QCD analysis of DIS and SIDIS data

The combined analysis of polarized DIS and SIDIS data is performed in NLO QCD using two alternative procedures: standard fitting procedure and the recently developed direct new method. The results from two methods are compared. The especial attention is paid to the light sea and strange PDFs.     

Small x QCD effects in DIS with a forward jet or a forward \pi^{0}

We compare predictions based on small x (QCD) dynamics with recent data for deep inelastic events containing forward jets or forward mesons. We quantify the effect of imposing the (higher order) consistency condition on the BFKL equation and study uncertainties inherent in the QCD predictions. We also estimate the cross-section for the forward production of two jets. Received: 1 March 1999 / Published online: 30 June 1999     

QCD analysis of DIS and SIDIS data with two alternative methods

The global fit analysis of all published data on DIS and semi-inclusive DIS (SIDIS) asymmetries is performed in the next to leading (NLO) QCD order. The respective parameterization on polarized PDFs is constructed. The especial attention is paid to the impact of novel SIDIS data on the polarized distributions of light sea and strange quarks as well as on the polarized gluon distributions. The first moments of these distributions entering the nucleon spin are found to be surprisingly small quantities. The alternative direct (free of any fitting procedure) method of NLO QCD analysis is elaborated. Method is especially important for analysis of SIDIS data because it allows to avoid the problems arising in the conventional fitting procedure: functional arbitrariness at initial scale and ambiguities in the error band calculation. Within the alternative method the central values and uncertainties of the measured asymmetries directly propagate to the central values and uncertainties of the polarized PDFs we are interested in. The method is applied to all existing SIDIS data on pion production for an estimation in NLO QCD of the valence and sea quark contributions to the proton spin. As a result one arrives at the conclusion that, contrary to the valence contributions, the sea contributions to the proton spin are compatible with zero within the errors.     

QCD Factorization of Semi-Inclusive DIS Process at Operator Level

The operator level proof of factorization theorem exhibited in [ar Xiv:hep-ph/1307.4194] is extended to the semi-inclusive deep inelastic scattering process(SIDIS). Factorization theorem can be proved at operator level if there are not detected soft hadrons. The key point is that the initial one-nucleon state is the eigenstate of QCD.     

Gottfried Sum Rule in QCD Nonsinglet Analysis of DIS Fixed-Target Data

Deep-inelastic-scattering data from fixed-target experiments on the structure function F₂ were analyzed in the valence-quark approximation at the next-to-next-to-leading-order accuracy level in the strong-coupling constant. In this analysis, parton distributions were parametrized by employing information from the Gottfried sum rule. The strong-coupling constant was found to be α _s (M²_Z) = 0.1180 ± 0.0020 (total expt. error), which is in perfect agreement with the world-averaged value from an updated Particle Data Group (PDG) report, α^PDG _s (M²_Z) = 0.1181 ± 0.0011. Also, the value of 〈x〉_u?d = 0.187 ± 0.021 found for the second moment of the difference in the u- and d-quark distributions complies very well with the most recent lattice result 〈x〉^LATTICE_u?d = 0.208 ± 0.024.     

QCD factorization for the DIS structure functions at small x

We suggest a new form of QCD factorization for inclusive processes at high energies and then reduce it to K _T -factorization. In turn, K _T -factorization can be reduced to collinear factorization when the unintegrated parton distributions have at least one maximum in k _⊥. This property of the parton distributions can be checked with studying data of experiment. The sharper the maximum is, the more accurate the transition to collinear factorization can be done. We apply our results to deduce theoretical restrictions requirements on fits for parton distributions in both K _T - and collinear factorizations contrary to the present situation where the fits are introduced from purely phenomenological consideration so that any formula for them is acceptable if it matches experimental data.     

Polarized parton distributions from NLO QCD analysis of world DIS and SIDIS data

The combined analysis of polarized DIS and SIDIS data is performed in NLO QCD. The new parametrization on polarized PDFs is constructed. The uncertainties on PDFs and their first moments are estimated applying the modified Hessian method. Special attention is paid to the impact of novel SIDIS data on the polarized distributions of light sea and strange quarks. In particular, the important question of polarized sea symmetry is studied in comparison with the latest results on this subject.     

The neutron transversity from semi-inclusive DIS off 3He

A calculation of nuclear effects in the extraction of neutron single spin asymmetries in semi-inclusive deep inelastic scattering off ³He is described. In the kinematics of forth-coming experiments at JLab, it is found that the nuclear effects arising within an impulse approximation approach are under control.     

