American Institute of Physics
SEARCH AIP
home contact us sitemap
Physics News Update
Number 547, July 12, 2001 by Phil Schewe, James Riordon, and Ben Stein

CP Violation in the Decay of B Mesons

CP violation in the decay of B mesons has now definitively been observed at SLAC. CP is an abstract abbreviation for a mathematical operation in which particles undergo a change in charge conjugation (C) and parity (P). The combined CP operation essentially turns a particle into an antiparticle.

To say that CP symmetry is violated is to say that the physical properties of particles and antiparticles are not fully the same, a fact discovered first almost 40 years ago in the decay of K mesons. Since then B factories at SLAC and the Japanese KEK lab, where colliding electrons and positrons produce copious amounts of B mesons, have zeroed in on producing the same result with the rarer B mesons.

Both groups submitted preliminary results in February of this year (Update 525) with very limited data sets. Now SLAC is offering a more robust measurement of the CP-violating parameter, referred to as sine (2beta); the value it reports is 0.59 with an uncertainty of 0.14. (SLAC press release, 6 July 2001 and paper submitted to Physical Review Letters.)

Why Don't Circadian Rhythms Coincide With the 24-Hour Day?

Found almost ubiquitously in living organisms, circadian rhythms are generated by body clocks with natural periods ranging roughly from 22 to 28 hours. However, they rarely match the 24-hour length of a day (hence the word "circadian," meaning "about a day"). This is strange, since species whose body clocks are resonant with the Earth's 24-hour cycle of light intensity and temperature variations ought to be the best ones for adapting to the environment and organizing daily activities.

To address this issue, a physicist in Japan (Hiroaki Daido, Kyushu Institute of Technology, now at the University of Osaka Prefecture, daido@ms.osakafu-u.ac.jp, 011-81-722-54-9366) has devised a mathematical model that explores competition between species with body clocks of different periods. Daido makes two major assumptions in his model: First, the population growth rate of a species depends on the time difference between its body clock period and the 24-hour day (for example, a creature vulnerable to harmful ultraviolet rays during the day will have a maximum growth rate if it has a 24-hour cycle and therefore stays perfectly nocturnal). Second, the amount of competition between pairs of species becomes more severe with a smaller time difference between their body clocks (two species looking for food will have an easier time if they do it 12 hours apart).

Daido's model shows in particular that a 24-hour body clock can actually turn out to be a disadvantage as long as the benefits of being in sync with the environment are not large enough. That's because competition with other species turns out to be most intense for species with 24-hour body clocks. Daido's model can also address other biological rhythms such as circannual rhythms which control, for example, animals' hibernation timings.

However, he points out that other factors, such as the effects of natural disasters, may also have contributed to the existing circadian rhythms, and he calls for testing the results of his model with biological observations and experiments. (Daido, Physical Review Letters, 23 July 2001; text at Physics News Select).

Caution: Slippage Might Occur

Caution: slippage might occur in tightly confined fluids. Fluid mechanics is one of the most mature and successful branches of physics---a study vital for understanding phenomena ranging from ocean currents to the lift generated by butterfly wings. It would likely startle many physicists to learn that one of the venerated discipline's fundamental assumptions is sometimes wrong.

But that's exactly what a group of researchers from the Australian National University found in an experimental test of fluid behavior in small spaces. It has long been thought that the fluid molecules adjacent to a surface are stationary, regardless of the motion of the rest of the fluid. This no-slip boundary condition, as the assumption is known, leads to precise and detailed descriptions of nearly all fluid mechanical systems.

Recently, however, a number of studies have suggested that the assumption breaks down for flow in confined spaces, such as the insides of capillaries or the channels of microfluidic chips used in cutting-edge bioanalysis. The Australian researchers (V. S. J. Craig, vince.craig@anu.edu.au, 61-2-6125-3359) set out to explicitly test the age-old conjecture by measuring the motion of a ten micron silica sphere as they drove it through liquid toward a flat wall. Subsequent analysis showed that a fluid model incorporating boundary slip explained the data better than the classical no-slip model. No-slip boundaries are still close enough to the truth in most circumstances.

The new study, however, reveals that descriptions of the blood moving through our capillaries, lubricants in nanomachines, and flow in other tiny systems must include boundary slip conditions to achieve precision at such small scales. (V. S. J. Craig et al., Physical Review Letters, 30 July; text at Physics News Select).