Deep Inelastic Data Research Articles

Leading and higher twists in the proton polarized structure functiong1pat large Bjorkenx

A phenomenological parametrization of the proton polarized structure function ${g}_{1}^{p}{(x,Q}^{2})$ is developed for $x\ensuremath{\gtrsim}0.02$ using deep inelastic data up to $\ensuremath{\sim}50 (\mathrm{GeV}{/c)}^{2}$ as well as available experimental results on both photo- and electroproduction of proton resonances. According to the new parametrization the generalized Drell-Hearn-Gerasimov sum rule is predicted to have a zero-crossing point at ${Q}^{2}=0.16\ifmmode\pm\else\textpm\fi{}0.04 (\mathrm{GeV}{/c)}^{2}.$ Then, low-order polarized Nachtmann moments are estimated and their ${Q}^{2}$ behavior is investigated in terms of leading and higher twists for ${Q}^{2}\ensuremath{\gtrsim}1 (\mathrm{GeV}{/c)}^{2}.$ The leading twist is treated at NLO in the strong coupling constant and the effects of higher orders of the perturbative series are estimated using soft-gluon resummation techniques. In the case of the first moment, higher-twist effects are found to be quite small for ${Q}^{2}\ensuremath{\gtrsim}1 (\mathrm{GeV}{/c)}^{2},$ and the singlet axial charge has been determined to be ${a}_{0}[10 (\mathrm{GeV}{/c)}^{2}]=0.16\ifmmode\pm\else\textpm\fi{}0.09.$ In the case of higher order moments, which are sensitive to the large-x region, higher-twist effects are significantly reduced by the introduction of soft gluon contributions, but they are still relevant at ${Q}^{2}\ensuremath{\sim}$ few $(\mathrm{GeV}{/c)}^{2}$ at variance with the case of the unpolarized transverse structure function of the proton. Our finding suggests that spin-dependent correlations among partons may have more impact than spin-independent ones. As a by-product, it is also shown that the Bloom-Gilman local duality is strongly violated in the region of polarized electroproduction of the $\ensuremath{\Delta}(1232)$ resonance.

The compatibility of diffractive hard scattering in [formula omitted] and ep collisions

We show that the data for diffractive dijet production at the Tevatron p p ̄ collider are consistent with diffractive deep inelastic ep data collected at HERA. The breakdown of factorization between the two data sets is naturally explained by a strong tendency to populate the rapidity gap in the p p ̄ diffractive process by secondaries from soft underlying interactions and by bremsstrahlung associated with the presence of the hard dijet subprocess.

First Regge parameterisation of polarized DIS cross section

The first Regge description of the virtual photon absorption cross section difference Δσ(γ ∗, N) = [σ 1 2 (γ ∗, N) − σ 3 2 (γ ∗, N)] was obtained from a global fit of all the data collected by the experiments measuring spin asymmetries in polarized lepton — polarized nucleon deep inelastic scattering. This work present a phenomenological and a numerical description of all the polarized deep inelastic data ( Δσ(γ ∗, N), g 1 spin structure function) on the whole measured kinematical range (0.3 GeV 2 < Q 2 < 70 GeV 2, 4 GeV 2 < W 2 < 300 GeV 2). The fit also provide reliable predictions for the photo-production limit through a smooth Q 2-transition.

Deep inelastic neutron scattering from fluid hydrogen and deuterium: From vibrational excitations to the impulse approximation

Experimental deep inelastic neutron-scattering data from fluid deuterium at a temperature of 20.7 K, collected over a wide range of momentum transfer $q$ $(28{\AA{}}^{\ensuremath{-}1}&lt;q&lt;70{\AA{}}^{\ensuremath{-}1}),$ are presented. These experimental results, together with data previously collected from fluid hydrogen, are compared with a quantum-mechanical calculation, based on a Morse anharmonic intramolecular interaction. For both fluids it is shown that the present calculation is able to reproduce the experimental response function $F(y,q)$ over a wide momentum-transfer range, which spans from the molecular rotovibrational structure up to the molecular dissociation regime. Thus this calculation represents a clear improvement on previous models, based on harmonic or impulse approximations, which provide a good description of these fluids in the two opposite limits only, i.e., at the lowest and highest $q$ values, respectively.

