Wednesday, 3 April 2013

First results of AMS-02

Today we saw the first physics results from the AMS-02 collaboration. AMS is a particle detector attached to the International Space Station where it collects more than 10 billion cosmic ray events per year. The data released today concern the energy spectrum of cosmic ray positrons. Before discussing the AMS results it's worth taking a historical detour to understand the wider context.

The Universe we see is made of matter, but some small amounts of antimatter are being constantly produced by all sorts of violent processes: the scattering of high energy cosmic rays on the interstellar medium, the creation of  electron-positron pairs in the electromagnetic field of pulsars,  proton-proton collisions at the LHC, etc. Another possible production mechanism is annihilation of dark matter in the center of our galaxy, hence the interest of particle physicists in the subject.  Dark matter may show up as an excess of high-energy positrons over the background predicted from common astrophysical processes. Assuming we understand the background.

Until a few years ago the common lore was that the dominant production of positrons in our galaxy is via the scattering of high energy cosmic protons off particles in the galactic disc. This predicts the positron fraction decreasing with energy. For this reason, when back in summer 2008 PAMELA  reported a sharp rise of the positron fraction between 10 and 100 GeV we thought  for a moment we had a smoking-gun signal of dark matter. Later, the Fermi satellite confirmed the excess and showed that the rise extends at least up to 200 GeV. However, now we don't consider the excess an evidence of dark matter. One reason is that models of dark matter that quantitatively explain the PAMELA and Fermi signal are rather baroque. Firstly, one needs a large annihilation cross section, of order 10^-24 cm^3/sec, 2 orders of magnitude larger than the one required for dark matter to be a thermal relic. Moreover, dark matter needs to annihilate mostly into leptons  and, unlike what happens in typical models, very little into hadrons (as no excess in the cosmic ray antiproton spectrum is observed).  Another reason for skepticism is  that any dark matter model explaining PAMELA and Fermi is in tension with constraints on the gamma ray flux from the galactic center and from the dwarf galaxies. In the meantime, astrophysicists went back to the drawing board and proposed more ordinary sources of high energy positrons. The current lore is that a few nearby pulsars could be responsible for the observed rise of the positron fraction. Thus, after the initial excitement, things have settled down in a limbo: we're sure the  positron excess is real, but we cannot prove that it's a signature of dark matter, and neither we can prove that it isn't.

So, what have we learned today? Qualitatively, not much, quantitatively, a bit. AMS, with its full-fledged multi purpose detector, has better particle identification capabilities compared to the previous missions, which allows them to reduce systematic errors in the positron fraction down to 1% at low energies (compared to 2% in PAMELA) and explore a larger energy range. Currently, their positron measurement extends up to 350 GeV, so almost a factor of 2 beyond the highest data point from Fermi. AMS shows that the rise continues at least up to 250 GeV. The flux of high energy positrons seems to be isotropic, although their current constraints on the dipole component do not yet exclude a local (pulsar) origin of the positron flux. They also see a hint of flattening of the positron fraction above 250 GeV, although at this point this is not significant. If the positron excess originates from annihilation of dark matter particles with the mass of several hundred GeV one should see a drop in the positron spectrum at energies above the dark matter mass, but it is not said pulsars or other astrophysical phenomena could not produce a similar drop. (Note also that  a sharp drop in the positron spectrum will typically be accompanied by a similar feature in the electron+positron spectrum, but according to the measurements by Fermi nothing dramatic is happening up to 1 TeV.)

So, AMS-02 made some bold claims today. Dark matter is mentioned 9 times in the  press release, supersymmetry twice. They say that “...over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter...”. However this is just a lot of smoke without fire. There's absolutely no way that measurements of the  positron spectrum may give us a reliable evidence for dark matter: not now, and not anytime soon. We simply have no robust way of telling a dark matter signal from a boring astrophysics background in that channel, because we don't know the shape nor the normalization of the background.  It doesn't mean that AMS cannot provide a tantalizing signature of dark matter in the future. The most important thing we learned today is that AMS works and exceeds in precision the previous instruments (which wasn't that obvious: it's the first time a serious experiment is performed on a space station, and besides the mission underwent a dramatic downgrade shortly before the launch). We're waiting most eagerly on the AMS measurements of the antiproton  and anti-deuterium spectra. A correlated excess in several channels could give us more confidence in the dark matter origin. Until that happens, the history has taught us to be skeptical about any evidence of dark matter from astrophysics experiments.

