44477244
Lorentz invariance and quantum gravity: an additional fine-tuning problem?
Trying to combine standard quantum field theories with gravity leads to a breakdown of the usual structure of space time at around the Planck length, 1.6x10(-35) m, with possible violations of Lorentz invariance. Calculations of preferred-frame effects in quantum gravity have further motivated high precision searches for Lorentz violation. Here, we explain that combining known elementary particle interactions with a Planck-scale preferred frame gives rise to Lorentz violation at the percent level, some 20 orders of magnitude higher than earlier estimates, unless the bare parameters of the theory are unnaturally strongly fine tuned. Therefore an important task is not just the improvement of the precision of searches for violations of Lorentz invariance, but also the search for theoretical mechanisms for automatically preserving Lorentz invariance.
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.
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.
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.
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.