Inconsistencies in Proton Size have Scientists Puzzled
Speaking at the American Physical Society on April 13th, scientists say they need to conduct further research to figure out why recent calculations of proton size aren't matching old ones.
Randolf Pohl, a researcher at Max Planck Institute of Quantum Optics says, "The discrepancy is rather severe." The main query Pohl and his co-workers are wondering about is whether the explanation for this discrepancy is as simple as someone messing up the measurements - or whether this mismatch will lead to new theories in physics.
The proton is shrinkingProtons are positively charged particles located in the nucleus of an atom. They are understood to be the foundation of everything from inanimate objects in nature to life in general. Years of research and calculations resulted in giving the proton a static size of 0.8768 femtometers (millionth of a billionth of a meter) in radius.
That number changed in 2009 when scientists used a new method and found that the measurement reduced to 0.84087, which is approximately a difference in radius of 4 percent.
The previous research had used electrons, which are essentially negatively charged particles that orbit the nucleus, to determine the radius of a proton. Making measurements with electrons can enable researchers to pick one of two options. Either they can fire a bunch of electrons at a bunch of protons to see how the electrons get deflected. This scattering method provides an in-depth look into the size of the proton.
The alternative is to move the electron. Electrons fly around the nucleus of an atom at different levels. These levels are known as orbitals. Protons also reside in this general area. An electron can jump from one orbital to the next by decreasing or increasing it's energy, which is done through gaining or losing a photon, a basic particle of light. The exact amount of energy used to move an electron between orbitals tells scientists how much power the proton has, thus resulting in a method to figure out the proton's size.
However, Pohl and his fellow researchers lead this experiment without using electrons at all. Instead, they used muons, which is another negatively charged particle. A muon is two hundred times heavier than electrons, and so they orbit protons two hundred times closer. This makes it much easy for scientists to figure out which general orbit a muon revolves around, making for a more sensitive and accurate measurement of proton size.
Pohl said, "The muon is closer to the proton and it has a better view."
Possible explanationsThese sensitive measurements using muons is the reason the scientists stumbled across this totally unexpected discovery and got a smaller-than-expected number for the size of the proton. Physicists are now trying their best to figure out and explain the discrepancy.
The possibility that the measurements were simply wrong is most probable. Pohl calls this the "boring explanation," though not all scientists necessarily agree.
Jan Bernauer, a physicist at Massachusetts Institute of Technology, says "I would say it's not the experimental side."
If human error turns out to be the main cause of this inconsistency, issues with calculations need to be remedied because, as Bernauer explained, "we actually know everything that goes on but we are just not calculating it quite right."
Most exciting possibility in this discovery is that it could quite possibly reveal some new theories in the laws of physics previously unexplained by the general physics theory, which is the Standard Model. Quite possibly, there is something we don't know about how electrons and muons interact with particles.
Another possibility might involve other unknown particles we don't know about that pull and carry force with other particles, which could be an explanation for the measurement discrepancies.
What's NextPhysicists are not scrambling to launch new sets of experiments across multiple labs to figure out what's going on. One major method of research involves testing the scattering of electrons as mentioned earlier just to make sure the results were recorded properly.
Another experiment involves repeating scatter experiments, but with firing muons at the proton instead of electrons. This project called the "Muon Scattering Experiment," is going to be conducted at the Paul Scherrer Institute in Switzerland. The labs will allow researchers to simultaneously execute and measure muon and electron scattering all in one experiment.
Arrington said, "The hope is that on the electron-scattering side, we'll have double-checked all the things that are challenging in these measurements. If we still have this discrepancy, we'll be able to fill in this last box and look at the muon-scattering and see, independent of how you make the measurement, do electrons and muons give you something different?"
The plan is to start collecting data in that experiment in 2015 or 2016, Arrington said, meaning the size of the proton will remain in limbo for a little longer.
The plan to start experimenting and collecting data for this experiment begins in 2015 or 2016, which means the size of a proton will have to stay a mystery for a bit longer.
Arrington said, "It's not easy. We hope to do it in a little less than 10 years, but maybe we're being optimistic."