Search for lepton flavour number violating $Z^0$ -decays

Abstract

A search for lepton avour number violating Z 0 decays in the channels Z 0 ! , Z 0 ! e , Z 0 ! e , using the DELPHI detector with data collected during the 1991{94 LEP runs, is described. No signal was found. Upper limits at 95% con dence level for the respective branching fractions of 1:2 10 5 , 2 : 2 10 5 , and 0:25 10 5 , w ere obtained.

Lepton flavor violating Z decays: A promising window to low scale seesaw neutrinos

“…Present Bound (90%CL) Future Sensitivity BR(µ → eγ) 4.2 × 10 −13 (MEG 2016) [24] 4 × 10 −14 (MEG-II) [25] BR(τ → eγ) 3.3 × 10 −8 (BABAR 2010) [26] 10 −9 (BELLE-II) [27] BR(τ → µγ) 4.4 × 10 −8 (BABAR 2010) [26] 10 −9 (BELLE-II) [27] BR(µ → eee) 1.0 × 10 −12 (SINDRUM 1988) [28] 10 −16 Mu3E (PSI) [29] BR(τ → eee) 2.7 × 10 −8 (BELLE 2010) [30] 10 −9,−10 (BELLE-II) [27] BR(τ → µµµ) 2.1 × 10 −8 (BELLE 2010) [30] 10 −9,−10 (BELLE-II) [27] BR(τ → µη) 2.3 × 10 −8 (BELLE 2010) [31] 10 −9,−10 (BELLE-II) [27] CR(µ − e, Au) 7.0 × 10 −13 (SINDRUM II 2006) [32] CR(µ − e, Ti) 4.3 × 10 −12 (SINDRUM II 2004) [33] 10 −18 PRISM (J-PARC) [34] CR(µ − e, Al) 3.1 × 10 −15 COMET-I (J-PARC) [35] 2.6 × 10 −17 COMET-II (J-PARC) [35] 2.5 × 10 −17 Mu2E (Fermilab) [36] BR(Z → µe) 1.7 × 10 −6 (LEP 1995) [37], 7.5 × 10 −7 (ATLAS 2014) [9] BR(Z → τ e) 9.8 × 10 −6 (LEP 1995) [37] BR(Z → τ µ) 1.2 × 10 −5 (LEP 1995) [38], 1.69 × 10 −5 (ATLAS 2014) [39] BR(H → µe) 3.6 × 10 −3 (CMS 2015) [40] BR(H → τ e) 1.04 × 10 −2 (ATLAS 2016) [39], 0.7 × 10 −2 (CMS 2015) [40] BR(H → τ µ) 1.43 × 10 −2 (ATLAS 2016) [39], 1.51 × 10 −2 (CMS 2015) [41] In this work, we consider the Inverse Seesaw (ISS) [14,[42][43][44] as a specific realization of the low scale seesaw models. In particular, the ISS extends the SM spectrum with three pairs of RH neutrinos with opposite lepton numbers and considers Majorana masses for some of these new fields, which are assumed to be naturally small since they are the only masses that violate LN.…”

Sensitivity of future lepton colliders to the search for charged lepton flavor violation

“…H → τ [10,11], and derived upper bounds on the off-diagonal Yukawa couplings of the order of 10 −3 . The ATLAS experiment currently provides the most stringent limit for Z → eµ [12] with BR < 7.5 × 10 −7 , while experiments at the Large Electron Positron (LEP) collider set the most stringent limits for Z → eτ (Z → µτ ) with BR < 9.8 × 10 −6 [13] (BR < 1.2 × 10 −5 [14]). Lepton colliders with their clean environment and well-understood backgrounds can outperform hadron colliders with less integrated luminosity and thus provide an ideal facility to probe rare CLFV events.…”

Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

“…It is also important to note that a relatively large Br(H → µτ) can be achieved without any particular tuning of the effective couplings, while a large Br(H → eτ) is possible only at the cost of some fine-tuning of the corresponding couplings [25]. Upper bounds on the LFV Z → eµ, Z → µτ and Z → eτ decays were set by the LEP experiments [26,27]: Br(Z → eµ) < 1.7×10 −6 , Br(Z → eτ) < 9.8×10 −6 , and Br(Z → µτ) < 1.2×10 −5 at the 95% CL. The ATLAS experiment set the most stringent upper bound on the LFV Z → eµ decays [28]: Br(Z → eµ) < 7.5 × 10 −7 at 95% CL.…”