Studies of jet quenching using isolated-photon + jet correlations in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV

TL;DR

This work reports the first CMS measurement of isolated-photon+jet correlations in PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV, using $150\ \mu\text{b}^{-1}$ to quantify jet quenching while exploiting the photon as an in-situ parton energy calibrator. By constructing photon+jet pairs with $p_T^γ>60$ GeV and $p_T^{\text{Jet}}>30$ GeV and analyzing $\Delta\phi_{Jγ}$ and $x_{Jγ}$, the study finds no significant angular broadening relative to pp references, but observes centrality-dependent energy loss manifested as a lower $\langle x_{Jγ} \rangle$ and a reduced photon-jet matching fraction $R_{Jγ}$ in the most central events. The results, supported by data-driven background subtraction and MC references (PYTHIA+HYDJET and pyquen+hydjet), provide compelling evidence for in-medium parton energy loss and demonstrate the photon+jet channel as an unbiased tomographic probe of the quark-gluon plasma. These findings offer quantitative benchmarks for jet-quenching models and constrain fragmentation and energy-deposition mechanisms in heavy-ion collisions.

Abstract

Results from the first study of isolated-photon + jet correlations in relativistic heavy ion collisions are reported. The analysis uses data from PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair corresponding to an integrated luminosity of 150 inverse microbarns recorded by the CMS experiment at the LHC. For events containing an isolated photon with transverse momentum pt(gamma) > 60 GeV and an associated jet with pt(Jet) > 30 GeV, the photon + jet pt imbalance is studied as a function of collision centrality and compared to pp data and PYTHIA calculations at the same collision energy. Using the pt(gamma) of the isolated photon as an estimate of the momentum of the associated parton at production, this measurement allows an unbiased characterisation of the in-medium parton energy loss. For more central PbPb collisions, a significant decrease in the ratio pt(Jet)/pt(gamma) relative to that in the PYTHIA reference is observed. Furthermore, significantly more pt(gamma) > 60 GeV photons in PbPb are observed not to have an associated pt(Jet) > 30 GeV jet, compared to the reference. However, no significant broadening of the photon + jet azimuthal correlation is observed.

Figures (4)

• Figure 1: Azimuthal correlation $\Delta\phi_{J\gamma}\xspace$ between the photon and associated jet after background subtraction. The area of each distribution is normalised to unity. All panels show $\text{PbPb}$ data (filled circles) compared to pp data at 2.76$\,\text{Te\spaceV}$ (filled squares), and to the pythia + hydjet MC simulation (shaded histogram) in bins of increasing centrality left to right. The error bars on the points represent the statistical uncertainty.
• Figure 2: Fitted $\Delta\phi_{J\gamma}$ width ($\sigma$ in Eq. \ref{['eq:sig']}) between the photon and associated jet after background subtraction as a function of $N_{\text{part}}\xspace$. The fit range was restricted to $\Delta\phi_{J\gamma}\xspace > \frac{2}{3}\pi$. The yellow boxes indicate point-to-point systematic uncertainties and the error bars denote the statistical uncertainty.
• Figure 3: Ratio of $p_{\mathrm{T}}$ between the photon ($p_{\mathrm{T}}\xspace^{\gamma} > 60$${\,\text{Ge\spaceV\space/\space}c}$) and jet ($p_{\mathrm{T}}\xspace^{\text{Jet}} > 30$${\,\text{Ge\spaceV\space/\space}c}$, $\Delta\phi_{J\gamma}\xspace > \frac{7}{8}\pi$) after subtracting background. The area of each distribution is normalised to unity. All panels show $\text{PbPb}$ data (filled circles) compared to pp data at 2.76$\,\text{Te\spaceV}$ (filled squares), and to the pythia + hydjet MC simulation (shaded histogram) in bins of increasing centrality left to right. The error bars on the points represent the statistical uncertainty. See text for an explanation of the open and shaded red systematic uncertainty boxes.
• Figure 4: (a) Average ratio of jet transverse momentum to photon transverse momentum, $\langle x_{J\gamma}\rangle\xspace$, as a function of $N_{\text{part}}\xspace$. The empty box at the far right indicates the correlated systematic uncertainty. (b) Average fraction of isolated photons with an associated jet above 30${\,\text{Ge\spaceV\space/\space}c}$, $R_{J\gamma}\xspace$, as a function of $N_{\text{part}}\xspace$. In both panels, the yellow boxes indicate point-to-point systematic uncertainties and the error bars denote the statistical uncertainty.