Cosmic Antimatter Rays!

by Nicholas Mee on April 11, 2013

In 1912 the Austrian physicist Victor Hess conclusively demonstrated the existence of cosmic rays when he took an electrometer to an altitude of over 5000 metres in a hot-air balloon. At this altitude Hess detected four times as much radiation as at ground level.

We now know that the Earth is continually bombarded by high energy particles from the depths of space. On reaching the Earth’s atmosphere most of these cosmic rays collide with molecules of nitrogen or oxygen in the air, which is why only a small proportion of the rays reach the Earth’s surface. The atmosphere does a great job of protecting us from this harmful radiation, but it means that Earth-based detectors are unable to gain a clear picture of the incoming radiation. This is why the Alpha Magnetic Spectrometer (AMS) has been mounted on the International Space Station. It is designed to detect cosmic rays before they enter the atmosphere.

Since May 2011 AMS has been collecting data. It has already recorded the impact of far more cosmic ray particles than any previous space-based experiment. Cosmic rays do not all have the same identity. They are simply stable massive particles that are racing across the universe. These particles include protons, electrons and the nuclei of various types of atom.

Antimatter

The feature of cosmic rays that is getting physicists excited is the proportion of them that are antimatter particles.

It has been known since the 1930s that there is a mirror image particle, or antiparticle, for each matter particle. For instance, the antiparticle of the proton is known as the antiproton. It has exactly the same mass as the proton, but the opposite electric charge, so whereas the proton is positively charged, the antiproton is negatively charged. Similarly, the electron has an antiparticle, which is known as the positron. Whereas the electron is negatively charged, the positron is positively charged, as its name might suggest.

One of the mysteries that AMS is revealing is that there are far more cosmic ray positrons than expected.

Where do cosmic rays come from?

Most cosmic rays are produced in a supernova explosion in which a star has blasted itself apart in its death throes. All the elements in the Periodic Table are cooked up in these explosions and the shock waves send protons and atomic nuclei flying off into space with extremely high energies. These incredibly violent events might account for most of the particles that form cosmic rays, but the large quantities of cosmic ray positrons are thought to require a different explanation. There are two main ideas about their origin.

The Crab Nebula which was produced by a supernova explosion observed in 1054. (Copyright NASA)

Cosmic Lighthouses

Following a supernova explosion all that is left of the original star is an ultra-compact remnant known as a neutron star. The neutron star consists of a star’s worth of material compressed to the density of an atomic nucleus. This is the equivalent of the Sun being squeezed into a sphere the size of a major city such as London. The most famous example of a neutron star lies at the heart of the crab nebula, which was produced by a supernova explosion seen almost 1000 years ago in 1054.

Neutron stars spin extremely rapidly – the neutron star within the crab nebula is more massive than the Sun, but it rotates 30 times a second. This generates incredibly strong magnetic fields that produce two oppositely directed beams firing radiation into space like a cosmic lighthouse. When these beams point in our direction, once every rotation, a pulse of radiation may be detected. For this reason, these objects are known as pulsars. The pulsar beams are thought to act like gigantic particle accelerators, and they might be the source of the excess positrons detected by AMS.

Computer generated simulation of a pulsar. (Copyright Nicholas Mee)

Dark Matter

But there is another intriguing possibility. Many theorists think that the next big discovery at the Large Hadron Collider will be supersymmetry and that the lightest new particle predicted by the theory would be a stable particle that was produced in great profusion in the very early universe. If this is correct, the universe would still be full of these particles. In fact they would have all the characteristics of dark matter. Some theories of supersymmetry suggest that when two of these particles meet they will mutually annihilate to produce other particles that will then decay, the net result being the production of positrons. This might explain the surprisingly large quantity of cosmic ray positrons detected by AMS.

So the results from AMS could be the first signs of the existence of particles of dark matter.

Which Idea Is Correct?

Fortunately, there is a way to distinguish between these two possible solutions. If the hypothetical dark matter particle has a mass of say 175 GeV, which is perfectly possible and is about 200 times the mass of a proton, then when two such particle mutually annihilate a total of around 350 GeV would be released. This is the energy that is available to any new particles, such as positrons, that would be produced. This means that if dark matter particles are the origin of the positrons then there will be a maximum energy for such particles. Beyond this energy none will be detected.

If the positrons are being produced by pulsars, on the other hand, then there might be fewer with very high energies, but the numbers would be expected to tail off gradually with no abrupt limit to the energy of the positrons.

AMS has not yet collected sufficient data to distinguish between these two possibilities, but within the next few years it should be able to provide us with a definitive answer.

Further Information

My article Super Symmetry! includes more information about supersymmetry and how it might explain the origin of dark matter.

The latest results from the AMS detector were announced by Nobel laureate Sam Ting in a seminar at CERN on 3rd April. More details are given here:  http://home.web.cern.ch/about/updates/2013/04/ams-experiment-measures-antimatter-excess-space

For more detailed information, take a look at this paper: Viewpoint: Positrons Galore

 

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