Test of lepton flavour violation at LHC

We study lepton flavour violating decays of neutralinos and sleptons within the Minimal Supersymmetric Standard Model, taking into account the recent bounds on flavour violating lepton decays. Taking the SPS1a’ scenario as an example, we show that some of these lepton flavour violating branching ratios of neutralinos and sleptons can be ∼5-10%. We study the impact of the lepton flavour violating neutralino and slepton decays on the di-lepton mass distributions measured at LHC. We find that they can result in novel and characteristic edge structures in the distributions. The appearance of these remarkable structures provides a powerful test of supersymmetric lepton flavour violation at LHC. 1 [email protected] 2 [email protected] 3 [email protected] 4 [email protected] 5 [email protected] 6 [email protected] Preprint submitted to Elsevier Science 28 February 2008 There are stringent experimental constraints on lepton flavour violation (LFV) in the charged lepton sector, the strongest coming from the decay branching ratio of μ → eγ, BR(μ → eγ) < 1.2×10−11 [1]. Others are BR(μ → eee) < 10 [2], BR(τ → eγ) < 1.1 × 10 [3], BR(τ → μγ) < 6.8 × 10 [4], BR(τ → μμμ) < 1.9×10−7 [5] and the limit on μN → eN , Rμe < 7.8×10−13 [6], with Rμe = Γ[μ N(Z,A) → eN(Z,A)]/Γ[μN(Z,A) → νμN(Z − 1, A)]. On the other hand, the various neutrino experiments have clearly established that individual lepton flavour is violated (for a recent review see e.g. [7]). In supersymmetric (SUSY) extensions of the Standard Model LFV can originate in the slepton sector due to soft SUSY breaking parameters, e.g. mass matrices with flavour offdiagonal entries. Several studies along this line have been performed assuming either specific high-scale models or specifying the LFV parameters at the low scale (see for instance [8–11]). For the LHC it has been shown that flavour violating decay chains can be reconstructed despite considerable backgrounds and the stringent experimental bounds on flavour violating lepton decays [12]. In this letter, we study signals of LFV in the slepton sector which should be detectable at LHC. In particular, we study the impact of LFV on the di-lepton mass distributions from neutralino cascade decays. We take into account the constraints on the LFV parameters from experimental limits on the rare decays l → lγ. This practically implies that the constraints from the rare three-body decays are fulfilled as they are dominated by virtual photon exchange [8]. The only possible exception is that from τ → μμμ enhanced by the Higgs boson exchange for large tanβ [13]. This, however, does not apply to our case as we study a scenario with tanβ = 10 in our numerical analysis. In the considered parameter range also the rate for μ− e conversion is well below the corresponding experimental limit [8]. The most general charged slepton mass matrix including left-right mixing as well as flavour mixing in the basis of (ẽL, μ̃L, τ̃L, ẽR, μ̃R, τ̃R) ≡ (l̃1L, l̃2L, l̃3L, l̃1R, l̃2R, l̃3R) is given by: