Prof. Gavin Salam FRS

Abstract:

The tiny droplets of quark gluon plasma (QGP) created in high-energy nuclear collisions experience fast expansion and cooling with a lifetime of a few fm=c. Despite the information provided by probes such as jet quenching and quarkonium suppression, and the excellent description by hydrodynamical models, direct access to the time evolution of the system remains elusive. We point out that the study of hadronically decaying W bosons, notably in events with a top-antitop quark pair, can provide key novel insight into the time structure of the QGP. This is because of a unique feature, namely a time delay between the moment of the collision and that when the W-boson decay products start interacting with the medium. Furthermore, the length of the time delay can be constrained by selecting specific reconstructed top-quark momenta. We carry out a Monte Carlo feasibility study and find that the LHC has the potential to bring first limited information on the time structure of the QGP. Substantially increased LHC heavy-ion luminosities or future higher-energy colliders would open opportunities for more extensive studies.

Abstract:

This corrects the article DOI: 10.1103/PhysRevLett.115.082002.

Abstract:

The photon PDF of the proton is needed for precision comparisons of LHC cross sections with theoretical predictions. In a recent paper, we showed how the photon PDF could be determined in terms of the electromagnetic proton structure functions F2 and FL measured in electron-proton scattering experiments, and gave an explicit formula for the PDF including all terms up to next-to-leading order. In this paper we give details of the derivation. We obtain the photon PDF using the factorisation theorem and applying it to suitable BSM hard scattering processes. We also obtain the same PDF in a process-independent manner using the usual definition of PDFs in terms of light-cone Fourier transforms of products of operators. We show how our method gives an exact representation for the photon PDF in terms of F2 and FL, valid to all orders in QED and QCD, and including all non-perturbative corrections. This representation is then used to give an explicit formula for the photon PDF to one order higher than our previous result. We also generalise our results to obtain formulæ for the polarised photon PDF, as well as the photon TMDPDF. Using our formula, we derive the Pγi subset of DGLAP splitting functions to order ααs and α2, which agree with known results. We give a detailed explanation of the approach that we follow to determine a photon PDF and its uncertainty within the above framework.