Recent investigations of QCD at HERA

The latest results from the H1 and ZEUS collaborations which challenge the QCD description of high energy $ep$ collisions are presented. Data from HERA continue to provide precision measurements and are compared to the latest theoretical predictions. Measurements of new processes are also presented as well as investigation of regions where perturbative QCD fails to describe the data. Four themes are presented here. Measurements of hard QCD processes, prompt photon and jet production, are used to compare to the latest theoretical predictions and, in the case of jet production, used to make high-precision extractions of the strong coupling constant up to next-next-to-leading order in QCD. All H1 and ZEUS charm and beauty cross sections in deep inelastic scattering have been combined and used to extract heavy-quark masses, including the running of the charm-quark mass with the scale of the process. Factorisation in diffraction has been investigated in charm production in deep inelastic scattering and prompt photon production in diffractive photoproduction has been measured for the first time. Finally, the inclusive data on deep inelastic scattering is presented in various forms in order to allow investigation of the underlying mechanism at very low photon virtuality $Q^2$ and low Bjorken $x$.Comment: 11 pages, 9 figures. submitted to the proceedings of the "DIS2017" Workshop, Birmingham, UK. Updated with some additional reference

Heavy quark production at HERA and the LHC

Measurements of heavy quark production, particularly from HERA, their theoretical understanding and their relevance for the LHC are reviewed. The status of beauty and charm production is discussed in the context of the different components of the production process: the parton density function of the colliding hadrons; the hard scatter; and the fragmentation of the quarks into hadrons. The theory of QCD at next-to-leading order generally describes well the hadronic structure and the production of heavy quarks although sometimes fails in details which are highlighted. The fragmentation of heavy quarks measured at HERA is consistent with that at LEP and hence supports the notion of universality.Comment: Write-up of talk at the workshop, "A workshop on the implications of HERA for LHC physics". To appear in proceedings. 10 pages, 7 figure

The Hadronic Final State at HERA

The hadronic final state in electron-proton collisions at HERA has provided a rich testing ground for development of the theory of the strong force, QCD. In this review, over 200 publications from the H1 and ZEUS Collaborations are summarised. Short distance physics, the measurement of processes at high energy scales, has provided rigorous tests of perturbative QCD and constrained the structure of the proton as well as allowing precise measurements of the strong coupling constant to be made. Non-perturbative or low energy processes have also been investigated and results on hadronisation interpreted together with those from other experiments. Searches for exotic QCD objects, such as pentaquarks, glueballs and instantons have been performed. The subject of diffraction has been re-invigorated through its precise measurement, such that it can now be described by perturbative QCD. After discussion of HERA, the H1 and ZEUS detectors and the techniques used to reconstruct differing hadronic final states, the above subject areas are elaborated. The major achievements are then condensed further in a final section summarising what has been learned.Comment: 60 pages, 65 figures, submitted to Reviews of Modern Physics. Updated version includes comments to the text from journal referee

VHEeP: A very high energy electron-proton collider based on proton-driven plasma wakefield acceleration

Based on current CERN infrastructure, an electron-proton collider is proposed at a centre-of-mass energy of about 9 TeV. A 7 TeV LHC bunch is used as the proton driver to create a plasma wakefield which then accelerates electrons to 3 TeV, these then colliding with the other 7 TeV LHC proton beam. The basic parameters of the collider are presented, which although of very high energy, has integrated luminosities of the order of 1 pb$^{-1}$/year. For such a collider, with a centre-of-mass energy 30 times greater than HERA, parton momentum fractions, $x$, down to about $10^{-8}$ are accessible for $Q^2$ of 1 GeV$^2$ and could lead to effects of saturation or some other breakdown of DGLAP being observed. The total photon-proton cross section can be measured up to very high energies and also at different energies as the possibility of varying the electron beam energy is assumed; this could have synergy with cosmic-ray physics. Other physics which can be pursued at such a collider are contact interaction searches, such as quark and electron substructure, and measurements of the proton structure as well as other more conventional measurements of QCD at high energies and in a new kinematic regime. The events at very low $x$ will lead to electrons and the hadronic final state produced at very low angles and so a novel spectrometer device will be needed to measure these. First ideas of the physics programme of such a collider are given.Comment: 7 pages, 4 figures, presented at the DIS 2015 Workshop, Dalla