CMS-EXO-17-025

CMS-EXO-17-025 ; CERN-EP-2018-326
Search for contact interactions and large extra dimensions in the dilepton mass spectra from proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
26 December 2018
JHEP 04 (2019) 114
Abstract: A search for nonresonant excesses in the invariant mass spectra of electron and muon pairs is presented. The analysis is based on data from proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the CMS experiment in 2016, corresponding to a total integrated luminosity of 36 fb$^{-1}$. No significant deviation from the standard model is observed. Limits are set at 95% confidence level on energy scales for two general classes of nonresonant models. For a class of fermion contact interaction models, lower limits ranging from 20 to 32 TeV are set on the characteristic compositeness scale $\Lambda$. For the Arkani-Hamed, Dimopoulos, and Dvali model of large extra dimensions, the first results in the dilepton final state at 13 TeV are reported, and values of the ultraviolet cutoff parameter $\Lambda_{\text{T}}$ below 6.9 TeV are excluded. A combination with recent CMS diphoton results improves this exclusion to $\Lambda_{\text{T}}$ below 7.7 TeV, providing the most sensitive limits to date in nonhadronic final states.
Links: e-print arXiv:1812.10443 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Electron (left) and muon (right) pair invariant mass spectra for the combined barrel-barrel and barrel-endcap event categories. Example model predictions are given for CI (left) and ADD (right). The lower panel shows the relative difference between the data and predicted background. The gray band gives the fractional uncertainty (statistical and systematic) in the prediction.
png pdf	Figure 1-a: Electron pair invariant mass spectrum for the combined barrel-barrel and barrel-endcap event categories. Example model predictions are given for CI. The lower panel shows the relative difference between the data and predicted background. The gray band gives the fractional uncertainty (statistical and systematic) in the prediction.
png pdf	Figure 1-b: Muon pair invariant mass spectrum for the combined barrel-barrel and barrel-endcap event categories. Example model predictions are given for ADD. The lower panel shows the relative difference between the data and predicted background. The gray band gives the fractional uncertainty (statistical and systematic) in the prediction.
png pdf	Figure 2: Dilepton exclusion limits at 95% CL on the CI scale ($\Lambda $) for the six CI models considered for the electron (left) and muon (right) channels. The limits are obtained for $ {m_{\ell \ell}} > $ 400 GeV in the case of constructive (destructive) interference.
png pdf root	Figure 2-a: Dilepton exclusion limits at 95% CL on the CI scale ($\Lambda $) for the six CI models considered for the electron channel. The limits are obtained for $ {m_{\ell \ell}} > $ 400 GeV in the case of constructive (destructive) interference.
png pdf root	Figure 2-b: Dilepton exclusion limits at 95% CL on the CI scale ($\Lambda $) for the six CI models considered for the muon channel. The limits are obtained for $ {m_{\ell \ell}} > $ 400 GeV in the case of constructive (destructive) interference.
png pdf	Figure 3: Combined dilepton 95% CL exclusion limits on the cross section for the left-left constructive CI model (left), and on the CI scale ($\Lambda $) for the six different CI models considered (right). The red curve in the left plot shows the theoretical cross section as a function of $\Lambda $. The limits are obtained for $ {m_{\ell \ell}} > $ 400 GeV in the case of constructive (destructive) interference.
png pdf root	Figure 3-a: Combined dilepton 95% CL exclusion limits on the cross section for the left-left constructive CI model. The red curve shows the theoretical cross section as a function of $\Lambda $. The limits are obtained for $ {m_{\ell \ell}} > $ 400 GeV in the case of constructive (destructive) interference.
png pdf root	Figure 3-b: Combined dilepton 95% CL exclusion limits on the CI scale ($\Lambda $) for the six different CI models considered. The limits are obtained for $ {m_{\ell \ell}} > $ 400 GeV in the case of constructive (destructive) interference.
png pdf	Figure 4: Exclusion limits at 95% CL on the UV cutoff for the electron (left) and muon (right) channels with $ {m_{\ell \ell}} > $ 1.8 TeV in the GRW, Hewett, and HLZ conventions for the ADD model. Signal model cross sections are calculated up to leading order and a correction factor of 1.3 is applied. The results are compared to the previous combined result from CMS [8].
png pdf	Figure 4-a: Exclusion limits at 95% CL on the UV cutoff for the electron channel with $ {m_{\ell \ell}} > $ 1.8 TeV in the GRW, Hewett, and HLZ conventions for the ADD model. Signal model cross sections are calculated up to leading order and a correction factor of 1.3 is applied. The results are compared to the previous combined result from CMS [8].
png pdf	Figure 4-b: Exclusion limits at 95% CL on the UV cutoff for the muon channel with $ {m_{\ell \ell}} > $ 1.8 TeV in the GRW, Hewett, and HLZ conventions for the ADD model. Signal model cross sections are calculated up to leading order and a correction factor of 1.3 is applied. The results are compared to the previous combined result from CMS [8].
png pdf	Figure 5: Combined dilepton 95% CL exclusion limit on the cross section in the GRW convention (left) and on the UV cutoff for all parameter conventions (right) with $ {m_{\ell \ell}} > $ 1.8 TeV for the ADD model. The curves labeled ADD in the left plot show the theoretical signal cross section calculated by PYTHIA, as a function of the cutoff parameter ${\Lambda _{\mathrm {T}}}$, and signal contributions with $ {m_{\ell \ell}} > {\Lambda _{\mathrm {T}}} $ are set to 0. Signal model cross sections are calculated up to leading order and, where indicated by the appropriate label, a correction factor of 1.3 is applied. The results are compared to previous ones from CMS [8].
png pdf	Figure 5-a: Combined dilepton 95% CL exclusion limit on the cross section in the GRW convention, with $ {m_{\ell \ell}} > $ 1.8 TeV for the ADD model. The curves labeled ADD show the theoretical signal cross section calculated by PYTHIA, as a function of the cutoff parameter ${\Lambda _{\mathrm {T}}}$, and signal contributions with $ {m_{\ell \ell}} > {\Lambda _{\mathrm {T}}} $ are set to 0. Signal model cross sections are calculated up to leading order and, where indicated by the appropriate label, a correction factor of 1.3 is applied.
png pdf	Figure 5-b: Combined dilepton 95% CL exclusion limit on the UV cutoff for all parameter conventions, with $ {m_{\ell \ell}} > $ 1.8 TeV for the ADD model. Signal model cross sections are calculated up to leading order and, where indicated by the appropriate label, a correction factor of 1.3 is applied. The results are compared to previous ones from CMS [8].
png pdf	Figure 6: Individual and combined dilepton (this analysis) and diphoton [10] 95% CL expected (left) and observed (right) exclusion limits as a summary of all parameter conventions for the ADD model. Signal model cross sections are calculated up to leading order. The dilepton limits from the $\sqrt {s} = $ 8 TeV measurement [8] are also shown.
png pdf	Figure 6-a: Individual and combined dilepton (this analysis) and diphoton [10] 95% CL expected exclusion limits as a summary of all parameter conventions for the ADD model. Signal model cross sections are calculated up to leading order. The dilepton limits from the $\sqrt {s} = $ 8 TeV measurement [8] are also shown.
png pdf	Figure 6-b: Individual and combined dilepton (this analysis) and diphoton [10] 95% CL observed exclusion limits as a summary of all parameter conventions for the ADD model. Signal model cross sections are calculated up to leading order. The dilepton limits from the $\sqrt {s} = $ 8 TeV measurement [8] are also shown.

