pΞ−p\Xi^- Correlation in Relativistic Heavy Ion Collisions with Nucleon-Hyperon Interaction from Lattice QCD

On the basis of the $p\Xi^-$ interaction extracted from (2+1)-flavor lattice QCD simulations at the physical point, the momentum correlation of $p$ and $\Xi^-$ produced in relativistic heavy ion collisions is evaluated. $C_{\rm SL}(Q)$ defined by a ratio of the momentum correlations between the systems with different source sizes is shown to be largely enhanced at low momentum due to the strong attraction between $p$ and $\Xi^-$ in the $I=J=0$ channel. Thus, measuring this ratio at RHIC and LHC and its comparison to the theoretical analysis will give a useful constraint on the $p\Xi^-$ interaction.Comment: 4 pages, 2 figures; proceedings contribution for Quark Matter 201

Weak Decay of Λ\Lambda in Nuclei: Direct Quark Mechanism vs Meson Exchange

Nonmesonic decays of $\Lambda$ in nuclear medium and light hypernuclei are studied by using the $\Lambda N \to NN$ weak transition potential derived from the meson exchange mechanism and the direct quark mechanism. The long range part of the transition potential is described by exchanges of the pseudoscalar mesons ($\pi$, $K$, $\eta$), while the vector mesons ($\rho$, $\omega$, $K^\ast$) may be considered as the medium- and short-range part in the meson exchange picture. We propose the direct quark transition potential as the short range part, which is derived from the matrix elements of the $\Delta S=1$ effective weak Hamiltonian in the two baryon states. The results indicate that the direct quark contribution is significantly large and its behavior is qualitatively different from the vector meson exchanges. We also find that the decay rate is sensitive to the choice of form factor and that a soft cutoff must be used for the pion-baryon verteces so that the strong tensor transition is suppressed. We find that the $\pi + K + DQ$ results are compatible with experiment although the $n/p$ ratio is still too large. The $\pi^+$ decays of light hypernuclei are related to the \DI=3/2 amplitudes of the nonmesonic decay. The role of chiral symmetry for the pionic decays are discussed.Comment: 10 pages, Talk presented by Makoto Oka at the APCTP Workshop on Strangeness Nuclear Physics (SNP'99), February, 199

Experimental Determination of the Gain Distribution of an Avalanche Photodiode at Low Gains

A measurement system for determining the gain distributions of avalanche photodiodes (APDs) in a low gain range is presented. The system is based on an ultralow-noise charge--sensitive amplifier and detects the output carriers from an APD. The noise of the charge--sensitive amplifier is as low as 4.2 electrons at a sampling rate of 200 Hz. The gain distribution of a commercial Si APD with low average gains are presented, demonstrating the McIntyre theory in the low gain range.Comment: 3 pages, 4 figure