Search for vector-like leptons in multilepton final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper reports a search for vector-like leptons in multilepton final states at the LHC, setting the most stringent limits to date on their existence within a specific mass range, using CMS data from 2016-2017.

Contribution

It presents the first comprehensive search for vector-like leptons coupling to third-generation leptons at 13 TeV, excluding masses from 120 to 790 GeV.

Findings

No significant excess observed over the standard model background.

Excludes vector-like lepton doublets with masses between 120 and 790 GeV at 95% CL.

Sets the most stringent limits on such particles to date.

Abstract

A search for vector-like leptons in multilepton final states is presented. The data sample corresponds to an integrated luminosity of 77.4 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2016 and 2017. Events are categorized by the multiplicity of electrons, muons, and hadronically decaying $\tau$ leptons. The missing transverse momentum and the scalar sum of the lepton transverse momenta are used to distinguish the signal from background. The observed results are consistent with the expectations from the standard model hypothesis. The existence of a vector-like lepton doublet, coupling to the third generation standard model leptons in the mass range of 120-790 GeV, is excluded at 95% confidence level. These are the most stringent limits yet on the production of a vector-like lepton doublet, coupling to the third generation standard model leptons.

Full text

0.1 Introduction

The standard model (SM) of particle physics is a quantum field theory that describes the known fundamental particles and their interactions. The predictions of the SM have been experimentally tested with great precision [1]. However, the SM does not explain several observations, such as the existence of dark matter and the baryon asymmetry in the universe. In addition, there exist theoretical issues such as the hierarchy problem, that suggest that an extension of the SM, predicting new particles, is needed to provide a more complete description of nature.

In one class of new particles there are nonchiral color singlet fermions that couple to the SM leptons. The term nonchiral implies that the left- and right-handed components of these particles transform identically under gauge symmetries. These particles are thus referred to as vector-like leptons (VLLs). They arise in a wide variety of models invoking, for example, supersymmetry or extra dimensions [2, 3, 4, 5]. The VLLs are often classified by the SM lepton generation with which they are associated. VLLs and their associated SM leptons have identical lepton numbers.

This paper presents a search for an SU(2) doublet VLL extension [6] of the SM with couplings to the third generation SM leptons. The search is carried out in final states with multiple charged leptons (\Pe, \Pgm, \Pgt), using proton-proton ($\Pp\Pp$) collision data collected by the CMS detector at the LHC in 2016 and 2017. The model that we consider introduces a vector-like \Pgtlepton $(\Pgt^{\prime-})$, its antiparticle $(\Pgt^{\prime+})$, and the corresponding neutrinos ($\PGn^{\prime}_{\Pgt}$ and $\PAGn^{\prime}_{\Pgt}$). At the LHC, they can be produced in $\Pgt^{\prime\pm}\PGn^{\prime}_{\Pgt}$, $\Pgt^{\prime+}\Pgt^{\prime-}$, and $\PGn^{\prime}_{\Pgt}\PAGn^{\prime}_{\Pgt}$ channels, with subsequent decays of $\Pgt^{\prime}$ to \PZ\Pgtor \PH\Pgtand of $\PGn^{\prime}_{\Pgt}$ to \PW\Pgt, where \PW, \PZ, and \PHare the SM \PW, \PZ, and Higgs bosons. At tree-level, the $\Pgt^{\prime}$ and $\PGn^{\prime}_{\Pgt}$ are mass degenerate, whereas higher order radiative corrections predict $<$0.3% relative mass splitting between these two states, for VLL masses greater than 100\GeV. In this paper, $\Pgt^{\prime}$ and $\PGn^{\prime}_{\Pgt}$ are assumed to be mass degenerate. The mass of the VLL is the only free parameter both in the production cross section and in the branching fraction calculations. The tree-level Feynman diagrams for associated and pair production of the doublet model VLLs are shown in Fig. 1 along with possible subsequent decay chains that would result in a multilepton final state.

The ATLAS Collaboration performed a search for heavy lepton resonances decaying into a \PZboson and a lepton in a multilepton final state at a center-of-mass energy of 8\TeV [7], constraining a singlet VLL model and excluding VLLs in the mass range of 114–176\GeV. However, to date, there are no such constraints on the doublet VLL model from any of the LHC experiments. The L3 Collaboration at LEP placed a lower bound of $\approx$100\GeVon additional heavy leptons [8]. Given these existing constraints, this analysis focuses on VLL masses greater than 100\GeV.

