J/$\psi$ and other Hidden Heavy Flavours

ISUB =

86	$\mathrm{g}\mathrm{g}\to \mathrm{J}/\psi \mathrm{g}$
87	$\mathrm{g}\mathrm{g}\to \chi_{0 \c } \mathrm{g}$
88	$\mathrm{g}\mathrm{g}\to \chi_{1 \c } \mathrm{g}$
89	$\mathrm{g}\mathrm{g}\to \chi_{2 \c } \mathrm{g}$
104	$\mathrm{g}\mathrm{g}\to \chi_{0 \c }$
105	$\mathrm{g}\mathrm{g}\to \chi_{2 \c }$
106	$\mathrm{g}\mathrm{g}\to \mathrm{J}/\psi \gamma$
107	$\mathrm{g}\gamma \to \mathrm{J}/\psi \mathrm{g}$
108	$\gamma \gamma \to \mathrm{J}/\psi \gamma$

In PYTHIA one may distinguish between three main sources of $\mathrm{J}/\psi$ production.

75.

Decays of $\mathrm{B}$ mesons and baryons.

76.

Parton-shower evolution, wherein a $\c$ and a $\overline{\mathrm{c}}$ quark produced in adjacent branchings (e.g. $\mathrm{g}\to \mathrm{g}\mathrm{g}\to \c\overline{\mathrm{c}}\c\overline{\mathrm{c}}$) turn out to have so small an invariant mass that the pair collapses to a single particle.

77.

Direct production, where a $\c$ quark loop gives a coupling between a set of gluons and a $\c\overline{\mathrm{c}}$ bound state. Higher-lying states, like the $\chi_c$ ones, may subsequently decay to $\mathrm{J}/\psi$.

The first two sources are implicit in the production of $\b$ and $\c$ quarks, although the forcing specifically of $\mathrm{J}/\psi$ production is difficult. In this section are given the main processes for the third source, intended for applications at hadron colliders. Processes 104 and 105 are the equivalents of 87 and 89 in the limit of $p_{\perp}\to 0$; note that $\mathrm{g}\mathrm{g}\to \mathrm{J}/\psi$ and $\mathrm{g}\mathrm{g}\to \chi_{1 \c }$ are forbidden and thus absent. As always one should beware of double-counting between 87 and 104, and between 89 and 105, and thus use either the one or the other depending on the kinematical domain to be studied. The cross sections depend on wave function values at the origin, see PARP(38) and PARP(39). A review of the physics issues involved may be found in [Glo88] (note, however, that the choice of $Q^2$ scale is different in PYTHIA).

It is known that the above sources are not enough to explain the full $\mathrm{J}/\psi$ rate, and further production mechanisms have been proposed, extending on the more conventional treatment here [Can97].

While programmed for the charm system, it would be straightforward to apply these processes instead to bottom mesons. One needs to change the codes of states produced, which is achieved by KFPR(ISUB,1)=KFPR(ISUB,1)+110 for the processes ISUB above, and changing the values of the wave functions at the origin, PARP(38) and PARP(39).