CMS-PAS-FSQ-15-007

CMS-PAS-FSQ-15-007
Underlying event measurements with leading particles and jets in pp collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
November 2015

Abstract: A measurement of the underlying event (UE) activity is performed in proton-proton collisions at the centre-of-mass energy of 13 TeV with the CMS experiment at the LHC. The measurement is performed using leading charged-particles as well as leading charged-particle jets as reference objects. A leading charged-particle or charged-particle jet is required to be produced in the central pseudorapidity region ($\|\eta\| <$ 2) and with transverse momentum $p_{\mathrm{T}} \geq $ 0.5 ($p_{\mathrm{T}}^{jet} \geq$ 1) GeV for leading charged-particle (charge particle jet). The UE activity is measured in terms of the average multiplicity and scalar sum of $p_{\mathrm{T}}$ of the charged-particles, with $\|\eta\| <$ 2 and $p_{\mathrm{T}} \geq $ 0.5 GeV, in the azimuthal region transverse to the direction of the leading reference object. The measurements are compared to predictions of several Monte Carlo event generators providing constraints on the modelling of the UE dynamics.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Summary	CMS Publications

Summary

A measurement of the underlying event activity is made for proton-proton collisions at a centre-of-mass energy of 13 TeV. Average particle and energy densities are calculated in the transverse region and measured as a function of the $ p_{\mathrm{T}} $ of the leading charged-particle as well as of the leading charged-particles jet. All charged particles with $ p_{\mathrm{T}} \geq $ 0.5 GeV and $ | \eta | < $ 2.0 are used to calculate the UE activity. Measured distributions are corrected for the detector effects and the selection efficiencies. The corrected distributions are compared with several theory predictions and Monash tune of PYTHIA8 is found to be in best agreement with the measurements. A comparison of the UE densities at 0.9, 2.76, 7, and 13 TeV shows a strong growth in the activities with the centre-of-mass energy, with transMIN densities having a stronger $ \sqrt{s} $ dependence than transDIF.