Search Results (28)

Within the type-I seesaw mechanism, it is possible to have large (order one) light–heavy neutrino mixing even in the case of low right-handed neutrino mass scale (of the order of GeV). This implies large lepton flavor violation. As an example, we consider the process $\mu \to e\gamma$ that can have a branching of up to ${10}^{-8}$ within type-I seesaw (in contrast with the tiny value ${10}^{-54}$ expected). Such an enhancement of lepton flavor violation can be used to constraint the parameter space of long-lived particle experiments. Full article

Interest in searches for Charged Lepton Flavor Violation (CLFV) has continued in the past few decades since the observation of CLFV would indicate a new physics Beyond the Standard Model (BSM). As several future lepton colliders with high luminosity have been proposed, the search for CLFV will reach an unprecedented level of precision. Many BSM models allow CLFV processes at the tree level, such as the R-parity-violating (RPV) Minimal Supersymmetric Standard Model (MSSM), which is a good choice for benchmarking. In this paper, we perform a detailed fast Monte Carlo simulation study on RPV-induced CLFV processes at future lepton colliders, including a 240 $\mathrm{GeV}$ circular electron positron collider (CEPC) and a 6 or 14 $\mathrm{TeV}$ Muon Collider. As a result, we found that the upper limits on the $\tau$-related RPV couplings will be significantly improved, while several new limits on RPV couplings can be set, which are inaccessible by low-energy experiments. Full article

The neutrino sector offers one of the most sensitive probes of new physics beyond the Standard Model of Particle Physics (SM). The mechanism of neutrino mass generation is still unknown. The observed suppression of neutrino masses hints at a large scale, conceivably of the order of the scale of a rand unified theory (GUT), which is a unique feature of neutrinos that is not shared by the charged fermions. The origin of neutrino masses and mixing is part of the outstanding puzzle of fermion masses and mixings, which is not explained ab initio in the SM. Flavor model building for both quark and lepton sectors is important in order to gain a better understanding of the origin of the structure of mass hierarchy and flavor mixing, which constitute the dominant fraction of the SM parameters. Recent activities in neutrino flavor model building based on non-Abelian discrete flavor symmetries and modular flavor symmetries have been shown to be a promising direction to explore. The emerging models provide a framework that has a significantly reduced number of undetermined parameters in the flavor sector. In addition, such a framework affords a novel origin of $\mathcal{C}$$\mathcal{P}$ violation from group theory due to the intimate connection between physical $\mathcal{C}$$\mathcal{P}$ transformation and group theoretical properties of non-Abelian discrete groups. Model building based on non-Abelian discrete flavor symmetries and their modular variants enables the particle physics community to interpret the current and anticipated upcoming data from neutrino experiments. Non-Abelian discrete flavor symmetries and their modular variants can result from compactification of a higher-dimensional theory. Pursuit of flavor model building based on such frameworks thus also provides the connection to possible UV completions: in particular, to string theory. We emphasize the importance of constructing models in which the uncertainties of theoretical predictions are smaller than, or at most compatible with, the error bars of measurements in neutrino experiments. While there exist proof-of-principle versions of bottom-up models in which the theoretical uncertainties are under control, it is remarkable that the key ingredients of such constructions were discovered first in top-down model building. We outline how a successful unification of bottom-up and top-down ideas and techniques may guide us towards a new era of precision flavor model building in which future experimental results can give us crucial insights into the UV completion of the SM. Full article

Lepton flavor universality exists in the Standard Model, and hence any observation of the violation of this universality will be a hint for new physics. Recent experimental searches for processes violating this symmetry have attracted much attention among theorists and experimentalists alike. In recent years, such hints have been observed in flavor changing neutral current weak processes such as $b\to sll$ and charged current weak processes such as $b\to cl\nu$ processes by collider experiments like Belle, Belle II, BaBar, LHCb, ATLAS, and CMS collaborations, where $b,s,c$ are the bottom, strange, and charm quarks, respectively, and $l,\nu$ stand for lepton and the corresponding lepton neutrino, respectively. This article is a review of some of the interesting anomalies observed in the B-sector and includes decays of $B_s$ mesons. Full article

