Nano-Biotechnology

Protons Spin Around the World

Missing quantum momentum

By Gabriel Gache, Science News Editor

18th of December 2007, 08:46 GMT

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Artistic impression of the proton's spin in relation with that of the constituent quarks

One of the most intriguing physical puzzles related to quantum physics and particle physics is a property of the protons, which all elementary particles share, called spin. The spin number, or quantum momentum, represents the particle's rotation around its own axis, but unlike the gyroscopic instruments and spinning top toys, the spin of an elementary particle determines certain properties of the world we experience every day.

All the elementary particles have this property that is responsible for the magnetic interactions and determines the stability of the universe. However, physicists have some difficulties when it comes to measuring the proton's spin. This may be due to the fact that protons are not true elementary particles, but composite ones formed of other elementary particles. For example, each proton consists of three quarks bound together by gluons.

While trying to measure the spin of the proton physicists observed that 30 percent of the quantum momentum predicted was missing. The spin of a composite particle is determined by the spin of the constituent particles. Therefore, the current models predict that about 60 percent of the spin should be produced by the quarks while the remaining 40 percent would originate in the binding gluons. Nevertheless, it seems that 30 percent of the quantum momentum produced by the quarks cannot be observed.

Experiments revealed that this might be the result of inaccurate interpretation; however, in reality there is no measurement error. This would mainly be generated by the influence of the gluons' spin, shielding the quark spin during the gluon polarization process, thus preventing the measurement of the quark spin, which results in inaccurate calculations.

But the theory behind this explanation has flaws, as recent experiments showed that the gluon polarization phenomenon is not strong enough to be responsible for the whole 30 percent missing in the proton's spin, thus further more accurate experiments and calculations are needed in order to create a new theory to explain the current observations.

TAGS:

protons | elementary particles | quantum momentum | string theory | quantum physics

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