Underlying Event measurements in pp collisions at $ \sqrt {s} = 0.9 $ and 7 TeV with the ALICE experiment at the LHC

Abstract

We present measurements of Underlying Event observables in pp collisions at √ s = 0.9 and 7 TeV. The analysis is performed as a function of the highest charged-particle transverse momentum p T,LT 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%.

Experimental overview on small collision systems at the LHC

“…Other effects, in particular related to jet quenching [166], are not straightforward to measure because of difficulties to define a rigorous normalization for nuclear effects, which is already a challenge in the case of pPb due to the contribution from multiple-parton interactions [121]. In pp even more than in pPb collisions there is a complex interplay between hard, multiple semi-hard and soft processes [113,164,[167][168][169][170][171], which needs to be disentangled from nuclear modification.…”

Thermal and non-thermal charmed meson production in heavy ions collisions at the LHC

“…Our interpretation of this is that the significant part of J/ψ-mesons (up to p T ∼ 3 GeV) is thermalized, but without being in a kinetic equilibrium with the medium. The points are ALICE data [22], histograms are simulated HYDJET++ events (blue -soft component, red -hard component, blackboth components). TeV.…”

The Onset of Jet Quenching Phenomenon

“…; the nuclear modifications factor, etc.. (R AA , R CP , I AA , I CP ). However, the absence of different levels of suppression for light vs. heavy quarks [10,11], and quark vs. gluon jets [12], in contrast to the QCD predictions, has necessitated the need for more sensitive observables and differential studies in order to have better quantification for the formed medium properties, and accordingly better constraints for the medium parameters. Moreover, other interesting measurements that have shown similar phenomena in the high multiplicity events in proton-proton collisions as in central heavy ion collisions, e.g.…”