Centrality and pseudorapidity dependence of the transverse energy density in $p\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02$ TeV

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 $p\text{Pb}$ collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02\text{TeV}$. 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 $p\text{Pb}$ collisions the maximum value of $d{E}_{\mathrm{T}}/d\eta$ is $22\text{GeV}$, which implies an $E_{\mathrm{T}}$ per participant nucleon pair comparable to that of peripheral PbPb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=2.76\text{TeV}$. 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.

Figure 1

Transverse energy density versus $\eta$ from minimum bias $p\text{Pb}$ collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02\text{TeV}$. The proton is moving toward positive $\eta$. The statistical uncertainties are smaller than the size of the data points and the total errors are dominated by the systematics. The systematic uncertainties are largely correlated point to point within the central and with the HF regions and so shown by gray bands there. The systematic uncertainties for the most forward and backward data points, i.e., $\eta =\pm 5.9$ are uncorrelated with those of central and HF regions and so are shown as vertical bars. Predictions from the epos-lhc (red solid), qgsjet ii (green dashed), and hijing (blue dotted) event generators are also shown.

Figure 2

Transverse energy density per participating nucleon-nucleon pair evaluated at ${\eta }_{\text{c.m.}}$ versus $\sqrt{s_{{}_{\mathrm{NN}}}}$ for minimum bias $p\text{Au}$, $p\text{U}$, $d\text{Au}$, and $p\text{Pb}$ collisions. For the CMS $p\text{Pb}$ data at $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02\text{TeV}$, $N_{\text{part}}$ was estimated to be $8.0\pm 0.2$ using the method described in Ref. [31]. The uncertainties are generally smaller than the size of the data points. Also shown are the corresponding results for central AuAu and PbPb collisions, as well as simulation for minimum bias $p\text{Pb}$ collisions from three event generators [3, 4, 18, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46].

Figure 3

Transverse energy density versus $\eta$ and centrality from $5.02\text{TeV}$ $p\text{Pb}$ collisions for the HF-double (left), HF-single (center), and $N_{\text{track}}$ (right) centrality definitions for data and for predictions from the epos-lhc, qgsjet ii, and hijing event generators, for 0–10% (upper), 40–50% (middle), and 70–80% (lower) central collisions. The uncertainties are dominated by the systematic components, which are largely correlated point-to-point in the central region and in HF and which are shown by gray bands there.

Figure 4

Transverse energy density per participating nucleon-nucleon pair versus $N_{\text{part}}$ for different $\eta$ ranges. The $N_{\text{part}}$ values are based on the method described in Ref. [31]. The HF-single method was used to define centrality. The total experimental uncertainties are shown by gray bands. The values of $N_{\text{part}}$ were calculated using the method described in Ref. [31].

Figure 5

Ratio of peripheral to central $E_{\mathrm{T}}$ production, $S_{\text{PC}}$, as a function of $\eta$ for three centrality ranges for HF-double (left), HF-single (middle), and $N_{\text{track}}$ (right) for data, and for the epos-lhc, qgsjet ii, and hijing event generators. The systematic uncertainties are dominant and are of comparable size to the data points.