Double parton scattering in pair production of $J/\psi$ mesons at the LHC revisited

Double parton scattering (DPS) is studied for the example of $J/\psi$ pair production in the LHCb and ATLAS experiments of the Large Hadron Collider (LHC) at center-of-mass energies of $\sqrt{S}=7$, 8, and 13 TeV. We report theoretical predictions delivered to the LHCb and ATLAS Collaborations adjusted for the fiducial volumes of the corresponding measurements during run I, and we provide new predictions at 13 TeV collision energy. It is shown that DPS can lead to noticeable contributions in the distributions of longitudinal variables of the $\text{di-}J/\psi$ system, especially at 13 TeV. The increased DPS rate in double $J/\psi$ production at high energies will open up more possibilities for the separation of single parton scattering and DPS contributions in future studies.

Figure 1

Schematical representation of SPS (left) and DPS (right) for a proton-proton collision. Whereas in the SPS case, the two final-state particles (grey ellipses) originate from the same scattering process (dark grey circle), in the DPS case, two scattering processes of two independent partons from each proton occur.

Figure 2

Total cross section dependence of the center-of-mass energy of the collider for the LHCb (a) and the ATLAS cuts (b). Shown are the central values of Table 1 for the SPS prediction with parton showering and intrinsic transverse momentum of the initial-state partons and the DPS contribution. The lines for SPS are interpolated between the points.

Figure 3

Differential distributions for the LHCb cuts at a collider energy of $\sqrt{S}=7\mathrm{TeV}$. Shown are the transverse momentum of the $\text{di-}J/\psi$ system (a), the azimuthal angular separation (b), the invariant mass spectrum (c), and the rapidity separation (d) for various SPS and DPS predictions (see text for an explanation of the lines).

Figure 4

Differential distributions for the LHCb cuts at a collider energy of $\sqrt{S}=13\mathrm{TeV}$. The distributions are the same as for Fig. 3, with the transverse momentum of the $\text{di-}J/\psi$ system (a), the azimuthal angular separation (b), the invariant mass spectrum (c), and the rapidity separation (d).

Figure 5

Differential distributions for the ATLAS cuts at a collider energy of $\sqrt{S}=8\mathrm{TeV}$. The distributions are the same as for Fig. 3, with the transverse momentum of the $\text{di-}J/\psi$ system (a), the azimuthal angular separation (b), the invariant mass spectrum (c), and the rapidity separation (d).

Figure 6

The distribution showing the transverse momentum of a single $J/\psi$ for the ATLAS cuts at a collider energy of $\sqrt{S}=8\mathrm{TeV}$.

Figure 7

Differential distributions for the ATLAS cuts at a collider energy of $\sqrt{S}=13\mathrm{TeV}$. The distributions are the same as for Fig. 3, with the transverse momentum of the $\text{di-}J/\psi$ system (a), the azimuthal angular separation (b), the invariant mass spectrum (c), and the rapidity separation (d).

Figure 8

Comparison of SPS and DPS predictions to the CMS data at a collider energy of $\sqrt{S}=7\mathrm{TeV}$ for the invariant mass (a), the rapidity separation (b), and the transverse momentum of the $\text{di-}J/\psi$ system (c). Shown are bins which are normalized to the corresponding total cross sections of the different SPS and DPS distributions and the data.

Figure 9

Comparison between our results and [35] for the LHCb cuts. Shown are the transverse momentum distribution of the $\text{di-}J/\psi$ system (a), the rapidity separation (b), and the invariant mass (c). The “SPS NLO (Lansberg et al.)” and “DPS (Lansberg et al.)” uncertainty bands are read off from the corresponding plots in [35]. The cross sections here are not multiplied by the squared branching ratio ${\mathrm{BR}}^2(J/\psi \to 2\mu )$.

Figure 10

Same as Fig. 9, with the transverse momentum distribution of the $\text{di-}J/\psi$ system (a), the rapidity separation (b), and the invariant mass (c), but for the case of the ${\mathrm{ATLAS}}^{*}$ cuts. These differ from the ATLAS cuts defined in Sec. 2c2 by imposing a lower $p_{T,J/\psi }$ cut of $p_{T,J/\psi }>5\mathrm{GeV}$ instead of $p_{T,J/\psi }>8.5\mathrm{GeV}$. Furthermore, the predictions are shown for $\sqrt{S}=7\mathrm{TeV}$ instead of 8 TeV.