Search For Lepton Flavor Violation Research Articles

Search for lepton-flavor-violating decays of the Z boson into a τ lepton and a light lepton with the ATLAS detector

Direct searches for lepton flavor violation in decays of the $Z$ boson with the ATLAS detector at the LHC are presented. Decays of the $Z$ boson into an electron or a muon and a hadronically decaying $\tau$ lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb$^{-1}$ at a center-of-mass energy of $\sqrt{s} = 13$ TeV. No significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: $\mathcal{B} (Z\to e\tau) < 5.8 \times 10^{-5}$ and $\mathcal{B} (Z\to \mu\tau) < 2.4 \times 10^{-5}$. This is the first limit on $\mathcal{B} (Z\to e\tau)$ with ATLAS data. The upper limit on $\mathcal{B} (Z\to \mu\tau)$ is combined with a previous ATLAS result based on 20.3 fb$^{-1}$ of proton-proton collision data at a center-of-mass energy of $\sqrt{s} = 8$ TeV and the combined upper limit at 95% confidence level is $\mathcal{B} (Z\to \mu\tau) < 1.3 \times 10^{-5}$.

Probing TeV scale origin of neutrino mass at future lepton colliders via neutral and doubly-charged scalars

We point out how future lepton colliders can provide unique insight into the scalar sector of TeV scale models for neutrino masses with local $B-L$ symmetry. Our specific focus is on the TeV scale left-right model, which naturally embeds this $B-L$ symmetry. In particular, we make a detailed study of the lepton collider implications of the neutral ($H_3$) and doubly-charged ($H^{\pm\pm}$) scalars from the right-handed triplet Higgs that is responsible for the spontaneous breaking of the $B-L$ symmetry and implementing the seesaw mechanism. Due to mixing with other scalars, the neutral scalar $H_3$ could acquire sizable flavor violating couplings to the charged leptons. Produced on-shell or off-shell at the planned $e^+e^-$ colliders, it would induce distinct lepton flavor violating signals like $e^+e^- \to \mu^\pm \tau^\mp ~ (+H_3)$, with the couplings probed up to $\sim 10^{-4}$ for a wide range of neutral scalar mass, which is well beyond the reach of current searches for charged lepton flavor violation. The Yukawa couplings of the doubly-charged scalar $H^{\pm\pm}$ to the charged leptons might also be flavor-violating, which is correlated to the heavy right-handed neutrino masses and mixings. With a combination of the pair, single and off-shell production of $H^{\pm\pm}$ like $e^+e^- \to H^{++} H^{--},\, H^{\pm\pm} e^\mp \mu^\mp,\, \mu^\pm \tau^\mp$, the Yukawa couplings can be probed up to $10^{-3}$ at future lepton colliders, which is allowed by current lepton flavor data in a large region of parameter space. For both the neutral and doubly-charged cases, the scalar masses could be probed up to the few-TeV range in the off-shell channel.

Doubly-charged scalars in the type II seesaw mechanism: Fundamental symmetry tests and high-energy searches

We analyze the sensitivity of low-energy fundamental symmetry tests to interactions mediated by doubly-charged scalars that arise in type II seesaw models of neutrino mass and their left-right symmetric extensions. We focus on the next generation measurement of the parity-violating asymmetry in M\o{}ller scattering planned by the MOLLER collaboration at Jefferson Laboratory. We compare the MOLLER sensitivity to that of searches for charged lepton flavor violation (CLFV) and neutrinoless double beta-decay ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$-decay) as well as present and possible future high-energy collider probes. We show that for the simplest type-II seesaw scenario, CLFV searches have the greatest sensitivity. However, in a left-right symmetric extension where the scale of parity-breaking is decoupled from the $SU(2{)}_{R}$-breaking scale, the MOLLER experiment will provide a unique probe of scalar triplet interactions in the right-handed sector for a doubly-charged scalar mass up to $\ensuremath{\sim}10\text{ }\text{ }\mathrm{TeV}$ and help elucidate the mechanism of $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$-decay.

