CMS-PAS-HIN-24-011

CMS-PAS-HIN-24-011
Measurement of the tau g$- factor in ultraperipheral PbPb collisions recorded by the CMS experiment
CMS Collaboration
9 October 2024

Abstract: Measurements of the anomalous magnetic moment of leptons provide stringent tests of the standard model and potential hints of physics beyond the standard model. These measurements for electrons and muons are among the most precisely measured quantities in physics. However, due to the short lifetime of the tau lepton, its anomalous magnetic moment cannot be determined through precession measurements. We report the latest measurement of the anomalous magnetic moment of the tau lepton based on a data sample of ultraperipheral PbPb collisions with an integrated luminosity of up to 1.70 nb $^{-1}$ , depending on the decay channel, 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 dependence on $Z^4$ ( $Z =$ 82 for lead) enhances the cross section for photoproduction with respect to proton-proton and electron-positron collisions. This measurement is improved with respect to the previous CMS measurement via the addition of three tau pair decay modes, a factor of four in luminosity, and the incorporation of both cross section and kinematic distributions in the determination of g $-$ 2. The measured value of the $\gamma\gamma \to \tau^{+}\tau^{-}$ fiducial cross section is the most precise to date, while the g $-$ 2 measurement is one of the most precise both in PbPb and $\mathrm{e^{+}e^{-}}$ collisions.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the $\mathrm{Pb}\mathrm{Pb}\to\mathrm{Pb}^{(\star)}(\gamma\gamma \to \tau^{+}\tau^{-})\mathrm{Pb}^{(\star)}$ . The star in $\text{Pb}^\star$ indicates the possible excitation of the lead ion.
png pdf	Figure 2: Neutron emission probability as a function of the dimuon invariant mass for each category of neutron emission.
png pdf	Figure 3: Postfit distributions of lepton $p_{\mathrm{T}}$ for the $\mu$ +1 prong (upper left), $\mu$ +3 prong (upper right), $\mu$ +e (lower left), and e+3 prong (lower right) channels. Signal distributions are stacked on top of the background model, consisting of $\gamma\gamma \to \mu^{+}\mu^{-}\gamma$ , $\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-$ , and/or the inclusive background predicted by the ABCD method. The stacked distributions are compared with data (black points).
png pdf	Figure 3-a: Postfit distributions of lepton $p_{\mathrm{T}}$ for the $\mu$ +1 prong (upper left), $\mu$ +3 prong (upper right), $\mu$ +e (lower left), and e+3 prong (lower right) channels. Signal distributions are stacked on top of the background model, consisting of $\gamma\gamma \to \mu^{+}\mu^{-}\gamma$ , $\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-$ , and/or the inclusive background predicted by the ABCD method. The stacked distributions are compared with data (black points).
png pdf	Figure 3-b: Postfit distributions of lepton $p_{\mathrm{T}}$ for the $\mu$ +1 prong (upper left), $\mu$ +3 prong (upper right), $\mu$ +e (lower left), and e+3 prong (lower right) channels. Signal distributions are stacked on top of the background model, consisting of $\gamma\gamma \to \mu^{+}\mu^{-}\gamma$ , $\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-$ , and/or the inclusive background predicted by the ABCD method. The stacked distributions are compared with data (black points).
png pdf	Figure 3-c: Postfit distributions of lepton $p_{\mathrm{T}}$ for the $\mu$ +1 prong (upper left), $\mu$ +3 prong (upper right), $\mu$ +e (lower left), and e+3 prong (lower right) channels. Signal distributions are stacked on top of the background model, consisting of $\gamma\gamma \to \mu^{+}\mu^{-}\gamma$ , $\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-$ , and/or the inclusive background predicted by the ABCD method. The stacked distributions are compared with data (black points).
png pdf	Figure 3-d: Postfit distributions of lepton $p_{\mathrm{T}}$ for the $\mu$ +1 prong (upper left), $\mu$ +3 prong (upper right), $\mu$ +e (lower left), and e+3 prong (lower right) channels. Signal distributions are stacked on top of the background model, consisting of $\gamma\gamma \to \mu^{+}\mu^{-}\gamma$ , $\gamma\gamma \to \mathrm{e}^+\mathrm{e}^-$ , and/or the inclusive background predicted by the ABCD method. The stacked distributions are compared with data (black points).
png pdf	Figure 4: Left: Simultaneous limits on cross section of $\mathrm{Pb}\mathrm{Pb}\to\mathrm{Pb}^{(\star)}(\gamma\gamma \to \tau^{+}\tau^{-})\mathrm{Pb}^{(\star)}$ and $a_{\tau}$ , using the sensitivity of lepton $p_{\mathrm{T}}$ to $a_{\tau}$ . Middle and right: Limits on the signal cross section and $a_{\tau}$ , using the sensitivity of both cross section and lepton $p_{\mathrm{T}}$ to $a_{\tau}$ .
png pdf	Figure 4-a: Left: Simultaneous limits on cross section of $\mathrm{Pb}\mathrm{Pb}\to\mathrm{Pb}^{(\star)}(\gamma\gamma \to \tau^{+}\tau^{-})\mathrm{Pb}^{(\star)}$ and $a_{\tau}$ , using the sensitivity of lepton $p_{\mathrm{T}}$ to $a_{\tau}$ . Middle and right: Limits on the signal cross section and $a_{\tau}$ , using the sensitivity of both cross section and lepton $p_{\mathrm{T}}$ to $a_{\tau}$ .
png pdf	Figure 4-b: Left: Simultaneous limits on cross section of $\mathrm{Pb}\mathrm{Pb}\to\mathrm{Pb}^{(\star)}(\gamma\gamma \to \tau^{+}\tau^{-})\mathrm{Pb}^{(\star)}$ and $a_{\tau}$ , using the sensitivity of lepton $p_{\mathrm{T}}$ to $a_{\tau}$ . Middle and right: Limits on the signal cross section and $a_{\tau}$ , using the sensitivity of both cross section and lepton $p_{\mathrm{T}}$ to $a_{\tau}$ .
png pdf	Figure 4-c: Left: Simultaneous limits on cross section of $\mathrm{Pb}\mathrm{Pb}\to\mathrm{Pb}^{(\star)}(\gamma\gamma \to \tau^{+}\tau^{-})\mathrm{Pb}^{(\star)}$ and $a_{\tau}$ , using the sensitivity of lepton $p_{\mathrm{T}}$ to $a_{\tau}$ . Middle and right: Limits on the signal cross section and $a_{\tau}$ , using the sensitivity of both cross section and lepton $p_{\mathrm{T}}$ to $a_{\tau}$ .
png pdf	Figure 5: Breakdown of the 68% and 95% limits on $a_{\tau}$ in the four considered decay channels and the combined result, compared to a set of best previous measurements.