The higher beam intensity available for Mu2e-II will require a substantially different target design. This paper discusses our recent advances in conceptual R&D for a Mu2e-II target station. The design is based on energy deposition and radiation damage simulations, as well as thermal and mechanical analyses, to estimate the survivability of the system. We considered rotated targets, fixed granular targets and a novel conveyor target with tungsten or carbon spherical elements that are circulated through the beam path. The motion of the spheres can be generated either mechanically or both mechanically and by a He gas flow. The simulations identified the conveyor target as the preferred approach, and that approach has been developed into a prototype. We describe this first prototype for the Mu2e-II target and report on its mechanical tests performed at Fermilab, which indicate the feasibility of the design, and discuss its challenges as well as suggest directions for further improvement. Full article

Permanent electric dipole moments (EDMs) are excellent probes of physics beyond the Standard Model, especially on new sources of CP violation. The muon EDM has recently attracted significant attention due to discrepancies in the magnetic anomaly of the muon, as well as potential violations of lepton-flavor universality in B-meson decays. At the Paul Scherrer Institute in Switzerland, we have proposed a muon EDM search experiment employing the frozen-spin technique, where a radial electric field is exerted within a storage solenoid to cancel the muon’s anomalous spin precession. Consequently, the EDM signal can be inferred from the upstream-downstream asymmetry of the decay positron count versus time. The experiment is planned to take place in two phases, anticipating an annual statistical sensitivity of $3\times {10}^{-21}$$e·$cm for Phase I and $6\times {10}^{-23}$$e·$cm for Phase II. Going beyond ${10}^{-21}$$e·$cm will enable us to probe various Standard Model extensions. Full article

This experiment to search for the one of the charged lepton flavor-violating processes, muon-electron conversion, DeeMe, is being conducted at the J-PARC MLF H-Line in Japan. This experiment utilizes a pulsed proton beam from the Rapid Cycling Synchrotron (RCS). A graphite target is bombarded with a pulsed proton beam, negative pion production and pion-in-flight-decay to negative muon; then, the creation of muonic atoms is caused in the same pion production target. A converted electron is expected to be emitted after 1 ∼ 2 micro second-delayed timing. And two-body reaction of the new process, ${\mu }^{-}+(\mathrm{A},\mathrm{Z})\to e^{-}+(\mathrm{A},\mathrm{Z})$, results in 105 MeV monoenergetic electron. Thus, 1 ∼ 2 micro second-delayed 105 MeV monoenergetic electron is a searched signal. Electrons around 105 MeV are transported by the H-Line and analyzed using the dipole magnet (0.4 T) and four multi-wire proportional chambers (MWPCs). However, the burst pulse reaching ${10}^8$ charged particles/pulse attributable to the RCS pulse leads to significant dead time for the MWPC. Thus, the HV switching scheme is introduced to handle the prompt burst. The target single event sensitivity is ${10}^{-13}$. The H-Line construction was completed, and commissioning went well. The overview of the experiment and the current status are described in this article. Full article

We report on the MEG II experiment, a search for the charged lepton flavor violating (CLFV) decay ${\mu }^{+}\to e^{+}\gamma$. The experiment is designed to improve upon the most sensitive search for the decay, i.e., the MEG experiment, by an order of magnitude. The MEG II experiment aims to reach a final sensitivity of $6\times {10}^{-14}$ at the 90% confidence level. The experiment completed its first year of data collection in 2021. This proceedings discusses preliminary positron and photon data-driven kinematic resolution measurements and compares them to those of the MEG experiment and the MEG II design expectation. Preliminary estimates of the first year and final experiment sensitivity are presented. Full article

In this talk, I discuss a possible future for the global muon physics program. I focus on the future of flavor studies, precision measurements and searches that can be pursued with a new class of muonium beam sources, and emerging practical applications of muons in the industrial, academic, and government sectors. Full article

Behind succeeding measurements of anomalies in semileptonic decays at LHCb and several collider experiments hinting at the possible violation of lepton flavor universality, we undertake a concise review of theoretical foundations of the tree- and loop-level b-hadron decays, $b\to cl{\nu }_l$ and $b\to s{l}^{+}{l}^{-}$ along with experimental environments. We revisit the world averages for $R_{D\left(D^{*}\right)}$, $R_{K\left(K^{*}\right)}$, $R_{J/\psi }$, and $R_{{\eta }_c}$, for the semileptonic transitions and provide results within the framework of the relativistic independent quark model in addition to the results from model-independent studies. If the ongoing evaluation of the data of LHC Run 2 confirms the measurements of Run 1, then the statistical significance of the effect in each decay channel is likely to reach 5 $\sigma$. A confirmation of these measurements would soon turn out to be the first remarkable observation of physics beyond the Standard Model, providing a wider outlook on the understanding of new physics. Full article

