Search for lepton flavour violating decays of heavy resonances and quantum black holes to an \(\mathrm {e}\mu \) pair in proton–proton collisions at \(\sqrt{s}=8~\text {TeV} \)

The European Physical Journal C, Jun 2016

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\(~\text {fb}^\text {-1}\) collected in proton-proton collisions at a centre-of-mass energy of 8\(~\text {TeV}\) 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 \(\mathrm {\tau }\) sneutrino in R-parity violating supersymmetry is considered. The \(\mathrm {\tau }\) sneutrino is excluded for masses below 1.28\(~\text {TeV}\) for couplings \(\lambda _{132}=\lambda _{231}=\lambda '_{311}=0.01\), and below 2.30\(~\text {TeV}\) 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 \({\mathrm {Z}} \) 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\(~\text {TeV}\). In models that invoke extra dimensions, the bounds range from 2.36\(~\text {TeV}\) for one extra dimension to 3.63\(~\text {TeV}\) for six extra dimensions. This is the first search for QBHs decaying into the \(\mathrm {e}\) \({\mu }\) final state.

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Search for lepton flavour violating decays of heavy resonances and quantum black holes to an \(\mathrm {e}\mu \) pair in proton–proton collisions at \(\sqrt{s}=8~\text {TeV} \)

Eur. Phys. J. C Search for lepton flavour violating decays of heavy resonances and quantum black holes to an eµ pair in proton-proton √ collisions at s = 8 TeV CMS Collaboration 0 1 2 3 4 5 7 8 11 12 13 0 CERN , 1211 Geneva 23 , Switzerland 1 Ghent University , Ghent , Belgium K. Beernaert , L. Benucci, A. Cimmino, S. Crucy, D. Dobur, A. Fagot, G. Garcia, M. Gul, J. Mccartin, A. A. Ocampo Rios, D. Poyraz, D. Ryckbosch, S. Salva, M. Sigamani, M. Tytgat, W. Van Driessche, E. Yazgan, N. Zaganidis 2 Helsinki Institute of Physics , Helsinki, Finland J. Härkönen, V. Karimäki, R. Kinnunen, T. Lampén, K. Lassila-Perini, S. Lehti, T. Lindén, P. Luukka, T. Peltola, J. Tuominiemi, E. Tuovinen, L. Wendland 3 University of Cyprus , Nicosia, Cyprus A. Attikis, G. Mavromanolakis, J. Mousa, C. Nicolaou, F. Ptochos, P. A. Razis, H. Rykaczewski 4 Faculty of Science, University of Split , Split , Croatia Z. Antunovic, M. Kovac 5 Lappeenranta University of Technology , Lappeenranta , Finland J. Talvitie, T. Tuuva 6 , T. Peiffer, A. Perieanu , N. Pietsch, J. Poehlsen, D. Rathjens, C. Sander, C. Scharf, P. Schleper, E. Schlieckau, A. Schmidt, S. Schumann, J. Schwandt, V. Sola, H. Stadie, G. Steinbrück, F. M. Stober, H. Tholen, D. Troendle, E. Usai, L. Vanelderen, A. Vanhoefer, B. Vormwald 7 University of Hamburg , Hamburg, Germany V. Blobel, M. Centis Vignali, A. R. Draeger, J. Erfle, E. Garutti, K. Goebel, D. Gonzalez, M. Görner, J. Haller, M. Hoffmann, R. S. Höing, A. Junkes, R. Klanner, R. Kogler, N. Kovalchuk, T. Lapsien, T. Lenz, I. Marchesini, D. Marconi, M. Meyer, D. Nowatschin, J. Ott, F. Pantaleo 8 Institute for Research in Fundamental Sciences (IPM) , Tehran , Iran H. Bakhshiansohi , H. Behnamian, S. M. Etesami 9 , M. Khakzad , M. Mohammadi Najafabadi, M. Naseri, S. Paktinat Mehdiabadi, F. Rezaei Hosseinabadi, B. Safarzadeh 10 , M. Zeinali 11 University of Canterbury , Christchurch , New Zealand P. H. Butler 12 Universidad de Oviedo , Oviedo , Spain J. Cuevas, J. Fernandez Menendez, S. Folgueras, I. Gonzalez Caballero, E. Palencia Cortezon, J. M. Vizan Garcia 13 Institute of Experimental Physics, Faculty of Physics, University of Warsaw , Warsaw , Poland G. Brona, K. Bunkowski, A. Byszuk 14 , K. Doroba , A. Kalinowski, M. Konecki, J. Krolikowski, M. Misiura, M. Olszewski, M. Walczak A search for narrow resonances decaying to an electron and a muon is presented. The eμ 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 eμ 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 τ sneutrino in R-parity violating supersymmetry is considered. The τ sneutrino is excluded for masses below 1.28 TeV for couplings λ132 = λ231 = λ311 = 0.01, and below 2.30 TeV for λ132 = λ231 = 0.07 and λ311 = 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 eμ mass spectrum excludes QBH production below a threshold mass Mth 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 eμ final state. 1 Introduction Several extensions of the standard model (SM) predict the existence of heavy, short-lived states that decay to the eμ final state, and motivate the search for lepton flavour violating (LFV) signatures in interactions involving charged leptons. This paper reports a search for phenomena beyond the SM in the invariant mass spectrum of eμ pairs. The analysis is based on data with an integrated luminosity of 19.7 fb−1 collected in proton-proton (pp) collisions at √s = 8 TeV with the CMS detector at the CERN LHC [ 1 ]. The results are interpreted in terms of three theoretically predicted objects: a τ sneutrino (ν˜τ ) lightest supersymmetric particle (LSP) in R-parity violating (RPV) supersymmetry (SUSY) [ 2 ], interfering LFV Z and γ bosons [ 3 ], and quantum black holes (QBHs) [ 4–6 ]. In RPV SUSY, lepton number can be violated at tree level in interactions between fermions and sfermions, and the ν˜τ may be the LSP [ 7 ]. For the resonant ν˜τ signal, the following trilinear RPV part of the superpotential is considered: WRPV = 21 λi jk Li L j E¯k + λi jk Li Q j D¯ k , where i , j , and k ∈ {1, 2, 3} are generation indices, L and Q are the SU (2)L doublet superfields of the leptons and quarks, and E¯ and D¯ are the SU (2)L singlet superfields of the charged leptons and down-like quarks. We assume that all RPV couplings vanish, except for λ132, λ231, and λ311, and consider a SUSY mass hierarchy with a ν˜τ LSP. In this model, the ν˜τ can be produced resonantly in pp collisions via the λ311 coupling and it can decay either into an eμ pair via the λ132 and λ231 couplings, or into a dd pair via the λ311 coupling. In this analysis we consider only the eμ final state and, for simplicity, we assume λ132 = λ231. The LFV Z signal is based on a model with two extra dimensions [ 3,8 ], where the three generations of the SM arise from a single generation in higher-dimensional spacetime. Flavour changing processes are introduced through the Kaluza–Klein modes of gauge fields that are not localised on a brane. In four-dimensional space-time, an effective Lagrangian can be obtained that contains two complex vector fields Z and γ . These vector fields generate transitions between the families in which the generation number changes by unity, such as the process d + s → Z /γ → e− + μ+ and its charge conjugate. The structure of the terms in the Lagrangian for the production and decay of the Z and γ bosons is analogous to that describing the interactions of the Z boson and the photon with quarks and charged leptons, respectively. The coupling strengths g12 and e12 are related to their SM counterparts through a multiplicative coupling modifier κ. For simplicity, the masses MZ and Mγ are assumed to be equal, and the model is referred to as the LFV Z model. It is characterized by the two independent parameters MZ and κ. Theories that have a fundamental Planck