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.
9067395
Measurement of dijet electroproduction at small jet separation
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.
122702137
Elastic Electroproduction of $\rho$ Mesons at HERA
The elastic electroproduction of rho mesons is studied at HERA with the H1 detector for a photon virtuality in the range 1 Q^2 60 GeV^2 and for a hadronic centre of mass energy in the range 30 W 140 GeV. The shape of the pipi mass distribution in the rho resonance region is measured as a function of Q^2. The full set of rho spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of 8 +- 3 % of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on Q^2, W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large Q^2, the gamma^*p cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron-hadron cross sections.
14063112
Measurement and QCD Analysis of Neutral and Charged Current Cross Sections at HERA
The inclusive e(+)p single and double differential cross sections for neutral and charged current processes axe measured with the H1 detector at HERA. The data were taken in 1999 and 2000 at a centre-of-mass energy of roots = 319 GeV and correspond to an integrated luminosity of 65.2 pb(-1). The cross sections are measured in the range of four-momentum transfer squared Q(2) between 100 and 30 000 GeV2 and Bjorken x between 0.0013 and 0.65. The neutral current analysis for the new e(+)p data and the earlier e-p data taken in 1998 and 1999 is extended to small energies of the scattered electron and therefore to higher values of inelasticity y, allowing a determination of the longitudinal structure function F-L at high Q(2) (110 - 700 GeV2). A new measurement of the structure function x (F) over tilde (3) is obtained using the new e(+)p and previously published e p neutral current cross section data at high Q(2). These data together with H1 low Q(2) precision data are further used to perform new next-to-leading order QCD analyses in the framework of the Standard Model to extract flavour separated parton distributions in the proton.
13227053
Search for leptoquark bosons in ep collisions at HERA
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.
2554439
On the rise of the proton structure function F2 towards Low x
Abstract A measurement of the derivative ( ∂ ln F 2 / ∂ ln x ) Q 2 ≡− λ ( x , Q 2 ) of the proton structure function F 2 is presented in the low x domain of deeply inelastic positron–proton scattering. For 5×10 −5 ⩽ x ⩽0.01 and Q 2 ⩾1.5 GeV 2 , λ ( x , Q 2 ) is found to be independent of  x and to increase linearly with ln Q 2 .
14441689
Time resolved emittance measurement in the University of Maryland Electron Ring
Initial experimental results are presented for measurements of the emittance of an electron beam in the University of Maryland Electron Ring (UMER). Two independent sets of slit-collector systems are used to measure the emittance in the vertical and horizontal directions respectively. Results are then compared to the time integrated emittance measurements obtained from a pepperpot. The UMER electron beam has an energy of 10 keV, with as much as 100 mA of current, and pulse length of 100 ns. The beam is confined by a periodic lattice of focusing/defocusing quadrupole magnets at 16 cm intervals. Beam steering is achieved with dipole magnets at 32 cm intervals and additional small steering dipoles as needed.
18948934
INITIATION OF THE DETONATION IN THE GRAVITATIONALLY CONFINED DETONATION MODEL OF TYPE Ia SUPERNOVAE
We study the initiation of the detonation in the gravitationally confined detonation (GCD) model of Type Ia supernovae (SNe Ia). In this model, ignition occurs at one or several off-center points, resulting in a burning bubble of hot ash that rises rapidly, breaks through the surface of the star, and collides at a point on the stellar surface opposite the breakout, producing a high-velocity inwardly directed flow. Initiation of the detonation occurs spontaneously in a region where the length scale of the temperature gradient extending from the flow (in which carbon burning is already occurring) into unburned fuel is commensurate to the range of critical length scales which have been derived from one-dimensional simulations that resolve the initiation of a detonation. By increasing the maximum resolution in a truncated cone that encompasses this region, beginning somewhat before initiation of the detonation occurs, we successfully simulate in situ the first gradient-initiated detonation in a whole-star simulation. The detonation emerges when a compression wave overruns a pocket of fuel situated in a Kelvin-Helmholtz cusp at the leading edge of the inwardly directed jet of burning carbon. The compression wave preconditions the temperature in the fuel in such a way that the Zel'dovichmore » gradient mechanism can operate and a detonation ensues. We explore the dependence of the length scale of the temperature gradient on spatial resolution and discuss the implications for the robustness of this detonation mechanism. We find that the time and the location at which initiation of the detonation occurs varies with resolution. In particular, initiation of a detonation had not yet occurred in our highest resolution simulation by the time we ended the simulation because of the computational demand it required. However, it may detonate later. We suggest that the turbulent shear layer surrounding the inwardly directed jet provides the most favorable physical conditions, and therefore the most likely location, for initiation of a detonation in the GCD model.« less
15277016
Chandra Observations of the Optically Dark GRB 030528
The X‐ray‐rich GRB 030528 was detected by the HETE satellite and its localization was rapidly disseminated. However, early optical observations failed to detect a counterpart source. In a 2‐epoch ToO observation with Chandra, we discovered a fading X‐ray source the likely counterpart to GRB 030528. The source brightness was typical of X‐ray afterglows observed at similar epochs. Other observers detected an IR source at a location consistent with the X‐ray source. The X‐ray spectrum is not consistent with a large absorbing column.
