Search Results (15)

Tests of lepton flavour universality in rare decays of b hadrons mediated by flavour-changing neutral-current transitions constitute sensitive probes for physics beyond the standard model. In recent years, such tests have become increasingly precise and have attracted significant theoretical and experimental attention. In this article, we review the status of searches for lepton flavour universality violations in these processes and discuss prospects for future measurements at various facilities. Full article

We review the experimental status of searches for lepton-flavour violation in the charged sector. We give an overview of searches for lepton-flavour violation in purely leptonic decays, hadron decays, and decays of heavy bosons. We focus on the most stringent constraints on lepton-flavour violating processes in these areas and give prospects for ongoing and future experiments. Full article

In many modern particle physics experiments, high-rate data handling is one of the most critical challenges due to the increase in particle intensity required to achieve higher statistics. We will tackle the challenge in the COMET experiment by developing the sub-microseconds ultra-fast machine learning (ML) algorithm implemented inside FPGAs to search for the lepton flavour violation process, a $\mu$-e conversion, using the world’s most intense muon beam. Our previous study showed that a trigger algorithm based on a gradient-boosted decision tree will realise the sufficient trigger performance within 3.2 $\mathsf{\mu }$s with a cut-based event classification. In this paper, we further investigated neural network algorithms as event classifications. For the feasibility test, a multi-layer perceptron (MLP) model was implemented inside the FPGA, and the preliminary results are presented. Full article

The observation of lepton flavour violation (LFV) in the charged lepton sector would be an unambiguous sign of physics beyond the Standard Model (BSM), and thus, it is the channel of choice for many BSM searches. LFV searches in muon decays in particular benefit from the fact that muons can be easily produced at high rates. There is a global effort to search for LFV at high-intensity muon sources to which the upcoming Mu3e experiment at the Paul Scherrer Institute (PSI) will contribute. The Mu3e Collaboration aims to perform a background-free search for the LFV decay ${\mu }^{+}\to e^{+}{e}^{-}{e}^{+}$ with an unprecedented sensitivity in the order of 10⁻¹⁵ in the first phase of operation and 10⁻¹⁶ in the final phase—an improvement over the preceding SINDRUM experiment by four orders of magnitude. The high muon stopping rates and low momenta of the decay electrons make high demands on momentum and time resolution and on the data acquisition. The innovative experimental concept is based on a tracking detector built from novel ultra-thin silicon pixel sensors and scintillating fibres and tiles as well as online event reconstruction and filtering in real time. Full article

The discovery of neutrino oscillations is the first laboratory evidence of New Physics beyond the Standard Model. Oscillating neutrinos necessarily imply that neutrinos are massive and that (neutral) lepton flavour is violated. However, a signal of charged lepton flavour violation (cLFV) has so far eluded experimental discovery. In this proceeding, we review some phenomenological implications of the current experimental bounds (and future sensitivities) on observables related to muons, with particular attention to charged lepton flavour violating processes. In connection to neutrino masses, we also highlight some phenomenological implications of leptonic CP violation on cLFV observables. Full article

Charged lepton flavour violation processes provide a well-motivated experimental probe into new physics beyond the Standard Model. Muon to electron conversion is one example that the COMET experiment aims to measure with increased sensitivity over previous searches. Taking a staged approach, the COMET experiment will measure muon to electron conversion with sensitivities of $\mathcal{O}\left({10}^{-15}\right)$ and $\mathcal{O}\left({10}^{-17}\right)$ in Phase-I and Phase-II, respectively. An important initial low-intensity beam run, Phase-$\alpha$, is also planned to begin in 2023 with Phase-I following in 2024. This article summarises the COMET experiment and the recent progress made towards the beginning of physics runs. Full article