scale of the order of a TeV [ 9–13 ] offer the possibility of producing microscopic black holes [ 14–16 ] at the LHC. In contrast to semiclassical, thermal black holes, which would decay to high-multiplicity final states, QBHs are non-thermal objects expected to decay predominantly to pairs of particles. We consider the production of a spin-0, colourless, neutral QBH in a model with lepton flavour violation, in which the cross section for QBH production is extrapolated from semiclassical black holes and depends on the threshold mass Mth for QBH production and the number of extra dimensions n. For n = 0, it corresponds to a 3+1-dimensional model with low-scale quantum gravity, where a renormalization of Newton’s constant leads to a Planck scale at the TeV scale [ 13,17,18 ]; n = 1 corresponds to the Randall– Sundrum (RS) brane world model [ 9,10 ]; and n > 1 to the Arkani-Hamed–Dimopoulos–Dvali (ADD) model [ 11,12 ]. We consider flat-space black holes (black holes that are spherical both in the brane and in the bulk dimensions) and, in the case of RS-type black holes (n = 1), consider only the regime in which almost flat five-dimensional space is an applicable metric. This is the case for rS 1/(ke−krc ), where rS is the Schwarzschild radius, k denotes the Anti-de Sitter curvature, and rc is the size of the extra dimension. The threshold Mth is assumed to be at the Planck scale in the definition of the Particle Data Group [ 19 ] for n = 0 and n > 1, whereas for n = 1 both the PDG and RS definitions [ 4 ] are adopted. In this model, the branching fraction of QBH decays to the e±μ∓ final state is 1.1 %, which is twice that of the dimuon or dielectron decay modes, making the e±μ∓ signature the most promising leptonic decay channel. While the resonant ν˜τ and LFV Z signals result in a narrow peak in the invariant mass spectrum of the eμ pair, the mass distribution of the QBH signal is characterized by an edge at the threshold for QBH production, and a monotonically decreasing tail. Direct searches for resonances in the eμ invariant mass spectrum with interpretations in terms of ν˜τ production have been carried out by the CDF [ 20 ] and D0 [ 21 ] collaborations at the Fermilab Tevatron and most recently by the ATLAS collaboration [ 22 ] using pp collision data at a centre-of-mass energy of 8 TeV at the LHC. For couplings λ132 = 0.07 and λ311 = 0.11, the most stringent of these limits stems from the search performed by the ATLAS collaboration, excluding at 95 % confidence level (CL) a ν˜τ below a mass of 2.0 TeV. Low-energy muon conversion experiments [ 23 ] yield strong limits as a function of the τ sneutrino mass on the product of the two RPV couplings of λ132λ311 < 3.3 × 10−7 Mν˜τ /1 TeV 2 at 90 % CL [ 24 ]. In the case of the Z signal, searches for K0L → eμ decays constrain the coupling modifier κ. For the choice MZ = Mγ , a bound of κ MZ /100 TeV is obtained at 90 % CL [ 3,25 ]. There have been searches for QBHs decaying hadronically, by the CMS [ 26–28 ] and ATLAS [ 29,30 ] collaborations, and in the photon plus jet, lepton plus jet, dimuon, and dielectron final states, by the ATLAS collaboration [ 31–34 ]. This is the first search for QBH decays into the eμ final state. The search for the phenomena beyond the SM described above is carried out for invariant masses of the eμ pair of Meμ ≥ 200 GeV, which is the relevant region in light of existing constraints from other direct searches. Using the same event selection, the eμ invariant mass spectrum is searched for two different signal shapes: the shape associated with a narrow resonance that may be interpreted in terms of any model involving a resonance decaying promptly into an electron and a muon, and the more model-specific QBH signal shape. With a relative eμ invariant mass resolution ranging from 1.6 % at Meμ = 200 GeV to 6 % at Meμ = 3 TeV, the CMS detector is a powerful tool for searches for new physics in the eμ invariant mass spectrum. 2 The CMS detector The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. The silicon tracker consists of 1440 silicon pixel and 15 148 silicon strip detector modules and measures charged particles within the pseudorapidity range |η| < 2.5. The ECAL consists of 75 848 lead tungstate crystals and provides coverage for |η| < 1.479 in a barrel region and 1.479 < |η| < 3.0 in two endcap regions. Muons are measured in the range |η| < 2.4, with detection planes using three technologies: drift tubes, cathode strip chambers, and resistive plate chambers. A two-level trigger system is used by the CMS experiment. The first level is composed of custom hardware processors and uses information from the calorimeters and muon detectors to select interesting events and to reduce the event rate from the initial bunch crossing frequency of 20 MHz to a maximum of 100 kHz. The high-level trigger processor farm further decreases the event rate to 400 Hz before data storage. A detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in Ref. [ 35 ]. 3 Event selection The search is designed in a model-independent way by requiring only one prompt, isolated muon and one prompt, isolated electron in the event selection. This minimal selection allows for a reinterpretation of the results in terms of models with more complex event topologies than the single eμ pair present in the signals considered in this paper. The data sample is selected using a single-muon trigger with a minimum transverse momentum ( pT) requirement of pT > 40 GeV. In order to allow the trigger to remain unprescaled, the pseudorapidity of the muons is constrained to values |η| < 2.1. Offline, each event is required to have a reconstructed pp collision vertex with at least four associated tracks, located less than 2 cm from the centre of the detector in the plane transverse to the beam and less than 24 cm from it in the direction along the beam. The primary vertex is defined as the vertex with the largest sum of squared transverse momenta of its associated tracks. The reconstruction and identification of electrons and muons is carried out using standard CMS algorithms, described in more detail in Refs. [ 36–40 ]. Reconstruction of the muon track starts from two tracks, one built in the silicon tracker and one built in the muon system. Hits used to reconstruct the tracks in the two systems are then used to reconstruct a track spanning over the entire detector [ 36 ]. Muon candidates are required to have a transverse momentum of pT > 45 GeV with a measured uncertainty of δ( pT)/ pT < 0.3 and must fall into the acceptance of the trigger of |η| < 2.1. The candidate’s track must have transverse and longitudinal impact parameters with respect to the primary vertex position of less than 0.2 and 0.5 cm, respectively. At least one hit in the pixel detector, six or more hits in silicon-strip tracker layers, and matched segments in at least two muon detector planes are required to be associated with the reconstructed track. In order to suppress backgrounds from muons within jets, the scalar pT sum of all other tracks within a cone of size 0.3 in ΔR = (Δη)2 + (Δφ)2 (where φ is the