This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

Share this

Free weekly newswire

Sign up to receive all our latest news direct to your inbox.

Physics on film

100 Second Science Your scientific questions answered simply by specialists in less than 100 seconds.

Watch now

Bright Recruits

At all stages of your career – whether you're an undergraduate, graduate, researcher or industry professional – can help find the job for you.

Find your perfect job

Physics connect

Are you looking for a supplier? Physics Connect lists thousands of scientific companies, businesses, non-profit organizations, institutions and experts worldwide.

Start your search today


Antiprotons pass latest symmetry test

The Antiprotonic Decelerator at CERN (Courtesy: CERN)

By Hamish Johnston

For something that is rare in the universe, antimatter has certainly been in the news a lot lately.

The latest breakthrough involves antiprotonic helium and is published in Nature today. This exotic “atom” is formed when one electron in a helium atom is replaced with an antiproton, which is negatively charged.

For two decades physicists have known that antiprotonic helium is formed in a metastable state that sticks around for a few milliseconds before decaying. This should make it to possible to study its energy levels and measure the ratio of the antiproton mass to the electron mass. This could then be compared with the well-known proton-to-electron mass ratio to see if the proton and antiproton have different masses. Such an asymmetry goes against the Standard Model of particle physics and its discovery could help physicists understand why the universe is dominated by matter.

Now, physicists working on the Antiprotonic Decelerator at CERN have done just that. Masaki Hori of the Max Planck Institute of Quantum Optics in Garching, Germany, and an international team made laser-spectroscopy measurements and worked out the mass ratio to a remarkable degree of precision.

The experiment begins with pulses of antiprotons being injected into helium gas to create the exotic atoms. The team then fires laser pulses at the atoms to knock the antiproton from its metastable state to an unstable state, causing it to annihilate with the helium nucleus. This produces pions, which are easily detected. By varying the wavelength of the lasers to find the maximum rate of pion production, the team found the exact energy of the transition.

The big challenge for the researchers was that the atoms are moving about, which causes a Doppler broadening of the transition wavelength. Scientists get around this with normal atoms by firing two identical lasers in opposite directions at the target. The atom absorbs one photon from each beam – which is only likely to occur if the atom has no relative motion in the direction of the lasers, eliminating the Doppler broadening.

This is trickier to do with antiprotonic helium, and Hori and colleagues instead used lasers at two different frequencies to eliminate much of the Doppler broadening.

So after all that hard work, did they discover any new physics? I’m afraid not. The antiproton-to-electron mass ratio is the same as the proton-to-electron ratio to an impressive nine significant figures.

The work is described in Nature 475 484.

This entry was posted in General. Bookmark the permalink.
View all posts by this author  | View this author's profile

Comments are closed.


  • Comments should be relevant to the article and not be used to promote your own work, products or services.
  • Please keep your comments brief (we recommend a maximum of 250 words).
  • We reserve the right to remove excessively long, inappropriate or offensive entries.

Show/hide formatting guidelines

Tag Description Example Output
<a> Hyperlink <a href="">google</a> google
<abbr> Abbreviation <abbr title="World Health Organisation" >WHO</abbr> WHO
<acronym> Acronym <acronym title="as soon as possible">ASAP</acronym> ASAP
<b> Bold <b>Some text</b> Some text
<blockquote> Quoted from another source <blockquote cite="">IOP</blockquote>
<cite> Cite <cite>Diagram 1</cite> Diagram 1
<del> Deleted text From this line<del datetime="2012-12-17"> this text was deleted</del> From this line this text was deleted
<em> Emphasized text In this line<em> this text was emphasised</em> In this line this text was emphasised
<i> Italic <i>Some text</i> Some text
<q> Quotation WWF goal is to build a future <q cite="">
where people live in harmony with nature and animals</q>
WWF goal is to build a future
where people live in harmony with nature and animals
<strike> Strike text <strike>Some text</strike> Some text
<strong> Stronger emphasis of text <strong>Some text</strong> Some text