Quark Distribution Functions Research Articles

Parton distributions: a study of the new HERA data, αs, the gluon and [formula omitted] jet production

New data, especially HERA measurements of the proton structure function at small x, allow the opportunity to improve our knowledge of the gluon and quark distribution functions. We perform a global analysis which incorporates these new precise data, and which extends down to Q 2 = 1.5 GeV 2. We discuss the sensitivity to the value of α s and the improvement in the determination of the gluon. We compare the predictions of the single jet inclusive cross section with recent measurements from the Fermilab Tevatron.

QCD corrections to W boson plus heavy quark production at the Tevatron

The next-to-leading order QCD correction to the production of a W boson in association with a jet containing a heavy quark are presented. The calculation is fully differential in the final state particle momenta and includes the mass of the heavy quark. We study for the case of the Tevatron the sensitivity of the cross section to the strange quark distribution function, the dependence of the cross section on the heavy quark mass, the transverse momentum distribution of the jet containing the heavy quark, and the momentum distribution of the heavy quark in the jet.

Modelling the nucleon wave function from soft and hard processes

Current light-cone wave functions for the nucleon are unsatisfactory since they are in conflict with the data of the nucleon's Dirac form factor at large momentum transfer. Therefore, we attempt a determination of a new wave function respecting theoretical ideas on its parameterization and satisfying the following constraints: It should provide a soft Feynman contribution to the proton's form factor in agreement with data; it should be consistent with current parameterizations of the valence quark distribution functions and lastly it should provide an acceptable value for the $\jp \to N \bar N$ decay width. The latter process is calculated within the modified perturbative approach to hard exclusive reactions. A simultaneous fit to the three sets of data leads to a wave function whose $x$-dependent part, the distribution amplitude, shows the same type of asymmetry as those distribution amplitudes constrained by QCD sum rules. The asymmetry is however much more moderate as in those amplitudes. Our distribution amplitude resembles the asymptotic one in shape but the position of the maximum is somewhat shifted.

Quark model description of the N-N system: Momentum distributions, structure functions, and the EMC effect.

We employ a relativistic quark bag picture, the chromo-dielectric soliton model, to discuss the quarks' symmetry structure and momentum distribution in the $N$-$N$ system. Six-quark clusters are constructed in a constrained mean-field calculation. The corresponding Hamiltonian contains not only an effective interaction between the quarks and a scalar field, which is assumed to parameterize all non-perturbative effects due to the non-linearity of QCD, but also quark-quark interactions mediated through one-gluon-exchange. We also evaluate the quark light-cone distribution functions, characterizing inclusive deep-inelastic lepton scattering, for the nucleon as well as for the six-quark structures. We find a competition between a softening of the quarks' momenta through the increase of the confinement volume, and a hardening via the admixture of higher symmetry configurations due to the color-electrostatic one-gluon-exchange. These findings suggest an unexpected absence of many-nucleon, multi-quark effects, even though six-quark structures should represent a non-negligible part of the nuclear ground state.

New quark distributions and semi-inclusive electroproduction on polarized nucleons

The quark-parton model calculation including the effects of intrinsic transverse momentum and of all six twist-two distribution functions of quarks in polarized nucleons is performed. It is demonstrated that new twist-two quark distribution functions and polarized quark fragmentation functions can be investigated in semi-inclusive DIS at leading order in Q 2. The general expression for the cross-section of semi-inclusive DIS of polarized leptons on polarized nucleons in terms of structure functions is also discussed.

Hadron structure and left-right asymmetry in inclusive production in single-spin hadron-hadron collisions.

Theoretical arguments and experimental facts are presented which show the following. Left-right asymmetries are expected to exist in a number of single-spin inclusive production processes. Measuring such asymmetries by using different types of projectile-target combinations, one can obtain useful information on the spin structure of hadrons in general, and determine the spin-dependent quark distribution functions in particular. Quantitative predictions for these asymmetries in various reactions are presented.

