Charged Lepton Flavor Violation Research Articles

The calorimeter of the Mu2e experiment at Fermilab

The Mu2e experiment at Fermilab looks for Charged Lepton Flavor Violation (CLFV) improving by 4 orders of magnitude the current experimental sensitivity for the muon to electron conversion in a muonic atom. A positive signal could not be explained in the framework of the current Standard Model of particle interactions and therefore would be a clear indication of new physics. In 3 years of data taking, Mu2e is expected to observe less than one background event mimicking the electron coming from muon conversion. Achieving such a level of background suppression requires a deep knowledge of the experimental apparatus: a straw tube tracker, measuring the electron momentum and time, a cosmic ray veto system rejecting most of cosmic ray background and a pure CsI crystal calorimeter, that will measure time of flight, energy and impact position of the converted electron. The calorimeter has to operate in a harsh radiation environment, in a 10−4 Torr vacuum and inside a 1 T magnetic field. The results of the first qualification tests of the calorimeter components are reported together with the energy and time performances expected from the simulation and measured in beam tests of a small scale prototype.

Observable lepton number violation with predominantly Dirac nature of active neutrinos

We study a specific version of SU(2)R × SU(2)L × U(1)B−L models extended by discrete symmetries where the new physics sector responsible for tiny neutrino masses at leading order remains decoupled from the new physics sector that can give rise to observable signatures of lepton number violation such as neutrinoless double beta decay. More specifically, the dominant contribution to light neutrino masses comes from a one-loop Dirac mass. At higher loop level, a tiny Majorana mass also appears which remains suppressed by many order of magnitudes in comparison to the Dirac mass. Such a model where the active neutrinos are predominantly of Dirac type, also predicts observable charged lepton flavour violation like μ → 3e, μ → eγ and multi-component dark matter.

Common origin of neutrino mass, dark matter and Dirac leptogenesis

We study the possibility of generating tiny Dirac neutrino masses at one loop level through the scotogenic mechanism such that one of the particles going inside the loop can be a stable cold dark matter (DM) candidate. Majorana mass terms of singlet fermions as well as tree level Dirac neutrino masses are prevented by incorporating the presence of additional discrete symmetries in a minimal fashion, which also guarantee the stability of the dark matter candidate. Due to the absence of total lepton number violation, the observed baryon asymmetry of the Universe is generated through the mechanism of Dirac leptogenesis where an equal and opposite amount of leptonic asymmetry is generated in the left and right handed sectors which are prevented from equilibration due to tiny Dirac Yukawa couplings. Dark matter relic abundance is generated through its usual freeze-out at a temperature much below the scale of leptogenesis. We constrain the relevant parameter space from neutrino mass, baryon asymmetry, Planck bound on dark matter relic abundance, and latest LUX bound on spin independent DM-nucleon scattering cross section. We also discuss the charged lepton flavour violation (μ → e γ) and electric dipole moment of electron in this model in the light of the latest experimental data and constrain the parameter space of the model.

Flavon-induced connections between lepton flavour mixing and charged lepton flavour violation processes

In leptonic flavour models with discrete flavour symmetries, couplings between flavons and leptons can result in special flavour structures after they gain vacuum expectation values. At the same time, they can also contribute to the other lepton-flavour-violating processes. We study the flavon-induced LFV 3-body charged lepton decays and radiative decays and we take as example the $A_4$ discrete symmetry. In $A_4$ models, a $Z_3$ residual symmetry roughly holds in the charged lepton sector for the realisation of tri-bimaximal mixing at leading order. The only processes allowed by this symmetry are $\tau^-\to \mu^+ e^- e^-, e^+ \mu^- \mu^-$, and the other 3-body and all radiative decays are suppressed by small $Z_3$-breaking effects. These processes also depend on the representation the flavon is in, whether pseudo-real (case i) or complex (case ii). We calculate the decay rates for all processes for each case and derive their strong connection with lepton flavour mixing. In case i, sum rules for the branching ratios of these processes are obtained, with typical examples $\text{Br}(\tau^-\to \mu^+ e^- e^-) \approx \text{Br}(\tau^-\to e^+ \mu^- \mu^-)$ and $\text{Br}(\tau^-\to e^- \gamma) \approx \text{Br}(\tau^-\to \mu^- \gamma)$. In case ii, we observe that the mixing between two $Z_3$-covariant flavons plays an important role. All processes are suppressed by charged lepton masses and current experimental constraints allow the electroweak scale and the flavon masses to be around hundreds of GeV. Our discussion can be generalised in other flavour models with different flavour symmetries.