azimuthal angle in radians) around the muon candidate’s track is required to be less than 10 % of the candidate’s pT. In the electron reconstruction, ECAL clusters are matched to silicon pixel detector hits, which are then used as seeds for the reconstruction of tracks in the tracker. Electron candidates are built from clusters with associated tracks and must lie within the barrel or endcap acceptance regions, with pseudorapidities of |η| < 1.442 and 1.56 < |η| < 2.5, respectively, with a transverse energy ET > 35 GeV. The transverse energy is defined as the magnitude of the projection on the plane perpendicular to the beam of the electron momentum vector normalized to the electron energy measured in the ECAL. Misidentification of jets as electrons is suppressed by requiring that the scalar sum of the pT of all other tracks in a cone of size 0.3 in ΔR around the electron candidate’s track is less than 5 GeV. In addition, the sum of the ET of calorimeter energy deposits in the same cone that are not associated with the electron candidate must be less than 3 % of the candidate’s ET (plus a small η-dependent offset). To minimise the impact of additional pp interactions in the same bunch crossing (pileup) on the selection efficiency, the calorimeter isolation is corrected for the average energy density in the event [ 41 ]. Further reduction of electron misidentification is achieved by requiring the transverse profile of the energy deposition in the ECAL to be consistent with the expected electron profile, and the sum of HCAL energy deposits in a cone of size 0.15 in ΔR to be less than 5 % of the electron’s ECAL energy. The transverse impact parameter of the electron candidate’s track with respect to the primary vertex must not exceed 0.02 cm and 0.05 cm, for barrel and endcap candidates, respectively, and the track must not have more than one missing hit in the layers of the pixel detector it crossed. The trigger efficiency has been measured using the “tagand-probe” technique in dimuon events from Z decays described in [ 36,38,39 ]. The trigger efficiency for muons that pass the selection requirements is 92.9 % within |η| < 0.9, 83.1 % within 0.9 < |η| < 1.2, and 80.3 % within 1.2 < |η| < 2.1. The muon identification efficiency, including the isolation requirement, is measured with the tag-andprobe technique applied to muons from Z boson decays using tracks in the inner silicon tracker as probes. The same efficiency of 95 ± 1 % (syst) is obtained in the three pseudorapidity regions |η| < 0.9, 0.9 < |η| < 1.2, and 1.2 < |η| < 2.1, with corresponding efficiency ratios between data and the simulation of 0.990 ± 0.005 (syst), 0.992 ± 0.005 (syst), and 0.995 ± 0.005 (syst). A pT range up to 300 GeV has been probed with the tag-and-probe method and the muon identification efficiencies remain constant within the statistical precision, as do the corresponding efficiency ratios between data and simulation. The evolution of the muon reconstruction and identification efficiencies and the muon trigger efficiency for muon pT > 300 GeV is based on simulation. Using dielectron events from Z boson decays [ 37 ], the total efficiency to reconstruct and select electrons with pTe > 100 GeV is found to be 88 ± 2 % (syst) in the barrel region and 84 ± 4 % (syst) in the endcaps. According to Monte Carlo (MC) simulation, the variation of these efficiencies with electron pT is less than ±1 % in the barrel and ±2 % in the endcaps. The corresponding efficiency ratios for pTe > 100 GeV between data and simulation are 0.985 ± 0.014 (syst) in the barrel and 0.981 ± 0.004 (syst) in the endcaps. These efficiencies and efficiency ratios have been measured up to an electron pT of 1 TeV in the barrel and 500 GeV in the endcap regions. In the event selection, at least one isolated muon and one isolated electron that both pass the identification criteria described above are required. After the application of all efficiency scale factors that correct the simulation to the efficiencies measured in data, the combined dilepton reconstruction and identification efficiency for RPV ν˜τ signal events within the detector acceptance is expected to be 80.6 % at Mν˜τ = 200 GeV and the full selection efficiency including the trigger requirement is 71.2 %. The MC simulation predicts that this efficiency is constant within 3 % for masses between 200 GeV and 3 TeV. The electron and the muon are not required to have opposite charge, in order to avoid a loss in signal efficiency due to possible electron charge misidentification at high electron pT. Since highly energetic muons can produce bremsstrahlung resulting in an associated supercluster in the calorimeter in the direction of the muon’s inner track, they can be misidentified as electrons. Therefore, an electron candidate is rejected if there is a muon with pT greater than 5 GeV within ΔR < 0.1 of the candidate. Only one eμ pair per event is considered. For about 1 % of the events passing the event selection there is more than one eμ pair in the event, in which case the pair with the highest invariant mass is selected. 4 Signal simulation The RPV and QBH signal samples are generated with the CalcHEP (v. 3.4.1) event generator [ 42 ]. A cross section calculation at next-to-leading order (NLO) in perturbative QCD is used for the RPV signal [ 43 ], in which the factorization and renormalization scales are set to Mν˜τ and the CTEQ6M [ 44 ] set of parton distribution functions (PDF) is used. The invariant mass distributions of reconstructed eμ pairs from simulated QBH signal samples are presented in Fig. 1 for different signal masses and numbers of extra dimensions. A more detailed description of the implemented QBH model including the dependence of the Meμ spectrum from QBH decays on the model parameters is presented in Ref. [ 45 ]. The LFV Z signal events are produced with the MadGraph (v. 5.1.5.9) generator [ 46 ]. The effects of the interference resulting from the MZ = Mγ mass degeneracy on the cross section and signal acceptance are taken into account, and the coupling parameters of the model are taken to be the same as in Ref. [ 3 ]. All signal samples use the CTEQ6L1 [ 44 ] PDF, pythia (v. 6.426) [ 47 ] for hadronization with the underlying event tune Z2*, and are processed through a simulation of the full CMS detector based on Geant4 (v. 9.4) [ 48 ]. The pythia Z2* tune is derived from the Z1 tune [ 49 ], which uses the CTEQ5L PDF set, whereas Z2* adopts CTEQ6L. . 