120295589
Tranverse momentum distribution of dimuons in the Drell‐Yan process
We describe recent progress in the theory of the QT distribution in the Drell‐Yan process for 0?QT2≲Q2. The basic theoretical result is in transverse position (b) space. In this talk we discuss in some detail how to get from b‐space to QT‐space.
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)
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.
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.
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.
8084858
HIGHLY IONIZED HIGH-VELOCITY CLOUDS: HOT INTERGALACTIC MEDIUM OR GALACTIC HALO?
We use spectroscopic data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) to study the wide range of ionization states of the "highly ionized high-velocity clouds" (HVCs). Studied extensively in O VI absorption, these clouds are usually assumed to be infalling gas in the Galactic halo at distances less than 50 kpc. An alternative model attributes the O VI (and O VII X-ray absorption) to cosmological structures of low-density, shock-heated intergalactic gas, distributed over 1-3 Mpc surrounding the Milky Way. The latter interpretation is unlikely, owing to the enormous required mass of gas (4 × 1012 M☉). Our detection, in 9 of 12 sight lines, of low-ionization stages (C II/III/IV; Si II/III/IV) at similar high velocities as O VI requires gas densities far above that (nH ≈ 5 × 10-6 cm-3) associated with the warm-hot intergalactic medium (WHIM). These HVCs are probably cooling, multiphase gas in the Galactic halo, bow shocks, and interfaces between clouds falling through a hot, rotating gaseous halo. The velocity segregation of these HVCs in Galactic coordinates is consistent with a pattern in which infalling clouds reflect the sense of Galactic rotation, with peculiar velocities superposed.
1954406
Hubble Space Telescope Survey of Interstellar High-Velocity Si III
We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong ?1206.500 line of Si III using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Because the Si III line is 4-5 times stronger than O VI ?1031.926, it provides a sensitive probe of ionized gas down to column densities N Si III 5 ? 1011 cm?2 at Si III equivalent width 10 m?. We detect high-velocity Si III over 91% ? 4% of the sky (53 of 58 sight lines); 59% of the HVCs show negative local standard of rest velocities. The mean HVC column density per sight line is log N Si III = 13.19 ? 0.45, while the mean for all 90 velocity components is 12.92 ? 0.46. Lower limits due to Si III line saturation are included in this average, so the actual mean/median values are even higher. The Si III appears to trace an extensive ionized component of Galactic halo gas at temperatures 104.0-4.5 K indicative of a cooling flow. Photoionization models suggest that typical Si III absorbers with 12.5 < log N Si III < 13.5 have total hydrogen column densities N H 1018-1019 cm?2 for gas of hydrogen density n H 0.1 cm?3 and 10% solar metallicity. With typical neutral fractions N H I /N H 0.01, these HVCs may elude even long-duration 21 cm observations at Arecibo, the EVLA, and other radio facilities. However, if Si III is associated with higher density gas, n H ? 1 cm?3, the corresponding neutral hydrogen could be visible in deep observations. This reservoir of ionized gas may contain 108 M ? and produce a mass infall rate of 1 M ? yr?1 to the Galactic disk.