Physics News Update

Article Tools

Enlarge text

Shrink text
Print this page

E-mail

Subscribe
E-mail alert
RSS feed

Save and Share
Save to Digg

Digg this

Del.icio.us
Save to Furl

Furl

Number 525, February 13, 2001 by Phil Schewe, James Riordon, and Ben Stein

CP Violations in B Mesons: First Official Experimental Results

B mesons and anti-B mesons have the same intrinsic lifetimes, but subtle differences in the way they decay is indicative of a preference in the overall scheme of things for matter over anti-matter, at least in our sector of the universe. The explanation is believed to arise from certain basic symmetries, or lack thereof, in the way particle interactions occur. One such symmetry is called parity (P): a physical interaction ought to be the same whether viewed directly or in a (three-dimensional) mirror. A second important symmetry proposition, called charge conjugation (C), says that an interaction should be the same even if we replace all the participating particles with antiparticles.

Experiments in the 1950s and 60s showed that interactions via the weak nuclear force not only can violate the C and P propositions, but even the combined CP symmetry, and it is this fact, like a rarely expressed defective gene, which over cosmological time leads to the apparent large-scale extinction of antimatter. CP violation was first studied in the 60s with the asymmetrical decays of K mesons, which possess strange quarks (for the latest on CP violation in K's see Update 420).

Theorists believe that CP violation in B mesons (carrying a much heavier quark, the bottom quark) should be more prominent, although the Bs themselves are harder to manufacture than Ks. Two years ago Fermilab issued a rudimentary measurement of CP violation in B mesons based on rare events culled from proton-antiproton collisions (Update 405). At the B Factory at SLAC producing B and anti-B mesons is the main business. Now the scientists at the BaBar Detector at the B Factory have just released their first official results in a seminar at SLAC (contact Stewart Smith, 650-926-4775, ajsmith@slac.stanford.edu; web site), and they constitute the best evidence yet for CP violation in B mesons.

The chief parameter used to indicate CP violation is called sin (2 beta), and BaBar's measured value is 0.34, with an uncertainty of 0.20, an accuracy about twice as good as the previous value (Update 497). Results from the BELLE detector group at the KEK lab in Japan, the other premier lab dedicated to studying B mesons, are also just now available; a preprint cites a value for sin (2 beta) of 0.58, with an uncertainty of about 0.33 (http://www.lanl.gov/list/hep-ex/new). A value of zero would have implied that there were no CP violation. The uncertainties in these early measured values would therefore preclude a definite statement on the size of CP violation or on any likely agreement with theoretical estimates. Both BaBar and Belle have submitted their work to Physical Review Letters, which (pending approval) plans to publish them in the same issue.

Micro-Coliseums of Light

Using laser light, physicists have created "optical billiards" for gas atoms, traps in which atoms bounce back and forth like balls on a billiard table. With shapes ranging from circles to wedges, such arrangements provide "coliseums" for testing important ideas in physics. Groups at the University of Texas at Austin (Mark Raizen, 512-471-4753, raizen@physics.utexas.edu) and the Weizmann Institute of Science in Israel (Nir Davidson, 011-972-8-9342034, fedavid@wisemail.weizmann.ac.il) use rapidly scanning laser beams in two dimensions to draw out the desired patterns. Laser light induces electric dipoles, or a separation of electrical charge, to occur in the atoms; the dipoles in turn cause the atoms to become repelled from certain regions of the light beams' electrical field which correspond to the "walls" of the coliseum.

By studying how the trajectory of the atomic atoms depends on the shape of the billiard table, both groups tested aspects of classical chaos theory. They probed atomic trajectories indirectly, by creating a little hole in the optical billiard and measuring the escape rate of the atoms. Trapping ultracold cesium atoms in a micron-scale V-shaped wedge whose vertex faced downward toward the ground, the Texas researchers confirmed theoretical predictions that the trajectory of the atoms shifted from stable to chaotic as the angle of the vertex was changed. Confining rubidium atoms in micron-scale billiard tables oriented perpendicular to the ground, the Weizmann group found that circle and ellipse shapes promoted stable, non-chaotic motion, while a "tilted stadium," consisting of two half circles connected by two non-parallel straight lines, caused the atoms to exhibit essentially chaotic motion.

In future studies, both teams plan to use optical billiards to test such things as quantum chaos and the effects of noise on the trajectories of atoms. (Milner et al. and Friedman et al., Physical Review Letters, 19 Feb. 2001; text at Physics News Select).

Atomic Slide Puzzles

Vacancies in crystal surfaces are holes where atoms are missing from otherwise regular and uniform crystal lattices. Scientists have suspected for some time that vacancies are responsible for motion in crystals as the holes trade places with atoms, leading to atom-sized bubbles that percolate across crystal facets.

Now, researchers in the Netherlands (R. van Gastel, Universiteit Leiden, 011-3171-527-5700, gastel@phys.leidenuniv.nl) have managed to measure vacancy motion in a copper crystal, and they found that the holes are surprisingly mobile. The discovery has important implications for the semiconductor industry and technologies that rely on tiny surface structures that may be gradually destroyed through vacancy mediated motion.

The researchers used a scanning tunneling microscope (STM) to study vacancy motion by monitoring the positions of Indium atoms embedded in a copper lattice. Because vacancies move rapidly, changing places with atoms roughly a hundred million times each second at room temperature, comparatively slow STMs cannot image vacancies directly. Instead, the researchers calculated vacancy motion by tracking the positions of the indium atoms. From one image to another, indium atoms exhibited long jumps which result from multiple vacancy interactions. Essentially, the indium atoms move across the copper crystal in much the same way that individual pieces may be maneuvered from one place to another in toys known as slide puzzles (see image at Physics News Graphics).

Although high vacancy mobility may be bad news for manufacturers of microstructures, the new insights will potentially help to optimize crystal growing procedures vital to the semiconductor industry. In future work, the researchers plan to create vacancies artificially by selectively removing atoms from a chilled crystal surface. Provided that crystal is sufficiently cooled, the vacancies should move slowly enough to show up in STM images. (R. van Gastel et al., Physical Review Letters, 19 February 2001; text at Physics News Select).