Investigating the Role of Coherence Effects on Jet Quenching in Pb-Pb Collisions at $\sqrt{s_{NN}} =2.76$ TeV using Jet Substructure

We report measurements of two jet shapes, the ratio of 2-Subjettiness to 1-Subjettiness ( τ2/τ1 ) and the opening angle between the two axes of the 2-Subjettiness jet shape, which is obtained by reclustering the jet with the exclusive- k T algorithm [S.D.Ellis and D.E.Soper, Phys.Rev.B 48, 3160] and undoing the final clustering step. The aim of this measurement is to explore a possible change in the rate of 2-pronged objects in Pb-Pb compared to pp due to colour coherence. Coherence effects [Y.Mehtar-Tani, C.A.Salgado and K.Tywoniuk Phys. Rev. Lett. 106:122002, 2011] relate to the ability of the medium to resolve a jet's substructure, which has an impact on the energy loss magnitude and mechanism of the traversing jet. In both collision systems charged jets are found with the anti- k T algorithm [M.Cacciari, G.P.Salam and G.Soyez JHEP 0804:063, 2008], a resolution parameter of R=0.4 and a constituent cut off of 0.15 GeV. This analysis uses hadron-jet coincidence techniques in Pb-Pb collisions to reject the combinatorial background and corrects further for background effects by employing various jet shape subtraction techniques and two dimensional unfolding. Measurements of the Nsubjettiness for jet momenta of 40–60 GeV/ c in Pb-Pb collisions at sNN=2.76 TeV and pp collisions at s=7 TeV will be presented and compared to PYTHIA simulations.