Lepton Flavour Physics using Intense Muon Beams
P. Dauncey, P. Dornan, E. Gillies, P Jonsson, B. Krikler, A. Kurup, J. Nash, J. Pasternak, Y. Uchida
The COMET (Coherent Muon-to-Electron Transition) Experiment is the next flagship experiment for the J-PARC proton beam laboratory in Japan, replacing T2K/Super-K/Hyper-K. Studying the process of muon-electron conversion at a sensitivity of 10-17, 10,000 times better than the current limit, COMET is on the frontier for the search for new physics through types of decays known as charged lepton flavour violating processes.
Producing roughly 1018 muonic aluminium atoms (atoms with a muon orbiting them, just like the electrons around any ordinary atom), COMET will watch to see if any of the muons spontaneously convert into an electron. Observing this would be one of the most fundamentally significant results in particle physics, opening up a whole new era of investigation where we would be able to study the Universe at its smallest scales, well beyond levels that are probed by "direct" methods such as at the ATLAS and CMS experiments at the LHC.
Muon-to-Electron conversion has been studied since the 1950s but COMET (and its cousin at Fermilab, Mu2E) represents a new generation of experiment where modern, high-power proton beams are used to produce immense quantities of muons in dedicated beamlines that are designed expressly for this purpose. Previous experiments, such as the current record holder, SINDRUM II, were placed in existing particle beamlines and ran a little more like traditional "fixed-target" experiments, where targets are placed in beams and the outgoing particles observed.
The figure above shows a schematic of COMET, showing the various component that produce the muonic atoms and observe the electrons emerging from them. An incoming beam of protons is collided with a solid target producing large amounts of pions, which then decay to muons. This high intensity muon beam is then directed onto an aluminium target, where they are stopped and form muonic atoms. A detector observes the particles emitted from the aluminium over the time-scale of a microsecond - the lifetime of the muonic atoms.
The curved parts of the experiment after the proton target and the muon target allow us to choose the momentum and charge of the particles reaching the target. The first curved section allows only slow negative muons to reach the muon target, whereas the second selects only the most interesting electrons (whose energy is close to the mass of the muon) for the particle detector.
Imperial and UK activities
COMET at Imperial College and in the UK
The Imperial College group was the first UK group to get involved in the COMET/PRISM programme, with a Royal Society joint project in 2007 between Imperial and the University of Osaka. In the years since then we have been joined by UCL, Manchester and Oxford universities, which together form the COMET-UK collaboration.
The Imperial College group is involved with COMET through:
Overseeing the software to run simulations, optimise the experiment's setup and eventually to analyse the data
Developing the trigger and data acquisition system to extract data from the actual experiment
Helping the design and construction of several parts of the experiment including the superconducting magnets needed to control the beam and the trigger system.
The UK is also involved in further design studies for the COMET beamline and its detectors, and the engineering of the pion production system, in which our colleagues at Rutherford Appleton Laboratory lead the world through their experience at ISIS, T2K and the Neutrino Factory. We also have strong superconducting magnet expertise and teams of engineers and technicians who can supply COMET with the valuable experience gained by building T2K and other experiments.
As of early 2013, 27 institutes from 12 countries participate in COMET, and Dr Y. Uchida from our group was elected by the collaboration as Chair of the Collaboration Board, to lead the experiment alongside the Spokesperson Y. Kuno from Osaka University and Project Manager S. Mihara of KEK.
Muon-to-Electron Conversion may be observed at COMET Phase-I, or the full COMET Experiment. In either case, it is important that high-precision measurements be made of the process, and also that it be investigated for different muonic atoms to aluminium. This is because the way the rate of muon-to-electron conversion changes for different target materials, depends on the kind of new physics driving the process.
PRISM is a future experiment that allows us to observe a hundred times the number of possible muon-to-electron conversions than COMET. This enables us to vary the target material in which the muonic atoms are produced to include those that are harder to measure. It uses an FFAG accelerator to produce a very well-behaved muon beam that is optimised for muon-to-electron conversion studies, which would also provide valuable experience for future muon collider designs.
COMET is a highly challenging experiment, where a single stray electron can spoil the whole measurement if it is not understood. Several aspects of particle behaviour, such as hadronic interactions at the several hundred MeV scale and the properties of pions and muons as they impinge on matter, need to be well controlled and yet, are poorly understood at present.
Therefore, it has been decided to pursue COMET in a staged approach, with Phase-I currently being built.
This involves creating the full proton beamline and the target on which the pions are produced, and part of the novel structure that sends these pions along as they decay into muons. There, we will position a newly-designed set-up that will work with following two objectives:
- Measure the pion and muon beam at the end of the partial beamline, and physics measurements of the high intensity beam's interaction with matter to prepare for Phase-II,
- Perform physics studies of lepton flavour violation, including muon-to-electron conversion, but also other process that cannot be studied at the full COMET experiment.
The expected sensitivity to muon-electron conversion for Phase-I is around 100 times better than that of SINDRUM-II.
Muon electron conversion
What is muon-electron conversion?
"Who ordered that" — I.I. Rabi, on the discovery of the muon
The search for such processes goes right back to the 1930's. At first, muons, being identical to the electron though roughly 200 times more massive, were understood to be exactly that — a heavy electron. This led to searches for the process, mu → e + γ, with the expectation that the muon could simply shed it's mass by emitting a photon, leaving behind the much lighter electron.
As scientists came back emtpy handed from these searches, the notion of lepton number was developed. If only one electron goes into a reaction, then the total number of electrons and electron-neutrinos coming out of the reaction, minus the total number of positrons (anti-electrons) and electron-antineutrinos, must also be one. The same is true for muons and taus. Lepton flavour conservation became a fundamental part of the Standard Model, one of our best and most tested theories, used by physicists to explain the sub-atomic world we see.
Neutrinos that change type
At least, that was true until recently. T2K/Super-K/Hyper-K , among other experiments, observed a phenomena known as neutrino oscillations. A beam of one type of neutrinos was found to contain neutrinos of a different flavour, a while later. Imagine kicking a football yet seeing your friend catch a basketball. Straight away, we see that the notion of lepton number conservation cannot be correct. Yet so far this has only been seen amongst neutrinos. Can the charged leptons (the electron, the muon and the tau) do the same thing? This is the question that COMET will try to answer.
What is more, if we do see this process at COMET, this is physics beyond not only the Standard Model, but also neutrino oscillations, as the prediction of the rate for muon-electron conversion using neutrino oscillations alone is smaller than we can observe. Yet almost any theory that extends the Standard Model, allows for the process to occur at rates comfortably within the range visible to COMET.