Searches for heavy neutral lepton production and lepton flavour violation in kaon decays at the NA62 experiment

Searches for heavy neutral lepton production and lepton flavour violation in kaon decays at the NA62 experiment

Flavorful leptoquarks at hadron colliders

$B$-physics data and flavor symmetries suggest that leptoquarks can have masses as low as few ${\cal{O}}(\mbox{TeV})$, predominantly decay to third generation quarks, and highlight $pp \to b \mu \mu$ signatures from single production and $pp \to b b \mu \mu$ from pair production. Abandoning flavor symmetries could allow for inverted quark hierarchies, and cause sizable $pp \to j \mu \mu$ and $jj \mu \mu $ cross sections, induced by second generation couplings. Final states with leptons other than muons including lepton flavor violation (LFV) ones can also arise. The corresponding couplings can also be probed by precision studies of the $B \to (X_s, K^*,\phi) ee $ distribution and LFV searches in $B$-decays. We demonstrate sensitivity in single leptoquark production for the LHC and extrapolate to the high luminosity HL-LHC. Exploration of the bulk of the phase space requires a hadron collider beyond the reach of the LHC, with $b$-identification capabilities.

Search for charged lepton flavor violation of vector mesons in the BLMSSM model

We analyze the charged lepton flavor violating (CLFV) decays of vector mesons $V\ensuremath{\rightarrow}{l}_{i}^{\ifmmode\pm\else\textpm\fi{}}{l}_{j}^{\ensuremath{\mp}}$ with $V\ensuremath{\in}{\ensuremath{\phi},J/\mathrm{\ensuremath{\Psi}},\mathrm{\ensuremath{\Upsilon}},{\ensuremath{\rho}}^{0},\ensuremath{\omega}}$ in the BLMSSM model. This new model is introduced as a supersymmetric extension of the Standard Model (SM), where local gauged baryon number $B$ and lepton number $L$ are considered. The numerical results indicate the BLMSSM model can produce significant contributions to such two-body CLFV decays, and the branching ratios to these CLFV processes can easily reach the present experimental upper bounds. Therefore, searching for CLFV processes of vector mesons may be an effective channel to study new physics.

Search for Charged Lepton Flavour Violation at CMS

Lepton flavour is a conserved quantity in the standard model of particle physics, but it does not follow from an underlying gauge symmetry. After the discovery of neutrino oscillation, it has been established that lepton flavour is not conserved in the neutral sector. Thus the lepton sector is an excellent place to look for New Physics, and in this perspective the Charged Lepton Flavour Violation is interesting. Various extensions of the standard model predict lepton flavour violating decays that can be observed at LHC. This report summarises several searches for lepton flavour violation with data collected by the CMS detector.

Charged lepton flavour violation searches at the Paul Scherrer Institut: Status of the MEGII and Mu3e experiments

The MEG experiment has recently set a new upper limit on the branching ratio of the μ+ → e+γ decay, B(μ+ → e+γ) &lt; 4.2 × 10-13 (at 90% confidence level) and un upgrade of the experiment (the MEGII experiment) is ongoing with the aim of improving the single event sensitivity (SES) by one order of magnitude with respect to the previous MEG experiment’s SES. The strong scientific motivation associated with the charged Lepton Flavour Violation (cLFV) searches pushes also towards searching for the complementary muon cLFV μ+ → e+e+e- decay with the Mu3e experiment aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (phase I) up to an ultimate SES of few ×10-16. Both experiments will be hosted at the Paul Scherrer Institut which delivers the most intense continuous low energy muon beam in the world up to few ×108 μ/s. The status of both the MEGII and Mu3e experiments is given.

Development of Muon Accelerators for Neutrino Experiments

High-brilliance muon beams offer a unique potential for precision neutrino studies by providing intense neutrino beams with well-defined flavor content and energy spectrum. They also offer a path to improved precision searches for charged lepton flavor violation, and provide a basis for a next generation lepton-antilepton collider. The R&D for these muon facilities involves several technologies of which cooling the muon beam is a critical component. This talk will review progress on the development of the key technologies and their demonstration experiments.

