arXiv subjects

Yoshitaka Kuno

Publications and source records attributed to Yoshitaka Kuno.

At least 19 recordsLinked to original sources

arXiv2025

$μ^- \to e^-γ$ in a muonic atom as a probe for effective lepton flavor violating operators involving photon fields

We propose the $μ^-\to e^-γ$ process in a muonic atom as a novel means to investigate charged lepton flavor violation (CLFV). We demonstrate its sensitivity in probing effective CLFV operators associated with both single and double photon fields. In comparison to $μ^+\to e^+γ$ using free positive muon decays at rest, the emitted electron and photon in $μ^-\to e^-γ$ exhibit non-monochromatic spectra, presenting non-trivial case. We derive the decay rate formula and demonstrate that the potential rate of the process is significant enough to motivate exploration in future muon CLFV experiments.

hep-ph↗

arXiv2024

Extracting Signal Electron Trajectories in the COMET Phase-I Cylindrical Drift Chamber Using Deep Learning

We present a pioneering approach to tracking analysis within the COMET Phase-I experiment, which aims to search for the charged lepton flavor violating $μ\to e$ conversion process in a muonic atom, at J-PARC, Japan. This paper specifically introduces the extraction of signal electron trajectories in the COMET Phase-I cylindrical drift chamber (CDC) amidst a high background hit rate, with more than $40\,\%$ occupancy of the total CDC cells, utilizing deep learning techniques of semantic segmentation. Our model achieved remarkable results, with a purity rate of $98\,\%$ and a retention rate of $90\,\%$ for CDC cells with signal hits, surpassing the design-goal performance of $90\,\%$ for both metrics. This study marks the initial application of deep learning to COMET tracking, paving the way for more advanced techniques in future research.

hep-ex↗

arXiv2022

A Measurement of Proton, Deuteron, Triton and Alpha Particle Emission after Nuclear Muon Capture on Al, Si and Ti with the AlCap Experiment

Heavy charged particles after nuclear muon capture are an important nuclear physics background to the muon-to-electron conversion experiments Mu2e and COMET, which will search for charged lepton flavor violation at an unprecedented level of sensitivity. The AlCap experiment measured the yield and energy spectra of protons, deuterons, tritons, and alpha particles emitted after the nuclear capture of muons stopped in Al, Si, and Ti in the low energy range relevant for the muon-to-electron conversion experiments. Individual charged particle types were identified in layered silicon detector packages and their initial energy distributions were unfolded from the observed energy spectra. Detailed information on yields and energy spectra for all observed nuclei are presented in the paper.

physics.ins-det↗

arXiv2021

An FPGA-based Trigger System with Online Track Recognition in COMET Phase-I

An FPGA-based online trigger system has been developed for the COMET Phase-I experiment. This experiment searches for muon-to-electron conversion, which has never been observed yet. A drift chamber and trigger counters detect a mono-energetic electron from the conversion process in a 1-T solenoidal magnetic field. A highly intense muon source is applied to reach unprecedented experimental sensitivity. It also generates undesirable background particles, and a trigger rate due to these particles is expected to be much higher than an acceptable trigger rate in the data acquisition system. By using hit information from the drift chamber too, the online trigger system efficiently suppresses a background trigger rate while keeping signal-event acceptance large. A characteristic of this system is the utilization of the machine learning technique in the form of look-up tables on hardware. An initial simulation study indicates that the signal-event acceptance of the online trigger is 96% while the background trigger rate is reduced from over $90\,\mathrm{kHz}$ to $13\,\mathrm{kHz}$. For this scenario, we have produced trigger-related electronics that construct a distributed trigger architecture. The total latency of the trigger system was estimated to be $3.2\,\mathrm{μs}$, and the first operation test was carried out by using a part of the drift-chamber readout region.

