A search for an unexpected asymmetry in the production of $e^+ \mu^-$ and $e^- \mu^+$ pairs in proton-proton collisions recorded by the ATLAS detector at $\sqrt s = 13$ TeV

The collaboration
Phys.Lett.B 830 (2022) 137106, 2022.

Abstract
This search, a type not previously performed at ATLAS, uses a comparison of the production cross sections for $e^+ \mu^-$ and $e^- \mu^+$ pairs to constrain physics processes beyond the Standard Model. It uses $139 \text{fb}^{-1}$ of proton$-$proton collision data recorded at $\sqrt{s} = 13$ TeV at the LHC. Targeting sources of new physics which prefer final states containing $e^{+}\mu^{-}$ to $e^{-}\mu^{+}$, the search contains two broad signal regions which are used to provide model-independent constraints on the ratio of cross sections at the 2% level. The search also has two special selections targeting supersymmetric models and leptoquark signatures. Observations using one of these selections are able to exclude, at 95% confidence level, singly produced smuons with masses up to 640 GeV in a model in which the only other light sparticle is a neutralino when the $R$-parity-violating coupling $\lambda'_{231}$ is close to unity. Observations using the other selection exclude scalar leptoquarks with masses below 1880 GeV when $g_{\text{1R}}^{eu}=g_{\text{1R}}^{\mu c}=1$, at 95% confidence level. The limit on the coupling reduces to $g_{\text{1R}}^{eu}=g_{\text{1R}}^{\mu c}=0.46$ for a mass of 1420 GeV.

• #### SR-RPV and SR-LQ yields

Data from Table 1 (Also seen in Figure 5)

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Observed yields, and (post-fit) expected yields for the data-driven SM estimates. Yields are shown for the benchmark RPV-supersymmetry signal points...

• #### Obs RPV-SUSY limit lambdap231 1p0

Data from Figure 6 (Also seen in Figure 7)

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The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=1.0$.

• #### Exp RPV-SUSY limit lambdap231 1p0

Data from Figure 6 (Also seen in Figure 7)

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The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=1.0$.

• #### Exp RPV-SUSY 1sigma_exp limit band lambdap231 1p0

Data from Figure 6

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The $1\sigma_{\text{exp}}$ variation of the expected exclusion contour at 95% CL as a function of the smuon and neutralino masses,...

• #### Obs RPV-SUSY limit lambdap231 0p1

Data from Figure 7

10.17182/hepdata.115579.v1/t5

The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.1$.

• #### Exp RPV-SUSY limit lambdap231 0p1

Data from Figure 7

10.17182/hepdata.115579.v1/t6

The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.1$.

• #### Obs RPV-SUSY limit lambdap231 0p15

Data from Figure 7

10.17182/hepdata.115579.v1/t7

The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.15$.

• #### Exp RPV-SUSY limit lambdap231 0p15

Data from Figure 7

10.17182/hepdata.115579.v1/t8

The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.15$.

• #### Obs RPV-SUSY limit lambdap231 0p2

Data from Figure 7

10.17182/hepdata.115579.v1/t9

The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.2$.

• #### Exp RPV-SUSY limit lambdap231 0p2

Data from Figure 7

10.17182/hepdata.115579.v1/t10

The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.2$.

• #### Obs RPV-SUSY limit lambdap231 0p4

Data from Figure 7

10.17182/hepdata.115579.v1/t11

The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.4$.

• #### Exp RPV-SUSY limit lambdap231 0p4

Data from Figure 7

10.17182/hepdata.115579.v1/t12

The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.4$.

• #### Obs RPV-SUSY limit lambdap231 0p6

Data from Figure 7

10.17182/hepdata.115579.v1/t13

The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0.6$.

• #### Exp RPV-SUSY limit lambdap231 0p6

Data from Figure 7

10.17182/hepdata.115579.v1/t14

The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=0p6$.

• #### Obs RPV-SUSY limit lambdap231 1p5

Data from Figure 7

10.17182/hepdata.115579.v1/t15

The observed exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=1.5$.

• #### Exp RPV-SUSY limit lambdap231 1p5

Data from Figure 7

10.17182/hepdata.115579.v1/t16

The expected exclusion contour at 95% CL as a function of the smuon and neutralino masses, for $\lambda_{231}^{'}=1.5$.

• #### Obs leptoquark limit

Data from Figure 8

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The observed exclusion contour at 95% CL as a function of the leptoquark mass and coupling strength.

• #### Exp leptoquark limit

Data from Figure 8

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The expected exclusion contour at 95% CL as a function of the leptoquark mass and coupling strength.

• #### Obs leptoquark -1sigma_theory limit

Data from Figure 8

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The minus $1\sigma_{\text{theory}}$ variation of the observed exclusion contour at 95% CL as a function of the leptoquark mass and...

• #### Obs leptoquark 1sigma_theory limit

Data from Figure 8

10.17182/hepdata.115579.v1/t20

The plus $1\sigma_{\text{theory}}$ variation of the observed exclusion contour at 95% CL as a function of the leptoquark mass and...

• #### Exp leptoquark 1sigma_exp limit band

Data from Figure 8

10.17182/hepdata.115579.v1/t21

The $1\sigma_{\text{exp}}$ variation of the expected exclusion contour at 95% CL as a function of the leptoquark mass and coupling...

• #### $\rho$ measurement results in SR-MET

Data from Table 1 of the auxiliary material (Also seen in Figures 2 and 3)

10.17182/hepdata.115579.v1/t22

Observed yields, and fake lepton background yields in the $e^{+}\mu^{-}$ and $e^{-}\mu^{+}$ channels of SR-MET, along with the results of...

• #### $\rho$ measurement results in SR-JET

Data from Table 1 of the auxiliary material (Also seen in Figures 2 and 3)

10.17182/hepdata.115579.v1/t23

Observed yields, and fake lepton background yields in the $e^{+}\mu^{-}$ and $e^{-}\mu^{+}$ channels of SR-JET, along with the results of...

• #### Upper Limits SR-MET, $M_{\text{T2}}$

Data from Table 2 of the auxiliary material (Also shown in Figure 4)

10.17182/hepdata.115579.v1/t24

Observed and expected 95% CL upper limits on the total number of signal events entering the $e^{+}\mu^{-}$ and $e^{-}\mu^{+}$ channels...

• #### Upper Limits SR-JET, $H_{\text{P}}$

Data from Table 2 of the auxiliary material (Also shown in Figure 4)

10.17182/hepdata.115579.v1/t25

Observed and expected 95% CL upper limits on the total number of signal events entering the $e^{+}\mu^{-}$ and $e^{-}\mu^{+}$ channels...

• #### Signal Cutflow

Data from Table 3 of the auxiliary material

10.17182/hepdata.115579.v1/t26

Signal yields following each cut in the analysis, for representative $R$-parity-violating supersymmetry and leptoquark signals. All yields are MC generator-weighted...

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