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| CMS-EXO-18-013 ; CERN-EP-2019-280 | ||
| Search for an excited lepton that decays via a contact interaction to a lepton and two jets in proton-proton collisions at ${\sqrt{s}} = $ 13 TeV | ||
| CMS Collaboration | ||
| 13 January 2020 | ||
| JHEP 05 (2020) 052 | ||
| Abstract: Results are presented from a search for events containing an excited lepton (electron or muon) produced in association with an ordinary lepton of the same flavor and decaying to a lepton and two hadronic jets. Both the production and the decay of the excited leptons are assumed to occur via a contact interaction with a characteristic energy scale $\Lambda$. The branching fraction for the decay mode under study increases with the mass of the excited lepton and is the most sensitive channel for very heavy excited leptons. The analysis uses a sample of proton-proton collisions collected by the CMS experiment at the LHC at ${\sqrt{s}} = $ 13 TeV, corresponding to an integrated luminosity of 77.4 fb$^{-1}$. The four-body invariant mass of the two lepton plus two jet system is used as the primary discriminating variable. No significant excess of events beyond the expectation for standard model processes is observed. Assuming that $\Lambda$ is equal to the mass of the excited leptons, excited electrons and muons with masses below 5.6 and 5.7 TeV, respectively, are excluded at 95% confidence level. These are the best limits to date. | ||
| Links: e-print arXiv:2001.04521 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Feynman diagram for the production of an excited lepton in association with an SM lepton in a hadron collider. The excited lepton decays via a contact interaction to one SM lepton and two resolved jets, which result from the hadronization of the quarks. |
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Figure 2:
Branching fractions, $\mathcal {B}$, of excited lepton decay channels as a function of the ratio of the excited lepton mass $({M_{\ell ^*}})$ and compositeness scale ($\Lambda $) for fixed values of the model parameters $f=f'$, which represent the couplings of excited leptons to SM particles. The branching fraction calculation is based on Ref. [6]. The contact interaction decay to one lepton and two jets, subject of this analysis, is dominating the region of high ${M_{\ell ^*}} /\Lambda $. Couplings $f$ and $f'$ are assumed to be equal to 1 in the left graph, and 0.1 in the right graph. |
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Figure 2-a:
Branching fractions, $\mathcal {B}$, of excited lepton decay channels as a function of the ratio of the excited lepton mass $({M_{\ell ^*}})$ and compositeness scale ($\Lambda $) for fixed values of the model parameters $f=f'$, which represent the couplings of excited leptons to SM particles. The branching fraction calculation is based on Ref. [6]. The contact interaction decay to one lepton and two jets, subject of this analysis, is dominating the region of high ${M_{\ell ^*}} /\Lambda $. Couplings $f$ and $f'$ are assumed to be equal to 1 in the left graph, and 0.1 in the right graph. |
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Figure 2-b:
Branching fractions, $\mathcal {B}$, of excited lepton decay channels as a function of the ratio of the excited lepton mass $({M_{\ell ^*}})$ and compositeness scale ($\Lambda $) for fixed values of the model parameters $f=f'$, which represent the couplings of excited leptons to SM particles. The branching fraction calculation is based on Ref. [6]. The contact interaction decay to one lepton and two jets, subject of this analysis, is dominating the region of high ${M_{\ell ^*}} /\Lambda $. Couplings $f$ and $f'$ are assumed to be equal to 1 in the left graph, and 0.1 in the right graph. |
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Figure 3:
Definition of the two validation regions (VR) and the high-mass signal region (SRT). |
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Figure 4:
Event distributions as a function of the four-body invariant $M_{{\ell \ell jj}}$ mass for the electron (left) and muon (right) channels, for the low-mass validation region defined by $M_{\ell \ell} < $ 200 GeV. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panels show the ratio of data to the simulated SM background, with the shaded band representing the uncertainty. |
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Figure 4-a:
Event distributions as a function of the four-body invariant $M_{{\ell \ell jj}}$ mass for the electron (left) and muon (right) channels, for the low-mass validation region defined by $M_{\ell \ell} < $ 200 GeV. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panels show the ratio of data to the simulated SM background, with the shaded band representing the uncertainty. |
