Search for lepton flavour violating decays of the Higgs boson to μτ and eτ in proton-proton collisions at $$ \sqrt{s}=13 $$ TeV

Abstract

A search for lepton flavour violating decays of the Higgs boson in the µτ and eτ decay modes is presented. The search is based on a data set corresponding to an integrated luminosity of 35.9 fb −1 of proton-proton collisions collected with the CMS detector in 2016, at a centre-of-mass energy of 13 TeV. No significant excess over the standard model expectation is observed. The observed (expected) upper limits on the lepton flavour violating branching fractions of the Higgs boson are B(H → µτ ) < 0.25% (0.25%) and B(H → eτ ) < 0.61% (0.37%), at 95% confidence level. These results are used to derive upper limits on the off-diagonal µτ and eτ Yukawa couplings √ |Y µτ | 2 + |Y τ µ | 2 < 1.43×10 −3 and |Y eτ | 2 + |Y τ e | 2 < 2.26×10 −3 at 95% confidence level. The limits on the lepton flavour violating branching fractions of the Higgs boson and on the associated Yukawa couplings are the most stringent to date. The CMS collaboration 33 IntroductionThe discovery of the Higgs boson (H) at the CERN LHC [1][2][3] has stimulated further precision measurements of the properties of the new particle. A combined study of the 7 and 8 TeV data sets collected by the CMS and ATLAS collaborations shows consistency between the measured couplings of the Higgs boson and the standard model (SM) predictions [4]. However, the constraint on the branching fraction to non-SM decay modes derived from these measurements, B(non-SM) < 34% at 95% confidence level (CL), still allows for a significant contribution from exotic decays [4]. In this paper a search for lepton flavour violating (LFV) decays of the Higgs boson in the µτ and eτ channels is presented. These decays are forbidden in the SM but occur in many new physics scenarios. These include supersymmetric [5][6][7][8][9][10][11][12][13], composite Higgs [14,15], or Randall-Sundrum models [16][17][18], SM extensions with more than one Higgs boson doublet [19,20] or with flavour symmetries [21], and many other scenarios [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. The presence of LFV Higgs boson couplings would allow τ → µ and τ → e to proceed via a virtual Higgs boson [37,38]. Consequently the experimental limits on rare τ lepton -1 - JHEP06(2018)001decays, such as τ → eγ and τ → µγ [39], provide upper limits on B(H → µτ ) and B(H → eτ ) [40,41] of O(10%). Measurements of the electron and muon magnetic moments, and exclusion limits on the electric dipole moment of the electron also provide complementary constraints [42]. The LFV Higgs boson decay to µe is strongly constrained by the µ → eγ limit, B(H → eµ) < O(10 −9 ) [43].The CMS experiment published the first direct search for H → µτ [44], followed by searches for H → eτ and H → eµ decays [45], using proton-proton (pp) collision data corresponding to an integrated luminosity of 19.7 fb −1 at a centre-of-mass energy of 8 TeV.A small excess of data with respect to the SM background-only hypothesis at m H = 125 GeV was observed in the H → µτ channel, with a significance of 2.4 standard deviations (σ), and the bes...

Flavor Techniques for LFV Processes: Higgs Decays in a General Seesaw Model

“…Present Upper Bounds (95% C.L.) BR(H → µe) -3.5 × 10 −4 CMS (2016) [3] BR(H → τ e) 4.7 × 10 −3 ATLAS (2019) [4] 6.1 × 10 −3 CMS (2018) [5] BR(H → τ µ) 2.8 × 10 −3 ATLAS (2019) [4] 2.5 × 10 −3 CMS (2018) [5] we mentioned before. The reason is that, in this basis, particles have well-defined propagators, meaning that they will keep their identity unless they interact with other two -or more-particles.…”

Collider constraints on lepton flavor violation in the 2HDM

“…The dash-dotted, dashed and solid lines represent the boundaries of the excluded regions for mH = 130, 200 and 300 GeV respectively. 0.000 0.005 0.010 0.015 0.020 0.025 0.030 [6]. The allowed regions from the CMS heavy Higgs LFV search [10] are denoted by the area between the two red lines for each corresponding mH .…”

“…In the previous section, we have discussed the case where the production of the heavy scalar is enhanced by the presence of Y tt U . In that optimistic scenario, we see that the CMS heavy Higgs LFV search can put bounds [6]. The allowed regions from the CMS heavy Higgs LFV search [10] are denoted by the area between the two red lines for each corresponding mH .…”

“…The excluded parameter space is shown in Fig. 9 [6]. The red region represents the boundary of excluded regions from the CMS heavy Higgs LFV search [10].…”

“…Moreover, this particular channel has attracted great interests [2][3][4][5] and remains important for constraining many models beyond the SM. The most recent bound of this τ − µ channel, found by using 35.9 fb −1 data at 13 TeV center-of-mass energy, is set by CMS, i.e., BR(h → µτ ) < 0.25% at 95% CL [6]. A slightly weaker bound is obtained by ATLAS, BR(h → µτ ) < 0.28% [7].…”