Charm, Beauty and Top at HERA

TL;DR

This review synthesizes HERA's heavy-flavour program, linking open charm and beauty measurements in photoproduction and DIS to the core questions of QCD: the gluon content of the proton, heavy-quark masses, and fragmentation. It surveys a spectrum of theoretical frameworks (FFNS, ZMVFNS, GMVFNS) and emphasizes the role of scale choices and fragmentation modeling in interpreting data, with NLO fixed-flavour calculations generally providing the baseline for differential observables and GMVFNS guiding inclusive quantities. The measurements across tagging methods and kinematic regimes show broad agreement with QCD predictions, while highlighting areas where higher-order corrections or improved fragmentation are needed, and they demonstrate the impact of heavy-flavour data on PDF determinations and LHC predictions. The work underscores how HERA’s heavy-flavour program informs both current collider phenomenology and future explorations of QCD dynamics, including potential extrapolations to LHeC or similar facilities.

Abstract

Results on open charm and beauty production and on the search for top production in high-energy electron-proton collisions at HERA are reviewed. This includes a discussion of relevant theoretical aspects, a summary of the available measurements and measurement techniques, and their impact on improved understanding of QCD and its parameters, such as parton density functions and charm- and beauty-quark masses. The impact of these results on measurements at the LHC and elsewhere is also addressed.

Figures (55)

• Figure 1: (a) The dominant production process for charm and beauty quarks in $ep$ collisions at HERA, the boson-gluon fusion (BGF) reaction. (b) The simplest Quark-Parton-Model diagram for deeply inelastic scattering on a light quark.
• Figure 2: Quark-parton-model view of heavy flavour production in $ep$ collisions at HERA.
• Figure 3: Possible hard scales in the boson-gluon-fusion process. For the explanation of the symbols see text.
• Figure 4: Illustration of event kinematic variables for $ep$ scattering at HERA with heavy-quark production via the boson-gluon-fusion process.
• Figure 5: Example for the factorisation of heavy-flavour production in QCD into proton structure, photon structure, hard matrix element and fragmentation.