Last Friday Samuel Ting, the winner of the 1975 Nobel prize in Physics for the co-discovery of the J/ψ particle, gave a seminar in the packed CERN main auditorium on the latest results from AMS, the Alpha Magnetic Spectrometer installed on the international space station.

The seminar began with a well-crafted exposition of AMS, detailing how the spectrometer is constructed with a set of subdetectors capable among other things of identifying and measuring positrons from cosmic rays with extremely high purity. Ting spent the first 20 slides or so of his talk providing detail on the tracker, the transition radiation detector, the electromagnetic calorimeter, and the other components, giving credit to as many of his colleagues as he could. I found this a bit pedantic but really appropriate, given that Ting is well beyond retirement age and yet he is still the one attracting all the spotlights.

So AMS is a very nice detector - as good as we can afford to put in orbit these days. Here suffices to say that it is a 5 by 4 by 3 meters thing, weighing 7.5 tons, comprising 300,000 electronics channels. Its design is optimized for the task of detecting and measuring the flux of electrons, positrons, protons and antiprotons in cosmic rays in a very redundant way. The transition radiation detector (TRD) identifies electrons and positrons, the silicon tracker subsequently measures the atomic number Z and momentum P of the incident particle, then the ECAL calorimeter measures energy of electromagnetic showers. In addition, a time-of-flight (TOF) system measures Z as well as E, and an excellent ring-imaging Cerenkov (RICH) detector also measures Z and E; the charge is determined by the curvature of particle trajectories inside a magnet. Below is a sketch showing the various components.

(Image credit: AMS collaboration)

In order to study the positron flux, the detector must separate protons from positrons effectively. The combination of TRD and ECAL does this at the level of 10^6. Ting explained that for 1-TeV particles there is still a factor of 50 rejection of protons in the positron signal by using the TRD alone. The maximum measurable momentum of the tracker is 2 TeV.

In 40 months of data taking, AMS collected 54 billion cosmic rays. That is a lot of cosmics - Ting said it is more than all the cosmic rays collected in the last 100 years altogether, but I question whether his claim is indeed true. In any case, it is an impressive dataset!

One objective of AMS is to find the origin of dark matter. The idea is that the collision of ordinary cosmic rays may produce electrons and positrons, which have a predictable spectrum in energy. The collision of dark matter would produce additional ones. If one measures the ratio of positrons over sum of positrons and electrons, one expects a smooth behaviour as energy increases, while the addition of a neutralino annihilation signal would produce a smooth increase with energy and then a sharp cutoff. A similar signal would be observable with antiprotons, but with much smaller yields.

 There are six features one may study in the data as a function of energy. The energy at which the ratio begins to increase, the rate of increase with energy, the existence of sharp structures, the energy beyond which the ratio ceases to increase, the isotropy in the signal, and the rate at which the ratio falls beyond the turning point. Ting showed results for all but the last feature -saying it is still being analyzed. In fact, the latter is the single most interesting feature, as a sharp drop would be very difficult to explain with any non-dark-matter model for positron fluxes.

The data showed an increase of the flux above 10 GeV, no sharp structures, a positive slope up to 275+-32 GeV. He mentioned that there seems to be no anisotropy in the direction of the flux, with a limit at 0.03. Ting teased us by explaining how the sharp decrease would be a tale-telling feature, by showing simulations of pulsar signals and of the signal of a neutralino; but the data he showed was sapiently constrained to the part which does not fall off -if it does.

He demonstrated, by showing the individual fluxes, that the rise of the positron fraction is due to an excess of positrons, and not to a loss of electrons. In summary, the measurement of the fluxes show that they are different in magnitude and in energy dependence. They cannot be described by single power laws; but a single power law can describe the total flux.

The interpretation of what the data may mean of course is an open issue. Ting showed the simplest possible model that can fit the data: the sum of a diffuse flux and a source flux for both electrons and positrons. Once one does a simultaneous fit to the positron flux and the total flux, one gets the parameters of the diffuse and the source fluxes. Everything does seem to fit in nicely with the simple model he proposed. However, the highest-energy data which is still to be released is the one which may make the AMS data a credible first indication of dark matter detection. That is why I believe Ting is bound to return to the CERN auditorium soon... Unless there is no decrease!