Electromagnetic processes at the LHC : nuclear parton distributions from deep inelastic pair production and exclusive photoproduction of single W bosons

Ultra-peripheral collisions at ion colliders, such as the Large Hadron Collider (LHC) at CERN, offer the possibility to study electromagnetic processes at hitherto unexplored energies. In the framework of the equivalent photon approximation (EPA), where relativistic charged particles are treated as beams of equivalent photons, the collisions of ions can be used to study two-photon (γγ collisions) or photonuclear processes (γp and γA collisions). This thesis contains two studies of processes which might be observed at the LHC. The main purpose in these studies is the estimation of the total rates of these processes at LHC energies and the investigation of their event signature, in order to find out whether an observation of these processes is feasible and what can be learnt from them. Our first study [1] addresses the production of lepton pairs in heavy ion collisions in the region of large momentum transfer of one of the photons and the feasibility to study the quark content of nucleons and nuclei from this process. This is particularly interesting with regard to how the quark parton distributions of nucleons change when affected by nuclear modifications. However, to be able to learn from this process, it is important to isolate it experimentally. This means, one has to find a way to distinguish the deep inelastic scattering (DIS) process from possible background, mainly coming from the elastic (doubly coherent) two-photon pair production. To this purpose we calculate the cross section for two-photon lepton pair production in relativistic heavy ion collisions in the region of large momentum transfer of one of the photons in a plane wave Born approximation. We compare differential cross sections from this calculation to those of a calculation of doubly coherent pair production. We find the elastic contribution to be dominant at small momentum transfers of the photons. However, at transverse momenta of the leptons of above ∼GeV the two contributions become comparable in size. Furthermore, the two processes show distinct event signatures: In the doubly coherent process, the leptons are produced back-to-back in the transverse plane, i. e. having equal transverse momenta. In the deep inelastic contribution, in contrast, the leptons show an asymmetric distribution of their transverse momenta. One lepton is produced with relatively large transverse momentum, reflecting the Q2 distribution of the photon, while the transverse momentum distribution of the other lepton peaks at small transverse momenta. Furthermore, lepton pair production from deep inelastic scattering in heavy ion collisions can be related to deep inelastic lepton scattering off nuclei by means of a combination of the equivalent photon approximation and photon splitting into a lepton pair. This provides an intuitive picture of the process and moreover simplifies the calculation considerably. In order to test these approximations, we calculate differential cross sections as functions of various kinematic variables in the equivalent photon approximation (EPA) and the equivalent lepton approximation (ELA) and compare the results to those of the full calculation. As result, the EPA is found to be satisfying, whereas the ELA consistently overestimates the results from the full calculation. Despite the quantitative failure of the ELA, our results confirm that it may still serve as a qualitative picture of the process. Furthermore, we include two parameterizations which account for nuclear modifications in our full calculation in order to study their effect on the cross section. We find the differential cross section as function of the rapidity to be most sensitive to the differences between the two sets. In our second study [2], we are concerned with the exclusive photoproduction of single W bosons in p-p or p-A collisions, where the proton is converted into a neutron, which proceeds in forward direction. This process is very sensitive to the coupling of two W bosons and a photon and thus offers the possibility to test this coupling for contributions which go beyond the Standard Model provided a reasonable rate of events can be achieved. This study aims to estimate the total rate of events for this process at the LHC. To this purpose, we calculate the cross section for real photoproduction of single W bosons and cross-check the
results with those of former studies of this process. We extend those studies by including, in
addition, a weak magnetic form factor. Convolving the photoproduction cross sections with the
equivalent photon spectra of ions and protons yields the cross sections for p-p and p-A collisions.
We estimate the rates of events expected at the LHC from this process within the Standard
Model. Furthermore, we compare different choices of the (almost unknown) timelike weak form
factors and examine how they affect the sensitivity of the total cross section to the triple gauge
boson coupling.
Since the W decays almost immediately, it can only be measured through its decay products.
The most important decay channel, in the sense that it has experimentally the cleanest signal,
is the decay into leptons. We include the decay of the W into a light anti-lepton (e+ or μ+)
and the corresponding neutrino into our calculations and calculate differential cross sections as
functions of the energies, rapidities, and transverse momenta of all final state particles in this
process. Based on the rate predicted for the p-p case, we conclude that an observation of this
process should be possible in the very high-luminosity runs at the LHC.