The highest energy particle in nature

The highest energy particle in nature

Ordinary white light from the sun or a bulb at home has an energy of about 1 eV. X-rays and gamma rays have energies of about 1,000 and 10,00,000 times that of ordinary light. The masses of electrons and protons are around 0.5 MeV and 938 MeV. High energies for these particles could be obtained when humans started building accelerators. At first, one could manage to get only several GeV. However, by 1990, the particles could reach an energy of 1,000 GeV in the Tevatron accelerator in Fermilab. The biggest accelerator today is the Large Hadron Collider in Switzerland in which protons are accelerated to 14 TeV.

While humans have struggled to get to this energy, nature seems to be doing it easily at several sources in the universe and sending them to earth as Primary Cosmic Rays (PCR). In fact, these rays, which are actually particles, come from all directions in space. These have energies starting from MeV to millions of TeV and more. As could be expected, the rate of PCR decreases with increasing energy. Typically the rate for a square metre detector is about one per year at around 1,000 TeV. This further decreases by a factor of 1,000 at around 1 EeV. The fact that the energy of particle with 10 EeV is equivalent to 1.6 joules shows the enormous velocity of these particles.

These high energy particles can be detected only indirectly through the billions of secondary particles they produce in the atmosphere. These secondary particles are collectively called Extensive Air Showers (EAS). These can be detected on ground by spreading many detectors with large separation. Measuring various parameters of the EAS gives information about the mass, energy, the arrival direction, etc, of the primary particle. There are several such EAS arrays in the world including one at Ooty. But most of them are designed to probe medium energy PCR. In the region above 10 EeV, the flux of primaries is extremely low and because of this, there are several unanswered questions like 'where are these PCRs made?'

To detect particles of higher energy, one needs a huge experimental array since the particle rates are low. James Cronin, a physicist at the University of Chicago, USA, along with Alan Watson of University of Leeds, UK, decided to study the highest energy PCR by building a very large array in 2003. The array they envisaged and eventually built had a detection area of around 3,000 sq km. Particle detection is done with 1,700 water Cerenkov detectors which are essentially water-filled plastic tanks fitted with optical sensors. These are deployed on a grid spacing of
1.5 km. The array is spread across the plains
of Pampa Amarillo at the foot of the
Andes in Argentina. The experiment, named after French physicist Pierre Auger, started in 2008 and has recorded several thousand particles of EeV energy.

The energy limit

At the highest energies, the cosmic ray proton flux is expected to get depleted because of the interaction with the low energy photons of the Cosmic Microwave Background. This suppression is called the GZK (Greisen, Zatsepin and Kuzmin limit). Thus, one of the important aims of the experiment is to measure the flux of cosmic ray particles at these high energies and check for the proposed suppression. Data collected for the last decade shows about two lakh quality events having energy greater than 3 EeV. Furthermore, analysis has revealed that a depleted flux is clearly beyond 40 EeV. The observed flux becomes half of what one expects from the lower energies. Thus, the experiment shows a limit in energy for PCR events.

However, the Auger experiment has also shown that the fraction of protons decreases around 1 EeV with the composition becoming increasingly heavy. So, the decrease in number of PCR seen in the experiment is possibly due to a suppression of proton flux due to GZK cut off and the lack of higher energy heavier particles due to possible source inefficiencies. Thus, the important conclusion is that cosmic accelerators may not be able to generate particles with energy greater than 50 EeV!

In September 2017, James and his colleagues announced a large scale anisotropy in the arrival direction of PCR. It was found that the rate of arrival is about 6% greater from one side of the sky than from the opposite direction, with the excess lying 120 degrees from the centre of our galaxy. This means that the PCR arrives with a greater rate from a direction distant from the galactic centre and thus, is not from our galaxy.

It has been speculated for quite some time that powerful active galactic nuclei (AGN) like quasars can accelerate particles to very high energies. The analysis of the direction of these energetic Auger events has shown that the source of some of these particles could indeed be AGNs. With much to learn, this experiment is only the beginning.

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