Forward Jet Production in Deep Inelastic Scattering at HERA

TL;DR

This study investigates forward-jet production in deep inelastic $ep$ scattering at HERA to probe parton dynamics at low $x_{Bj}$. It analyzes single and triple differential cross sections, including events with an additional di-jet system, and confronts data with LO/NLO QCD calculations and several QCD-based models (DGLAP direct/resolved, BFKL, CCFM, CDM). Hadronisation corrections and theoretical uncertainties are quantified, revealing that non-DGLAP dynamics (notably CDM and DGLAP-resolved) provide a better description across much of the phase space, especially at low $x_{Bj}$ and for forward kinematics. The results provide evidence for breaking of strict $k_t$ ordering beyond the RG-DIR framework and demonstrate the need for higher-order or non-ordered emissions to accurately model forward-jet radiation patterns at small $x_{Bj}$.

Abstract

The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (x_{Bj}) and as triple differential cross sections d^3 σ/ dx_{Bj} dQ^2 dp_{t,jet}^2, where Q^2 is the four momentum transfer squared and p_{t,jet}^2 is the squared transverse momentum of the forward jet. Also cross sections for events with a di-jet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with next-to-leading order QCD calculations and with the predictions of various QCD-based models.

Figures (10)

• Figure 1: Schematic diagram of $ep$ scattering with a forward jet taking a fraction $x_{jet}=E_{jet}/E_p$ of the proton momentum. The evolution in the longitudinal momentum fraction, $x$, from large $x_{jet}$ to small $x_{Bj}$ is indicated.
• Figure 2: Control plots for the forward jet selection. The sample with no $p_{t,jet}^2/Q^2$-cut applied (upper) and the sample with the $0.5 < p_{t,jet}^2/Q^2 < 5$-cut applied (lower) are shown. The distributions are at detector level and normalised to unity. All variables are measured in the laboratory frame. Comparisons are made to the predictions of the DJANGO (full line) and RAPGAP (dashed line) Monte Carlo programs.
• Figure 3: The hadron level cross section for forward jet production as a function of $x_{Bj}$ compared to NLO predictions from DISENT (a) and to QCD Monte Carlo models (b and c). The shaded band around the data points shows the error from the uncertainties in the calorimetric energy scales. The hatched band around the NLO calculations illustrates the theoretical uncertainties in the calculations, estimated as described in the text. The dashed line in (a) shows the LO contribution.
• Figure 4: The hadron level triple differential cross section for forward jet production as a function of $x_{Bj}$, in bins of $Q^2$ (GeV$^2$) and $p_{t,jet}^2$ (GeV$^2$). The data are compared to the prediction of NLO (full line) and LO (dashed line) calculations from DISENT. Both calculations are corrected for hadronisation effects. The band around the data points illustrates the error due to the uncertainties in the calorimetric energy scales. The band around the NLO calculations illustrates the theoretical uncertainties in the calculations. In each bin the range in and the average value of $r=p_{t,jet}^2/Q^2$ is shown.
• Figure 5: The hadron level triple differential cross section for forward jet production as a function of $x_{Bj}$, in bins of $Q^2$ (GeV$^2$) and $p_{t,jet}^2$ (GeV$^2$). The data are compared to the predictions of CASCADE. The band around the data points illustrates the error due to the uncertainties in the calorimetric energy scales. In each bin the range in and the average value of $r=p_{t,jet}^2/Q^2$ is shown.