0.2 U . A0.18 0.16 CMS Simulation (8 TeV) QBH simulation n = 0 , Mth = 0.5 TeV n = 1 , Mth = 0.5 TeV n = 6 , Mth = 0.5 TeV n = 6 , Mth = 1 TeV n = 6 , Mth = 2 TeV n = 6 , Mth = 3 TeV The total acceptance times efficiency for each of the three signal models considered in this analysis is determined using MC simulation with selection efficiencies corrected to the values measured in data. The signal acceptance, as defined by the selection on the lepton pT and η applied to the generated leptons in the signal simulation, and the product of acceptance and selection efficiency, are shown in Tables 1 and 2, evaluated for selected signal masses. The acceptance of the RPV ν˜τ model is that of a generic spin-0 resonance. In the case of the LFV Z model, the acceptance is more modelspecific due to the interference between the Z and the γ . This interference shapes the η distributions of the leptons in the final state, which leads to a smaller acceptance compared to a generic spin-1 resonance. Table 3 lists the parameterizations of the acceptance times efficiency as a function of signal mass for the RPV ν˜τ and LFV Z resonance signals, resulting from fits in the mass range from 200 GeV to 2.5 TeV. These parameterizations are used later in the statistical interpretation of the resonance search. The SM backgrounds contributing to the eμ final state can be divided into two classes of events. The first class comprises events with at least two prompt, isolated leptons. The second class consists of events with either jets or photons that are misidentified as isolated leptons, and events with jets containing non-prompt leptons. This second class of background is referred to as “non-prompt background” in this paper. The expected SM background from processes with two prompt leptons is obtained from MC simulations. It consists mostly of events from tt production and WW production; the former process is dominant at lower masses and the latter becomes equally important above Meμ ∼ 1 TeV. Other background processes estimated from MC simulation are the additional diboson processes WZ and ZZ, single top tW production, and Drell–Yan (DY) τ τ events with subsequent decay of the τ τ pair into an electron and a muon. The tt, tW, and WW simulated samples are generated using powheg (v. 1.0) [ 50–52 ] with the CT10 PDF [ 53 ], and the DY, WZ, and ZZ background samples are generated using the MadGraph (v. 5.1.3.30) event generator with the CTEQ6L1 PDF. All background samples use pythia (v. 6.426) for hadronization with the underlying event tune Z2∗. The generated events are processed through a full simulation of the CMS detector based on Geant4 (v. 9.4). Pileup interactions are included in the simulation and event-dependent weights are applied in order to reproduce the number of pp interactions expected for the measured instantaneous luminosity. After this procedure, the distribution of the number of vertices per event observed in data is well described by the simulation. The simulated samples are normalized to the integrated luminosity of the data sample, 19.7 fb−1. The cross sections are calculated to next-to-next-to-leading order (NNLO) accuracy in perturbative QCD for tt [ 54 ] and DY [ 55 ] and to NLO accuracy for the tW [ 56 ], WW, WZ, and ZZ [ 57 ] processes. The main sources of non-prompt background in the eμ selection arise from W+jet and Wγ production with a jet or photon that are misidentified as an electron. The Z+jet, QCD multijet, and tt processes yield subleading contributions to the background with non-prompt leptons. The Wγ background is estimated from simulation based on the MadGraph (v. 5.1.3.30) event generator. A background estimation based on control samples in data, using the jet-to-electron misidentification rate (MR) method explained below, is used to determine the Meμ distributions from W+jet and QCD multijet production. The measurement of the jet-to-electron misidentification rate has been carried out in the context of Ref. [ 40 ]. It starts from a sample collected using a prescaled single electromagnetic cluster trigger, in which the presence of an electron candidate with relaxed electron identification criteria is required. The events of the sample must have no more than one reconstructed electron with ET > 10 GeV, in order to suppress the contribution from Z decays. The misidentification measurement can be biased by selecting genuine electrons from W+jet events or converted photons from γ +jet events. Processes that can give a single electron, such as tt, tW, WW, WZ, Z → τ τ , and Z → ee where, if a second electron is produced, it fails to be reconstructed, give another less significant source of contamination. Simulated samples are used to correct for this contamination and its effect on the MR. After these corrections, the electron MR, measured in bins of ET and η, is the number of electrons passing the full selection over the number of electron candidates in the sample. Using the measured electron MR, the W+jet and QCD multijet contributions can be estimated from a sample with a muon passing the single-muon trigger and the full muon selection, and an electron candidate satisfying the relaxed selection requirements but failing the full electron selection. Each event in the sample is weighted by the factor MR/(1−MR) to determine the overall contribution of the jet backgrounds. Contributions from processes other than W+jet and QCD multijet are subtracted from the sample to which the MR is applied, to avoid double counting. This subtraction is based on MC simulated background samples. A systematic uncertainty of 30 % is applied to the jet background estimate, based on cross-checks and closure tests. An uncertainty of 50 % is assigned to the background estimate for the Wγ process, which is taken from simulation at leading order (LO) in perturbative QCD. CMS CMS 19.7 fb-1 (8 TeV) Data tt WW DY Jets tW Wγ WZ, ZZ >104 V eG103 0 /1102 s t ven10 E < 1 10-1 with same-charge eμ pairs are expected, most of which stem from the W+jet process, followed by tt and diboson production WZ/ZZ. The systematic uncertainties assigned to backgrounds obtained from simulation include the integrated luminosity (2.6 %) [ 58 ] and the acceptance times efficiency (5 %). The latter is based on the uncertainties in the various efficiency scale factors that correct the simulation to the efficiencies measured in data. According to simulation, the evolution of the lepton selection efficiencies from the Z pole, where they are measured, to high lepton pT is covered within this uncertainty. The uncertainty in the muon momentum scale is 5 % After the event selection, 28 925 events are observed in data. The eμ invariant mass distribution is shown in Fig. 2, together with the corresponding cumulative distribution. A comparison of the observed and expected event yields is given in Table 4. The dominant background process is tt, which contributes 69 % of the total background yield after selection, followed by WW production, contributing 11 %. The two selected leptons carry opposite measured electric charge in 26 840 events and carry the same charge in 2085 events. According to the background estimation, 2100 ± 360 events per TeV. Electron energy scale uncertainties are 0.6 % in the barrel and 1.5 % in the endcap. These momentum and energy scale uncertainties cumulatively lead to an uncertainty in the total background yield of 2 % at Meμ = 500 GeV and 3.5 % at Meμ = 1 TeV. Uncertainties in the electron ET and muon pT resolutions have a negligible impact on the total background yield. The uncertainty associated with the choice of PDF in the background simulation is evaluated according to the PDF4LHC prescription [ 59,60 ] and translates into an uncertainty in the background yield ranging from 5 % at