CMS-PAS-EXO-18-013

CMS-PAS-EXO-18-013
Search for excited leptons decaying via contact interaction to two leptons and two jets
CMS Collaboration
March 2019

Abstract: A search is presented for excited electrons and excited muons decaying via a contact interaction to final states of two leptons and two jets. This channel complements other searches for excited leptons with radiative or neutral current decays. In all cases, excited leptons would be produced via contact interaction with a characteristic energy scale $\Lambda$ . The branching fraction of this decay channel increases with excited lepton mass and thus is expected to be the most sensitive search channel for very heavy excited leptons. The analysis uses proton-proton collision data collected in 2016 and 2017 with the CMS detector at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated total luminosity of 77.4 fb $^{-1}$ . The four-body invariant mass distribution is used as the discriminating variable for this search and no significant deviation from the standard model prediction is found. Exclusion limits are set at the 95% confidence level. For the case where the characteristic scale is equal to the excited lepton mass, excited electrons and muons below 5.6 TeV and 5.7 TeV, respectively, are excluded. These are the most stringent exclusion limits to date on the excited lepton mass.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 05 (2020) 052. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram for the production of an excited lepton in association with a SM lepton in a hadron collider. It subsequently decays via a CI to one SM lepton and two resolved jets.
png pdf	Figure 2: Branching fraction of excited lepton decay channels as a function of the ratio of excited lepton mass ( ${M_{\ell ^*}}$ ) and compositeness scale ( $\Lambda$ ) for fixed values of the model parameters $f=f^{\prime}=$ 1, which represent the coupling of excited leptons to SM particles, as described in Sec. 3.
png pdf	Figure 3: Definition of the two validation regions (VR) and the high-mass signal region.
png pdf	Figure 4: Signal selection efficiency after all selection steps.
png pdf	Figure 5: The distribution of the four-body invariant mass is shown for the electron (left) and muon (right) channels, for the low-mass validation region defined by $M_{\ell \ell} <$ 200 GeV. The lower panel shows the ratio of data to the simulated SM background with the shaded band representing the uncertainty.
png pdf	Figure 5-a: The distribution of the four-body invariant mass is shown for the electron channel, for the low-mass validation region defined by $M_{\ell \ell} <$ 200 GeV. The lower panel shows the ratio of data to the simulated SM background with the shaded band representing the uncertainty.
png pdf	Figure 5-b: The distribution of the four-body invariant mass is shown for the muon channel, for the low-mass validation region defined by $M_{\ell \ell} <$ 200 GeV. The lower panel shows the ratio of data to the simulated SM background with the shaded band representing the uncertainty.
png pdf	Figure 6: The distribution of the four-body invariant mass is shown for the electron (left) and muon (right) channels, for the medium-mass validation region defined by 200 $< M_{\ell \ell} <$ 500 GeV. The lower panel shows the ratio of data to the simulated SM background with the shaded band representing the uncertainty.
png pdf	Figure 6-a: The distribution of the four-body invariant mass is shown for the electron channel, for the medium-mass validation region defined by 200 $< M_{\ell \ell} <$ 500 GeV. The lower panel shows the ratio of data to the simulated SM background with the shaded band representing the uncertainty.
png pdf	Figure 6-b: The distribution of the four-body invariant mass is shown for the muon channel, for the medium-mass validation region defined by 200 $< M_{\ell \ell} <$ 500 GeV. The lower panel shows the ratio of data to the simulated SM background with the shaded band representing the uncertainty.
png pdf	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 data correspond to 77.4 fb $^{-1}$ recorded during 2016 and 2017. The example signal shape for two excited lepton masses is indicated as a grey line with the parameters given in the legend. The panel below shows the data/MC ratio with the total uncertainty in grey.
png pdf	Figure 7-a: Distribution of the two lepton two jet invariant mass in the signal region ( $M_{\ell \ell} >$ 500 GeV) for the electron channel. The data correspond to 77.4 fb $^{-1}$ recorded during 2016 and 2017. The example signal shape for two excited lepton masses is indicated as a grey line with the parameters given in the legend. The panel below shows the data/MC ratio with the total uncertainty in grey.
png pdf	Figure 7-b: Distribution of the two lepton two jet invariant mass in the signal region ( $M_{\ell \ell} >$ 500 GeV) for the muon channel. The data correspond to 77.4 fb $^{-1}$ recorded during 2016 and 2017. The example signal shape for two excited lepton masses is indicated as a grey line with the parameters given in the legend. The panel below shows the data/MC ratio with the total uncertainty in grey.
png pdf	Figure 8: Cross section limits on excited leptons using the $M_{{\ell \ell jj}}$ distribution for the electron (left) and muon (right) channels. The expectation from the model is represented by two cases, $\Lambda = {M_{\ell ^*}}$ and $\Lambda =$ 10 TeV for which the signal events are simulated. The cross section scales with $\Lambda$ .
png pdf	Figure 8-a: Cross section limits on excited leptons using the $M_{{\ell \ell jj}}$ distribution for the electron channel. The expectation from the model is represented by two cases, $\Lambda = {M_{\ell ^*}}$ and $\Lambda =$ 10 TeV for which the signal events are simulated. The cross section scales with $\Lambda$ .
png pdf	Figure 8-b: Cross section limits on excited leptons using the $M_{{\ell \ell jj}}$ distribution for the muon channel. The expectation from the model is represented by two cases, $\Lambda = {M_{\ell ^*}}$ and $\Lambda =$ 10 TeV for which the signal events are simulated. The cross section scales with $\Lambda$ .
png pdf	Figure 9-a: Limits on the compositeness scale $\Lambda$ for the electron channel. In addition, the $\Lambda$ limits from a recent CMS search in the ${\ell \ell \gamma}$ channel [23] are shown as a blue, dashed line. The latter addresses only the scenario of $f=f^{\prime}$ while the hadronic CI channel is sensitive to this and the scenario $f = -f^{\prime}$ . The model is not valid in the hatched area.
png pdf	Figure 9-b: Limits on the compositeness scale $\Lambda$ for the muon channel. In addition, the $\Lambda$ limits from a recent CMS search in the ${\ell \ell \gamma}$ channel [23] are shown as a blue, dashed line. The latter addresses only the scenario of $f=f^{\prime}$ while the hadronic CI channel is sensitive to this and the scenario $f = -f^{\prime}$ . The model is not valid in the hatched area.

