Photon and leptons induced processes at the LHC

Abstract

We study a few basic photon- and lepton-initiated processes at the LHC which can be computed using the recently developed photon and lepton parton densities. First, we consider the production of a massive scalar particle initiated by lepton-antilepton annihilation and photon-photon fusion as representative examples of searches of exotic particles. Then we study lepton-lepton scattering, since this Standard-Model process may be observable at the LHC. We examine these processes at leading and next-to-leading order and, using the POWHEG method, we match our calculations to parton shower programs that implement the required lepton or photon initial-states. We assess the typical size of cross-sections and their uncertainties and discuss the preferred choices for the factorization scale. These processes can also be computed starting directly from the lepto-production hadronic tensor, leading to a result where some collinear-enhanced QED corrections are missing, but all strong corrections are included. Thus, we are in the unique position to perform a comparison of results obtained via the factorization approach to a calculation that does not have strong corrections. This is particularly relevant in the case of lepton-scattering, that is more abundant at lower energies where it is affected by larger strong corrections. We thus compute this process also with the hadronic-tensor method, and compare the results with those obtained with POWHEG. Finally, for some lepton-lepton scattering processes, we compare the size of the signal to the main quark-induced background, which is double Drell-Yan production, and outline a preliminary search strategy to enhance the signal to background ratio.

Modelling $W^+ W^-$ production with rapidity gaps at the LHC

“…The key point is that the p → γp vertex can be decomposed at the amplitude level (linearly) in terms of F 1,2 . In particular, we can see that at the small Q 2 i value relevant for elastic scattering the factor of τ i 1 in (47), and indeed numerically it turns out that F 2 is rather subleading with respect to F 1 even at larger Q 2 i . Therefore we can safely drop the second term in (47), and can then see that the factor of…”

“…More precisely, F 1 and F 2 are associated with the amplitudes where the proton helicity is conserved and flipped, respectively. Therefore, one should account for these amplitudes independently, with the incoherent squared sum giving precisely the sum (47). However, the effect of doing this on the survival factor is numerically very close (at the 1% level) to simply working with F E (Q 2 i ) 1/2 directly at the amplitude level, and hence in practice we can do this.…”

“…In [29] this approach was also applied for the first time to the production of a back-to-back same-sign lepton pair of the same flavour, or a lepton pair of differing flavours and arbitrary signs, that is via lepton-lepton scattering. This has subsequently been extended in [47] to include further kinematically subleading contributions 2 .…”

“…The input for the proton structure functions comes from noting that the same density matrix ρ appears in the cross section for lepton-proton scattering. One can therefore make use of 2 The additional diagram included in [47] is suppressed by the back-to-back requirement, as described in [29]. We in addition note that our formulation of the SF approach, as given in terms of the photon density matrix ρ, is taken for consistency with the original work of [44] (see in particular (5.1) and Appendix D); however, as discussed below (2) the integral over Mi is understood as being performed simultaneously with the phase space integral in (1), i.e.…”

On-shell Z boson production at hadron colliders through 𝒪(ααs)

“…In turn, we observe a reduction of the cross section of the partonic channels induced by quarks and gluons, which in general should be compensated by the positive cross section of the additional photon-induced channels. We remark that a similar O(1%) reduction of the quark-and gluon-induced DY cross section for the production of a lepton pair is balanced by the contribution of the γγ → + − process, of comparable size [25,27,111]. In the specific case of on-shell Z production, the γγ initial state does not contribute at O(αα s ) , but only at O(α 2 ) ; its absence explains, at technical level, the size and sign of the observed effects.…”