An Experimental Review of Charged Lepton Flavor Violation in Muon Channel

The flavor violating transition between generations of charged leptons is highly suppressed in the Standard Model of the elementary particle physics, whereas many of the promising new physics beyond the Standard Model predict sizable rates of the flavor violating processes within a reach of the ongoing or proposed experiments. The status and the perspectives of the experimental quests for the charged lepton flavor violation in muon channel are reviewed focusing on the three major processes, \(\mu ^{ - }N \to e^{ - }N\), \(\mu ^{ + } \to e^{ + }\gamma \), and \(\mu ^{ + } \to e^{ + }e^{ - }e^{ + }\).

Neutrino Masses from a Pseudo-Dirac Bino.

We show that, in U(1)_{R}-symmetric supersymmetric models, the bino and its Dirac partner (the singlino) can play the role of right-handed neutrinos and generate the neutrino masses and mixing, without the need for traditional bilinear or trilinear R-parity violating operators. The two particles form a pseudo-Dirac pair, the "biνo." An inverse seesaw texture is generated for the neutrino-biνo sector, and the lightest neutrino is predicted to be massless. Unlike in most models with heavy right-handed neutrinos, the biνo can be sizably produced at the LHC through its interactions with colored particles, while respecting low energy constraints from neutrinoless double-beta decay and charged lepton flavor violation.

Charged lepton flavour violcxmation and neutrinoless double beta decay in left-right symmetric models with type I+II seesaw

We study the new physics contributions to neutrinoless double beta decay ($0\nu\beta \beta$) half-life and lepton flavour violation (LFV) amplitude within the framework of the minimal left-right symmetric model (MLRSM). Considering all possible new physics contributions to $0\nu\beta \beta$ and charged lepton flavour violation $\mu \rightarrow e \gamma, \mu \rightarrow 3e$ in MLRSM, we constrain the parameter space of the model from the requirement of satisfying existing experimental bounds. Assuming the breaking scale of the left-right symmetry to be $\mathcal{O}(1)$ TeV accessible at ongoing and near future collider experiments, we consider the most general type I+II seesaw mechanism for the origin of tiny neutrino masses. Choosing the relative contribution of the type II seesaw term allows us to calculate the right handed neutrino mass matrix as well as Dirac neutrino mass matrix as a function of the model parameters, required for the calculation of $0\nu\beta \beta$ and LFV amplitudes. We show that such a general type I+II seesaw structure results in more allowed parameter space compared to individual type I or type II seesaw cases considered in earlier works. In particular, we show that the doubly charged scalar masses $M_{\Delta}$ are allowed to be smaller than the heaviest right handed neutrino mass $M_N$ from the present experimental bounds in these scenarios which is in contrast to earlier results with individual type I or type II seesaw showing $M_{\Delta} > M_N$.

The MuPix system-on-chip for the Mu3e experiment

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ+→e+e−e+. Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on 50μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1T solenoidal magnetic field provides precise vertex and momentum information. The MuPix chip combines pixel sensor cells with integrated analog electronics and a periphery with a complete digital readout. The MuPix7 is the first HV-MAPS prototype implementing all functionalities of the final sensor including a readout state machine and high speed serialization with 1.25Gbit/s data output, allowing for a streaming readout in parallel to the data taking. The observed efficiency of the MuPix7 chip including the full readout system is ⩾99% in a high rate test beam.