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Figure 4-b:
Event distributions as a function of the four-body invariant $M_{{\ell \ell jj}}$ mass for the electron (left) and muon (right) channels, for the low-mass validation region defined by $M_{\ell \ell} < $ 200 GeV. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panels show the ratio of data to the simulated SM background, with the shaded band representing the uncertainty. |
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Figure 5:
Event distribution as a function of the four-body invariant mass $M_{{\ell \ell jj}}$ for the electron (left) and muon (right) channels, for the medium-mass validation region defined by 200 $ < M_{\ell \ell} < $ 500 GeV. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panel shows the ratio of data to the simulated SM background, with the shaded band representing the uncertainty. |
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Figure 5-a:
Event distribution as a function of the four-body invariant mass $M_{{\ell \ell jj}}$ for the electron (left) and muon (right) channels, for the medium-mass validation region defined by 200 $ < M_{\ell \ell} < $ 500 GeV. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panel shows the ratio of data to the simulated SM background, with the shaded band representing the uncertainty. |
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Figure 5-b:
Event distribution as a function of the four-body invariant mass $M_{{\ell \ell jj}}$ for the electron (left) and muon (right) channels, for the medium-mass validation region defined by 200 $ < M_{\ell \ell} < $ 500 GeV. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panel shows the ratio of data to the simulated SM background, with the shaded band representing the uncertainty. |
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Figure 6:
Signal efficiency after all selections are applied, as a function of the excited lepton mass ${M_{\ell ^*}}$, based on simulated events. |
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Figure 7:
Distribution of the two-lepton two-jet invariant mass in the signal region ($M_{\ell \ell} > $ 500 GeV) for the electron (left) and muon (right) channels. The example signal shape for two excited lepton masses is indicated as a gray line with the parameters given in the legend and for the benchmark case where the couplings $f$ and $f'$ are set to unity. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panels show the ratio of data to simulation with the total uncertainty in gray. |
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Figure 7-a:
Distribution of the two-lepton two-jet invariant mass in the signal region ($M_{\ell \ell} > $ 500 GeV) for the electron (left) and muon (right) channels. The example signal shape for two excited lepton masses is indicated as a gray line with the parameters given in the legend and for the benchmark case where the couplings $f$ and $f'$ are set to unity. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panels show the ratio of data to simulation with the total uncertainty in gray. |
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Figure 7-b:
Distribution of the two-lepton two-jet invariant mass in the signal region ($M_{\ell \ell} > $ 500 GeV) for the electron (left) and muon (right) channels. The example signal shape for two excited lepton masses is indicated as a gray line with the parameters given in the legend and for the benchmark case where the couplings $f$ and $f'$ are set to unity. The bin content is normalized to the width of the first bin, i.e., 100 GeV. The lower panels show the ratio of data to simulation with the total uncertainty in gray. |
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Figure 8:
Limits at 95% CL on the product of the production cross section and branching fraction for $\ell {\ell ^*} \to {\ell \ell jj} $, as a function of the invariant mass $M_{{\ell \ell jj}}$, for the electron (left) and muon (right) channels. The expectation from the model is represented for $ {| f |} = {| f' |} = $ 1 by two cases, $\Lambda = $ 10 TeV, and $\Lambda = {M_{\ell ^*}} $. |
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Figure 8-a:
Limits at 95% CL on the product of the production cross section and branching fraction for $\ell {\ell ^*} \to {\ell \ell jj} $, as a function of the invariant mass $M_{{\ell \ell jj}}$, for the electron (left) and muon (right) channels. The expectation from the model is represented for $ {| f |} = {| f' |} = $ 1 by two cases, $\Lambda = $ 10 TeV, and $\Lambda = {M_{\ell ^*}} $. |
