Abstract
In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher $p_{\mathrm{T}}$ trigger particle. Measurements of these correlations as a function of pseudorapidity ($\Delta\eta$) and azimuthal ($\Delta\varphi$) differences are used to extract the centrality and $p_{\mathrm{T}}$ dependence of the shape of the near-side peak in the $p_{\mathrm{T}}$ range $1 < p_{\mathrm{T}} <$ 8 GeV/c in Pb-Pb and pp collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the $\Delta\varphi$ direction is almost independent of centrality, a significant broadening in the $\Delta\eta$ direction is found from peripheral to central collisions. This feature is prominent for the low $p_{\mathrm{T}}$ region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp in the $\Delta\varphi$ direction and above 3 GeV/c in the $\Delta\eta$ direction. Furthermore, for the 10\% most central collisions and $1 < p_{\mathrm{T, assoc}} <$ 2 GeV/c, $1 < p_{\mathrm{T, trig}} <$ 3 GeV/c a departure from a Gaussian shape is found: a depletion develops around the centre of the peak. The results are compared to AMPT model simulations as well as other theoretical calculations indicating that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
Highlights
In elementary interactions with large momentum transfer
The transfer of energy from the leading parton to the medium and/or into additional gluon radiation leads to effects that can be exploited to characterize the color density and scattering power of the medium
No dependence of the results presented in this paper on the polarity of the magnetic field was observed
Summary
They cannot exist freely and, instead, evolve from high to low virtuality, producing parton showers. These eventually hadronize into a spray of collimated hadrons called jets. High-pT partons are produced at the early stages of heavy-ion collisions. They propagate and evolve through the dense and hot medium created in these collisions and are expected to lose energy due to induced gluon radiation and elastic scatterings, a process commonly referred to as jet quenching. The transfer of energy from the leading parton to the medium and/or into additional gluon radiation leads to effects that can be exploited to characterize the color density and scattering power of the medium
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