Observation of $\tau$ Lepton Pair Production in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$

We present an observation of photon-photon production of $\tau$ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of $404\mathrm{\mu }{\mathrm{b}}^{-1}$ collected by the CMS experiment at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$. The $\gamma \gamma \to {\tau }^{+}{\tau }^{-}$ process is observed for ${\tau }^{+}{\tau }^{-}$ events with a muon and three charged hadrons in the final state. The measured fiducial cross section is $\sigma (\gamma \gamma \to {\tau }^{+}{\tau }^{-})=4.8\pm 0.6(\text{stat})\pm 0.5(\text{syst})\mathrm{\mu }\mathrm{b}$, where the second (third) term corresponds to the statistical (systematic) uncertainty in $\sigma (\gamma \gamma \to {\tau }^{+}{\tau }^{-})$ in agreement with leading-order QED predictions. Using $\sigma (\gamma \gamma \to {\tau }^{+}{\tau }^{-})$, we estimate a model-dependent value of the anomalous magnetic moment of the $\tau$ lepton of $a_{\tau }=0.001_{-0.089}^{+0.055}$.

Figure 1

Left: transverse momentum of the muon originating from the ${\tau }_{\mu }$ candidate. Middle: invariant mass of the three pions forming the ${\tau }_{3\mathrm{prong}}$ candidate. Right: ${\tau }^{+}{\tau }^{-}$ invariant mass. In all plots, the signal component (magenta histogram) is stacked on top of the background component (green histogram), considering their initial normalizations, as described in the text. The sum of signal and background is displayed by a blue line, and the shaded area shows the statistical uncertainty. The data are represented with black points, and the uncertainty is statistical only. The lower panels show the ratios of data to the signal-plus-background prediction, and the shaded bands represent the statistical uncertainty in the prefit expectation.

Figure 2

Difference in azimuthal opening angle between the ${\tau }_{\mu }$ and ${\tau }_{3\mathrm{prong}}$ candidates. The data are represented by the points with the vertical bars showing the statistical uncertainties. The signal (background) contribution is given by the magenta (green) histogram, after the application of the fit procedure. The total is displayed by a blue line, and the shaded area shows the combined statistical and systematic uncertainties. The lower panel shows the ratio of data to the signal plus background prediction, and the shaded band represents the total uncertainty in the postfit prediction.

Figure 3

The cross section, $\sigma (\gamma \gamma \to {\tau }^{+}{\tau }^{-})$, measured in a fiducial phase space region at $\sqrt{s_{\mathit{NN}}}=5.02\mathrm{TeV}$. The theoretical predictions [3, 4] are computed with leading-order accuracy in QED and are represented by the vertical solid lines that can be compared with the vertical dotted line representing this measurement. The outer blue (inner red) error bars surrounding the data point indicate the total (statistical) uncertainties, whereas the green hatched bands correspond to the uncertainty in the theoretical predictions as described in the main text. The potential electromagnetic excitation of the outgoing Pb ions is denoted by ${(}^{*})$.