Search for lepton flavor violation at future lepton colliders

Lepton flavor violating (LFV) processes, [Formula: see text] and [Formula: see text] ([Formula: see text] or [Formula: see text]), via four-Fermi contact interactions at future International Linear Collider (ILC) are studied. Taking account of previous experimental results of LFV processes [Formula: see text] and [Formula: see text], we find that the upper limits on the LFV parameters for [Formula: see text] could be improved at the ILC experiment using the polarized electron beam. The improvement of the upper limits could be nearly an order of magnitude as compared to previous ones.

Search for Lepton Flavor Violation in Z and Higgs decays with the CMS Experiment

Searches for charged lepton flavor violation in decays of the Z and Higgs boson using data corresponding to an integrated luminosity of 19.7 (2.3) fb−1 collected by the CMS detector in proton-proton collisions at s=8(13) TeV are described.

Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: Hrightarrow etau , Hrightarrow mu tau , and Zrightarrow mu tau . The searches are based on the data sample of proton–proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 mathrm{fb}^{-1} at a centre-of-mass energy of sqrt{s}=8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(Hrightarrow etau )<1.04%, Br(Hrightarrow mu tau )<1.43%, and Br(Zrightarrow mu tau )<1.69times 10^{-5}.

The tracker and calorimeter systems of the Mu2e experiment

The Mu2e experiment at Fermilab is a sensitive search for lepton flavor violation via muon-to-electron conversion in the field of an aluminum nucleus. The requirements for the tracking and calorimeter systems of the experiment, as well as the design of systems that fulfill these requirements, are briefly described.

Conversions of bound muons: Lepton flavor violation from doubly charged scalars

We present the first detailed computation of the conversion of a bound muon into an electron mediated by a doubly charged $SU(2)$ singlet scalar. Although such particles are not too exotic, up to now their contribution to $\mu$-$e$ conversion is unknown. We close this gap by presenting a detailed calculation, which will allow the reader not only to fully comprehend the discussion but also to generalise our results to similar cases if needed. We furthermore compare the predictions, for both the general case and for an example model featuring a neutrino mass at 2-loop level, to current experimental data and future sensitivities. We show that, depending on the explicit values of the couplings as well as on the actual future limits on the branching ratio, $\mu$-$e$ conversion may potentially yield a lower limit on the doubly charged singlet scalar mass which is stronger than what could be obtained by colliders. Our results considerably strengthen the case for low-energy lepton flavour violation searches being a very valuable addition to collider experiments.

Searches for New Physics with Lepton Flavours and Multi-lepton Final States in ATLAS

Lepton flavours and multi-lepton signatures are powerful probes to search for new physics. Also, the lepton sector of the Standard Model has to be extended to describe the observed neutrino oscillations. ATLAS experiment searches for lepton flavour violation, new gauge boson allowing to unify quark and lepton fields, heavy charged and neutral leptons as well as for new phenomena in multiphoton final states are summarized in this document. The most recent results will be discussed and compared with existing results.

Lepton Universality and Lepton Flavour Violation tests at theB-factories

We review the experimental status of the lepton universality tests and lepton flavour violation searches after the completion of the data-taking and most of the data analysis of the B -factories BABAR and Belle. The universality of the Standard Model charged weak couplings has been confirmed and moderately improved in precision by the B -factories results. Lepton Flavour violation in the τ lepton decays has been searched in several decay modes and no evidence has been found. We set 90% confidence level upper limits on the Lepton Flavour violating branching ratios in the 10−7 –10−8 range, greatly extending the previous limits set mainly by CLEO in the 10−6 range.

