Variation of Molecular Cloud Properties across the Spiral Arm in M 51

We present the results of high-resolution 13CO(1-0) mapping observations with the NRO 45m telescope of the area toward the southern bright arm region of M51, including the galactic center. The obtained map shows the central depression of the the circumnuclear ring and the spiral arm structure.The arm-to-interarm ratio of the 13CO(1-0) integrated intensity is 2-4. We also have found a feature different from that found in the 12CO results. The 12CO/13CO ratio spatially varies, and shows high values (~20) for the interarm and the central region, but low values(~10) for the arm. These indicate that there is a denser gas in the spiral arm than in the interarm. The distribution of the 13CO shows a better correspondence with that of the H\alpha emission than with the 12CO in the disk region, except for the central region. We found that the 13CO emission is located on the downstream side of the 12CO arm, namely there is an offset between the 12CO and the 13CO as well as the H\alpha emission. This suggests that there is a time delay between the accumulation of gas caused by the density wave and dense gas formation, accordingly star formation. This time delay is estimated to be ~10^7 yr based on the assumption of galactic rotation derived by the rotation curve and the pattern speed of M51. It is similar to the growth timescale of a gravitational instability in the spiral arm of M51, suggesting that the gravitational instability plays an important role for dense gas formation.Comment: 23 pages, 10 figures, PASJ Vol.54, No.2 (2002), in pres

Search for lepton flavor violation via the intense high-energy muon beam

A deep inerastic scattering process \mutau is discussed to study lepton flavor violation between muons and tau leptons. In supersymmetric models, the Higgs boson mediated diagrams could be important for this reaction. We find that at a muon energy ($E_{\mu}$) higher than 50 GeV, the predicted cross section significantly increases due to the contribution from sea $b$-quarks. The number of produced tau leptons can be $\mathcal{O}(10^4)$ at $E_{\mu}$= 300 GeV from $10^{20}$ muons, whereas $\mathcal{O}(10^2)$ events are given at $E_{\mu}= 50$ GeV.Comment: Contribution to the 6th International Workshop on Neutrino Factories & Superbeams(NuFact04), Jul. 26-Aug. 1, 2004, Osaka Univerisity, Osaka, Japan, talk given by S.K., to appear in the Proceedings, 3 pages, 4 figure

Probing Left-handed Slepton Flavor Mixing at Future Lepton Colliders

It has been argued in the literature that the search for the slepton oscillation phenomenon can be a powerful probe of intergenerational mixing between sleptons, once sleptons are found at future colliders. In this article we estimate possible reach of future lepton colliders in probing left-handed slepton flavor mixing, especially mixing between the first and third generations, on which constraints imposed by other processes like $\tau \to e \gamma$ are very weak. $e^+e^-$ collider is suitable for this purpose, since it can produce, if kinematically allowed, sleptons of the first generation via t-channel, in addition to s-channel. Utilizing e^+e^- \to \tau e + 4jets + \E signal at $e^+e^-$ linear collider with integrated luminosity L=50 fb^{-1}(500 fb^{-1}) it may be possible to reach mixing angle $\sin 2\theta_{\tilde{\nu}} \gtrsim 0.06 (0.04)$ and mass difference $\Delta m_{\tilde{\nu}} \gtrsim 0.07 (0.04)$ GeV for sneutrinos in the first and third generations at the statistical significance of 5 \sigma.Comment: 27 pages, 6 figures. A new section added. Conclusion unchanged. To appear in Phys. Rev.