$J/\psi$-meson production within improved color evaporation model with the $k_T$-factorization approach for $c\overline{c}$ production

We use a new approach for the color evaporation model for quarkonium production. The production of $c\overline{c}$ pairs is performed within the $k_T$-factorization approach using different unintegrated gluon distribution functions (UGDF) from the literature. We include all recent improvements to the color evaporation model. We cannot describe simultaneously midrapidity and forward rapidity data measured at the LHC when using the Kimber-Martin-Ryskin (KMR) UGDF based on the collinear MMHT2014lo parton distribution function (PDF) with the same normalization parameter. Furthermore, we get a somewhat too hard distribution in the $J/\psi$ transverse momentum. Correcting the standard KMR-MMHT2014lo distributions for saturation effects at small values of $x$ improves $J/\psi$ rapidity distributions. When using CT14lo collinear PDFs a better agreement with the LHCb data can be achieved without a clear need for implementing saturation effects. We get a poor description of the large transverse momentum distributions of $J/\psi$ with the JH-2013 Ciafaloni-Catani-Fiorani-Marchesini (CCFM)-based UGDF. Here an explicit inclusion of the $2\to 3$ processes considerably improves the situation. Similar effects are discussed in the context of the KMR UGDF.

Figure 1

A diagrammatic representation of the $g^{*}{g}^{*}\to c\overline{c}$ (left) and $g^{*}{g}^{*}\to gc\overline{c}$ (right) mechanisms under consideration.

Figure 2

Distributions in rapidity and transverse momentum of $D$ meson for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization approach.

Figure 3

Distributions in rapidity and transverse momentum of prompt $J/\psi$ for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization realization of the ICEM for different UGDFs. The LHCb data were measured on only one side of $y=0$. We have added them symmetrically on the other side as is often done in the literature.

Figure 4

The double-differential cross section as a function of the transverse momentum of initial gluon $k_t$ and the transverse momentum of the meson $p_T^{J/\psi }$ for the KMR (left) and for the JH-2013-set2 (right) UGDFs.

Figure 5

Distributions in the transverse momentum of prompt $J/\psi$ for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization realization of the improved (solid lines) and original (dashed lines) color evaporation model.

Figure 6

The same as in Fig. 3 but for different collinear PDFs used in the calculation of the KMR UGDF.

Figure 7

The same as in Fig. 3 but for different choices of the renormalization and factorization scales specified in the figure. Here the probabilities are adjusted separately for each choice of the scale.

Figure 8

Distributions in rapidity and transverse momentum of prompt $J/\psi$ for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization realization of the ICEM. The shaded bands represent uncertainties of our model due to the variation of the default set of the scales and of the charm quark mass. Here $P_{J/\psi }$ is kept unchanged. Details are specified in the figure.

Figure 9

Distributions in transverse momentum of the $J/\psi$ meson for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization realization of the ICEM for different UGDFs. Here, the shaded bands represent the uncertainty related to the choice of the factorization scale only.

Figure 10

Distributions in rapidity and transverse momentum of prompt $J/\psi$ for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization realization of the ICEM for different UGDFs. Here, both the $g^{*}{g}^{*}\to c\overline{c}$ and the $g^{*}{g}^{*}\to gc\overline{c}$ mechanisms are taken into account.

Figure 11

Distributions in the transverse momentum of the prompt $J/\psi$ at midrapidities for $\sqrt{s}=7\mathrm{TeV}$ together with the ALICE, ATLAS, and CMS data (left panel) and for $\sqrt{s}=1.96\mathrm{TeV}$ together with the CDF data (right panel). The theoretical predictions are obtained within the $k_T$-factorization realization of the ICEM.

Figure 12

Distributions in transverse momentum of prompt $J/\psi$ at midrapidities for $\sqrt{s}=7\mathrm{TeV}$ together with the ALICE, ATLAS, and CMS data for the JH-2013-set2 (left panel) and the KMR-CT14lo (right panel) UGDFs. Here, both the $g^{*}{g}^{*}\to c\overline{c}$ and the $g^{*}{g}^{*}\to gc\overline{c}$ mechanisms are taken into account.

Figure 13

Distributions in rapidity and transverse momentum of prompt $J/\psi$ for $\sqrt{s}=7\mathrm{TeV}$ obtained within the $k_T$-factorization realization of the ICEM for the KMR-MMHT2014lo UGDFs modified for saturation effects.