Deep Inelastic Scattering Structure Functions Research Articles

Structure functions are not parton probabilities

We explain why contrary to common belief, the deep inelastic scattering structure functions are not related to parton probabilities in the target.

Renormalon model predictions for power corrections to flavor-singlet deep-inelastic structure functions

We analyze power corrections to flavor-singlet deep-inelastic scattering structure functions in the framework of the infrared renormalon model. Our calculations, together with previous results for the non-singlet contribution, allow us to model the x-dependence of higher twist corrections to F 2, F L and g 1 in the whole x domain.

QCD coherence and jet rates in small x deep inelastic scattering

The contributions to the deep inelastic scattering structure function which arise from emission of zero, one, two or three resolvable gluons and any number of unresolvable ones are computed to order α ̄ S 3 . Coherence effects are taken into account via angular ordering and are demonstrated to yield (at the leading logarithm level) the identical results to those obtained assuming the multi-Regge kinematics of BFKL.

Large-order behaviour due to ultraviolet renormalons in QCD

Ultraviolet renormalons, contrary to their infrared counterparts, lead to a universal contribution to the large-order behaviour of perturbative expansions in QCD. In this letter, we determine the nature of the leading ultraviolet renormalon singularity for the inclusive hadroproduction cross section in e + e - annihilation, for hadronic τ decays and the moments of deepinelastic scattering structure functions. We comment on the relevance of ultraviolet renormalons to estimates in low orders of perturbation theory.

DIS structure functions in lattice QCD

In this talk I present the complete 1-loop perturbative computation of the renormalization constants and mixing coefficients of quark and gluon lattice operators of rank two and three whose hadronic elements enter in the determination of the first and second moment of Deep Inelastic Scattering Structure Functions, making use of the nearest-neighbor improved “clover-leaf” lattice QCD action. To perform the huge amount of calculations required for the evaluation of al the relevant Feynman diagrams, extensive use of symbolic manipulation languages like Schoonschip and Form has been made.

Improved parton distributions from global analysis of recent deep inelastic scattering and inclusive jet data

The impact of recent precision measurements of DIS structure functions and inclusive jet production at the Fermilab Tevatron on the global QCD analysis of parton distribution functions is studied in detail. Particular emphasis is placed on exploring the range of variation of the gluon distribution $G(x,Q)$ allowed by these new data. The strong coupling of $G(x,Q)$ with ${\ensuremath{\alpha}}_{s}$ is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets [modified minimal subtraction $(\overline{\mathrm{MS}})$, deep inelastic scattering (DIS), and leading order (LO)], a series that gives a range of parton distributions with corresponding ${\ensuremath{\alpha}}_{s}$'s, and a set with a low starting value of $Q$. Previously obtained gluon distributions that are consistent with the high ${E}_{t}$ jet cross section are also discussed in the context of this new global analysis.

Deep inelastic scattering in improved lattice QCD (II). The second moment of structure functions

In this paper we present the 1-loop perturbative computation of the renormalization constants and mixing coefficients of the lattice quark operators of rank three whose hadronic elements enter in the determination of the second moment of Deep Inelastic Scattering (DIS) structure functions. We have employed in our calculations the nearest-neighbor improved “clover-leaf” lattice QCD action. The interest of using this action in Monte Carlo simulations lies in the fact that all terms which in the continuum limit are effectively of order a ( a being the lattice spacing) have been demonstrated to be absent from on-shell hadronic lattice matrix elements. We have limited our computations to the quenched case, in which quark operators do not mix with gluon operators. We have studied the transformation properties under the hypercubic group of the operators up to the rank five (which are related to moments up to the fourth of DIS structure functions), and we discuss the choice of the operators considered in this paper together with the feasibility of lattice computations for operators of higher ranks. To perform the huge amount of calculations required for the evaluation of all the relevant Feynman diagrams, we have extensively used the symbolic manipulation languages Schoonschip and Form.

Deep inelastic scattering in improved lattice QCD (I). The first moment of structure functions

We present the complete 1-loop perturbative computation of the renormalization constants and mixing coefficients of the operators that measure the first moment of deep inelastic scattering structure functions, employing the nearest neighbor improved lattice QCD action. The interest of using this action in Monte Carlo simulations lies in the fact that all terms which in the continuum limit are effectively of order a ( a being the lattice spacing) have been proven to be absent from [p]Because of the complexity of the calculations, we have checked the analytical expression of all Feynman diagrams using Schoonschip. To this end we have developed a suitable code designed to automatically carry out all the necessary lattice algebraic manipulations, starting from the elementary building blocks of each diagram. [p]We have found discrepancies with some of the published numbers, but we are in agreement with the known results on the energy-momentum tensor.

Polarized parton distributions

We analyze spin dependent parton distributions consistent with the most recent measurements of the spin dependent deep inelastic scattering structure functions and obtained in the framework of the spin dilution model. Predictions for the doubly polarised proton-proton Drell-Yan asymmetry, for the high $p_{T}$ photon production mechanism and $J/\Psi$ excitation are calculated using these distributions and are shown to be particularly adequate to unveil the polarisation of partons in the proton.

Spin-isospin asymmetry in nucleon structure

The deep-inelastic scattering structure function ratio F 2 n / F 2 p and the nucleon spin asymmetries are found to be strongly x-dependent. In the context of the parton model, the observed inhomogeneities mean that quarks with different flavors and spins carry different fractions of the nucleon momentum. We show that all of these asymmetry effects can be qualitatively understood in terms of the admixtures of excited-state configurations in the nucleon wave function due to color hyperfine interactions.

