Cosmic rays surprise us again

The Alpha Magnetic Spectrometer being tested at CERN by being exposed to a beam of positrons.

The Alpha Magnetic Spectrometer (AMS-02)being tested at CERN by being exposed to a beam of positrons. (Picture from Wikipedia)

[Text and figures updated on April 9th 2013 due to insight from Ryan Nichol: Thanks]

The team running the Alpha Magnetic Spectrometer (AMS-02) have produced their first set of results. And as expected, they are full of surprises.

AMS-02 is an awesomely complex device – too power-hungry, heavy and complex to be placed on its own space platform, it was attached to the International Space Station 18 months ago on the last space shuttle mission. I wrote about this here.

It has with 650 separate microprocessors, 1118 temperature sensors and 298 active thermostatically-controlled heaters. It is basically a general-purpose particle detector like those found at CERN, and represents the culmination of nearly one hundred years of ‘fishing for particles’ in the high atmosphere.

  • First we flew balloons and found that ‘radiation levels’ increased as we went higher.
  • Then we discovered a ‘zoo’ of particles not yet observed on Earth – positrons, muons, pions, and anti-protons.
  • Then we discovered that ‘cosmic rays’ were not ‘rays’ but particles. And we realised that at the Earth’s surface we only observed the debris of collisions of ‘cosmic ray’ particles with the atoms in the upper atmosphere.

Where did these primary cosmic ray particle from?  What physical process accelerated them? Why did they have the range of energies that we observed? What were they? Protons? Electrons? Positrons? We just didn’t know. The AMS-02 was sent up to answer these questions.

I have found much of the comment on the results incomprehensible (BBC Example) with the discussion being exclusively focussed on ‘dark matter’.  So I thought I would try to summarise the results as I see them based on reading the original paper.

Over the last 18 months (roughly 50 million seconds) AMS-02 has observed 25 billion ‘events’  (roughly 600 per second). However, the results they report concern only a tiny fraction of these events – around 6.8 million observations of positrons or electrons believed to be ‘primary’ – coming straight from outer space.

  • They found that – as is usual for cosmic rays – there were fewer and fewer particles with high energies (Figure 1 below)
  • Looking at just the electrons and positrons (i.e. ignoring the protons and other particles they observed) there were only about 10% the number of positrons compared with electrons, but that the exact fraction changed with energy (See Figure 2 below)
  • They found that there were no ‘special’ energies – the spectrum was smooth.
  • They observed that the particles came uniformly from all directions  – the distribution was uniform with variations of greater 4% very unlikely.
  • The electron and positron fluxes followed nearly the same ‘power law’ i.e. the number of particles observed with a given energy changes in nearly the same way – indicating that they probably have the same source.

They conclude very modestly that the detailed observation of this positron ‘spectrum’ demonstrates…

“…the existence of new physical phenomena, whether from a particle physics of astrophysical origin.”

I like this experiment because it represents a new way to observe the Universe – and our observations of the Universe have always surprised us. Observations have the power to puncture the vast bubbles of speculation and fantasy that constitute much of cosmology. I am sure that over the 20 year lifetime of this experiment, AMS-02 will surprise us again and again.


Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

AMS Figure 6

Figure 2: Graph of the fraction of positrons compared with electrons as a function of energy in billions of electron volts (GeV). The ‘error’ bars show the uncertainty in the fraction due to the small number of events detected.


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4 Responses to “Cosmic rays surprise us again”

  1. Ryan Nichol Says:

    Hello Michael, I just wanted to offer a small clarification to your description. The ~10% positron fraction is not a fractional excess of a positrons over electrons but an absolute fraction. That is to say AMS, Pamela and Fermi observe approximately 10 electrons for each positron.

    p.s I confess, I couldn’t quickly determine if the number quoted is N_e+/N_e- or N_e+(N_e- + N_e+).

    • protonsforbreakfast Says:

      Whoops, Thanks I have clearly misunderstood this. And I now can’t see what is so special about this observation. A fractional excess of positrons speaks of a marginally asymmetric process. A small fraction of positrons – well … it could be anything?!

      Anyway: I will amend the text.

      Oh and congratulations on the baby by the way – I hope you are all well. M

  2. Why Measuring Stuff Matters | Protons for Breakfast Blog Says:

    […] Measurements of the type and energy of particles in Cosmic rays may reveal details about nature of the Universe. […]

  3. gdw100 Says:

    Reblogged this on garywilliamsblog.

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