Measurement of isolated photon-hadron correlations in $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV $pp$ and $p$-Pb collisions

This paper presents isolated photon-hadron correlations using $pp$ and $p$-Pb data collected by the ALICE detector at the LHC. For photons with $\left|\eta \right|<0.67$ and $12<p_{\mathrm{T}}<40$ GeV/$c$, the associated yield of charged particles in the range $\left|\eta \right|<0.80$ and $0.5<p_{\mathrm{T}}<10$ GeV/$c$ is presented. These momenta are much lower than previous measurements at the LHC. No significant difference between $pp$ and $p$-Pb is observed, with Pythia 8.2 describing both data sets within uncertainties. This measurement constrains nuclear effects on the parton fragmentation in $p$-Pb collisions, and provides a benchmark for future studies of Pb-Pb collisions.

Figure 1

${\sigma }_{\mathrm{long}}^2\mathrm{distribution}$ of isolated clusters (black) and template fit results for $p$-Pb data in various $p_{\mathrm{T}}$ ranges. The stacked histograms (yellow for background, blue for signal) show the predicted counts corresponding to the best fit. The bottom panels show the normalized residuals of the fit, with the statistical uncertainty on the isolated cluster data and the background template added in quadrature. The gray shaded region indicates the signal region for the isolated-photon selection. See text for additional details.

Figure 2

Purity of the ${\gamma }^{\mathrm{iso}}$ sample as a function of transverse momentum for $pp$ (red) and $p$-Pb (blue) data. The error bars represent statistical uncertainties only. The red shaded area represents systematic uncertainties in $pp$, while the blue empty boxes represent systematic uncertainties in $p$-Pb. The smooth lines correspond to a three-parameter error function fit to the data.

Figure 3

${\gamma }^{\mathrm{iso}}$–hadron signal region (black circles) and background region (gray squares) correlations in $pp$ collisions at $\sqrt{s}$ = 5.02 TeV as measured by the ALICE detector. The shower signal region photons correspond to isolated clusters with ${\sigma }_{\mathrm{long}}^2<0.3$, while the shower background region photons correspond to isolated clusters with ${\sigma }_{\mathrm{long}}^2>0.4$. The vertical bars represent statistical uncertainty only. The horizontal bars represent the bin width in $\left|\mathrm{\Delta }\phi \right|$. The background correlation is subtracted from the signal correlation according to the numerator in Eq. (10).

Figure 4

${\gamma }^{\mathrm{iso}}$-hadron correlation functions for $pp$ (red) and $p$-Pb (blue) data at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV as measured by the ALICE detector. The different panels represent three different $z_{\mathrm{T}}$ bins. The correlation functions are projected over the range $\left|\mathrm{\Delta }\eta \right|<1.2$. The darker bands at zero represents the uncertainty from the underlying event estimation in $pp$ and $p$-Pb. The underlying event was estimated over the range $0.4<\left|\mathrm{\Delta }\phi \right|<1.6$. The vertical bars represent statistical uncertainties only. The boxes indicate the systematic uncertainties. The dashed green line represents the ${\gamma }^{\mathrm{iso}}$–hadron correlation function obtained with PYTHIA 8.2 Monash Tune. “$p$” is the $p$ value for the hypothesis that the $pp$ and $p$-Pb data follow the same true correlation function.

Figure 5

${\gamma }^{\mathrm{iso}}$-tagged fragmentation function for $pp$ (red) and $p$-Pb data (blue) at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV as measured by the ALICE detector. The boxes represent the systematic uncertainties while the vertical bars indicate the statistical uncertainties. The dashed green line corresponds to PYTHIA 8.2. The ${\chi }^2$ test for the comparison of $pp$ and $p$-Pb data incorporates correlations among different $z_{\mathrm{T}}$ intervals. A constant that was fit to the ratio including statistical and systematic uncertainties is shown as gray band, with the width indicating the uncertainty on the fit.