Wing Sheung Chan

The Standard Model and lepton flavour violation 23 neutral Higgs bosons ( h 0 and H 0 , one of which is the observed 125-GeV Higgs boson), a CP-odd pseudoscalar ( A ) and two charged Higgs bosons ( H ± ). Two-Higgs-doublet models can be characterised by six independent parameters, whereas the SM Higgs sector only needs two. In the SM, the Yukawa coupling matrix can always be diagonalised, meaning that the Higgs boson cannot induce flavour-changing neutral currents. However, with two Higgs doublets and thus two Yukawa coupling matrices in 2HDM, simultaneous diagonalisation of both matrices is no longer trivial. In fact, in the most generic scenario where the two Higgs doublets can both couple to all the up-type quarks, down-type quarks and charged leptons, simultaneous diagonalisation of the Yukawa couplings is generally impossible [46] . This is the so-called Type-III 2HDM. Flavour-changing neutral currents are possible at tree level in Type-III 2HDM. At the one-loop level, LFV Z interactions can also be induced. A study has shown that with constraints from proton–proton ( pp ) collision data at a centre-of-mass energy √ s = 8 TeV , the branching fraction of the LFV Z → µτ decay is bound from above by 10 − 6 [47] . As examples, a few Feynman diagrams for the mentioned possible BSM LFV processes are shown in Figure 1.2. W W N R Z µ τ (a) Heavy right-handed neutrinos ˜ ` 0 ˜ ` ˜ χ 0 Z µ τ (b) SUSY (slepton flavour mixing) h 0 , H 0 Z τ Z µ τ (c) Type-III 2HDM Figure 1.2.: Examples of Feynman diagrams for possible BSM LFV processes. BSM particles are highlighted in red.