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35 pages, 2385 KiB

Open AccessArticle

Mu2e Run I Sensitivity Projections for the Neutrinoless μ⁻ → e⁻ Conversion Search in Aluminum

by Mu2e Collaboration

Universe 2023, 9(1), 54; https://doi.org/10.3390/universe9010054 - 13 Jan 2023

Cited by 15 | Viewed by 3232

Abstract

The Mu2e experiment at Fermilab will search for the neutrinoless

μ^{-} \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 [...] Read more.

The Mu2e experiment at Fermilab will search for the neutrinoless

μ^{-} \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 σ

discovery sensitivity is

R_{μ 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_{μ e} < 6.2 \times 10^{- 16}

at 90% CL. This represents a three order of magnitude improvement over the current experimental limit of

R_{μ e} < 7 \times 10^{- 13}

at 90% CL set by the SINDRUM II experiment. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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16 pages, 21719 KiB

Open AccessArticle

Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning

by Marco Chiappini, Marco Francesconi, Satoru Kobayashi, Manuel Meucci, Rina Onda, Patrick Schwendimann and on behalf of the MEG II Collaboration

Universe 2021, 7(12), 466; https://doi.org/10.3390/universe7120466 - 30 Nov 2021

Cited by 8 | Viewed by 2314

Abstract

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating

μ^{+} \to e^{+} γ

decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper [...] Read more.

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating

μ^{+} \to e^{+} γ

decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR

(μ^{+} \to e^{+} γ) \leq 4.2 \times 10^{- 13}

at

90 %

confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of

6 \times 10^{- 14}

. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (

7 \times 10^{7}

μ^{+}

/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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18 pages, 22360 KiB

Open AccessArticle

Searching for the Muon Decay to Three Electrons with the Mu3e Experiment

by Cristina Martin Perez and Luigi Vigani

Universe 2021, 7(11), 420; https://doi.org/10.3390/universe7110420 - 3 Nov 2021

Cited by 3 | Viewed by 1850

Abstract

Mu3e is a dedicated experiment designed to find or exclude the charged lepton flavor violating

μ \to

eee decay at branching fractions above

10^{- 16}

. The search is pursued in two operational phases: Phase I uses an existing beamline at the [...] Read more.

Mu3e is a dedicated experiment designed to find or exclude the charged lepton flavor violating

μ \to

eee decay at branching fractions above

10^{- 16}

. The search is pursued in two operational phases: Phase I uses an existing beamline at the Paul Scherrer Institute (PSI), targeting a single event sensitivity of

2 \cdot 10^{- 15}

, while the ultimate sensitivity is reached in Phase II using a high intensity muon beamline under study at PSI. As the

μ \to

eee decay is heavily suppressed in the Standard Model of particle physics, the observation of such a signal would be an unambiguous indication of the existence of new physics. Achieving the desired sensitivity requires a high rate of muons (

10^{8}

stopped muons per second) along with a detector with large kinematic acceptance and efficiency, able to reconstruct the low momentum of the decay electrons and positrons. To achieve this goal, the Mu3e experiment is mounted with an ultra thin tracking detector based on monolithic active pixel sensors for excellent momentum and vertex resolution, combined with scintillating fibers and tiles for precise timing measurements. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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Review

Jump to: Research

24 pages, 4115 KiB

Open AccessReview

Searches for Lepton Flavor Violation in Tau Decays at Belle II

by Swagato Banerjee

Universe 2022, 8(9), 480; https://doi.org/10.3390/universe8090480 - 13 Sep 2022

Cited by 6 | Viewed by 1572

Abstract

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 [...] Read more.

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

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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39 pages, 8404 KiB

Open AccessReview

Introduction to Charged Lepton Flavor Violation

by Marco Ardu and Gianantonio Pezzullo

Universe 2022, 8(6), 299; https://doi.org/10.3390/universe8060299 - 25 May 2022

Cited by 11 | Viewed by 2041

Abstract

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 [...] Read more.

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

μ \to e

sector. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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16 pages, 964 KiB

Open AccessReview

Muon to Positron Conversion

by MyeongJae Lee and Michael MacKenzie

Universe 2022, 8(4), 227; https://doi.org/10.3390/universe8040227 - 7 Apr 2022

Cited by 5 | Viewed by 2004

Abstract

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 [...] Read more.

