Abstract
The almost hermetic coverage of the CMS detector is used to measure the distribution of transverse energy, $E_\mathrm{T}$, over 13.2 units of pseudorapidity, $\eta$, for pPb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV. The huge angular acceptance exploits the fact that the CASTOR calorimeter at $-6.6$ $<$ $\eta$ $<$ $-5.2$ is effectively present on both sides of the colliding system because of a switch in the proton-going and lead-going beam directions. This wide acceptance enables the study of correlations between well-separated angular regions and makes the measurement a particularly powerful test of event generators. For minimum bias pPb collisions the maximum value of d$E_\mathrm{T}/$d$\eta$ is 22 GeV, which implies an $E_\mathrm{T}$ per participant nucleon pair comparable to that of peripheral PbPb collisions at $\sqrt{s_\mathrm{NN}} =$ 2.76 TeV. The increase of d$E_\mathrm{T}/$d$\eta$ with centrality is much stronger for the lead-going side than for the proton-going side. The $\eta$ dependence of d$E_\mathrm{T}/$d$\eta$ is sensitive to the $\eta$ range in which the centrality variable is defined. Several modern generators are compared to these results but none is able to capture all aspects of the $\eta$ and centrality dependence of the data and the correlations observed between different $\eta$ regions.
Highlights
In a heavy-ion or proton nucleus collision the total transverse energy, ET, is a measure of the energy liberated by the deceleration, or “stopping power” of the colliding nucleons while dET/dy measures the total energy carried by the system of particles or medium, produced in the collision, which is moving with longitudinal rapidity y [1]
The time τ0 at which it is first appropriate to speak about an energy density is a model assumption
Some collaborations have chosen to report the product of energy density and proper time BJτ0 [2,3] while others have used τ0
Summary
In a heavy-ion or proton nucleus collision the total transverse energy, ET, is a measure of the energy liberated by the deceleration, or “stopping power” of the colliding nucleons while dET/dy measures the total energy carried by the system of particles or medium, produced in the collision, which is moving with longitudinal rapidity y [1]. For symmetric heavy-ion collisions, the shape of dET/dη vs η has only a weak dependence on the η region, which is used to classify the centrality of the events [4] To test whether this is the case for the much smaller system created in pPb collisions, events are classified according to the ET or charged-particle multiplicity in several different η regions, and the dET/dη distributions produced by the different classification procedures are compared to each other. The comparison of these collider data with modern event generator calculations is a significant motivation for this work. The data are compared in detail to calculations from three event generators: HIJING v2.1, EPOS-LHC, and QGSJET II-04 [23,24,25]
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