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CMS-EXO-23-015 ; CERN-EP-2025-021

Search for vector-like leptons with long-lived particle decays in the CMS muon system in proton-proton collisions at $\sqrt{s} =$ 13 TeV

CMS Collaboration

20 March 2025

Submitted to J. High Energy Phys.

Abstract: A first search is presented for vector-like leptons (VLLs) decaying into a light long-lived pseudoscalar boson and a standard model $\tau$ lepton. The pseudoscalar boson is assumed to have a mass of 2 GeV and to decay exclusively into a pair of photons. It is identified using the CMS muon system. The analysis is carried out using a data set of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment in 2016--2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. Selected events contain at least one pseudoscalar boson decaying electromagnetically in the muon system and at least one hadronically decaying $\tau$ lepton. No significant excess of data events is observed compared to the background expectation. Upper limits are set at 95% confidence level on the vector-like lepton production cross section as a function of the VLL mass and the pseudoscalar boson mean proper decay length. The observed and expected exclusion ranges of the VLL mass extend up to 700 and 670 GeV, respectively, depending on the pseudoscalar boson lifetime.

Links: e-print arXiv:2503.16699 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: Feynman diagram of pair production of singlet vector-like leptons ({\HepParticle$\tau$\prime} ), which in turn both decay into an SM $\tau$ lepton and a new long-lived pseudoscalar boson ($\mathrm{a}_{\tau}$).
png pdf	Figure 2: Distributions of the number of hits in the cluster ($N_\text{hits}$) for the DT (left) and CSC (right) cluster categories in the signal region (SR). The black markers represent the data. The solid orange line and associated orange band show the background prediction and corresponding uncertainty. The red dotted, blue dashed and green dashed-dotted lines denote different signal hypotheses for a pseudoscalar boson with a mass of 2 GeV. The last histogram bin contains all overflow events. The lower panel in each plot shows the ratio of the data to the estimated background.
png pdf	Figure 2-a: Distributions of the number of hits in the cluster ($N_\text{hits}$) for the DT (left) and CSC (right) cluster categories in the signal region (SR). The black markers represent the data. The solid orange line and associated orange band show the background prediction and corresponding uncertainty. The red dotted, blue dashed and green dashed-dotted lines denote different signal hypotheses for a pseudoscalar boson with a mass of 2 GeV. The last histogram bin contains all overflow events. The lower panel in each plot shows the ratio of the data to the estimated background.
png pdf	Figure 2-b: Distributions of the number of hits in the cluster ($N_\text{hits}$) for the DT (left) and CSC (right) cluster categories in the signal region (SR). The black markers represent the data. The solid orange line and associated orange band show the background prediction and corresponding uncertainty. The red dotted, blue dashed and green dashed-dotted lines denote different signal hypotheses for a pseudoscalar boson with a mass of 2 GeV. The last histogram bin contains all overflow events. The lower panel in each plot shows the ratio of the data to the estimated background.
png pdf	Figure 3: Distributions of the number of hits in the cluster ($N_\text{hits}$) for the DT (left) and CSC (right) cluster categories in the out-of-time (OOT) region. The black markers represent the data. The solid orange line and associated orange band show the background prediction and corresponding uncertainty. The red dotted, blue dashed and green dashed-dotted lines denote different signal hypotheses for a pseudoscalar boson with a mass of 2 GeV. The last histogram bin contains all overflow events. The lower panel in each plot shows the ratio of the data to the estimated background.
png pdf	Figure 3-a: Distributions of the number of hits in the cluster ($N_\text{hits}$) for the DT (left) and CSC (right) cluster categories in the out-of-time (OOT) region. The black markers represent the data. The solid orange line and associated orange band show the background prediction and corresponding uncertainty. The red dotted, blue dashed and green dashed-dotted lines denote different signal hypotheses for a pseudoscalar boson with a mass of 2 GeV. The last histogram bin contains all overflow events. The lower panel in each plot shows the ratio of the data to the estimated background.
png pdf	Figure 3-b: Distributions of the number of hits in the cluster ($N_\text{hits}$) for the DT (left) and CSC (right) cluster categories in the out-of-time (OOT) region. The black markers represent the data. The solid orange line and associated orange band show the background prediction and corresponding uncertainty. The red dotted, blue dashed and green dashed-dotted lines denote different signal hypotheses for a pseudoscalar boson with a mass of 2 GeV. The last histogram bin contains all overflow events. The lower panel in each plot shows the ratio of the data to the estimated background.
png pdf	Figure 4: The 95% CL observed and expected upper limits on the VLL production cross section as a functions of the VLL mass for the pseudoscalar boson mean proper decay length $c\tau_\mathrm{a}=$ 0.025\unitm (left), and as a functions of $c\tau_\mathrm{a}$ for VLL mass of 700 GeV (right). The pseudoscalar boson mass is 2 GeV. The NLO theoretical prediction [29] is shown as a red line.
png pdf	Figure 4-a: The 95% CL observed and expected upper limits on the VLL production cross section as a functions of the VLL mass for the pseudoscalar boson mean proper decay length $c\tau_\mathrm{a}=$ 0.025\unitm (left), and as a functions of $c\tau_\mathrm{a}$ for VLL mass of 700 GeV (right). The pseudoscalar boson mass is 2 GeV. The NLO theoretical prediction [29] is shown as a red line.
png pdf	Figure 4-b: The 95% CL observed and expected upper limits on the VLL production cross section as a functions of the VLL mass for the pseudoscalar boson mean proper decay length $c\tau_\mathrm{a}=$ 0.025\unitm (left), and as a functions of $c\tau_\mathrm{a}$ for VLL mass of 700 GeV (right). The pseudoscalar boson mass is 2 GeV. The NLO theoretical prediction [29] is shown as a red line.
png pdf	Figure 5: The 95% CL observed upper limits on the VLL production cross section as a function of the VLL mass and the pseudoscalar boson mean proper decay length $c\tau_\mathrm{a}$. The pseudoscalar boson mass is 2 GeV. The area enclosed by the white line corresponds to the excluded region.

Summary

The first search for singlet vector-like leptons (VLLs) that decay into a light long-lived pseudoscalar boson and a $\tau$ lepton has been presented. It is performed using the CMS data set of proton-proton collisions at 13 TeV collected in 2016--2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. This analysis targets a reconstructed signature with at least one hadronically decaying tau lepton and with at least one muon detector shower resulting from the pseudoscalar boson decay in the CMS muon system. Selected events are categorized based on the presence of a cluster of muon detector hits in the barrel or the endcap region. No significant deviation from the background-only hypothesis is observed. The results of each category are combined to derive upper limits on the VLL production cross section as a function of the VLL mass and the pseudoscalar boson proper decay length. The VLL masses below 700 GeV are excluded at 95% confidence level, depending on the pseudoscalar boson lifetime. These are the most stringent constraints on the production of singlet VLLs with long-lived decays.

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