Antiquark Densities Research Articles

Matter, Dark Matter and Quartic Potential Generated by Unstable Confinement of Quarks

The Standard Model for particle physics is here extended by making a non-equilibrium filling of the empty vacuum after the start of Big Bang. The process is described as an unstable binding of massless quarks to massless antiquarks. When the filled part of vacuum condenses, the system becomes stabilized, quarks acquire mass and become confined and a quartic potential is induced, which hence need not be introduced ad hoc. The coupling and scale parameters in this potential have become asymmetric microscopic functions of the quark and antiquark densities. The so obtained dynamics can explain how the matter-antimatter asymmetry in the Universe and dark matter emerged. Quantum corrections are included and the model then gives ordinary matter, dark matter and dark energy contents at correct orders of magnitude.

Strange quark suppression from a simultaneous Monte Carlo analysis of parton distributions and fragmentation functions

We perform the first simultaneous extraction of unpolarized parton distributions and fragmentation functions from a Monte Carlo analysis of inclusive and semi-inclusive deep-inelastic scattering, Drell-Yan lepton-pair production, and single-inclusive $e^+ e^-$ annihilation data. We use data resampling techniques to thoroughly explore the Bayesian posterior distribution of the extracted functions, and use $k$-means clustering on the parameter samples to identify the configurations that give the best description across all reactions. Inclusion of the semi-inclusive data reveals a strong suppression of the strange quark distribution at parton momentum fractions $x \gtrsim 0.01$, in contrast with the ATLAS observation of enhanced strangeness in $W^\pm$ and $Z$ production at the LHC. Our study reveals significant correlations between the strange quark density and the strange $\to$ kaon fragmentation function needed to simultaneously describe semi-inclusive $K^\pm$ production data from COMPASS and inclusive $K^\pm$ spectra in $e^+ e^-$ annihilation from ALEPH and SLD, as well as between the strange and light antiquark densities in the proton.

Gluons and sea quarks in the proton at low scales

We study the evolution of parton distributions down to low scales by considering several of their Mellin moments. For the initial conditions, we use a broad array of current parton density fits. Confirming earlier findings in the literature, we conclude that current determinations of parton distributions are incompatible with the idea that gluon or antiquark densities are generated by purely perturbative radiation as it is encoded in the DGLAP evolution equations.

Phenomenology of leading nucleon production in ep collisions at HERA in the framework of fracture functions

In recent years, several experiments at the $e^-p$ collider HERA have collected high precision deep inelastic scattering (DIS) data on the spectrum of leading nucleon carrying a large fraction of the proton's energy. In this paper, we have analyzed recent experimental data on the production of forward proton and neutron in DIS at HERA in the framework of a perturbative QCD. We propose a technique based on the fractures functions framework, and extract the nucleon fracture functions (nucleon FFs) ${\cal M}_2^{(n/p)} (x, Q^2; x_L)$ from global QCD analysis of DIS data measured by ZEUS collaboration at HERA. We have shown that an approach based on the fracture functions formalism allows us phenomenologically parametrize the nucleon FFs. Considering both leading neutron as well as leading proton production data at HERA, we present the results for the separate parton distributions for all parton species, including valence quark densities, the anti-quark densities, the strange sea distribution, and the gluon distribution functions. We proposed several parameterizations for the nucleon FFs and open the possibility of these asymmetries. The obtained optimum set of nucleon FFs is accompanied by Hessian uncertainty sets which allow one to propagate uncertainties to other observables interest. The extracted results for the $t$-integrated leading neutron $F_2^{\rm LN(3)} (x, Q^2; x_L)$ and leading proton $F_2^{\rm LP(3)} (x, Q^2; x_L)$ structure functions are in good agreement with all data analyzed, for a wide range of fractional momentum variable $x$ as well as the longitudinal momentum fraction $x_L$.

Asymmetric sea quark and gluon densities, based on the polarized chiral quark model

We investigate the polarized structure of the proton based on the modified chiral quark model (). We calculate the polarized quark, antiquark and gluon densities inside the proton at low Q2 values using the . We then evolve these densities to high energy scales. The polarized antiquark and gluon distributions are obtained using the . These distributions can be calculated in our model, considering the gluon cloud which surrounds the bare quarks. This is a new feature which was not taken into account previously in the polarized . Using the ‘Pauli-blocking’ ansatz, we are able to extract asymmetric antiquark densities which are flavor dependent. It is shown that our results are in good agreement with the experimental data and also compatible with the different fitting parameterizations. What makes our results more distinctive is that all initial parton densities are obtained via a completely theoretical framework in which we do not need to resort to any specific parameterizations.

Symmetry breaking effect on determination of polarized and unpolarized parton distributions

We perform a new extraction for unpolarized and polarized parton distribution functions considering a flavor decompositions for sea quarks and applying very recent deep inelastic scattering (DIS) and semi inclusive deep inelastic scattering (SIDIS) data in the fixed flavor number scheme (FFNS) framework. In the new symmetry breaking scenario the light quark and antiquark densities are extracted separately and new parametrization forms are determined for them. The heavy flavors contribution, including charm and bottom quarks, are also taken to be account for unpolarized distributions.

NNLO corrections to massive lepton-pair production in longitudinally polarized proton–proton collisions

We present the full next-to-next-to-leading order (NNLO) coefficient functions for the polarized cross section dΔ σ/ dQ for the Drell–Yan process p+ p→ l + l −+′ X′. Here ′ X′ denotes any inclusive hadronic state and Q represents the invariant mass of the lepton pair. All QCD partonic subprocesses have been included provided the lepton pair is created by a virtual photon, which is a valid approximation for Q<50 GeV. Unlike the differential distribution w.r.t. transverse momentum the dominant subprocess for the integrated cross section is given by q+ q ̄ →γ ∗+′X′ and its higher order corrections so that massive lepton pair production provides us with an excellent tool to measure the polarized anti-quark densities. Our calculations are carried out using the method of n-dimensional regularization by making a special choice for the γ 5 matrix. We give predictions for double longitudinal spin asymmetry measurements at the RHIC.

A QCD analysis of HERA and fixed target structure function data

The parton momentum densities in the proton are determined from a NLO QCD analysis of structure functions measured by HERA and fixed target experiments. Also included are data on the difference of the up and down anti-quark densities. The uncertainties in the parton densities, structure functions and related cross sections are estimated from the experimental errors, taking into account all correlations. Standard Model predictions of the charged current Born cross sections at large x and Q 2 are calculated and compared with recent data from ZEUS.

Dynamical generation of the constituent mass in expanding plasma

We investigate the dynamics of the chiral transition in expanding quark-antiquark plasma produced in an ultra-relativistic heavy ion collision. The chiral symmetry break-down and dynamical generation of the constitutent quark mass are studied within the linear sigma model and Nambu-Jona-Lasinio model. The time dependence of the quark and antiquark densities is obtained from the scaling solution of the relativistic Vlasov equation. Fast initial growth and strong oscillations of the constituent quark mass are found in the linear sigma model as well as in the NJL model, when derivative terms are taken into account.

Scaling violations for the spin-dependent structure function G1

With the formalism of Altarelli and Parisi, we study scaling violations for the spin dependent structure function G 1. We find that the variation of G 1 with Q 2 is essentially determined by that of the valence quarks helicity distributions, the antiquark densities contributing negligibly.