giovedì 4 aprile 2013

AMS-02 confirms PAMELA positron data extending the measurement at higher energy

(This is a bad english translation of the italian post)

PAMELA is a magnetic spectrometer launched more than six years ago aboard the Russian satellite Resurs-DK1. Since 2006 has been providing high precision data on cosmic rays of solar, galactic and trapped in the geomagnetic field.
The main result is that the size of the component of antimatter, antiprotons and positrons, present in cosmic rays. The importance of this measure lies in the possibility indirect detection of  dark matter.

Possible production of secondary particles as a result of the annihilation of dark matter particles. 
  PAMELA data, published in Nature in 2009   ( Nature 458, 607-609 (2009)   a copy is available here ), reached for the first time the energy of 100 GeV. The picture that emerges is challenging for dark matter: the number of antiprotons is consistent with what is expected from a normal production, while the positrons shows a significant increase above 10 GeV. Various hypotheses have been put forward (several hundred in more than 1000 papers). Indeed, on the nature of this unexpected increase of high-energy positrons the most interesting is the possibility that they may be produced by the annihilation of dark matter, although astrophysical sources such as pulsars could contribute in part the positron flux observed.

Measurement of the positron / (positrons + electrons) (top) and antiproton / proton (bottom) measured by PAMELA. The measurement of the ratio allows for a more precise measurement, reducing the instrumental effects. Note the rarity of antiparticles than particles. The component of antiprotons is consistent with the various hypotheses on the interaction of cosmic rays in the interstellar medium (solid and dashed curves), while that of positrons shows an unexpected excess above 10 GeV. The black curve shows the expected contribution in the absence of anomalous contributions, while the green curve suggests the contribution of a particle of dark matter, which - annihilate - produces an excess of positrons. The blue curve shows an estimate of the possible contribution due to Pulsar.


The results of AMS compared with those of PAMELA and GLAST
(From here )

It is therefore very   nice (and mostly reassuring ....) to have a confirmation of the PAMELA 2009 data. The first confirmation came with the satellite FERMI / GLAST in 2011 ( here the preprint, here an explanation of the method of measurement) which extended the energy. range 
The independent measurements of AMS , presented by Sam Ting at Cern April 3 (here the press release), here an excellent post by Roberto Battiston, in fact, show a good agreement with those of PAMELA. The greater size of AMS allows to have a greater number of positrons and therefore a minor statistical error and reach higher energies. At low energies the difference is due to the solar modulation. At higher energies it seems that the ratio positron / electron tends towards a constant value (and lower than that of GLAST), indicating that the mechanism of production of positrons, whatever it is, is going towards a constant value. Unfortunately there are no clear structures  (with a lot of imagination something to 100 GeV) or sharp drops that  might be due to  dark matter annihilation. 
This first result shows the potential of AMS, the most important experiment on board the  International Space Station and bodes well for the continuation of the mission.
It will be interesting to see if the results of AMS on antiprotons confirm -as it is plausible to expect - those of PAMELA.