From: as on
Source: National Geographic
http://news. nationalgeograph ic.com/news/ 2010/07/100707- science-
proton- smaller-standard -model-quantum- physics/

Proton Smaller Than Thought—May Rewrite Laws of Physics

Scientists "totally surprised" by "significant shake-up."

Kate Ravilious

for National Geographic News <http://news. nationalgeograph ic.com/
news>

Published July 7, 2010

*Protons, among the building blocks of atoms, are even smaller than
we
thought—and the unexpected discovery may alter some of the most
trusted
laws of physics.*

All atoms are made up of nuclei orbited by electrons. The nuclei, in
turn, are made of neutrons and protons, which are themselves made of
particles called quarks. (Related: "'God Particle' May Be Five
Distinct
Particles, New Evidence Shows."
<http://news. nationalgeograph ic.com/news/ 2010/06/100616- large-
hadron- collider- lhc-higgs- boson-god- particle/>)

For years the accepted value for the radius of a proton has been
0.8768
femtometers, where a femtometer equals one quadrillionth of a meter.

The size of a proton is an essential value in equations that make up
the
60-year-old theory of quantum electrodynamics, a cornerstone of the
Standard Model of particle physics. The Standard Model describes how
all
forces, except gravity, affect subatomic particles. (See "Einstein's
Gravity Confirmed on a Cosmic Scale."
<http://news. nationalgeograph ic.com/news/ 2010/03/100310- einstein-
theory-general- relativity- gravity-dark- matter-proof/>)

But the proton's current value is accurate only by plus or minus one
percent—which isn't accurate enough for quantum electrodynamics, or
QED,
theory to work perfectly. So physicists have been searching for ways
to
refine the number. **

*Smaller Proton Size Revealed by Lasers*

In a ten-year experiment
<https://muhy. web.psi.ch/ wiki/index. php/Main/ HomePage>, a team led
by
Randolf Pohl
<http://www.mpq. mpg.de/cms/ mpq/en/people/ Pohl_Randolf. shtml> of
the
Max-Planck Institute of Quantum Optics in Garching, Germany, used a
specialized particle accelerator to alter hydrogen atoms, which are
each
made of a single proton orbited by an electron.

(Related: "Large Hadron Collider Smashes Protons, Sets Record."
<http://news. nationalgeograph ic.com/news/ 2010/03/100330- large-
hadron- collider- lhc-record- higgs-boson/>)

For each hydrogen atom, the team replaced the atom's electron with a
particle called a muon, which is 200 times more massive than an
electron.

"Because the muon is so much heavier, it orbits very close to the
proton, so it is sensitive to the proton's size," said team member
Aldo
Antognini, of the Paul-Scherrer Institute in Switzerland.

Muons are unstable, and they decay into other particles in just 2.2
microseconds. The team knew that firing a laser at the atom before
the
muon decays should excite the muon, causing it to move to a higher
energy level—a higher orbit around the proton. The muon should then
release the extra energy as x-rays and move to a lower energy level.

The distance between these energy levels is determined by the size of
the proton, which in turn dictates the frequency of the emitted x-
rays.

But based on the accepted proton radius, the experiment failed to
produce x-rays at the anticipated frequency.

In the summer of 2009 the team decided to widen their search to
include
other possible proton sizes. To their astonishment, the scientists
detected x-rays at an assumed proton radius of 0.8418 femtometers—4
percent smaller than expected.

"We were totally surprised and don't have any explanation for it
currently," Antognini said.**

*Smaller Proton a "Significant Shake-up"*

The proton finding won't impact most people's daily lives. But if it
proves correct, it means something fundamental is wrong in particle
physics.

It's possible the smaller proton means the Rydberg constant hasn't
been
correctly measured. This value describes the way light gets emitted
from
various elements—a key component of spectroscopy, which is used, for
instance, to tell which kinds of elements exist in galaxies and the
vast
interstellar gas-and-dust clouds called nebulae.

(Related: "Particles Larger Than Galaxies Fill the Universe?"
<http://news. nationalgeograph ic.com/news/ 2009/06/090602- particles-
larger-than- galaxies. html>)

Or, if the Rydberg constant is correct, the smaller size of a proton
could mean the equations in QED theory will fail to work.

"It is a significant shakeup and could mean a complete rethink of
QED,
potentially opening the door to a new theory," said Jeff Flowers, a
scientist with the National Physical Laboratory <http://www.npl. co.uk/
>
in the U.K., who wasn’t involved with the experiment.

Over the coming weeks physicists all over the world will be
scrutinizing
the experimental setup and complex calculations, making sure that
there
are no mistakes.

Assuming no errors are found, the scientists may have to get to work
rebuilding the Standard Model. //

/Findings appear in this week's issue of the journal/ Nature
<http://www.nature. com/>.