Measurements of transverse energy distributions in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

Transverse energy $\left(E_T\right)$ distributions have been measured for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ by the STAR Collaboration at RHIC. $E_T$ is constructed from its hadronic and electromagnetic components, which have been measured separately. $E_T$ production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of $E_T$ per charged particle agrees well with measurements at lower collision energy, indicating that the growth in $E_T$ for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total $E_T$ is consistent with a final state dominated by mesons and independent of centrality.

Figure 1

Typical MIP spectrum. x-axis corresponds to ADC channel number. The hits correspond to isolated tracks with $p>1.25\mathrm{GeV}∕c$ which project to EMC towers. The peak corresponds to the energy deposited by nonshowering hadrons (MIP peak).

Figure 2

$p∕E_{\mathrm{\text{tower}}}$ spectrum for electron candidates, selected through $dE∕dx$ from the TPC, with $1.5<p<5.0\mathrm{GeV}∕c$. A well defined electron peak is observed. The dashed line corresponds to the hadronic background in the $dE∕dx$-identified electron sample.

Figure 3

Upper plot: points are measured $p∕E_{\mathrm{\text{tower}}}$ electron peak position as a function of the distance to the center of the tower. The solid line is from a calculation based on a full GEANT simulation of the detector response to electrons. Lower plot: points show measured energy deposited by electrons in the tower as a function of the momentum for distances to the center of the tower smaller than 2.0 cm. The first point is the electron equivalent energy of the minimum ionizing particles. The solid line is a second order polynomial fit of the data.

Figure 4

Mean values from GEANT simulations of the energy deposited in the EMC by various hadronic species as a function of momentum.

Figure 5

Spatial profiles of energy deposition in the EMC as a function of distance $\left(d\right)$ from the hit point to the center of the tower for ${\pi }^{+}$ and ${\pi }^{-}$ from simulations and for positive and negative hadrons from data. The arrow indicates the distance corresponding to the border of a tower in $0<\eta <0.2$. An overall agreement between the shapes of the profiles is observed, with a small normalization difference (see text).

Figure 6

Upper panel: Event-by-event ratio of the reconstructed electromagnetic energy and the input from the event generator as a function of the raw energy measured by the EMC. At 150 GeV, count numbers vary from 10 to 40 counts from the outer to the inner contour lines in steps of $\sim 10$ counts. Lower panel: The same ratio distribution for the most central events. The solid line is a Gaussian fit.

Figure 7

Total transverse energy for $0<\eta <1$. The minimum bias distribution is presented as well as the distributions for the different centrality bins (see Table 3). The shaded area corresponds to the 5% most central bin. The main axis scale corresponds to the $E_T$ measured in the detector acceptance and the bottom axis is corrected to represent the extrapolation to full azimuthal acceptance.

Figure 8

$⟨d{E}_T∕d\eta {\mid }_{\eta =0.5}⟩$ per $N_{\mathrm{part}}$ pair vs $N_{\mathrm{part}}$. Upper panel: $N_{\mathrm{part}}$ is obtained from Monte Carlo Glauber calculations. The lines show calculations using the HIJING model [27] (solid), the EKRT saturation model [7] [dotted, Eq. (8)], and the two-component fit (dashed, see text). Results from WA98 [14] and PHENIX [15] are also shown. The gray bands correspond to overall systematic uncertainties, independent of $N_{\mathrm{part}}$. Error bars are the quadrature sum of the errors on the measurements and the uncertainties on $N_{\mathrm{part}}$ calculation. Lower panel: the same data are shown as in the upper panel but using and optical Glauber model calculation for $N_{\mathrm{part}}$. The line shows the same result from EKRT model calculation.

Figure 9

$d{E}_T∕dy$ (see text for details) per $N_{\mathrm{part}}$ pair vs $\sqrt{s_{NN}}$ for central events. In this figure, $d{E}_T∕dy∕\left(0.5N_{\mathrm{part}}\right)$ is seen to grow logarithmicaly with $\sqrt{s_{NN}}$. The error bar in the STAR point represents the total systematic uncertainty. The solid line is a EKRT model prediction [7], corrected for $d\eta ∕dy$, for central $\mathrm{Au}+\mathrm{Au}$ collisions.

Figure 10

Upper panel: $⟨d{E}_T∕d\eta ⟩∕⟨d{N}_{\mathit{ch}}∕d\eta ⟩$ vs $N_{\mathrm{part}}$. Predictions from HIJING simulations for $\mathrm{Au}+\mathrm{Au}$ at 200 GeV are presented. Results from WA98 [14] and PHENIX [15] are also shown. The gray band corresponds to an overall normalization uncertainty for the STAR measurement. Bottom panel: Charged hadrons mean transverse momentum as a function of $N_{\mathrm{part}}$ [40].

Figure 11

$⟨d{E}_T∕d\eta ⟩∕⟨d{N}_{\mathit{ch}}∕d\eta ⟩$ vs $\sqrt{s_{NN}}$ for central events. The error bar in the STAR point corresponds to the systematic uncertainty. A constant value of $\sim 800\mathrm{MeV}$ per charged particle, within errors, characterizes transverse energy production over this full energy range.

Figure 12

Energy dependence of the electromagnetic fraction of the transverse energy for a number of systems spanning SPS to RHIC energy for central events.

Figure 13

Participant number dependence of the electromagnetic fraction of the total transverse energy. The results are consistent with HIJING within errors over the full centrality range.