Abstract
A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with charged-particle jets produced in the central pseudorapidity region (|η jet| < 2) and with transverse momentum 1 ≤ p T jet < 100 GeV. The analysis uses a data sample collected at a centre-of-mass energy of 2.76 TeV with the CMS experiment at the LHC. The UE activity is measured as a function of p T jet in terms of the average multiplicity and scalar sum of transverse momenta (p T) of charged particles, with |η| < 2 and p T > 0.5 GeV, in the azimuthal region transverse to the highest p T jet direction. By further dividing the transverse region into two regions of smaller and larger activity, various components of the UE activity are separated. The measurements are compared to previous results at 0.9 and 7 TeV, and to predictions of several Monte Carlo event generators, providing constraints on the modelling of the UE dynamics.
Highlights
A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with charged-particle jets produced in the central pseudorapidity region (|ηjet| < 2) and with transverse momentum 1 ≤ pjTet < 100 GeV
We study the UE activity in pp collisions at s = 2.76 TeV by measuring the average multiplicity and scalar transverse momentum sum (ΣpT) densities of charged particles in the azimuthal region orthogonal to the direction of the leading charged-particle jet, referred to as the transverse region
At high pT, the distributions show a slow rise in the transMAX region, while for transMIN the flattening of the UE activity as a function of pjTet is more pronounced
Summary
A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with charged-particle jets produced in the central pseudorapidity region (|ηjet| < 2) and with transverse momentum 1 ≤ pjTet < 100 GeV. The UE activity is measured as a function of pjTet in terms of the average multiplicity and scalar sum of transverse momenta (pT) of charged particles, with |η| < 2 and pT > 0.5 GeV, in the azimuthal region transverse to the highest pT jet direction. We study the UE activity in pp collisions at s = 2.76 TeV by measuring the average multiplicity and scalar transverse momentum sum (ΣpT) densities of charged particles in the azimuthal region orthogonal to the direction of the leading charged-particle jet, referred to as the transverse region. At a given centre-of-mass energy, the UE activity is expected to increase with the momentum transfer between the interacting partons (hard scale). Studying the UE as a function of the hard scale at several centre-of-mass energies provides an insight into the UE dynamics and its evolution with the collision energy, further constraining the model parameters
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