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Figure 8-b:
Limits at 95% CL on the product of the production cross section and branching fraction for $\ell {\ell ^*} \to {\ell \ell jj} $, as a function of the invariant mass $M_{{\ell \ell jj}}$, for the electron (left) and muon (right) channels. The expectation from the model is represented for $ {| f |} = {| f' |} = $ 1 by two cases, $\Lambda = $ 10 TeV, and $\Lambda = {M_{\ell ^*}} $. |
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Figure 9:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the benchmark case where the GI couplings $ {| f |}$ and $ {| f' |}$ are set to one. The model is not valid in the hatched area. |
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Figure 9-a:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the benchmark case where the GI couplings $ {| f |}$ and $ {| f' |}$ are set to one. The model is not valid in the hatched area. |
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Figure 9-b:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the benchmark case where the GI couplings $ {| f |}$ and $ {| f' |}$ are set to one. The model is not valid in the hatched area. |
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Figure 10:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the case where the GI couplings $ {| f |}$ and $ {| f' |}$ are set to 0.1. The model is not valid in the hatched area. |
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Figure 10-a:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the case where the GI couplings $ {| f |}$ and $ {| f' |}$ are set to 0.1. The model is not valid in the hatched area. |
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Figure 10-b:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the case where the GI couplings $ {| f |}$ and $ {| f' |}$ are set to 0.1. The model is not valid in the hatched area. |
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Figure 11-a:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the benchmark case where the GI couplings $f$ and $f'$ vanish. The model is not valid in the hatched area. |
png pdf |
Figure 11-b:
Limits on the compositeness scale $\Lambda $ for the electron (left) and muon (right) channels, as a function of the mass of the excited lepton, for the benchmark case where the GI couplings $f$ and $f'$ vanish. The model is not valid in the hatched area. |
| Tables | |
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Table 1:
Observed event yields in bins of four-body mass compared to the expected SM background, for the $2e2j$ and $2\mu 2j$ final states. Also shown are the expected event yields for two simulated signal samples with the given masses and couplings. All yields are given in bins of the discriminating four-body mass ($2\ell 2j$) distribution, with lower and upper value for each bin given in units of GeV. Systematic uncertainties, as described in the text, are shown. |
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Table 2:
Summary of the observed (expected) limits on ${\ell ^*}$ mass, assuming ${M_{\ell ^*}} = \Lambda $, for the cases $f = f'$ and $f = -f'$. The limits evaluated in terms of the compositeness scale $\Lambda $ are shown in the right column. |
| Summary |
|
A search for excited leptons decaying via a contact interaction to final states of two electrons or two muons and two resolved jets has been performed. This channel complements other searches for excited leptons. It has greatest sensitivity at large values of the excited lepton mass $ {M_{\ell^*}} $. The data for this analysis were recorded with the CMS detector in the years 2016 and 2017, corresponding to a total integrated luminosity of 77.4 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV. No significant deviations from SM expectations are observed in the signal region and 95% exclusion limits have been set. Excited electrons (muons) up to masses of $M_{{\mathrm{e}^*} } = $ 5.6 TeV ($M_{\mu^{*}} = $ 5.7 TeV) are excluded with the usual assumption of ${M_{\ell^*}} = \Lambda$. These are the best limits to date. The limit was also re-evaluated in terms of the substructure scale $\Lambda$, leading to limits of $\Lambda = $ 11 and 12 TeV for excited electrons and muons, respectively, for mass values around 2 TeV and couplings of unity. When studying weaker gauge couplings, the limit on the maximum $ {M_{\ell^*}} $ does not change, but the larger cross section increases the $\Lambda$ sensitivity at lower masses. For couplings around zero, where the ${\ell\ell\gamma} $ decay has no sensitivity, limits around 20 TeV for the compositeness scale $\Lambda$ are achieved. |
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