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December 7, 2006

New Light Particle(s) Discovered?

Posted by John Baez

The lightest known particle heavier than the electron is the muon: it’s about 207 times as heavy. This paper claims to have found evidence for new neutral particles that are only about 14 and 37 times as heavy as the the electron:

Here’s a news story:

The title of this story makes the discovery sound like a sure thing, but I think we should be careful. The evidence is not iron-clad — and particle physics beyond the Standard Model is serious stuff!

The authors fired nuclei of lead at an emulsion that served both as target and detector, and looked for excess production of low-energy electron-positron pairs. Looking at a sample of 1220 electron-positron pairs, they found peaks at energies of 7±1 MeV and 19±1 MeV:

These peaks are more than 3 standard deviations above the mean - enough to be interesting, but not enough to clinch the case.

For theorists, part of the excitement is the possibility that a light particle like this could be the long-sought axion. Previous hints of the axion may have been seen earlier this year, in a very different experiment:

Alas, an axion several times heavier than the electron could not serve to explain dark matter, because it wouldn’t be stable: it could decay into an electron-positron pair, as Jain and Singh claim to see. Indeed, they claim lifetimes of less than 10-13 seconds.

I thank my student Mike Stay for pointing out Jain and Singh’s paper.

Posted at December 7, 2006 5:51 AM UTC

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8 Comments & 1 Trackback

Re: New Light Particle(s) Discovered?

the possibility that a light particle like this could be the long-sought axion.

nn-category theorists might enjoy to note that quite a few models of “physics beyond the standard model” obtain the axion as an incarnation of the existence of the “BB-field”, i.e. of a 2-form gauge field, also known as the connection on a gerbe.

So the detection of an axion in nature might be a first experimental indication for higher gauge theory in high energy physics - if you like.

Posted by: urs on December 7, 2006 12:14 PM | Permalink | Reply to this

Re: New Light Particle(s) Discovered?

I was checking to see what else Planet Musings had to deliver about this fascinating news item, and it brought me to Superweak’s entry “No axions for you!” One paragraph in particular is worth quoting, I think:

To this physicist, the bumps in question look like perfectly normal statistical fluctuations in a small sample: if you don’t know beforehand where to look for a peak, you take a penalty factor in how significant you can claim your result to be because you get to search in many places, and some of them are going to fluctuate. (The whole blind analysis craze has apparently passed the authors by.) Even if you take the paper’s claims of the statistical significance at face value, though, this does not count as a “discovery”; it is at best “evidence.” Discoveries require five standard deviations (more or less) and reliable, repeatable independent confirmation; that last part is critical (witness the pentaquark fiasco, as belligerently reported and retracted by the New Scientist).

(All links are from the original.)

Posted by: Blake Stacey on December 7, 2006 4:19 PM | Permalink | Reply to this

Re: New Light Particle(s) Discovered?

Blake Stacey wrote that Superweak wrote:

Even if you take the paper’s claims of the statistical significance at face value, though, this does not count as a “discovery”; it is at best “evidence.”

Thanks for this link!

I hope it’s clear I was saying the same thing. Even the authors of the paper don’t claim to have ‘discovered’ anything. Still, it’s fun to hope, and dream.

It’s also fun to learn about axions and why they were introduced: to solve the ‘strong CP problem’, which is that the CP-violating topological term tr(FF) tr(F \wedge F) one can add to the Lagrangian for the strong force doesn’t seem to be present in Nature.

Posted by: John Baez on December 7, 2006 5:49 PM | Permalink | Reply to this

Re: New Light Particle(s) Discovered?

John Baez wrote:

I hope it’s clear I was saying the same thing. Even the authors of the paper don’t claim to have ‘discovered’ anything. Still, it’s fun to hope, and dream.

Agreed on all counts. Yes, I got the same message from your post as from the paragraph I quoted; what really grabbed my attention was the mention of New Scientist. As it turns out, the linguists and computer science people have also had cause to be upset with that magazine. I’ve been trying to keep track of such things, and although I’ve stumbled across interesting sites — such as MIT’s Knight Science Journalism Tracker — a big part of watching and correcting science journalism seems to be missing.

Well, if nothing else, should this axion business work out then Greg Egan should be happy: we’ve found the basic particles of his Bubble Makers.

Posted by: Blake Stacey on December 7, 2006 9:37 PM | Permalink | Reply to this

Re: New Light Particle(s) Discovered?

There are several interesting things here [1st article
below]. The electron-positron pair production being
just one. Not stated at all is that this system can
be used to test the Malcolm Fairbairn theory recently
summarized in New Scientist [see 2nd article below]
“Dark-matter particles could ‘X-ray’ the Sun”. That
is, if some high-energy gamma rays from the putative
black hole/accretion disk turn into axions, those
could completely penetrate the massive blue star, turn
back into gamma rays on the other side, and perhaps be
detectable in the signal.

– Jonathan Vos Post


Source: Particle Physics and Astronomy Research Council
Date: November 28, 2006

Astronomers Find First Ever Gamma Ray Clock

Astronomers using the H.E.S.S. telescopes have
discovered the first ever modulated signal from space
in Very High Energy Gamma Rays – the most energetic
such signal ever observed. Regular signals from space
have been known since the 1960s, when the first radio
pulsar (nicknamed Little Green Men-1 for its regular
nature) was discovered. This is the first time a
signal has been seen at such high energies – 100,000
times higher than previously known - and is reported
today (24th November) in the Journal Astronomy and

Map of the gamma ray sky in the region of LS5039. The
green star shows the position of LS5039 as measured
using radio telescopes and the white ellipse shows the
gamma ray position. (Image courtesy of Particle
Physics and Astronomy Research Council)

The signal comes from a system called LS 5039 which
was discovered by the H.E.S.S. team in 2005. LS5039 is
a binary system formed of a massive blue star (20
times the mass of the Sun) and an unknown object,
possibly a black hole. The two objects orbit each
other at very short distance, varying between only 1/5
and 2/5 of the separation of the Earth from the Sun,
with one orbit completed every four days.

