Measurement of dijet azimuthal decorrelation in pp collisions at [Formula: see text]

Abstract

A measurement of the decorrelation of azimuthal angles between the two jets with the largest transverse momenta is presented for seven regions of leading jet transverse momentum up to 2.2[Formula: see text]. The analysis is based on the proton-proton collision data collected with the CMS experiment at a centre-of-mass energy of 8[Formula: see text] corresponding to an integrated luminosity of 19.7[Formula: see text]. The dijet azimuthal decorrelation is caused by the radiation of additional jets and probes the dynamics of multijet production. The results are compared to fixed-order predictions of perturbative quantum chromodynamics (QCD), and to simulations using Monte Carlo event generators that include parton showers, hadronization, and multiparton interactions. Event generators with only two outgoing high transverse momentum partons fail to describe the measurement, even when supplemented with next-to-leading-order QCD corrections and parton showers. Much better agreement is achieved when at least three outgoing partons are complemented through either next-to-leading-order predictions or parton showers. This observation emphasizes the need to improve predictions for multijet production.

Fig. 1

Distribution of $E_{\mathrm{T}}//\sum E_{\mathrm{T}}$ for data (points) in comparison with simulated jet production and other processes with large $E_{\mathrm{T}}/$ (stacked) separately for the two regions $\mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}<\mathit{\pi }/2$ (top) and $\mathit{\pi }/2<\mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}<\mathit{\pi }$ (bottom). The main contribution of events with large $E_{\mathrm{T}}/$ in the final state is caused by processes such as Z/W + jet(s) with $\mathrm{Z}\to \mathit{\nu }\overline{\mathit{\nu }}$ and $\mathrm{W}\to \ell \mathit{\nu }$

Fig. 2

Normalized dijet cross section differential in $\mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}$ for seven $p_{\mathrm{T}}^{\text{max}}$ regions, scaled by multiplicative factors for presentation purposes. The error bars on the data points include statistical and systematic uncertainties. Overlaid on the data (points) are predictions from LO (dashed line; $\mathit{\pi }/2\le \mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}<2\mathit{\pi }/3$) and NLO (solid line; $2\mathit{\pi }/3\le \mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}\le \mathit{\pi }$) calculations using the CT10 NLO PDF set. The PDF, ${\mathit{\alpha }}_S$, and scale uncertainties are added in quadrature to give the total theoretical uncertainty, which is indicated by the downwards-diagonally (LO) and upwards-diagonally (NLO) hatched regions around the theory lines

Fig. 3

Ratios of the normalized dijet cross section differential in $\mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}$ to LO (triangles) and NLO (squares) pQCD predictions using the CT10 PDF set at next-to-leading evolution order for all $p_{\mathrm{T}}^{\text{max}}$ regions. The error bars on the data points represent the total experimental uncertainty, which is the quadratic sum of the statistical and systematic uncertainties. The uncertainties of the theoretical predictions are shown as inner band (PDF & ${\mathit{\alpha }}_S$) and outer band (scales). The predictions using various other PDF sets relative to CT10 are indicated with different line styles

Fig. 4

Normalized dijet cross section differential in $\mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}$ for seven $p_{\mathrm{T}}^{\text{max}}$ regions, scaled by multiplicative factors for presentation purposes. The error bars on the data points include statistical and systematic uncertainties. Overlaid on the data are predictions from the pythia6, herwig++, pythia8, MadGraph + pythia6, and powheg + pythia8 event generators

Fig. 5

Ratios of pythia6, herwig++, pythia8, MadGraph + pythia6, and powheg + pythia8 predictions to the normalized dijet cross section differential in $\mathrm{\Delta }{\mathit{\varphi }}_{\text{dijet}}$, for all $p_{\mathrm{T}}^{\text{max}}$ regions. The solid band indicates the total experimental uncertainty and the error bars on the MC points represent the statistical uncertainties of the simulated data