Color Evaporation Model Research Articles

Color evaporation and elasticΥphotoproduction at DESY HERA

The diffractive photoproduction of vector mesons is usually described by considering the two-gluon (Pomeron) exchange, the nondiagonal parton distributions, and the contribution of the real part to the cross section. In this Brief Report we analyze the diffractive photoproduction of the $\ensuremath{\Upsilon}$ at DESY HERA using an alternative model, the color evaporation model (CEM), where the cross section is simply determined by the boson-gluon cross section and an assumption for the production of the colorless state. We verify that, as in the $J/\ensuremath{\psi}$ case, the HERA data for this process can be well described by the CEM. Moreover, we propose the analysis of the ratio $R={\ensuremath{\sigma}}_{\ensuremath{\Upsilon}}/{\ensuremath{\sigma}}_{J/\ensuremath{\psi}}$ to discriminate between the different approaches.

Color evaporation induced rapidity gaps

We show that soft color rearrangement of final states can account for the appearance of rapidity gaps between jets. In the color evaporation model the probability to form a gap is simply determined by the color multiplicity of the final state. This model has no free parameters and reproduces all data obtained by the ZEUS, H1, DØ, and CDF collaborations.

Color evaporation induced rapidity gaps

We show that soft color rearrangement of final states can account for the appearance of rapidity gaps between jets. In the color evaporation model the probability to form a gap is simply determined by the color multiplicity of the final state. This model has no free parameters and reproduces all data obtained by the ZEUS, H1, D\O{}, and CDF Collaborations.

The color evaporation model in diffractive J/ ψ photoproduction

Our goal is the study of color effects on the J/ ψ and D mesons production, as well as the study of their elastic production. We use the color evaporation model (CEM), where a single factor takes into account the probability that the c c produced in a color octet state turns into a physical (colorless) quarkonium or continues as a color octet, producing open charm. The main result of this work, besides investigating the range of applicability of the CEM, is to show that this model can be used to describe the elastic processes of J/ ψ and open charm photoproduction without the use of a Pomeron.

NUCLEAR SHADOWING EFFECTS ON CHARMONIUM PRODUCTION IN QCD

It is shown that the effects on charmonium production of the modification of nucleon parton distributions inside nuclei are similar in the factorization approach based on nonrelativistic QCD and in the color evaporation model. In the first model, a separate study of the color octet and color singlet contributions to the yields of the various charmonium states as well as the contributions of these states to the total J/ψ production is performed. It is found that a clear xF dependence of these contributions which, together with an overall normalization given by absorption of the pre-resonant [Formula: see text] state inside the nuclear medium, can reproduce experimental data a for moderate xF. However, at large xF, there is bigger suppression that cannot be fit by this overall normalization together with the shadowing of structure functions.

Testing quarkonium production with photoproducedJ/ψ+γ

I compute the leading color-octet contributions to the process $\gamma + p \to J/\psi + \gamma (+ X)$ within the non-relativistic QCD (NRQCD) factorization formalism. In the color-singlet model, $J/\psi + \gamma$ can only be produced when the photon interacts through its structure function, while the color-octet mechanism allows for production of $J/\psi + \gamma$ via direct photon-gluon fusion. Resolved photon processes can be easily be distinguished from direct photon processes by examining the fraction of the incident photon energy carried away by the $J/\psi$ in the event. Therefore, this process provides a conclusive test of the color-octet mechanism. $J/\psi + \gamma$ production is particularly sensitive to the NRQCD matrix element which figures prominently in the fragmentation production of $J/\psi$ at large $p_{\perp}$ in hadron colliders. I also examine the predictions of the color evaporation model (CEM) of quarkonium production and find that this process can easily discriminate between the NRQCD factorization formalism and the CEM.

Prompt charmonium production in Z dexays

The color-evaporation model quantitatively describes all data on photoproduction and hadroproduction of charmonium. Although the model is in part nonperturbative, the associated parameters can for instance be determined from the charmonium photoproduction data. At this point its predictions for the prompt production of ψ's at the Z pole are made with no free parameters. We show here that this approach successfully describes all available data on the production of prompt ψ's in Z decays. We also comment on the prompt Z decays into other charmonium states.

Quantitative tests of color evaporation: charmonium production

The color evaporation model simply states that charmonium production is described by the same dynamics as D D production, i.e., by the formation of a colored c c pair. Its color happens to be bleached by soft final-state interactions. We show that the model gives a complete picture of charmonium production including low-energy production by proton, photon and antiproton beams, and high-energy production at the Tevatron and HERA.

Pt dependence of J/ psi enhancement by plasma formation.

$\frac{J}{\ensuremath{\psi}}$ enhancement in relativistic heavy-ion collisions by a quark-gluon plasma is analyzed for all ${P}_{t}$ using the color-evaporation model. At both low and high ${P}_{t}$, appreciable enhancement is found for all plasma temperatures.

InclusiveJψ's ine+e−annihilations

Within the framework of perturbative quantum chromodynamics we calculate the cross section as well as energy and angular distributions for inclusive $\frac{J}{\ensuremath{\psi}}$ production in ${e}^{+}{e}^{\ensuremath{-}}$ annihilations. The correct color-singlet assignment of the $\frac{J}{\ensuremath{\psi}}$ is taken into account. Our results are compared with the predictions of the "color-evaporation model."