Penetration of the Earth by ultrahigh energy neutrinos predicted by lowxQCD

We calculate the cross sections for neutrino interactions with (isoscalar) nuclear targets in the energy domain all the way up to 10^12 GeV. Small x QCD effects are included by using a unified BFKL/DGLAP formalism which embodies non-leading log(1/x) contributions. The few free parameters which specify the input parton distributions are determined by fitting to HERA deep inelastic data. The attenuation of neutrinos transversing the Earth at different nadir angles is calculated for a variety of energy spectra for neutrinos originating from different sources (from Active Galactic Nuclei, Gamma ray bursts, top-down models), as well as for atmospheric neutrinos. For this purpose we solve the transport equation which includes regeneration due to neutral current neutrino interactions, besides attenuation.

Saturation effects in deep inelastic scattering at lowQ2and its implications on diffraction

We present a model based on the concept of saturation for small $Q^2$ and small $x$. With only three parameters we achieve a good description of all Deep Inelastic Scattering data below $x=0.01$. This includes a consistent treatment of charm and a successful extrapolation into the photoproduction regime. The same model leads to a roughly constant ratio of diffractive and inclusive cross section.

STRUCTURE FUNCTIONS OF THE NUCLEON AND THEIR INTERPRETATION

The current status of measurements of the nucleon structure functions and their understanding is reviewed. The fixed target experiments E665, CCFR and NMC and the HERA experiments H1 and ZEUS are discussed in some detail. The extraction of parton momentum distribution functions from global fits is described, with particular attention paid to much improved information on the gluon momentum distribution. The status of αsmeasurements from deep inelastic data is reviewed. Models and nonperturbative approaches for the parton input distributions are outlined. The impact on the phenomenology of QCD of the data at very low values of the Bjorken x variable is discussed in detail. Recent advances in the understanding of the transition from deep inelastic scattering to photoproduction are summarized. Some brief comments are made on the recent HERA measurements of the ep NC and CC cross-sections at very high Q2.

Contact interaction explanation of HERA events and SU(3)C × SU(2)L × U(1)Y invariance

We reexamine the contact interaction hypothesis for excess high-$Q^2$ events seen at HERA in $e^+ p \to e^+ X$ scattering. We consider the most general structure of contact interactions insisting on $SU(3)_C\times SU(2)_L \times U(1)_Y$ invariance. Apart from constraints for $e^+ e^- \to q\bar q$ , Drell-Yan and atomic parity violation experiments, we find stringent constraints from neutrino deep inelastic data. A possible choice of contact terms that escape present bounds is still possible, althought new data from FERMILAB and LEP should further constrain such a possibility.

Measurement of the Proton and Deuteron Spin Structure Functiong1in the Resonance Region

We have measured the proton and deuteron spin structure functions g_1^p and g_1^d in the region of the nucleon resonances for W^2 < 5 GeV^2 and $Q^2\simeq 0.5$ and $Q^2\simeq 1.2$ GeV^2 by inelastically scattering 9.7 GeV polarized electrons off polarized $^{15}NH_3$ and $^{15}ND_3$ targets. We observe significant structure in g_1^p in the resonance region. We have used the present results, together with the deep-inelastic data at higher W^2, to extract $\Gamma(Q^2)\equiv\int_0^1 g_1(x,Q^2) dx$. This is the first information on the low-Q^2 evolution of Gamma toward the Gerasimov-Drell-Hearn limit at Q^2 = 0.

Diffractive heavy flavour production at the Tevatron and the LHC

We give predictions for diffractive heavy flavour production at the Tevatron and the LHC in leading-order approximation. In the framework of these studies we use three different models for the partonic structure of the Pomeron recently proposed by Stirling and Kunszt. These Pomeron models are, despite being fitted to the same diffractive deep inelastic HERA data, very different in their parton content and taken together provide a powerful tool to probe the structure of the Pomeron. All models satisfy GLAP evolution and show a significant Q2 dependence. We give numerical predictions for single as well as double diffractive cross sections assuming a Donnachie-Landshoff-type Pomeron flux factor.

Can partons describe the CDF jet data?

The recent CDF single jet inclusive measurements at Fermilab are incorporated in a global next-to-leading order parton analysis of the available deep inelastic and related data. Using physically motivated parametric forms of the input parton distributions, we find that it is impossible to achieve a simultaneous QCD description of both the CDF jet distribution for transverse energies E T > 200 GeV and the deep inelastic structure function data for x > 0.3. However, the CDF data for E T < 200 GeV and the deep inelastic data are adequately described provided that the QCD coupling α S ( M Z 2) is increased from its preferred deep inelastic value to α S(M Z 2) ⋍ 0.116−0.120 .

