46016111
Factorization of Hard Processes in QCD
We summarize the standard factorization theorems for hard processes in QCD, and describe their proofs.
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.
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)
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.
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.
1002044
Ionization Sources and Physical Conditions in the Diffuse Ionized Gas Halos of Four Edge-On Galaxies
Deep long-slit spectra of the diffuse ionized gas halos of the edge-on spiral galaxies NGC 4302 and UGC 10288 are presented. Emission lines are detected up to about z = 2 kpc in NGC 4302 and to nearly z = 3 kpc on the north side of UGC 10288. For both galaxies, the line ratios [N II]/Hα and [S II]/Hα increase with z in accordance with dilute photoionization models. Runs of [S II]/[N II] and for UGC 10288 the run of [O III]/Hα, however, are not explained by the models. Scale height determinations of their diffuse ionized gas (DIG) halos are generally lower than those of galaxies with more prominent extraplanar DIG features. These data, along with previously presented data for NGC 5775 and NGC 891, are used to address the issue of how DIG halos are energized. Composite photoionization/shock models are generally better at explaining runs of line ratios in these galaxies than photoionization models alone. Models of line ratios in NGC 5775 require a greater contribution from shocks for filamentary regions than for nonfilamentary regions to explain the run of [O III]/Hα. In either case, the [S II]/[N II] ratio is not well fit by the models. Composite models for UGC 10288 are successful at reproducing the run of [S II]/[N II] for all but the the highest values of [N II]/Hα; however, the run of [O III]/Hα versus [N II]/Hα does not show any discernible trend, making it difficult to determine whether or not shocks contribute to the layer's maintenance. We also examine whether the data can be explained simply by an increase in temperature with z in a pure photoionization model without a secondary source of ionization. Runs of [S II]/Hα, [N II]/Hα, and [S II]/[N II] in each of the four galaxies are consistent with such an increase. However, the runs of [O III]/Hα versus z in NGC 5775 and UGC 10288 require unusually high-ionization fractions of O++ that cannot be explained without invoking a secondary ionization source or at the very least a much higher temperature for the [O III]-emitting component than for the [S II]- and [N II]-emitting component. An increase in temperature with z is generally more successful at explaining the [O III]/Hα run in NGC 891, with the ionization fraction of O++ remaining relatively low and nearly constant with z.
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.