Successful Demonstration of Ionization Cooling by MICE Collaboration
06 Feb 2020
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Nature has published a paper by the MICE collaboration, in which they demonstrate a technique that could carry the search for new phenomena in lepton-antilepton collisions to extremely high energy and provide uniquely well-characterized neutrino beams.  

 

We’ve made great progress colliding beams of high-energy particles; we’ve gained a better understanding of matter and its interactions, refined the standard model and, in expanding the applications, we can analyze chemical structures or even treat cancers.

This has predominantly been done using beams of stable particles; protons, anti-protons, electrons and positrons, but there’s potential using muon-antimuon collisions to produce beam energies that would dwarf the best of our current efforts. 


16EC2630_MICE_facility.jpg 


What makes a Muon different?

Almost identical to electrons, muons are a fundamental particle, but they’re around 200 times heavier than an electron and unstable too. It’s this extra mass that allows for higher energies and if muons were stable, they would be the perfect particle to accelerate!


When accelerating particles, you also need to take synchrotron radiation into account; the radiation given off when a charged particle is accelerated perpendicular to its velocity. This radiation takes the form of photons being emitted (so energy is lost) and since most particle accelerators are circular, it can massively take away from the energy that ends up going into the final collision*. Synchrotron radiation increases on a scale of 1/m4; so electrons will emit more than their heavier brother the muon, but the proton will emit the least. This means we can keep building more and more powerful proton accelerators, but since they’re not a fundamental particle the collisions are much “messier”.

*n.b Synchrotron Radiation isn't always a problem! For example, the Diamond Light Source creates synchrotron light to be used for a range of experiments.


There's also the advantage that the Higgs coupling is proportional to the mass of the muon. In the Standard Model, fundamental particles (electrons, muons, quarks, short-lived particles like the Z boson) are massless. The Higgs particle gives them mass by how strongly it couples to them: light particles have low couplings and heavy particles have high couplings. The masses of the particles seem to be quite random with a range of values - electrons are over 300,000 times lighter than the heaviest known particle, the top quark.


As the muon mass is proportional to the Higgs Coupling, the chances of directly producing a Higgs boson at a muon collider are much higher - around 200 times more than with electrons. What's more, the theoretical description of muon decay isare so precise that the beams of neutrinos created when muons decay can be used to try and understand aspects of big bang and the beginning of our universe – in particular, why there’s so much more matter than anti-matter left behind.


However, for us to avoid using muons for so long, you can imagine there are numerous difficulties they come with; for example, muons have a lifetime of only 2.2μs, in which time they have to be captured and accelerated.
 

MICEphotoTarget-26.jpg14EC3149_MICE_build_1_Aug.jpg


The problem with Muons..

The main breakthrough made by the MICE (Muon Ionization Cooling Experiment) collaboration is in overcoming difficulties arising from the way in which muons are produced. When, for example, protons are created(from separating hydrogen atoms/ions), they all head in the same direction, rather like a military marching band. Muons on the other hand, form more of a ‘cloud’ and can be likened to a mixed group of pedestrians crossing a bridge; moving side to side, some distracted by the sights, as they travel.  
 
If we used a cloud of muons as is, we would get a beam that is 10s of centimeters in diameter. The magnets needed for an accelerator that could accept this would be expensive both to build and run. Furthermore, the particle intensity would be too low and so the chances of them colliding and producing interesting physics is also low.  

 


 ..And how to fix it

The process of reducing the beam diameter, making this cloud less diffuse and getting the muons moving in the same direction is called cooling. “Ionization cooling” was first proposed in the 1970s and in early 1980s Simon Van Der Meer solved a similar problem for anti-protons; called “stochastic cooling” and it earned him a Nobel Prize. The problem is that stochastic cooling takes a few seconds, but muons live for a few millionths of a second. In the 1990s ionization cooling became a theoretically operable technique but putting it into practice was still formidable.  

 

The MICE collaboration was formed in the early 2000s and developed the novel technique shown below, which has now been proven to work.

