Measurement of isolated prompt photon production in pp and p-Pb collisions at the LHC

TL;DR

This work reports ALICE measurements of isolated prompt photon production in pp and p--Pb collisions at the LHC, using a charged isolation threshold $p_T^{iso,\,ch}<1.5$ GeV/$c$ in a cone of $R=0.4$ at midrapidity. By employing shower-shape discrimination and data-driven purity corrections (ABCD and template fits), the study provides cross sections that align with NLO pQCD predictions using modern PDFs and nuclear PDFs, and examines the nuclear modification factor $R_{pA}$. The results show $R_{pA}\approx 1$ for $p_T>20$ GeV/$c$, with hints of suppression at lower $p_T$ compatible with gluon-shadowing in Pb, and extend the low-$x$ reach to $x\approx 0-3$ through $2 p_T/\sqrt{s}$ scaling. These findings offer meaningful constraints for nuclear PDF fits and demonstrate the viability of isolated prompt photons as probes of the initial state in nuclear collisions.

Abstract

This paper presents the measurement of the isolated prompt photon inclusive production cross section in pp and p-Pb collisions by the ALICE Collaboration at the LHC. The measurement is performed in p-Pb collisions at centre-of-mass energies per nucleon pair of $\sqrt{s_{\rm{NN}}} = 5.02$ TeV and $8.16$ TeV, as well as in pp collisions at $\sqrt{s} = 5.02$ TeV and $8$ TeV. The cross section is obtained at midrapidity $(|y|<0.7)$ using a charged-track based isolation momentum $p_{\rm T}^{\rm{iso, ch}} < 1.5$ GeV/$c$ in a cone with radius $R=0.4$. The data for both collision systems are well reproduced by perturbative QCD (pQCD) calculations at next-to-leading order (NLO) using recent parton distribution functions for free (PDF) and bound (nPDF) nucleons. Furthermore, the nuclear modification factor $R_{\text{pA}}$ for both collision energies is consistent with unity for $p_{\rm T} > 20$ GeV/$c$. However, deviations from unity ($R_{\rm pA}<1$) of up to 20% are observed for $p_{\rm T} < 20$ GeV/$c$ with limited significance, indicating the possible presence of nuclear effects in the initial state of the collision. The suppression increases with decreasing \pt with a significance of $2.3σ$ for a non-zero slope and yields $R_{\rm pA}<1$ with a significance of $1.8σ$ at $\sqrt{s_{\rm NN}} = 8.16$ TeV for $p_{\rm T} < 20$ GeV/$c$. In addition, a significance of $1.1σ$ is observed for $R_{\rm pA}<1$ at the lower collision energy $\sqrt{s_{\rm NN}} = 5.02$ TeV for $p_{\rm T} < 14$ GeV/$c$. The magnitude and shape of the suppression are consistent with pQCD predictions at NLO using nPDFs that incorporate nuclear shadowing effects in the Pb nucleus.

Figures (6)

• Figure 1: Probability distribution of the charged isolation momentum $p_{\text{\tiny T }}^{\text{iso,~ch}}$ (see Eq. \ref{['eq:isolation']}) in pp collisions at $\sqrt{s}=8TeV$. The isolation threshold $p_{\text{\tiny T }}^{\text{iso,~ch}}<1.5GeV/c$ is drawn as a dashed grey line. The $p_{\text{\tiny T }}^{\text{iso,~ch}}$ distribution is shown for data (black) and PYTHIA 8 simulations for a signal ($\gamma$--jet) and background (jet--jet) dominated population.
• Figure 2: Isolated prompt photon reconstruction efficiencies calculated according to Eq. \ref{['eq:efficiency']} using PYTHIA 8 simulations of $\gamma$--jet (signal) processes.
• Figure 3: Isolated prompt photon purity as a function of cluster $p_{\rm T}$. The purity in p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02TeV$ (left) is calculated using the template fit approach, whereas for $\sqrt{s_{\mathrm{NN}}}\xspace= 8$ and 8.16TeV (right) the purity is obtained using the ABCD method. The purity is fitted using a modified sigmoid function given in Eq. \ref{['eq:sigmoid']} and an error function for $\sqrt{s_{\mathrm{NN}}}\xspace=5.02TeV$. Vertical lines and boxes denote statistical and systematic uncertainties, respectively.
• Figure 4: Isolated prompt photon inclusive production cross section in pp collisions at $\sqrt{s}=5.02$isophotonsppandPbPb5TeV and 8TeV, and p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02$ and 8.16TeV. Vertical bars and boxes denote statistical and systematic uncertainties, respectively. Coloured lines denote JETPHOX pQCD calculations at NLO using the recent NNPDF4.0 NNPDF40 proton PDF and the nNNPDF3.0 nNNPDF30 nuclear PDF. The BFG II BFGII fragmentation function is used to describe parton-to-photon fragmentation. Dashed coloured bands denote the theoretical scale uncertainties, and solid bands denote PDF uncertainties at 90% CL.
• Figure 5: Nuclear modification factor ($R_{\text{pA}}$) of isolated prompt photon production in p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02$ and 8.16TeV. For illustration purposes, the data points at $\sqrt{s_{\text{NN}}}=5.02TeV$ are displaced by $\Delta p_{\rm T}=+300MeV/c$. Vertical bars and boxes denote the statistical and systematic uncertainties, respectively. Coloured boxes around unity denote the respective normalisation uncertainties. The measurement is compared to pQCD calculations using recent (n)PDFs, where the shaded band denotes the nPDF uncertainties. The nPDF uncertainties of the prediction at $\sqrt{s_{\mathrm{NN}}}\xspace=5.02$ coincide with those at 8.16TeV and are therefore omitted for visibility. Theoretical scale uncertainties are fully correlated between both collision systems and are therefore not shown. For improved visibility, the right panel shows only the $R_{\text{pA}}$ at $\sqrt{s_{\text{NN}}}=8.16TeV$ compared to pQCD calculations at NLO using the nNNPDF3.0 nNNPDF30 and nCTEQ15HQ nCTEQ15HQ nPDFs.