Charged Lepton Flavor Violation Research Articles

Low‐energy precision tests of the standard model: a snapshot

This brief review describes a class of uniquely crafted particle physics experiments that typically each tackle just one investigation—and they do that very well. The aim of these experiments is to both establish Standard Model parameters and also to provide unique tests in search of new physics. I provide a brief snapshot of many of the current activities, selected with a bias toward low‐energy and high precision. These include searches for permanent electric dipole moments, charged lepton flavor violation, tests of the weak interaction, and other broad searches for deviations from very precise Standard Model predictions, such as the muon's anomalous magnetic moment. I highlight what drives these efforts and how they might impact a new Standard Model. image

Diamond detector for beam profile monitoring in COMET experiment atJ-PARC

We present the design and initial prototype results of a proton beam profile monitor for the COMET experiment at J-PARC. The goal of COMET is to look for charged lepton flavor violation by direct μ to e conversion at a sensitivity of 10−19. The 8 GeV proton beam pulsed at 100 ns with 1010 protons/s will be used to create muons through pion production and decay. In the final experiment, the proton flux will be raised to 1014 protons/s to increase the sensitivity. These requirements of harsh radiation tolerance and fast readout make diamond a good choice for constructing a beam profile monitor in COMET. We present first results of the characterization of single crystal diamond (scCVD) sourced from a new company 2A SYSTEMS Singapore. Our measurements indicate excellent charge collection and high carrier mobility down to cryogenic temperatures.

Charged lepton flavor violation in the semi-constrained NMSSM with right-handed neutrinos

We study the μ→eγ decay in the Z3-invariant next-to-minimal supersymmetric (SUSY) Standard Model (NMSSM) with superheavy right-handed neutrinos. We assume that the soft SUSY breaking parameters are generated at the GUT scale, not universally as in the minimal supergravity scenario but in such a way that those soft parameters which are specific to the NMSSM can differ from the soft parameters which involve only the MSSM fields while keeping the universality at the GUT scale within the soft parameters for the MSSM and right-handed neutrino fields. We call this type of boundary conditions “semi-constrained”. In this model, the lepton-flavor-violating off-diagonal elements of the slepton mass matrix are induced by radiative corrections from the neutrino Yukawa couplings, just like as in the MSSM extended with the right-handed neutrinos, and these off-diagonal elements induce sizable rates of μ→eγ depending on the parameter space. Since this model has more free parameters than the MSSM, the parameter region favored from the Higgs boson mass can slightly differ from that in the MSSM. We show that there is a parameter region in which the μ→eγ decay can be observable in the near future even if the SUSY mass scale is about 4 TeV.

Status and implications of neutrino masses: A brief panorama

With the historic discovery of the Higgs boson our picture of particle physics would have been complete were it not for the neutrino sector and cosmology. I briefly discuss the role of neutrino masses and mixing upon gauge coupling unification, electroweak breaking and the flavor sector. Time is ripe for new discoveries such as leptonic CP violation, charged lepton flavor violation and neutrinoless double beta decay. Neutrinos could also play a role in elucidating the nature of dark matter and cosmic inflation.

Flavour physics: Status and perspectives

The pattern of quark flavour violation predicted by the Standard Model agrees within theoretical and experimental uncertainties with the available data except for a number of intriguing anomalies that require further investigation. In the coming flavour precision era a multitude of new observables will be measured and the accuracy of the measurements of quark flavour violating rare processes and of the relevant lattice QCD calculations will be significantly improved. This will allow us not only to clarify these anomalies but also hopefully to discover new physics (NP). On the other hand a discovery of charged lepton flavour violation and of non‐vanishing electric dipole moments (EDMs) of particles would be a clear signal of NP. Most importantly the unique role of quark and lepton flavour physics and of EDMs in the coming years will be to allow us to get insight into the dynamics well beyond the reach of high energy processes at the LHC. In fact scales as short as 10−21 m (Zeptouniverse) corresponding to energy scale of 200 TeV or even shorter distance scales can be explored in this manner. We discuss the requirements that have to be met for such a flavour expedition to the Zeptouniverse to be successful. In particular we emphasize the power of correlations between flavour observables in the search for NP. In this context the proposed DNA charts allow for a clear distinction between various alternatives for the new dynamics at the LHC scales and at much shorter distance scales. image