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

μ^{-} + N (A, Z) \to e^{+} + N^{'} (A, Z - 2)

(referred to as

μ^{-} \to e^{+}

) complements

0 ν β β

decay searches, and is sensitive to potential flavor effects in the neutrino mass-generation mechanism. A theoretical motivation for

μ^{-} \to e^{+}

is presented along with a review of the status of past

μ^{-} \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

μ^{-} \to e^{+}

work. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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17 pages, 7303 KiB

Open AccessReview

Search for Muon-to-Electron Conversion with the COMET Experiment

by Manabu Moritsu

Universe 2022, 8(4), 196; https://doi.org/10.3390/universe8040196 - 22 Mar 2022

Cited by 6 | Viewed by 2253

Abstract

Charged Lepton Flavor Violation is expected to be one of the most powerful tools to reveal physics beyond the Standard Model. The COMET experiment aims to search for the neutrinoless coherent transition of a muon into an electron in the field of a [...] Read more.

Charged Lepton Flavor Violation is expected to be one of the most powerful tools to reveal physics beyond the Standard Model. The COMET experiment aims to search for the neutrinoless coherent transition of a muon into an electron in the field of a nucleus. Muon-to-electron conversion has never been observed, and can be, and would be, clear evidence of new physics if discovered. The experimental sensitivity of this process, defined as the ratio of the muon-to-electron conversion rate to the total muon capture rate, is expected to be significantly improved by a factor of 100 to 10,000 in the coming decade. The COMET experiment will take place at J-PARC with single event sensitivities of the orders of 10⁻¹⁵ and 10⁻¹⁷ in Phase-I and Phase-II, respectively. The ambitious goal of the COMET experiment is achieved by realizing a high-quality pulsed beam and an unprecedentedly powerful muon source together with an excellent detector apparatus that can tolerate a severe radiation environment. The construction of a new beam line, superconducting magnets, detectors and electronics is in progress towards the forthcoming Phase-I experiment. We present the experimental methods, sensitivity and backgrounds along with recent status and prospects. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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14 pages, 354 KiB

Open AccessReview

Studying ΔL = 2 Lepton Flavor Violation with Muons

by Alexey A. Petrov, Renae Conlin and Cody Grant

Universe 2022, 8(3), 169; https://doi.org/10.3390/universe8030169 - 8 Mar 2022

Cited by 1 | Viewed by 1550

Abstract

Flavor violating processes in the lepton sector have highly suppressed branching ratios in the standard model. Thus, observation of lepton flavor violation (LFV) constitutes a clear indication of physics beyond the standard model (BSM). We review new physics searches in the processes that [...] Read more.

Flavor violating processes in the lepton sector have highly suppressed branching ratios in the standard model. Thus, observation of lepton flavor violation (LFV) constitutes a clear indication of physics beyond the standard model (BSM). We review new physics searches in the processes that violate the conservation of lepton (muon) flavor by two units with muonia and muonium–antimuonium oscillations. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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19 pages, 1718 KiB

Open AccessReview

Charged Lepton Flavor Violation at the High-Energy Colliders: Neutrino Mass Relevant Particles

by Yongchao Zhang

Universe 2022, 8(3), 164; https://doi.org/10.3390/universe8030164 - 6 Mar 2022

Viewed by 1932

Abstract

We summarize the potential charged lepton flavor violation (LFV) from neutrino mass relevant models, for instance the seesaw mechanisms. In particular, we study, in a model-dependent way, the LFV signals at the high-energy hadron and lepton colliders originating from the beyond standard model [...] Read more.

We summarize the potential charged lepton flavor violation (LFV) from neutrino mass relevant models, for instance the seesaw mechanisms. In particular, we study, in a model-dependent way, the LFV signals at the high-energy hadron and lepton colliders originating from the beyond standard model (BSM) neutral scalar H, doubly charged scalar

H^{\pm \pm}

, heavy neutrino N, heavy

W_{R}

boson, and the

Z^{'}

boson. For the neutral scalar, doubly charged scalar and

Z^{'}

boson, the LFV signals originate from the (effective) LFV couplings of these particles to the charged leptons, while for the heavy neutrino N and

W_{R}

boson, the LFV effects are from flavor mixing in the neutrino sector. We consider current limits on these BSM particles and estimate their prospects at future high-energy hadron and lepton colliders. Full article

(This article belongs to the Special Issue Charged Lepton Flavor Violation)

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