Screenshot 2020-02-04 at 14.51.18.png

Figure above shows downstream to upstream ratio of number of events. A ratio greater than unity in the beam core is evidence for ionization cooling and is evident for 6-140 and 10-140 beams with both the full LH2 absorber and the LiH absorber. The effect predicted from simulation is shown in red, while that measured is shown in black. Taken from https://arxiv.org/pdf/1907.08562.pdf


Making the muon beam

The experiment took place in the UK using the MICE muon beam-line, part of the ISIS neutron and muon source. Fast moving protons strike a target (such as tungsten or mercury) which produces a debris of pions, kaons and muons. The pions produced are separated and directed a long a series of magnetic lenses where they quickly decay to produce extra muons (and neutrinos). Muons are then captured by beamline elements to form a third beam. - And all of this must be done in under 2.2μs, the lifetime of a muon.



MICE infographic FINAL.jpg




Links for more information:

MICE Collaboration home page

MICE's most up to date page (as of 06/02/2020) for the public

Our MICE page

MICE Infographic

STFC article following MICE's publication in Nature

CERN Courier article on MICE



Quotes from members of the collaboration

“The enthusiasm, dedication, and hard work of the international collaboration and the outstanding support of laboratory personnel at STFC and from institutes across the world have made this game-changing breakthrough possible,” commented Professor Ken Long from Imperial College London, spokesperson for the experiment.


Dr Chris Rogers, based at ISIS and the collaboration’s Physics Coordinator, explained: “MICE has demonstrated a completely new way of squeezing a particle beam into a smaller volume. This technique is necessary for making a successful muon collider, which could outperform even the LHC.”


Professor Alain Blondel, spokesperson of MICE from 2001 to 2013, and Emeritus Professor at the University of Geneva, commented: “We started MICE studies in 2000 with great enthusiasm and a strong team from all continents.  It is a great pride to see the demonstration achieved, just at a time when it becomes evident to many new people that we must include muon machines in the future of particle physics."


“In this era of ever more-expensive particle accelerators, MICE points the way to a new generation of cost-effective muon colliders,” said Professor Dan Kaplan, Director of the IIT Center for Accelerator and Particle Physics in Chicago.


Professor Paul Soler from the University of Glasgow and UK Principal Investigator said: "Ionization cooling is a game-changer for the future of high-energy muon accelerators, such as a muon collider, and we are extremely grateful to all the international funding agencies, including STFC in the UK, for supporting the experiment and to the staff at the ISIS neutron and muon source for hosting the facility that made this result possible.”


Dr Chris Booth, of the University of Sheffield, who led the development of the particle-production target on ISIS, said: “The techniques demonstrated by the MICE experiment will allow new particle accelerators to be constructed which could explore fundamental physics in areas ranging from neutrinos to the Higgs boson and the energy frontier that lies beyond the reach of the LHC.”


Dr. Maurizio Bonesini, who led the effort of National Institute for Nuclear Physics (INFN) Italy, to develop the high-performance particle-identification system of the experiment, said: "It is a great pleasure to see the successful completion of such an interdisciplinary effort pointing to a fruitful international collaboration  in the field of new accelerators."

 

Professor Emeritus John Cobb of Oxford University Physics Department and the John Adams Institute for Accelerator Science, who jointly led the design and operation of one of the superconducting magnets, said: “Even though the physics underlying ionization cooling has been understood for many years, its practical implementation is extremely difficult. MICE successfully confronted the formidable technical challenges of designing and operating a system of coupled, high-field superconducting magnets, subject to potentially enormous forces, which tightly enveloped a delicate vessel containing many litres of (potentially explosive) liquid hydrogen. The practical experience gained will be invaluable to the further development of muon accelerators.”

 

Professor Jingyu Tang of Institute of High Energy Physics, Chinese Academy of Sciences who led China’s participation in MICE said, “We are happy that China contributed to this fascinating project. We all benefit from the international efforts in the execution of the MICE experiment.”


Professor Yoshitaka Kuno of the University of Osaka, leader of the Japanese team that designed and built the liquid hydrogen absorber congratulated the collaboration on its fantastic success in demonstrating muon ionization cooling and said: "It is a great step towards a future muon collider. The ionization cooling technique can also be used to make a cold muon beam to enhance significantly the future development of low-energy muon physics, including searches for charged lepton flavour violation through the observation of .”

 


Contact: Taylor, Josephine (STFC,RAL,PPD)