Intrinsic charm contribution to [formula omitted] plus open charm associate hadroproduction at high energies

We present the results of a calculation of the intrinsic charm quark contribution to J ψ plus open charm associated production at large transverse momentum in hadron collisions at high energies. It is shown that the value of the cross section for such process and its energy dependence significantly determine by the choice of charm quark distribution function. We found that intrinsic charm contribution to inclusive J ψ hadroproduction at high energies is about several per cent of the total contribution of all mechanisms.

Large time-scale fluctuations of the quark condensate at high temperature.

The transport theory for a quark-antiquark plasma is used to study large time-scale fluctuations of the quark condensate. The interactions between quarks are assumed to be of the Nambu--Jona-Lasino type and we restrict ourselves to the mean field (Vlasov) approximation. The space-time behavior of the fluctuations of the mean field and, consequently, of the condensate depends strongly on the form of the initial conditions for the quark distribution function. If the latter can be initially represented as a sum of the thermal background distribution plus a perturbation which is broad in momentum, then the fluctuations are suppressed in time by a mechanism analogous to that of Landau damping. On the other hand, if we add a perturbation which is peaked in momentum to the thermal background distribution, the fluctuations are not damped and behave similarly to Van Dampen modes in ordinary plasmas.

Infrared factorization in inclusive B meson decays

We perform infrared factorization of differential rates of rediative and semileptonic inclusive decays of the B meson in the end-point region of photon and charged lepton spectrum, respectively, in the leading heavy quark mass limit. We find that the differential rates are expressed in terms of hard, jet and soft functions, which satisfy evolution equations. Solving these equations, we find expressions for the moments of the differential rates in their end-point regions, which take into account all leading and nonleading logarithmic corrections in perturbation theory, as well as large nonperturbative power corrections. Expanded to the one-loop level, our predictions coincide with the results of existing lowest order calculations for B→ γX s and B→l vX u . Nonperturbative corrections appear in our formalism from the boundary value of the soft function in the evolution equation. The soft function is a universal process-independent function, which describes the distribution of the b quark in the B meson. Its behavior in the end-point region is governed by the nonperturbative asymptotics of a Wilson line expectation value. By considering the contributions of infrared renormalons, we find an ansatz for the Wilson line, which leads to a Gaussian model for the heavy quark distribution function.

Quark dynamics and spin structure in the chiral chromodielectric model

The dynamical structure of the nucleon is studied in the chiral version of the chromodielectric model. The color-dielectric field and the meson clouds are described by hedgehog coherent states. Standard projection techniques are used to construct zero-linear-momentum eigenstates with the nucleon quantum numbers of angular momentum and isospin. Both the unpolarized and the polarized quark distribution functions are computed. Results are in good agreement with the data and a noticeable improvement with respect to the predictions of the non-chiral model is observed.

Erratum: Model calculation of nucleon structure functions

Deep inelastic polarized and unpolarized structure functions for a free nucleon are obtained in a modified Center-of-Mass bag model, which includes the symmetry breaking effects from spin-dependent interactions. The quark distribution functions, calculated at $Q_0^2\simeq$ (0.9GeV/c)$^2$, are evolved to higher $Q^2$ region and compared with the data and other models. The model gives a reasonable description for the valence part of the structure functions at $x>0.3$. For small-$x$ region, the contributions from the sea are necessary. The spin-dependent effects are important in describing the existing data.

Measurement of the double-spin asymmetry ALL for inclusive multi-γ pair production with 200 GeV/ c polarized proton beam and polarized proton target

The invariant double-differential cross section, E 1E 2d 6σ / d p 3 1d p 3 2 , and the double-spin asymmetry, A LL , for inclusive multi-γ pair production in which γ-rays came from neutral mesons were measured with a 200 GeV/ c longitudinally-polarized proton beam and a longitudinally-polarized proton target. Most of the multi-γ pairs comes from two-jet type events which are sensitive to partonic interaction. The A LL values were found to be consistent with zero. The invariant double-differential cross section for inclusive π 0 π 0 production was also measured. These measured cross sections are consistent with LUND Monte Carlo simulations. Using the LUND Monte Carlo simulation package with the Carlitz-Kaur model of spin dependent distribution functions of valence quarks, the A LL values have been compared with theoretical predictions of gluon polarization, ΔG/ G. The results put restrictions on the size of ΔG/ G in the region of 0.05 ⪅ x ⪅ 0.35.

