CMS-PAS-EXO-22-007

CMS-PAS-EXO-22-007
Search for excited tau leptons in the $ \tau\tau\gamma $ final state in pp collisions at $ \sqrt{s}= $ 13 TeV
CMS Collaboration
5 June 2024

Abstract: We report the results of a test of the compositeness of the heaviest charged lepton, $ \tau $, using data collected in proton-proton collisions at a center of mass energy of 13 TeV at the Large Hadron Collider at CERN in 2016--2018, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. We seek tau lepton pair production in which one tau lepton is produced in an excited state and decays to a ground state tau lepton and photon. The analysis selects events with two isolated tau lepton decay candidates plus a high energy photon. The mass of the excited tau lepton is reconstructed using the missing transverse momentum in the event, assuming the momentum of the neutrinos from each tau lepton decay lie along the visible tau lepton decay product direction. We observe no excess of events above the standard model background prediction and use this null result to set lower bounds on the excited tau lepton mass. For a compositeness scale $ \Lambda $ equal to the excited tau lepton mass (10 TeV), we exclude excited tau leptons with masses below 4700 GeV (2800 GeV).
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram representing the contact-interaction production of a single excited tau lepton ($ \tau^* $) in association with a tau lepton. The excited tau lepton decays to the ground state via the emission of a photon.
png pdf	Figure 2: Left, 2-dimensional collinear mass distributions for example signal mass points. Right, a diagram showing the signal binning scheme.
png pdf	Figure 2-a: Left, 2-dimensional collinear mass distributions for example signal mass points. Right, a diagram showing the signal binning scheme.
png pdf	Figure 2-b: Left, 2-dimensional collinear mass distributions for example signal mass points. Right, a diagram showing the signal binning scheme.
png pdf	Figure 3: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (left), $ \mu\tau_\mathrm{h}\gamma $ (center) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (right) final states. Of the two mass pairings, the minimum value is along the x-axis and the maximum value is along the y-axis.
png pdf	Figure 3-a: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (left), $ \mu\tau_\mathrm{h}\gamma $ (center) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (right) final states. Of the two mass pairings, the minimum value is along the x-axis and the maximum value is along the y-axis.
png pdf	Figure 3-b: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (left), $ \mu\tau_\mathrm{h}\gamma $ (center) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (right) final states. Of the two mass pairings, the minimum value is along the x-axis and the maximum value is along the y-axis.
png pdf	Figure 3-c: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (left), $ \mu\tau_\mathrm{h}\gamma $ (center) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (right) final states. Of the two mass pairings, the minimum value is along the x-axis and the maximum value is along the y-axis.
png pdf	Figure 4: Observed upper limits on the cross section times branching fraction, as a function of the $ \tau^* $ mass, for single $ \tau^* $ production via a contact interaction in association with a SM $ \tau $. The dashed line shows the expected exclusion limit. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.
png pdf	Figure 5: Observed upper limits on the cross section times branching fraction, as a function of the $ \tau^* $ mass, for single $ \tau^* $ production via a contact interaction in association with a SM $ \tau $. The limits are shown for when the final states are treated as independent.

Tables
png pdf	Table 1: Boundaries of the signal bin for each $ \tau^* $ mass considered.
png pdf	Table 2: Post-fit $ \kappa $ values as calculated in the simultaneous background-only fit of all signal and control regions.
png pdf	Table 3: Summary of systematic uncertainties that affect the analysis by at least 1%. Values are in $ % $ of the inclusive yield. For the $ \tau_\mathrm{h} $ containing columns the first number is the value for the background and the second number is the value for the signal ($ m_{\tau^{*}}= $ 1750 GeV and $ \Lambda= $ 10 TeV).
png pdf	Table 4: Observed events and expected background yields. The expected background yields are post-fit under the background-only hypothesis, inclusive in the collinear mass. The expected $ \tau^{*} $ yields are evaluated at $ \Lambda=$ 10 TeV and errors are statistical.

Summary

We have performed a search for excited tau leptons ($ \tau^* $) using events with two tau leptons and a photon in the final state, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. We assume that the excited tau lepton is produced via a contact interaction in association with a ground state tau lepton. The mass of the $ \tau^* $ is reconstructed from the missing transverse momentum, assuming the momenta of the neutrinos from the $ \tau $ decays lie along the visible $ \tau $ decay product directions. No excess of events is observed above the expected SM backgrounds, and we set upper limits on the cross section times branching fraction for this process. Assuming a compositeness scale $ \Lambda $ equal to the $ \tau^* $ mass (10 TeV) we exclude $ \tau^* $ masses less than 4700 GeV (2800 GeV).

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