Measurement of the underlying event activity using charged particle jets in proton-proton collisions at √s = 2.76 TeV

Abstract

A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with a leading charged particle jet produced at central pseudo rapidity () and of transverse momentum in the range of 1 to 100 GeV. The analysis uses 0.3 of integrated luminosity at the centre-of-mass energy of 2.76 TeV at the LHC by the CMS experiment. The UE activity is measured as a function of in terms of the average multiplicity and scalar- sum of charged particles with and GeV in the azimuthal region transverse to the leading jet direction. By dividing the transverse region into the minimum and maximum activity sides with respect to the leading jet, further information on the dynamics of the UE is obtained. The measurements are compared to previous results at 0.9 and 7 TeV and to predictions of several QCD-inspired models providing constraints for the tuning parameters involved.

Results at = 2.76 TeV

Transverse densities

PAS Plots	Additional tunes	Description
png pdf	png pdf	Measured particle density in the transvers region (), as a function leading charged-particle jet . The data are compared to various Monte Carlo predictions of PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.
png pdf	png pdf	Measured density in the transvers region (), as a function leading charged-particle jet . The data are compared to various Monte Carlo predictions of PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.

TransMAX densities

PAS Plots	Additional tunes	Description
png pdf	png pdf	Measured particle density in the transMAX region with minimum activity within , as a function of leading charged-particle jet . PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.
png pdf	png pdf	Measured density in the transMAX region with minimum activity within , as a function of leading charged-particle jet . The data are compared to various Monte Carlo predictions of PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.

TransMIN densities

PAS Plots	Additional tunes	Description
png pdf	png pdf	Measured particle density in the transMIN region with minimum activity within , as a function of leading charged-particle jet . PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.
png pdf	png pdf	Measured density in the transMIN region with minimum activity within , as a function of leading charged-particle jet . The data are compared to various Monte Carlo predictions of PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.

TransDIF densities

PAS Plots	Additional tunes	Description
png pdf	png pdf	Measured difference of the activity in the transDIF region, (), between maximum and minimum activity regions for particle density as a function of leading charged-particle jet . PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.
png pdf	png pdf	Measured difference of the activity in the transDIF region, (), between maximum and minimum activity regions for density as a function of leading charged-particle jet . The data are compared to various Monte Carlo predictions of PYTHIA6 (version 6.426; tunes Z2* and CUETP6S1), PYTHIA8 (version 8.175; tunes 4C, CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data. The ratios of MC predictions to the measurements are shown on the bottom of the figure. The inner errors correspond to the statistical uncertainties and the outer errors represent the statistical and systematic uncertainties added in quadrature.

Comparison with = 0.9 TeV and 7 TeV data

PAS Plots	Additional tunes	Description
png pdf	png pdf	Comparison of UE activity at 0.9, 2.76 and 7 TeV for particle density as a function of leading charged-particle jet . Predictions of PYTHIA6 (version 6.426; tune Z2*), PYTHIA8 (version 8.175; tunes CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data.
png pdf	png pdf	Comparison of UE activity at 0.9, 2.76 and 7 TeV for for density as a function of leading charged-particle jet . Predictions of PYTHIA6 (version 6.426; tune Z2*), PYTHIA8 (version 8.175; tunes CUETP8S1, Monash and CUETP8M1) and Herwig++ (version 2.7; tune UE-EE-5C) are compared to the data.

Additional plots

Plot	Description
png pdf	Distributions of events passing the minimun bias, jet20 and jet40 triggers weigthed by trigger prescales as a function of the leading jet .
png pdf	Comparision of track distributions for data and PYTHIA6 Z2* and PYTHIA8 4C Monte Carlo simulations.
png pdf	Comparision of leading track jet distributions for data and PYTHIA6 Z2* and PYTHIA8 4C Monte Carlo simulations.