Reconciling(g−2)μand charged lepton flavor violating processes through a doubly charged scalar

In this work, we investigate the phenomenological consequences of a doubly charged scalar which may belong to different uncoloured scalar multiplets. This doubly charged scalar couples to the charged leptons as well as gauge bosons, which we parametrize in a model independent way. Restricting ourselves in the regime of conserved charged-parity (CP), we assume only a few non-zero Yukawa couplings ($y_{\mu \ell}$, where $\ell=e,\mu,\tau$) between the doubly charged scalar and the charged leptons. Our choices allow the doubly charged scalar to impinge low-energy processes like anomalous magnetic moment of muon and a few possible charged lepton flavour violating (CLFV) processes. These same Yukawa couplings are also instrumental in producing same-sign dilepton signatures at the LHC. In this article we examine the impact of individual contributions from the diagonal and off-diagonal Yukawa couplings in the light of muon $(g-2)$ excess. Subsequently, we use the derived information to inquire the possible CLFV processes and finally the collider signals from the decay of a doubly charged scalar. Our simplified analyses, depending on the mass of doubly charged scalar, provide a good estimate for the magnitude of the concerned Yukawa couplings. Our findings would appear resourceful to test the phenomenological significance of a doubly charged scalar by using complementary information from muon $(g-2)$, CLFV and the collider experiments.

Search for lepton flavour violating decays of heavy resonances and quantum black holes to an [Formula: see text] pair in proton-proton collisions at [Formula: see text

A search for narrow resonances decaying to an electron and a muon is presented. The mathrm {e} {mu } mass spectrum is also investigated for non-resonant contributions from the production of quantum black holes (QBHs). The analysis is performed using data corresponding to an integrated luminosity of 19.7~text {fb}^text {-1} collected in proton-proton collisions at a centre-of-mass energy of 8~text {TeV} with the CMS detector at the LHC. With no evidence for physics beyond the standard model in the invariant mass spectrum of selected mathrm {e}mu pairs, upper limits are set at 95 % confidence level on the product of cross section and branching fraction for signals arising in theories with charged lepton flavour violation. In the search for narrow resonances, the resonant production of a mathrm {tau } sneutrino in R-parity violating supersymmetry is considered. The mathrm {tau } sneutrino is excluded for masses below 1.28~text {TeV} for couplings lambda _{132}=lambda _{231}=lambda '_{311}=0.01, and below 2.30~text {TeV} for lambda _{132}=lambda _{231}=0.07 and lambda '_{311}=0.11. These are the most stringent limits to date from direct searches at high-energy colliders. In addition, the resonance searches are interpreted in terms of a model with heavy partners of the {mathrm {Z}} boson and the photon. In a framework of TeV-scale quantum gravity based on a renormalization of Newton’s constant, the search for non-resonant contributions to the mathrm {e} {mu } mass spectrum excludes QBH production below a threshold mass M_{mathrm {th}} of 1.99~text {TeV}. In models that invoke extra dimensions, the bounds range from 2.36~text {TeV} for one extra dimension to 3.63~text {TeV} for six extra dimensions. This is the first search for QBHs decaying into the mathrm {e} {mu } final state.

Higgs-flavon mixing and h → μτ

ATLAS and CMS have reported an excess in the flavor violating decay of the Higgs boson, h → μτ . We show that this result can be accommodated through a mixing of the Higgs with a flavon, the field responsible for generating the Yukawa matrices in the lepton sector. We employ a version of the Froggatt-Nielsen mechanism at the electroweak scale, with only the leptons and the flavon transforming non-trivially under the corresponding symmetry group. Non-observation of charged lepton flavor violation (LFV) in other processes imposes important constraints on the model, which we find to be satisfied in substantial regions of parameter space.

Improved analyses forμ−e−→e−e−in muonic atoms by contact interactions

The charged lepton flavor violating (CLFV) processes of ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{e}^{\ensuremath{-}}$ decay by four Fermi contact interactions in a muonic atom for various atoms are investigated. The wave functions of bound and scattering state leptons are properly treated by solving Dirac equations with Coulomb interaction of the finite nuclear charge distributions. This new effect contributes significantly in particular for heavier atoms, where the obtained decay rate is about one order of magnitude larger than the previous estimation for $^{208}\mathrm{Pb}$. We find that, as the atomic number $Z$ increases, the ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{e}^{\ensuremath{-}}$ decay rates increase more rapidly than the result of the previous work of ${Z}^{3}$, suggesting this decay as one of the promising processes to search for CLFV interaction.