An approach to NLO QCD analysis of the semi-inclusive DIS data with the modified Jacobi polynomial expansion method

The modification of the Jacobi polynomial expansion method (MJEM) is proposed on the basis of the application of the truncated moments instead of the full ones. This allows us to reconstruct the local quark helicity distributions with high precision even for the narrow Bjorken x region accessible for measurement, using as an input only the four first moments extracted from the data in the next to leading order QCD. The variational (extrapolation) procedure is also proposed allowing us to reconstruct the distributions outside the accessible Bjorken x region using the distributions obtained with MJEM in the accessible region. The numerical calculations encourage one that the proposed variational (extrapolation) procedure could be applied to estimate the full first (especially important) quark moments.     

Baryon production in QCD jets

Recent data from several sources—Υ decays, high energy deep inelastic scattering,e ⁺ e ^? annihilation at PETRA—reveal substantial baryon production in QCD jets. We discuss the likely mechanisms within the context of QCD, and formulate experimental tests to distinguish amongst them.     

Inclusive cross section and single transverse-spin asymmetry of very forward neutron production at PHENIX

The cross section and x _F dependence of A _N of very forward neutron production in polarized p + p collisions at √s = 200 GeV were measured in the PHENIX experiment at RHIC. The measured cross sections were consistent with the x _F scaling claimed by the ISR experiment. Significant negative A _N was observed in the forward region, and no significant backward A _N was observed. We also measured √s and p _T dependence of A _N of very forward neutron production at √s from 62.4 to 500 GeV.     

Study of heavy hyperons production in DIS at COMPASS

Samples of Λ and $\bar \Lambda$ hyperons produced in deep-inelastic muon scattering off ⁶LiD target were collected with the COMPASS experimental setup at CERN. The relative yields of Σ-(1385), $\bar \Sigma ^ - (1385)$ , $\bar \Sigma ^ + (1385)$ , Ξ^?(1321), and $\bar \Xi ^ + (1321)$ hyperons decaying into the $\Lambda \left( {\bar \Lambda } \right)\pi$ were measured. The yield ratios of heavy hyperons to Λ were found to be in the range 3.8%–5.6% with a precision of about 10%. The experimental values are used to tune the parameters of the LEPTO Monte-Carlo generator relevant for strange particle production.     

The proton and neutron magnetic moments in lattice QCD

We have computed the proton and neutron magnetic moments on the lattice by a Monte Carlo simulation of QCD in the quenched approximation. The results are in remarkable agreement with the experimental values.     

Top-quark production and QCD

We review theoretical calculations for top-quark production that include complete next-to-leading-order QCD corrections as well as higher-order soft-gluon corrections from threshold resummation. We discuss in detail the differences between various approaches that have appeared in the literature and review results for top-quark total cross sections and differential distributions at the Tevatron and the LHC.     

A neutron electric dipole moment from supersymmetric QCD

Supersymmetric QCD naturally contains CP-violating interactions which can contribute to a neutron electric dipole moment. An analysis of the two-loop graphs results in a non-zero EDM whose magnitude is estimated and found to be compatible with experiment.     

Diffractive production of charm in DIS and gluon nuclear shadowing

We evaluate nuclear shadowing of the total cross section of charm particles production in DIS within the framework of Gribov theory of nuclear shadowing generalized to account for the QCD evolution. We use as an input the recent QCD Pomeron parton density analysis of the HERA diffractive data. Assuming that the QCD factorization theorem is applicable to the charm production off nuclei we also calculate shadowing of the gluon densities in nuclei and find it sufficiently large for heavy nuclei: G_A~200(x,Q²)/AG_N(x,Q²) ~ 0.45 m 0.5 · (A/200)^т.15 for x ~ 10^х1ц, Q² ~ 20 1 40 GeV² to influence significantly the physics of heavy ion collisions at LHC. We evaluate also suppression of minijet and hidden charm production in the central AA collisions. We also discuss some properties of the final states for %^*A processes dominated by the scattering off small x gluons like the high p_t jet and charm production.     

Lepton pair production in two-dimensional QCD

Large q² lepton pair production is calculated in leading order in 1/N_c in two-dimensional quantum chromodynamics (QCD). From this confinement model the (Drell-Yan) parton model result is recovered.