Searcharxiv⌕ Search

arXiv subjects

A. Ishikawa

Publications and source records attributed to A. Ishikawa.

333 records · Page 19Linked to original sources
arXiv2006

Measurement of Forward-Backward Asymmetry and Wilson Coefficients in $B \to K^* \ell^+ \ell^-$

We report the first measurement of the forward-backward asymmetry and the ratios of Wilson coefficients $A_9/A_7$ and $A_{10}/A_7$ in $B \to K^* \ell^+ \ell^-$, where $\ell$ represents an electron or a muon. We observe a large integrated forward-backward asymmetry with a significance of 3.4$σ$. The results are obtained from a data sample containing 386 $\times 10^6$ $B\bar{B}$ pairs that were collected on the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider.

hep-ex↗
arXiv2003

Observation of $B \to K^* \ell^+ \ell^-$

We report the first observation of the flavor-changing neutral current decay $B \to K^* \ell^+ \ell^-$ and an improved measurement of the decay $B \to K \ell^+ \ell^-$, where $\ell$ represents an electron or a muon, with a data sample of 140 fb${}^{-1}$ accumulated at the $Υ(4S)$ resonance with the Belle detector at KEKB. The results for the branching fractions are ${\cal B}(B \to K^* \ell^+ \ell^-)=(11.5^{+2.6}_{-2.4} \pm 0.8 \pm 0.2) \times 10^{-7}$ and ${\cal B}(B \to K \ell^+ \ell^-)=(4.8^{+1.0}_{-0.9} \pm 0.3 \pm 0.1) \times 10^{-7}$, where the first error is statistical, the second is systematic and the third is from model dependence.

hep-ex↗
arXiv2003

Electron correlation and two dimensionality in the spin-density-wave phase of (TMTTF)$_2$Br under pressure

The incommensurate spin-density-wave (SDW) phase in (TMTTF)$_2$Br was investigated through transport measurements under pressure and magnetic fields parallel to the $c^{\ast}$ axis. For the incommensurate SDW phase of (TMTTF)$_2$Br stabilized above 0.5 GPa, the SDW transition temperature $T_{\rm SDW}$ increases with the applied magnetic field. The field dependence of $T_{\rm SDW}$ is described by a quadratic behavior and the coefficient of the quadratic term increases with increasing pressure. These results are consistent with the prediction of the mean-field theory based on the suppression of the SDW transition by two-dimensionality. From the relation between the coefficient of the quadratic term and $T_{\rm SDW}$ at zero magnetic field, we determined the role of electron correlation and two dimensionality in the SDW phase of (TMTTF)$_2$Br under pressure and found that the SDW transition in (TMTTF)$_2$Br can be well explained within the mean field theory by taking into account the reduction of the coupling constant $N(0)I$ by pressure.

cond-mat.str-el↗
arXiv2002

Role of the dimerized gap due to anion ordering in spin-density wave phase of (TMTSF)$_2$ClO$_4$ at high magnetic fields

Magnetoresistance measurements have been carried out along the highly conducting a axis in the FISDW phase of hydrogened and deuterated (TMTSF)$_2$ClO$_4$ for various cooling rates through the anion ordering temperature. With increasing the cooling rate, a) the high field phase boundary $β_{\rm {HI}}$, observed at 27 T in hydrogened samples for slowly cooled, is shifted towards a lower field, b) the last semimetallic SDW phase below $β_{\rm {HI}}$ is suppressed, and c) the FISDW insulating phase above $β_{\rm {HI}}$ is enhanced in both salts. The cooling rate dependence of the FISDW transition and of $β_{\rm {HI}}$ in both salts can be explained by taking into account the peculiar SDW nesting vector stabilized by the dimerized gap due to anion ordering.

cond-mat.str-el↗
arXiv2000

SDW and FISDW transition of (TMTSF)$_2$ClO$_4$ at high magnetic fields

The magnetic field dependence of the SDW transition in (TMTSF)$_2$ClO$_4$ for various anion cooling rates has been measured, with the field up to 27T parallel to the lowest conductivity direction $c^{\ast}$. For quenched (TMTSF)$_2$ClO$_4$, the SDW transition temperature $T_{\rm {SDW}}$ increases from 4.5K in zero field up to 8.4K at 27T. A quadratic behavior is observed below 18T, followed by a saturation behavior. These results are consistent with the prediction of the mean-field theory. From these behaviors, $T_{\rm {SDW}}$ is estimated as $T_{\rm {SDW_0}}$=13.5K for the perfect nesting case. This indicates that the SDW phase in quenched (TMTSF)$_2$ClO$_4$, where $T_{\rm {SDW}}$ is less than 6K, is strongly suppressed by the two-dimensionality of the system. In the intermediate cooled state in which the SDW phase does not appear in zero field, the transition temperature for the field-induced SDW shows a quadratic behavior above 12T and there is no saturation behavior even at 27T, in contrast to the FISDW phase in the relaxed state. This behavior can probably be attributed to the difference of the dimerized gap due to anion ordering.

cond-mat.str-el↗
arXiv1995

Macroscopoic Three-Loop Amplitudes and the Fusion Rules from the Two-Matrix Model

From the computation of three-point singlet correlators in the two-matrix model, we obtain an explicit expression for the macroscopic three-loop amplitudes having boundary lengths $\ell_{i}$ $(i = 1\sim 3)$ in the case of the unitary series $(p,q)= (m+1,m)$ coupled to two-dimensional gravity. The sum appearing in this expression is found to conform to the structure of the CFT fusion rules while the summand factorizes through a product of three modified Bessel functions. We briefly discuss a possible generalization of these features to macroscopic $n$-loop amplitudes.

hep-th↗
arXiv1994

Continuum Annulus Amplitude from the Two-Matrix Model

An explicit expression for continuum annulus amplitudes having boundary lengths $\ell_{1}$ and $\ell_{2}$ is obtained from the two-matrix model for the case of the unitary series; $(p,q) = (m + 1, m)$. In the limit of vanishing cosmological constant, we find an integral representation of these amplitudes which is reproduced, for the cases of the $m = 2~(c=0)$ and the $m \rightarrow \infty~(c=1)$, by a continuum approach consisting of quantum mechanics of loops and a matter system integrated over the modular parameter of the annulus. We comment on a possible relation to the unconventional branch of the Liouville gravity.

hep-th↗