54078870
GLOBAL PROPERTIES OF X-RAY FLASHES AND X-RAY-RICH GAMMA-RAY BURSTS OBSERVED BY SWIFT
We describe and discuss the spectral and temporal characteristics of the prompt emission and X-ray afterglow emission of X-ray flashes (XRFs) and X-ray-rich gamma-ray bursts (XRRs) detected and observed by Swift between 2004 December and 2006 September. We compare these characteristics to a sample of conventional classical gamma-ray bursts (C-GRBs) observed during the same period. We confirm the correlation between Epeakobs and fluence noted by others and find further evidence that XRFs, XRRs, and C-GRBs form a continuum. We also confirm that our known redshift sample is consistent with the correlation between the peak energy in the GRB rest frame (Epeaksrc) and the isotropic radiated energy (Eiso), the so-called Epeaksrc-Eiso relation. The spectral properties of X-ray afterglows of XRFs and C-GRBs are similar, but the temporal properties of XRFs and C-GRBs are quite different. We found that the light curves of C-GRB afterglows show a break to steeper indices (shallow-to-steep break) at much earlier times than do XRF afterglows. Moreover, the overall luminosity of XRF X-ray afterglows is systematically smaller by a factor of 2 or more compared to that of C-GRBs. These distinct differences between the X-ray afterglows of XRFs and C-GRBs may be the key to understanding not only the mysterious shallow-to-steep break in X-ray afterglow light curves, but also the unique nature of XRFs.
118961575
Bayesian analysis of neutrinos observed from supernova SN-1987A
We present a Bayesian analysis of the energies and arrival times of the neutrinos from supernova SN 1987A detected by the Kamiokande II, IMB, and Baksan detectors, and find strong evidence for two components in the neutrino signal: a long time scale component from thermal Kelvin-Helmholtz cooling of the nascent neutron star, and a brief $(\ensuremath{\sim}1 \mathrm{s}),$ softer component similar to that expected from emission by accreting material in the delayed supernova scenario. In the context of this model, we show that the data constrain the electron antineutrino rest mass to be less than 5.7 eV with 95% probability. Our analysis takes advantage of significant advances that have occurred in the years since the detections in both our understanding of the supernova mechanism and our ability to analyze sparse data. This has led to significant improvement over previous studies in two important respects. First, our comparison of the data with parametrized models of the neutrino emission uses a consistent and straightforward Bayesian statistical methodology. This methodology helps us distinguish the complementary tasks of parameter estimation and model assessment, and fully accounts for the strong, nonlinear correlations between inferred values of neutrino emission model parameters. It also clarifies and improves the derivation of the likelihood function (the probability for the data), improving on earlier derivations in two ways: more consistent accounting for the energy-dependent efficiencies of the detectors; and inclusion of the empirically measured detector background spectra. These improvements lead to significant differences between our inferences and those found in earlier studies. Inclusion of detector background spectra proves crucial for proper analysis of the Baksan data and for demonstrating its consistency with data from other detectors. Second, we compare the data with a much wider variety of neutrino emission models than was explored previously, several of them inspired by recent numerical calculations of collapse and explosion based on the delayed supernova mechanism. This allows us to compare predictions of both the prompt and delayed mechanisms with the data, and ensures that our conclusions are robust. We find that two-component models for the neutrino signal are $\ensuremath{\sim}100$ times more probable than single-component models. Moreover, single-component models imply a radius and binding energy for the nascent neutron star significantly larger than those implied by even the stiffest acceptable equations of state for neutron star matter. In contrast, the radius and binding energy implied by two-component models are in agreement with predictions. Taking this agreement with prior expectations into account increases the odds in favor of two-component models by more than an order of magnitude. The inferred characteristics of the neutrino emission are in spectacular agreement with the salient features of the theory of stellar collapse and neutron star formation that had developed over several decades in the absence of direct observational data. We compare our work with previous work that used more conventional ``frequentist'' methods (including our own previous maximum likelihood analysis). We identify several methodological and technical weaknesses in earlier analyses, and show how these are overcome in our Bayesian analysis.