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| CMS-PAS-HIN-21-009 | ||
| Observation of $\tau$ lepton pair production in ultraperipheral nucleus-nucleus collisions | ||
| CMS Collaboration | ||
| March 2022 | ||
| Abstract: The first observation of $\tau$ lepton pair production in ultraperipheral nucleus-nucleus collisions, a pure quantum electrodynamics (QED) process, is presented. The measurement is based on a data sample collected by the CMS experiment at a per nucleon center-of-mass energy of 5.02 TeV, and corresponding to an integrated luminosity of 404 $\mu$b$^{-1}$. The photon-induced $\gamma\gamma\to\tau^{+}\tau^{-}$ production is observed with a statistical significance of at least five standard deviations for $\tau^{+}\tau^{-}$ events with a muon and three charged hadrons in the final state. The cross section is measured in a fiducial phase space region, and is found to be $\sigma(\gamma\gamma\to\tau^{+}\tau^{-}) = $ 4.8 $\pm$ 0.6 (stat) $\pm$ 0.5 (syst) $\mu$b, in agreement with leading-order QED predictions. The measurement, produced with a fraction of the expected integrated luminosity of the LHC program, establishes the potential for a substantially more precise determination of the anomalous magnetic moment of the $\tau$ lepton, which is currently poorly constrained. | ||
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These preliminary results are superseded in this paper, PRL 131 (2023) 151803. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
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. A potential electromagnetic excitation of the outgoing Pb ions is denoted by $(^{\ast})$. |
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Figure 2:
Left: Transverse momentum of the muon originating from the ${\tau _\mu}$ candidate. Middle: Invariant mass of the three pions forming the $\tau _\mathrm {3prong}$ candidate. Right: $\tau \tau $ invariant mass. In all plots, the signal component (pink histogram) is stacked on top of the background component (green histogram), considering their initial normalizations as described in the text. The total 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. |
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Figure 2-a:
Left: Transverse momentum of the muon originating from the ${\tau _\mu}$ candidate. Middle: Invariant mass of the three pions forming the $\tau _\mathrm {3prong}$ candidate. Right: $\tau \tau $ invariant mass. In all plots, the signal component (pink histogram) is stacked on top of the background component (green histogram), considering their initial normalizations as described in the text. The total 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. |
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Figure 2-b:
Left: Transverse momentum of the muon originating from the ${\tau _\mu}$ candidate. Middle: Invariant mass of the three pions forming the $\tau _\mathrm {3prong}$ candidate. Right: $\tau \tau $ invariant mass. In all plots, the signal component (pink histogram) is stacked on top of the background component (green histogram), considering their initial normalizations as described in the text. The total 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. |
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Figure 2-c:
Left: Transverse momentum of the muon originating from the ${\tau _\mu}$ candidate. Middle: Invariant mass of the three pions forming the $\tau _\mathrm {3prong}$ candidate. Right: $\tau \tau $ invariant mass. In all plots, the signal component (pink histogram) is stacked on top of the background component (green histogram), considering their initial normalizations as described in the text. The total 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. |
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Figure 3:
Difference in azimuthal opening angle between the ${\tau _\mu}$ and $ {\tau _\mathrm {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 pink (green) histogram, after the application of the fit procedure. The total is displayed by a blue line and the shaded area shows the total uncertainty. The lower panel shows the ratio of data to the signal plus background prediction, and the shaded band represents the total uncertainty. |
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Figure 4:
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 are computed with leading-order accuracy in QED and represented by the dark and light gray bands [3,4]. |
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Figure A1:
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 $. |
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Figure A2:
Initial agreement between the data and the signal and background models before the fit procedure is applied. Difference in azimuthal opening angle between the $\tau _\mu $ and $\tau _\mathrm {3prong}$ candidates. The signal component (pink histogram) is stacked on top of the background component (green histogram), before the application of the fit procedure. The total is displayed by a blue line and the shaded area shows the statistical uncertainty. The data are represented by the points with the vertical bars showing the statistical uncertainties. The lower panel shows the ratios of data to the signal plus background prediction, and the shaded bands represent the statistical uncertainty. |
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Figure A3:
Comparison of the constraints on $ {a_\tau} $ at 68% confidence level from this analysis (using the parametrization from Ref. [3]) and the DELPHI experiment at LEP. The projection to the integrated PbPb luminosity expected from the High-Luminosity LHC program is included. 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 luminosity. |
| Tables | |
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Table 1:
Summary of the event selection, which coincides with the definition of the fiducial phase space region for the $ {\sigma ({\gamma {}\gamma \to {\tau^{+} {}\tau^{-} {}}})} $ measurement. |
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Table 2:
Postfit contributions to the systematic uncertainty of the ${\sigma ({\gamma {}\gamma \to {\tau^{+} {}\tau^{-} {}}})}$ measurement, in percent. The last row gives the sum in quadrature of all components. |
| Summary |
| In summary, the first observation of $\tau$ lepton pair production in ultraperipheral nucleus-nucleus collisions is reported. Events with a final state of one muon and three charged particles identified as pions are reconstructed from a lead-lead data sample collected by the CMS experiment at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV in 2015, and corresponding to an integrated luminosity of 404 $\mu$b$^{-1}$. The statistical significance of the signal relative to the background-only expectation is above five standard deviations. The cross section for the $\gamma\gamma\tau\tau$ process, within a fiducial phase space region, is in agreement with predictions from quantum electrodynamics at leading-order accuracy. This measurement introduces a novel experimental strategy using heavy ion collisions already recorded by the LHC, which is expected, with the incorporation of additional data, to surpass the precision on the $\tau$ magnetic moment attained previously at lepton-lepton colliders. Using the measured cross section and its corresponding uncertainties, we estimate a limit of ($-$2.4 $ < a_\tau< $ 1.7) $\times $ 10$^{-2}$ with 68% confidence level. |
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