You can find the AMS paper here. See also Matt's blog. Reading the mainstream press it seems that Sam Ting with some help from CERN succeeded in fooling the journalists. I'm glad  that CERN already shook off the faster-than-light neutrinos trauma and is ready for another hoopla.... life is going to be  more interesting for bloggers :-) 


Anonymous said...

Some of Ting's comments could (or not) be taken as hints that the steep drop expected in the DM annihilation scenario may be showing up at slightly higher energies than presented today. Then the pressing question that may make people's minds up would be: is there any working example of a potential boring astrophysical background giving rise + sudden drop? From the discussion I understood that the latter is not provided by the pulsar hypothesis...

Also: if one sticks to a simple DM annihilation explanation, can the mass be extrapolated from the shape of the rise? The data look impressively precise for fitting.

cb said...

Quotation from :

"The main conclusion of our work is therefore that future electron/positron data will likely be insufficient to discriminate between the dark matter and the single pulsar interpretations of the cosmic-ray lepton excess. One caveat to this statement would be the detection of several bumps in the electron-positron spectrum at high energies that could be associated to the contribution of several nearby pulsars, and that would be diffcult to mimic with dark matter annihilations or decays..."

-Are there already some bumps visible in the spectra (near 90, 70, 30 GeV)?
-Is this quoted work reliable ?

Whatever happens, AMS-02 has definitely proved to be an efficient new tool for precision astroparticle physics (to paraphrase the cosmologist Michael Turner who coined the phrase precision cosmology)!

James Salsman said...

In February, Ting was widely quoted as saying, "It will not be a minor paper." What's major about this paper?

Daniel Fischer said...

The paper is currently available open-access at by the way.

Anonymous said...

@James, the amount of associated hoopla... (yawn)

Anonymous said...

Odd, , which is a not-so-old paper by Delahaye and some others, shows the positron fraction has a component that rises as energy increases. Figure 14, page 22 of 31, lower right panel.

Seems Ockhams' razor might favor enhancement of lowly secondaries as the culprit.

Jester said...

Anon #1, we don't know the details of the mechanism of positron emission from pulsars. But there must be an energy cutoff somewhere, so naively pulsars also predict a drop. On the other hand, there's probably no reason why different pulsars should have a drop at the same energy, thus a spectrum with several drops would be more natural (it's hand-waving however, there's no theorem that pulsars cannot give a sharp drop). Yes, if you assume dark matter annihilation then the position of the drop roughly gives you the dark matter mass.
cb, yes, this paper is reliable, and yes, by eye the spectrum looks bumpy, but I don't know how significant it is. I'm sure within a week there will be papers with a more quantitative analysis.

Anonymous said...

Does it mean anything that the low energy points seem very significantly inconsistent?

Anonymous said...

I meant PAMELA vs AMS-02 are inconsistent...

Jester said...

The positron flux at low energies (below 10 GeV) depends on the activity and the polarity of the Sun which changes with the 11/22 years period. Thus the low points are not even supposed to be the same in PAMELA and AMS-02

Robert L. Oldershaw said...

Another possible explanation for the positron excess is that the Galaxy's dark matter halo is composed of stellar-mass primordial black holes. Kerr-Newman black holes would be prodigious sources of cosmic ray nuclei and particles, including positrons.

Robert L. Oldershaw
Discrete Scale Relativity/Fractal Cosmology

Anonymous said...

Why do you write "the mission underwent a dramatic downgrade", how can you state it was a "downgrade"?

Anonymous said...

@Robert L. Oldershaw - The MACHO collaboration didn't find stellar mass BHs (0.3 - 30 solar masses)to be a significant contribution to the halo's dark matter.
You're suggesting that halo BHs below their detection limit would still be enough to account for the background?