Indirect searches for sterile neutrinos at a high-luminosity Z-factory

A future high-luminosity Z-factory will offer the possibility to study rare Z decays, as those leading to lepton flavour violating final states. Processes such as Z → ℓ 1 ∓ ℓ 2 ± are potentially complementary to low-energy (high-intensity) observables of lepton flavour violation. In this work we address the impact of new sterile fermions on lepton flavour violating Z decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints on the sterile states. We consider a minimal extension of the Standard Model by one sterile fermion state, and two well-motivated frameworks of neutrino mass generation, the Inverse Seesaw embedded into the Standard Model, and the νMSM. Our study shows that sterile neutrinos can give rise to contributions to BR(Z → ℓ 1 ∓ ℓ 2 ± ) within reach of the FCC-ee. We also discuss the complementarity between a high-luminosity Z-factory and low-energy charged lepton flavour violation facilities.

Way to crosscheckμ−econversion in the case of no signals ofμ→eγandμ→3e

We consider the case that the $\ensuremath{\mu}\ensuremath{-}e$ conversion signal is discovered but other charged lepton flavor violating (cLFV) processes will never be found. In such a case, we need other approaches to confirm the $\ensuremath{\mu}\ensuremath{-}e$ conversion and its underlying physics without conventional cLFV searches. We study R-parity violating (RPV) SUSY models as a benchmark. We briefly review that our interesting case is realized in RPV SUSY models with reasonable settings in light of current theoretical and experimental status. We focus on the exotic collider signatures at the LHC ($pp\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ and $pp\ensuremath{\rightarrow}jj$) as the other approaches. We show the correlations between the branching ratio of the $\ensuremath{\mu}\ensuremath{-}e$ conversion process and cross sections of these processes. It is the first time that these correlations have been graphically shown. We exhibit the RPV parameter dependence of the branching ratio and the cross sections and discuss the feasibility of determining the parameters.

The MuPix high voltage monolithic active pixel sensor for the Mu3e experiment

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ → eee. In order to reduce background by up to 16 orders of magnitude, decay vertex position, decay time and particle momenta have to be measured precisely. A pixel tracker based on 50 μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field will deliver precise vertex and momentum information. Test beam results like an excellent efficiency of >99.5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chip developed for the Mu3e pixel tracker are presented.

Charged Lepton Flavor Violation: Latest Results and Future Plans of the MEG Experiment

Charged lepton flavor violation processes are ideal probes for new physics due to the suppression of Standard Model backgrounds. Currently, the MEG collaboration is searching for μ+ → e+γ decay with unprecedented precision and has recently published a new analysis based on data collected in the years 2009–2011. This resulted in an upper limit on the branching ratio of 5.7 · 10−13 at 90% confidence level, which represents a four times more stringent limit than the previous world best limit set by MEG. The details behind this result, as well as the current status and future plans of theMEG experiment are reported here.

Proton decay and new contribution to 0ν2β decay in SO(10) with low-mass Z′ boson, observable n − n ¯ $$ n-\overline{n} $$ oscillation, lepton flavor violation, and rare kaon decay

In the conventional approach to observable $$ n-\overline{n} $$ oscillation through Pati-Salam intermediate gauge symmetry in SO(10), the canonical seesaw mechanism is also constrained by the symmetry breaking scale M R ∼ M C ≤ 106 GeV which yields light neutrino masses several orders larger than the neutrino oscillation data. A method to evade this difficulty is through TeV scale gauged inverse seesaw mechanism which has been recently exploited while predicting experimentally verifiable W ± , Z R bosons with a new dominant contribution to neutrinoless double beta decay in the W L −W L channel and other observable phenomena, but with proton lifetime far beyond the accessible limits. In the present work, adopting the view that W ± may be heavy and currently inaccessible to accelerator tests, we show how a class of non-supersymmetric SO(10) models allows a TeV scale Z′ boson, experimentally testable proton decay along with observable $$ n-\overline{n} $$ oscillation, and lepto-quark gauge boson mediated rare kaon decays without resorting to additional fine-tuning of parameters. The occurrence of Pati-Salam gauge symmetry with unbroken D-parity and two gauge couplings at the highest intermediate scale guarantees precision unification with vanishing GUT-threshold or gravitational corrections on sin2 θ W (M Z ) prediction in this model. Predictions for neutrinoless double beta decay in the W L − W L channel is analysed in detail including light and heavy sterile neutrino exchange contributions by means of normal and band plots and also by scattered plots while a new formula for half-life is derived. Comparison with available data from various groups by normal and scattered plots reveals how the existing experimental bounds are satisfied irrespective of the mass hierarchy in the light neutrino sector leading to the lower bound on the lightest sterile neutrino mass, $$ {\widehat{M}}_{S_1}\ge 18\pm 2.9 $$ GeV. The model also predicts branching ratios for charged lepton flavor violation verifiable by ongoing search experiments. We also derive new renormalisation group equations constraining the lepto-quark gauge boson mass in the presence of SU(2) L × U(1) R × U(1) B−L × SU(3) C symmetry, specific to the occurrence of extra Z′ boson, leading to a new lower bound on the lepto-quark gauge boson mass mediating rare kaon decay, M LQ ≥ (1.54±0.06) × 106 GeV. We also discuss the symmetry breaking of non-SUSY SO(10) through the well known flipped SU(5) × Ũ(1) path and show, for the first time, how TeV scale Z′ is predicted with gauged inverse seesaw ansatz for neutrino masses and substantial lepton flavor and lepton number violations. As a significant new result along this path, we report a successful unification of the two gauge couplings of SU(5) × Ũ(1) into the single GUT coupling of SO(10).