The Mu2e experiment at Fermi National Accelerator Laboratory will search for the charged-lepton flavour-violating neutrinoless conversion of negative muons into electrons in the Coulomb field of an Al nucleus. The conversion electron with a monoenergetic 104.967 MeV signature will be identified by a complementary measurement carried out by a high-resolution tracker and an electromagnetic calorimeter, improving by four orders of magnitude the current single-event sensitivity. The calorimeter—composed of 1348 pure CsI crystals arranged in two annular disks—has a high granularity, 10% energy resolution and 500 ps timing resolution for 100 MeV electrons. The readout, based on large-area UV-extended SiPMs, features a fully custom readout chain, from the analogue front-end electronics to the digitisation boards. The readout electronics design was validated for operation in vacuum and under magnetic fields. An extensive radiation hardness certification campaign certified the FEE design for doses up to 100 krad and 10${}^{12}$ n${}_{1\mathrm{MeVeq}}$/cm${}^2$ and for single-event effects. A final vertical slice test on the final readout chain was carried out with cosmic rays on a large-scale calorimeter prototype. Full article

The Mu2e experiment at Fermilab will search for the standard model-forbidden, charged lepton flavour-violating conversion of a negative muon into an electron in the field of an aluminium nucleus. The distinctive signal signature is represented by a mono-energetic electron with an energy near the muon’s rest mass. The experiment aims to improve the current single-event sensitivity by four orders of magnitude by means of a high-intensity pulsed muon beam and a high-precision tracking system. The electromagnetic calorimeter complements the tracker by providing high rejection power in muon to electron identification and a seed for track reconstruction while working in vacuum in presence of a 1 T axial magnetic field and in a harsh radiation environment. For 100 MeV electrons, the calorimeter should achieve: (a) a time resolution better than 0.5 ns, (b) an energy resolution <10%, and (c) a position resolution of 1 cm. The calorimeter design consists of two disks, each loaded with 674 undoped CsI crystals read out by two large-area arrays of UV-extended SiPMs and custom analogue and digital electronics. We describe here the status of construction for all calorimeter components and the performance measurements conducted on the large-sized prototype with electron beams and minimum ionizing particles at a cosmic ray test stand. A discussion of the calorimeter’s engineering aspects and the on-going assembly is also reported. Full article

Oscillation of two-flavour neutrinos is considered within a quantum mechanical framework of consistent (decoherent) dynamic histories. We investigate how consistency of selected three-time histories is affected by oscillation parameters. We show that the presence of normal matter is crucial to maintain consistency of certain classes of neutrinos’ dynamic histories and that the consistency does not depend on a Majorana phase and remains insensitive to a potential CP violation. Full article

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating ${\mathsf{\mu }}^{+}\to {\mathrm{e}}^{+}\mathsf{\gamma }$ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR $({\mathsf{\mu }}^{+}\to {\mathrm{e}}^{+}\mathsf{\gamma })\le 4.2\times {10}^{-13}$ 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 ($7\times {10}^7$ ${\mathsf{\mu }}^{+}$/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

The LHCb experiment has been able to collect the largest sample ever produced of charm-hadron decays, performing a number of measurements of observables related to $CP$ violation in the charm sector. In this document, the most recent results from LHCb on the search of direct $CP$ violation in $D^0\to K_s^0{K}_s^0$, $D_{\left(s\right)}^{+}\to h^{+}{\pi }^0$ and $D_{\left(s\right)}^{+}\to h^{+}\eta$ decays are summarised, in addition to the most precise measurement of time-dependent $CP$ asymmetry in $D^0\to h^{+}{h}^{-}$ decays and the first observation of mass difference between neutral charm-meson eigenstates. Full article

Flavour anomalies have attracted a lot of attention over recent years as they provide unique hints for possible New Physics. Here, we consider a supersymmetric (SUSY) extension of the Standard Model (SM) with an additional anomaly-free gauge $U\left(1\right)$ group. The key feature of our model is the particular choice of non-universal charges to the gauge boson $Z^{\prime }$, which not only allows a relaxation of the flavour discrepancies but, contrary to previous studies, can reproduce the SM mixing matrices both in the quark and lepton sectors. We pay special attention to the latter and explicitly enumerate all parameters relevant for our calculation in the low-energy effective theory. We find regions in the parameter space that satisfy experimental constraints on meson mixing and LHC $Z^{\prime }$ searches and can alleviate the flavour anomalies. In addition, we also discuss the predictions for lepton-flavour violating decays $B^{+}\to K^{+}\mu \tau$ and $B^{+}\to K^{+}e\tau$. Full article

