Measurement of three-jet production cross-sections in pp collisions at 7 TeV centre-of-mass energy using the ATLAS detector

TL;DR

This ATLAS study presents double-differential measurements of three-jet production in $pp$ collisions at $\sqrt{s}=7$ TeV, as functions of $m_{jjj}$ and $|Y^{*}|$ with jets defined via the anti-$k_t$ algorithm for $R=0.4$ and $R=0.6$. The data, corresponding to $4.51\,\mathrm{fb}^{-1}$, are unfolded to the particle level and compared to NLO QCD predictions with non-perturbative corrections across several PDF sets; NPC corrections are found to be small ($<10\%$) across the studied kinematic range. Overall, the NLO QCD predictions describe the data well over nearly seven orders of magnitude in cross section, with CT10, NNPDF2.3, MSTW2008, GJR08, and HERAPDF1.5 in good agreement, while ABM11 tends to undershoot. The results provide constraints on PDFs in a phase space region complementary to inclusive and dijet measurements and test perturbative QCD in a multi-jet environment at high $m_{jjj}$ and large $|Y^{*}|$.

Abstract

Double-differential three-jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV using the ATLAS detector at the Large Hadron Collider. The measurements are presented as a function of the three-jet mass $(m_{jjj})$, in bins of the sum of the absolute rapidity separations between the three leading jets $(|Y^\ast|)$. Invariant masses extending up to 5 TeV are reached for $8< |Y^\ast| < 10$. These measurements use a sample of data recorded using the ATLAS detector in 2011, which corresponds to an integrated luminosity of 4.51 fb$^{-1}$. Jets are identified using the anti-$k_t$ algorithm with two different jet radius parameters, R=0.4 and R=0.6. The dominant uncertainty in these measurements comes from the jet energy scale. Next-to-leading-order QCD calculations corrected to account for non-perturbative effects are compared to the measurements. Good agreement is found between the data and the theoretical predictions based on most of the available sets of parton distribution functions, over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values.

Figures (8)

• Figure 1: Total systematic uncertainty in the three-jet cross-sec-tion for anti-$k_{t}$$R=0.6$ jets as a function of $m_{jjj}$ (a) in $\left|Y^{*}\right|<2$ and (b) $8<\left|Y^{*}\right|<10$ bins. The bands shows the uncertainties due to jet energy scale, jet angular resolution, jet energy resolution and the combined uncertainty due to jet quality selection and unfolding. The outer band represents the total experimental uncertainty.
• Figure 2: Non-perturbative corrections obtained using various MC generators and tunes for the differential three-jet cross-sec-tion as a function of three-jet mass in the range $\left|Y^{*}\right|<2$ for anti-$k_{t}$ jet (a) $R=0.4$ and (b) $R=0.6$.
• Figure 3: The three-jet double-differential cross-sec-tion as a function of $m_{jjj}$ in bins $\left|Y^{*}\right|$, as denoted in the legend. The jets are identified using the anti-$k_{t}$ algorithm with $R=0.4$. For convenience, the cross-sec-tions are multiplied by the factors indicated in the legend. Also shown is the comparison with the NLOJET++ prediction with the CT 10 PDF set corrected for non-perturbative effects. The statistical uncertainties are smaller than the size of the symbols. Where visible, the sum in quadrature of the statistical and experimental systematic uncertainties is plotted.
• Figure 4: The three-jet double-differential cross-sec-tion as a function of $m_{jjj}$ in bins $\left|Y^{*}\right|$, as denoted in the legend. The jets are identified using the anti-$k_{t}$ algorithm with $R=0.6$. For convenience, the cross-sec-tions are multiplied by the factors indicated in the legend. Also shown is the comparison with the NLOJET++ prediction with the CT 10 PDF set corrected for non-perturbative effects. The statistical uncertainties are smaller than the size of the symbols. Where visible, the sum in quadrature of the statistical and experimental systematic uncertainties is plotted.
• Figure 5: The ratio of NLO QCD predictions, obtained by using NLOJET++ with different PDF sets (CT 10, MSTW 2008, GJR 08) and corrected for non-perturbative effects, to data as a function of $m_{jjj}$ in bins of $\left|Y^{*}\right|$, as denoted in the legend. The ratios are for jets identified using the anti-$k_{t}$ algorithm with $R=0.4$. The experimental error bands are centered at one and designate the relative statistical (thin dashed line) and total (statistical and systematic uncertainties added in quadrature) experimental uncertainties (thick solid line). The theoretical predictions are represented by thick lines with the hatched or filled band around it. The line show the central values and the band represent the total theory uncertainty.