The Standard Model (SM) of particle physics is the most general renormalizable theory which is built on a few general principles and fundamental symmetries with the given particle content. However, multiple symmetries are not built into the model and are simply consequences of renormalizabilty, gauge invariance, and particle content of the theory. It is crucial to test the validity of these types of symmetries and related conservation laws experimentally. The CERN LHC provides the highest sensitivity for testing the SM symmetries at high energy scales involving heavy particles such as the top quark. In this article, we are going to review the recent experimental searches of charge–parity and charged-lepton flavor violation in the top quark sector. Full article

The Mu2e experiment at Fermilab will search for the neutrinoless ${\mu }^{-}\to e^{-}$ conversion in the field of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I and Run II, separated by an approximately two-year-long shutdown. This paper presents an estimate of the expected Mu2e Run I search sensitivity and includes a detailed discussion of the background sources, uncertainties of their prediction, analysis procedures, and the optimization of the experimental sensitivity. The expected Run I $5\sigma$ discovery sensitivity is $R_{\mu e}=1.2\times {10}^{-15}$, with a total expected background of $0.11\pm 0.03$ events. In the absence of a signal, the expected upper limit is $R_{\mu e}<6.2\times {10}^{-16}$ at 90% CL. This represents a three order of magnitude improvement over the current experimental limit of $R_{\mu e}<7\times {10}^{-13}$ at 90% CL set by the SINDRUM II experiment. Full article

Searches for lepton flavor violation in tau decays are unambiguous signatures of new physics. The branching ratios of tau leptons at the level of ${10}^{-10}$–${10}^{-9}$ can be probed using 50 $a{b}^{-1}$ of electron-positron annihilation data being collected by the Belle II experiment at the world’s highest luminosity accelerator, the SuperKEKB, located at the High Energy Accelerator Research Organization, KEK, in Tsukuba, Japan. Searches with such expected sensitivity will either discover new physics or strongly constrain several new physics models. Full article

Neutrino masses are evidence of lepton flavor violation, but no violation in the interactions among the charged leptons has been observed yet. Many models of Physics Beyond the Standard Model (BSM) predict Charged Lepton Flavor Violation (CLFV) in a wide spectrum of processes with rates in reach of upcoming experiments. The experimental searches that provide the current best limits on the CLFV searches are reviewed, with a particular emphasis on the muon-based experiments that give the most stringent constraints on the BSM parameter space. The next generation of muon-based experiments (MEG-II, Mu2e, COMET, Mu3e) aims to reach improvements by many orders of magnitude with respect to the current best limits, thanks to several technological advancements. We review popular heavy BSM theories, and we present the calculations of the predicted CLFV branching ratios, focusing on the more sensitive $\mu \to e$ sector. Full article

Lepton-flavor violation (LFV) has been discovered in the neutrino sector by neutrino oscillation experiments. The minimal extension of the Standard Model (SM) to include neutrino masses allows LFV in the charged sector (CLFV) at the loop level, but at rates that are too small to be experimentally observed. Lepton-number violation (LNV) is explicitly forbidden even in the minimally extended SM, so the observation of an LNV process would be unambiguous evidence of physics beyond the SM. The search for the LNV and CLFV process ${\mu }^{-}+N(A,Z)\to e^{+}+N^{\prime }(A,Z-2)$ (referred to as ${\mu }^{-}\to e^{+}$) complements $0\nu \beta \beta$ decay searches, and is sensitive to potential flavor effects in the neutrino mass-generation mechanism. A theoretical motivation for ${\mu }^{-}\to e^{+}$ is presented along with a review of the status of past ${\mu }^{-}\to e^{+}$ experiments and future prospects. Special attention is paid to an uncertain and potentially dominant background for these searches, namely, radiative muon capture (RMC). The RMC high energy photon spectrum is theoretically understudied and existing measurements insufficiently constrain this portion of the spectrum, leading to potentially significant impacts on current and future ${\mu }^{-}\to e^{+}$ work. Full article