Charged lepton flavor violation in light of the muon magnetic moment anomaly and colliders

Any observation of charged lepton flavor violation (CLFV) implies the existence of new physics beyond the SM in charged lepton sector. CLFV interactions may also contribute to the muon magnetic moment and explain the discrepancy between the SM prediction and the recent muon $$g-2$$ g - 2 precision measurement at Fermilab. We consider the most general SM gauge invariant Lagrangian of $$\Delta L=0$$ Δ L = 0 bileptons with CLFV couplings and investigate the interplay of low-energy precision experiments and colliders in light of the muon magnetic moment anomaly. We go beyond previous work by demonstrating the sensitivity of the LHC, the MACE experiment, a proposed muonium-antimuonium conversion experiment, and a muon collider. Currently-available LHC data is already able to probe unexplored parameter space via the CLFV process $$pp\rightarrow \gamma ^*/Z^*\rightarrow \ell _1^\pm \ell _1^\pm \ell _2^\mp \ell _2^\mp $$ p p → γ ∗ / Z ∗ → ℓ 1 ± ℓ 1 ± ℓ 2 ∓ ℓ 2 ∓ .

The singly-charged scalar singlet as the origin of neutrino masses

We consider the generation of neutrino masses via a singly-charged scalar singlet. Under general assumptions we identify two distinct structures for the neutrino mass matrix. This yields a constraint for the antisymmetric Yukawa coupling of the singly-charged scalar singlet to two left-handed lepton doublets, irrespective of how the breaking of lepton-number conservation is achieved. The constraint disfavours large hierarchies among the Yukawa couplings. We study the implications for the phenomenology of lepton-flavour universality, measurements of the W-boson mass, flavour violation in the charged-lepton sector and decays of the singly-charged scalar singlet. We also discuss the parameter space that can address the Cabibbo Angle Anomaly.

Implications of SU(2)L gauge invariance for constraints on Lorentz violation

Lorentz invariance may only be broken far above the electroweak scale, since violations are experimentally stringently constrained. Therefore, the Standard-Model Extension parameterizing Lorentz violation (LV) via (higher-dimensional) field theory operators is manifestly SU(2)L gauge-invariant. As a consequence, LV in neutrinos implies LV in charged leptons and vice versa. This allows us to obtain estimated sensitivities for flavour-changing operators in the charged-lepton sector from neutrino oscillations as well as sensitivities for flavour-diagonal neutrino effects from high-precision electron experiments. We also apply this method to an analysis of time-of-flight data for neutrinos (detected by IceCube) and photons from gamma ray bursts where discrepancies have been observed. Our conclusion is that an explanation of the arrival time difference between neutrino and photon events by dim-5 operators in the neutrino sector would lead to unacceptably large LV effects in the charged-lepton sector.

COMET status and plans

Lepton Flavour Violation in the charged lepton sector (CLFV) is forbidden in the Standard Model. Therefore, the observation of CLFV process would be clear evidence of physics beyond the Standard Model. The COMET (COherent Muon to Electron Transitions) experiment will measure one of these processes: µN → eN at the Japan Proton Accelerator Research Complex in Tokai, Japan. The COMET experiment will be carried out using a two-staged approach. Phase-I of the experiment is aiming at a signal sensitivity of 3.1 × 10−15. Phase-II will use much more intense beam and a more complex transport system to achieve a single-event sensitivity of 3 × 10−17. The article gives an overview of construction and status of the COMET experiment.

LEPTON FLAVOUR VIOLATION BEYOND THE PRESENT LIMITS

The status of the theoretical predictions for processes with flavor violation in the charged lepton sector is discussed in the light of the recent determination of the reactor angle θ13, as well as the improved limit set by MEG on BR (μ → eγ).

LEPTON FLAVOR VIOLATION DECAYS OF VECTOR MESONS IN UNPARTICLE PHYSICS

We investigate the lepton flavor violation decays of vector mesons in the scenario of the unparticle physics by considering the constraint from μ-e conversion. In unparticle physics, the predictions of LFV decays of vector mesons depend strongly on the scale dimension [Formula: see text]. The predictions of LFV decays of vector mesons can reach the detective sensitivity in experiment in region of [Formula: see text], while the prediction of μ-e conversion rate can meet the experimental upper limit. For the searching of the lepton flavor violation processes of charged lepton sector in experiment, the process ϒ→eμ may be a promising one to be observed.

NEUTRINO TRIBIMAXIMAL MIXING FROM A4 ALONE

Neutrino tribimaximal mixing is obtained from the breaking of A4 to Z3 in the charged-lepton sector and to Z2 in the neutrino sector. To enforce this conflicting pattern, extra particles and symmetries are usually invoked, often accompanied by nonrenormalizable interactions and even extra dimensions. It is shown here in a specific renormalizable model how A4 alone will accomplish this, with only the help of lepton number.