0.2 The CMS detector

The central feature of the CMS apparatus is a superconducting solenoid of 6\unitm internal diameter, providing a magnetic field of 3.8\unitT. 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, each composed of a barrel and two endcap sections. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. The inner tracker measures charged particles with $\abs{\eta}<2.5$ and provides an impact parameter resolution of $\approx$15\mumand a transverse momentum (\pt) resolution of about 1.5% for 100\GeVcharged particles. Extensive forward calorimetry complements the barrel and endcap detectors by covering the pseudorapidity range $3.0<\abs{\eta}<5.2$. Collision events of interest are selected using a two-tiered trigger system [9]. The first level, composed of custom hardware processors, selects events at a rate of around 100\unitkHz. The second level, based on an array of microprocessors running a version of the full event reconstruction software optimized for fast processing, reduces the event rate to around 1\unitkHz before data storage. A detailed description of the CMS detector, along with a definition of the coordinate system and relevant kinematic variables, can be found in Ref. [10].

0.3 Event reconstruction and particle identification

Events collected for this search are recorded using a combination of triggers requiring a single electron or a single muon. For events collected in 2016 (2017), the electron trigger requires an electron with $\pt>27$ (35)\GeV, while the muon trigger requires a muon with $\pt>24$ (27)\GeV. Information from all subdetectors is combined using the CMS particle-flow (PF) algorithm [11] to reconstruct and identify individual particles (charged hadrons, neutral hadrons, photons, electrons, and muons). Collectively these are referred to as PF objects.

For each event, PF objects originating from the same interaction vertex are clustered into jets using the infrared- and collinear-safe anti-\ktalgorithm [12, 13], with a radius parameter of 0.4. The momenta of all PF objects in each jet are summed vectorially to determine the jet momentum. The reconstructed vertex with the largest value of summed physics-object $\pt^{2}$ is taken to be the primary $\Pp\Pp$ interaction vertex. The physics objects are the jets, clustered using the jet finding algorithm [12, 13] with the tracks assigned to the vertex as inputs, and the associated missing transverse momentum, taken as the negative vector sum of the \ptof those jets. Additional interactions within the same or nearby bunch crossings (pileup) can contribute spurious extra tracks and calorimetric energy depositions to the jet momentum. Hence, charged particles identified as originating from pileup vertices are discarded and an offset correction [14] is applied to account for the remaining neutral pileup particle contributions. Additional jet energy corrections are applied to account for the nonlinear response of the detectors [15].

The missing transverse momentum vector (\ptvecmiss) is calculated as the negative vectorial \ptsum of all the PF objects belonging to the primary vertex. The \ptmissis defined as the magnitude of this vector. For calculating \ptmissin 2016, we use PF objects located in the full fiducial volume of the detector, whereas for 2017, PF objects within $2.5<\abs{\eta}<3.0$ and with $\pt<50\GeV$ are excluded to mitigate noise effects related to the aging of the CMS ECAL.

Electron candidates are reconstructed by combining ECAL superclusters and Gaussian sum filter [16] tracks from the silicon tracker [17]. Muon candidates are reconstructed by combining the information from both the silicon tracker and the muon spectrometer [18]. Hadronically decaying \Pgtlepton candidates (\tauh) are selected using the hadron-plus-strips algorithm [19]. This algorithm has been designed to optimize the performance of \tauhreconstruction by considering specific \tauhdecay modes. It starts with hadronic jets and reconstructs \tauhcandidates from the tracks (“prongs”) and energy deposits in strips of the ECAL, in the 1-prong, 1-prong + $\pi^{0}$, and 3-prong decay modes. We require the reconstructed leptons to lie within the region of pseudorapidity $\abs{\eta}<2.5$, $2.4$, and $2.3$ for the electron, muon, and \tauhcandidates, respectively.

Lepton candidates arising from $\Pp\Pp$ collisions can be broadly categorized into prompt, nonprompt, and conversion leptons. A prompt lepton can be produced in the decay of a \PW, \PZor Higgs boson. Events from background processes such as $\PW\PZ$ and $\PZ\PZ$ contain multiple prompt leptons and thus these backgrounds are classified as prompt backgrounds. A nonprompt lepton can arise in heavy flavor hadron decays within a jet, or from hadrons that punch through to the muon system, or from hadronic showers with large electromagnetic fractions. A small fraction of reconstructed leptons from nonprompt sources mimic leptons from prompt sources and are referred to as misidentified leptons. The background arising from such sources is referred to as the misidentified background (MisID). A conversion lepton is one which is produced when a radiated photon converts to a pair of leptons. The background arising from such processes is referred to as the conversion background.

