119484177
Parton Distribution and Decay Functions
Abstract We define parton distribution and decay functions in QCD and prove some of their basic properties. These include renormalization, light-cone expansions and sum rules.
17338636
Light-cone Variables, Rapidity and All That
I give a pedagogical summary of the methods of light-cone variables, rapidity and pseudo-rapidity. Then I show how these methods are useful in analyzing diffractive high-energy collisions.
119047717
The Status of Perturbative QCD
Progress in quantum chromodynamics in the past year is reviewed in these specific areas: proof of factorization for hadron-hadron collisions, fast calculation of higher order graphs, perturbative Monte Carlo calculations for hadron-hadron scattering, applicability of perturbative methods to heavy quark production, and understanding of the small-x problem. 22 refs. (LEW)
7579254
Parton distribution functions suitable for Monte-Carlo event generators
In the usual factorization theorems, which give predictions only for inclusive cross sections, there is considerable freedom in the choice of the scheme to define the parton distribution functions. These theorems do not directly apply to Monte-Carlo event generators, and more general factorization theorems which give predictions for fully exclusive cross sections are needed. It has been shown that appropriate parton distribution functions are uniquely defined by the showering algorithm. In this paper, we present results of calculations of the Monte-Carlo parton distribution functions in terms of the commonly used parton distribution functions. At small x the differences are large, which demonstrates the importance of using the correct parton distribution functions in an event generator rather than parton distribution functions. We present some simple approximations that enable an understanding of the sizes of the results to be obtained.
2620123
Double pomeron jet cross sections
Abstract We treat hadron-hadron collisions where the final state is kinematically of the kind associated with double-pomeron-exchange (DPE) and has large transverse momentum jets. We show that in addition to the conventional factorized (FDPE) contribution, there is a nonfactorized (NDPE) contribution which has no pomeron beam jet. Within a simple model we compute DPE-two-jet total and differential cross sections at Tevatron energy scales, and show that the NDPE contribution is dominant.
28842411
Transversity distribution does not contribute to hard exclusive electroproduction of mesons
We show that in hard exclusive electroproduction, ep{yields}eVp, the leading-twist hard-scattering coefficient for the production of a transversely polarized vector meson V vanishes to all orders of perturbation theory. This implies that this process cannot be used to measure the skewed transversity distribution of quarks in a hadron. In contrast, a recent calculation obtained a non-zero value at NLO. We show that this calculation is incorrect because it failed to include the necessary collinear subtractions. Our method of proof also applies to other processes whose hard-scattering coefficients are constrained by chirality and helicity conservation, and thus validates helicity selection rules based on these symmetries. (c) 2000 The American Physical Society.
5948490
Highly Ionized High‐Velocity Clouds toward PKS 2155−304 and Markarian 509
To gain insight into four highly ionized high-velocity clouds (HVCs) discovered by Sembach et al., we have analyzed data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) for the PKS 2155-304 and Mrk 509 sight lines. We measure strong absorption in O VI and column densities of multiple ionization stages of silicon (Si II, III, and IV) and carbon (C II, III, and IV). We interpret this ionization pattern as a multiphase medium that contains both collisionally ionized and photoionized gas. Toward PKS 2155-304, for HVCs at -140 and -270 km s-1, respectively, we measure log N(O ) = 13.80 ± 0.03 and log N(O ) = 13.56 ± 0.06; from Lyman series absorption, we find log N(H ) = 16.37 and 15.23. The presence of high-velocity O VI spread over a broad (100 km s-1) profile, together with large amounts of low-ionization species, is difficult to reconcile with the low densities, ne ≈ 5 × 10-6 cm-3, in the collisional/photoionization models of Nicastro et al., although the HVCs show a similar relation in N(Si IV)/N(C IV) versus N(C II)/N(C IV) to that of high-z intergalactic clouds. Our results suggest that the high-velocity O VI in these absorbers does not necessarily trace the warm-hot intergalactic medium but instead may trace HVCs with low total hydrogen column density. We propose that the broad high-velocity O VI absorption arises from shock ionization, at bow shock interfaces produced from infalling clumps of gas with velocity shear. The similar ratios of high ions for HVC Complex C and these highly ionized HVCs suggest a common production mechanism in the Galactic halo.
