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Shrinking Protons

July 13, 2010

If the results of new study  holds then the size of proton is smaller than what it was thought to be or measured in earlier experiments, roughly 4% smaller. This can have serious implications on the quantum mechanic theories itself. The experiment the group of Randolf Pohl at Max Planck Institute, Munich, Germany, performed was very neat, even though the possibility of any mistake or missing out on some factor while calculating the radius of proton cannot be ruled out.

Protons along with neutrons form the nuclei of atom. While protons are very common particle and has been studied for a while, still not everything is clearly understood about them, especially the inernal structure. Protons are positively charged particles which further consists of smaller fundamental particles called quarks. The charge is roughly spread around in a spherical area and when we say the size of proton, it’s the size of this charge cloud. Proton size is extremely small, roughly 10,000 times smaller than orbit of the electrons.

So, how do we go about measuring the size of proton which is so tiny- spectroscopic measurement is the answer. Scientists can measure the size of the proton by meausring the interactions of electron with proton. According to quantum mechanics, the electrons orbitting around protons can occupy certain discrete energy levels and the size of the proton also contributes in determining these energy levels. Many such measurements have been performed to measure the size of proton, the most recent estimate being 0.8768 femtometers [Nature].

In the current sudy, researchers measured the size of protons to be 0.84184 fm which is 10 times more precise than previous estimates, but numbers don’t match. In order to do so, they created an exotic version of Hydrogen, muonic Hydrogen, which is exactly like Hydrogen with the lone electron replaced by muon. Muons have same charge as electrons but they are bout 200 times heavier than electrons, thus they orbit more close to the proton and are smaller in size and thereby interactions between them are more sensitive and pronounced and more dominated by the protons. To create muonic hydrogen, they bombarded muons at hydrogen and roughly 1% of hydrogen was converted to muonic hydrogen.

As soon as the muonic hydrogen was formed, they performed spectroscopic measurements to measure the Lamb shift  which ultimately can be used to measure the size of proton. Lamb shift is a very important phenomena in quantum mechanics. Basically, simple QED says that the 2S and 2P energy levels will have same energy, but in 1947 it was discovered by Rutherford and Lamb that it’s not so and there are energy differences between the two due to result of various interactions including the effect of proton size. In muonic hydrogen, this effect or Lamb shift is more pronounced. When laser is fired at munoic hudrogen, the electron gets excited and jump to higher energy level and thus they can measure the energy difference between the 2S and 2P levels, the Lamb shift, which ultimately can be used to estimate the size of proton.

A short time after the atoms are created, they blast in a pulse of laser light with its frequency tuned close to the frequency corresponding to the splitting between the 2S and 2P states. The 2P state has a very short lifetime, so any atoms that get excited by the laser will decay very quickly, and emit an x-ray in the process (because the energy difference between the ground state and the 2P state is enormous, thanks to the heavy muon). When the laser is tuned to exactly the right frequency, they see lots of x-rays from decaying atoms. When it’s a little bit off, the number of x-rays drops off dramatically. Then they just need to measure the laser frequency, and they get the Lamb shift directly. And with a bit of math, they can convert that to a measurement of the proton size. [Science Blogs ]

Something can be wrong in these experiments or earlier experiments including some gaps in the theory itself, so more experiments need to be carried out to clarify this discrepancy. But the experiment is very neat which I liked a lot. Hope this study and further studies will result in providing better picture of proton size and inner structure.

Reference: Pohl, R. et al. Nature 466, 213-217 (2010). | Article

Picture credit: Flickr user fatllama (picture not from this experiment) Used under creative commons license.

Filed under: Research,Science

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4 Comments Leave a Comment

  • 1. Chun-Xuan Jiang  |  July 31, 2010 at 2:46 am
    In 1996 Jiang counts proton radius is 0.015 femtometre

  • 2. Chun-Xuan Jiang  |  July 31, 2010 at 2:53 am

    Jiang,C-X.determination of proton and neutron radii,Apeoron,3,Nr.3-4,126(1996)
    Jiang counts proton and neutron radii are 0.015 femtometres

  • 3. Chun-Xuan Jiang  |  August 16, 2010 at 10:09 pm

  • 4. Chun-Xuan Jiang  |  August 16, 2010 at 10:10 pm

    many physical problems are false,because physicists do not consider tachyonic theory

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