arXiv subjects
Publications and source records attributed to M. Masuda.
We search for the rare radiative decay $D^0\toγγ$ using a data sample with an integrated luminosity of $832{\rm fb^{-1}}$ recorded by the Belle detector at the KEKB $e^+e^-$ asymmetric-energy collider. We find no statistically significant signal and set an upper limit on the branching fraction of ${\cal B}(D^0\toγγ)<8.5\times10^{-7}$ at $90\%$ confidence level. This is the most restrictive limit on the decay channel to date.
We report the first observation of the decays $B^0 \rightarrow p\barΛ D^{(*)-}$. The data sample of $711$ fb$^{-1}$ used in this analysis corresponds to $772$ million $B\bar{B}$ pairs, collected at the $Υ(4S)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We observe $19.8σ$ and $10.8σ$ excesses of events for the two decay modes and measure the branching fractions of $B^0 \rightarrow p\barΛ D^{-}$ and $B^0 \rightarrow p\barΛ D^{*-}$ to be $(25.1\pm2.6\pm3.5)\times10^{-6}$ and $(33.6\pm6.3\pm4.4)\times10^{-6}$, respectively, where the first uncertainties are statistical and the second are systematic. These results are not compatible with the predictions based on the generalized factorization approach. In addition, a threshold enhancement in the di-baryon ($p\barΛ$) system is observed, consistent with that observed in similar $B$ decays.
We report the first double differential cross sections of two charged pions and kaons ($e^+e^- \rightarrow hhX$) in electron-positron annihilation as a function of the fractional energies of the two hadrons for any charge and hadron combination. The dependence of these di-hadron cross sections on the topology (same, opposite-hemisphere or anywhere) is also studied with the help of the event shape variable thrust and its axis. The ratios of these di-hadron cross sections for different charges and hadron combinations directly shed light on the contributing fragmentation functions. For example, we find that the ratio of same-sign pion pairs over opposite-sign pion pairs drops toward higher fractional energies where disfavored fragmentation is expected to be suppressed. These di-hadron results are obtained from a $655\,{\rm fb}^{-1}$ data sample collected near the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. Extending the previously published single-pion and single-kaon cross sections, single-proton ($e^+e^- \rightarrow pX$) cross sections are extracted from a $159\,{\rm fb}^{-1}$ data sub-sample.
We report the performance of an instrument that employs a torsion balance for probing a non-standard force in the sub-micrometre range. High sensitivity is achieved by using a torsion balance that has a long torsional period, strong magnetic damping of all vibrational motions and a feedback system that employs an optical lever. In torsion balance experiments, the distance fluctuations during measurements and the accuracy to which the absolute distance is determined are crucial for determining the sensitivity of the balance to a macroscopic force in the sub-micrometre range. We have estimated the root mean square amplitude of the distance fluctuation to be 18 nm by considering the effects due to seismic motions, tilt motions, residual angular fluctuations and thermal fluctuations. We have also estimated the error of the absolute distance to be 13 nm and the statistical error of the force to be 3.4$\times$10$^{-12}$ N by measuring the electrostatic forces. As a result of this systematic study, we have evaluated the sensitivity of the balance to both a non-standard force and to the Casimir force.
The process $e^+e^- \to γχ_{cJ}$ ($J$=1, 2) is studied via initial state radiation using 980 fb$^{-1}$ of data at and around the $Υ(nS)$ ($n$=1, 2, 3, 4, 5) resonances collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. No significant signal is observed except from $ψ(2S)$ decays. Upper limits on the cross sections between $\sqrt{s}=3.80$ and $5.56~{\rm GeV}$ are determined at the 90% credibility level, which range from few pb to a few tens of pb. We also set upper limits on the decay rate of the vector charmonium [$ψ(4040$), $ψ(4160)$, and $ψ(4415)$] and charmoniumlike [$Y(4260)$, $Y(4360)$, and $Y(4660)$] states to $γχ_{cJ}$.
We report the first observational search for tau neutrinos from gamma ray bursts (GRBs) using one of the Ashra light collectors. The earth-skimming tau-neutrino technique of imaging Cherenkov tau showers was applied as a detection method. We set stringent upper limits on the tau-neutrino fluence in PeV-EeV region for 3780 s (between 2.83 and 1.78 hours before) and another 3780 s (between 21.2 and 22.2 hours after) surrounding GRB081203A triggered by the Swift satellite. This first search for PeV-EeV tau neutrino complements other experiments in energy range and methodology, and suggests the prologue of "multi-particle astronomy" with a precise determination of time and location.
We report the search for a nonstandard force by measuring the Casimir forces in the 0.48--6.5 $μ$m range. By comparing the data and the theory of the Casimir force, we have obtained constraints for the parameter $α$ of the Yukawa-type deviations from Newtonian gravity. The obtained limits are more stringent than previous limits in the 1.0--2.9 $μ$m range. Furthermore, we have obtained lower limits for the fundamental scale $M_{*}$ for gauged baryon number in the bulk. In particular, for six extra dimensions, the limits on $M_{*}$ are stringent in the range $6.5\times10^{-6}<ρ<2.5\times10^{-4}$.
It is shown that in general the gauge given by the limit of $ξ\to 0$ in $R_ξ$ gauge does not necessarily agree with the unitary gauge by examining the symmetry breaking of two steps from ${\rm SU(3) \otimes U(1)}_N$ to ${\rm SU(2) \otimes U(1)}_Y$ and then to ${\rm U(1)}_{em}$.
The gauge invariant and anomaly-free 331 model is studied with $R_ξ$ gauges in order to see which of the right-handed singlet between the case (i), assigning the bosonic singlet to the counterpart of 3 and $\bar 3$ representations, and the case (ii), assigning fermionic siglet to the counterpart of the left-handed 3 representation and assigning the antifermionic singlet to that of the left-handed $\bar 3$ representaion, should be assigned in physics but not mathematics to the counterparts of the left-handed fundamental 3 representation and the left-handed complex conjugate $\bar 3$ representation. It is shown that through an effect on the Yukawa interaction of the transformation property of the singlet it depends on the choices of the case (i) or the case (ii) whether the anomaly coefficients even after the spontaneous symmetry breaking (SSB) vanish or not, and the coefficients vanish only in the case (ii) but do not in the case (i). Furthermore, it is pointed out that the BRS invariance after the SSB holds for the case (ii) and (i-b) of two possibilities in the choice for the mass eigenstates in the case (i) and thus the BRS invariance does not necessarily ensure renormalizability of the theory though the inverse of it holds.