Unlike prompt leptons, misidentified leptons are expected to have significant nearby hadronic activity. An isolation requirement that compares the \ptof a lepton to the \ptsum of particles in its immediate neighborhood strongly reduces the backgrounds from misidentified leptons. We use relative isolation criteria for both electrons and muons. Relative isolation is defined as the scalar \ptsum of photons, and charged and neutral hadrons, as reconstructed by the PF algorithm within a specified $\Delta R$ cone around the lepton candidate, normalized to the lepton candidate $\pt$. The $\Delta R$ between a particle and the lepton is defined as $\Delta R=\sqrt{\smash[b]{(\Delta\eta)^{2}+(\Delta\phi)^{2}}}$, where $\Delta\eta$ is the difference in pseudorapidity, and $\Delta\phi$ is the difference in the azimuthal angle (in radians). This relative isolation is required to be less than 7 or 8% within a cone of size $\Delta R=0.3$ for electrons whose energy deposits are reconstructed in the ECAL barrel ($\abs{\eta}<1.48$) or in the endcap ($1.48<\abs{\eta}<3.00$), respectively, and less than 15% within a cone of size $\Delta R=0.4$ for muons. The \tauhcandidates are required to pass an isolation requirement based on a multivariate analysis [20]. The isolation quantities are corrected for pileup by considering only those charged PF candidates that are consistent with having originated from the primary vertex, and by subtracting a per-event average pileup contribution to the neutral PF components. We further reduce the MisID backgrounds by imposing requirements on the longitudinal ($d_{z}$) and transverse ($d_{xy}$) impact parameters of the leptons with respect to the primary vertex in the event. Electrons in the barrel (endcap) must satisfy $\abs{d_{z}}<0.1$ $(0.2)\cm$ and $\abs{d_{xy}}<0.05$ $(0.1)\cm$. Muons must satisfy $\abs{d_{z}}<0.1\cm$ and $\abs{d_{xy}}<0.05\cm$. For \tauhleptons, we require $\abs{d_{z}}<0.2\cm$.

0.4 Signal and background simulation

Simulated samples are used to estimate the contribution of all prompt and conversion background processes. The $\PW\PZ$ and $\PZ\PZ$ processes are generated at next-to-leading order (NLO) using \POWHEGv2 [21, 22, 23, 24, 25]. The $\PZ/\gamma^{*}$, $\PZ/\gamma^{*}+\gamma$, \ttbar, $\ttbar+\gamma$, and triboson processes are generated at NLO using \MGvATNLOv5.2.2 [26] and processes with the Higgs boson are generated using \POWHEGv2 [27, 28] and the JHUGen v6.2.8 generator [29, 30, 31, 32]. Signal events are generated using \MGvATNLOat leading order (LO) precision. For all simulation data, the parton showering, fragmentation, and hadronization steps are done using \PYTHIA8.230 [33] with tune CUETP8M1 [34] for 2016 samples, and CP5 [35] for 2017 samples.

All 2016 samples are generated with the same order of the NNPDF3.0 parton distribution function (PDF) [36] as the order of the MC generator. All 2017 samples are generated with the NNPDF3.1 next-to-next-to-leading (NNLO) order PDF [37], irrespective of the order of the MC generator. The response of the CMS detector is simulated using dedicated software based on the \GEANTfourtoolkit [38]. Additional weights are applied to all simulated events to account for differences in the trigger and lepton identification efficiencies between data and simulation. For the simulated events, additional minimum bias interactions are superimposed on the primary collision, reweighted in such a way that the frequency distribution of the extra interactions matches that observed in data.

0.5 Event selection criteria

We collectively refer to electrons and muons as light-leptons to distinguish them from \tauhleptons. Events are then categorized as those with four or more light-leptons (4L), exactly three light-leptons (3L), and exactly two light-leptons along with at least one \tauhlepton (2L1T). In the 2L1T channel, we have a further division based on whether the two light-leptons are of opposite sign (OS) or same sign (SS). In all categories, the leptons are ordered by decreasing transverse momenta and those with the largest \ptare labeled as the leading leptons. The leading light-lepton is required to satisfy $\pt>38$ (28)\GeVif it is an electron (muon). These thresholds are imposed so that the corresponding single lepton triggers are fully efficient for events that would subsequently satisfy the offline selection. All of the other leptons are required to satisfy $\pt>20\GeV$.

We use the scalar \ptsum of the leptons (denoted as $L_{\mathrm{T}}$) to discriminate signal from SM backgrounds in all channels. The $L_{\mathrm{T}}$ distribution is divided into 150\GeVbins, each of which is treated as a separate experiment. In the 2L1T and 4L categories that contain more than one \tauhand more than four light-lepton candidates, respectively, only the leading \tauhand the leading four light-leptons are used in the calculation of $L_{\mathrm{T}}$.

In order to improve sensitivity for the signal, in each of the 4L, 3L, and 2L1T (OS, SS) categories, the events are divided into low- and high-\ptmissregions. While the 4L category is divided into $\ptmiss<50\GeV$ and $>$50\GeVregions, the 3L and 2L1T (OS, SS) categories are divided into $\ptmiss<150\GeV$ and $>$150\GeVregions. These categories form the bases of signal regions (SR) that would be sensitive to the presence of a VLL signal. They are complemented by orthogonal control regions (CR) that are expected to be dominantly populated by backgrounds. Additionally, all events with a light-lepton pair invariant mass below 12\GeVare vetoed regardless of the flavor and sign of the pair, in order to suppress low mass quarkonia resonances. The SRs are described in Table 0.5, where OSSF refers to an opposite-sign, same-flavor lepton pair. A detailed description of the CRs is given in Section 0.6.