Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at $\sqrt{s_{NN}}=2.76$ TeV measured with the ATLAS detector

Prompt photon production in $\sqrt{s_{NN}}=2.76$-TeV $\mathrm{Pb}+\mathrm{Pb}$ collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of $0.14 {\mathrm{nb}}^{-1}$. Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, $\left|\eta \right|<1.37$ and $1.52\le \left|\eta \right|<2.37$. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading-order perturbative QCD (pQCD) calculations. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and the ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei and another which includes nuclear modifications to the nucleon parton distribution functions.

Figure 1

Comparisons of distributions of three shower-shaped variables ($w_{\mathrm{s},3},w_{\eta ,2}$, and $R_{\mathrm{had}}$) from data (black points) with simulation results after shower shape corrections (yellow histogram) for tight and isolated photons with reconstructed $35\le p_{\text{T}}<44.1$ GeV and $\left|\eta \right|<1.37$. Events from the 0%–10% centrality interval are shown in the top row (a)–(c), while those from the 40%–80% interval are shown in the bottom row (d)–(f).

Figure 2

Distributions of photon isolation transverse energy in a ${\mathrm{\Delta }R}_{\mathrm{iso}}=0.3$ cone for the four centrality bins in data (black points, normalized by the number of events and by the histogram bin width) for photons with $35\le p_{\text{T}}<44.1$ GeV. In the left column (a)–(d) simulations (yellow histogram) are normalized to the data so that the integrals in the range $E_{\mathrm{T}}^{\mathrm{iso}}<0$ are the same. The corresponding sample of nontight photon candidates, normalized to the distribution of tight photons for $E_{\mathrm{T}}^{\mathrm{iso}}\ge 8$ GeV is shown overlaid on the tight photon data in the right column (e)–(h) to illustrate the source of the photons with large $E_{\mathrm{T}}^{\mathrm{iso}}$.

Figure 3

Illustration of the double-sideband approach, showing the two axes for partitioning photon candidates: region A is the “signal region” (tight and isolated photons); region B contains tight, nonisolated photons, region C contains nontight isolated photons; and region D contains nontight and nonisolated photons.

Figure 4

Photon purity as a function of collision centrality (left to right) and photon $p_{\text{T}}$ for photons measured in $\left|\eta \right|<1.37$ [(a)–(d)] and $1.52\le \left|\eta \right|<2.37$ [(e)–(h)]. The $p_{\text{T}}$ intervals to the right of the vertical dotted line indicated in some bins use the extrapolation method described in the text to account for low event counts in the sidebands.

Figure 5

Total photon efficiency as a function of photon $p_{\text{T}}$ and event centrality averaged over $\left|\eta \right|<1.37$ [(a)–(d)] and $1.52\le \left|\eta \right|<2.37$ [(e)–(h)]. Variations of the efficiency from removing the small corrections to the simulated shower-shape variable, and from removing fragmentation photons from the simulations are shown by dotted and dashed lines, respectively.

Figure 6

Fully corrected yields of prompt photons in four centrality intervals as a function of $p_{\text{T}}$ in $\left|\eta \right|<1.37$ (a) and $1.52\le \left|\eta \right|<2.37$ (b) using tight selection, isolation cone size ${\mathrm{\Delta }R}_{\mathrm{iso}}=0.3$, and isolation transverse energy of less than 6 GeV. jetphox calculations, for proton-proton collisions and using the same isolation criterion, are shown by the yellow bands. Statistical uncertainties are shown by the error bars. Systematic uncertainties on the photon yields are shown by braces, which are smaller than the markers for some points. The scale uncertainties owing to $\langle T_{\mathrm{AA}}\rangle$ are tabulated for each bin in Table 1.

Figure 7

Fully corrected normalized yields of prompt photons as a function of $p_{\text{T}}$ in $\left|\eta \right|<1.37$ [(a)–(d)] and $1.52\le \left|\eta \right|<2.37$ [(e)–(h)] using tight photon selection, isolation cone size ${\mathrm{\Delta }R}_{\mathrm{iso}}=0.3$, and isolation transverse energy of less than 6 GeV, divided by jetphox predictions for $pp$ collisions, which implement the same isolation selection. The combined scale and PDF uncertainty on the jetphox calculation is shown by the gray line with yellow area. In addition, two other jetphox calculations are shown, also divided by the $pp$ results: $\mathrm{Pb}+\mathrm{Pb}$ collisions with no nuclear modification (black line with gray area) and $\mathrm{Pb}+\mathrm{Pb}$ collisions with EPS09 nuclear modifications (gray line with blue area). Statistical uncertainties are shown by the bars. Systematic uncertainties on the photon yields are combined and shown by the upper and lower braces. The scale uncertainties owing only to $\langle T_{\mathrm{AA}}\rangle$ are tabulated for each bin in Table 1.

Figure 8

Fully corrected yields of prompt photons as a function of $p_{\text{T}}$ in $1.52\le \left|\eta \right|<2.37$ divided by that measured in $\left|\eta \right|<1.37$ using the tight photon selection, isolation cone size ${\mathrm{\Delta }R}_{\mathrm{iso}}=0.3$, and isolation transverse energy of 6 GeV for four centrality intervals [(a)–(d)]. The yield ratio is compared to jetphox 1.3 predictions that implement the same isolation selection for three different configurations: $pp$ collisions (gray line with yellow area), $\mathrm{Pb}+\mathrm{Pb}$ collisions with no nuclear modification (black line with grey area), and $\mathrm{Pb}+\mathrm{Pb}$ collisions with EPS09 nuclear modifications (gray line with blue area). Statistical uncertainties are shown by the bars. Systematic uncertainties on the photon yields are combined and shown by the braces.