Search for charged lepton flavour violation with the MEG and MEG II experiments

In the next decade Charged Lepton Flavour Violation (CLFV) experiments will play a key role in the search for new physics. Given that flavour oscillations occur among neutrinos, unmeasurably small amplitudes are expected for CLFV processes, while in many new physics scenarios such processes are strongly enhanced. A new generation of experiments looking for CLFV muon decays will be performed at the Paul Scherrer Institut (MEG II, Mu3e), at Fermilab (Mu2e) and at J-Parc (DeeMee, COMET), with sensitivities improved by one or two order of magnitudes with respect to current limits. The MEG experiment has performed the most recent improvement in the search for the CLFV decay μ + → e + γ . With the analysis of half of the collected statistics, a new upper limit on the branching ratio has been set BR ( μ → eγ ) −13 at 90% CL, and the final result is soon to be released. A substantial improvement of MEG results requires an improvement of detector performances, in order to reject the background contributions which limit the signal sensitivity. This will be carried out by a short-term upgrade of the apparatus, MEG II. The major modifications of the experimental apparatus consist in the replacement of the current positron tracking system with a new high transparency and high granularity spectrometer and the substitution and rearrangement of some of the photosensors in the liquid-xenon photon detector. MEG II will start taking data in 2016, with a sensitivity on the μ + → e + γ decay of about 5 × 10 −14 . Therefore as MEG performed the last improvement in the search for CLFV, MEG II will start a new era probing unexplored regions of new physics scenarios.

Neutrino masses from neutral top partners

We present theories of `Natural Neutrinos' in which neutral fermionic top partner fields are simultaneously the right-handed neutrinos (RHN), linking seemingly disparate aspects of the Standard Model structure: a) The RHN top partners are responsible for the observed small neutrino masses, b) They help ameliorate the tuning in the weak scale and address the little hierarchy problem, and c) The factor of $3$ arising from $N_c$ in the top-loop Higgs mass corrections is countered by a factor $3$ from the number of vector-like generations of RHN. The RHN top partners may arise in pseudo-Nambu-Goldstone-Boson (pNGB) Higgs models such as the Twin Higgs, as well as more general Composite, Little, and Orbifold Higgs scenarios, and three simple example models are presented. This framework firmly predicts a TeV-scale seesaw, as the RHN masses are bounded to be below the TeV scale by naturalness. The generation of light neutrino masses relies on a collective breaking of lepton number, allowing for comparatively large neutrino Yukawa couplings and a rich associated phenomenology. The structure of the neutrino mass mechanism realizes in certain limits the Inverse or Linear classes of seesaw. Natural Neutrino models are testable at a variety of current and future experiments, particularly in tests of lepton universality, searches for lepton flavor violation, and precision electroweak and Higgs coupling measurements possible at high energy $e^+ e^-$ and hadron colliders.

Looking for hints of a reconstructible seesaw model at the Large Hadron Collider

We study the production of heavy neutrinos at the Large Hadron Collider (LHC) through the dominant s-channel production mode as well as the vector boson fusion (VBF) process. We consider the TeV scale minimal linear seesaw model containing two heavy singlets with opposite lepton number. This model is fully reconstructible from oscillation data apart from an overall normalization constant which can be constrained from meta-stability of the electroweak vacuum and bounds coming from lepton flavor violation (LFV) searches. Dirac nature of heavy neutrinos in this model implies suppression of the conventional same-sign-dilepton signal at the LHC. We analyze the collider signatures with tri-lepton final state and missing transverse energy as well as VBF type signals which are characterized by two additional forward tagged jets. Our investigation reveals that due to stringent constraints on light-heavy mixing coming from LFV and meta-stability bounds, the model can be explored only for light to moderate mass range of heavy neutrinos. We also note that in case of a positive signal, flavor counting of the final tri-lepton channel can give information about the mass hierarchy of the light neutrinos.

The ATLAS Level-1 Muon Topological Trigger Information for Run 2 of the LHC

For the next run of the LHC, the ATLAS Level-1 trigger system will include topological information on trigger objects from the calorimeters and muon detectors. In order to supply coarse grained muon topological information, the existing MUCTPI (Muon-to-Central-Trigger-Processor Interface) system has been upgraded. The MIOCT (Muon Octant) module firmware has been then modified to extract, encode and send topological information through the existing MUCTPI electrical trigger outputs. The topological information from the muon detectors will be sent to the Level-1 Topological Trigger Processor (L1Topo) through the MUCTPI-to-Level-1-Topological-Processor (MuCTPiToTopo) interface. Examples of physics searches involving muons are: search for Lepton Flavour Violation, Bs-physics, Beyond the Standard Model (BSM) physics and others. This paper describes the modifications to the MUCTPI and its integration with the full trigger chain.