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Nakao, M.

Publications and source records attributed to Nakao, M..

16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

A search for $$\upmu ^+ \rightarrow \textrm{e}^+ \upgamma $$ with the first dataset of the MEG II experiment

Abstract The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay$$\upmu ^+ \rightarrow {\textrm{e}}^+ \upgamma $$ μ + → e + γ from data taken in the first physics run in 2021. No excess of events over the expected background is observed, yielding an upper limit on the branching ratio of$${\mathcal {B}} (\upmu ^+ \rightarrow {\textrm{e}}^+ \upgamma ) < 7.5 \times 10^{-13}$$ B ( μ + → e + γ ) < 7.5 × 10 - 13 (90% CL). The combination of this result and the limit obtained by MEG gives$${\mathcal {B}} (\upmu ^+ \rightarrow {\textrm{e}}^+ \upgamma ) < 3.1 \times 10^{-13}$$ B ( μ + → e + γ ) < 3.1 × 10 - 13 (90% CL), which is the most stringent limit to date. A ten-fold larger sample of data is being collected during the years 2022–2023, and data-taking will continue in the coming years.

Physics↗
DOE OSTI2024

Operation and performance of the MEG II detector

Abstract The MEG II experiment, located at the Paul Scherrer Institut (PSI) in Switzerland, is the successor to the MEG experiment, which completed data taking in 2013. MEG II started fully operational data taking in 2021, with the goal of improving the sensitivity of the$$\upmu ^+ \rightarrow {\textrm{e}}^+ \upgamma $$ μ + → e + γ decay down to$$\sim 6 \times 10^{-14}$$ ∼ 6 × 10 - 14 almost an order of magnitude better than the current limit. In this paper, we describe the operation and performance of the experiment and give a new estimate of its sensitivity versus data acquisition time.

Physics↗
DOE OSTI2022

Novel X-ray scanning technique for in-situ alignment of photo-detectors in the MEGII calorimeter

Here, we describe and show results of a novel technique to measure with high precision the positions of photodetectors installed in the upgraded liquid xenon calorimeter of the MEG II experiment. The measurement was done by detecting the signal in individual photo-detectors as the front surface of the calorimeter was scanned in axial (Z) and azimuthal (Φ) directions with a precisely controlled, thin, collimated beam of X-rays obtained from a 57 CoX-ray source. The measurements were made when the calorimeter was cold and operational, under which conditions they could not otherwise be made. This technique is used to check for changes in the photodetector positions due to the effects of cooling and thermal cycling of the support structure and to provide a reference between the positions of the operational photodetectors and alignment markers on the cryostat exterior. The use of an essentially monochromatic X-ray source allowed ancillary measurements of photodetector properties, including pulse shape, relative gain, and variation in response over the photodetector surface. Measurement uncertainties below 0.6mm in Z and 0.7 mrad in Φ have been achieved, well within the requirements for the MEG II performance.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Azimuthal asymmetries of back-to-back π ± - ( π 0 , η , π ± ) pairs in e + e - annihilation

This work reports the first observation of azimuthal asymmetries around the thrust axis in e + e – annihilation of pairs of back-to-back charged pions in one hemisphere, and π 0 and η mesons in the opposite hemisphere. These results are complemented by a new analysis of pairs of back-to-back charged pions. The π 0 and η asymmetries rise with the relative momentum z of the detected hadrons as well as with the transverse momentum with respect to the thrust axis. These asymmetries are sensitive to the Collins fragmentation function $H$ $^{1}_{⊥}$ and provide complementary information to previous measurements with charged pions and kaons in the final state. In particular, the η final states will provide additional information on the flavor structure of $H$ $^{1}_{⊥}$. This is the first measurement of the explicit transverse-momentum dependence of the Collins fragmentation function from Belle data. It uses a dataset of 980.4 fb –1 collected by the Belle experiment at or near a center- of-mass energy of 10.58 GeV.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

First measurement of the CKM angle Φ 3 with $B^±→ D({K}_{\mathrm{S}}^0π^+π^–π^0)K^±$ decays

We present the first model-independent measurement of the CKM unitarity triangle angle Φ 3 using B ± → D ( \( {K}_{\mathrm{S}}^0 \) π + π – π 0 ) K ± decays, where D indicates either a D 0 or \( \overline{D} \) 0 meson. Measurements of the strong-phase difference of the D → \( {K}_{\mathrm{S}}^0 \) π + π – π 0 amplitude obtained from CLEO-c data are used as input. This analysis is based on the full Belle data set of $772 × 10^6 B$ \( \overline{B} \) events collected at the Υ(4 S ) resonance. We obtain Φ 3 = ( \( {5.7}_{-8.8}^{+10.2} \) ± 3 . 5 ± 5 . 7) ° and the suppressed amplitude ratio r B = 0 . 323 ± 0 . 147 ± 0 . 023 ± 0 . 051. Here the first uncertainty is statistical, the second is the experimental systematic, and the third is due to the precision of the strong-phase parameters measured from CLEO-c data. The 95% confidence interval on Φ 3 is ( – 29 . 7 , 109 . 5) ° , which is consistent with the current world average.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

First measurements of beam backgrounds at SuperKEKB

The high design luminosity of the SuperKEKB electron-positron collider is expected to result in challenging levels of beam-induced backgrounds in the interaction region. Properly simulating and mitigating these backgrounds is critical to the success of the Belle~II experiment. We report on measurements performed with a suite of dedicated beam background detectors, collectively known as BEAST II, during the so-called Phase 1 commissioning run of SuperKEKB in 2016, which involved operation of both the high energy ring (HER) of 7 GeV electrons as well as the low energy ring (LER) of 4 GeV positrons. We describe the BEAST II detector systems, the simulation of beam backgrounds, and the measurements performed. The measurements include standard ones of dose rates versus accelerator conditions, and more novel investigations, such as bunch-by-bunch measurements of injection backgrounds and measurements sensitive to the energy spectrum and angular distribution of fast neutrons. We observe beam-gas, Touschek, beam-dust, and injection backgrounds. We do not observe significant synchrotron radiation, as expected. Measured LER beam-gas backgrounds and Touschek backgrounds in both rings are slightly elevated, on average three times larger than the levels predicted by simulation. HER beam-gas backgrounds are on on average two orders of magnitude larger than predicted. Systematic uncertainties and channel-to-channel variations are large, so that these excesses constitute only 1-2 sigma level effects. Neutron background rates are higher than predicted and should be studied further. We will measure the remaining beam background processes, due to colliding beams, in the imminent commissioning Phase 2. These backgrounds are expected to be the most critical for Belle II, to the point of necessitating replacement of detector components during the Phase 3 (full-luminosity) operation of SuperKEB

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

The design of the MEG II experiment

The MEG experiment, designed to search for the &#x03BC;<!-- μ --> + &#x2192;<!-- → --> e + &#x03B3;<!-- γ --> decay, completed data-taking in 2013 reaching a sensitivity level of 5.3 &#x00D7;<!-- × --> 10 &#x2212;<!-- - --> 13 for the branching ratio. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6 &#x00D7;<!-- × --> 10 &#x2212;<!-- - --> 14 , a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