56099774
Construction of a Prototype Spark Chamber
A small demonstration spark chamber is to be built at the Cavendish laboratory. A prototype chamber consisting of five 20x22.5cm plates has been built and descriptions of its properties and construction are given, while a second chamber with a somewhat novel design is nearly complete. A discussion of the issues surrounding the final design is presented, and recommendations are made in light of the results of this work.
14712521
A LARGE RESERVOIR OF IONIZED GAS IN THE GALACTIC HALO: IONIZED SILICON IN HIGH-VELOCITY AND INTERMEDIATE-VELOCITY CLOUDS
The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M ☉ yr–1) cooling inflow to replenish star formation in the disk. Using absorption spectra from the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer toward 37 active galactic nuclei at high latitude, we detect widespread interstellar Si III λ1206.50 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at l = 180°, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81% ± 5% for 30 out of 37 directions) with Si III optical depth typically 4-5 times that of O VI λ1031.926. The mean HVC column density per sight line, log NSi III = 13.42 ± 0.21, corresponds to total column density N HII (6 × 1018 cm–2)(Z Si/0.2Z ☉)–1 of ionized low-metallicity gas, similar to that inferred in O VI. This reservoir could total 108 M ☉ and produce a mass infall rate ~1 M ☉ yr–1. By modeling Si II, Si III, Si IV, and H I in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, log U = –3.0+0.3 –0.4, approximately 10 times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [Si II/H I], are 10%-30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. These formally lower matallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The HVC and IVC metallicities may be compared with the median photometric metallicity, [Fe/H] = –1.46 ± 0.30, for ~200,000 halo F/G stars in the Sloan Digital Sky Survey.
118344053
Identifying the colour of TeV-scale resonances
A bstractWe explore how the colour of any new TeV-scale resonances that decay into top quark pairs can be identified by studying the dependence of the observed cross-section on a central jet veto. To facilitate this study, colour octet resonance production was implemented in Pythia8 and colour singlet resonance production is simulated after minor modifications. We find that the colour of a 2 TeV resonance can be identified with 10 fb−1 of data at a centre-of-mass energy of 14 TeV for a wide range of couplings, but only if the uncertainty in the theoretical prediction is dramatically reduced from its current level.
16974063
Metallicity and Ionization in High-Velocity Cloud Complex C
We analyze HST and FUSE ultraviolet spectroscopic data for 11 sight lines passing through the infalling high-velocity cloud (HVC) Complex C. These sight lines pass through regions with H I column densities in the range N = 1018.1-1020.1 cm-2. From [O I/H I] abundances, we find that Complex C metallicities range from 0.09 to 0.29 Z☉, with a column density weighted mean of 0.13 Z☉. Nitrogen (N I) is underabundant by factors of (0.01-0.07)(N/H)☉, significantly less than oxygen relative to solar abundances. This pattern suggests nucleosynthetic enrichment by Type II SNe, consistent with an origin in the Galactic fountain or infalling gas produced in winds from Local Group galaxies. The range of metallicity and its possible (2 σ) dependence on N could indicate some mixing of primordial material with enriched gas from the Milky Way, but the mixing mechanism is unclear. We also investigate the significant highly ionized component of Complex C, detected in C IV, Si IV, and O VI, but not in N V. High-ion column density ratios show little variance and are consistent with shock ionization or ionization at interfaces between Complex C and a hotter surrounding medium. Evidence for the former mechanism is seen in the Mrk 876 line profiles, where the offset in line centroids between low and high ions suggests a decelerating bow shock.