Tables
png pdf	Table 1: Systematic uncertainties in the predicted SM yields for the electron and the muon channels, for two dilepton mass thresholds. Where noted, uncertainties are provided separately for events where both leptons are in the barrel region (BB), or where at least one of the leptons is in the endcap region (BE). Uncertainties that are mass-dependent affect both the event yield and the shape of the invariant mass distribution. The systematic uncertainties in the signal yields are largely the same as for the background, with a few exceptions as discussed in the text.
png pdf	Table 2: Exclusion limits at 95% CL for the electron and muon channels, their combination, and the combination with the diphoton [10] analysis, in multiple parameter conventions of the ADD model. Signal model cross sections are calculated up to leading order and, where indicated by the appropriate label, a correction factor of 1.3 is applied. For each of the model parameters, the first value is the observed limit followed by the expected limit in parentheses.

Summary

A search for nonresonant excesses in the invariant mass spectra of electron and muon pairs has been presented. The data set recorded with the CMS detector during 2016 is analyzed, corresponding to an integrated luminosity of {35.9} ({36.3}) fb$^{-1}$ for the electron (muon) channel. No significant deviations from standard model expectations are observed.

A contact interaction (CI) model, taking into account both constructive and destructive interference scenarios, has been used for interpreting the experimental measurements. The 95% confidence level exclusion limits on the compositeness scale range from $\Lambda_{\text{LL}} > $ 20 TeV for the destructive case to $\Lambda_{\text{RR}} > $ 32 TeV for the constructive one, for the left-left and the right-right helicity currents, respectively.

For the Arkani-Hamed-Dimopoulos-Dvali (ADD) model of large extra dimensions, values of the ultraviolet cutoff parameter $\Lambda_{\text{T}}$ (in the Giudice-Rattazzi-Wells, GRW, convention) below 6.9 TeV have been excluded at the 95% confidence level. This corresponds to an exclusion on the string scale $M_{\text{S}}$ below 6.1 TeV in the Hewett convention; in the Han-Lykken-Zhang (HLZ) convention, lower limits are set on $M_{\text{S}}$ that range from 5.5 to 8.2 TeV, depending on the number of extra dimensions. When combined with the results from the latest CMS diphoton analysis [10], these limits improve to 7.7 TeV (GRW), 6.9 TeV (Hewett), and the range 6.1 to 9.3 TeV (HLZ), respectively.

The results presented here for the CI and ADD models improve on previous CMS results at $\sqrt{s} = $ 8 TeV in the dilepton final state [8]. The CI limits on $\Lambda$ are compatible with the dilepton results reported by the ATLAS Collaboration [12,13]. However, an exact comparison is not possible because the ATLAS limits are based on priors for $\Lambda$, whereas the limits reported here are based on a prior that is flat in cross section. For the ADD model, the results reported here are the first measurements at $\sqrt{s} = $ 13 TeV in the dilepton final state. The combination with the CMS diphoton analysis yields the most sensitive results in nonhadronic final states to date.

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