Abstract
The jet radial structure and particle transverse momentum (pT) composition within jets are presented in centrality-selected Pb–Pb collisions at sNN=2.76 TeV. Track-based jets, which are also called charged jets, were reconstructed with a resolution parameter of R=0.3 at midrapidity |ηch jet|<0.6 for transverse momenta pT,ch jet=30–120 GeV/c. Jet–hadron correlations in relative azimuth and pseudorapidity space (Δφ,Δη) are measured to study the distribution of the associated particles around the jet axis for different pT,assoc-ranges between 1 and 20 GeV/c. The data in Pb–Pb collisions are compared to reference distributions for pp collisions, obtained using embedded PYTHIA simulations. The number of high-pT associate particles (4<pT,assoc<20 GeV/c) in Pb–Pb collisions is found to be suppressed compared to the reference by 30 to 10%, depending on centrality. The radial particle distribution relative to the jet axis shows a moderate modification in Pb–Pb collisions with respect to PYTHIA. High-pT associate particles are slightly more collimated in Pb–Pb collisions compared to the reference, while low-pT associate particles tend to be broadened. The results, which are presented for the first time down to pT,ch jet=30 GeV/c in Pb–Pb collisions, are compatible with both previous jet–hadron-related measurements from the CMS Collaboration and jet shape measurements from the ALICE Collaboration at higher pT, and add further support for the established picture of in-medium parton energy loss.
Highlights
At energy densities above approximately 0.5 GeV/fm3 and temperatures above approximately 160 MeV [1], Quantum Chromodynamics (QCD) calculations on the lattice predict the existence of a phase transition from normal nuclear matter to a new state of matter called the Quark–Gluon Plasma (QGP), where the partonic constituents, quarks and gluons, are no longer confined in hadrons
The centrality-dependent linear slope of the distribution for producing high transverse momentum (pT), ch jet = 40–60 GeV/c is more than one standard deviation away from zero, taking into account statistical and systematic uncertainties added in quadrature, indicating that there is a slightly stronger suppression for more central collisions in this case
The same observable was measured for jets with several higher minimum pT, const-cuts, i.e. 1, 2, and 3 GeV/c, which are less affected by the underlying event
Summary
At energy densities above approximately 0.5 GeV/fm and temperatures above approximately 160 MeV [1], Quantum Chromodynamics (QCD) calculations on the lattice predict the existence of a phase transition from normal nuclear matter to a new state of matter called the Quark–Gluon Plasma (QGP), where the partonic constituents, quarks and gluons, are no longer confined in hadrons. Hard scatterings are expected to occur early in the collision evolution, producing high transverse momentum (pT) partons that propagate through the expanding medium and eventually fragment into jets of hadrons. High-pT partons lose energy in interactions with the medium due to elastic scattering and induced gluon radiation [18,19]. Besides a reduction of the jet energy, this can result in a broadening of the transverse jet profile and a softening of the fragmentation [20]
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