The hadronic final state at HERA

Abstract

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 determinations 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.

Measurement of pseudorapidity distributions of charged particles in proton–proton collisions at $$\sqrt{s} = 8$$ s = 8 TeV by the CMS and TOTEM experiments

“…In pythia8, the same model is used to generate the cross section and the diffractive mass, but particle production is modelled differently. For lowmass states, the string model is used, but for higher masses (M X > 10 GeV/c 2 ) a perturbative description of pomeronproton scattering is introduced, based on diffractive PDFs [33][34][35], which represent probability distributions for partons in the proton under the constraint that the proton emerges intact from the collision. The non-perturbative string model introduces a mass dependence on the relative probability for a pomeron to couple to a quark or a gluon [36].…”

Exclusive processes with a leading neutron inepcollisions

“…Ref. [1]). In particular, the study of diffractive processes has been one of the most successful areas at HERA, with vector meson production and Deeply Virtual Compton Scattering (DVCS) in exclusive processes (γ * p → Ep, with E = ρ, φ, J/Ψ, γ) being important probes of the transition between the soft and hard regimes of QCD.…”

Next-to-Next-to Leading Order QCD Predictions for Single Jet Inclusive Production at the LHC

“…The inclusive cross section in DIS involves the exchange of a virtual photon coupling to quarks at lowest order via the electroweak coupling constant. The electrical neutrality of the gluon means that the gluon PDF can only be constrained using specific final states, such as heavy quarks or jets [2], or indirectly through the evolution of the flavor singlet distribution. In contrast, jet production at the Tevatron [3,4] and LHC directly probes the gluon PDF and is Oðα 2 s Þ at leading order (LO).…”