Search for physics beyond the standard model in multilepton final states in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV

Abstract

A search for physics beyond the standard model in events with at least three charged leptons (electrons or muons) is presented. The data sample corresponds to an integrated luminosity of 137 fb−1 of proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV, collected with the CMS detector at the LHC in 2016–2018. The two targeted signal processes are pair production of type-III seesaw heavy fermions and production of a light scalar or pseudoscalar boson in association with a pair of top quarks. The heavy fermions may be manifested as an excess of events with large values of leptonic transverse momenta or missing transverse momentum. The light scalars or pseudoscalars may create a localized excess in the dilepton mass spectra. The results exclude heavy fermions of the type-III seesaw model for masses below 880 GeV at 95% confidence level in the scenario of equal branching fractions to each lepton flavor. This is the most restrictive limit on the flavor-democratic scenario of the type-III seesaw model to date. Assuming a Yukawa coupling of unit strength to top quarks, branching fractions of new scalar (pseudoscalar) bosons to dielectrons or dimuons above 0.004 (0.03) and 0.04 (0.03) are excluded at 95% confidence level for masses in the range 15–75 and 108–340 GeV, respectively. These are the first limits in these channels on an extension of the standard model with scalar or pseudoscalar particles.

Muon and electron g − 2 and lepton masses in flavor models

“…Of course for models involving the third generation, decay chains of this kind involving taus are possible and, in particular, flavons coupling to a FN diagram for the tau would mostly decay into τ + τ − . Searches based on multi-lepton final states have been performed by the LHC collaborations [61][62][63][64], and employed to constrain a variety of new-physics models. In particular the analysis in [63] was interpreted in terms of production of third generation vector-like lepton doublets decaying to SM gauge/Higgs bosons and taus/tau neutrinos.…”

( g−2 ) anomalies and neutrino mass

“…11 of Ref. [52] gives a lower limit on m Ψ in the range of (800-900) GeV for Ψ þþ decaying to e or μ.…”

“…For Σ decaying to τ's the limit is considerably worse; see Ref. [52] and for more details the earlier Ref. [53]).…”

“…Several "exotic" searches at ATLAS and CMS can provide lower mass limits on Ψ. The currently most stringent one is, to our knowledge, the multilepton search by CMS [52]. This work uses a total of 137=fb of statistics to search for three charged leptons with missing energy in the final state and no hadronic activity associated with the events.…”

Stealth decaying spin-1 dark matter

“…This leads to clean signatures with multiple charged leptons in the final state, assuming the Z and W to decay leptonically. While a detailed analysis is beyond the scope of the this work, we note that current bounds from multilepton searches at the LHC does not reach yet the TeV scale mass region [82][83][84]. For instance, in the ZZe + e − channel, taking at least one Z to decay into e + e − or µ + µ − , the expected number of multilepton events is less than one for a data set of about 140 fb −1 using the above estimate of the pp → EĒ cross section at √ s = 13 TeV.…”