I think the simplest explanation for the results is that it's a "boring astrophysical process" and not some exotic, magical DM particle.

Anonymous said...

"it's the first time a serious experiment is performed on a space station"

I assume you mean a serious experiment in particle physics, seeing as how science of one kind or another is mostly what they do up there, and they're not joking as far as I can tell.

Robert L. Oldershaw said...

"You're suggesting that halo BHs below their detection limit would still be enough to account for the background?"


The Kerr-Newman ultracompacts I have proposed as candidates for the galactic dark matter have masses that are predominantly below 0.3 solar masses. Specifically, the two biggest classes have masses of 8 x 10^-5 and 0.145 solar masses.

Also, I do not think the last word is in from microlensing research, e.g., Sumi et al recently discovered on the order of trillions of unbound planetary-mass objects that no one had a clue about.

Finally, very large numbers of black holes and other ultracompact astrophysical objects have been observed and can be inferered.

To date, no exotic particle (WIMP, axion, sterile neutrino, etc.) has ever appeared - and we have looked everywhere and in every way.

Anonymous said...

non superconducting magnet

Anonymous said...

@Robert L. Oldershaw

While I agree that the search for exotic DM particles has come up empty handed, the claim that there are "trillions of unbound planets" is like saying that AMS-02 has found proof of DM.

Microlensing has seen about 10 - 20 objects that MAY be planets (could be low mass brown dwarfs) and MAY be unbound to their parent stars.

The Nice theory of planetary formation implies that some planets may get ejected from their system early on so yes, there are probably many unbound planet mass objects out there, but it's a stretch to claim that there are trillions of them. I suspect that many of these rogue planets had unhappy endings when they met nearby stars in their natal cluster.

Anonymous said...

If the next AMS-02 measurements will show asymmetry < 1% then the pulsar explanation will be almost ruled out. So what is left? Thus, the importance of these data and probably the other that are coming.

Jonathan Miller said...

You note that there are problems with the positron source being from dark matter, but that is only with the traditional models of dark matter (single constituent which self annihilates) which are most likely too simplistic. Which you start considering things like hidden sectors and composite dark matter these problems are not significant.

Nomadd said...

Was getting rid of the cryogenic magnet a downgrade? It lost sensitivity mostly in the lower energy areas, right? But it gained the potential to run for 18 year instead of three, and raw detection numbers are what it needs.

Robert L. Oldershaw said...


I am wondering if you have read the paper by Sumi et al (MOA, etc. collaboration) published in Nature in 2011. They estimated approximately 400 billion unbound planetary-mass objects in the Galaxy.

Subsequent papers by others (all are available at had higher estimates - in the trillions.

Jester said...

Nomadd, I think replacing the superconducting magnet with the weaker one hurts them especially at high energies where things are most interesting. Yes, it was a downgrade (contrary to what Sam Ting says in public) forced upon them by NASA safety measures.
Jonathan, sure, there exist models (e.g. with light hidden sector) that can explain the presence of the signal in positrons and the lack thereof in antiprotons. However the tension with the gamma ray constraints is rather model independent.
Anon, yes constraints on the dipole anisotropy at the level of delta<0.01 should exclude the Geminga pulsar as the main source of the positrons (the current AMS limit is delta < 0.036). But maybe there are several pulsars, or maybe another astrophysical process is responsible (supernovae?)... it will be very hard to exclude astrophysical explanations of the positron excess.

wolfgang said...

Lubos estimated the drop off from the maximum energies for e- and e+ mentioned in the talk.
His procedure is actually quite interesting - anybody (anonymous) from AMS who could confirm or deny that his estimates are similar to what was really seen?

Anonymous said...

@Robert L. Oldershaw - The Sumi microlensing paper claims of *at most* 400 billion rogue planets is based on a limited dataset; this is a far cry from trillions.
If you're talking about Wick­ra­mas­inghe's paper... the less said the better.