“The way in which the gamma ray signal varies makes
LS5039 a unique laboratory for studying particle
acceleration near compact objects such as black
holes.” Explained Dr Paula Chadwick from the
University of Durham, a British team member of

Different mechanisms can affect the gamma-ray signal
that reaches Earth and by seeing how the signal
varies, astronomers can learn a great deal about
binary systems such as LS 5039 and also the effects
that take place near black holes.

As it dives towards the blue-giant star, the compact
companion is exposed to the strong stellar ‘wind’ and
the intense light radiated by the star, allowing on
the one hand particles to be accelerated to high
energies, but at the same time making it increasingly
difficult for gamma rays produced by these particles
to escape, depending on the orientation of the system
with respect to us. The interplay of these two effects
is at the root of the complex modulation pattern.

The gamma-ray signal is strongest when the compact
object (thought to be a black hole) is in front of the
star as seen from Earth and weakest when it is behind
the star. The gamma rays are thought to be produced as
particles which are accelerated in the star’s
atmosphere (the stellar wind) interact with the
compact object. The compact object acts as a probe of
the star’s environment, showing how the magnetic field
varies depending on distance from the star by
mirroring those changes in the gamma ray signal.

In addition, a geometrical effect adds a further
modulation to the flux of gamma-rays observed from the
Earth. We know since Einstein derived his famous
equation (E=mc²) that matter and energy are
equivalent, and that pairs of particles and
antiparticles can mutually annihilate to give light.
Symmetrically, when very energetic gamma rays meet the
light from a massive star, they can be converted into
matter (an electron-positron pair in this case). So,
the light from the star resembles, for gamma rays, a
fog which masks the source of the gamma rays when the
compact object is behind the star, partially eclipsing
the source. “The periodic absorption of gamma-rays is
a nice illustration of the production of
matter-antimatter pairs by light, though it also
obscures the view to the particle accelerator in this
system” (Guillaume Dubus, Astrophysical Laboratory of
the Grenoble Observatory, LAOG).

UK work on H.E.S.S. is funded by the Particle Physics
and Astronomy Research Council.


Dark-matter particles could ‘X-ray’ the Sun

* 27 November 2006
* news service
* Marcus Chown

An intriguing particle first glimpsed last year, which
could be related to particles that make up the
universe’s dark matter, might help us see right
through the sun.

The particle in question is the axion. It was
originally proposed to fix a problem with the strong
force in particle physics, but has more recently been
considered as a candidate for dark matter, the
mysterious, unseen stuff thought to make up nearly 90
per cent of galaxies’ mass. Last year an experiment at
the Legnaro National Laboratory in Legnaro, Italy,
provided tantalising hints of an axion, but it
interacted too strongly with matter to be a good fit
for dark matter (New Scientist, 15 July, p 35).

However, the axions found by the Italian team would
still interact so rarely with normal matter that they
would be able to slip right through even the
super-dense plasma of the sun. Now Malcolm Fairbairn
of Stockholm University in Sweden and his colleagues
say this could allow such axions to “X-ray” the sun.

The team base their claim on a model of how gamma-ray
photons interact with the sun’s magnetic field. This
showed that when gamma rays pass through the sun’s
outer layers, a small fraction would be transformed
into axions. These particles would easily penetrate
the sun and emerge on the far side, where the magnetic
field would flip a small fraction of the axions back
to gamma-ray photons. “If this happens, it means that
even if a gamma-ray source is eclipsed by the sun, we
should still be able to see it, just as if the sun is
partially transparent,” Fairbairn says

In 1991, the quasar 3C 279 was eclipsed by the sun
while astronomers were observing it using the orbiting
Energetic Gamma Ray Experiment Telescope (EGRET).
While gamma rays were seen during the occultation, the
detectors on the instrument were nowhere near
discriminating enough to pin them uniquely to 3C 279.

Fairbairn thinks that the much more accurate Gamma-Ray
Large Area Space Telescope (GLAST), to be launched in
2007, will be able to keep tabs on a gamma-ray source
as it goes behind the sun. “Observing [such] gamma
rays would confirm the existence of an entirely new
particle,” says Fairbairn. “Potentially it might allow
us to ‘X-ray’ the interior of the sun,” he adds.

Nigel Smith of the Rutherford Appleton Laboratory in
the UK and a member of the UK Dark Matter
Collaboration is intrigued. “The calculations -
especially on the magnetic fields that the axion would
be exposed to - look rather complex, but the idea
certainly looks sound,” he says.

From issue 2579 of New Scientist magazine, 27 November
2006, page 15


Posted by: Jonathan Vos Post on December 7, 2006 6:05 PM | Permalink | Reply to this
Read the post a la caza del axión
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Re: New Light Particle(s) Discovered?

More comments on that all around the blogosphere. For instance Chad Orzel’s Axions and the Problem of EurekAlert.

Posted by: urs on December 8, 2006 9:52 AM | Permalink | Reply to this

Corrected URL; Re: New Light Particle(s) Discovered?

Took me a while to find. Your link is broken. A valid link is

Now I see the problem, and feel duped.

Posted by: Jonathan Vos Post on December 11, 2006 4:52 AM | Permalink | Reply to this

Re: Corrected URL; Re: New Light Particle(s) Discovered?

Your link is broken.

Oops, sorry. Corrected now.

Posted by: urs on December 11, 2006 10:59 AM | Permalink | Reply to this

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