CMS-PAS-B2G-21-004

CMS-PAS-B2G-21-004
Search for pair-produced vector-like leptons in $\geq $ 3b $+$ N $\tau$ final states
CMS Collaboration
March 2022

Abstract: A search for vector-like leptons (VLLs) is presented in the context of the 4321 model, a UV-complete model with the potential to explain existing B-physics measurements that are in tension with standard model predictions. The analyzed data correspond to an integrated luminosity of 97 fb$^{-1}$, and were recorded by the CMS detector at the LHC in proton-proton collisions at $\sqrt{s}=$ 13 TeV at the LHC. Final states with $\geq$3 b jets and two third-generation leptons ($\tau\tau$, $\tau\nu_{\tau}$, or $\nu_{\tau}\nu_{\tau}$) are targeted. Expected upper limits are derived on the VLL production cross section in the VLL mass range 500-1050 GeV, assuming only electroweak production. At the low end of this mass range, the expected limits are below the expected production cross section, whereas at the high end of the mass range the expected upper limits on the production cross section are several times higher than the expected cross section for electroweak production. A mild excess, consistent with a possible signal, is observed in the data, such that the observed upper limits are approximately double the expected limits. The maximum likelihood fit prefers the presence of signal at the level of 2.8$ \sigma$, for a representative VLL mass point of 600 GeV.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PLB. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Left and centre: example Feynman diagrams showing production of VLL pairs through s-channel bosons, as expected at the LHC. In these diagrams, L represents either the neutral VLL, N, or the charged VLL, E. Right: vector-like lepton decays proceed through their interactions with the vector leptoquark, U. These decays are primarily to third-generation leptons and quarks.
png pdf	Figure 1-a: Left and centre: example Feynman diagrams showing production of VLL pairs through s-channel bosons, as expected at the LHC. In these diagrams, L represents either the neutral VLL, N, or the charged VLL, E. Right: vector-like lepton decays proceed through their interactions with the vector leptoquark, U. These decays are primarily to third-generation leptons and quarks.
png pdf	Figure 1-b: Left and centre: example Feynman diagrams showing production of VLL pairs through s-channel bosons, as expected at the LHC. In these diagrams, L represents either the neutral VLL, N, or the charged VLL, E. Right: vector-like lepton decays proceed through their interactions with the vector leptoquark, U. These decays are primarily to third-generation leptons and quarks.
png pdf	Figure 1-c: Left and centre: example Feynman diagrams showing production of VLL pairs through s-channel bosons, as expected at the LHC. In these diagrams, L represents either the neutral VLL, N, or the charged VLL, E. Right: vector-like lepton decays proceed through their interactions with the vector leptoquark, U. These decays are primarily to third-generation leptons and quarks.
png pdf	Figure 2: Diagram of the event categorization and the various signal and control regions used in the analysis. The regions within the solid box are all used in the maximum likelihood fit. The regions in the dashed box are used to determine some parameters used in data-driven background estimations. The selections are mutually exclusive between all regions, so that each event only enters a single region. For brevity, not all selection criteria are shown; the detailed selection criteria are described in the text.
png pdf	Figure 3: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-a: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-b: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-c: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-d: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-e: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-f: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-g: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-h: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-i: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-j: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-k: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 3-l: Postfit plots for 2017 (two leftmost columns) and 2018 (two rightmost columns) showing the distributions of the main observables in the signal region for the different ${{\tau _\mathrm {h}}}$ multiplicity channels (from top to bottom: 0-${{\tau _\mathrm {h}}}$, 1-${{\tau _\mathrm {h}}}$, 2-${{\tau _\mathrm {h}}}$). The first and third column show the background-only fits and the second and fourth columns show the fit including the signal model.
png pdf	Figure 4: Expected and observed 95% confidence level upper limits on the electroweak vector-like lepton cross section times branching fraction, combining the 2017 and 2018 data and all ${\tau _\mathrm {h}}$ multiplicity channels combined.

Tables
png pdf	Table 1: Illustrative contributions from different VLL production and decay modes to the 0-, 1-, and 2-$\tau$ signal regions. The decay products in parentheses represent the objects coming from the intermediate vector leptoquark, U, in the decay. For brevity, no distinction is made between particles and antiparticles, the multiplicities of each decay mode are not shown, and the impacts of object misidentification are not considered in the table. E and N represent the charged and neutral VLLs; t, b, $\tau$, and $\nu_\tau$ represent top quarks, bottom quarks, tau leptons and tau neutrinos; and j represents any quark other than t or b.
png pdf	Table 2: List of input features used during training of the ABCNet model for VLL classification.

Summary

The first search for vector-like leptons in the context of the 4321 model has been presented, using proton-proton collision data collected with the CMS detector at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 96.5 fb$^{-1}$. The probed model consists of an extension of the standard model with an $\textrm{SU}(4) \times \textrm{SU}(3)' \times \textrm{SU}(2)_L \times \textrm{U}(1)'$ gauge sector that can provide a combined explanation to multiple anomalies observed in B hadron decays, which point to lepton flavour nonuniversality. In the model, a leptoquark is predicted as the primary source of lepton flavour violation while the UV-completion predicts additional vector-like fermion families. In particular, vector-like leptons are investigated by their coupling to standard model fermions through leptoquark interactions, resulting in third generation fermion signatures. Final states containing at least three b-tagged jets and varying $\tau$ lepton multiplicities are considered. To improve the search sensitivity, machine learning methods based on graph neural networks were used to learn the kinematic relationship between particles in a large jet multiplicity environment.

Vector-like lepton masses up to 640 GeV were expected to be excluded at the 95% confidence level. Mild excesses in the data, compared with the expectation, are observed in the signal-sensitive bins of the 1-${{\tau_\mathrm{h}}}$ and 2-${{\tau_\mathrm{h}}}$ regions for both 2017 and 2018 data. As a result, no VLL masses are excluded at the 95% confidence level and limits are set between 10 and 30 fb, depending on the VLL mass hypothesis. The observed excess is consistent with the presence of VLLs in the context of the 4321 model, and the excess of events over the background-only hypothesis corresponds to a significance of 2.8$\sigma$.

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