physics.ins-det↗

arXiv2021

Probing $μe γγ$ contact interactions with $μ\to e$ conversion

Contact interactions of a muon, an electron and two photons can contribute to the decay $μ\to e γγ$, but also to the conversion of a muon into an electron in the electric field of a nucleus. We calculate the $μ\to e$ conversion rate, and show that for the coefficients of operators involving the combination $FF \propto |\vec{E}|^2$ (as opposed to $F\tilde{F} \propto \vec{E} \cdot \vec{B}$), the current bound on $μ\to e$ conversion is more sensitive than the bound on $μ\to e γγ$.

hep-ph↗

arXiv2020

GPU-Accelerated Event Reconstruction for the COMET Phase-I Experiment

This paper discusses a parallelized event reconstruction of the COMET Phase-I experiment. The experiment aims to discover charged lepton flavor violation by observing 104.97 MeV electrons from neutrinoless muon-to-electron conversion in muonic atoms. The event reconstruction of electrons with multiple helix turns is a challenging problem because hit-to-turn classification requires a high computation cost. The introduced algorithm finds an optimal seed of position and momentum for each turn partition by investigating the residual sum of squares based on distance-of-closest-approach (DCA) between hits and a track extrapolated from the seed. Hits with DCA less than a cutoff value are classified for the turn represented by the seed. The classification performance was optimized by tuning the cutoff value and refining the set of classified hits. The workload was parallelized over the seeds and the hits by defining two GPU kernels, which record track parameters extrapolated from the seeds and finds the DCAs of hits, respectively. A reasonable efficiency and momentum resolution was obtained for a wide momentum region which covers both signal and background electrons. The event reconstruction results from the CPU and GPU were identical to each other. The benchmarked GPUs had an order of magnitude of speedup over a CPU with 16 cores while the exact speed gains varied depending on their architectures.

physics.ins-det↗

arXiv2019

GPU Tracking in the COMET Phase-I Cylindrical Drift Chamber

The GPU-accelerated track finding method is investigated to track electrons from neutrinoless muon decay in the COMET Phase-I experiment. Inside the cylindrical drift chamber, one third of the signal electron trajectories are composed of multiple turns where the correct hit assignments to each turn partition are significant in the track finding. Scanning all possible track seeds of position and momentum can resolve the hit-to-turn assignment problem with a high robustness, but requires a huge computational cost: The initial track seeds $(θ,z,p_x,p_y,p_z)$ have broad uncertainties, so there exists many number of seeds that should be compared. In this article, this problem of massive computations are mitigated with 1) the parallel computing of Runge-Kutta-Nyström track propagation with the GPU, and 2) an initial guess on the seeds using the Hough transform and the detector geometry. The computation speed enhancement compared to the CPU is also presented.

physics.ins-det↗

arXiv2019

Momentum distribution of the electron pair from the charged lepton flavor violating process $μ^-e^-\to e^-e^-$ in muonic atoms with a polarized muon

The $μ^-e^-\to e^-e^-$ process in a muonic atom is one of the promising probes to study the charged lepton flavor violation (CLFV). We have investigated the angular distribution of electrons from the polarized muon of the atomic bound state. The parity violating asymmetric distribution of electrons is analyzed by using lepton wave functions under the Coulomb interaction of a finite nuclear charge distribution. It is found that the asymmetry parameters of electrons are very sensitive to the chiral structure of the CLFV interaction and the contact/photonic interaction. Therefore, together with the atomic number dependence of the decay rate studied in our previous work, the angular distribution of electrons from a polarized muon should be a very useful tool to constrain the model beyond the standard model.

hep-ph↗

arXiv2019

Selecting mu -> e Conversion Targets to distinguish Lepton Flavour-Changing Operators

