Measurement of the triple-differential dijet cross section in proton-proton collisions at sqrt(s) = 8 TeV and constraints on parton distribution functions

TL;DR

This study measures the triple-differential dijet cross section in pp collisions at $\sqrt{s}=8$ TeV with CMS, as a function of $p_{\mathrm{T,avg}}$, $y^{*}$, and $y_{\mathrm{b}}$, correcting for detector effects and comparing to NLO pQCD with EW and NP corrections. It shows that the data largely agree with predictions across phase space, except in highly boosted configurations where PDF uncertainties are large, highlighting sensitivity to high-$x$ parton dynamics. A combined fit with HERA DIS data demonstrates reduced gluon uncertainties at high $x$ and a revised gluon shape, while simultaneously extracting $\alpha_S(M_Z)=0.1199^{+0.0015}_{-0.0016}$ (exp) ${}_{-0.0020}^{+0.0031}$ (theo/scale), in agreement with other measurements and world averages. Overall, the work advances PDF constraints and provides a precise, high-$x$ probe of proton structure using LHC jet data.

Abstract

A measurement is presented of the triple-differential dijet cross section at a centre-of-mass energy of 8 TeV using 19.7 inverse femtobarns of data collected with the CMS detector in proton-proton collisions at the LHC. The cross section is measured as a function of the average transverse momentum, half the rapidity separation, and the boost of the two leading jets in the event. The cross section is corrected for detector effects and compared to calculations in perturbative quantum chromodynamics at next-to-leading order accuracy, complemented with electroweak and nonperturbative corrections. New constraints on parton distribution functions are obtained and the inferred value of the strong coupling constant is alpha[S](M[Z]) = 0.1199 +/- 0.0015 (exp) -0.0020 +0.0031 (theo), where M[Z] is the mass of the Z boson.

Figures (11)

• Figure 1: Illustration of the dijet event topologies in the $y^{*}$ and $y_{\mathrm{b}}$ kinematic plane. The dijet system can be classified as a same-side or opposite-side jet event according to the boost $y_{\mathrm{b}}$ of the two leading jets, thereby providing insight into the parton kinematics.
• Figure 2: Relative contributions of all subprocesses to the total cross section at NLO as a function of $p_{\mathrm{T,avg}}$ in the various $y^{*}$ and $y_{\mathrm{b}}$ bins. The subprocess contributions are grouped into seven categories according to the type of the incoming partons. The calculations have been performed with NLOJet++. The notation implies the sum over initial-state parton flavors as well as interchanged quarks and antiquarks.
• Figure 3: Overview of all experimental uncertainties affecting the cross section measurement in six bins of $y_{\mathrm{b}}$ and $y^{*}$. The error bars indicate the statistical uncertainty after unfolding. The different lines show the uncertainties resulting from jet energy corrections, jet energy resolution, integrated luminosity, non-Gaussian tails in the resolution, and from residual effects included in the uncorrelated uncertainty. The total uncertainty is obtained by adding all uncertainties in quadrature.
• Figure 4: Overview of the theoretical correction factors. For each of the six analysis bins the NLO QCD (top left), the electroweak (top right), and the NP correction factor (bottom) are shown as a function of $p_{\mathrm{T,avg}}$. The NLO QCD correction has been derived with the same NLO PDF in numerator and denominator and is included in the NLO prediction by NLOJet++.
• Figure 5: Overview of the theoretical uncertainties. The scale uncertainty dominates in the low-$p_{\mathrm{T,avg}}$ region. At high $p_{\mathrm{T,avg}}$, and especially in the boosted region, the PDFs become the dominant source of uncertainty.