Meμ = 200 GeV to 9 % at Meμ = 1 TeV. Among the uncertainties in the cross sections used for the normalization of the various simulated background samples, the 5 % uncertainty in the NNLO QCD cross section of the dominant tt background [54] is the most relevant. Further uncertainties associated with the modelling of the shape of the eμ invariant mass distribution are taken into account for the two leading backgrounds: tt (higher-order corrections on the top- pT description discussed in [ 61 ]) and WW (scale uncertainties studied with the powheg generator). These lead to an uncertainty in the total background yield of up to 13 % at Meμ = 1 TeV. A further systematic uncertainty arises from the limited sizes of the simulated background samples at high invariant mass, where the background expectation is small. Taking all systematic uncertainties into account, the resulting uncertainty in the background yield ranges from 9 % at Meμ = 200 GeV to 18 % at Meμ = 1 TeV. As shown in the cumulative invariant mass distribution in Fig. 2, we observe a deficit in data compared to the background expectation for Meμ ≥ 700 GeV. In this invariant mass region, 17 events are observed and the background estimate yields 27 ± 4 (syst) events. Combining the systematic and statistical uncertainties, the local significance of this discrepancy is below 2σ . No significant excess with respect to the expectation is found in the measured eμ invariant mass distribution, and we set limits on the product of signal cross section and branching fraction for signal mass hypotheses above 200 GeV. Two types of signal shapes are considered for the limit setting: a narrow resonance and the broader eμ invariant mass spectrum from QBH decays. The RPV ν˜τ and Z signals both result in a narrow resonance. For coupling values not excluded by existing searches, the intrinsic widths of these signals are small compared to the detector resolution. Therefore, Gaussian functions are used to model the signal shapes. For each probed resonance signal mass, the two parameters, acceptance times efficiency (Table 3) and invariant mass resolution, define the signal shape used for limit setting. The invariant mass resolution is derived from fits of Gaussian distributions to the eμ invariant mass spectra from MC simulated signal samples and ranges from 1.6 % at a resonance mass of Mres = 200 GeV to 6 % at Mres = 3 TeV. For high values of eμ pair invariant mass, it is dominated by the resolution on the measurement of the muon pT, which ranges from about 2 % at pT = 200 GeV to 6 % at pT = 500 GeV and 10 % at pT = 1 TeV. These values are obtained from MC simulations and agree within the uncertainties with measurements using cosmic ray muons. This model of the narrow resonance allows for a scan of the invariant mass spectrum with a fine spacing of the signal mass hypothesis that corresponds to the invariant mass resolution. Unlike the ν˜τ and Z signals, the QBH signal exhibits a broader shape with a sharp edge at the threshold mass Mth and a tail towards higher masses (Fig. 1). The QBH signal shapes are obtained directly from simulated samples. The systematic uncertainties in the signal entering the limit calculation are the 2.6 % uncertainty in the integrated luminosity, the 5 % uncertainty in the product of acceptance and efficiency, and the relative uncertainty in the mass resolution, which ranges from 2 % at Mres = 200 GeV to 40 % at Mres = 3 TeV. The uncertainty in the signal acceptance times efficiency is dominated by the uncertainty in the trigger, lepton reconstruction, and identification efficiencies, and includes the subleading PDF uncertainty in the signal acceptance. Upper limits at 95 % CL on the product of cross section and branching fraction are determined using a binned likelihood Bayesian approach with a positive, uniform prior for the signal cross section [ 62 ]. The signal and background shapes enter the likelihood with a binning of 1 GeV, well below the invariant mass resolution for masses above 200 GeV. For the resonant signals ν˜τ and Z , search regions in the invariant mass spectrum are defined as ±6 times the invariant mass resolution evaluated at the hypothetical resonance mass. Only events in these search regions enter the binned likelihood in the limit calculation. The impact of a further broadening of the signal window size on the median expected limit has been found to be negligible within the uncertainties. For mass hypotheses above 800 GeV, the upper bound of the search region is dropped. In the case of the QBH signal, the search region is defined by a lower bound at Mth − 6σM , where σM is the invariant mass resolution, and there is no upper bound. The nuisance parameters associated with the systematic uncertainties are modelled with log-normal distributions, and a Markov Chain MC method is used for integration. For each mass hypothesis considered, the posterior probability density function is derived as a function of the signal cross section times branching fraction and yields the 95 % CL upper limit on this parameter of interest. The 95 % CL limits on the signal cross section times branching fraction for the RPV ν˜τ resonance signal are shown in Fig. 3 (left). The signal cross section shown is calculated at NLO in perturbative QCD with the RPV couplings set to λ132 = λ231 = 0.01 and λ311 = 0.01. For these couplings, a lower mass limit of 1.28 TeV is obtained. At this mass, the observed limit on the cross section times branching fraction is )103 b f ( Β ×σ102 10 1 10−1 Fig. 3 Left The 95 % CL upper limit on the product of signal cross section and branching fraction for the RPV ν˜τ signal as a function of the mass of the resonance Mν˜τ . Right The 95 % CL limit contours for the RPV ν˜τ signal in the (Mν˜τ , λ311) parameter plane. The values of the 0.25 fb. For a comparison with earlier searches at hadron colliders [ 20,22 ], the two coupling benchmarks λ132 = λ231 = 0.07, λ311 = 0.11 and λ132 = λ231 = 0.05, λ311 = 0.10 are considered. For RPV couplings λ132 = λ231 = 0.07 and λ311 = 0.11, we set a mass limit of 2.30 TeV, and improve the lower bound of 2.0 TeV previously set [22]. The lower bound on the signal mass for λ132 = λ231 = 0.05 and λ311 = 0.10 is 2.16 TeV. In the narrow width approximation, the cross section times branching fraction scales with the RPV couplings as: σ B ∼ λ311 2 [(λ132)2 + (λ231)2]/(3 λ311 +[(λ132)2 + (λ231)2]). 