Advances in atomic physics, such as cooling and trapping of atoms and molecules and developments in frequency metrology, have added orders of magnitude to the precision of atom-based clocks and sensors. Applications extend beyond atomic physics and this article reviews using these new techniques to address important challenges in physics and to look for variations in the fundamental constants, search for interactions beyond the standard model of particle physics, and test the principles of general relativity.

Search for New Physics with Atoms and Molecules

3.3.3. Ionic Bonding or Interaction 5141 3.3.4. Multiple Hydrogen Bonding Interactions 5142 4. Stereospecific Living Radical Polymerization 5143 4.1. Acrylamides and Methacrylamides 5143 4.2. Methacrylates and Acrylates 5146 4.3. Vinyl Esters, Vinyl Amides, and Vinyl Chloride 5148 4.4. Stereoblock and Stereogradient Polymers 5149 4.4.1. Stereoblock Polymers 5149 4.4.2. Stereogradient Polymers 5151 5. Conclusions 5151 6. Acknowledgments 5152 7. References 5152

Stereospecific living radical polymerization: dual control of chain length and tacticity for precision polymer synthesis.

The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.

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Experimental progress in positronium laser physics

A sharp and spatially extended peak in an astrophysical gamma ray spectrum would provide very strong evidence for the existence of dark matter (DM), given that there are no known astrophysical processes that could mimic such a signal. From the particle physics perspective, perhaps the simplest explanation for a gamma ray peak is internal bremsstrahlung in DM annihilation through a charged t-channel mediator η close in mass to the DM particle χ. Since DM annihilation to quarks is already tightly constrained in this scenario, we focus here on the leptophilic case. We compute the electromagnetic anapole and dipole moments that DM acquires at 1-loop, and we find an interesting enhancement of these moments if the DM particle and the mediator are close in mass. We constrain the DM anapole and dipole moments using direct detection data, and then translate these limits into bounds on the DM annihilation cross section. Our bounds are highly competitive with those from astrophysical gamma ray searches. In the second part of the paper, we derive complementary constraints on internal bremsstrahlung in DM annihilation using LEP mono-photon data, measurements of the anomalous magnetic moments of the electron and the muon, and searches for lepton flavor violation.more » We also comment on the impact of the internal bremsstrahlung scenario on the hyperfine splitting of true muonium.« less

https://iopscience.iop.org/article/10.1088/1475-7516/2014/04/022/pdf

Loopy constraints on leptophilic dark matter and internal bremsstrahlung

Progress in the structure control of polymethacrylates and polyacrylates through stereospecific living polymerization are described. Three types of stereospecific living polymerizations have been developed for methacrylate polymerization: isotactic with t-C4H9MgBr, syndiotactic with ­t-C4H9Li/R3Al, and heterotactic with t-C4H9Li/bis(2,6-di-t-butylphenoxy)methylaluminium [MeAl(ODBP)2]. The last initiator system has been proved effective for monomer-selective living copolymerization of methacrylates. The living nature of these polymerizations allows extensive use for the syntheses of stereoregular block polymers and copolymers, and end-functionalized polymers such as macromonomers. Through stereospecific polymerizations and copolymerizations of macromonomers, comb polymers and graft copolymers with defined stereoregularities in the main chain and side chains could be obtained. Some properties of these stereoregular polymers are also described, including stereocomplex formation and solution viscosity. Stereospecific polymerizations of crotonates leading to diisotactic, diheterotactic and disyndiotactic polymers are also discussed. Supercritical fluid chromatography (SFC) has been proven to be useful for isolating uniform polymers from stereoregular poly(methyl methacrylate)s (PMMAs) with narrow molecular weight distribution. Uniform end-functionalized polymers have been used to construct more elaborate uniform polymer architectures such as stereoblock, star, and comb polymers, and copolymers. The uniform polymers have been proven quite useful for the studies of the relationship between structures and properties such as glass transition temperature, melting temperature and solution viscosity. Particularly interesting is the use of isotactic and syndiotactic uniform PMMAs for the understanding of stereocomplex formation in certain solvents such as acetone. Furthermore, a uniform stereoblock PMMA was found to undergo intramolecular complexation in addition to intermolecular complexation in acetone. Uniform star and comb PMMAs were also prepared and found useful for discussing the effect of branching on the solution viscosity.



© 2000 Society of Chemical Industry

Structurally controlled polymerizations of methacrylates and acrylates

New precision measurement of hyperfine splitting of positronium

The stereoregularity of poly(methyl acrylate) and poly(methyl acrylate-αd) was determined from the NMR spectra. A method of quantitative determination of stereoregularity of poly(methyl acrylate) proposed in this paper is based on the fact that in the 100 Mc./sec. NMR spectrum the absorption peaks due to methylene protons in syndiotactic configurations overlap absorptions due to only one of two methylene protons in isotactic configurations. The stereostructure of poly(methy1 acrylates) polymerized with anionic catalysts such as Grignard reagents, n-butyllithium, and LiAlH4 is generally richer in isotactic diads than in syndiotactic diads. For example, poly(methyl acrylate) polymerized with phenylmagnesium bromide as catalyst at −20°C. consists of 99% isotactic and 1% syndiotactic diads. In radical polymerization, the isotacticity of poly(methyl acrylate) is independent of polymerization temperature. Poly(methyl acrylates) polymerized with a Ziegler-Natta catalyst consisting of Al(C2H5)2Cl and VCl4 have configurations similar to those polymerized by radical initiators. The stereoregularity of poly(methyl acrylate-α-d) resembled that of poly(methyl acrylate) polymerized under the same conditions.

Stereoregularity of poly(methyl acrylate)

Measurement of positronium hyperfine splitting with quantum oscillation

Stereoregularity of polymethyl acrylate and polyacrylonitrile

l-Menthyl methacrylate (l-MnMA) and dl-MnMA and their mixtures were poly merized with benzoyl peroxide (BPO), phenylmagnesium bromide (PhMgBr), and lithium aluminum hydride (LiAlH4), as catalyst. The isotacticity of the polymers of dl-MnMA polymerized with PhMgBr was independent of the polymerization temperature and low. The tacticity of the polymers from the mixtures of l- and dl-MnMA's polymerized with BPO hardly changed with monomer compositions, while that with LiAlH4 showed increase with the increase of l-monomer contents. Thechange of the optical activity of PMnMA's with the stereoregularity is slight as compared with large variation of the tacticity of the polymers. Although both in radical and anionic polymerizations preference of one monomer to another monomer does not occur, in isotactic polymerization with LiAlH4 asymmetric induction of menthyl groups on the main chain configurations takes place. The tacticities of some poly(methacrylic acid esters) polymerized with PhMgBr under similar conditions were compared.

A. Ishida

Papers

New precision measurement of hyperfine splitting of positronium

Stereoregularity of poly(methyl acrylate)

Measurement of positronium hyperfine splitting with quantum oscillation

Stereoregularity of polymethyl acrylate and polyacrylonitrile

Stereoregularity of poly(methacrylic acid esters)