Lepton flavour violation in RS models with a brane- or nearly brane-localized Higgs

We perform a comprehensive study of charged lepton flavour violation in Randall–Sundrum (RS) models in a fully 5D quantum-field-theoretical framework. We consider the RS model with minimal field content and a “custodially protected” extension as well as three implementations of the IR-brane localized Higgs field, including the non-decoupling effect of the KK excitations of a narrow bulk Higgs. Our calculation provides the first complete result for the flavour-violating electromagnetic dipole operator in Randall–Sundrum models. It contains three contributions with different dependence on the magnitude of the anarchic 5D Yukawa matrix, which can all be important in certain parameter regions. We study the typical range for the branching fractions of μ→eγ, μ→3e, μN→eN as well as τ→μγ, τ→3μ and the electron electric dipole moment by a numerical scan in both the minimal and the custodial RS model. The combination of μ→eγ and μN→eN currently provides the most stringent constraint on the parameter space of the model. A typical lower limit on the KK scale T is around 2 TeV in the minimal model (up to 4 TeV in the bulk Higgs case with large Yukawa couplings), and around 4 TeV in the custodially protected model, which corresponds to a mass of about 10 TeV for the first KK excitations, far beyond the lower limit from the non-observation of direct production at the LHC.

Mu2e Transport Solenoid Prototype Tests Results

The Fermilab Mu2e experiment has been developed to search for evidence of charged lepton flavor violation through the direct conversion of muons into electrons. The transport solenoid is an s-shaped magnet that guides the muons from the source to the stopping target. It consists of 52 superconducting coils arranged in 27 coil modules. A full-size prototype coil module, with all the features of a typical module of the full assembly, was successfully manufactured by a collaboration between INFN-Genoa and Fermilab. The prototype contains two coils that can be powered independently. To validate the design, the magnet went through an extensive test campaign. Warm tests included magnetic measurements with a vibrating stretched wire and electrical and dimensional checks. The cold performance was evaluated by a series of power tests and temperature dependence and minimum quench energy studies.

Diminuer l’intensité d’une douleur chronique : « what’else » ?

The charged lepton flavor violation process is a clean and sensitive probe of new physics beyond the Standard Model. A sensitivity study is performed to the process τ−→ γμ− based on a 3 fb−1 inclusive Monte Carlo sample of e+e− collisions at a center-of-mass energy of 4.26 or 4.6 GeV, in the framework of the BESIII software system. The 90% confidence level upper limits on ℬ(τ−→γμ−) are estimated assuming no signal is produced. We also obtain the sensitivity on ℬ(τ−→γμ−) as a function of the integrated luminosity, to serve as a reference for the HIEPA being proposed in China. It is found that 6.34 ab−1 are needed to reach the current best upper limit of 4.4 × 10−8 and about 2510 ab−1 are needed to reach a sensitivity of 10−9 if the detector design is similar to that of BESIII.

Sensitivity study of searching for τ− → γμ− at HIEPA

The charged lepton flavor violation process is a clean and sensitive probe of new physics beyond the Standard Model. A sensitivity study is performed to the process τ−→ γμ− based on a 3fb−1 inclusive Monte Carlo sample of e+e− collisions at a center-of-mass energy of 4.26 or 4.6GeV, in the framework of the BESIII software system. The 90% confidence level upper limits on ℬ(τ−→γμ−) are estimated assuming no signal is produced. We also obtain the sensitivity on ℬ(τ−→γμ−) as a function of the integrated luminosity, to serve as a reference for the HIEPA being proposed in China. It is found that 6.34 ab−1 are needed to reach the current best upper limit of 4.4 × 10−8 and about 2510 ab−1 are needed to reach a sensitivity of 10−9 if the detector design is similar to that of BESIII.