2 Using this relation and the observed upper cross section bounds, we derive the limit contour in the (Mν˜τ , λ311) parameter plane as a function of a fixed value of λ132 = λ231. For the results presented in Fig. 3 (right), values of the couplings λ311 and λ132 = λ231 up to 0.2 and 0.07 are considered, respectively. The ratio of decay width to mass of the τ sneutrino is less than 0.5 % for these coupling values and finite-width effects are small. Searches for resonant dijet production [ 27,29 ] that cover the τ sneutrino decay to a dd pair via the coupling λ311 do not exclude this region of parameter space. In the model considered here with resonant production of the ν˜τ , we do not reach the sensitivity of muon conversion experiments, which lead to a bound on the coupling product of λ132λ311 < 3.3 × 10−7(Mν˜τ /1 TeV)2 at 90 % CL, assuming λ132 = λ231. For comparison, with a signal mass of Mν˜τ = 1 TeV and the assumption λ132 = λ231 = λ311, we 19.7 fb-1 (8 TeV) 95% CL limit λ132=λ231=0.07 95% CL limit λ132=λ231=0.05 95% CL limit λ132=λ231 =0.01 95% CL limit λ132=λ231=0.007 parameter λ132 = λ231 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 obtain a limit of λ132λ311 < 4.1 × 10−5 at 90 % CL. We present results in terms of the product of the production cross section and branching fraction of the ν˜τ that do not depend on a specific production mechanism of the sneutrino. The 95 % CL limits on the signal cross section times branching fraction for the Z signal, which exhibits a different acceptance from the spin-0 resonance in the RPV model, are presented in Fig. 4 (left). For the coupling modifier κ = 0.05, a lower bound on the signal mass MZ = Mγ of 1.29 TeV is obtained. Figure 4 (right) shows the corresponding limit contour in the (MZ , κ) parameter plane. Since this resonance is produced dominantly in the ds initial state, the bound from searches for muon conversion is not as strong as for the RPV ν˜τ signal, but searches for K0L → eμ decays yield a stringent exclusion limit of κ MZ /100 TeV at 90 % CL. This can be compared to our bound of κ = 0.031 at 90 % CL for MZ = Mγ = 1 TeV. In the QBH search, we set limits on the mass threshold for QBH production, Mth, in models with n = 0 to n = 6 extra dimensions. The 95 % CL limits on the signal cross section times branching fraction for the QBH signal are shown in Fig. 5. For n = 0 in a model with a Planck scale at the TeV scale from a renormalization of the gravitational constant, we exclude QBH production below a threshold mass Mth of 1.99 TeV. For n = 1, two signal cross sections are considered with the Schwarzschild radius evaluated in the RS and PDG conventions. The resulting limits on Mth are 2.36 TeV and 2.81 TeV, respectively. For ADD-type black holes with n > 1, we obtain lower bounds on Mth ranging from 3.15 TeV for n = 2 to 3.63 TeV for n = 6. A summary of the 95 % ) f(b105 Β ×σ104 103 102 10 1 10−1 )103 b f ( Β CL lower mass limits set for all signal models is presented in Table 5. 7 Summary A search has been reported for heavy states decaying promptly into an electron and a muon using 19.7 fb−1 of protonproton collision data recorded with the CMS detector at the CMS 19.7 fb-1 (8 TeV) Excluded at ≥ 95% CL 2000 2500 MZ' (GeV) κ 10−1 10−2 Table 5 The 95 % CL observed and expected lower bounds on the signal masses of τ sneutrinos in RPV SUSY, resonances in the LFV Z model, and QBHs, each with subsequent decay into an eμ 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 Signal model Lower limit signal mass (TeV) Observed Expected RPV ν˜τ (λ132 = λ231 = λ311 = 0.01) RPV ν˜τ (λ132 = λ231 = 0.05, λ311 = 0.10) 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 τ sneutrinos in R-parity violating supersymmetry with subsequent decay into eμ pairs. For couplings λ132 = λ231 = 0.01 and λ311 = 0.01, τ sneutrino lightest supersymmetric particles for masses Mν˜τ below 1.28 TeV are excluded at 95 % CL. For couplings λ132 = λ231 = 0.07 and λ311 = 0.11, masses Mν˜τ 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 MZ = Mγ = 1.29 TeV is set at 95 % CL for the coupling modifier κ = 0.05. This direct search for resonant production of an eμ pair at the TeV scale does not reach the sensitivity of dedicated lowenergy 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 eμ pair in models with zero to six extra dimensions, assuming the threshold mass to be at the Planck scale, ranging from Mth = 1.99 TeV (n = 0) to 3.63 TeV (n = 6). These are the first limits on quantum black holes decaying into eμ final states. Acknowledgments We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS institutes for their contributions to the success of the CMS effort. In addition, we gratefully acknowledge the computing centres and personnel of the Worldwide LHC Computing Grid for delivering so effectively the computing infrastructure essential to our analyses. Finally, we acknowledge the enduring support for the construction and operation of the LHC and the CMS detector provided by the following funding agencies: BMWFW and FWF (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES and CSF (Croatia); RPF (Cyprus); MoER, ERC IUT and ERDF (Estonia); Academy of Finland, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NIH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); MSIP and NRF (Republic of Korea); LAS (Lithuania); MOE and UM (Malaysia); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico); MBIE (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR (Dubna); MON, RosAtom, RAS and RFBR (Russia); MESTD (Serbia); SEIDI and CPAN (Spain); Swiss Funding Agencies (Switzerland); MST (Taipei); ThEPCenter, IPST, STAR and NSTDA (Thailand); TUBITAK and TAEK (Turkey); NASU and SFFR (Ukraine); STFC (United Kingdom); DOE and NSF (USA). Individuals have received support from the Marie-Curie programme and the European Research Council and EPLANET (European Union); the Leventis Foundation; the A. P. Sloan Foundation; the Alexander von Humboldt Foundation; the Belgian Federal Science Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technologie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Science and Industrial Research, India; the HOMING PLUS programme of the Foundation for Polish Science, cofinanced from European Union, Regional Development Fund; the Mobility Plus programme of the Ministry of Science and Higher Education (Poland); the OPUS programme of the National Science Center (Poland); MIUR project 20108T4XTM (Italy); the Thalis and Aristeia programmes cofinanced by EU-ESF and the Greek NSRF; the National Priorities Research Program by Qatar National Research Fund; the Rachadapisek Sompot Fund for Postdoctoral Fellowship, Chulalongkorn University (Thailand); the Chulalongkorn Academic into Its 2nd Century Project Advancement Project (Thailand); and the Welch Foundation, contract C-1845. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecomm ons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Funded by SCOAP3. CMS Collaboration Yerevan Physics Institute, Yerevan, Armenia V. Khachatryan, A. M. Sirunyan, A. Tumasyan National Centre for Particle and High Energy Physics, Minsk, Belarus V. Mossolov, N. Shumeiko, J. Suarez Gonzalez Universidade Estadual Paulistaa , Universidade Federal do ABCb, São Paulo, Brazil S. Ahujaa , C. A. Bernardesb, A. De Souza Santosb, S. Dograa , T. R. Fernandez Perez Tomeia , E. M. Gregoresb, P. G. Mercadanteb, C. S. Moona,7, S. F. Novaesa , Sandra S. Padulaa , D. Romero Abadb, J. C. 