Search for R-parity violating supersymmetry and quantum black- holes in eμ final state in CMS

A search for narrow resonances decaying to an electron and a muon is performed using an integrated luminosity of 2.7 fb−1 of 13 TeV proton-proton collision data recorded with the CMS detector at the LHC. The eμ mass spectrum is also investigated for non-resonant contributions from the production of quantum black holes (QBH). 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 1.0 TeV for couplings λ132 = λ231 = λ311 = 0.01 and below 3.3 TeV for λ132 = λ231 = λ311 = 0.2. In a framework of TeV-scale quantum gravity, for models that invoke extra dimensions, the observed exclusion limits for the threshold mass of QBH production range from 2.5 TeV for one extra dimension to 4.5 TeV for six extra dimensions.


Introduction
Many extensions of the standard model (SM) predict the existence of heavy particles that decay promptly to the eµ final state, and motivate the search for lepton flavour violating (LFV) signatures in interactions involving charged leptons. This document reports a search for phenomena beyond the SM in the invariant mass spectrum of eµ pairs. The analysis [1] is based on data with an integrated luminosity of 2.7 fb −1 collected in proton-proton collisions at √ s = 13 TeV with the CMS detector [2] at the LHC. The results are interpreted in terms of two theoretical models: a τ sneutrino, which is assumed to be the lightest supersymmetric particle (LSP) in R-parity violating (RPV) supersymmetry (SUSY) and another interpretation comes from quantum black holes (QBH) with models that invoke extra spatial dimensions.
In RPV SUSY, lepton number can be violated at tree level in interactions between fermions and sfermions. We assume that all R-parity violating couplings are zero, except for λ 132 , λ 231 and λ 311 , which are connected to the production and decay of the τ sneutrino. In this model, the τ sneutrino can decay either into the final state under study, an eµ pair, via the couplings λ 132 and λ 231 or into dd Theories that invoke extra spatial dimensions, allow for a fundamental Planck scale at the TeV scale. These theories offer the possibility of the production of microscopic black holes at the LHC. In contrast to semiclassical, thermal black holes, which would decay to high-multiplicity final states, QBHs are non-thermal objects which are expected to decay predominantly into two-particle final states. We consider the production of a spin-0, colourless, charge-neutral QBH in a model with lepton flavour violation, in which the cross section for QBH production is extrapolated from the semiclassical case and depends on the threshold mass for QBH production, M th , and the number of extra dimensions n. The n = 1 case corresponds to the Randall-Sundrum (RS) [3] model and n > 1 to the Arkani-Hamed-Dimopoulos-Dvali (ADD) [4] model.

Analysis Strategy
The search is designed in a inclusive and model-independent way by only requiring one prompt muon and one prompt electron in the event selection. The data sample is selected using a single-muon trigger. In the offline analysis, muon candidates are required to have p T > 53 GeV and must fall into the acceptance region |η| < 2.4. The muon and electron candidates are required to pass the high p T Figure 1. Left: The invariant mass distribution of selected eµ pairs [1]. The black points with error bars represent data and the stacked histograms represent the expectations from SM processes. The combined statistical and systematic uncertainty is labeled total uncertainty. Right: The cumulative distribution of the invariant mass of selected eµ pairs [1], where all events above the mass value on the X-axis are summed up.
identification criteria. 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 E T . Since highly energetic muons can produce bremsstrahlung and an associated supercluster in the calorimeter in the direction of the muons inner track, they can be misidentified as electrons. Therefore, an electron candidate is rejected if there is a muon with p T greater than 5 GeV within ∆R < 0.1 of the

