Introducing MICE

The international Muon Ionization Cooling Experiment (MICE) is a high energy physics research experiment based at Rutherford Appleton Laboratory in Oxfordshire, U.K., with collaborators from institutes across the globe.

Muons are fundamental particles just like electrons, however they are heavier and they are unstable – they decay into photons (the particles which make up light), electrons and another type of fundamental particle known as neutrinos. The feynman diagram for this decay is shown below. Muons decay extremely quickly; on average they only live for 2.2 millionths of a second (2.2μs) before disappearing.

The feynman diagram for muon decay.

Now, neutrinos, one the particles which muons decay into, are very interesting fellows. If you were hosting a dinner party for fundamental particles, they would be top of your list. In particular neutrinos interact with normal matter and its mirror counterpart part, anti-matter differently (what physicists call charge-parity asymmetry). Understanding how differently could help us solve a fundamental problem in particle physics – why there is so much more matter in the Universe than anti-matter.

In order to study neutrinos and how they behave, we need to first generate a beam of them. There are number of ways to do this using particle accelerators. The way which lets you best understand the neutrino beam  is to make a beam of muons, and to wait while the muons decay, producing neutrinos. A facility which generates neutrinos using this method would be known as a Neutrino Factory.

Muons themselves are quite easy to make if you have a particle accelerator handy. You start off with hydrogen gas and heat it up to a very high temperature. When the hydrogen gets hot enough, the electrons orbiting the hydrogen atom nuclei (which is just one proton) detach themselves, and you have protons and electrons moving freely in a state known as a plasma. Using electric fields, protons can be pulled out of this plasma and accelerated to high energy by more electric and magnetic fields. Once they are traveling fast enough, this proton beam can smashed into a solid target (we use titanium) which generates a shower of subatomic particles, including another type of unstable particle known as pions. Finally, pions in turn quickly decay into muons.

Sounds simple enough, I hear no one saying. In practice however there are a few more complications. The muon beam which comes from pion decay is very spread out and divergent. Imagine trying to direct the water from a hosepipe through a house window. Easy enough if the pressure is high, but what if you partly cover the nozzle with your finger and the water is spraying everywhere? That is a very rough picture of what muon beams are like when they are first created – it is very hard to use them effectively unless we can make the beam more like the water from a hosepipe without a finger on the nozzle – narrow and all travelling in the same direction.

The purpose MICE of is to show that we can take a beam of muons and then shrink this beam’s size and divergence (known as emittance) to get the sort of well behaved beam we would need to do neutrino science. Normally, when accelerator physicists want to shrink the size of a particle beam, they used magnetic fields to push the particles into a smaller configuration. Muons are more tricky however, because they decay so very quickly – by the time magnets would have shrunk the beam, all the muons would be gone!

MICE is testing a new way of shrinking emittance, called ionization cooling. The idea is to send the muons through a material (typically hydrogen or lithium hydride), causing the muons to lose energy as they interact with the material (knocking electrons off the atoms, ionizing them). After the muons have lost energy in this manner they can be given energy back, via radio frequency electric fields, but only in the direction of the beam. Thus the muons lose energy in all directions, but get back get it back only in the direction we want them to travel, causing the beam to shrink.

Ionization Cooling – muons lose energy in all directions in a low atomic number absorber. Energy is then restored only in the direction of the beam via radio frequency (RF) electric fields. This shrinks the beam emittance.

MICE is built, commissioned and presently taking data on the first part of this process, passing a muon beam through a low atomic number material.

As off August 2017, lithium hydride running is done, and we are preparing the experiment for liquid hydrogen running in September and October. Results will be published soon, so watch this space for more updates from the research frontier!

For more information on MICE, including our publications, and live data plots, see http://mice.iit.edu.

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