The Chamber of Secrets

by Nicholas Mee on April 29, 2018

By the mid-1930s, just five fundamental particles were known. This concise collection of building blocks revealed the true nature of matter and light. Three types of particle: electrons, protons and neutrons, form the wide array of atoms known to chemistry, and the whole electromagnetic spectrum including light is composed of photons. The fifth particle is the positron, the anti-particle of the electron, predicted by Paul Dirac and discovered by Carl Anderson in cosmic rays.

Inside the CMS detector during maintenance. The outer polygonal rings are the muon chambers that track muons created within the LHC.

Who Ordered That?

The world of particle physics seemed neat and tidy with everything in its place. Then, in 1936, Carl Anderson and Seth Neddermeyer announced the discovery of another new particle in cosmic rays, a particle now known as the muon. The whole physics community was taken aback at the unexpected arrival of a particle with no obvious role in the grand scheme of things. Nuclear physicist Isador Rabi captured their surprise in his memorable reaction: Who ordered that?!

Our understanding of particle physics has come a long way since the 1930s. The Large Hadron Collider (LHC) blasts high energy protons together and a whole variety of particles are produced in these collisions. Most such particles are composed of quarks and antiquarks and interact via the strong force. These particles are known as hadrons. There are just a handful of particles, known as leptons, that do not feel the strong force.

The fundamental particles fit neatly into the standard model table of particles. There are three generations of matter particles plus their antiparticles. Each generation contains two types of quark and two leptons. The first generation consists of the up and down quarks, the electron and the electron neutrino, as shown in the first column below.

The particles of the Standard Model. The three generations of fundamental matter particles (fermions) are shown on the left. The force exchange particles (bosons) are shown on the right.

Muons behave just like very heavy electrons, with around 207 times as much mass. Like electrons, they feel the electromagnetic and weak forces, but not the strong force. Unlike the electron, the muon is unstable, decaying into an electron and two neutrinos with a lifetime of around 2 microseconds. The muon is the second generation equivalent of the electron as shown in the standard model table of particles above.

The tauon is the third generation equivalent of the electron. Discovered in 1975, it has almost 3500 times the mass of an electron. It is highly unstable because of its greater mass and decays in less than a trillionth of a second. The charged leptons, that is the electron, the muon, the tauon, and their antiparticles, interact via both the electromagnetic and weak forces. There are also three types of uncharged lepton and they only interact via the weak force. They are known as neutrinos.

The Dragon Of Smoke Escaping From Mount Fuji by Katsushika Hokusai.

Muons undergo numerous interactions when passing through matter, but lose very little energy in each collision. Being so much more massive than electrons, they just brush the electrons to one side as they pass by. Muons are also deflected much less than electrons by the electromagnetic fields within a solid material, so they generate much smaller electromagnetic ripples and lose much less energy in this way as well.

Tauons are more massive than muons and would pass even further through matter, but they decay so rapidly they do not have enough time to travel a significant distance. The upshot is that muons are the most penetrating of the particles produced in the LHC apart from the will-o-the-wisp neutrinos that head off into deep space and disappear without a trace. The outermost parts of the two main detectors at the LHC, known as ATLAS and CMS (Compact Muon Solenoid), are dedicated to tracking the muons created in the proton-proton collisions within the machine.

Enter the Dragon

Muons are continually created in our atmosphere as high energy cosmic rays from distant regions of the galaxy, mainly protons, collide with atomic nuclei in the atmosphere. Every second dozens of these muons pass through our bodies. It is estimated that about 150 muons pass through every square metre of the Earth’s surface every second. These ultra high energy muons can penetrate up to a kilometre of solid ground. They travel further through less dense materials such as air and this has given rise to an important practical application.

Japanese physicists have adapted sensitive muon detectors designed for particle physics experiments to monitor the innards of active volcanoes in a technique known as muon transmission imaging or muography. Detectors are positioned around the volcano and the flux of muons from different directions is measured. There is a greater transmission of muons through low density material such as a cavity within the volcano, so a 3D image of the interior of the volcano can be generated much as an X-ray is used in medicine. By imaging the volcano over a period of time it is possible to see the magma chamber filling with molten lava which offers the potential to save lives by evacuating the area prior to an eruption.

Muograph of Mount Iwo-dake on Satsuma-Iwojima Island. Credit: Hiroyuki Tanaka.


The Riddle of the Sphinx

Muons are now being used to investigate the mysteries of the ancient world. The Pharaoh Khufu ruled Egypt over 4500 years ago. Khufu is remembered for building the Great Pyramid on the plateau of Giza close to present day Cairo. Khufu’s monumental tomb is believed to have been constructed in just twenty years. Within the pyramid are three chambers whose names are modern inventions with no historical significance. They are the King’s Chamber, the Queen’s Chamber and an unfinished chamber cut into the bedrock, as shown in the diagram below. The pyramid was looted in antiquity and all that remains inside is the base of a sarcophagus in the King’s Chamber. But the Great Pyramid is vast and there has long been speculation that other hidden chambers may exist deep within its structure.

The muographers have taken up the challenge of revealing secret rooms hidden within the Great Pyramid. Last year a team of physicists led by Kunihiro Morishima of Nagoya University, Japan announced the discovery of a large cavity about 30 metres long above the Grand Gallery within the pyramid. They made this remarkable discovery by placing muon detectors in the Queen’s chamber and using the imaging methods developed for monitoring volcanoes. To confirm the reality of the discovery three different techniques for detecting muons were used, so the physicists are confident that their results are correct.

Perhaps, this is the secret chamber where the royal treasures of pharaoh Khufu were hidden 4500 years ago and they have rested there undisturbed ever since. As yet, no-one knows. The team is currently designing a mini flying robot with the aim of investigating further.


Further Information

A video version of this post is now available here: The Riddle of the Sphinx on The Cosmic Mystery Tour YouTube Channel. Please don’t forget to click the subscribe button.

There is more information about how the standard model was developed in my book Higgs Force: Cosmic Symmetry Shattered.


{ 2 comments… read them below or add one }

Julius Mazzarella April 29, 2018 at 3:40 pm

Wow…great article. You have a great talent expressing science related concepts …..

As I was reading about the three generations of particles something that came to mind…..I was wondering if the Standard Model predicted the three generations or did they just show up and could there be anymore?

Also from time to time I read about “quasi particles” such as the fracton, exciton, majorana fermion, ……long list….maybe you can write an article sometime as to how these differ from the particles of the Standard Model. or just how do they fit it! I bet some other readers may be interested as well.

Thank you
Julius Mazzarella


Nicholas Mee April 29, 2018 at 3:58 pm

Dear Julius
The standard model is only consistent if each generation has its full complement of four particles, so when the tauon was discovered in 1975 physicists realised the other three members of the generation were awaiting discovery. The final member of the generation, the tauon neutrino, was eventually tracked down by particle physicists at Fermilab 25 years later.
By studying the decays of the Z boson the number of neutrinos has been determined as three and no more. This was one of the most important results from the LEP collider which operated in the 1990s in the tunnel that is now home to the LHC. As there are only three types of neutrino there can only be three generations of matter particles. The universe seems to work quite well with three generations, but physicists have no explanation for why there are three and no more.


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