Green's function in the color field of a large nucleus.

We compute the Green's functions for scalars, fermions and vectors in the color field associated with the infinite momentum frame wavefunction of a large nucleus. Expectation values of this wavefunction can be computed by integrating over random orientations of the valence quark charge density. This relates the Green's functions to correlation functions of a two dimensional, ultraviolet finite, field theory. We show how one can compute the sea quark distribution functions, and explictly compute them in the kinematic range of transverse momenta, $\alpha_s^2 \mu^2 << k_t^2 << \mu^2$, where $\mu^2$ is the average color charge squared per unit area. When $m_{quark}^2 << \mu^2 \sim A^{1/3}$, the sea quark contribution to the infinite momentum frame wave function saturates at a value that is the same as that for massless sea quarks.

Heavy quark distribution function in QCD and the AC 2M 2 model

We show the phenomenological AC 2M 2 ansatz to be consistent with QCD through order 1/ m b in the description of B → l v l + X u and B → γ + X s transitions, including their energy spectra and differential distributions. This suggests a concrete realization of the QCD distribution function, which we call the “Roman” function. Yet the AC 2M 2 description of the end-point domain in B → l v l + X c is, formally speaking, incompatible with QCD: a different distribution function enters the description of b → c decays as compared to the transitions to massless quarks. The intrinsic limitation of the AC 2M 2 model could reveal itself in different values of the effective b quark mass from fits of the Λ b and B decays.

Model calculation of nucleon structure functions.

Deep inelastic polarized and unpolarized structure functions for a free nucleon are obtained in a modified center-of-mass bag model, which includes the symmetry-breaking effects from spin-dependent interactions. The quark distribution functions, calculated at ${Q}_{0}^{2}\ensuremath{\simeq}{(0.9 \frac{\mathrm{GeV}}{c})}^{2}$, are evolved to a higher ${Q}^{2}$ region and compared with the data and other models. The model gives a reasonable description for the valence part of the structure functions at $x&gt;0.3$. For the small-$x$ region, the contributions from the sea are necessary. The spin-dependent effects are important in describing the existing data.

Computing quark and gluon distribution functions for very large nuclei.

We argue that the distribution functions for quarks and gluons are computable at small x for sufficiently large nuclei, perhaps larger than can be physically realized. For such nuclei, we argue that weak coupling methods may be used. We show that the computation of the distribution functions can be recast as a many body problem with a modified propagator, a coupling constant which depends on the multiplicity of particles per unit rapidity per unit area, and for non-abelian gauge theories, some extra media dependent vertices. We explicitly compute the distribution function for gluons to lowest order, and argue how they may be computed in higher order.

Can Magnetic Moment of Nucleons Be Correlated with Quark Distribution Functions?

We show that the magnetic moment of nucleons should not be correlated with the quark distribution functions, Δq. The assumption inherent in such correlations is that the strange sea polarization is zero in a proton.

Can Magnetic Moment of Nucleons Be Correlated with Quark Distribution Functions?

We show that the magnetic moment of nucleons should not be correlated with the quark distribution functions, Llq. The assumption inherent in such correlations is that the strange sea polarization is zero in a proton.

Quark correlation functions in deep-inelastic semi-inclusive processes.

We investigate one-particle semi-inclusive processes in lepton-hadron scattering. In unpolarized scattering order $Q^{-1}$ corrections appear only when transverse momenta are detected. We consider the twist two and three matrix elements and calculate the semi-inclusive structure functions in terms of quark correlation functions. We find that at twist three level not only the standard quark distribution and fragmentation function contribute, but also two new transverse "profile functions". We demonstrate explicit gauge invariance of the hadronic tensor at the twist three level. The results of our approach are used to calculate expressions for some cross sections for semi-inclusive processes.

Direct vs. resolved photon. An exercise in factorization

A theoretically consistent analysis of jet production in γp and ep collisions, different from the conventional approach in which the quark distribution function inside the photon is assumed to behave like O( α/ αs), is proposed. This analysis is based on the observation that direct and resolved photon interactions are intimately related through the factorization mechanism. A brief discussion of its implications for the interpretation of recent data from HERA is given.