I am currently visiting my friend and colleague Pengjie Zhang at the Shanghai Observatory. Shanghai is a truly amazing city; the largest in China in terms of population, with more than 18,000,000 people living in its municipality, and an amazing density of 2,621.9/km² (!!).

The lakefront promenade (“Waitan”, or “the Bund”), is truly enchanting. The picture on the left shows it as it can be seen from the 490 meters-high ‘sky walk’ of the World Financial center, that opened about a month ago.

There is everything here. From the traditonal to the modern, from poor suburbs to tall sky-scrapers, from open air dancing to glamorous clubs. Maybe not the most international city in Asia (and maybe too many “Starbucks” “Friday’s” and “McDonald’s”) but certainly a striking demonstration of the chinese economic growth. As a chinese saying goes, “jiude buku, xinde bulai”, or “if the old doesn’t go, the new won’t come”. Here, you feel that the new is actually coming.

Many particle astrophysicists have eagerly awaited the release of the first PAMELA data on high-energy positrons. Now the data have been quasi-released, in the sense that they do appear in a number of papers, but none of them is authored by the PAMELA collaboration. The full story can be read in a number of blogs over the web, such as

http://www.sciencenews.org/view/generic/id/36202/title/PAMELA_may_have_spotted_the_dark_stuff
http://www.cosmosmagazine.com/news/2180/is-dark-matter-about-be-explained
http://www.nature.com/news/2008/080902/full/455007a.html?s=news_rss
http://www.andrewjaffe.net/blog/science/000368.html

but it can be summarized as follows: the data have been shown at the IDM08 conference. The authors of this paper have taken a picture of the relevant slide, and asked permission to the speaker to use the (digitally extracted) data in their paper.

Aside from all the fuss about how data were taken, embargo policies, and all that, the data (if confirmed) appear very interesting. The positron ratio (i.e. the relative number of positrons with respect to the sum electrons+positrons) appears to increase above 1 GeV, contrary to the expectations based on simple astrophysical models. There is then room for an “exotic” explanation, in terms of the annihilation of Dark Matter particles.

Other experiments, such as ATIC and PPB-BETS may have found interesting features in the positrons + electrons spectrum at higher energies as detailed in this paper.

If a decrease in the spectrum is confirmed (thus if we are actually dealing with a ‘bump’), and if we can validate the DM intepretation, e.g. through the observation of a consistent gamma-ray spectrum, this would represent an extraordinary result with dramatic consequences on our understanding of Dark Matter.