Speaker
Description
The matrix elements for $g^* g^* \to \chi_c(J_i) \chi_c(J_j)$
for off-shell gluons were derived [1].
The matrix elements are used then in the $k_t$-factorization
approach for the $p p \to \chi_c(J_i) \chi_c(J_j)$ reaction.
Different combination of the $chi_c$ mesons are considered.
We use the Kimber-Martin-Ryskin (KMR) unintegrated gluon distributions
to evaluate cross sections. We concentrate on large rapidity separation
between $\chi_c$ mesons.
Several differential distributions for a selected value of
the center of mass energy are calculated and shown.
A feed-down from double $\chi_c$ production to the double $J/\psi$
channel is estimated and compared to the dominant direct production
of two $J/\psi$ quarkonia.
This mechanism is important in the context of very small $\sigma_{eff}$
found from the analysis of $J/\psi$ production.
Similar analysis is repeated for the collinear factorization approach
[2]. The leading order contributions (2 $\to$ 2 processes)
are rather small, compared to the $k_t$-factorization result.
We include higher-order contributions of $2 \to 3$ processes with
two $\chi_c$ mesons and one gluon. Several differential distributions
will be presented and discussed. Again we will focus on large rapidity
distances between the $\chi_c$ mesons.
A comparison to the $k_t$-factorization approach will be
made. Conclusions will be formulated.
1) A. Cisek, W. Sch\"afer, A. Szczurek,
Phys. Rev. D97 (2018) 114018.
2) I. Babiarz, W. Sch\"afer and A. Szczurek, arXiv:1902.08426 (hep-ph).
in print in Phys. Rev. D.
