Tuesday, 17 May 2011

Fermi confirms PAMELA

The annual sabbat of the Fermi collaboration took place last week in Rome. One of the new results presented there was the measurement of the positron fraction in the cosmic rays. This is a very hot observable given PAMELA's claim that the positron fraction at high energies is larger than the one predicted by models of cosmic ray propagation. The PAMELA excess can be interpreted as a signature of dark matter, although boring astrophysical explanations are also possible. But a skeptic could doubt the PAMELA result. Measuring the positron fraction requires discriminating between positrons and much numerous protons; one needs the proton rejection power at the level of 1 in 100 000. PAMELA claims to have that rejection power, but the possibility of an unaccounted for systematic effect did exist.

Now the PAMELA result has been confirmed by a cute measurement performed by the Fermi satellite. Fermi, unlike PAMELA, does not have a magnet to distinguish positively charged particles from negatively charged ones. That's why, until now, they were presenting only the combined flux of electrons and positrons. Nevertheless, they are able to some extent separate electrons and positrons by borrowing the magnet from the Earth. The configuration of the Earth magnetic field lines happens to be such that for certain energies and certain arrival directions only electrons or only positrons are expected, see the picture. Using this effect, Fermi was able to produce the following measurement of the positron fraction:
The black data points are from PAMELA, and the grey band is the Fermi measurement with their systematical uncertainties. The two are nicely consistent. So, we still don't know whether the positron excess is due to dark matter or pulsars or old newspapers, but at least we know for sure it is real.

Another result presented in Rome deserves some advertisement. One of the main goals of Fermi is to search for gamma rays produced by annihilation of dark matter. A good strategy is to look in the direction of one of the dwarf galaxies. These are small satellites of our galaxy that are vastly dominated by dark matter, therefore the annihilation signal can be significant while the astrophysical backgrounds are less pesky. So far, no anomalous gamma ray flux from dwarf galaxies has been detected. From that, Fermi is able to put quite stringent limits on the annihilation cross section for various hypotheses about the final state into which dark matter annihilates:
The point is that for certain hypotheses, like for light dark matter annihilating into tau leptons or b-quarks, they are already excluding the cross sections expected if the dark matter is a thermal relic. So they're really closing in on the parameter space where a signal may be lurking if the WIMP paradigm is true. No luck so far, but they'll try again next year :-)

7 comments:

Luboš Motl said...

Exciting news. Re-reported on TRF. I first thought that the sabbath was a serious name haha before I made a Google search to find out that your last name is probably Jewish. ;-)

Matti Pitkanen said...

No one can deny that colored excitations of leptons would not be doing whatever they can to open the eyes of particle physicists. See this.

Anonymous said...

How does this last exclusion plot affect the light WIMP interpretation of DAMA/LIBRA & COGENT experiments?? The mass range that fit their data was about 7-10 GeV, and Fermi data seems to forbid a 10 GeV mass particle to be a thermal relic...

Jester said...

There is no straightforward connection between DAMA/CoGeNT and PAMELA/Fermi, the former points to GeVish and the latter to TeVish dark matter. There is many options on the table: DAMA/CoGeNT could be wrong and/or PAMELA/Fermi could be boring astrophysics, or dark matter is 2-component, or something even more complicated is happening....

Michel said...

Jester,
IMHO, actually there is a clear connection between the possible WIMP interpretations of CoGeNT/DAMA and the updated analysis of Fermi-LAT on dSphs. Their new staking analysis excludes light WIMP candidates, M < 10 GeV, annihilating in b-bbar (or some mixture of tau+/tau-) that are also candidates for CoGeNT/DAMA. This encompasses many models of DM, but the one that comes to my mind is a singlet scalar interacting through the Higgs, which is clearly challenged by the Fermi data on dSphs (see for instance arXiv:1012.0576). A possible caveat is their choice of NFW (ie vs a less cuspy profile), but this should not affect much the integrated gamma-ray flux, so I believe that the constraints reported by Fermi must be quite robust. As you emphasize, this nice result deserves some advertisement.
Michel Tytgat

Jester said...

Right Michel, I misread Anon's question and I thought he was asking about the connection between the PAMELA/Fermi positron excess and the CoGeNT signal. I totally agree with what you say: a large class of models explaining CoGeNT is going down the drain. I guess 2 light jets would be similar to b-bbar, so that's also getting excluded at small DM mass. Barring muon/electron/neutrino 2-body final states (which seem less natural) one needs to consider annihilation into many-body final states (e.g. via dark photons) or non-thermal DM production.

Anonymous said...

Assuming a dark matter corotating flux in the solar neighborhood (as in case of dark disk) the annual modulation of DAMA (and maybe CoGeNT) could be consistent with TeV mass particles suggested by PAMELA/Fermi positron excesses. (arXiv:1103.6091)