and theory with modest contributions from colour octet processes. The polarisation of the \(J/\psi\) meson is measured as a function of z and \(p_{t, \psi}\) and is reasonably described by the theoretical predictions.
208867227
Investigation of power corrections to event shape variables measured in deep-inelastic scattering
As the high abstention rate during the last european election shows, it seems that a large part of the european population is rather sceptical towards the european integration process. One ascribe this "euroscepticism" to the European Union communication. Our paper aims to evaluate this hypothesis which is not as obvious as it seems first. To do so we have evaluated the communication policy which as been set up by the European Commission (before the Romano Prodi Presidence) to conclude that the lack of a real european public sphere is probably a better explanation of this phenomenon.
122604063
Search for excited neutrinos at HERA
We present a search for excited neutrinos using ep data taken by the H1 experiment at HERA at a center-of-mass energy of 318 GeV with an integrated luminosity of 15 pb1. No evidence for excited neutrino production is found. Mass dependent exclusion limits are determined for the ratio of the coupling to the compositeness scale, f/, independently of the relative couplings to the SU(2) and U(1) gauge bosons. These limits extend the excluded region to higher masses than has been possible in previous searches at other colliders.
122848941
Deep inelastic inclusive e p scattering at low x and a determination of alpha(s)
A precise measurement of the inclusive deep-inelastic \(e^+ p\) scattering cross section is reported in the kinematic range \(1.5 \le Q^2 \le 150\) GeV\(^2\) and \( 3 \cdot 10^{-5} \le x \le 0.2\). The data were recorded with the H1 detector at HERA in 1996 and 1997, and correspond to an integrated luminosity of 20 pb\(^{-1}\). The double differential cross section, from which the proton structure function \(\) and the longitudinal structure function \(F_L(x,Q^2)\) are extracted, is measured with typically 1% statistical and 3% systematic uncertainties. The measured derivative \((\partial F_2(x,Q^2) / \partial \ln Q^2)_x \) is observed to rise continuously towards small x for fixed \(Q^2\). The cross section data are combined with published H1 measurements at high \(Q^2\) for a next-to-leading order DGLAP QCD analysis. The H1 data determine the gluon momentum distribution in the range \(3 \cdot 10^{-4} \le x \le 0.1\) to within an experimental accuracy of about 3% for \(Q^2 =20\) GeV\(^2\). A fit of the H1 measurements and the \(\mu p \) data of the BCDMS collaboration allows the strong coupling constant \(\alpha_s\) and the gluon distribution to be simultaneously determined. A value of \(\alpha_s (M_Z^2) =0.1150 \pm 0.0017 (exp)^{+0.0009}_{-0.0005} (model)\) is obtained in NLO, with an additional theoretical uncertainty of about \(\pm 0.005\), mainly due to the uncertainty of the renormalisation scale.
118934693
The ensemble photometric variability of ∼25,000 quasars in the Sloan Digital Sky Survey
Using a sample of over 25,000 spectroscopically confirmed quasars from the Sloan Digital Sky Survey, we show how quasar variability in the rest-frame optical/UV regime depends on rest-frame time lag, luminosity, rest wavelength, redshift, the presence of radio and X-ray emission, and the presence of broad absorption line systems. Imaging photometry is compared with three-band spectrophotometry obtained at later epochs spanning time lags up to about 2 yr. The large sample size and wide range of parameter values allow the dependence of variability to be isolated as a function of many independent parameters. The time dependence of variability (the structure function) is well fitted by a single power law with an index γ = 0.246 ± 0.008, on timescales from days to years. There is an anticorrelation of variability amplitude with rest wavelength—e.g., quasars are about twice as variable at 1000 A as at 6000 A—and quasars are systematically bluer when brighter at all redshifts. There is a strong anticorrelation of variability with quasar luminosity—variability amplitude decreases by a factor of about 4 when luminosity increases by a factor of 100. There is also a significant positive correlation of variability amplitude with redshift, indicating evolution of the quasar population or the variability mechanism. We parameterize all of these relationships. Quasars with ROSAT All-Sky Survey X-ray detections are significantly more variable (at optical/UV wavelengths) than those without, and radio-loud quasars are marginally more variable than their radio-quiet counterparts. We find no significant difference in the variability of quasars with and without broad absorption line troughs. Currently, no models of quasar variability address more than a few of these relationships. Models involving multiple discrete events or gravitational microlensing are unlikely by themselves to account for the data. So-called accretion disk instability models are promising, but more quantitative predictions are needed.
123595176
Neutrinos from SN 1987A - Implications for cooling of the nascent neutron star and the mass of the electron antineutrino
Data on neutrinos from SN 1987A are compared here with parameterized models of the neutrino emission using a consistent and straightforward statistical methodology. The empirically measured detector background spectra are included in the analysis, and the data are compared with a much wider variety of neutrino emission models than was explored previously. It is shown that the inferred neutrino emission model parameters are strongly correlated. The analysis confirms that simple models of the neutrino cooling of the nascent neutron star formed by the SN adequately explain the data. The inferred radius and binding energy of the neutron star are in excellent agreement with model calculations based on a wide range of equations of state. The results also raise the upper limit of the electron antineutrino rest mass to roughly 25 eV at the 95 percent confidence level, roughly 1.5-5 times higher than found previously.
119406486
Physical processes in photon-starved nonthermal pair plasmas
Compton spectra produced by relativistic, unmagnetized, nonthermal electron-positron pairs injected into photon-starved plasmas, where the luminosity in soft photons is much less than the power in the nonthermal pair injection, are studied. The overall form of the spectra consists of a rising spectrum at low energies, a single or double peak in the 0.1-1 MeV energy range, a cutoff above the peak, and a gamma-ray tail with a varying slope depending on the plasma parameters. The low-energy power law is mostly due to the primary soft photons being repeatedly upscattered by the hot thermal electrons. Comparisons are made with observations of compact objects. 37 refs.
16504497
Gamma-Ray Bursts as a Probe of the Epoch of Reionization
GRBs are expected to occur at redshifts far higher than the highest quasar redshifts so far detected. And unlike quasars, GRB afterglows may provide "clean" probes of the epoch of reionization because no complications (such as the presence of a strong Ly-alpha emission line) or "proximity effects" (such as the Stromgren sphere produced by ionizing photons from the quasar) are expected. Thus NIR and optical observations of GRB afterglows may provide unique information about the epoch of reionization. In particular, the flux at wavelengths shortward of Ly-alpha provides a direct measure of the density fluctuations of the IGM at the GRB redshift, while the flux at wavelengths longward of Ly-alpha provides an integrated measure of the number of ionizing photons produced by stars in the host galaxy of the GRB up until the burst occurs. A comparison of the sizes of the Stromgren spheres produced by stars in the host galaxies of GRBs and by quasars then provides an estimate of the relative contributions of star formation and quasars to reionization. We use detailed calculations of the expected shape of the GRB afterglow spectrum in the vicinity of Ly-alpha for GRBs at a variety of redshifts to illustrate these points.
54072956
Sloan digital sky survey: early data release
The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of ≈106 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux- and wavelength-calibrated, with 4096 pixels from 3800 to 9200 A at R ≈ 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.