Thursday, July 29, 2010


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The proton, one of the primary components of matter, could be smaller than previously thought. This is the surprising result experimentally established by an international collaboration of physicists, in which the Laboratoire Kastler Brossel (ENS Paris / UPMC / CNRS) is actively involved. This new measurement of the radius of the proton, obtained with an extreme accuracy, could call into question certain predictions of quantum electrodynamics, one of the fundamental theories of quantum physics, or even the value of the Rydberg constant (the most accurate physical constant to date).

Published in Nature on 8 July, this work is featured on the journal's cover.

The nuclei of atoms are made up of protons and neutrons, around which electrons orbit. These three elements (protons, neutrons and electrons) constitute practically all of the Earth's matter. Whereas the electron is considered as a “sizeless” particle, the proton, which is composed of quarks, is an extended object. Until now, only two methods have been used to measure its radius. Based on the study of the interactions between a proton and an electron, they focus either on the collisions between an electron and a proton or on the hydrogen atom (constituted of an electron and a proton). The value obtained, and that used by physicists, is 0.877 femtometers (+/- 0.007).

In order to determine the radius of protons more accurately, the physicists used “muonic hydrogen” in which the electron is replaced by a muon, a negatively charged elementary particle. “This idea goes back to the 1970s”, explains François Nez, CNRS researcher at the Laboratoire Kastler Brossel (LKB). “However, techniques needed to improve in order to make it possible.” The hydrogen atom, which is the simplest of existing atoms, has often been the best object for studying fundamental questions in physics. But why replace the electron by a muon? Negatively charged, a muon is 200 times heavier than an electron. Therefore, according to the laws of quantum physics, it should move 200 times nearer the proton than an electron in “normal” hydrogen does. The muon is “much more sensitive” to the size of the proton than an electron. Consequently, its atomic binding energy is highly dependent on the size of the proton. The measurement of this energy allows scientists to determine the radius of the proton in a much more accurate manner (0.1 % accuracy) than measurements using electrons (around 1 % accuracy).

To achieve this, an infrared laser had to be specifically designed. The six LKB researchers, from CNRS and UPMC, mainly provided their expertise in its manufacturing, essentially with regard to the “titanium-sapphire” part of the laser chain. The objective was to design a laser in which the emission wavelength (in other words the color of the laser light) can be adjusted at will. Since a muon disintegrates in 2 millionths of a second, it is necessary to be able to carry out the measurement on the muonic hydrogen during this very short lapse of time. The laser shot therefore needs to be triggered very rapidly (in around 1 millionth of a second). A first measurement campaign at the end of 2002 allowed the experimental set up developed by LKB to be put through its paces. LKB was also responsible for measuring the emission wavelength of the complete laser system. This involved targeting the different wavelengths absorbed by the muonic hydrogen one by one, making it possible to deduce the energy of the muon around the proton and thus the size of the proton.

After several series of measurements conducted with the accelerator of the Paul Scherrer Institute (PSI) in Switzerland, where the beam of muons is particularly intense , the researchers obtained an unexpected value for the radius of the proton. In fact, this result differs from that obtained using electrons. It amounts to 0.8418 femtometers (+/- 0.0007) instead of 0.877 femtometers for measurements using electrons. “We did not envisage that there could be any divergence between known values and our measurements”, points out LKB director Paul Indelicato. This difference is far too big to be put down to measurement inaccuracies and the team of scientists is currently attempting to explain this discrepancy. It could call into question the most accurately tested theory in physics, namely the theory of quantum electrodynamics, which is one of the cornerstones of modern physics. Another possibility is that the current value of the Rydberg constant, the physical constant determined with the greatest accuracy so far, could need to be revised. The researchers plan to repeat this experiment in the near future with muonic helium (instead of hydrogen), which could shed new light on these unexpected results.



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The key to losing weight could lie in manipulating our beliefs about how filling we think food will be before we eat it, suggesting that portion control is all a matter of perception.

BBSRC-funded studies showed that participants were more satisfied for longer periods of time after consuming varying quantities of food when they were led to believe that portion sizes were larger than they actually were.

Memories about how satisfying previous meals were also played a causal role in determining how long they staved off hunger. Together, these results suggest that memory and learning play an important role in governing our appetite.

In the first experiment, participants were shown the ingredients of a fruit smoothie. Half were shown a small portion of fruit and half were shown a large portion. They were then asked to assess the 'expected satiety' of the smoothie and to provide ratings before and three hours after consumption. Participants who were shown the large portion of fruit reported significantly greater fullness, even though all participants were given the same quantity of fruit.

In a second experiment, researchers manipulated the 'actual' and 'perceived' amount of soup that people thought that they had consumed. Using a soup bowl connected to a hidden pump beneath the bowl, the amount of soup in the bowl was increased or decreased as participants ate, without their knowledge. 3 hours after the meal, it was the perceived (remembered) amount of soup in the bowl and not the actual amount of soup consumed that predicted post-meal hunger and fullness ratings.

The findings, which will be presented by researchers from the University of Bristol at this month's annual conference of the Society for the Study of Ingestive Behaviour (SSIB), could have implications for more effective labelling of diet foods.

"The extent to which a food can alleviate hunger is not determined solely by its physical size, energy content, and so on. Instead, it is influenced by prior experience with a food, which affects our beliefs and expectations about satiation. This has an immediate effect on the portion sizes that we select and an effect on the hunger that we experience after eating," said Dr Jeff Brunstrom, Reader in Behavioural Nutrition at Bristol university's Department of Experimental Psychology.

"Labels on 'light' and 'diet' foods might lead us to think we will not be satisfied by such foods, possibly leading us to eat more afterwards," added Dr Brunstrom. "One way to militate against this, and indeed accentuate potential satiety effects, might be to emphasise the satiating properties of a food using labels such as 'satisfying' or 'hunger relieving'."



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Don't scoff at those lucky rabbit feet. New research shows that having some kind of lucky token can actually improve your performance – by increasing your self-confidence.

"I watch a lot of sports, and I read about sports, and I noticed that very often athletes – also famous athletes – hold superstitions," says Lysann Damisch of the University of Cologne. Michael Jordan wore his college team shorts underneath his NBA uniform for good luck; Tiger Woods wears a red shirt on tournament Sundays, usually the last and most important day of a tournament. "And I was wondering, why are they doing so?" Damisch thought that a belief in superstition might help people do better by improving their confidence. With her colleagues Barbara Stoberock and Thomas Mussweiler, also of the University of Cologne, she designed a set of experiments to see if activating people's superstitious beliefs would improve their performance on a task.

In one of the experiments, volunteers were told to bring a lucky charm with them. Then the researchers took it away to take a picture. People brought in all kinds of items, from old stuffed animals to wedding rings to lucky stones. Half of the volunteers were given their charm back before the test started; the other half were told there was a problem with the camera equipment and they would get it back later. Volunteers who had their lucky charm did better at a memory game on the computer, and other tests showed that this difference was because they felt more confident. They also set higher goals for themselves. Just wishing someone good luck – with "I press the thumbs for you," the German version of crossing your fingers – improved volunteers' success at a task that required manual dexterity. The research is published in Psychological Science, a journal of the Association for Psychological Science

Of course, even Michael Jordan lost basketball games sometimes. "It doesn't mean you win, because of course winning and losing is something else," says Damisch. "Maybe the other person is stronger."

Psychological Science




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