CMS-PAS-EXO-23-015

CMS-PAS-EXO-23-015
Search for vector-like leptons with long-lived particle decays in the CMS muon system
CMS Collaboration
20 July 2024

Abstract: This note presents a search for vector-like leptons (VLLs), which in turn decay into a long-lived pseudoscalar and a standard model tau lepton. The pseudoscalar with a mass of 2 GeV exclusively decays into a pair of photons, and is identified using the muon detector shower signature. The data analysis is carried out using a dataset of proton-proton collisions at 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 reconstructed muon detector hit cluster and at least one reconstructed hadronic tau lepton. No significant excess of data events are observed with respect to the background expectation, and 95% CL limits on the vector-like lepton production cross section are derived as a function of the VLL mass and pseudoscalar lifetime. Observed (expected) exclusion of the VLL mass is up to around 690 (640) GeV, depending on the pseudoscalar lifetime.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of pair production singlet vector-like leptons ($ \tau^{\prime} $), which in turn both decay into a SM $ \tau $ lepton and a new long-lived pseudoscalar ($ a_{\tau} $).
png pdf	Figure 2: Distributions of the number of hits in the cluster ($ \mathrm{N_{hits}} $) for the CSC- (left) and DT-cluster (right) categories in the in-time (IT) signal region. The black markers represent the data. The blue, green and red lines denote different signal hypotheses. The background prediction is shown in the orange histogram. The last histogram bin contains all overflow events.
png pdf	Figure 2-a: Distributions of the number of hits in the cluster ($ \mathrm{N_{hits}} $) for the CSC- (left) and DT-cluster (right) categories in the in-time (IT) signal region. The black markers represent the data. The blue, green and red lines denote different signal hypotheses. The background prediction is shown in the orange histogram. The last histogram bin contains all overflow events.
png pdf	Figure 2-b: Distributions of the number of hits in the cluster ($ \mathrm{N_{hits}} $) for the CSC- (left) and DT-cluster (right) categories in the in-time (IT) signal region. The black markers represent the data. The blue, green and red lines denote different signal hypotheses. The background prediction is shown in the orange histogram. The last histogram bin contains all overflow events.
png pdf	Figure 3: Distributions of the number of hits in the cluster ($ \mathrm{N_{hits}} $) for the CSC- (left) and DT-cluster (right) categories in the out-of-time (OOT) control region. The black markers represent the data. The blue, green and red lines denote different signal hypotheses. The background prediction is shown in the orange histogram. The last histogram bin contains all overflow events.
png pdf	Figure 3-a: Distributions of the number of hits in the cluster ($ \mathrm{N_{hits}} $) for the CSC- (left) and DT-cluster (right) categories in the out-of-time (OOT) control region. The black markers represent the data. The blue, green and red lines denote different signal hypotheses. The background prediction is shown in the orange histogram. The last histogram bin contains all overflow events.
png pdf	Figure 3-b: Distributions of the number of hits in the cluster ($ \mathrm{N_{hits}} $) for the CSC- (left) and DT-cluster (right) categories in the out-of-time (OOT) control region. The black markers represent the data. The blue, green and red lines denote different signal hypotheses. The background prediction is shown in the orange histogram. The last histogram bin contains all overflow events.
png pdf	Figure 4: 95% CL observed and expected upper limits on the VLL production cross section as a function of the VLL mass for a pseudoscalar $ c\tau_{a}= $ 0.025 m (left), and as a function of the pseudoscalar lifetime ($ c\tau_{a} $) for VLL mass of 700 GeV (right). The pseudoscalar mass is 2 GeV. The generator LO theoretical prediction is shown (pink line).
png pdf	Figure 4-a: 95% CL observed and expected upper limits on the VLL production cross section as a function of the VLL mass for a pseudoscalar $ c\tau_{a}= $ 0.025 m (left), and as a function of the pseudoscalar lifetime ($ c\tau_{a} $) for VLL mass of 700 GeV (right). The pseudoscalar mass is 2 GeV. The generator LO theoretical prediction is shown (pink line).
png pdf	Figure 4-b: 95% CL observed and expected upper limits on the VLL production cross section as a function of the VLL mass for a pseudoscalar $ c\tau_{a}= $ 0.025 m (left), and as a function of the pseudoscalar lifetime ($ c\tau_{a} $) for VLL mass of 700 GeV (right). The pseudoscalar mass is 2 GeV. The generator LO theoretical prediction is shown (pink line).

Summary

A first search for singlet vector-like leptons (VLLs) that decay into a long-lived pseudoscalar and a tau lepton has been performed using the dataset of pp 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 at least one muon detector shower resulting from the pseudoscalar 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 proper decay length of the pseudoscalar. VLL masses are excluded up to around 690 GeV, depending on the pseudoscalar lifetime.

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