Muon charged Lepton Flavor Violation search in Europe: the μ+ → e+γ and the μ+ → e+e−e+ decays

Lepton flavor violation (LFV) research is currently one of the most exciting branches of particle physics. Flavor violating processes, such as μ+ → e+γ and μ+ → e+e−e+, which are strongly suppressed in the Standard Model (SM), are very sensitive to new physics. The MEG experiment and the Mu3e experiment, which search for the μ+ → e+γ and the μ+ → e+e−e+ decay respectively, are two precision physics experiments at the forefront of field. They are housed at the Paul Scherrer Institut (PSI), in Switzerland, which provides the most intense continuous muon beam in the world. A summary of the status of the two experiments is given.

Effective theory of a doubly charged singlet scalar: complementarity of neutrino physics and the LHC

We consider a rather minimal extension of the Standard Model involving just one extra particle, namely a single $SU(2)_L$ singlet scalar $S^{++}$ and its antiparticle $S^{--}$. We propose a model independent effective operator, which yields an effective coupling of $S^{\pm \pm}$ to pairs of same sign weak gauge bosons, $W^{\pm} W^{\pm}$. We also allow tree-level couplings of $S^{\pm \pm}$ to pairs of same sign right-handed charged leptons $l^{\pm}_Rl'^{\pm}_R$ of the same or different flavour. We calculate explicitly the resulting two-loop diagrams in the effective theory responsible for neutrino mass and mixing. We propose sets of benchmark points for various $S^{\pm \pm}$ masses and couplings which can yield successful neutrino masses and mixing, consistent with limits on charged lepton flavour violation (LFV) and neutrinoless double beta decay. We discuss the prospects for $S^{\pm \pm}$ discovery at the LHC, for these benchmark points, including single and pair production and decay into same sign leptons plus jets and missing energy. The model represents a minimal example of the complementarity between neutrino physics (including LFV) and the LHC, involving just one new particle, the $S^{\pm \pm}$.

Lepton flavor violation in low-scale seesaw models: SUSY and non-SUSY contributions

Taking the supersymmetric inverse seesaw mechanism as the explanation for neutrino oscillation data, we investigate charged lepton flavor violation in radiative and 3-body lepton decays as well as in neutrinoless $\mu-e$ conversion in muonic atoms. In contrast to former studies, we take into account all possible contributions: supersymmetric as well as non-supersymmetric. We take CMSSM-like boundary conditions for the soft supersymmetry breaking parameters. We find several regions where cancellations between various contributions exist, reducing the lepton flavor violating rates by an order of magnitude compared to the case where only the dominant contribution is taken into account. This is in particular important for the correct interpretation of existing data as well as for estimating the reach of near future experiments where the sensitivity will be improved by one to two orders of magnitude. Moreover, we demonstrate that ratios like BR($\tau\to 3 \mu$)/BR($\tau\to \mu e^+ e^-$) can be used to determine whether the supersymmetric contributions dominate over the $W^\pm$ and $H^\pm$ contributions or vice versa.