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Vesztergombi20, A. J. Zsigmond Institute of Nuclear Research ATOMKI, Debrecen, Hungary N. Beni, S. Czellar, J. Karancsi21, J. Molnar, Z. Szillasi2 University of Debrecen, Debrecen, Hungary M. Bartók22, A. Makovec, P. Raics, Z. L. Trocsanyi, B. Ujvari National Institute of Science Education and Research, Bhubaneswar, India S. Choudhury23, P. Mal, K. Mandal, D. K. Sahoo, N. Sahoo, S. K. Swain Indian Institute of Science Education and Research (IISER), Pune, India S. Chauhan, S. Dube, A. Kapoor, K. Kothekar, S. Sharma University College Dublin, Dublin, Ireland M. Felcini, M. Grunewald INFN Sezione di Baria , Università di Barib, Politecnico di Baric, Bari, Italy M. Abbresciaa,b, C. Calabriaa,b, C. Caputoa,b, A. Colaleoa , D. Creanzaa,c, L. Cristellaa,b, N. De Filippisa,c, M. De Palmaa,b, L. Fiorea , G. Iasellia,c, G. Maggia,c, M. Maggia , G. Minielloa,b, S. Mya,c, S. Nuzzoa,b, A. Pompilia,b, G. Pugliesea,c, R. Radognaa,b, A. Ranieria , G. Selvaggia,b, L. Silvestrisa,2, R. 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Tosia,b INFN Sezione di Napolia , Università di Napoli ‘Federico II’b, Naples, Italy, Università della Basilicatac, Potenza, Italy, Università G. Marconid , Rome, Italy S. Buontempoa , N. Cavalloa,c, S. Di Guidaa,d,2, M. Espositoa,b, F. Fabozzia,c, A. O. M. Iorioa,b, G. Lanzaa , L. Listaa , S. Meolaa,d,2, M. Merolaa , P. Paoluccia,2, C. Sciaccaa,b, F. Thyssen INFN Sezione di Padovaa , Università di Padovab, Padova, Italy, Università di Trentoc, Trento, Italy P. Azzia,2, N. Bacchettaa , L. Benatoa,b, D. Biselloa,b, A. Bolettia,b, A. Brancaa,b, R. Carlina,b, P. Checchiaa , M. Dall’Ossoa,b,2, T. Dorigoa , U. Dossellia , F. Gasparinia,b, U. Gasparinia,b, A. Gozzelinoa , K. Kanishcheva,c, S. Lacapraraa , M. Margonia,b, A. T. Meneguzzoa,b, F. Montecassianoa , J. Pazzinia,b,2, N. Pozzobona,b, P. Ronchesea,b, F. Simonettoa,b, E. Torassa a , M. Tosia,b, M. Zanetti, P. Zottoa,b, A. Zucchettaa,b,2, G. Zumerlea,b INFN Sezione di Paviaa , Università di Paviab, Pavia, Italy A. Braghieria , A. Magnania,b, P. Montagnaa,b, S. P. Rattia,b, V. Rea , C. Riccardia,b, P. Salvinia , I. Vaia,b, P. Vituloa,b INFN Sezione di Pisaa , Università di Pisab, Scuola Normale Superiore di Pisac, Pisa, Italy K. Androsova,30, P. Azzurria,2, G. Bagliesia , J. Bernardinia , T. Boccalia , R. Castaldia , M. A. Cioccia,30, R. Dell’Orsoa , S. Donatoa,c,2, G. Fedi, L. Foàa,c†, A. Giassia , M. T. Grippoa,30, F. Ligabuea,c, T. Lomtadzea , L. Martinia,b, A. Messineoa,b, F. Pallaa,, A. Rizzia,b, A. Savoy-Navarroa,31, A. T. Serbana , P. Spagnoloa , R. Tenchinia , G. Tonellia,b, A. Venturia , P. G. Verdinia Kangwon National University, Chunchon, Korea A. Kropivnitskaya, S. K. Nam Kyungpook National University, Daegu, Korea D. H. Kim, G. N. Kim, M. S. Kim, D. J. Kong, S. Lee, Y. D. Oh, A. Sakharov, D. C. Son Chonbuk National University, Jeonju, Korea J. A. Brochero Cifuentes, H. Kim, T. J. Kim32 Institute for Universe and Elementary Particles, Chonnam National University, Kwangju, Korea S. Song Korea University, Seoul, Korea S. Cho, S. Choi, Y. Go, D. Gyun, B. Hong, H. Kim, Y. Kim, B. Lee, K. Lee, K. S. Lee, S. Lee, J. Lim, S. K. Park, Y. Roh Seoul National University, Seoul, Korea H. D. Yoo University of Seoul, Seoul, Korea M. Choi, H. Kim, J. H. Kim, J. S. H. Lee, I. C. Park, G. Ryu, M. S. Ryu Sungkyunkwan University, Suwon, Korea Y. Choi, J. Goh, D. Kim, E. Kwon, J. Lee, I. Yu Vilnius University, Vilnius, Lithuania V. Dudenas, A. Juodagalvis, J. Vaitkus Centro de Investigacion y de Estudios Avanzados del IPN, Mexico City, Mexico E. Casimiro Linares, H. Castilla-Valdez, E. De La Cruz-Burelo, I. Heredia-De La Cruz35, A. Hernandez-Almada, R. Lopez-Fernandez, J. Mejia Guisao, A. Sanchez-Hernandez Universidad Iberoamericana, Mexico City, Mexico S. Carrillo Moreno, F. Vazquez Valencia Benemerita Universidad Autonoma de Puebla, Puebla, Mexico I. Pedraza, H. A. Salazar Ibarguen, C. Uribe Estrada Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico A. Morelos Pineda University of Auckland, Auckland, New Zealand D. Krofcheck National Centre for Physics, Quaid-I-Azam University, Islamabad, Pakistan A. Ahmad, M. Ahmad, Q. Hassan, H. R. Hoorani, W. A. Khan, S. Qazi, M. Shoaib, M. Waqas Laboratório de Instrumentação e Física Experimental de Partículas, Lisbon, Portugal P. Bargassa, C. Beirão Da Cruz E Silva, A. Di Francesco, P. Faccioli, P. G. Ferreira Parracho, M. Gallinaro, J. Hollar, N. Leonardo, L. Lloret Iglesias, F. Nguyen, J. Rodrigues Antunes, J. Seixas, O. Toldaiev, D. Vadruccio, J. Varela, P. Vischia Institute for Theoretical and Experimental Physics, Moscow, Russia V. Epshteyn, V. Gavrilov, N. Lychkovskaya, V. Popov, I. Pozdnyakov, G. Safronov, A. Spiridonov, E. Vlasov, A. Zhokin National Research Nuclear University ‘Moscow Engineering Physics Institute’ (MEPhI), Moscow, Russia M. Chadeeva, R. Chistov, M. Danilov, V. Rusinov, E. Tarkovskii P. N. Lebedev Physical Institute, Moscow, Russia V. Andreev, M. Azarkin38, I. Dremin38, M. Kirakosyan, A. Leonidov38, G. Mesyats, S. V. Rusakov Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia A. Baskakov, A. Belyaev, E. Boos, M. Dubinin41, L. Dudko, A. Ershov, A. Gribushin, V. Klyukhin, O. Kodolova, I. Lokhtin, I. Miagkov, S. Obraztsov, S. Petrushanko, V. Savrin, A. Snigirev State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, Russia I. Azhgirey, I. Bayshev, S. Bitioukov, V. Kachanov, A. Kalinin, D. Konstantinov, V. Krychkine, V. Petrov, R. Ryutin, A. Sobol, L. Tourtchanovitch, S. Troshin, N. Tyurin, A. Uzunian, A. Volkov Faculty of Physics and Vinca Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia P. Adzic42, P. Cirkovic, D. Devetak, J. Milosevic, V. Rekovic Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain J. Alcaraz Maestre, E. Calvo, M. Cerrada, M. Chamizo Llatas, N. Colino, B. De La Cruz, A. Delgado Peris, A. 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Zeyrek Bogazici University, Istanbul, Turkey E. Gülmez, M. Kaya56, O. Kaya57, E. A. Yetkin58, T. Yetkin59 Istanbul Technical University, Istanbul, Turkey A. Cakir, K. Cankocak, S. Sen60, F. I. Vardarlı Institute for Scintillation Materials of National Academy of Science of Ukraine, Kharkov, Ukraine B. Grynyov National Scientific Center, Kharkov Institute of Physics and Technology, Kharkov, Ukraine L. Levchuk, P. Sorokin University of Bristol, Bristol, UK R. Aggleton, F. Ball, L. Beck, J. J. Brooke, E. Clement, D. Cussans, H. Flacher, J. Goldstein, M. Grimes, G. P. Heath, H. F. Heath, J. Jacob, L. Kreczko, C. Lucas, Z. Meng, D. M. Newbold61, S. Paramesvaran, A. Poll, T. Sakuma, S. Seif El Nasr-storey, S. Senkin, D. Smith, V. J. Smith Imperial College, London, UK M. Baber, R. Bainbridge, O. Buchmuller, A. Bundock, D. Burton, S. Casasso, M. Citron, D. Colling, L. Corpe, P. Dauncey, G. Davies, A. De Wit, M. Della Negra, P. Dunne, A. Elwood, D. Futyan, G. Hall, G. Iles, R. Lane, R. 