AMS is on

AMS-02 is up and running, and first events have already been twitted to the Earth. AMS is a full fledged particle detector attached to the ISS whose goal is to measure the cosmic ray spectra. The mission has been plagued by ill fate (delay due to the Columbia crash, scrapping of their superconducting magnet), now the road seems to be clear at last. The final preparations and the launch have been widely reported in the mainstream media, however my impression was that the actual science that AMS may accomplish was not clearly exposed. Here is my understanding of what AMS could teach us.

The official page of AMS lists the following scientific goals

• Search for primordial antimatter
• Search for dark matter
• Search for exotic forms of matter
• Study of the cosmic ray composition

The first point situates somewhere between Sam Ting's fixation and crackpottery. AMS will search for anti-helium nuclei arriving from the outer space. Unlike antiprotons, positrons and anti-deuterium, heavier anti-nuclei are not expected to be produced by cosmic ray collisions; anti-helium would have to be produced by astrophysical objects made of anti-matter. The problem is that we know there is no such thing: all the primordial antimatter annihilated with matter around 1 sec after the big bang. This view is not only the consequence of the current cosmological model, but it is also firmly supported by several independent observations, such as the cosmic gamma-ray spectrum, the cosmic microwave background, and the near perfect agreement between the predictions of nucleosynthesis and the composition of visible matter in the universe. Given the current body of evidence, AMS has a better chance for a 3rd degree encounter than for finding primordial anti-matter.

The situation with dark matter is more subtle. The PAMELA and FERMI satellites launched in the previous decade have been providing us with precise measurements of the high energy cosmic ray spectra. One thing we definitely have learnt is that it is painstaking to search for dark matter this way. Several excesses over theoretical predictions have been reported so far: PAMELA's positrons, Fermi's electrons, Fermi's photons from the galactic centre. They all have a plausible interpretation in terms of models of dark matter and an equally plausible interpretation in terms of boring astrophysical phenomena. AMS may provide more input regarding the high energy spectra. As can seen in the plots of the projected sensitivity, after 10 years of data taking they expect to extend the measurement of the positron and antiproton spectra up to almost TeV (compared to the current reach of PAMELA of about 200 GeV). It's hard to say if these projections are realistic, since it is not clear how much the resolution at high energies is degraded due to the replacement of the superconducting magnet by a weaker permanent one. Assuming they are realistic, particle physicists will be able to refine their models of dark matter, and astrophysicists to refine their models of pulsars. In any case, the chances for a smoking gun signal of dark matter appear slim at this point.

Nevertheless, there is one area where AMS is clearly superior to all previous experiments. The instrumentation of AMS includes a calorimeter, trackers, a Cherenkov detector and a time-of-flight detector to measure the energy, charge and mass of incoming particles. All this gives them very good particle identification, in particular they can easily separate heavier nuclei from much more numerous protons and helium nuclei. Flux ratios of various heavy nuclei, for example the boron-to-carbon ratio, are an important input for the models of cosmic ray production and propagation. Furthermore, if there exists exotic matter with distinct charge-to-mass ratio, for example the hypothetical strangelets with small Z/A, AMS is well equipped to identify it.

In summary, high energy astrophysics is a crowded field, and AMS is unlikely to turn it upside down. Their best shot for a spectacular discovery is exotic forms of matter with distinct Z/A ratio, provided they exist. Furthermore, if AMS and the ISS last long enough, and if the performance of the detector is as good as they promise, they should be able to extend PAMELA and FERMI measurements of the antiproton and positron spectra to higher energies, which may or may not clarify the origin of the positron and electron excess in PAMELA and Fermi. In the worst case AMS will sort out the spectra of heavier cosmic ray nuclei, providing valuable input for cosmic ray propagation model. Critics may complain that 2 billion dollars for tuning GALPROP is a lot. Optimists may stress that so far it's the only hope for returns from the 200 billion dollars sunk into the ISS.

Figures are taken from the talk of Andrei Kounine at TeVPA'10.