121385558
Superdense Matter: Neutrons Or Asymptotically Free Quarks?
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)
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.
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.
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.
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.
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.
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.