Tables
png pdf	Table 1: Observed event yields for 77.4 fb $^{-1}$ compared to the expected SM background for the $2\mathrm{e}2\mathrm{j}$ and $2\mu 2\mathrm{j}$ final state, respectively. Also shown are the expected event numbers for two signal examples with the given parameters. The events are given in bins of the discriminating four-body mass ( $2\ell 2\mathrm{j}$ ) distribution, with lower and upper value for each bin given in units of GeV. Uncertainties are systematic as described in the text.
png pdf	Table 2: Summary of the observed (expected) limits on ${\ell ^}$ mass, assuming ${M_{\ell ^}} = \Lambda$ , for the cases $f = f^{\prime}$ and $f = -f^{\prime}$ . The latter case is not applicable to ${\ell \ell \gamma}$ .

Summary

A search for excited leptons decaying via contact interaction to final states of two electrons or two muons and two resolved jets was carried out. This channel complements other searches for excited leptons. It has greatest sensitivity at large values of

$M_{\ell^*}$ and/or

$\Lambda$ and is therefore most sensitive to very heavy excited leptons. The utilized data for this analysis were recorded with the CMS detector in the years 2016 (35.9 fb

$^{-1}$ ) and 2017 (41.5 fb

$^{-1}$ ), 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 were observed in the signal region and 95% exclusion limits are set. Excited electrons (muons) up to masses of

$M_{e^*} =$ 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 a limit of

$\Lambda =$ 13 TeV for masses around 2 TeV.

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