Symmetries in the Physical Laws of Nature lead to observable effects. Beyond the regularities and conserved magnitudes, the last few decades in particle physics have seen the identification of symmetries, and their well-defined breaking, as the guiding principle for the elementary constituents of matter and their interactions. Flavour SU(3) symmetry of hadrons led to the Quark Model and the antisymmetric requirement under exchange of identical fermions led to the colour degree of freedom. Colour became the generating charge for flavour-independent strong interactions of quarks and gluons in the exact colour SU(3) local gauge symmetry. Parity Violation in weak interactions led us to consider the chiral fields of fermions as the objects with definite transformation properties under the weak isospin SU(2) gauge group of the Unifying Electro-Weak SU(2) × U(1) symmetry, which predicted novel weak neutral current interactions. CP-Violation led to three families of quarks opening the field of Flavour Physics. Time-reversal violation has recently been observed with entangled neutral mesons, compatible with CPT-invariance. The cancellation of gauge anomalies, which would invalidate the gauge symmetry of the quantum field theory, led to Quark–Lepton Symmetry. Neutrinos were postulated in order to save the conservation laws of energy and angular momentum in nuclear beta decay. After the ups and downs of their mass, neutrino oscillations were discovered in 1998, opening a new era about their origin of mass, mixing, discrete symmetries and the possibility of global lepton-number violation through Majorana mass terms and Leptogenesis as the source of the matter–antimatter asymmetry in the universe. The experimental discovery of quarks and leptons and the mediators of their interactions, with physical observables in spectacular agreement with this Standard Theory, is the triumph of Symmetries. The gauge symmetry is exact only when the particles are massless. One needs a subtle breaking of the symmetry, providing the origin of mass without affecting the excellent description of the interactions. This is the Brout–Englert–Higgs Mechanism, which produces the Higgs Boson as a remnant, discovered at CERN in 2012. Open present problems are addressed with by searching the New Physics Beyond-the-Standard-Model. Full article

Anomalies recently observed in semileptonic $b\to c{\ell }^{-}{\overline{\nu }}_{\ell }$ and $b\to s{\ell }^{+}{\ell }^{-}$ transitions point to violation of Lepton Flavour Universality. Strategies for new analyses of different modes are required, in particular for the modes induced by the $b\to u$ transition. We describe the purely leptonic B decay, the $\overline{B}\to \pi {\ell }^{-}{\overline{\nu }}_{\ell }$ channel and the B semileptonic modes to $\rho (770)$ and $a_1(1260)$ in extensions of the Standard Model involving Lepton Flavour Universality violating $b\to u$ operators. In particular, we review the observables in the four-dimensional angular $\overline{B}\to \rho (\pi \pi ){\ell }^{-}{\overline{\nu }}_{\ell }$ and $\overline{B}\to a_1(\rho \pi ){\ell }^{-}{\overline{\nu }}_{\ell }$ distributions, suitable to pin down deviations from the Standard Model. We discuss the complementarity among the various modes for New Physics searches. Full article

We propose that gravitational interactions of cosmic neutrinos with the statistically homogeneous and isotropic fluctuations of space-time lead to decoherence. This working hypothesis, which we describe by means of a Lindblad operator, is applied to the system of two- and three-flavour neutrinos undergoing vacuum oscillations and the consequences are investigated. As a result of this decoherence we find that the neutrino entropy would increase as a function of initial spectral distortions, mixing angles and charge-parity (CP)-violation phase. Subsequently we discuss the chances to discover such an increase observationally (in principle). We also present the expected flavour composition of the cosmic neutrino background after decoherence is completed. The physics of two- or three-flavour oscillation of cosmological neutrinos resembles in many aspects two- or three-level systems in atomic clocks, which were recently proposed by Weinberg for the study of decoherence phenomena. Full article