Results
After the event selection, 9608 events are observed in data in the complete mass spectrum, where the expectation from SM background is 10379. The eµ invariant mass distribution is shown in Figure 2. Left: 95% CL upper limit on the signal cross section times branching fraction for the RPV τ sneutrino signal as a function of the mass of the resonance [1]. Right: 95% CL upper limit on the signal cross section times branching fraction for the QBH signal as a function of the threshold mass [1]. Other important sources of systematic uncertainties include integrated luminosity, background cross section, muon and electron momentum scale and resolution, choice of PDF etc. Taking all systematic uncertainties into account, the resulting uncertainty on the background yield ranges from 15% at M eµ = 200 GeV to 31% at M eµ = 2 TeV. No significant excess with respect to the SM expectation is found in the measured eµ invariant mass distribution, and limits are set on the signal cross section times branching fraction. The RPV SUSY and QBH signal samples have been generated with the CALCHEP [5] and the QBH 2.0 [6] event generators respectively. All simulated samples use PYTHIA 8 [7] for hadronization. The generated events are processed through a full simulation of the CMS detector based on GEANT 4 [8]. The full selection efficiency of RPV τ sneutrino signal is 65.8% at Mτ ν = 1 TeV and that of QBH signal is 66.6% at M th = 1 TeV. The SM backgrounds contributing to the eµ final state can be divided into two categories: • Real lepton: Backgrounds with at least two prompt, isolated leptons. The expected SM background from processes with two prompt leptons is obtained from MC simulations. It consists mostly of events from tt and WW production. Other background processes estimated from MC simulation are WZ, ZZ, single top, and Drell-Yan (DY) production. jet is misidentified as a lepton; and Wγ production, where a photon is misidentified as a lepton. The estimate of the Wγ background is obtained from MC simulation. A data-driven background estimation based on control samples, using the jet-to-electron fake rate method, is used to determine the contribution from W+jets and QCD multijet production.
An upper limit of 95% confidence level on the cross section times branching ratio is determined using a binned likelihood Bayesian approach with a uniform prior for the signal cross section. The nuisance parameters associated with the systematic uncertainties are marginalised, and a Markov Chain Monte Carlo method is used for integration.
In RPV SUSY, the τ sneutrino signal gives rise to a narrow resonance. For coupling values considered in this search, the intrinsic width of this signal is small compared to the detector resolution and a Gaussian is used to model the signal shape. For each probed resonance signal mass, the two parameters, acceptance times efficiency and invariant mass resolution, determine the normalization and shape of the signal model respectively. The parameterization of the narrow resonance allows for a scan of the invariant mass spectrum with a fine spacing of the signal mass hypothesis. The QBH signal exhibits a broader shape with a sharp edge at the threshold mass M th and a tail towards higher masses. The QBH signal shapes are obtained directly from simulated samples. The 95% CL limits on the signal cross section times branching ratio for the RPV τ sneutrino resonance signal are shown in Fig. 2-left. The corresponding mass limits are presented in table 1. The 95% CL limits on the signal cross section times branching ratio for the QBH non-resonant signal are shown in Fig. 2-right. The corresponding limits on threshold mass are presented in table 2. The limit contour is also derived in the (Mτ ν , λ 311 ) parameter plane as a function of fixed values of λ 132 = λ 231 . The result is given in Fig. 3.

Conclusion
A search for a heavy resonance decaying into an eµ pair has been carried out using 2.7 fb −1 of protonproton collision data recorded with the CMS detector at the LHC at √ s = 13 TeV. Agreement between the data and the SM expectation is observed. We set limits on the resonant production of τ sneutrinos in RPV SUSY with subsequent decay into eµ pair. For couplings λ 132 = λ 231 = λ 311 = 0.01, a τ sneutrino LSP is excluded for masses below 1.0 TeV, for couplings λ 132 = λ 231 = λ 311 = 0.1 masses below 2.7 TeV are excluded and for couplings λ 132 = λ 231 = λ 311 = 0.2, masses below 3.3 TeV are excluded. The corresponding expected limits are 1.0 TeV, 2.7 TeV and 3.3 TeV. Lower bounds are set on the mass thresholds for the production of quantum black holes with subsequent decay into eµ pair in models with one to six extra dimensions, assuming the threshold mass to be at the Planck scale. The limits range from M th = 2.5 TeV (n = 1, RS model) to 4.5 TeV (n = 6 ADD model).