Eur. Phys. J. C (2014) 74: 3214
https://doi.org/10.1140/epjc/s10052-014-3214-7

Regular Article - Theoretical Physics

Double parton interactions in $$\gamma p, \gamma A$$ collisions in the direct photon kinematics

¹ Department of Physics, Technion-Israel Institute of Technology, Haifa, Israel
² Physics Department, Penn State University, University Park, PA, USA

^* e-mail: blok@physics.technion.ac.il

Received: 23 October 2014
Accepted: 1 December 2014
Published online: 17 December 2014

Abstract

We derive expressions for the differential distributions and the total cross section of the double-parton interaction in direct photon interaction with proton and nuclei. We demonstrate that in this case the cross section is more directly related to the nucleon generalized parton distribution than in the case of double-parton interactions in the proton–proton collisions. We focus on the production of two dijets each containing charm (anticharm) quarks and carrying $$x_1,x_2>0.2$$ fractions of the photon momentum. Numerical results are presented for the $$e$$–$$p$$ collisions at LHeC, HERA, and for the ultraperipheral $$AA$$ and $$pA$$ collisions at the LHC. We find that the events of this kind would be abundantly produced at the LHeC. For $$\sqrt{s}=1.3$$ TeV the expected rate is $$2\times 10^9$$ events for 1 year ($$10^7$$ s) of running and the luminosity $$10^{34}$$ cm$$^{-2}~\hbox {s}^{-1}$$, for the transverse cutoff of $$p_t>5$$ GeV. This would make it feasible to use these processes for the model independent determination of two parton GPDs in nucleon and in nuclei. We also find that a significant number of such double-parton interactions should be produced in $$p$$–$$Pb$$ and $$Pb$$–$$Pb$$ collisions at the LHC: $${\sim } 6\times 10^4$$ for $$Pb$$–$$Pb$$, and $${\sim } 7 \times 10^3$$ for $$p$$–$$Pb$$ collisions for the same transverse momentum cutoff and running time $$10^{6}~\mathrm{s}$$, and in HERA where $$1.2\times 10^5$$ events were produced for the integrated HERA luminosity of 1.2 fb$$^{-1}$$. Further studies are necessary to identify the kinematics where these MPIs could be separated from conventional 2-to-4 multijet events.