CTEQ parton distributions and flavor dependence of sea quarks

This paper describes salient features of new sets of parton distributions obtained by the CTEQ Collaboration based on a comprehensive QCD global analysis of all available data. The accuracy of the new data on deep inelastic scattering structure functions obtained by the very high statistics NMC and CCFR experiments provides unprecedented sensitivity to the flavor dependence of the sea-quark distributions. In addition to much better determination of the small- x dependence of all parton distributions, we found: (i) the strange quark distribution is much softer than the non-strange sea quarks and rises above the latter at small- x; and (ii) the difference d- u changes sign as a function of x. A few alternative sets of viable distributions with conventional assumptions are also discussed.

Pion cloud of the nucleon and its effect on deep-inelastic structure.

In this paper we present the calculation of deep-inelastic scattering structure functions of nucleons dressed by pions. The calculation is performed within the convolution model. We present analytic results for quark and antiquark distributions of a given flavor and spin and numerical results of those structure functions that are likely to be most affected by the dressing of the nucleon. We allow for the probe scattering off both nucleons and \ensuremath{\Delta}'s. Diagrams where it scatters off the pions themselves are not taken into account. For the numerical results we look only at combinations of structure functions where these do not contribute.

Proton and neutron structure functions in a Fermi-gas approximation.

The proton and neutron deep-inelastic scattering structure functions are found to be fit reasonably well by a finite-temperature Fermi-gas (effective) description of the confined quarks. Support problems at negative $x$ are avoided completely and formal sum rules are satisfied. The characteristic difference between the $u$ and $d$ valence distributions, which is difficult to achieve in many models of nucleon structure, arises naturally as a consequence of the different densities of $u$ and $d$ quarks in the nucleon.

Deep-inelastic scattering in a modified bag model.

Deep-inelastic scattering structure functions are calculated in a cloudy bag model with surface dynamics. The model is solved in the adiabatic approximation, the resultant wave functions are projected onto states of zero momentum, and valence-quark distributions are calculated. These distributions are evolved to ${Q}^{2}$=15 ${\mathrm{GeV}}^{2}$ using QCD perturbation theory, where they are compared with experiment. Qualitative agreement is achieved.

QCD corrections to the parton-model sum rules for structure functions of deep inelastic scattering

The results of two-loop calculations of the O( α s 2) corrections to the first nonsinglet moments of the deep inelastic scattering structure functions for polarized and unpolarized targets are presented.

Phenomenology on the lattice: Composite operators in lattice gauge theory.

We study composite operators in lattice gauge theory that reduce to operators of the form \ensuremath{\psi}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\Gamma}D\ensuremath{\rightarrow}D\ensuremath{\rightarrow}...\ensuremath{\psi} in the continuum limit; such operators arise in perturbative analyses of quantum chromodynamics. Using our results and the data of a numerical simulation one could normalize exclusive processes and predict moments of deep-inelastic scattering structure functions. To initiate the program we construct and renormalize lattice operators to the one-loop level. We are encouraged that the hadronic matrix elements of the simpler operators are within reach of numerical simulations.

Smallx behaviour of deep inelastic scattering structure functions within and beyond perturbative QCD

The bound on the behaviour of deep inelastic scattering structure functions in the limitx≃0 is derived for arbitraryQ2. To this aim the generalised vector meson dominance representation is used with the total cross-sections assumed to obey the Froissart bound and constrained to follow perturbative QCD at largeQ2. Semiphenomenological eikonal prescription of implementing unitarity upon the leading QCD contribution is formulated and possible effect of absorptive corrections upon structure functions is estimated.

QCD asymptotics and kinematic thresholds in deep inelastic scattering

The Nachtmann moments of deep inelastic scattering structure functions are required by kinematics to contain a so far neglected threshold factor which is dependent on both n and q 2. Its presence significantly affects the “moment analysis” in the usual QCD phenomenology and it resolves the difficulties connected with improper threshold behavior of the “ξ-scaling” analysis of structure functions.

Properties of hadrons as quark bound-states on the null plane

Specific properties of the interacting quark fields when quantized on the null plane are exploited in order to construct one-hadron states. The Melosh transformation naturally appears in the definition of these relativistic quark bound states and it implies a definite pattern for SU(6) and SU(3) breaking in the various coupling constants. The only three parameters of the model, 〈 p ⊥ 2〉 and the two quark effective masses, are determined from the experimental values of g A, μ proton and μ Λ . Many results are then obtained and successfully compared with experiments, as for instance, Γ( ϱ → ππ) = 140 MeV, Γ(f → ππ) = 96 MeV, Γ(A 2 → ϱπ) = 61 MeV, Γ(B → ωπ) = 125 MeV, Γ(g → ππ) = 23 MeV, Γ(ϱ′ → ππ) ⋍ 0, Γ(ϱ′ → 4π) ⋍ 450 MeV , Γ( ω → π 0 γ) = 990 keV, Γ( φ → ηγ) = 115 keV, 〈 r 2〉 π + = 0.54 fm 2, 〈r 2〉 K 0 = −0.025 fm 2 , ƒ K = 1.12 ƒ π , ƒ ϱ = 1, 62ƒ π . An interesting connection between baryon radii and the neutron/proton ratio of the deep inelastic scattering structure functions is discussed. The inclusion of charmed quarks into the model gives a value Γ(ψ → e + e −) ⋍ 2 keV and, furthermore, an acceptably small value for the radiative decay of the ψ (⪅ 100 keV).