Marco Stratmann

Global analysis of fragmentation functions for pions and kaons and their uncertainties

We present new sets of pion and kaon fragmentation functions obtained in next-to-leading order combined analyses of single-inclusive hadron production in electron-positron annihilation, proton-proton collisions, and deep-inelastic lepton-proton scattering with either pions or kaons identified in the final state. At variance with all previous fits, the present analyses take into account data where hadrons of different electrical charge are identified, which allow one to discriminate quark from antiquark fragmentation functions without the need of nontrivial flavor symmetry assumptions. The resulting sets are in good agreement with all data analyzed, which cover a much wider kinematical range than in previous fits. An extensive use of the Lagrange multiplier technique is made in order to assess the uncertainties in the extraction of the fragmentation functions and the synergy from the complementary data sets in our global analysis.

Next-to-leading order radiative parton model analysis of polarized deep inelastic lepton - nucleon scattering

A next-to-leading-order QCD analysis of spin asymmetries and structure functions in polarized deep inelastic lepton-nucleon scattering is presented within the framework of the radiative parton model. A consistent NLO formulation of the {ital Q}{sup 2} evolution of polarized parton distributions yields two sets of plausible NLO spin-dependent parton distributions in the conventional MS factorization scheme. They respect the fundamental positivity constraints down to the low resolution scale {ital Q}{sup 2}={mu}{sup 2}{sub NLO}=0.34 GeV{sup 2}. The {ital Q}{sup 2} dependence of the spin asymmetries {ital A}{sup {ital p},{ital n},{ital d}}{sub 1}({ital x},{ital Q}{sup 2}) is similar to the leading-order (LO) one in the range 1{le}{ital Q}{sup 2}{le}20 GeV{sup 2} and is shown to be non-negligible for {ital x} values relevant for the analysis of the present data and possibly forthcoming data at DESY HERA. {copyright} {ital 1996 The American Physical Society.}

Extraction of spin-dependent parton densities and their uncertainties

We discuss techniques and results for the extraction of the nucleons spin-dependent parton distributions and their uncertainties from data for polarized deep-inelastic lepton-nucleon and proton-proton scattering by means of a global QCD analysis. Computational methods are described that significantly increase the speed of the required calculations to a level that allows one to perform the full analysis consistently at next-to-leading order accuracy. We examine how the various data sets help to constrain different aspects of the quark, antiquark, and gluon helicity distributions. Uncertainty estimates are performed using both the Lagrange multiplier and the Hessian approaches. We use the extracted parton distribution functions and their estimated uncertainties to predict spin asymmetries for high-transverse momentum pion and jet production in polarized proton-proton collisions at 500 GeV center-of-mass system energy at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, as well as for W boson production.

Global Analysis of Nuclear Parton Distributions

We present a new global QCD analysis of nuclear parton distribution functions and their uncertainties. In addition to the most commonly analyzed data sets for the deep-inelastic scattering of charged leptons off nuclei and Drell-Yan di-lepton production, we include also measurements for neutrino-nucleus scattering and inclusive pion production in deuteron-gold collisions. The analysis is performed at next-to-leading order accuracy in perturbative QCD in a general mass variable flavor number scheme, adopting a current set of free nucleon parton distribution functions, defined accordingly, as reference. The emerging picture is one of consistency, where universal nuclear modification factors for each parton flavor reproduce the main features of all data without any significant tension among the different sets. We use the Hessian method to estimate the uncertainties of the obtained nuclear modification factors and examine critically their range of validity in view of the sparse kinematic coverage of the present data. We briefly present several applications of our nuclear parton densities in hard nuclear reactions at BNL-RHIC, CERN-LHC, and a future electron-ion collider.

Global Analysis of Helicity Parton Densities and their Uncertainties

We present a new analysis of the helicity parton distributions of the nucleon. The analysis takes into account the available data from inclusive and semi-inclusive polarized deep inelastic scattering, as well as from polarized proton-proton (p-p) scattering at RHIC. For the first time, all theoretical calculations are performed fully at next-to-leading order (NLO) of perturbative QCD, using a method that allows incorporation of the NLO corrections in a very fast and efficient way in the analysis. We find evidence for a rather small gluon polarization in the nucleon, over a limited region of momentum fraction, and for interesting flavor patterns in the polarized sea.

Global analysis of fragmentation functions for protons and charged hadrons

We present new sets of fragmentation functions for protons and inclusive charged hadrons obtained in combined next-to-leading order QCD analyses of single-inclusive hadron production in electron-positron annihilation, proton-proton collisions, and deep-inelastic lepton-proton scattering. These analyses complement previous results for pion and kaon fragmentation functions with charge and flavor discrimination. The Lagrange multiplier technique is used to assess the uncertainties in the extraction of the new sets of fragmentation functions.

Evidence for polarization of gluons in the proton

We discuss the impact of recent high-statistics Relativistic Heavy Ion Collider data on the determination of the gluon polarization in the proton in the context of a global QCD analysis of polarized parton distributions. We find evidence for a nonvanishing polarization of gluons in the region of momentum fraction and at the scales mostly probed by the data. Although information from low momentum fractions is presently lacking, this finding is suggestive of a significant contribution of gluon spin to the proton spin, thereby limiting the amount of orbital angular momentum required to balance the proton spin budget.

Next-to-leading order QCD corrections to high p(T) pion production in longitudinally polarized pp collisions

We present a calculation for single-inclusive large-p_T pion production in longitudinally polarized pp collisions in next-to-leading order QCD. We choose an approach where fully analytical expressions for the underlying partonic hard-scattering cross sections are obtained. We simultaneously rederive the corresponding corrections to unpolarized scattering and confirm the results existing in the literature. Our results allow to calculate the double-spin asymmetry A_LL^pi for this process at next-to-leading order, which will soon be used at BNL-RHIC to measure the polarization of gluons in the nucleon.

Parton-to-Pion Fragmentation Reloaded

We present a new, comprehensive global analysis of parton-to-pion fragmentation functions at next-to-leading order accuracy in QCD. The obtained results are based on the latest experimental information on single-inclusive pion production in electron-positron annihilation, lepton-nucleon deep-inelastic scattering, and proton-proton collisions. An excellent description of all data sets is achieved, and the remaining uncertainties in parton-to-pion fragmentation functions are estimated based on the Hessian method. Extensive comparisons to the results from our previous global analysis are performed.

Inclusive Hadron Production in the CERN-LHC Era

We present a detailed phenomenological analysis of single-inclusive hadron production at the CERN-LHC in both proton-proton and proton-lead collisions. First data from the LHC experiments on charged hadron spectra are compared to next-to-leading order QCD expectations, and predictions are made for identified pion, kaon, and proton distributions differential in transverse momentum and rapidity for LHC energies from 900 GeV to 14 TeV. The results are obtained with the latest sets of vacuum fragmentation functions based on global QCD analyses, and recently proposed medium modified fragmentation functions are used to model hadronization in proton-lead collisions assuming standard QCD factorization. Besides estimating theoretical ambiguities due to the choice of factorization and renormalization scales and parton densities, we carefully assess uncertainties due to our present knowledge of parton-to-hadron fragmentation functions with the Lagrange multiplier technique. It is outlined to what extent future LHC data will contribute to further our quantitative understanding of hadronization processes.