[en] In a consistently formulated pQCD framework incorporating nonzero mass heavy quark partons, there is still the freedom to define parton distributions obeying either mass-independent or mass-dependent evolution equations, contrary to statements made in a recent paper by MRRS. With properly matched hard cross-sections, different choices merely correspond to different factorization schemes, and they yield the same physical cross-sections. We demonstrate this principle in a concrete order α_s calculation of the DIS charm structure function. We also examine the proper matching between parton definitions and subtractions in the hard cross-section near threshold where the calculation is particularly sensitive to mass effects of the heavy quark. The results obtained from the general-mass formalism are quite stable against different choices of scale and exhibit a smooth transition in the threshold region (using either mass-independent or mass-dependent evolution), in contrast to results of ref. [3]. copyright 1997 American Institute of Physics

[en] In a consistently formulated PQCD framework incorporating nonzero mass heavy quark partons, there is still the freedom to define parton distributions obeying either mass-independent or mass-dependent evolution equations, contrary to statements made in a recent paper. With properly matched hard cross sections, different choices merely correspond to different factorization schemes; they yield the same physical cross sections. We demonstrate this principle in a concrete order α_s calculation of the Deeply Inelastic Scattering charm structure function. We also examine the proper matching between parton definitions and subtractions in the hard cross section near threshold where the calculation is particularly sensitive to mass effects of the heavy quark. The results obtained from the general-mass formalism are quite stable against different choices of scale and exhibit a smooth transition in the threshold region (using either mass-independent or mass-dependent evolution), in contrast with results of another recently proposed scheme. copyright 1997 The American Physical Society

[en] Essential to QCD applications of the operator product expansion, etc., is a knowledge of those operators that mix with gauge-invariant operators. A standard theorem asserts that the renormalization matrix is triangular: Gauge-invariant operators have ''alien'' gauge-variant operators among their counterterms, but, with a suitably chosen basis, the necessary alien operators have only themselves as counterterms. Moreover, the alien operators are supposed to vanish in physical matrix elements. A recent calculation by Hamberg and van Neerven apparently contradicts these results. By explicit calculations with the energy-momentum tensor, we show that the problems arise because of subtle infrared singularities that appear when gluonic matrix elements are taken on shell at zero momentum transfer

[en] We compute the temperature and IR signal of particles of radius a and albedo α at heliocentric distance R, taking into account the emissivity effect, and give an interpolating formula for the result. We compare with analyses of COBE DIRBE data by others (including recent detection of the cosmic IR background) for various values of heliocentric distance R, particle radius a, and particle albedo α. We then apply these results to a recently developed picture of the Kuiper belt as a two-sector disk with a nearby, low-density sector (40< R<50 endash 90 AU) and a more distant sector with a higher density. We consider the case in which passage through a molecular cloud essentially cleans the solar system of dust. We apply a simple model of dust production by comet collisions and removal by the Poynting-Robertson effect to find limits on total and dust masses in the near and far sectors as a function of time since such a passage. Finally, we compare Kuiper belt IR spectra for various parameter values. Results of this work include: (1) numerical limits on Kuiper belt dust as a function of (R, a, α) on the basis of four alternative sets of constraints, including those following from recent discovery of the cosmic IR background by Hauser et al.; (2) application to the two-sector Kuiper belt model, finding mass limits and spectrum shape for different values of relevant parameters including dependence on time elapsed since last passage through a molecular cloud cleared the outer solar system of dust; and (3) potential use of spectral information to determine time since last passage of the Sun through a giant molecular cloud. copyright copyright 1999. The American Astronomical Society

[en] Existing calculations of heavy quark hadroproduction in perturbative QCD are either based on the conventional zero-mass perturbative QCD theory, valid for energy scales much higher than the quark mass, or on a next-to-leading order (NLO) fixed-flavor-number (FFN) scheme which is appropriate for energy scales comparable to the quark mass. We formulate this problem in the general mass variable-flavor-number scheme which incorporates initial (final) state heavy quark parton distribution (fragmentation) functions as well as exact mass dependence in the hard cross section. This formalism has the built-in feature of reducing to the FFN scheme near threshold and to the conventional zero-mass parton picture in the very high energy limit. Making use of existing calculations in the NLO FFN scheme, we obtain more complete results on bottom quark production in the general scheme to order α_s³ both for current accelerator energies and for CERN LHC. Modest improvement over the FFN results is observed in the reduced scale dependence of the cross section and in the increased magnitude of the cross section, in the direction of experimental measurement. It is shown that the general scheme represents a more efficient way of organizing the perturbation series, since the bulk of the large NLO (α_s³) FFN contribution to the single heavy-quark inclusive cross section is already contained in the (resummed) order α_s² 'heavy flavor excitation' term in this scheme. Remaining limitations of the present calculation and possible improvements are discussed. copyright 1998 The American Physical Society