Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$

The first measurement of the dependence of $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$, with an integrated luminosity of approximately $1.5{\mathrm{nb}}^{-1}$, are collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region $8<m_{\mu \mu }<60\mathrm{GeV}$ are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range $|\eta |>8.3$. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.

Figure 1

Neutron multiplicity dependence of acoplanarity distributions from $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ for $p_T^{\mu }>3.5\mathrm{GeV}$, $|{\eta }^{\mu }|<2.4$, $|y^{\mu \mu }|<2.4$, and $8<m_{\mu \mu }<60\mathrm{GeV}$ in ultraperipheral Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The $\alpha$ distributions are normalized to unit integral over their measured range. The dot-dot-dashed and dotted lines indicate the core and tail contributions, respectively, found using a fit to Eq. (1). The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties and horizontal bin widths are shown as gray boxes.

Figure 2

Neutron multiplicity dependence of (upper) $⟨{\alpha }^{\mathrm{core}}⟩$ and (lower) $⟨m_{\mu \mu }⟩$ of ${\mu }^{+}{\mu }^{-}$ pairs in ultraperipheral Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The vertical lines on data points depict the statistical uncertainties, while the systematic uncertainties of the data are shown as shaded areas. The dot-dashed line shows the starlight prediction, and the dashed line corresponds to the leading-order QED calculation of Ref. [48].

Figure 3

A direct comparison between data and model calculations for the $\alpha$ distributions with $\alpha <0.008$. The dot-dashed line shows the starlight prediction and the dashed line corresponds to the leading-order QED calculation of Ref. [48]. The dot-dot-dashed and dotted lines indicate the core and tail contributions, respectively. The vertical lines and shaded areas represent the statistical and systematic uncertainties, respectively.