Measurement of multi-jet cross sections in proton-proton collisions at a 7 TeV center-of-mass energy

TL;DR

This study reports a first ATLAS measurement of inclusive multi-jet cross sections in pp collisions at √s = 7 TeV using 2.4 pb^-1, spanning up to six jets and jet HT. It systematically compares LO matrix-element plus parton-shower predictions (ALPGEN, SHERPA, PYTHIA) and NLO pQCD (NLOJet++) to corrected particle-level data, emphasizing jet-energy-scale uncertainties and background modeling for new-physics searches. LO predictions normalized to the two-jet cross section reproduce general trends, but NLO pQCD better describes certain ratios (e.g., three-to-two-jet) while showing tensions at low p_T, mitigated by tighter cuts. The work highlights the importance of non-perturbative corrections, JES, and flavor composition effects in multi-jet cross sections and provides benchmarks for tuning MC models and PDFs at the LHC.

Abstract

Inclusive multi-jet production is studied in proton-proton collisions at a center-of-mass energy of 7 TeV, using the ATLAS detector. The data sample corresponds to an integrated luminosity of 2.4 pb^-1. Results on multi-jet cross sections are presented and compared to both leading-order plus parton-shower Monte Carlo predictions and to next-to-leading-order QCD calculations.

Figures (12)

• Figure 1: Jet trigger efficiency for the third leading jet as a function of $p_{\mathrm{T}}$ for anti-$k_t$ jets with $R=0.4$ (a), and $R=0.6$ (b). Jet trigger efficiency as a function of $y$ of the third leading jet with $p_{\mathrm{T}}>60\mathrm{\ Ge V} \textrm{Ge V}$ and $R=0.4$ (c). Jet trigger efficiency as a function of the minimum separation $\Delta R$ between the two closest jets (d). The efficiency is shown both as calculated in data, as described in the text, and in Monte Carlo simulations for the three-jet trigger with a level-1 cut on the jet transverse energy of $10\mathrm{\ Ge V} \textrm{Ge V}$.
• Figure 2: Event display of a six-jet event satisfying the analysis requirements. The towers in the bottom right figure represent transverse energy deposited in the calorimeter projected on a grid of $\eta$ and $\phi$. Jets with transverse momenta ranging from 84 to $203\mathrm{\ Ge V} \textrm{Ge V}$ are measured in this event.
• Figure 3: Bin-by-bin correction factors for the cross sections (a) and for the $n$ to $n-1$ cross-section ratios (b) as a function of the inclusive jet multiplicity. The correction factors calculated using the ALPGEN+HERWIG/JIMMY AUET1 sample are shown with the systematic uncertainty as a yellow band around the points. See the text for an explanation of the legend labels.
• Figure 4: Jet response (mean reconstructed jet $p_{\mathrm{T}}$ over true jet $p_{\mathrm{T}}$) as a function of the true $p_{\mathrm{T}}$ for jets tagged as originating from a light quark or a gluon. The jet response in a sample with at least two jets of $p_{\mathrm{T}} > 60\mathrm{\ Ge V} \textrm{Ge V}$ (and with those two jets within $|y|<2.8$) is also shown for those jets with $|\eta|<0.8$. The anti-$k_t$ algorithm with $R=0.4$ is used.
• Figure 5: Fraction of selected jets in each inclusive multiplicity bin with neighboring jets within $\Delta R= 1.0$. Data (solid circles) are compared to the ALPGEN+HERWIG/JIMMY AUET1 (open squares) and PYTHIA AMBT1 (open triangles) Monte Carlo simulations.