Branching fraction for 
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 decays to four leptons and constraints on new physics

Rainbolt, J. Lovelace; Schmitt, Michael Henry

doi:10.1103/physrevd.99.013004

Cited by 7 publications

(7 citation statements)

References 21 publications

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“…(29) and (30), we find that visible effects in μe scattering are excluded for Λ ≳ 40 GeV. Moreover, very stringent bounds on g e X g μ X are set by the LHC experiments for 10 ≲ M U ≲ 50 GeV, via the measurement of the branching fraction for Z decays to four leptons (electrons or muons) [45], and by the BABAR experiment for M U ≲ 10 GeV [46]. As a result, we find that observable effects in μe scattering induced by NP lying below the electroweak scale are very unlikely.…”

Section: A Effective Lagrangianmentioning

confidence: 81%

New physics at the MUonE experiment at CERN

2020

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A confirmation of the long-standing muon g − 2 discrepancy requires both experimental and theoretical progress. On the theory side, the hadronic corrections are under close scrutiny, as they induce the leading uncertainty of the Standard Model prediction. Recently, the MUonE experiment has been proposed at CERN to provide a new determination of the leading hadronic contribution to the muon g − 2 via the measurement of the differential cross section of muon-electron scattering. The precision expected at this experiment raises the question whether possible new physics (NP) could affect its measurements. We address this issue studying possible NP signals in muon-electron collisions due to heavy or light mediators, depending on whether their mass is higher or lower than Oð1 GeVÞ. We analyze the former in a modelindependent way via an effective field theory approach, whereas for the latter we focus on scenarios with light scalar and vector bosons. Using existing experimental bounds, we show that possible NP effects in muon-electron collisions are expected to lie below MUonE's sensitivity. This result confirms and reinforces the physics case of the MUonE proposal.

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Section: A Effective Lagrangianmentioning

confidence: 81%

New physics at the MUonE experiment at CERN

2020

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“…Secondly, since the new Higgs couplings to leptons are enhanced by large tan β, one might worry that the branching ratio Z → 4 might be changed due to new Higgs mediation. The measurement on Z decay branching ratio Z → 4 is (4.58 ± 0.26) × 10 −6 [62], where means e, µ. However, in their study, an invariant mass cut larger than 4 GeV is applied for all opposite-sign, same-flavor lepton pairs.…”

Section: Collider Constraintsmentioning

confidence: 99%

A light scalar explanation of (g − 2)μ and the KOTO anomaly

Liu

McGinnis

Wagner

et al. 2020

J. High Energ. Phys.

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The KOTO experiment has recently performed a search for neutral Kaons decaying into neutral pions and a pair of neutrinos. Three events were observed in the KOTO signal region, with an expected background of about 0.05. Since no clear signal of systematic errors have been found, the excess of events in the decay K L → π 0 νν is quite intriguing. One possibility to explain this anomaly would be the presence of a scalar φ with mass of the order of the pion mass and inducing decays K L → π 0 φ which mimic the observed signal. A scalar with mass of the order of the pion mass and a coupling to muons of the order of the Standard Model Higgs coupling could also explain the muon anomalous magnetic moment anomaly (g − 2) µ . We built on these facts to show that a light singlet scalar with couplings to the leptons and quarks as the ones induced by mixing with Higgs states in two Higgs doublet models may lead to an explanation of both anomalies. More specifically, we show that this is the case in the so-called type-X models in which leptons and quarks couple to two different Higgs doublets, and for scalar masses that are in the range between 40 and 70 MeV. Due to the relatively large coupling to leptons required to fit (g − 2) µ , the scalar lifetime accidentally falls into the sub-nanosecond range which is essential to evade the severe proton beam dump experiments and astrophysical constraints, though it becomes sensitive to constraints from electron beam dump experiments. The additional phenomenological properties of this model are discussed.

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“…These experiments are summarized in Ref. [24], where a combination of existing results is also given. The combined measurement of the Z → 4l branching ratio is:…”

Section: Single Wilson-coefficient Constraints From Inclusive Lhc Mea...mentioning

confidence: 99%

“…For each of the experimental measurements in Table 1 of Ref. [24] we scale the central value by the leading-order ratio Γ(Z → 4µ)/Γ(Z → 4l) computed using Madgraph, and the statistical uncertainty by Γ(Z → 4l)/Γ(Z → 4µ). Combining the results yields…”

Section: Single Wilson-coefficient Constraints From Inclusive Lhc Mea...mentioning

confidence: 99%

“…Four-muon Z boson decays at the LHC probe orthogonal combinations of these Wilson coefficients, allowing for a complete determination of the four-muon operators in the SMEFT. The potential of four-lepton decay modes to constrain physics beyond the SM has been inves-tigated previously, particularly in the context of Z models [23,24]. We study the constraints imposed by current LHC data, as well as potential future constraints at a high-luminosity LHC.…”

Section: Introductionmentioning

confidence: 99%

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Four-lepton Z-boson decay constraints on the SMEFT

Boughezal,

Chen,

Petriello

et al. 2020

Preprint

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We discuss how four-lepton decays of the Z boson probe currently unconstrained flat directions in the parameter space of the Standard Model Effective Field Theory (SMEFT). We derive the constraints from these decays on four-lepton operators in the SMEFT and show how the LHC data for this process complements probes from neutrino-trident production. Future differential measurements with high-luminosity data can strongly constrain four-lepton operators and remove all flat directions in the four-muon sector of the SMEFT. We comment briefly on the possibility of using rare Z-decays to τ -leptons to probe untested directions in the SMEFT parameter space.

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Branching fraction for Z decays to four leptons and constraints on new physics

Cited by 7 publications

References 21 publications

New physics at the MUonE experiment at CERN

New physics at the MUonE experiment at CERN

A light scalar explanation of (g − 2)μ and the KOTO anomaly

Four-lepton Z-boson decay constraints on the SMEFT

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