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John C. Collins

Publications and source records attributed to John C. Collins.

36 records · Page 2Linked to original sources
arXiv1998

Diffractive Production of Jets and Weak Bosons, and Tests of Hard-Scattering Factorization

We extract diffractive parton densities from data on diffractive deep inelastic scattering (DIS) and on diffractive photoproduction of jets. We explore the results of several ansaetze for the functional form of the parton densities. Then we use the fitted parton densities to predict the diffractive production of jets and of W's and Z's in p-pbar collisions at the Tevatron. To fit the photoproduction data requires a large gluon density in the Pomeron. The predictions for the Tevatron cross sections are substantially higher than data; this signals a breakdown of hard-scattering factorization in diffractive hadron-hadron collisions.

hep-ph↗
arXiv1998

Tests of Factorization in Diffractive Charm Production and Double Pomeron Exchange

We present predictions for the diffractive production of heavy quarks in deep-inelastic scattering and hadron-hadron collisions and for double diffractive dijet production in hadron-hadron collisions. With the assumption of hard scattering factorization, the predictions are made using diffractive parton densities that we have previously fitted to HERA data. Comparisons of our predictions with preliminary data from HERA and the Tevatron indicate that factorization is obeyed in diffractive DIS but fails in hadron-induced processes.

hep-ph↗
arXiv1998

Factorization is not violated

We show that existing proofs of factorization imply the cancellation of certain multiladder contributions that Gotsman, Levin, and Maor had suggested would invalidate the basic factorization theorem in QCD. No modifications of the original argument are necessary, although the details of the example offer useful insight into the mechanisms of factorization.

hep-ph↗
arXiv1997

Direct Estimation of Sizes of Higher-Order Graphs

With the aid of simple examples we show how to make simple estimates of the sizes of higher-order Feynman graphs. Our methods enable appropriate values of renormalization and factorization scales to be made. They allow the diagnosis of the source of unusually large corrections that are in need of resummation.

hep-ph↗
arXiv1997

Factorization for hard exclusive electroproduction of mesons in QCD

We formulate and prove a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD. The proof is valid for the leading power in Q and all logarithms. This generalizes previous work on vector meson production in the diffractive region of small x. The amplitude is expressed in terms of off-diagonal generalizations of the usual parton densities. The full theorem applies to all kinds of meson and not just to vector mesons. The parton densities used include not only the ordinary parton density, but also the helicity density (g_1 or $Δq$) and the transversity density ($h_1$ or $δq$), and these can be probed by measuring the polarization of the produced mesons with unpolarized protons.

hep-ph↗
arXiv1997

Diffractive Production of Jets and Weak Bosons, and Tests of Hard-Scattering Factorization

We extract diffractive parton densities from diffractive, deep inelastic (DIS) ep data from the ZEUS experiment. Then we use these fits to predict the diffractive production of jets and of W's and Z's in p\bar p collisions at the Tevatron. Although the DIS data require a hard quark density in the pomeron, we find fairly low rates for the Tevatron processes (a few percent of the inclusive cross section). This results from the combined effects of Q^{2} evolution and of a normalization of the parton densities to the data. The calculated rates for W production are generally consistent with the preliminary data from the Tevatron. However, the jet data from CDF with a ``Roman pot'' trigger are substantially lower than the results of our calculations; if confirmed, this would signal a breakdown of hard-scattering factorization.

hep-ph↗
arXiv1996

QCD Aspects of Polarized Hard Scattering Processes

I give an overview of what our present knowledge of QCD predicts and does not predict for polarized hard scattering. For experimental programs, a big issue is how much further we can expect our theoretical understanding of QCD to improve.

hep-ph↗
arXiv1995

Double Pomeron Jet Cross Sections

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 non-factorized (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.

hep-ph↗
arXiv1994

On pi-pi Correlations in Polarized Quark Fragmentation Using the Linear Sigma Model

Using the linear sigma model to describe quark--pion interactions, we compute polarization asymmetries in quark fragmentation. We show that the effects of transverse quark polarizations appear in the correlation between the two leading pions in a jet produced by the fragmentation of a quark. Such asymmetries provide a window to the nature of chiral symmetry breaking in QCD.

hep-ph↗
arXiv1994

Issues in the Determination of Parton Distribution Functions

The CTEQ and MRS parton distributions involve a substantial number (~30) of parameters that are fit to a large number (~900) of data. Typically, these groups produce fits that represent a good fit to the data, but there is no substantial attempt to determine the errors associated with the fits. Determination of errors would involve consideration of the experimental statistical and systematic errors and also the errors in the theoretical formulas that relate the measured cross sections to parton distributions. We discuss the principles that would be needed in such an error analysis. These principles are standard. However, certain aspects of the principles appear counter-intuitive in the case of a large number of data. Accordingly, we strive to devote careful attention to the logic behind the methods.

hep-ph↗
arXiv1994

The Renormalization of Composite Operators in Yang-Mills Theories Using General Covariant Gauge

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 infra-red singularities that appear when gluonic matrix elements are taken on-shell at zero momentum transfer. Keywords: twist-two covariant gluon operator, finite part, mixing, non-abelian, anomalous dimension, Ward identity, BRST, modified LSZ reduction, Dixon, Taylor, Joglekar, Lee.

hep-ph↗
arXiv1993

Physics of Polarized pp Collisions

I will summarize the physics that can be investigated with polarized pp collisions. It is technically feasible to use the RHIC collider for accelerating highly polarized protons to a center-of-mass energy of about 400 GeV, with high luminosity. Such collisions can be used to probe the spin-dependence of hard collisions and of partons in a hadron, including the gluons. There are interesting twist 3 phenomena that are likely to be significant in view of the large transverse spin asymmetries at lower energies; this will have important implications for the spin structure of the proton wave function. Recent theoretical developments include the possibility of probing the spin of transversely polarized quarks via asymmetries in the jets they make.

hep-ph↗
arXiv1992

Spin in Perturbative QCD; Coherent Hard Diffraction

I briefly review: (a) some recent developments in the theory of hard scattering in QCD with polarized beams, and (b) coherent hard diffraction (that is, hard scattering in diffractive events, with the Pomeron behaving in an apparently point-like fashion).

hep-ph↗
arXiv1992

Diffractive Hard Scattering with a Coherent Pomeron

Diffractive scattering involves exchange of a Pomeron to make a rapidity gap. It is normally assumed that to get a hard scattering in diffraction, one may treat the Pomeron as an ordinary particle, which has distributions of gluons and quarks. We show that this is not so: When we use perturbative QCD, there is a breakdown of the factorization theorem. The whole Pomeron can initiate the hard scattering, even though it is not point-like. Qualitatively, but not quantitatively, this gives the same effect as a delta function term in the gluon density in a Pomeron.

hep-ph↗
arXiv1992

Hard Scattering in QCD with Polarized Beams

I show that factorization for hard processes in QCD is also valid when the detected particles are polarized, and that the proof of the theorem determines the operator form for the parton densities. Particular attention is given to the case of transversely polarized incoming hadrons.

hep-ph↗