Phenomenology of proton structure

Recent progress in translating the increased precision and volume of deep inelastic data and information from hadronic collisions into parton distribution functions (PDFs) is reviewed. We discuss how small-x structure function (SF) data may be able to shed light on which large logarithms are relevant.

Structure functions and parton distributions

We review recent developments in the determination of parton densities from deep inelastic and related data. We show how the asymmetries observed in theW ± rapidity distributions and inpp/pn Drell-Yan production further constrain the partons at moderatex.

Renormalization of hadronic diffraction and the structure of the pomeron

A phenomenological renormalization scheme for hadronic diffraction is proposed, which unitarizes the triple-pomeron Regge amplitude while preserving its M 2 and t dependence. Predictions for pp p p single diffractive, double diffractive and double pomeron exchange cross sections are presented and compared with experimental results. A new interpretation of hard and deep inelastic diffractive data emerges in which the momentum sum rule is obeyed by the constituents of a pomeron described as a mixture of quark and gluon color singlets in a ratio dictated by asymptopia.

Spin-dependent parton distributions from polarized structure function data

In the past year, polarized deep inelastic scattering experiments at CERN and SLAC have obtained structure function measurements off proton, neutron and deuteron targets at a level of precision never before achieved. The measurements can be used to test the Bjorken and Ellis-Jaffe sum rules, and also to obtain information on the parton distributions in polarized nucleons. We perform a global leading-order QCD fit to the proton deep inelastic data in order to extract the spin-dependent parton distributions. By using parametric forms which are consistent with theoretical expectations at large and small $x$, we find that the quark distributions are now rather well constrained. We assume that there is no significant intrinsic polarization of the strange quark sea. The data are then consistent with a modest amount of the proton's spin carried by the gluon, although the shape of the gluon distribution is not well constrained, and several qualitatively different shapes are suggested. The spin-dependent distributions we obtain can be used as input to phenomenological studies for future polarized hadron-hadron and lepton-hadron colliders.

The αS dependence of parton distributions

We perform next-to-leading order global analyses of deep inelastic and related data for different fixed values of α S ( M Z 2). We present sets of parton distributions for six values of α S in the range 0.105 to 0.130. We display the (x, Q 2) domains with the largest parton uncertainty and we discuss how forthcoming data may be able to improve the determination of the parton densities.

Parton distributions of the proton.

To obtain improved parton densities of the proton, we present a new global analysis of deep inelastic and related data including, in particular, the recent measurements of $F_2$ at HERA, of the asymmetry of the rapidity distributions of $W^\pm$ production at the FNAL $pp$ collider and of the asymmetry in Drell-Yan production in $pp$ and $pn$ collisions. We also incorporate data to determine the flavour dependence of the quark sea distributions. We find that the behaviour of the partons at small $x$ is consistent with the precocious onset of BFKL leading $\log(1/x)$ dynamics. We discuss the ambiguities remaining in the gluon distribution. We present improved predictions for $W$ boson (and $t$ quark) production at the FNAL $\pp$ collider.

Improved QCD sum rule estimates of the higher twist contributions to polarised and unpolarised nucleon structure functions

We re-examine the estimates of the higher twist contributions to the integral of g 1, the polarised structure function of the nucleon, based on QCD sum rules. By including corrections both to the perturbative contribution and to the low energy contribution we find that the matrix elements of the relevant operators are more stable to variations of the Borel parameter M 2, allowing for a meaningful estimate of the matrix elements. We find that these matrix elements are typically twice as large as previous estimates. However, inserting these new estimates into the recently corrected expressions for the first moments of g 1 leads to corrections too small to affect the phenomenological analysis. For the unpolarised case the higher twist corrections to the GLS and Bjorken sum rules are substantial and bring the estimate of Λ QCD from the former into good agreement with that obtained from the Q 2 dependence of deep inelastic data.

Problem of the statistical model in deep inelastic scattering phenomenology

Recent deep inelastic data leads to an up-down quark asymmetry of the nucleon sea. Explanations of the flavour asymmetry and the di-lepton production in proton-nucleus collisions call for a temperature T ≈ 100 MeV in a statistical model. This T may be conjectured as being due to the Fulling-Davies-Unruh effect. But it is not possible to fit the structure function itself.

Parton distributions updated

We refine our recent determination of parton distributions with the inclusion of the new published sets of precise muon and neutrino deep inelastic data. Deuteron screening effects are incorporated. The t t ̄ cross section at the FNAL p p ̄ collider is calculated.