πNσ\pi N \sigma Term and Quark Spin Content of the Nucleon

We report results of our calculation on the $\pi N\sigma$ term and quark spin content of the nucleon on the quenched $16^3 \times 24$ lattice at $\beta = 6.0$. The disconnected insertions which involve contributions from the sea quarks are calculated with the stochastic $Z_2$ noise algorithm. As a physical test of the algorithm, we show that the forward matrix elements of the vector and pseudoscalar currents for the disconnected insertions are indeed consistent with the known results of zero. We tried the Wuppertal smeared source and found it to be more noisy than the point source. With unrenormalized $m_q=4.42(17)$MeV, we find the $\pi N\sigma$ term to be $39.2\pm 5.2$ MeV. The strange quark condensate in the nucleon is large, i.e. $\langle N|\bar{s}s|N\rangle = 1.16 \pm 0.54$. For the quark spin content, we find $\Delta u =0.78\pm 0.07$, $\Delta d =-0.42\pm 0.07$, and $\Delta s = -0.13\pm 0.06$. The flavor-singlet axial charge $g_A^1 = \Delta \Sigma =0.22\pm 0.09$.Comment: contribution to Lattice '94; 3 page uuencoded ps fil

Strange Magnetic Moment of The Nucleon from Lattice QCD

We calculate the strange magnetic moment of the nucleon on a quenched $16^3 \times 24$ lattice at $\beta = 6.0$, and with Wilson fermions at $\kappa$ = 0.148, 0.152, and 0.154. The strange quark contribution from the disconnected insertion is estimated stochastically by employing the $Z_2$ noise method. Using an unbiased subtraction along with the help of charge conjugation and hermiticity, we reduce the error by a factor of 2 with negligible overhead. Our result is $G_{M}^{s} = -0.28\pm0.10 \mu_{N}$.Comment: Lattice 2000 (Hadronic Matrix Elements), 4 pages 2 fig

Incorporating the standstill distance and time headway distributions into freeway car-following models and an application to estimating freeway travel time reliability

Standstill distances and following time headways are two important microsimulation model parameters associated with driver aggression. This paper investigates the distributions of standstill distances and time headways and incorporates these distributions into car-following models to estimate travel time reliability. By incorporating standstill distance and following headway into car-following models as stochastic parameters, a speed-density region can be generated, based on which various travel-time-reliability measures can be calculated. Key findings of this study are as follows: (1) Both standstill distances and time headways follow fairly dispersed distributions. Therefore, it is suggested that microsimulation models should include the option of allowing standstill distances and time headways to follow distributions as well as to be specified separately for different vehicle classes. (2) By incorporating stochastic standstill distance and time headway parameters in car-following models, travel-time-reliability measures can be estimated more precisely and faster compared with using VISSIM

A feedback-driven bubble G24.136+00.436: a possible site of triggered star formation

We present a multi-wavelength study of the IR bubble G24.136+00.436. The J=1-0 observations of $^{12}$CO, $^{13}$CO and C$^{18}$O were carried out with the Purple Mountain Observatory 13.7 m telescope. Molecular gas with a velocity of 94.8 km s$^{-1}$ is found prominently in the southeast of the bubble, shaping as a shell with a total mass of $\sim2\times10^{4}$ $M_{\odot}$. It is likely assembled during the expansion of the bubble. The expanding shell consists of six dense cores. Their dense (a few of $10^{3}$ cm$^{-3}$) and massive (a few of $10^{3}$ $M_{\odot}$) characteristics coupled with the broad linewidths ($>$ 2.5 km s$^{-1}$) suggest they are promising sites of forming high-mass stars or clusters. This could be further consolidated by the detection of compact HII regions in Cores A and E. We tentatively identified and classified 63 candidate YSOs based on the \emph{Spitzer} and UKIDSS data. They are found to be dominantly distributed in regions with strong emission of molecular gas, indicative of active star formation especially in the shell. The HII region inside the bubble is mainly ionized by a $\sim$O8V star(s), of the dynamical age $\sim$1.6 Myr. The enhanced number of candidate YSOs and secondary star formation in the shell as well as time scales involved, indicate a possible scenario of triggering star formation, signified by the "collect and collapse" process.Comment: 13 pages, 10 figures, 4 tables, accepted by Ap

Stochastic Estimation with Z2Z_2 Noise

We introduce a $Z_2$ noise for the stochastic estimation of matrix inversion and discuss its superiority over other noises including the Gaussian noise. This algorithm is applied to the calculation of quark loops in lattice quantum chromodynamics that involves diagonal and off-diagonal traces of the inverse matrix. We will point out its usefulness in its applications to estimating determinants, eigenvalues, and eigenvectors, as well as its limitations based on the structure of the inverse matrix.Comment: 6 pages, 1 postscript figure, UK/93-0