Abstract
The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a p_{T} region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated with a higher p_{T} trigger particle (1<p_{T,trig}<8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV. A significant broadening of the peak in the Δη direction at low p_{T} is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1<p_{T,assoc}<2 GeV/c, 1<p_{T,trig}<3 GeV/c a novel feature is observed: a depletion develops around the center of the peak. The results are compared to pp collisions at the same center of mass energy and ampt model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
Highlights
The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a pT region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated with a higher pT trigger particle (1 < pT;trig < 8 GeV=c)
In is found from peripheral to central collisions, which vanishes above 4 GeV=c, while in the Δφ direction the peak is almost independent of centrality
An inclusive jet suppression has been observed at the LHC [1,2,3] together with a large dijet energy asymmetry [4,5], while studies of the momentum and angular distributions of jet fragments show only a small modification of the jet core [6,7,8], and an excess of soft particles radiated to large angles from the jet axis [9]
Summary
; ð1Þ where Ntrig is the total number of trigger particles in the centrality class and the pT;trig interval, ranging from 0.18 to 36 per event. The background distribution BðΔη; ΔφÞ 1⁄4 αd2Nmixed=dΔηdΔφ accounts for the acceptance and efficiency of pair reconstruction. It is constructed by correlating the trigger particles in one event with the associated particles from other events. In order to characterize the near-side peak shape, a simultaneous fit of the peak, the combinatorial background, and the long-range correlation background stemming from collective effects is performed. This exploits that in twoparticle correlations the near-side peak is centered around Δφ 1⁄4 0, Δη 1⁄4 0, while long-range correlation structures. The fit function used is a combination of a constant, a generalized twodimensional Gaussian function and cosðnΔφÞ terms for n 1⁄4 2, 3, 4
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