CMS-HIN-21-009

CMS-HIN-21-009 ; CERN-EP-2022-098
Observation of $ \tau $ lepton pair production in ultraperipheral lead-lead collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV
CMS Collaboration
10 June 2022
Phys. Rev. Lett. 131 (2023) 151803
Abstract: 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 $\mu$b$^{-1}$ collected by the CMS experiment at a center-of-mass energy per nucleon pair of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 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 $ \pm $ 0.5 $\mu$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 $ ^{+0.055}_{-0.089} $.
Links: e-print arXiv:2206.05192 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Left: Transverse momentum of the muon originating from the ${\tau _{\mu}}$ candidate. Middle: Invariant mass of the three pions forming the ${\tau _{\text {3prong}}}$ 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.
png pdf	Figure 1-a: Transverse momentum of the muon originating from the ${\tau _{\mu}}$ candidate. 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 panel shows the ratio of data to the signal-plus-background prediction, and the shaded band represents the statistical uncertainty in the prefit expectation.
png pdf	Figure 1-b: Invariant mass of the three pions forming the ${\tau _{\text {3prong}}}$ candidate. 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 panel shows the ratio of data to the signal-plus-background prediction, and the shaded band represents the statistical uncertainty in the prefit expectation.
png pdf	Figure 1-c: ${\tau^{+} \tau^{-} }$ invariant mass. 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 panel shows the ratio of data to the signal-plus-background prediction, and the shaded band represents the statistical uncertainty in the prefit expectation.
png pdf	Figure 2: Difference in azimuthal opening angle between the ${\tau _{\mu}}$ and $ {\tau _{\text {3prong}}} $ 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.
png pdf	Figure 3: The cross section, ${\sigma ({\gamma \gamma \to {\tau^{+} \tau^{-} }})}$, measured in a fiducial phase space region at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 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 *.

Summary

In summary, an observation of $ \tau $ lepton pair production in ultraperipheral nucleus-nucleus collisions is reported. Events with a final state of one muon and three charged hadrons assumed to be pions are reconstructed from a lead-lead data sample with an integrated luminosity of 404 $\mu$b$^{-1}$ collected by the CMS experiment at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV in 2015. The statistical significance of the signal relative to the background-only expectation is far above five standard deviations. The cross section for the $ \gamma\gamma\to\tau^{+}\tau^{-} $ process, within a fiducial phase space region, is $ \sigma(\gamma\gamma\to\tau^{+}\tau^{-})= $ 4.8 $ \pm $ 0.6 $ \pm $ 0.5 $\mu$b, in agreement with leading-order quantum electrodynamics predictions. Using the measured cross section and its corresponding uncertainties, we estimate a model-dependent value of the anomalous magnetic moment of the $ \tau $ lepton of $ a_{\tau}= $ 0.001 $ ^{+0.055}_{-0.089} $. This measurement provides a novel experimental probe of the $ \tau $ anomalous magnetic moment using heavy ion collisions at the LHC.

Additional Figures
png pdf	Additional Figure 1: Event display of a candidate $\gamma \gamma \to \tau^{+} \tau^{-} $ event measured in PbPb UPC at CMS. The event is reconstructed as having a leptonic $\tau$ decay (red), $\tau \to \mu \nu _\mu \nu _\tau $, and a hadronic $\tau$ decay (yellow), $\tau \to \pi ^\pm \pi ^\mp \pi ^\pm \nu _\tau $.
png pdf	Additional Figure 2: Leading-order QED diagram (and charge conjugate) for the photoproduction of a pair of $\tau$ leptons ${\gamma \gamma \to {\tau^{+} \tau^{-} }}$ in ultraperipheral PbPb collisions. The presence of $\gamma \tau \tau $ vertices gives sensitivity to the anomalous electromagnetic couplings of the $\tau$ lepton. A possible deviation of the anomalous magnetic moment $\delta a_\tau $ is illustrated in each vertex. The $\tau$ leptons are reconstructed in an observed final state involving one muon ($\mu $) and three charged particles assumed as pions ($\pi $), while neutrinos ($\nu $) escape undetected. The potential electromagnetic excitation of the outgoing Pb ions is denoted by *.
png pdf	Additional Figure 3: Comparison of the constraints on $a_\tau $ at 68% confidence level from this analysis and the DELPHI experiment at LEP [12]. The projection to the integrated PbPb luminosity expected from the High-Luminosity LHC program is included (using the same procedure described in the text, based on the parametrization from Ref. [3]). For the latter, we foresee a $\lesssim $ 4 ($ < $ 2) % systematic (statistical) uncertainty with the improvements originating from lepton and tracking reconstruction, and the knowledge of the integrated luminosity [57].

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