[ExI] proton measurement upset

[Damien Broderick]

On 7/8/2010 2:04 PM, Gregory Jones wrote:

> A touch smaller?  4% is huuuge.  Did they mean 4 parts per billion?

Nope.

> I know it is premature judgment and all that, but this result must be
> wrong, waaaaay wrong.

<http://www.nature.com/news/2010/100707/full/news.2010.337.html>

The proton seems to be 0.00000000000003 millimetres smaller than 
researchers previously thought, according to work published in today's 
issue of Nature1.

The difference is so infinitesimal that it might defy belief that 
anyone, even physicists, would care. But the new measurements could mean 
that there is a gap in existing theories of quantum mechanics. "It's a 
very serious discrepancy," says Ingo Sick, a physicist at the University 
of Basel in Switzerland, who has tried to reconcile the finding with 
four decades of previous measurements. "There is really something 
seriously wrong someplace."

Protons are among the most common particles out there. Together with 
their neutral counterparts, neutrons, they form the nuclei of every atom 
in the Universe. But despite its everday appearance, the proton remains 
something of a mystery to nuclear physicists, says Randolf Pohl, a 
researcher at the Max Planck Institute of Quantum Optics in Garching, 
Germany, and an author on the Nature paper. "We don't understand a lot 
of its internal structure," he says.

 From afar, the proton looks like a small point of positive charge, but 
on much closer inspection, the particle is more complex. Each proton is 
made of smaller fundamental particles called quarks, and that means its 
charge is roughly spread throughout a spherical area.

Physicists can measure the size of the proton by watching as an electron 
interacts with a proton. A single electron orbiting a proton can occupy 
only certain, discrete energy levels, which are described by the laws of 
quantum mechanics. Some of these energy levels depend in part on the 
size of the proton, and since the 1960s physicists have made hundreds of 
measurements of the proton's size with staggering accuracy. The most 
recent estimates, made by Sick using previous data, put the radius of 
the proton at around 0.8768 femtometres (1 femtometre = 10-15 metres).

Small wonder

Pohl and his team have a come up with a smaller number by using a cousin 
of the electron, known as the muon. Muons are about 200 times heavier 
than electrons, making them more sensitive to the proton's size. To 
measure the proton radius using the muon, Pohl and his colleagues fired 
muons from a particle accelerator at a cloud of hydrogen. Hydrogen 
nuclei each consist of a single proton, orbited by an electron. 
Sometimes a muon replaces an electron and orbits around a proton. Using 
lasers, the team measured relevant muonic energy levels with extremely 
high accuracy and found that the proton was around 4% smaller than 
previously thought.

That might not sound like much, but the difference is so far from 
previous measurements that the researchers actually missed it the first 
two times they ran the experiment in 2003 and 2007. "We thought that our 
laser system was not good enough," Pohl says. In 2009, they looked 
beyond the narrow range in which they expected to see the proton radius 
and saw an unmistakable signal.

"What gives? I don't know," says Sick. He says he believes the new 
result, but that there is no obvious way to make it compatible with 
years of earlier measurements.

"Something is missing, this is very clear," agrees Carl Carlson, a 
theoretical physicist at the College of William & Mary in Williamsburg, 
Virginia. The most intriguing possibility is that previously undetected 
particles are changing the interaction of the muon and the proton. Such 
particles could be the 'superpartners' of existing particles, as 
predicted by a theory known as supersymmetry, which seeks to unite all 
of the fundamental forces of physics, except gravity.

But, Carlson says, "the first thing is to go through the existing 
calculations with a fine tooth comb". It could be that an error was 
made, or that approximations made in existing quantum calculation simply 
aren't good enough. "Right now, I'd put my money on some other 
correction," he says. "It's also where my research time will be going 
over the next month."

     *
       References
          1. Pohl, R. et al. Nature 466, 213-217 (2010). | Article