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 analysis of fragmentation functions considering double spin asymmetry of SIDIS processes

We extract new sets of Pion and Kaon fragmentation functions (FFs) at NLO applying combined analysis of single-inclusive electron-positron annihilation and semi-inclusive deep-inelastic lepton-nucleon scattering (SIDIS). We use zero-mass variable-flavor-number (ZM-VFN) scheme in our analysis.     

QCD analysis of forward neutron production in DIS

Observing light-by-light scattering at the large hadron collider (LHC) has received quite some attention and it is believed to be a clean and sensitive channel to possible new physics. In this paper, we study the diphoton production at the LHC via the process \({{pp}}\rightarrow {{p}}\gamma \gamma {{p}}\rightarrow {{p}}\gamma \gamma {{p}}\) through graviton exchange in the large extra dimension (LED) model. Typically, when we do the background analysis, we also study the double Pomeron exchange of \(\gamma \gamma \) production. We compare its production in the quark–quark collision mode to the gluon–gluon collision mode and find that contributions from the gluon–gluon collision mode are comparable to the quark–quark one. Our result shows, for extra dimension \(\delta =4\) , with an integrated luminosity \(\mathcal{L} = 200\,\mathrm{fb}^{-1}\) at the 14 TeV LHC, that diphoton production through graviton exchange can probe the LED effects up to the scale \({M}_{S}=5.06 (4.51, 5.11)\,\mathrm{TeV}\) for the forward detector acceptance \(\xi _1 (\xi _2, \xi _3)\) , respectively, where \(0.0015<\xi _1<0.5\) , \(0.1<\xi _2<0.5\) , and \(0.0015<\xi _3<0.15\) .     

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.     

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.     

QCD analysis of experimental polarized deep-inelastic-scattering data

A detailed review of the current state of investigations into polarized deep inelastic scattering (DIS) is presented. Special attention is given to the methods of the QCD analysis of experimental data on these processes and to the methods of extrapolation of polarized structure functions and polarized quark distributions in the regions inaccessible to current experiments. In the case of pure inclusive processes, the QCD analysis of all worldwide data, including the latest COMPASS data, is presented in detail. Special attention is given to such important components of the nucleon-spin problem as the polarized strangeness and polarized gluon distribution. The features of SIDIS processes are considered; in particular, the role of fragmentation functions in the analysis of the semi-inclusive data is discussed. The methods of extracting the fragmentation functions from experimental data are considered in detail, and the corresponding results are presented. The results of analysis of the existing semi-inclusive polarized data both in the QCD leading order and in the next-to-leading order are considered. Special attention is given to non-standard, so-called difference asymmetries, which make it possible to minimize the dependence of results of analysis on the fragmentation functions. The current methods of QCD analysis of semi-inclusive polarized data are critically reviewed. An alternative method of QCD analysis of semi-inclusive data is presented for next-to-leading order QCD. Advantages of the method in practical applications are illustrated by the example of analysis of the HERMES data.     

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.     

QCD analysis of the semi-inclusive COMPASS and HERMES data

The first moments of polarized valence parton distribution functions truncated to the wide Bjorken x region 0.004 < x < 0.7 are directly (without any fitting procedure) extracted in the NLO QCD from combined semi-inclusive DIS data of COMPASS and HERMES collaborations. Two scenarios for fragmentation functions are considered. Applying the proposed original procedure to these results we estimate the contributions of sea u and d quarks to the proton spin, which turn out to be simply zero within the errors.     

A QCD analysis of diffractive deep-inelastic scattering data

We perform a novel type of analysis of diffractive deep-inelastic scattering data, in which the input parton distributions of the pomeron are parameterised using the perturbative QCD expressions. In particular, we treat individually the components of the pomeron of different size. We are able to describe simultaneously both the recent ZEUS and H1 diffractive data. In addition to the usual two-gluon model for the perturbative pomeron, we allow for the possibility that it may be made from two sea quarks.Received: 12 July 2004, Revised: 3 September 2004, Published online: 24 September 2004     

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.     

