PAMELA's coming-out

Yesterday, PAMELA finally posted on ArXiv her results on the cosmic-ray positron fraction. In the last months there was a lot of discussion whether it is right or wrong to take photographs of PAMELA while she was posing. Here at CERN, people were focused on less philosophical aspects: a few weeks ago Marco Cirelli talked about the implications for dark matter searches, and Richard Taillet talked about estimating the positron background from astrophysical processes in our galaxy. Finally, PAMELA had her coming-out seminar two days ago. PAMELA is a satellite experiment that studies cosmic-ray positrons and anti-protons. She has a better energy reach (by design up to 300GeV, although the results presented so far extend only up to 100 GeV) and much better accuracy than the previous experiments hunting for cosmic anti-matter. Thanks to that, she was able to firmly establish that there is an anomaly in the positron flux above 10 GeV, confirming the previous hints from HEAT and AMS.

Here are the PAMELA positron data compared with the theoretical predictions. The latter assume that the flux is dominated by the secondary production of positrons due to collisions of high-energy cosmic rays with the interstellar medium. The two lines are almost perpendicular to each other :-). In fact, the discrepancy below 10 GeV is not surprising, and is interpreted as being due to solar modulation. It turns out that the solar wind modifies the spectrum of low-energy cosmic rays, and in consequence the flux depends on solar activity which changes in the course of the 22-years solar cycle. Above 10 GeV the situation is different, as solar modulation is believed to produce negligible effects. Even though the secondary production of positrons has large theoretical uncertainties, one expects that it decreases with energy. Such a power-law decrease has been observed in the flux of anti-protons who also may originate from secondary production. The positron fraction, instead, significantly increases above 10 GeV.

Thus, PAMELA shows that the secondary production is not the dominant source of high-energy positrons. The excess can be due to astrophysical sources, for example young near-by pulsars have been proposed as an explanation. But what makes particle physicists so aroused is that dark matter annihilation is a plausible explanation too. It might be that PAMELA is a breakthrough in indirect dark matter searches. It is less known that there are other experiments that see some excess in the cosmic ray flux. Most interestingly, two balloon-borne experiments called ATIC and PPB-BETS see an excess in the total electron+positron flux (they cannot distinguish the two) with a peak around 700 GeV. This adds to the EGRET gamma-ray excess at a few GeV, and to the WMAP haze - an excess of diffuse microwave background from the core of our galaxy.

A dark matter candidate that fits the PAMELA excess, must have rather unexpected properties. If the observed dark matter abundance has a thermal origin, the dark matter annihilation cross section naively seems too small to explain the observed signal. As usual, theorists have magic tricks to boost the annihilation rate today. One is using the so-called boost factor: if dark matter clumps, its density is locally higher than average, and then the average annihilation rate also increases with respect to the case of a uniform distribution. However, this does not save the day for the most popular dark matter candidates. For example, the MSSM neutralino typically requires a boost factor of order a few hundred, which is probably stretching the point. The latest trick is called the Sommerfeld enhancement: if the dark matter particle feels some attractive long range forces (other than electromagnetism, of course), a pair of particles may form a bound state, which enhances the annihilation rate.

Another challenge for particle models is the fact that PAMELA sees no excess in the antiproton flux. This means that the dark matter particle must be hadrophobic, that is to say, it should decay preferentially into leptons. Again, the most popular dark matter candidates, like the MSSM neutralino, do not satisfy this criterion. However, particle models compatible with the PAMELA data do exist, for example Minimal Dark Matter (though this one is not compatible with the ATIC/PPB-BETS peak), or recent Exciting Dark Matter.

So, it seems, we have to wait and see till the smoke clears up. Certainly, a single indirect detection signal has to be taken with all due scepticism (so many have died before). Only combined efforts of several experiments can lead to a convincing conclusion. As for the moment, if somebody pointed a gun to my face and made me choose one answer, I would probably go for an astrophysical explanation. On the other hand, if the PAMELA excess is really a manifestation of dark matter, the LHC could concentrate on more interesting issues than discovering and undiscovering the MSSM. It seems that astrophysicists have at least one more year to sort this thing out by themselves.