Having trillions of planetary or sub-stellar mass objects floating around should have observable effects; WISE found far fewer brown dwarfs than predicted and found no rogue gas giants between us and Alpha Centauri.
If WFIRST gets off the ground in 2020, it may be able to observe a few of these rogue planets. LSST may be able to do it at around the same time.

Even trillions of planets wouldn't eliminate the need for some sort of exotic dark matter (or something else) to account for galactic rotation curves and it really stretches belief that they could account for DM in galactic cluster (and larger) scales.

Anonymous said...

Hi Jester, I don't know on which grounds you state "it was a downgrade (..) forced upon them by NASA safety measures". This is completely untrue. I wonder which are your sources, because they gave you wrong information.

First, the AMS was designed since the beginning (late 90's) to be mechanically interfaced with both the magnet systems (so, this "last minute option" has always been on the table, actually).

Second: the decision to re-configure the instrument was taken after very intensive MC simulations, calibration beam tests at CERN and thermal-vacuum tests in ESTEC.

Finally, the permanent-magnet configuration does not give weaker performance at high energies, as many people like to believe. In fact, the rigidity resolution depends on many subtle details; mainly, very roughly, on the magnetic field intensity (weakened) as well as on the track "lever-arm" (improved). So it is not obvious to say what is better.

The final option resulted the best choice on scientific basis. Thus, it definitely represents an *upgrade* by the point of view of the AMS physics goals.

Jester said...

Interesting. Is there a pre-2010 reference where the permanent magnet option is discussed? Why 15 years of R&D on the superconducting magnet if the old permanent magnet ensures comparable capabilities?

Anonymous said...

AMS was supposed to be on the Space Station only for 3 years. The duration of the ISS has been extended recently (right before AMS's magnet change), giving the possibility to AMS to operate on the ISS for the whole long duration of the station. So the magnet was changed and the lever arm increased (reducing the acceptance) to keep the same MDR.

Jester said...

Yes, this is the official version, unfortunately it doesn't make sense. You know that AMS-02 was originally scheduled for launch in 2005, so that would be 10 years until 2015 (besides, nobody assumed even for a moment that the ISS will be shot down in 2015, right after being completed). Absurd explanations like the one above seriously undermined collaboration's credibility.

Anonymous said...

Hi Jester. The PM-option is discussed even in post-AMS01 early documents in 99/2000, when AMS02 was designed. Then the SC-option become the default one, as it was preferred by NASA, which was (and still is) interested in exploring such a technology in space: the SC-magnet was a technological challenge rather than a science requirement. And, at that time, NASA had ensured an additional shuttle-flight for AMS (to be used e.g. for superfluid-He refilling, or bringing AMS to ground). The rest is history: the shuttle-program was closed (no additional flight after the AMS delivery), while and the ISS-program was extended to 10+ years. So, AMS convinced NASA that the original PM-option was the best one.
The science goals prevailed on the space-tech goals. Well, it's the opposite of what you wrote.

Anonymous said...

Laol! And why 15 years of R&D to realize a SC-magnet, if at the end NASA imposed safety measures??? Come on, Jester, that's ridiculous...

Anonymous Snowboarder said...

Ting did a disservice by suggesting that this paper would be "major" (by claiming it would not be "minor"). As to the press, well Nobel Prize + dark matter + supersymmetry... how could anyone resist?

I'll grant I'm just a somewhat competent lay person with a background in physics but I do find a few of the comments amusing ...'but no its all ok if we use this even more complicated dm model!'

I've always found this statement to be useful:


Anonymous said...

The major change in AMS-02's revisions was caused by the unanticipated decision by the Bush administration to cancel the shuttle in the 2010 timeframe.

Originally the plan was to launch AMS-02 for a three year mission. It would launch on one shuttle flight and after three years would be replaced with another experiment and return home via the shuttle. An alternate plan would be to do a superfluid He resupply in orbit (which was tested on the STS-57 shuttle flight). Much of the decision of which approach to take would be dependent on funding and what payload would replace AMS. (one of the key - and controversial decisions - for why AMS flew at all was because it was funded by the DoE, so NASA didn't have to take major funds out of its science dollars).

