Measurement of four-jet differential cross sections in $\sqrt{s}=8$ TeV proton-proton collisions using the ATLAS detector

TL;DR

This ATLAS study measures differential cross sections for events with at least four jets in pp collisions at √s = 8 TeV, using anti-kt R=0.4 jets and defining a stringent phase space with ΔR_4j^min>0.65 and pT thresholds. The analysis unfolds detector effects to the particle-jet level and compares results to LO and NLO pQCD predictions, as well as to the all-order HEJ framework, across momentum-, mass-, angular-, and central-ΣpT observables. Normalization factors are applied to LO predictions to facilitate shape comparisons, while NLO results are presented at fixed order with their intrinsic uncertainties. Overall, NLO predictions (BlackHat/Sherpa and NJet/Sherpa) and MadGraph+Pythia describe the data well within uncertainties, with HEJ excelling in wide-angle and high-multiplicity configurations; LO PS approaches show more limited agreement, highlighting the importance of higher-order and all-order approaches for multi-jet final states. These measurements provide essential tests of QCD and inform modeling strategies for multi-jet backgrounds in searches for new phenomena.

Abstract

Differential cross sections for the production of at least four jets have been measured in proton-proton collisions at $\sqrt{s} = 8$ TeV at the Large Hadron Collider using the ATLAS detector. Events are selected if the four anti-$k_{t}$ R=0.4 jets with the largest transverse momentum ($p_{T}$) within the rapidity range $|y|<2.8$ are well separated ($dR^{\rm min}_{4j}>0.65$), all have $p_{T}>64$ GeV, and include at least one jet with $p_{T} >100$ GeV. The dataset corresponds to an integrated luminosity of 20.3 $fb^{-1}$. The cross sections, corrected for detector effects, are compared to leading-order and next-to-leading-order calculations as a function of the jet momenta, invariant masses, minimum and maximum opening angles and other kinematic variables.

Figures (19)

• Figure 1: Schematic of the kinematic regions in which the four different jet triggers are used, including the total luminosity that each of them recorded. The term 4j45 (4j65) refers to a trigger requiring at least four jets with $\pt>45$ GeV (65 GeV), where the is measured at the EF level of the triggering system. The term j280 (j360) refers to a trigger requiring at least one jet with $\pt> 280$ GeV (360 GeV) at the EF level. The horizontal and vertical axes correspond to $p_{\text{T}}^{(1)}$ and $p_{\text{T}}^{(4)}$ respectively, both calculated at the offline level (i.e., including the full object calibration).
• Figure 2: Detector-level distributions of (a) $p_{\text{T}}^{(1)}$ and (b) $p_{\text{T}}^{(4)}$ for data and for example MC predictions. The MC predictions have passed through detector simulation. The lower panel in each plot shows the ratios of the MC predictions to data. For better comparison, the predictions are multiplied by the factors indicated in the legend.
• Figure 3: Total systematic uncertainty in the four-jet cross section measurement for ${\textrm{anti-}}k_{t}$$R=0.4$ jets as a function of (a) and (b) $\Delta\phi_{\text{2j}}^{\min}$. In both cases the event selection corresponds to the inclusive analysis cuts, namely $p_{\text{T}}^{(4)}>64$ GeV, $p_{\text{T}}^{(1)}>100$ GeV and $\Delta R_{\text{4j}}^{\min}>0.65$. Separate bands show the jet energy scale (JES) and resolution (JER), and the unfolding uncertainty, as well as the combined total systematic uncertainty resulting from adding in quadrature all the components. The total statistical uncertainty of the unfolded data spectrum is also shown. The luminosity uncertainty is not shown separately but is included in the total uncertainty band.
• Figure 4: The four-jet differential cross section as a function of leading jet ($p_{\text{T}}^{(1)}$), compared to different theoretical predictions: Pythia, Herwig++ and MadGraph+Pythia (top), and HEJ, NJet/Sherpa and BlackHat/Sherpa (bottom). For better comparison, the predictions are multiplied by the factors indicated in the legend. In each figure, the top panel shows the full spectra and the bottom panel the ratios of the different predictions to the data. The solid band represents the total experimental systematic uncertainty centred at one. The patterned band represents the NLO scale and PDF uncertainties calculated from NJet/Sherpa centred at the nominal NJet/Sherpa values. The scale uncertainties for HEJ (not drawn) are typically $^{+50\%}_{-30\%}$. The ratio curves are formed by the central values with vertical uncertainty lines resulting from the propagation of the statistical uncertainties of the predictions and those of the unfolded data spectrum.
• Figure 5: Unfolded four-jet differential cross section as a function of $p_{\text{T}}^{(2)}$, compared to different theoretical predictions. The other details are as for figure \ref{['fig:unfolded_jetpt0']}.