CMS-EXO-22-007

CMS-EXO-22-007 ; CERN-EP-2024-249
Search for excited tau leptons in the $ \tau\tau\gamma $ final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
28 October 2024
Submitted to J. High Energy Phys.
Abstract: Results are presented for a test of the compositeness of the heaviest charged lepton, $ \tau $, using data collected by the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC. The data were collected in 2016--2018 and correspond to an integrated luminosity of 138 fb$ ^{-1} $. This analysis searches for tau lepton pair production in which one of the tau leptons is produced in an excited state and decays to a ground state tau lepton and a photon. The event selection consists of two isolated tau lepton decay candidates and 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 are aligned with the visible decay products. No excess of events above the standard model background prediction is observed. This null result is used to set lower bounds on the excited tau lepton mass. For a compositeness scale $ \Lambda $ equal to the excited tau lepton mass, excited tau leptons with masses below 4700 GeV are excluded at 95% confidence level; for $ \Lambda= $ 10 TeV this exclusion is set at 2800 GeV. This is the first experimental result covering this production and decay process in the excited tau mass range above 175 GeV.
Links: e-print arXiv:2410.21137 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: 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 regions contributing to the two bins in the statistical fit.
png pdf	Figure 2-a: Left, 2-dimensional collinear mass distributions for example signal mass points. Right, a diagram showing the regions contributing to the two bins in the statistical fit.
png pdf	Figure 2-b: Left, 2-dimensional collinear mass distributions for example signal mass points. Right, a diagram showing the regions contributing to the two bins in the statistical fit.
png pdf	Figure 3: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (upper), $ \mu\tau_\mathrm{h}\gamma $ (middle) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (lower) final states.
png pdf	Figure 3-a: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (upper), $ \mu\tau_\mathrm{h}\gamma $ (middle) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (lower) final states.
png pdf	Figure 3-b: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (upper), $ \mu\tau_\mathrm{h}\gamma $ (middle) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (lower) final states.
png pdf	Figure 3-c: Observed signal yields in the 2-D mass plane for the $ \mathrm{e}\tau_\mathrm{h}\gamma $ (upper), $ \mu\tau_\mathrm{h}\gamma $ (middle) and $ \tau_\mathrm{h}\tau_\mathrm{h}\gamma $ (lower) final states.
png pdf	Figure 4: Observed (solid-black line) and expected (dashed line) upper limits on the product of the cross section and branching fraction, as a function of the $\tau^{}$ mass, for single $\tau^{}$ production via a contact interaction in association with an SM $ \tau $. 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.

Tables
png pdf	Table 1: Boundaries of the signal region for each $\tau^{*}$ mass considered. All units are in GeV.
png pdf	Table 2: Summary of systematic uncertainties that affect the expected signal and background yields by at least 1%. Values are in % of the inclusive yield. For the columns containing no $ \tau_\mathrm{h} $ it is the size of the effect on the background. For the columns containing $ \tau_\mathrm{h} $, the first number is the value for the background and the second number is the value for the signal with $ m_{ \tau^{*}} =$ 1750 GeV and $ \Lambda = $ 10 TeV.
png pdf	Table 3: Post-fit $ \kappa $ values calculated in the simultaneous background-only fit of all signal and control regions.
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 the uncertainties are statistical only.

Summary

Results have been presented for 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. It is assumed 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 directions of the visible $ \tau $ decay products. No excess of events is observed above the expected SM backgrounds. This null result is used to set upper limits on the cross section times branching fraction for this process. Assuming a compositeness scale $ \Lambda $ equal to the $\tau^{*}$ mass, $\tau^{*}$ masses less than 4700 GeV are excluded at 95% confidence level; for $ \Lambda= $ 10 TeV this exclusion is set at 2800 GeV. This is the first experimental result covering this production and decay process in the $\tau^{*}$ mass range above 175 GeV.

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