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| CMS-EXO-13-002 ; CERN-EP-2016-068 | ||
| Search for lepton flavour violating decays of heavy resonances and quantum black holes to an $\mathrm{ e }\mu$ pair in proton-proton collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 16 April 2016 | ||
| Eur. Phys. J. C 76 (2016) 317 | ||
| Abstract: A search for narrow resonances decaying to an electron and a muon is presented. The $\mathrm{ e }\mu$ mass spectrum is also investigated for non-resonant contributions from the production of quantum black holes (QBHs). The analysis is performed using data corresponding to an integrated luminosity of 19.7 fb$^{-1}$ collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. With no evidence for physics beyond the standard model in the invariant mass spectrum of selected $\mathrm{ e }\mu$ pairs, upper limits are set at 95% confidence level on the product of cross section and branching fraction for signals arising in theories with charged lepton flavour violation. In the search for narrow resonances, the resonant production of a $\tau$ sneutrino in R-parity violating supersymmetry is considered. The $\tau$ sneutrino is excluded for masses below 1.28 TeV for couplings $\lambda_{132}=\lambda_{231}=\lambda'_{311}= $ 0.01 , and below 2.30 TeV for $\lambda_{132}=\lambda_{231}= $ 0.07 and $\lambda'_{311}= $ 0.11. These are the most stringent limits to date from direct searches at high-energy colliders. In addition, the resonance searches are interpreted in terms of a model with heavy partners of the Z boson and the photon. In a framework of TeV-scale quantum gravity based on a renormalization of Newton's constant, the search for non-resonant contributions to the $\mathrm{ e }\mu$ mass spectrum excludes QBH production below a threshold mass $M_{\mathrm{th}}$ of 1.99 TeV. In models that invoke extra dimensions, the bounds range from 2.36 TeV for one extra dimension to 3.63 TeV for six extra dimensions. This is the first search for QBHs decaying into the $\mathrm{ e }\mu$ final state. | ||
| Links: e-print arXiv:1604.05239 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Invariant mass distributions of reconstructed $\mathrm{ e } \mu$ pairs from simulated QBH signal events that pass the event selection, normalized to unit area. The steps at the threshold masses ${M_{\mathrm {th}}}$ are smeared out by the detector resolution. |
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Figure 2-a:
The invariant mass distribution of selected $\mathrm{ e } \mu$ pairs (a), and the corresponding cumulative distribution, where all events above the mass value on the $x$-axis are summed (b). The points with error bars represent the data and the stacked histograms represent the expectations from SM processes. The label 'Jets' refers to the estimate of the W+jet and QCD multijet backgrounds from data. The ratio of the data to the background for each bin is shown at the bottom. The horizontal lines on the data points indicate the bin width. |
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Figure 2-b:
The invariant mass distribution of selected $\mathrm{ e } \mu$ pairs (a), and the corresponding cumulative distribution, where all events above the mass value on the $x$-axis are summed (b). The points with error bars represent the data and the stacked histograms represent the expectations from SM processes. The label 'Jets' refers to the estimate of the W+jet and QCD multijet backgrounds from data. The ratio of the data to the background for each bin is shown at the bottom. The horizontal lines on the data points indicate the bin width. |
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Figure 3-a:
a: The 95% CL upper limit on the product of signal cross section and branching fraction for the RPV ${\tilde{\nu }_{\tau }}$ signal as a function of the mass of the resonance ${M_{\tilde{\nu }_{\tau }}}$. b: The 95% CL limit contours for the RPV ${\tilde{\nu }_{\tau }}$ signal in the (${M_{\tilde{\nu }_{\tau }}}$ , ${\lambda '_{311}}$) parameter plane. The values of the parameter $ {\lambda _{132}} = {\lambda _{231}} $ are fixed to 0.07 (red dashed and dotted), 0.05 (green small-dashed), 0.01 (blue dashed), and 0.007 (black solid). The regions above the curves are excluded. |
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Figure 3-b:
a: The 95% CL upper limit on the product of signal cross section and branching fraction for the RPV ${\tilde{\nu }_{\tau }}$ signal as a function of the mass of the resonance ${M_{\tilde{\nu }_{\tau }}}$. b: The 95% CL limit contours for the RPV ${\tilde{\nu }_{\tau }}$ signal in the (${M_{\tilde{\nu }_{\tau }}}$ , ${\lambda '_{311}}$) parameter plane. The values of the parameter $ {\lambda _{132}} = {\lambda _{231}} $ are fixed to 0.07 (red dashed and dotted), 0.05 (green small-dashed), 0.01 (blue dashed), and 0.007 (black solid). The regions above the curves are excluded. |
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Figure 4-a:
