Search for an L − L gauge boson using Z → 4μ events in proton-proton collisions at s=13 TeV

Abstract

Search for an L µ − L τ gauge boson using Z → 4µ events in proton-proton collisions at √ s = 13 TeVThe CMS Collaboration * Abstract A search for a narrow Z gauge boson with a mass between 5 and 70 GeV resulting from an L µ − L τ U(1) local gauge symmetry is reported. Theories that predict such a particle have been proposed as an explanation of various experimental discrepancies, including the lack of a dark matter signal in direct-detection experiments, tension in the measurement of the anomalous magnetic moment of the muon, and reports of possible lepton flavor universality violation in B meson decays. A data sample of proton-proton collisions at a center-of-mass energy of 13 TeV is used, corresponding to an integrated luminosity of 77.3 fb −1 recorded in 2016 and 2017 by the CMS detector at the LHC. Events containing four muons with an invariant mass near the standard model Z boson mass are analyzed, and the selection is further optimized to be sensitive to the events that may contain Z → Z µµ → 4µ decays. The event yields are consistent with the standard model predictions. Upper limits of 10 −8 -10 −7 at 95% confidence level are set on the product of branching fractions B(Z → Z µµ)B(Z → µµ), depending on the Z mass, which excludes a Z boson coupling strength to muons above 0.004-0.3. These are the first dedicated limits on L µ − L τ models at the LHC and result in a significant increase in the excluded model parameter space. The results of this search may also be used to constrain the coupling strength of any light Z gauge boson to muons.

Nonstandard neutrino interactions at COHERENT, DUNE, T2HK and LHC

“…The dashed purple lines show the expected 2σ exclusion limit from DUNE and T2HK combined. Regions above the brown curves are excluded by CMS [26] and BaBar [27] at 2σ and 90% CL, respectively, using pp/e + e − → µ + µ − Z searches. The brown dashed curves are the 2σ expected sensitivities from HL-LHC, with an integrated luminosity of 3000 fb −1 , in the µ + µ − Z channel, and the blue solid (dashed) curves correspond to the expected 2σ (5σ) limit using di-muon searches for Cases A and B, and di-tau searches for Case C. In the upper panels, the blue shaded regions are excluded at 90% CL by the LHCb dark photon searches [28] and at 2σ by the ATLAS di-muon searches [29] with 139 fb −1 .…”

“…The BaBar searches [27] set a 90% CL upper limit on the new gauge coupling based on a L µ − L τ model corresponding to Q q = Q e = 0, Q µ = −Q τ = 1 in our parameterization. The CMS searches [26] set a 2σ upper limit on g by assuming the branching ratio B(Z → µ + µ − ) = 1/3 and Q µ = 1. By rescaling the observed bounds according to the branching fractions and production cross section, we extract bounds for our scenarios.…”

“…We generate the leading process qq → 4µ at the leading order (LO). Following the CMS analysis [26], we require at least four well-identified and isolated muons to have p T > 5 GeV and to be in the central region of the detector |η| < 2.4, with at least two muons to have p T > 10 GeV and at least one to have p T > 20 GeV. Dimuon candidates formed from an opposite sign muon pair are required to have 4 < M µ + µ − < 120 GeV.…”

“…The four selected muons are required to have zero net charge and 80 < M 4µ < 100 GeV. The NNLO/LO K-factor is chosen to be 1.29 [26]. By following the CMS procedure in Ref.…”

“…By following the CMS procedure in Ref. [26], we are able to reconstruct M Z , whose distributions are shown in the left panel of Fig. 5.…”