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Terentyev, L. Thomas, J. Wang, S. Wang, J. Yelton Florida International University, Miami, USA S. Hewamanage, S. Linn, P. Markowitz, G. Martinez, J. L. Rodriguez Florida Institute of Technology, Melbourne, USA M. M. Baarmand, V. Bhopatkar, S. Colafranceschi66, M. Hohlmann, H. Kalakhety, D. Noonan, T. Roy, F. Yumiceva Lawrence Livermore National Laboratory, Livermore, USA D. Lange, F. Rebassoo, D. Wright University of Maryland, College Park, USA C. Anelli, A. Baden, O. Baron, A. Belloni, B. Calvert, S. C. Eno, C. Ferraioli, J. A. Gomez, N. J. Hadley, S. Jabeen, R. G. Kellogg, T. Kolberg, J. Kunkle, Y. Lu, A. C. Mignerey, Y. H. Shin, A. Skuja, M. B. Tonjes, S. C. Tonwar University of Minnesota, Minneapolis, USA A. C. Benvenuti, B. Dahmes, A. Evans, A. Finkel, A. Gude, P. Hansen, S. Kalafut, S. C. Kao, K. Klapoetke, Y. Kubota, Z. Lesko, J. Mans, S. Nourbakhsh, N. Ruckstuhl, R. Rusack, N. Tambe, J. Turkewitz University of Mississippi, Oxford, USA J. G. Acosta, S. Oliveros University of Nebraska-Lincoln, Lincoln, USA E. Avdeeva, R. Bartek, K. Bloom, S. Bose, D. R. Claes, A. Dominguez, C. Fangmeier, R. Gonzalez Suarez, R. Kamalieddin, D. Knowlton, I. Kravchenko, F. Meier, J. Monroy, F. Ratnikov, J. E. Siado, G. R. Snow Northeastern University, Boston, USA G. Alverson, E. Barberis, D. Baumgartel, M. Chasco, A. Hortiangtham, A. Massironi, D. M. Morse, D. Nash, T. Orimoto, R. Teixeira De Lima, D. Trocino, R.-J. Wang, D. Wood, J. Zhang University of Notre Dame, Notre Dame, USA N. Dev, M. Hildreth, C. Jessop, D. J. Karmgard, N. Kellams, K. Lannon, N. Marinelli, F. Meng, C. Mueller, Y. Musienko37, M. Planer, A. Reinsvold, R. Ruchti, G. Smith, S. Taroni, N. Valls, M. Wayne, M. Wolf, A. Woodard University of Puerto Rico, Mayaguez, USA S. Malik Purdue University Calumet, Hammond, USA N. Parashar, J. Stupak University of Tennessee, Knoxville, USA M. Foerster, G. Riley, K. Rose, S. Spanier, K. Thapa Texas Tech University, Lubbock, USA N. Akchurin, C. Cowden, J. Damgov, C. Dragoiu, P. R. Dudero, J. Faulkner, S. Kunori, K. Lamichhane, S. W. Lee, T. Libeiro, S. Undleeb, I. Volobouev Wayne State University, Detroit, USA C. Clarke, R. Harr, P. E. Karchin, C. Kottachchi Kankanamge Don, P. Lamichhane, J. Sturdy University of Wisconsin-Madison, Madison, WI, USA D. A. Belknap, D. Carlsmith, M. Cepeda, S. Dasu, L. Dodd, S. Duric, B. Gomber, M. Grothe, M. Herndon, A. Hervé, P. Klabbers, A. Lanaro, A. Levine, K. Long, R. Loveless, A. Mohapatra, I. Ojalvo, T. Perry, G. A. Pierro, G. Polese, T. Ruggles, T. Sarangi, A. Savin, A. Sharma, N. Smith, W. H. Smith, D. Taylor, P. Verwilligen, N. Woods † Deceased 1: Also at Vienna University of Technology, Vienna, Austria 2: Also at CERN, European Organization for Nuclear Research, Geneva, Switzerland 3: Also at State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing, China 4: Also at Institut Pluridisciplinaire Hubert Curien, Université de Strasbourg, Université de Haute Alsace Mulhouse, CNRS/IN2P3, Strasbourg, France 5: Also at Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia 6: Also at Universidade Estadual de Campinas, Campinas, Brazil 7: Also at Centre National de la Recherche Scientifique (CNRS)-IN2P3, Paris, France 8: Also at Laboratoire Leprince-Ringuet, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France 9: Also at Joint Institute for Nuclear Research, Dubna, Russia 10: Also at British University in Egypt, Cairo, Egypt 11: Now at Suez University, Suez, Egypt 12: Also at Cairo University, Cairo, Egypt 13: Also at Fayoum University, El-Fayoum, Egypt 14: Also at Université de Haute Alsace, Mulhouse, France 15: Also at Tbilisi State University, Tbilisi, Georgia 16: Also at RWTH Aachen University, III. Physikalisches Institut A, Aachen, Germany 17: Also at University of Hamburg, Hamburg, Germany 18: Also at Brandenburg University of Technology, Cottbus, Germany 19: Also at Institute of Nuclear Research ATOMKI, Debrecen, Hungary 20: Also at Eötvös Loránd University, Budapest, Hungary 21: Also at University of Debrecen, Debrecen, Hungary 22: Also at Wigner Research Centre for Physics, Budapest, Hungary 23: Also at Indian Institute of Science Education and Research, Bhopal, India 24: Also at University of Visva-Bharati, Santiniketan, India 25: Now at King Abdulaziz University, Jeddah, Saudi Arabia 26: Also at University of Ruhuna, Matara, Sri Lanka 27: Also at Isfahan University of Technology, Isfahan, Iran 28: Also at University of Tehran, Department of Engineering Science, Tehran, Iran 29: Also at Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran 30: Also at Università degli Studi di Siena, Siena, Italy 31: Also at Purdue University, West Lafayette, USA 32: Now at Hanyang University, Seoul, Korea 33: Also at International Islamic University of Malaysia, Kuala Lumpur, Malaysia 34: Also at Malaysian Nuclear Agency, MOSTI, Kajang, Malaysia 35: Also at Consejo Nacional de Ciencia y Tecnología, Mexico city, Mexico 36: Also at Warsaw University of Technology, Institute of Electronic Systems, Warsaw, Poland 37: Also at Institute for Nuclear Research, Moscow, Russia 38: Now at National Research Nuclear University ‘Moscow Engineering Physics Institute’ (MEPhI), Moscow, Russia 39: Also at Institute of Nuclear Physics of the Uzbekistan Academy of Sciences, Tashkent, Uzbekistan 40: Also at St. Petersburg State Polytechnical University, St. Petersburg, Russia 41: Also at California Institute of Technology, Pasadena, USA 42: Also at Faculty of Physics, University of Belgrade, Belgrade, Serbia 43: Also at INFN Sezione di Roma; Università di Roma, Rome, Italy 44: Also at National Technical University of Athens, Athens, Greece 45: Also at Scuola Normale e Sezione dell’INFN, Pisa, Italy 46: Also at National and Kapodistrian University of Athens, Athens, Greece 47: Also at Institute for Theoretical and Experimental Physics, Moscow, Russia 48: Also at Albert Einstein Center for Fundamental Physics, Bern, Switzerland 49: Also at Adiyaman University, Adiyaman, Turkey 50: Also at Mersin University, Mersin, Turkey 51: Also at Cag University, Mersin, Turkey 52: Also at Piri Reis University, Istanbul, Turkey 53: Also at Gaziosmanpasa University, Tokat, Turkey 54: Also at Ozyegin University, Istanbul, Turkey 55: Also at Izmir Institute of Technology, Izmir, Turkey 56: Also at Marmara University, Istanbul, Turkey 57: Also at Kafkas University, Kars, Turkey 58: Also at Istanbul Bilgi University, Istanbul, Turkey 59: Also at Yildiz Technical University, Istanbul, Turkey 60: Also at Hacettepe University, Ankara, Turkey 61: Also at Rutherford Appleton Laboratory, Didcot, UK 62: Also at School of Physics and Astronomy, University of Southampton, Southampton, UK 63: Also at Instituto de Astrofísica de Canarias, La Laguna, Spain 64: Also at Utah Valley University, Orem, USA 65: Also at University of Belgrade, Faculty of Physics and Vinca Institute of Nuclear Sciences, Belgrade, Serbia 66: Also at Facoltà Ingegneria, Università di Roma, Rome, Italy 67: Also at Argonne National Laboratory, Argonne, USA 68: Also at Erzincan University, Erzincan, Turkey 69: Also at Mimar Sinan University, Istanbul, Istanbul, Turkey 70: Also at Texas A&M University at Qatar, Doha, Qatar 71: Also at Kyungpook National University, Daegu, Korea 1. 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V. Khachatryan, A. M. Sirunyan, A. Tumasyan, W. Adam. Search for lepton flavour violating decays of heavy resonances and quantum black holes to an \(\mathrm {e}\mu \) pair in proton–proton collisions at \(\sqrt{s}=8~\text {TeV} \), The European Physical Journal C, 2016, 317, DOI: 10.1140/epjc/s10052-016-4149-y