#
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics

## Volume 56, Number 5

##
Monday–Friday, June 13–17, 2011;
Atlanta, Georgia

### Session J2: Fundamental Symmetry Tests

10:30 AM–12:30 PM,
Wednesday, June 15, 2011

Room: A602

Chair: David DeMille, Yale University

Abstract ID: BAPS.2011.DAMOP.J2.2

### Abstract: J2.00002 : The size of the proton

11:00 AM–11:30 AM

Preview Abstract
Abstract

####
Author:

Randolf Pohl

(Max-Planck-Institute of Quantum Optics, Garching, Germany)

A measurement of the Lamb shift (2S--2P energy difference) in
muonic hydrogen ($\mu$p, the exotic hydrogen atom made from a
proton and a negative muon $\mu^-$) has been on the physicists'
wish list for more than 40 years.
Due to its 200 times larger mass, the muon's Bohr radius in
$\mu$p is only 1/200 of the electron's Bohr radius in regular
hydrogen (H).
This enhances finite size effects by about $200^3$ in $\mu$p,
compared to H.
The proton's finite size $r_p$ affects the 2S Lamb shift in
$\mu$p by as much as 2\%, making $\mu$p a unique, clean, atomic
system to study $r_p$ using laser spectroscopy.
We have recently observed the first transitions in muonic
hydrogen~[1] and muonic deuterium~[2].
The $2S_{1/2}^{F=1}$ to $2P_{3/2}^{F=2}$ transition in $\mu$p was
found at 49881.88(76)\,GHz [1].
Even with this - by laser spectroscopy standards - very moderate
accuracy of 760\,MHz (4\% of the natural line width) we can
deduce $r_p$ 10 times more accurately than the CODATA world
average [3].
We obtain $r_p\,=\,0.84184(67)$\,fm~[1].
The accuracy of $r_p$ is limited by the uncertainty of the proton
polarizability which is enters the theory relating the measured
frequency to $r_p$.
We have also measured a second transition in $\mu$p (
$2S_{1/2}^{F=0}$ to $2P_{3/2}^{F=1}$ ) [2].
It confirms our value~[1] of $r_p$, and provides the first
determination of the 2S hyperfine splitting (HFS) in $\mu$p.
The HFS reveals the Zemach radius, i.e. the radius of the
magnetization distribution inside the proton.
Now there is a ``proton size puzzle.''
We found the resonance~[1] 75\,GHz (i.e. 4 natural line widths)
away from the expected position.
Our $r_p$ is 10 times more accurate, but 4\% ($5 \sigma$)
smaller than the CODATA value~[3].
There are still surprises in physics.\\[4pt]
[1] R. Pohl et al. (CREMA collaboration), Nature 466, 213 (July
2010).\\[0pt]
[2] CREMA collaboration, to be published.\\[0pt]
[3] P.J.~Mohr, B.N.~Taylor and D.B.~Newell, Rev.~Mod.~Phys. 80,
633 (2008).

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.DAMOP.J2.2