NASA has always been interested in developing new technology rather than replicating what's exists (vis a $2.5 Billion Mars rover rather than a fleet of Spirit-class rovers).

Historic example - When the NICMOS infrared instrument was put aboard Hubble it was always intended at some point to be able to "refuel" its block of cryogenic Nitrogen. It could have been new dewar of Nitrogen or He, but the Air Force wanted to develop a cryogenic fridge, presumably for infrared missile warning satellites. They provided money for the NICMOS cooler. The far less expensive approach would have been to just build a NICMOS-2 (without the heat short problem on NICMOS) and put in far better detectors in the new instrument for far better science (at lower cost).

When the Bush administration made the decision to shut down the shuttle and end US involvement in the ISS after the commitment to the international partners was finished AMS lost both its flight to the ISS and the return trip home after its three year mission was finished.

It literally took an act of Congress to override the Bush administration and find a ride up for AMS, but clearly by that point there was no way to return it to Earth, and no practical way to refuel the He supply for a superconductor magnet. Had they chosen to stick with the superconductor magnet the AMS-02 would literally become a non-functional hood ornament once the cryogens ran out. With the decision to change to the non-superconducting magnet AMS-02 could continue to run for the life of the space station.

There were also more subtle changes with AMS's reconfiguration. The original design called for shuttles to fly up new hard drives and return ones filled with data. With the retirement of the shuttle it's extremely difficult to return any significant amounts of cargo (anything bigger than a briefcase) from space. Of the five cargo ships for the space station only the SpaceX Dragon has cargo return capabilities. So instead of hard drives AMS had to be given more bandwidth on the high speed data transmissions to send back its data.

This is indicative of many changes that were needed with the shuttle's retirement. Instead of shipping up new components and returning failed components to the ground more spares were stored on the ISS, more systems were redesigned to be modular so they could be repaired in space, only needing much smaller (and easier to manifest) submodules.

It wasn't until Obama took over that the decision was made to extend ISS to 2020, and hopefully further extensions.

Instead of a 3 year superconducting AMS we've now got one which will last through the life of the ISS, I'd say that's a pretty decent tradeoff.

And it's incredibly arrogant to claim that AMS is the only (or even the first) science done on the ISS. Certainly it's the first significant astrophysics or astronomy, but there's plenty of other science being done (and with far less funding). Just because microgravity science isn't as glamorous as astrophysics doesn't mean it's any less important.

Able Lawrence said...

I was reading the paper by Nima Arkani-Hameed from 2009
Now let us see the current data presented by AMS-2 in the light of the results from ATIC where there is a sudden drop of electrons+positrons (ATIC could not distinguish the two) above ~850 GeV
From this we already know that there is a sudden drop in the light lepton signature. The only thing AMS-2 has done is confirm the excess. We will have to wait quite a long time to see the confirmation of dark matter but a sudden drop in leptons whether positron or electrons would be a big deal

Chris Kennedy said...

Do I understand you correctly that typical models (SUSY?)call for a higher fraction of hadrons which conflict with the actual level that is observed in the cosmic ray antiproton spectrum?
If I understand this correctly and it is true - then isn't this a much bigger deal than arguing over the energy curve for positron fraction anyway?

Anonymous said...

@Chris Kennedy:

In principle you're right, however you can (more or less) easily think of models where no or only very few hadrons (i.e. in particular antiprotons) are produced. One "famous" example is hep:ph/0810.0713, where the production of antiprotons is forbidden by kinematics.

Dr C Bremner said...

Fermi gets a ratio of greater than one for high energy positrons versus electrons yet AMS is much lower , the amorphic distribution is indicative of a zpf source of the positrons , this concurs with Sakharov and Noever and Bremner (NASA )work on quantum gravity born from their superconductor observations giving a 10 megahertz gravity frequency cutoff. In this model there is no need for the dark energy and matter fudges. It is logical the positron surplus is a result of the fundamental nature of the zpf ( vacuum) and a lorentz variant 2 dimensional time.