The ultra-lightweight support structure and gaseous helium cooling for the Mu3e silicon pixel tracker

The Mu3e experiment searches for charged lepton flavor violation in the rare decay μ→eee. In order to reach a sensitivity of better than 10−16, more than 109 muon decays per second have to be observed over a running time of one year. Precise determination of particle momentum, vertex position and time are necessary for background suppression. These requirements can be met by combining an ultra-lightweight tracker based on High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) with a timing system which consists of a scintillating fiber detector and a tile hodoscope. As the momentum of particles from muon decay at rest is below 53 MeV/c, the silicon pixel tracker resolution is dominated by multiple Coulomb scattering. This leads to extreme requirements for the material budget of the tracking detector of below 0.1% of a radiation length per layer. Even though the target power consumption of the HV-MAPS detector is as low as 150 mW/cm2, the detector cooling must be very efficient and at the same time avoid adding material inside the active tracking volume.

Lepton-Flavor Violation and Physics beyond the Standard Model

Searches for the charged lepton-flavor violating (LFV) processes have a long history, while their importance is increasing now. The studies are important whether new physics is discovered at the LHC or not, since they have potential to probe physics higher than TeV scale. Here, we discuss the charged LFV processes from viewpoints of non-minimal Higgs boson, physics beyond standard model, and origin of the neutrino masses.

Mu2e Magnetic Measurement Studies

The Mu2e experiment at Fermilab is designed to explore charged lepton flavor violation by searching for muon-to-electron conversion. The magnetic field generated by a system of solenoids is a crucial component of Mu2e and requires accurate characterization to detect any potential flaws and to produce a detailed field map. In order to design and build a precise field mapping system consisting of Hall and NMR probes, tolerances and precision for such a system need to be evaluated. To generate a final magnetic field map of the Mu2e solenoids, a continuous field has to be extracted from a discrete set of measurement points. A design for the Mu2e field mapping hardware, and results from simulations to specify parameters for Hall and NMR probes are presented. A fitting procedure for the analytical treatment of our expected magnetic measurements is introduced.

Tolerance Studies of the Mu2e Solenoid System

The muon-to-electron conversion experiment at Fermilab is designed to explore charged lepton flavor violation. It is composed of three large superconducting solenoids, namely, the production solenoid, the transport solenoid, and the detector solenoid. Each subsystem has a set of field requirements. Tolerance sensitivity studies of the magnet system were performed with the objective of demonstrating that the present magnet design meets all the field requirements. Systematic and random errors were considered on the position and alignment of the coils. The study helps to identify the critical sources of errors and which are translated to coil manufacturing and mechanical support tolerances.

Lepton flavor violation from right-handed neutrino thresholds

Charged lepton flavor violation is reappraised in the context of a supersymmetric seesaw mechanism. It is pointed out that a nontrivial flavor structure of right-handed neutrinos, whose effect has been thus far less studied, can give rise to significant slepton flavor transitions. Under the premise that the neutrino Yukawa couplings are of $\mathcal{O}(1)$, the right-handed neutrino mixing contribution could form a basis of the $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ amplitude, which by itself might lead to an experimentally accessible rate, given a typical low-energy sparticle spectrum. Emphasis is placed on the crucial role of the recently measured lepton mixing angle ${\ensuremath{\theta}}_{13}$ as well as the leptonic $CP$-violating phases.

A new idea to search for charged lepton flavor violation using a muonic atom

We propose a new process of μ−e− → e−e− in a muonic atom for a quest of charged lepton flavor violation. The Coulomb attraction from the nucleus in a heavy muonic atom leads to significant enhancement in its rate, compared to μ+e− → e+e−. The upper limit of the branching ratio is estimated to be of the orders of O(10−17 – 10−18) for the photonic and the four Fermi interactions from the present experimental constraints. The search for this process could serve complementarily with the other relevant processes to shed lights upon the nature of charged lepton flavor violation. This talk is based on the work [1].

Searching for charged lepton flavor violation: New results from the MEG experiment

Discovery of charged lepton flavor violation would be unambiguous evidence of new physics beyond the Standard Model. The MEG experiment focuses on μ+ → e+γ decay and has recently published a search result based on data collected in 2009 and 2010. No evidence of this decay mode is found in this data sample, resulting in an upper limit on the branching ratio of 2.4 · 10−12 at 90% confidence level, the best limit ever achieved for this process. This result as well as the current status and prospects of the MEG experiment are reported here.