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The proton, one of the most well-known and basic building
blocks of matter, turns out to be holding on to a few secrets. A new
measurement found that the radius of the proton is about 4 percent smaller than
previously thought.

Protons are positively charged elementary particles.
Together with neutrons and electrons, they make up the atoms
that build our universe.

Scientists discovered the surprising anomaly by shooting
laser beams at an exotic version of a hydrogen atom, which most often consists
of one proton and one electron. The new measurement has improved the accuracy of
the known proton
radius by a factor of ten, the researchers said.

The finding means that either the theory governing how light
and matter interact (called quantum electrodynamics, or QED) must be revised,
or that a constant used in many fundamental calculations is wrong, the
researchers said.

The scientists detailed their discovery in the July 8 issue
of the journal Nature.

” The authors' measurement uses a novel method that is
more sensitive than any of the earlier methods,” wrote Jeff Flowers of the
U.K.'s National Physical Laboratory in an accompanying essay in the same issue
of Nature. ” But it gives a result that is significantly discrepant from
that obtained by the next most accurate method, throwing doubt on the QED
calculations that underlie both methods.”

Flowers was not involved in the new measurement.

Exotic hydrogen

In the experiment, the researchers used a special version of
hydrogen that contains one proton and one muon – an exotic
cousin of the electron that weighs about 200 times more than an electron.
The muon, just like an electron, is a point-like particle that orbits around
the more extended proton.

” Think of the proton as a fuzzy cloud of charge,”
described the lead researcher, Randolf Pohl of Germany's Max-Planck Institute
of Quantum Optics. ” It's not a hard surface, not a ball.”

In fact, the muon can even pass straight through the proton,
which contains lots of open space between its constituent building blocks -
three particles called quarks.

The muon can exist in different energy states that affect
the way it orbits the proton. The size of the proton affects these states and
how much energy is required to knock a muon out of one and into another.

And these effects are amplified by the larger mass of the
muon compared to an electron, allowing the researchers a chance to peer into
the orbital mechanics of the atom.

Blasting with lasers

To home in on the size of the proton, the scientists finely
tuned a laser
beam to blast their hydrogen atoms with very specific amounts of energy,
hoping to stimulate the muons to jump from one energy state to another.

For a long time, they observed no effect in the range they
expected, and assumed their laser was faulty. Finally the researchers tried an
energy range completely removed from the expected region, and found exactly the
transition they were looking for.

” When it wasn't in the reasonable region, we extended
our search region to the unreasonable, and then we had this indication of a
signal,” Pohl told LiveScience. ” We were really stunned.”

If the new value is confirmed, it could mean some rewriting
of basic
physics is in order.

Perhaps the value of the so-called Rydberg constant, which
is used to calculate the proton's size, is off. If that is the case, other
fundamental calculations will need revising, too.

Or, perhaps the entire theory describing this and other
particles – quantum electrodynamics – is misunderstood.

” If experimental discrepancies are confirmed rather
than errors being found, high-accuracy work such as that by Pohl and
colleagues, not the high-energy collisions of giant accelerators, may have seen
beyond the standard model of particle physics,” Flowers wrote.

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Original Story: Proton is Smaller Than Thought, New Measurement Finds
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