The experimental sensitivity to $μ\to e$ conversion on nuclei is set to improve by four orders of magnitude in coming years. However, various operator coefficients add coherently in the amplitude for $μ\to e$ conversion, weighted by nucleus-dependent functions, and therefore in the event of a detection, identifying the relevant new physics scenarios could be difficult. Using a representation of the nuclear targets as vectors in coefficient space, whose components are the weighting functions, we quantify the expectation that different nuclear targets could give different constraints.We show that all but two combinations of the 10 Spin-Independent (SI) coefficients could be constrained by future measurements, but discriminating among the axial, tensor and pseudoscalar operators that contribute to the Spin-Dependent (SD) process would require dedicated nuclear calculations. We anticipate that $μ\to e$ conversion could constrain 10 to 14 combinations of coefficients; if $μ\to e γ$ and $μ\to 3e$ constrain eight more, that leaves 60 to 64 "flat directions" in the basis of QED$\times$QCD-invariant operators which describe $μ\to e$ flavour change below $m_W$.

hep-ph↗

arXiv2018

Spin-dependent $μ\to e$ conversion

The experimental sensitivity to $μ\to e$ conversion on nuclei is expected to improve by four orders of magnitude in coming years. We consider the impact of $μ\to e$ flavour-changing tensor and axial-vector four-fermion operators which couple to the spin of nucleons. Such operators, which have not previously been considered, contribute to $μ\to e$ conversion in three ways: in nuclei with spin they mediate a spin-dependent transition; in all nuclei they contribute to the coherent ($A^2$-enhanced) spin-independent conversion via finite recoil effects and via loop mixing with dipole, scalar, and vector operators. We estimate the spin-dependent rate in Aluminium (the target of the upcoming COMET and Mu2e experiments), show that the loop effects give the greatest sensitivity to tensor and axial-vector operators involving first-generation quarks, and discuss the complementarity of the spin-dependent and independent contributions to $μ\to e$ conversion

hep-ph↗

arXiv2018

Spin-dependent ${μ\to e}$ Conversion on Light Nuclei

The experimental sensitivity to $μ\to e$ conversion will improve by four or more orders of magnitude in coming years, making it interesting to consider the "spin-dependent" (SD) contribution to the rate. This process does not benefit from the atomic-number-squared enhancement of the spin-independent (SI) contribution, but probes different operators. We give details of our recent estimate of the spin dependent rate, expressed as a function of operator coefficients at the experimental scale, and explore the prospects for distinguishing coefficients by using different targets. For this purpose, a geometric representation of different targets as vectors in coefficient space is introduced. It is found that comparing the rate on isotopes with and without spin could allow to detect spin dependent coefficients that are at least a factor of few larger than the spin independent ones. Distinguishing among the axial, tensor and pseudoscalar operators that induce the SD rate would require calculating the nuclear matrix elements for the second two. Comparing the SD rate on nuclei with an odd proton vs odd neutron could allow to distinguish operators involving $u$ quarks from those involving $d$ quarks; this is interesting because the distinction is difficult to make for SI operators.

hep-ph↗

arXiv2017

Improved analysis for $μ^-e^-\to e^-e^-$ in muonic atoms by photonic interaction

Studies of the charged lepton flavor violating process of $μ^-e^-\to e^-e^-$ in muonic atoms by the four Fermi interaction [Y. Uesaka \textit{et al}., Phys. Rev. D {\bf 93}, 076006 (2016)] are extended to include the photonic interaction. The wave functions of a muon and electrons are obtained by solving the Dirac equation with the Coulomb interaction of a finite nuclear charge distribution. We find suppression of the $μ^-e^-\to e^-e^-$ rate over the initial estimation for the photonic interaction, in contrast to enhancement for the four Fermi interaction. It is due to the Coulomb interaction of scattering states and relativistic lepton wave functions. This finding suggests that the atomic number dependence of the $μ^-e^-\to e^-e^-$ rate could be used to distinguish between the photonic and the four Fermi interactions.

