CMS-PAS-EXO-16-009

CMS-PAS-EXO-16-009
Search for excited leptons in the $\ell\ell\gamma$ final state at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
August 2016

Abstract: This note presents a search for a possible compositness of electrons and muons using 2.7 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $ \sqrt{s} = $ 13 TeV collected with the CMS detector in 2015. Excited leptons ($\ell^$) are assumed to be produced via contact interactions in conjunction with the corresponding standard model lepton. The decays considered here are $\ell^ \to \ell \gamma$ with $\ell = e,\mu$. The number of events observed in data is consistent with background processes from the standard model of particle interactions and exclusion limits on the excited lepton mass, and the compositness scale $\Lambda$, are set. For the case $M_{\ell^} = \Lambda$, the existence of excited electrons (muons) up to masses of 2.8 (3.0) TeV is excluded. This analysis sets the most stringent limits on $\ell^$ to date.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram for the investigated ${\ell \ell \gamma }$ channels.
png pdf	Figure 2-a: ${M_{\ell \gamma }^\text {min}}$ and ${M_{\ell \gamma }^\text {max}}$ for the electron channel in the upper row and the muon channel below. The different background contributions are shown in colors according to the legend. Also shown are two examples of potential ${\ell ^*}$ signals.
png pdf	Figure 2-b: ${M_{\ell \gamma }^\text {min}}$ and ${M_{\ell \gamma }^\text {max}}$ for the electron channel in the upper row and the muon channel below. The different background contributions are shown in colors according to the legend. Also shown are two examples of potential ${\ell ^*}$ signals.
png pdf	Figure 2-c: ${M_{\ell \gamma }^\text {min}}$ and ${M_{\ell \gamma }^\text {max}}$ for the electron channel in the upper row and the muon channel below. The different background contributions are shown in colors according to the legend. Also shown are two examples of potential ${\ell ^*}$ signals.
png pdf	Figure 2-d: ${M_{\ell \gamma }^\text {min}}$ and ${M_{\ell \gamma }^\text {max}}$ for the electron channel in the upper row and the muon channel below. The different background contributions are shown in colors according to the legend. Also shown are two examples of potential ${\ell ^*}$ signals.
png pdf	Figure 3-a: 2D ${M_{\ell \gamma }^\text {min}}$ versus ${M_{\ell \gamma }^\text {max}}$ distribution after full selection except the L-shaped search window, with expected background (blue), data (black crosses) and two signal examples for ${\ell ^*}$ masses as given in the legend. The size of the blue squares for the background correspond to the number of events in a particular bin. On the left for the electron channel which has a high detector resolution over the full mass range. On the right for the muon channel where resolution degrades with mass. The background is weighted to the luminosity and the cross section.
png pdf	Figure 3-b: 2D ${M_{\ell \gamma }^\text {min}}$ versus ${M_{\ell \gamma }^\text {max}}$ distribution after full selection except the L-shaped search window, with expected background (blue), data (black crosses) and two signal examples for ${\ell ^*}$ masses as given in the legend. The size of the blue squares for the background correspond to the number of events in a particular bin. On the left for the electron channel which has a high detector resolution over the full mass range. On the right for the muon channel where resolution degrades with mass. The background is weighted to the luminosity and the cross section.
png pdf	Figure 4-a: Exclusion limits for electrons (a,b) and muons (c,d). On (a,c) the limit on the product of cross section and branching fraction. On (b,d) the limit on the compositness scale $\Lambda $.
png pdf	Figure 4-b: Exclusion limits for electrons (a,b) and muons (c,d). On (a,c) the limit on the product of cross section and branching fraction. On (b,d) the limit on the compositness scale $\Lambda $.
png pdf	Figure 4-c: Exclusion limits for electrons (a,b) and muons (c,d). On (a,c) the limit on the product of cross section and branching fraction. On (b,d) the limit on the compositness scale $\Lambda $.
png pdf	Figure 4-d: Exclusion limits for electrons (a,b) and muons (c,d). On (a,c) the limit on the product of cross section and branching fraction. On (b,d) the limit on the compositness scale $\Lambda $.

Tables
png pdf	Table 1: Number of observed events in data compared to expected backgrounds with uncertainties after all selection steps except the L-shaped search window.
png pdf	Table 2: Sources of systematic uncertainties. A range is provided for $\mathrm {p_T} $-dependent uncertainties.
png pdf	Table 3: Summary of the observed (expected) limits on ${\ell ^}$ mass, assuming ${M_\ell ^} = \Lambda $.

Summary

A search for excited leptons in the final state with two high $p_{\mathrm{T}}$ leptons and one photon has been performed using the 2015 data set of 2.7 fb$^{-1}$ at $\sqrt{s} =$ 13 TeV recorded with the CMS detector. Events with a dilepton invariant mass significantly above the Z-pole mass, above 116 GeV, are selected. No significant excess is found and limits on the product of cross section and branching fraction are shown. Limits on the $\ell^{*}$ mass as a function of the compositness scale $\Lambda$ exclude excited leptons below 2.8(3.0) TeV for the electron(muon) channel for $ M_{\ell^{*}} = \Lambda$ and values of $\Lambda$ up to more than 14(15) TeV at low $ M_{\ell^{*}} $.

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