Charged lepton flavor violation on target at GeV scale

We study the lepton flavor violating process, $e+ T \to \tau +T'$, at a few GeV. This process can be studied by experiments directing GeV scale electron or positron beams on internal or fixed targets. We study the effects of some low energy lepton flavor violating interactions on this process. We study the sensitivities of this process on these low energy lepton flavor violating interactions and compare them to the sensitivities of lepton flavor violating $\tau$ decay processes. Comparing with $\tau$ decay processes, this process provides another way to study the lepton flavor violating effects with $e-\tau$ conversion and it can be searched for in facilities with GeV scale electron or positron beams which are available in a number of laboratories in the world.

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.

Characterization of a prototype for the electromagnetic calorimeter of the Mu2e experiment

The Mu2e experiment at Fermilab searches the neutrinoless conversion of the muon into electron in the field of an Aluminum nucleus. The observation of this process would be a proof of the Charged Lepton Flavor Violation (CLFV). In case of no observation, the upper limit will be set to R_(μe) < 6×10^(−17) @ 90% CL, improving by a factor of 4 the previous best determination. The Mu2e detector apparatus consists of a straw tubes tracker that will measure the electrons momentum, and an electromagnetic calorimeter that provides a tracking-independent measurement of the electron energy, time and position. In this paper, we describe the baseline project of the EMC and present results in terms of performances and R&D.

Mu to electron conversion with the COMET experiment

The COMET experiment at J-PARC is under construction and will search for mu to electron conversion in aluminium at the 3.10 level, four orders of magnitude better than current limits. The experiment will take place in two phases with a first phase aiming at a two orders of magnitude improvement. 1 Charged lepton flavour violation with COMET The COMET experiment is currently in preparation at JPARC. The aim is to investigate charged lepton violation by searching for mu to e transitions from muons stopped in aluminium. The collaboration currently has 177 participants from 33 institutes from Belarus, Canada, China, Czech Republic, France, Georgia, Germany, India, Japan, Malaysia, Russia, Saudi Arabia, South Korea, United Kingdom and Vietnam. First data taking is expected in 2018. Lepton flavour violation does not feature in the standard model which only describes flavour violations in the charged current interactions of quarks. However neutrino oscillation shows that neutral lepton flavour violation does occur in nature and this, so far, is the one unambiguous observation in particle physics of an interaction beyond the basic standard model. The actual mechanism of lepton flavour violation in the neutrino sector remains to be discovered and so establishing if there is measurable lepton flavour violation in the charged sector is crucial in the search for a wider theory or at least the necessary extension of the standard model to encompass neutrino oscillation. Flavour is a crucial part of particle physics and whilst it is becoming increasingly well parameterised there is little real appreciation of its role. Consequently it is necessary to acquire as much information as possible if a valid theory is to emerge. An important consequence of flavour changing processes with three families is the mixing matrix with the crucial phase which can yield CP violating processes. Observation of charged lepton flavour processes will almost certainly lead to additional sources of CP violation which could contribute to the unexplained baryon – antibaryon asymmetry. 1.1 Mu to e transitions A search for a muon changing to an electron is the transition most accessible for experimental investigation of charged lepton violation. Of course it could be that the main transition is from the tau but the lower production rate and the very short lifetime make it much harder to achieve very low rates. Three processes for to e transitions are currently being investigated Present limits are currently: Br( → e < 5.7.10, Br( → eee) < 1.10 and to e conversion on gold lower than 6.10. Currently →e and →eee are being investigated at PSI. Two experiments are in construction to investigate to e conversion, COMET at J-PARC and Mu2e at FNAL. As a result of neutrino oscillation charged lepton violation must take place at some level This is shown in Figure 1, however the rate from this process is very small, ~(m /mW) and so effectively unobservable. Figure 1. Mu to e transition as a result of neutrino oscillation. Possible processes for charged lepton violation can be classified as ‘Photonic’, Figure 2, and Non-Photonic, Figure 3. Figure 2. Photonic processes for to e Transitions Figure 3. Non-photonic processes for to e Transitions These can be summarised by a Lagrangian[1] , 01010 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611801010 epj 118 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). 201 FCCP 5