hep-ph↗

arXiv2017

Future Experimental Improvement for the Search of LNV Process in $eμ$ Sector

Exploring the leptonic sector in frontier experiments is more of importance nowadays, since the conservation of lepton flavor and total lepton number are not guaranteed anymore in the Standard Model after the discovery of neutrino oscillations. $μ^- + N(A,Z) \rightarrow e^+ + N(A,Z-2)$ conversion in a muonic atom is one of the most promising channels to investigate the lepton number violation process, and the measurement of this process is planned in future $μ^--e^-$ conversion experiments with a muonic atom in a muon-stopping target. This paper discusses how to maximize the experimental sensitivity of the $μ^--e^+$ conversion by introducing the new requirement of the mass relation of $M(A,Z-2)<M(A,Z-1)$, where $M(A,Z)$ is the mass of the muon-stopping target nucleus, to get rid of the background from radiative muon capture. The sensitivity of the $μ^--e^+$ conversion is anticipated to have four orders of magnitude of improvement in forthcoming experiments using a proper target nucleus, which satisfies the mass relation. The most promising isotopes found are $^{40}$Ca and $^{32}$S.

hep-ex↗

arXiv2016

Improved analyses for $μ^-e^-\rightarrow e^-e^-$ in muonic atoms by contact interactions

The charged lepton flavor violating (CLFV) processes of $μ^-e^-\rightarrow e^-e^-$ 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 $^{208}$Pb. We find that, as the atomic number $Z$ increases, the $μ^-e^-\rightarrow e^-e^-$ 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.

hep-ph↗

arXiv2015

Improved analysis of the CLFV decay of muonic atoms $μ^-e^-\rightarrow e^-e^-$

Koike et al. proposed the charged lepton flavor violation (CLFV) decay of the muonic atom $μ^-e^-\rightarrow e^-e^-$ as one of the promising processes to search for new physics beyond the standard model. It was found that the attractive interaction of leptons with the nucleus enhances the transition rate of the $μ^-e^-\rightarrow e^-e^-$ process. We report on our improved analysis of this process by taking into account the distortion of the out-going electrons in the nuclear Coulomb potential and the relativistic treatment of the muon and the electrons. As results, we found significant enhancement of the transition rate. The transition rate for $^{208}$Pb becomes about 7 times larger than the previous estimation, which enhances the sensitivity of this process to discover the CLFV interaction. We also report on the energy spectrum of the out-going electron.

hep-ph↗

arXiv2012

Search for Lepton Number Violating Charged Current Processes with Neutrino Beams

We propose a new idea to test a class of loop-induced neutrino mass mechanisms by searching for lepton number violating charged current processes with incident of a neutrino beam. The expected rates of these processes are estimated based on some theoretical assumptions. They turn out to be sizable so that detection of such processes could be possible at near detectors in future highly intense neutrino-beam facilities.

hep-ph↗

arXiv2010

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

We propose a new process of mu(-) 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 mu(+) 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.

hep-ph↗

arXiv2004

A Study of Lepton Flavor Violating $μN (e N) \to τX$ Reactions in Supersymmetric Models

We study a lepton flavor violating $μN \to τX$ reaction in deep inelastic scattering region in supersymmetric models. The contribution from the Higgs boson mediation could be important for this reaction. For that case, the cross section is constrained by the experimental limit of the pseudo-scalar coupling from $τ\to μη$ decays. We find that at a muon energy ($E_μ$) higher than 50 GeV, the predicted cross section increases significantly due to the contribution from sea $b$-quarks. As a result, with $10^{20}$ muons per year, at most a number of $\mathcal{O}(10^4)$ is expected for $μN \to τX$ events at $E_μ$= 300 GeV, whereas $\mathcal{O}(10^2)$ events are given at $E_μ= 50$ GeV. Furthermore, the $μN \to τX$ phenomenology, in particular that for the signal and backgrounds, is briefly discussed. Another promising possibility to search for the $e N \to τX$ reaction at an electron-positron linear collider is also discussed. Searches for these reactions would be competitive to studies of rare tau decays and have potential to improve sensitivities to lepton flavor violation significantly.

hep-ph↗