Diffractive J/Psi production in high energy gamma-gamma collisions as a probe of the QCD pomeron

Abstract

The reaction γγ---> J/Psi J/Psi is discussed assuming dominance of the QCD BFKL pomeron exchange. We give prediction for the cross-section of this process for LEP2 and TESLA energies. We solve the BFKL equation in the non-forward configuration taking into account dominant non-leading effects which come from the requirement that the virtuality of the exchanged gluons along the gluon ladder is controlled by their transverse momentum squared. We compare our results with those corresponding to the simple two gluon exchange mechanism and with the BFKL pomeron exchange in the leading logarithmic approximation. The BFKL effects are found to generate a steeper t-dependence than the two gluon exchange. The cross-section is found to increase with increasing CM energy W as (W^2)^{2λ}. The parameter λis slowly varying with W and takes the values lambda \sim 0.23 - 0.28. The magnitude of the total cross-section for the process γγ--> J/Psi J/Psi is found to increase from 4 to 26 pb within the energy range accessible at LEP2. The magnitude of the total cross-section for the process e^+e^- ---> e^+e^- J/Psi J/Psi with antitagged e^+ and e^- is estimated to be around 0.1 pb at LEP2.

Figures (4)

• Figure 1: The QCD pomeron exchange mechanism of the process $\gamma \gamma \rightarrow J/\Psi J/\Psi$.
• Figure 2: The diagrams describing the coupling of two gluons to the $\gamma \rightarrow J/\Psi$ transition vertex.
• Figure 3: Energy dependence of the cross-section for the process $\gamma\gamma \rightarrow J/\Psi J/\Psi$. The two lower curves correspond to the calculations based on equation (\ref{['bfklkc0']}) which contains the non-leading effects coming from the constraint (\ref{['kc2']}). The continuous line corresponds to $s_0=0.04 {\rm \; GeV}^2$ and the dashed line to $s_0=0.16 {\rm \; GeV}^2$. The two upper curves correspond to equation (\ref{['bfkl']}) i.e. to the BFKL equation in the leading logarithmic approximation. The dashed-dotted line corresponds to $s_0=0.04 {\rm \; GeV}^2$ and short dashed line to $s_0=0.16{\rm \; GeV}^2$.
• Figure 4: The differential cross-section of the process $\gamma \gamma \rightarrow J/\Psi J/\Psi$ corresponding to the solution of equation (\ref{['bfklkc0']}) which contains the non-leading effects coming from the kinematical constraint (\ref{['kc2']}) shown for two values of the CM energy $W$, $W=50{\rm \; GeV}$ (continuous line) and $W=125 {\rm \; GeV}$ (dashed line). The short dashed line corresponds to the Born term i.e. to the elementary two gluon exchange mechanism which gives the energy independent cross-section. The parameter $s_0$ was set equal to $0.10{\rm \; GeV}^2$.