We present measurements of Underlying Event observables in pp collisions
at root s = 0 : 9 and 7 TeV. The analysis is performed as a function of
the highest charged-particle transverse momentum p(T),L-T in the event.
Different regions are defined with respect to the azimuthal direction of
the leading (highest transverse momentum) track: Toward, Transverse and
Away. The Toward and Away regions collect the fragmentation products of
the hardest partonic interaction. The Transverse region is expected to
be most sensitive to the Underlying Event activity. The study is
performed with charged particles above three different p(T) thresholds:
0.15, 0.5 and 1.0 GeV/c. In the Transverse region we observe an increase
in the multiplicity of a factor 2-3 between the lower and higher
collision energies, depending on the track p(T) threshold considered.
Data are compared to PYTHIA 6.4, PYTHIA 8.1 and PHOJET. On average, all
models considered underestimate the multiplicity and summed p(T) in the
Transverse region by about 10-30%