## Scope of the Working Group

The Working Group Neutrino Masses and Lepton Flavor Violation at the LHC is part of the Helmholtz Alliance

Physics at the Terascale. The ongoing research focuses on effects of physics beyond the Standard Model in the leptonic sector, in particular on the phenomenology of Lepton Number Violation (LNV) and Lepton Flavor Violation (LFV) at the LHC. The working group consists of several theoretical groups in collaboration with experimental groups from

ATLAS and

CMS experiments.

## Contributing universities and groups

## Meetings

- Neutrino-LHC working group meeting at 3rd Annual Workshop of the Helmholtz Alliance "Physics at the Terascale", 13. November 2009, DESY, Hamburg
- Kick-off Workshop 24./25. November 2009, Würzburg

## Models and phenomenology

#### Seesaw models

#### Flavor models

#### R-Parity violation

## Collider signatures for analysis

#### Invariant mass distribution of lepton pairs

The neutralino decay at the end of a

SUSY decay chain will provide precision mass measurements of the sleptons and neutralinos involved using endpoint determination of the lepton pair invariant mass, see e.g.

arXiv:hep-ph/9902201 and

arXiv:0711.4008. In the case of Lepton Flavor Conserving (LFC) neutralino decays, an exess of opposite sign same flavor (OSSF) over opposite sign different flavor (OSDF) lepton pairs is expected as compared to the Standard Model background. In the presence of LFV decays of the neutralino, more intermediate slepton states can contribute to this signal and opposite sign different flavor (OSDF) lepton pairs can arise. If the statistics at LHC allow high-precision measurements of dilepton invariant mass distributions, the ratios between possible lepton combinations are different from the LFC case depending on the LFV couplings and mass spectra. In that case one could also see more edges in the lepton pair invariant mass distributions as compared to the flavor conserving ones, see

arXiv:hep-ph/0510074.

#### Like-sign dilepton final states

If Majorana particles are involved in scattering processes, a signature of like-sign dileptons (LSD) can arise. In analogy to the process of neutrinoless double beta decay, LSDs can be produced e.g. via neutralino exchange if RPV is assumed. Therefore a signature of LSDs at LHC can provide further information on RPV couplings, as discussed in

arXiv:hep-ph/0007228 or

arXiv:0902.4697.

#### Displaced vertices

The reconstruction of a relatively long-lived particle will lead to a displaced vertex in the detector. In many models of RPV, the lightest

SUSY particle (LSP) will have a macroscopic decay length so that

SUSY decay chains involving the LSP would produce displaced vertices. Measurements of the displacement in conjunction with track reconstruction can thus provide information on the RPV couplings of the LSP, as outlined e.g. in

arXiv:0803.4405.

#### Rare decays of taus

Many models of LFV predict branching ratios for tau decays within the sensitivity of ongoing experiments, see e.g.

arXiv:hep-ex/0702017. Among the various LFV tau decay modes, the decay into three muons has been studied both at ATLAS and

CMS (

CERN-CMS-CR-2009-013) based on simulated samples. In order to probe this rare decay mode, very high statistics are necessary to separate signal events from SM signatures coming from charm or bottom production.

#### Exotic Higgs decays

A measurement of Higgs decays into leptons of different flavors would be a clear sign for LFV. This decay modes are predicted in general

SUSY Higgs models or in models with specific flavor symmetries. For the analysis of LFV Higgs decays in a two-Higgs doublet model see

arXiv:hep-ph/0207302.

## Tools and Tutorials

-- AndreasRedelbach - 26-Nov-2009