Wednesday 10 December 2008


I'm slowly recovering from the shock of starting a new life in a new time zone in a new haircut. Time to kick off with no-longer-from-CERN blogging. As a warm-up, I have an overdue rant. Some inspiration came from Tommaso's post who shares a few warm remarks about theorists teaching experimentalists. Here I elaborate on the opposite case.

The Nature magazine recently published a paper from the ATIC collaboration. ATIC is a balloon-borne experiment that studies high energy electrons and positrons (they cannot distinguish the two) coming from the cosmos. Many of these electrons and positrons are created by known astrophysical processes, mainly by cosmic rays scattering on interstellar matter (the secondary production). Astrophysicists can roughly estimate the flux due to the secondary production, although these estimates are subject to many uncertainties and should be taken with a whole container of salt. Anyway, assuming that the background estimates are correct, ATIC observes an excess of electrons at positrons at energies between 100 and 800 GeV. This fits well together with the positron excess between 10 and 100 GeV reported recently by the PAMELA satellite. Things are even more interesting. Rather than a mere excess, ATIC sees a distinct feature in the spectrum: a bump between 300 and 800 GeV. Astronomers are excited because this could be a signature of an interesting astrophysical object (a young nearby pulsar?, a microquasar?). Particle physicists are even more excited because the PAMELA and ATIC observations could be the first clear signals of annihilating or decaying dark matter particles with TeV scale masses.

The results from ATIC may turn out an important piece in the puzzle of what is the nature of dark matter. However, the collaboration must consider their results so uninteresting that they have to provide us with a cavalier theoretical interpretation. The signal is interpreted in the context of the so-called LKP - the lightest Kaluza-Klein particle in the Universal Extra Dimensions (UED) scenario. The names of Kaluza and Klein appear in the abstract and $e^N$ times in the main text. In case you missed it, they stress in conclusion that "if the Kaluza–Klein annihilation explanation proves to be correct, this will necessitate a fuller investigation of such multi-dimensional spaces, with potentially important implications for our understanding of the Universe." All in all, ATIC claims to have found in their data some hints toward the presence of extra dimensions of spacetime. What is their reason for such an extraordinary claim? It looks like they applied the modern tertium non datur: whatever is not the MSSM must be the UED.

That's philosophy. Physics, on the other hand, does not support ATIC's interpretation. From the theoretical point of view one can complain that the UED is an artificial and poorly motivated construction, that it does not address any problems of the SM (except for dark matter) while creating new problems of its own, and so on. But the main point here is that the LKP interpretation is not consistent with all available experimental data. Firstly, the LKP annihilation cross section is not large enough to explain the ATIC and PAMELA signals (if the dark matter abundance is of thermal origin). ATIC shrugs this off with a bla bla, where the former bla stands for a 100 boost factor from dark matter clumpiness (which does not come out of numerical simulations) and the latter for "other kids do it too". There is yet another serious problem that has to do with the fact that PAMELA observes no excess in cosmic anti-protons. Even though the LKP couples more strongly to leptons than to quarks (because the former have larger hypercharges to which the LKP couples), the decay rate into hadrons in the UED is still far too large. This issue is not addressed at all in ATIC's paper because it was submitted before PAMELA's data came out.

To summarize, it's such a pity to mar a beautiful experiment with a crappy theory.


Unknown said...

J.: we definitely want to see the new haircut. I'll be eagerly looking around in the internets for updates on that....

Anonymous said...

hi Jester

Great article, as usual! I'm glad you got back to blogging so fast.

Matti Pitkänen said...

Just a little comment knowing that anonymous ignorants are there and ready to make gestures of power.

p-Adic thermodynamics combined with p-adic length scale hypothesis allows to predict elementary particle masses with amazing accuracy (Dear Anonymous, before emitting venom please visit my web page and load the first parts of the book describing the calculations and show the mistakes). Just a couple of days ago Tommaso told about a new measurement of top quark mass bringing the best value of top quark mass to 165.1 GeV. This value is consistent with TGD prediction without any fine tuning the unknown second order contribution (few per cent). The earlier value 169.1 GeV required second order contribution to be maximal.

A fractal hierarchy of QCD type physics is predicted and CDF anomaly represents only one of these. A scaled up variant of ordinary hadron physics, which is of special interest as far as LHC is considered, corresponds to Mersenne prime M_89=2^89 -1. The mass scale for proton is predicted to be around 512 GeV by p-adic scaling from M_107 characterizing ordinary hadron physics. Maybe this physics is expressing itself via ATIC and Pamela anomalies.

Anonymous said...

A similar convention certainly seems to exist in accelerator-based experimental papers: the results of a search "must" be interpreted in some framework, e.g. something like "if we assume mSUGRA, then the upper limits we've set on the cross section for such-and-such a new particle would translate into the following limits on its mass." I've always thought this was a bit silly, but over time I've learned to ignore it and just look at the last bit of the conclusions before the model-dependent interpretation.

I guess that's not quite as troubling as what you're describing, though.

Anonymous said...

There is a big difference between a signal which seems to indicate new physics and one which doesn't. In the case of limits on extra contributions, there are usually many many models which could be limited, and since it is just a limit, they may as well present it for one oft-talked about model if nothing else so they can compare with each other.

For a signal, the stakes are much higher, and I think this tendency to leap onto one theory is much less reasonable. (ATIC is the not the first, the g-2 of the muon paper started fitting smuon masses to its excess, as I reall). So while I agree that ATIC went way overboard. There are scientific reasons why they considered UED more promising than (say) supersymmetric theories, though there are parameter regions where either option would probably do, even if they are far away from our favorite or expected corners.

I'm left wondering why you would bother spending half of your blog post worrying about the fact that they have proclaimed UED. Isn't the signal itself much more interesting? Wouldn't some of the issues or future prospects be more interesting to discuss?

Jester said...

I just think that ATIC deserves a kick in the butt for misleading people into an obviously wrong interpretation. It does not change the fact that their experimental results are very important and I will surely return to them in this blog.

Btw, experimentalists' fixation on mSUGRA that Seth mentions above is even more harmful and deserves a separate rant ;-)

Anonymous said...

In this plot the energy spectrum is scaled by E^3. I wonder how is the bump without this scaling.

Jester said...

There is no bump if you plot without E^3: the spectrum keeps decreasing and only the slope changes.

Anonymous said...

hi jester,

I think there is a bump.

I agree with the rest of your comments though, although the relevance or not of UED theory itself is not the point, its the spin of the DM which is the point, and UED seems to have taken the default role of the spin one candidate.

The very idea of extra dimensions, sorry, EXTRA DIMENSIONS!, motivates people and I don't see too much harm.

Also, the ATIC people have to claim the interpretation, or someone else who claims the interpretation would get the credit if it turned out to be right, even if the ATIC lot know that the interpretation is probably wrong.

So I can see both sides of the story,

best, Malcolm

p.s. did they make you cut your hair at the border? I will let Hannah know, (she will be pleased).