a: The 95% CL exclusion limit on the product of signal cross section and branching fraction for the Z' signal as a function of the mass $M_{\mathrm{Z}' }$. b: The 95% CL limit contour for the Z' signal in the ($M_{\mathrm{Z}' }$, ${\kappa}$) parameter plane. |
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Figure 4-b:
a: The 95% CL exclusion limit on the product of signal cross section and branching fraction for the Z' signal as a function of the mass $M_{\mathrm{Z}' }$. b: The 95% CL limit contour for the Z' signal in the ($M_{\mathrm{Z}' }$, ${\kappa}$) parameter plane. |
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Figure 5:
The 95% CL exclusion limit on the product of signal cross section and branching fraction for the QBH signal as a function of the threshold mass $M_{\mathrm {th}}$. The limits have been calculated using the signal shape of the QBH model without extra dimensions ($n= $ 0 ). For signal masses $M_{\mathrm {th}} \geq $ 1 TeV, the change in the QBH signal shape for different numbers of extra dimensions has a negligible impact on the limit. |
| Tables | |
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Table 1:
Signal acceptance ($A$) and the product of acceptance and efficiency($A \epsilon $) for different signal masses, for the RPV ${\tilde{\nu }_{\tau }}$ and LFV Z' models. The acceptance is defined as the fraction of signal events in the simulation passing the selection on lepton $ {p_{\mathrm {T}}} $ and $\eta $ applied to the generated leptons. |
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Table 2:
Signal acceptance ($A$) and the product of acceptance and efficiency($A \epsilon $) for different threshold masses $M_{\mathrm {th}}$, for the QBH models with $n= $ 0 and $n= $ 6 extra dimensions. The acceptance is defined as the fraction of signal events in the simulation passing the selection on lepton $ {p_{\mathrm {T}}} $ and $\eta $ applied to the generated leptons. |
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Table 3:
Parametrization of the product of signal acceptance and efficiency($A \epsilon $) as a function of signal mass $M$, for the RPV $ {\tilde{\nu }_{\tau }}$ and LFV Z' models. The value of $M$ is expressed in units of GeV . |
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Table 4:
The number of observed events compared to the background expectation in five invariant mass ranges and in the full invariant mass range. The yields obtained from simulations are normalized according to their expected cross sections. The background label 'Jets' refers to the estimate of the W+jet and QCD multijet backgrounds from data. |
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Table 5:
The 95% CL observed and expected lower bounds on the signal masses of $\tau$ sneutrinos in RPV SUSY, resonances in the LFV Z' model, and QBHs, each with subsequent decay into an $\mathrm{ e } \mu$ pair. For the QBH signal with $n =$ 1 , two signal cross sections are considered with the Schwarzschild radius evaluated in either the Randall-Sundrum(RS) or the Particle Data Group(PDG) convention. |
| Summary |
| A search has been reported for heavy states decaying promptly into an electron and a muon using 19.7 fb$^{-1}$ of proton-proton collision data recorded with the CMS detector at the LHC at a centre-of-mass energy of $8 TeV$. Agreement is observed between the data and the standard model expectation with new limits set on resonant production of $\tau$ sneutrinos in R-parity violating supersymmetry with subsequent decay into $\mathrm{ e }\mu$ pairs. For couplings ${\lambda_{132}} ={\lambda_{231}} =$ 0.01 and ${\lambda'_{311}} =$ 0.01, $\tau$ sneutrino lightest supersymmetric particles for masses ${M_{\tilde{\nu}_{\tau}}} $ below 1.28 TeV are excluded at 95% CL. For couplings ${\lambda_{132}} ={\lambda_{231}} =$ 0.07 and ${\lambda'_{311}} =$ 0.11, masses ${M_{\tilde{\nu}_{\tau}}} $ below 2.30 TeV are excluded. These are the most stringent limits from direct searches at high-energy colliders. For the Z' signal model, a lower mass limit of $M_{\mathrm{Z}'}=M_{\gamma^\prime}=$ 1.29 TeV is set at 95% CL for the coupling modifier ${\kappa} = $ 0.05. This direct search for resonant production of an $\mathrm{ e }\mu$ pair at the TeV scale does not reach the sensitivity of dedicated low-energy experiments, but complements such indirect searches and can readily be interpreted in terms of different signals of new physics involving a heavy state that decays promptly into an electron and a muon. Lower bounds are set on the mass threshold for the production of quantum black holes with subsequent decay into an $\mathrm{ e }\mu$ pair in models with zero to six extra dimensions, assuming the threshold mass to be at the Planck scale, ranging from $M_{\mathrm{th}}=$ 1.99 TeV ($n$=0) to 3.63 TeV ($n$=6). These are the first limits on quantum black holes decaying into $\mathrm{ e }\mu$ final states. |
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Compact Muon Solenoid LHC, CERN |
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