engleski

First measurements of N-subjettiness in central Pb- Pb collisions at √sNN = 2.76 TeV

The ALICE Collaboration reports the first fully- corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of- mass energies of √s = 7 TeV and √sNN = 2.76 TeV, respectively. In particular the ratio of 2- subjettiness to 1-subjettiness, τ2/τ1, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy- ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT, jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ2/τ1 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ2/ τ1, together with the splitting aperture angle ∆R, are also performed in pp collisions at √s = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at √s = 7 TeV, in order to validate the model as a vacuum reference for the Pb- Pb centre-of-mass energy. The PYTHIA references for τ2/τ1 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.

Heavy Ion Experiments ; jet: transverse momentum ; beam: width ; jet: energy loss ; heavy ion: scattering ; jet: resolution ; p p: scattering ; PYTHIA ; structure ; semi-inclusive reaction ; ALICE ; kinematics ; experimental results ; 2760 GeV-cms/nucleon ; 7000 GeV-cms/nucleon

nije evidentirano