Parton

About: Parton is a research topic. Over the lifetime, 13484 publications have been published within this topic receiving 368877 citations.

Systematic study of high $p_{T}$ hadron spectra in $p p$, $p$ A and A A collisions from SPS to RHIC energies

Abstract: High-p{sub T} particle spectra in p+p (p(bar sign)+p), p+A, and A+B collisions are calculated in a parton model with QCD-inspired hard scattering and evolution in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic k{sub T} and its broadening in p+A and A+B collisions due to initial multiple parton scattering are found to be very important at low energies ({radical}(s) 1 GeV/c) in A+B collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This suggests that hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at p{sub T}{approx}2-10 GeV/c. However, there is no evidence of jet quenching or parton energy loss. Assuming this model of parton scattering, nuclear broadening and parton energy loss, one can exclude an effective parton energy loss dE{sub q}/dx>0.01 GeV/fm and a mean free path {lambda}{sub q}<7 fm from the experimental data of A+B collisions at the SPS energies.more » Predictions for high p{sub T} particle spectra in p+A and A+A collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed. (c) 2000 The American Physical Society.« less

Uncertainties of predictions from parton distributions II: theoretical errors

Abstract: We study the uncertainties in parton distributions, determined in global fits to deep inelastic and related hard scattering data, due to so-called theoretical errors. Amongst these, we include potential errors due to the change of perturbative order (NLO $\to$ NNLO), $\ln(1/x)$ and $\ln(1-x)$ effects, absorptive corrections and higher-twist contributions. We investigate these uncertainties both by including explicit corrections to our standard global analysis and by examining the sensitivity to changes of the x,Q 2,W 2 cuts on the data that are fitted. In this way we expose those kinematic regions where the conventional DGLAP description is inadequate. As a consequence we obtain a set of NLO, and of NNLO, conservative partons where the data are fully consistent with DGLAP evolution, but over a restricted kinematic domain. We also examine the potential effects of such issues as the choice of input parametrisation, heavy target corrections, assumptions about the strange quark sea and isospin violation. Hence we are able to compare the theoretical errors with those uncertainties due to errors on the experimental measurements, which we studied previously. We use W and Higgs boson production at the Tevatron and the LHC as explicit examples of the uncertainties arising from parton distributions. For many observables the theoretical error is dominant, but for the cross section for W production at the Tevatron both the theoretical and experimental uncertainties are small, and hence the NNLO prediction may serve as a valuable luminosity monitor.

The Dual Parton Model at Cosmic Ray Energies

Abstract: The dual parton model for hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions is studied in the fragmentation region up to the cosmic ray energy region. Because of the excellent Feynman scaling behavior of the model outside the regions around [ital x][sub [ital F]]=1 and [ital x][sub [ital F]]=0, it is found that accelerator data in the fragmentation region are indeed relevant for the cosmic ray energy region. However, not enough data are available in the fragmentation region of hadron collisions with light target nuclei. Therefore many features of hadron production in collisions involving nuclei can only be extracted from the study of models.

QCD evolution of the Sivers function

Abstract: We extend the Collins-Soper-Sterman (CSS) formalism to apply it to the spin dependence governed by the Sivers function. We use it to give a correct numerical QCD evolution of existing fixed-scale fits of the Sivers function. With the aid of approximations useful for the nonperturbative region, we present the results as parametrizations of a Gaussian form in transverse-momentum space, rather than in the Fourier conjugate transverse coordinate space normally used in the CSS formalism. They are specifically valid at small transverse momentum. Since evolution has been applied, our results can be used to make predictions for Drell-Yan and semi-inclusive deep inelastic scattering at energies different from those where the original fits were made. Our evolved functions are of a form that they can be used in the same parton-model factorization formulas as used in the original fits, but now with a predicted scale dependence in the fit parameters. We also present a method by which our evolved functions can be corrected to allow for twist-3 contributions at large parton transverse momentum.