Measurement of the underlying event activity using charged-particle jets in proton-proton collisions at $\sqrt{s}=2.76$ TeV

A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with a charged-particle jet produced at central pseudorapidity ($\mid\eta^{\textrm{jet}}\mid < 2$) and with transverse momentum $1\leq p^{\textrm{jet}}_{\textrm{T}} < 100$ GeV. The analysis uses a data sample collected by the CMS experiment at the LHC at a centre-of-mass energy of $2.76$ TeV, corresponding to an integrated luminosity of $0.3$ nb$^{-1}$. The UE activity is measured as a function of $p^{\textrm{jet}}_{\textrm{T}}$ in terms of the average multiplicity and scalar-$p_{\textrm{T}}$ sum of charged particles, with $\mid\eta\mid < 2$ and $p_{\textrm{T}} > 0.5$ GeV, in the azimuthal region transverse to the highest-$p_{\textrm{T}}$ jet direction. By further dividing the transverse region into two halves of smaller and larger activity respectively, valuable information on different UE contributions is obtained. 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 theoretical modelling of the UE dynamics.