Summary

We report the best measurement of the

$\gamma\gamma \to \tau^{+}\tau^{-}$ fiducial cross section,

$\sigma(\gamma\gamma \to \tau^{+}\tau^{-})=$ 447

$^{+16}_{-11}$ (stat+sys)

$\mu$ b, using a data sample of PbPb collisions at a center-of-mass energy per nucleon pair of

$\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 5.02 TeV. The precision is partly made possible by the high signal efficiency obtained with relaxed requirements on the transverse momenta of the visible decay products of the tau lepton with respect to other measurements. We also report a measurement of

$a_{\tau}= -$ 35

$^{+18}_{-10}$ (stat+sys)

$\times$ 10

$^{-3}$ . This result represents more than a factor of four decrease in uncertainty on the limits of

$a_{\tau}$ as compared to the previous CMS measurement with the same colliding system. This is due to a factor of four increase in luminosity, the inclusion of the

$\mu$ +1 prong, e+3 prong, and

$\mu$ +e tau-tau decay modes, as well as a more sophisticated likelihood extraction using both the visible

$\tau$ lepton kinematics as well as the

$\tau^{+}\tau^{-}$ cross section. The measurement is made in a complementary phase space of

$\tau^{+}\tau^{-}$ invariant mass as compared to the CMS measurement in proton-proton collisions [5], is compatible with previous measurements, and is of similar sensitivity to the best measurements from ATLAS [23] and DELPHI [24].

References
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