Biomedical subjects

L B Weinstein

Publications and source records attributed to L B Weinstein.

At least 19 recordsLinked to original sources

PubMed2006

Measurement of two- and three-nucleon short-range correlation probabilities in nuclei.

The ratios of inclusive electron scattering cross sections of 4He, 12C, and 56Fe to 3He have been measured at 1 < xB <. At Q2 > 1.4 GeV2, the ratios exhibit two separate plateaus, at 1.5 < xB < 2 and at xB > 2.25. This pattern is predicted by models that include 2- and 3-nucleon short-range correlations (SRC). Relative to A = 3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A = 4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.

Journal Article↗

PubMed2005

Recoil polarization for delta excitation in pion electroproduction.

We measured angular distributions of recoil-polarization response functions for neutral pion electroproduction for W = 1.23 GeV at Q(2) = 1.0 (GeV/c)(2), obtaining 14 separated response functions plus 2 Rosenbluth combinations; of these, 12 have been observed for the first time. Dynamical models do not describe quantities governed by imaginary parts of interference products well, indicating the need for adjusting magnitudes and phases for nonresonant amplitudes. We performed a nearly model-independent multipole analysis and obtained values for Re (S(1+)/M(1+)) = -(6.84 +/- 0.15)% and Re (E(1+)/M(1+)) = -(2.91 +/- 0.19)% that are distinctly different from those from the traditional Legendre analysis based upon M1+ dominance and ll(pi) < or = 1 truncation.

Journal Article↗

PubMed2004

Two-nucleon momentum distributions measured in 3He(e,e'pp)n.

We have measured the 3He(e,e'pp)n reaction at 2.2 GeV over a wide kinematic range. The kinetic energy distribution for "fast" nucleons (p>250 MeV/c) peaks where two nucleons each have 20% or less, and the third nucleon has most of the transferred energy. These fast pp and pn pairs are back to back with little momentum along the three-momentum transfer, indicating that they are spectators. Calculations by Sargsian and by Laget also indicate that we have measured distorted two-nucleon momentum distributions by striking one nucleon and detecting the spectator correlated pair.

Journal Article↗

PubMed2003

Polarization transfer in the 4He(e-->,e'p-->)3H reaction up to Q2=2.6 (GeV/c)2.

We have measured the proton recoil polarization in the 4He(e-->,e(')p-->)4H reaction at Q(2)=0.5, 1.0, 1.6, and 2.6 (GeV/c)(2). The measured ratio of polarization transfer coefficients differs from a fully relativistic calculation, favoring the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model. In addition, the measured induced polarizations agree reasonably well with the fully relativistic calculation indicating that the treatment of final-state interactions is under control.

Journal Article↗

PubMed2003

Cross-section measurement of charged-pion photoproduction from hydrogen and deuterium.

We have measured the differential cross section for the gamman-->pi(-)p and gammap-->pi(+)n reactions at theta(c.m.)=90 degrees in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at E(gamma) greater, similar 3.3 GeV exhibit a global scaling behavior for both pi(-) and pi(+) photoproduction, consistent with the constituent counting rule and the existing pi(+) photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive pi(-) to pi(+) photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.

Journal Article↗

PubMed2001

Relativistic effects and two-body currents in (H)((-->)e(')p)n using out-of-plane detection.

Measurements of the (2)H((-->)e,e(')p)n reaction were performed with the out-of-plane magnetic spectrometers (OOPS) at the MIT-Bates Linear Accelerator. The longitudinal-transverse, f(LT) and f(')(LT), and the transverse-transverse, f(TT), interference responses at a missing momentum of 210 MeV/c were simultaneously extracted in the dip region at Q2 = 0.15 (GeV/c)(2). In comparison to models of deuteron electrodisintegration, the data clearly reveal strong effects of relativity and final-state interactions and the importance of two-body meson-exchange currents and isobar configurations. We demonstrate that such effects can be disentangled by extracting these responses using the novel out-of-plane technique.

Journal Article↗

PubMed2001

Search for quadrupole strength in the electroexcitation of the delta+(1232).

High-precision 1H(e,e'p)pi(0) measurements at Q2 = 0.126 (GeV/c)2 are reported, which allow the determination of quadrupole amplitudes in the gamma*N-->Delta transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I = 3/2) amplitude ratios, R(SM) = (-6.5+/-0.2(stat+sys)+/-2.5(mod))% and R(EM) = (-2.1+/-0.2(stat+sys)+/-2.0(mod))%, are dominated by model error. Previous R(SM) and R(EM) results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

Journal Article↗

PubMed2000

Mothers and methadone.

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Analgesics, Opioid↗

PubMed1999

Guided tours: from precursor snoRNA to functional snoRNP.

Small nucleolar RNAs (snoRNAs) use base pairing to guide modification of conserved nucleotides in functionally important regions of ribosomal RNA. The box C/D snoRNAs direct 2'-O-methylation and the box H/ACA snoRNAs direct pseudouridylation. Each snoRNA interacts with proteins, many of them newly identified. Progress in understanding how snoRNA sequences are stored within genomes, liberated from precursor molecules and targeted to the nucleolus has begun to elucidate each step in the biogenesis of these critical contributors to ribosome formation.

Animals↗

PubMed1997

A second catalytic metal ion in group I ribozyme.

Although only a subset of protein enzymes depend on the presence of a metal ion for their catalytic function, all naturally occurring RNA enzymes require metal ions to stabilize their structure and for catalytic competence. In the self-splicing group I intron from Tetrahymena thermophila, several divalent metals can serve structural roles, but only Mg2+ and Mn2+ promote splice-site cleavage and exon ligation. A study of a ribozyme reaction analogous to 5'-splice-site cleavage by guanosine uncovered the first metal ion with a definitive role in catalysis. Substitution of the 3'-oxygen of the leaving group with sulphur resulted in a metal-specificity switch, indicating an interaction between the leaving group and the metal ion. Here we use 3'-(thioinosylyl)-(3'-->5')-uridine, IspU, as a substrate in a reaction that emulates exon ligation. Activity requires the addition of a thiophilic metal ion (Cd2+ or Mn2+), providing evidence for stabilization of the leaving group by a metal ion in that step of splicing. Based on the principle of microscopic reversibility, this metal ion activates the nucleophilic 3'-hydroxyl of guanosine in the first step of splicing, supporting the model of a two-metal-ion active site.

Animals↗

PubMed1995