Azimuthal Correlations within Exclusive Dijets with Large Momentum Transfer in Photon-Lead Collisions

The structure of nucleons is multidimensional and depends on the transverse momenta, spatial geometry, and polarization of the constituent partons. Such a structure can be studied using high-energy photons produced in ultraperipheral heavy-ion collisions. The first measurement of the azimuthal angular correlations of exclusively produced events with two jets in photon-lead interactions at large momentum transfer is presented, a process that is considered to be sensitive to the underlying nuclear gluon polarization. This study uses a data sample of ultraperipheral lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, corresponding to an integrated luminosity of $0.38{\mathrm{nb}}^{-1}$, collected with the CMS experiment at the LHC. The measured second harmonic of the correlation between the sum and difference of the two jet transverse momentum vectors is found to be positive, and rising, as the dijet transverse momentum increases. A well-tuned model that has been successful at describing a wide range of proton scattering data from the HERA experiments fails to describe the observed correlations, suggesting the presence of gluon polarization effects.

Figure 1

Magnitudes of the vector sum ($Q_T$) and vector difference ($P_T$) of the two jets (left). The dashed blue line illustrates the $Q_T<25\mathrm{GeV}$ requirement. Invariant mass (center) and rapidity (right) of the dijet candidates after all selection requirements. The lines show the rapgap MC generated events including detector resolution effects. The statistical uncertainties are covered by the symbol size.

Figure 2

The unfolded $1/N_{\text{events}}dN/d\mathrm{\Phi }$ distribution (upper) and the unfolded $⟨\cos (2\mathrm{\Phi })⟩$ values as a function of $Q_T$ (lower). The corresponding distributions from the rapgap simulation at the generator level (blue lines) and theoretical calculation by Hatta et al. [20] for $⟨\cos (2\mathrm{\Phi })⟩$ (red dashed lines) are also shown for $Q_T<15\mathrm{GeV}$, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $0<{\eta }_{\text{jet}}<2.4$. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.