Sea quark polarization and semi-inclusive DIS data

We investigate the potential impact of forthcoming Jefferson Lab semi-inclusive polarized deep inelastic scattering proton measurements in the determination of the sea quark polarization in the nucleons by means of a next to leading order global QCD analysis. Specifically, we estimate the resulting improvement in the constraints on polarized parton densities for different flavors, which is found to be significant for up and strange quarks, and the correlation between remaining uncertainty ranges for each of the parton species. PACS 12.38.Bx, 13.85.Ni     

A QCD analysis of HERA and fixed target structure function data

The parton momentum density distributions of the proton are determined from a next-to-leading order QCD analysis of structure functions measured at HERA and by fixed target experiments. Also included are data on the difference of the up and down anti-quark distributions. The uncertainties in the parton densities, structure functions and related cross sections are estimated from the experimental errors and those on the input parameters of the fit. Several QCD predictions based on the parton densities obtained from this analysis are calculated and compared to data. Received: 21 December 1999 / Published online: 6 April 2000     

Towards a new global QCD analysis: low <Emphasis Type="Italic">x</Emphasis> DIS data from non-linear evolution

A new approach to global QCD analysis is developed. The main ingredients are two QCD-based evolution equations. The first one is the Balitsky-Kovchegov non-linear equation, which sums higher twists while preserving unitarity. The second equation is linear and it is responsible for the correct short distance behavior of the theory, namely it includes the DGLAP kernel. Our approach allows for extrapolation of the parton distributions to the very high energies available at the LHC as well as very low photon virtualities, . All existing low x data on the F₂ structure function are reproduced using two fitting parameters, the other parameters were taken as constants. The result is . Analyzing the parameter at very low x and Q² well below we find -0.1. This is a result which agrees with the “soft pomeron” intercept without involving soft physics. Received: 9 September 2002 / Revised version: 15 November 2002 / Published online: 14 February 2003     

Results from a QCD analysis of the new deep inelastic scattering data

Results from a numerical evolution of the Altarelli-Parisi equations are presented, covering the full range 0<x<1. The predictions of leading order and next-to-leading order in perturbative QCD, are compared with the most precise currently available data. All necessary assumptions are quantitatively discussed.     

Consistency in NLO analyses of inclusive and semi-inclusive polarized DIS data

We perform a detailed study of the consistency between different sets of polarized deep inelastic scattering data and theory, from the standpoint of a next to leading order QCD global analysis, and following the criteria proposed by Collins and Pumplin. In face of recent suggestions that challenge the usual assumption about parent parton spin independence of unpolarized fragmentation functions, we specially focus on polarized semi-inclusive data. Received: 2 December 2002 / Revised version: 6 February 2003 / Published online: 31 March 2003 RID="a" ID="a" e-mail: sassot@df.uba.ar ^* Partially supported by Conicet and Fundación Antorchas     

An NLO QCD analysis of inclusive cross-section and jet-production data from the ZEUS experiment

The ZEUS inclusive differential cross-section data from HERA, for charged and neutral current processes taken with e ⁺ and e^- beams, together with differential cross-section data on inclusive jet production in e ⁺ p scattering and dijet production in scattering, have been used in a new NLO QCD analysis to extract the parton distribution functions of the proton. The input of jet-production data constrains the gluon and allows an accurate extraction of at NLO; An additional uncertainty from the choice of scales is estimated as . This is the first extraction of from HERA data alone.     

QCD Analysis of the semi-inclusive HERMES and COMPASS data

The first moments of polarized valence PDFs, responsible for the respective contributions to the nucleon spin, are extracted in NLO QCD from the data of COMPASS and HERMES collaborations on the semi-inclusive difference asymmetries. To this end the new method of QCD analysis (alternative to the usual global fit analysis) is applied. Using the obtained results on valence PDFs the first moments of polarized sea PDFs are reconstructed. They turn out surprisingly small: compatible with zeros within the errors.     

Determination of QCD condensates from τ decay data

We have used the latest data from the ALEPH Collaboration to extract values for QCD condensates up to dimension d=12 in the V-A channel and up to dimension d=8 in the V, A and V+A channels. Performing two- and three-parameter fits, we obtain new results for the correlations of condensates. The results are consistent and agree with most of the previous results found in the literature.     

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